## Civilian Avionics

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gorcee
Posts: 1501
Joined: Sun Jul 13, 2008 3:14 am UTC

### Re: Civilian Avionics

nehpest wrote:
gorcee wrote:All this talk of gyros is kind of silly, considering any weight gain saved is going to be much smaller than engineering the control surfaces to meet free-play specifications. All-moving horizontal tail free-play limits are [imath]/pm 0.017^\circ[/imath], or a total of [imath]0.034^\circ[/imath]. In order to meet that, you're going to need some seriously beefy electrohydrostatic actuators. I don't believe that any current production aircraft meet this specification, because they are waived by wind-tunnel testing.

But to get such a waiver for a supersonic vehicle, you'll need to do transonic testing. There's one (well, two, kind of) wind tunnel in the US that has continuous dynamics testing, and it costs around \$80,000 per day to operate, and that doesn't include the cost of constructing the model.

I'm posting from my phone so I won't respond directly to everybody's posts, but I wanted to let ya'll know that our next full meeting, so I'll be able to provide a full update after that.

Also, re wind tunnel testing: this is one area in which we're quite fortunate. We have a supersonic wind tunnel right here on campus that can exceed our current design goals; I've read some of the masters theses from previous years, and learned that it's apparently pretty nicely appointed. University policy is that we're free to use it as long as we don't interrupt any faculty usage.

Supersonic and transonic wind tunnels are two different beasts. There is also the difference between continuous flow and blow-down tunnels. Blow-down tunnels can't be used for dynamics testing, as you only get a max of about 6 seconds of transonic speed.

Supersonic tunnels are, in general, much, much easier to build/operate than transonic. The reason is because you can put the sting/model aft of the convergent-divergent section, where the flow is always supersonic. Transonic is much more difficult, because you have to operate in a range of M=0.8-1.2. The transonic range is so difficult to work in that the best aerodynamicists and aeroelasticians in the world still do not understand all of the phenomena that take place.

NASA's Transonic Dynamics Tunnel (TDT) is basically the only continuous flow tunnel in the country. Calspan also operates a transonic tunnel, but it's a bit smaller, and privately owned. The last time I priced out testing, they both cost about the same.

Regarding vehicle dynamics testing:

In general, most physical phenomena are governed by a dynamical system. In general, you can break things down into a number of states (x) and outputs (y). For control systems, you also have control terms, (u) (x y and u are all vectors).

Typically, what we aim to do is write the system in the form [imath]\dot{x}=f(t,x(t),u(t))[/imath]. In a linear dynamical system, we can write this as [imath]\dot{x}=Ax(t)+Bu(t)[/imath], where A and B are matrices. Not every state may be observable. So our outputs are [imath]y=Cx+Du[/imath], where again, C and D are matrices (often, D is zero). C is the observability matrix. If you can directly measure a state with a sensor, [imath]x_i[/imath], then you set [imath]C_{ii}=1[/imath].

An example that I use quite often in my work is that of a simple air vehicle model. There are five states, representing orientation and velocity. There are three inputs: rudder, elevator, and ailerons. It is nonlinear in its states, with a final form of [imath]\dot{x}=Ax+f(x)+Bu[/imath]

So, my control system assumes measurements on these states, and then does things by changing the elements of u to make the vehicle do what I want. Of course, this is a high-level, simplified model. In reality, there is a similar model between the pilot and the ailerons, the pilot and the rudder, etc. And then there are even similar models for the sensors. This model is linear in its inputs, but in practice this is never the case. There are all sorts of nonlinearities in controls: dead-zones, backlash, saturation, etc.

For subsonic flight, all of this is more or less pretty simple to handle. For supersonic flight, it's more complicated, but simple to handle. For the range between M~=0.8 and M~=1.2, it's a goddamn nightmare.

But, the good news is that it is a fascinating research problem!

Anyhow, this is one of my areas of research, so I'd be more than happy to answer more questions and advise in any way. I'm not trying to piss in your cheerios. Instead, I'd rather help you drive your energies to the most immediately productive avenues.

Figuring out what sensors you need is usually something that happens later in the design process. Some things are obvious: a pitot-static probe for measuring airspeed, for instance. But others are not as obvious. And generally what is done is that the choices of sensors is dependent on the demands of the feedback control system. In other words, in order to close the loop, what elements of the observability matrix do you need to fill out?

Sometimes, you can't fill them all out. In a jet engine, for instance, it may be really hard (or cost prohibitive) to get a temperature reading, or an RPM reading. In such a case, you have to look at your state space model and see what you can do. Sometimes, you have to add (or subtract) a few different state measurements together to get the desired output.

But sensors aren't just there for the sake of being there. They have a purpose, and this purpose is to relay information to the pilot (or flight control computer). The starting point of avionics design is the flight controller. The sensors are what you plug into it. You don't start building a computer by buying a keyboard and a mouse. Likewise, you don't start by planning sensors without knowing what you need. So the first thing I would do in your situation is try to start with a basic flight dynamics model. You don't need to be an aero engineer to do this (though some knowledge does help). But you can start by building a computer model of the vehicle, perhaps in Simulink. This will give you a basic idea of the overall requirements for basic flight. As time goes on, you can expand this model to include other things (landing gear, engine controls, fuel monitoring, etc. etc. etc.).

But you've definitely got to start at the beginning, and the most fundamental question is: what am I measuring and why?

nehpest
Posts: 518
Joined: Fri Jun 12, 2009 9:25 pm UTC

### Re: Civilian Avionics

Sockmonkey wrote:IIRC a team working on an M3+ missle in the 60s used a commercially available car manifold paint for certain high temp areas. Remember, engineering solutions are everywhere. Even in apparently unrelated areas.

Bolded is too true. I'll keep that in mind!

p1t1o wrote:
nehpest wrote:However, I don't know what skin temp sensors are meant to accomplish. Does that have to do with the aerodynamic heating p1t1o mentioned?

Yup, temperature can effect structural strangth, flexibility and so on, you need to know that it is not going to fly apart if you speed up. Wind tunnel testing though will be a good indicator of how much heating will take place - see below.
<snip>
Hehehe, nope, the above is an extreme example. For a body at around mach one, I imagine the heating will be on the order of a hundred or so degrees celcius, give or take a bunch, depending on the planned altitude. Also your aircraft will accelerate alot more slowly, allowing heat to equilibrate somewhat. The sprint missile accelerated at 100gs and had a heat shield that boiled off to remove excess heat, plus, it only had to survive a maximum of about 15 seconds from launch to intercept anyway.

Right. We talked about heat a bit in the meeting, and it seems that they aren't too concerned about it.

gorcee wrote:Lemme drop some knowledge on ya.

I was beginning to suspect you work in aerospace, or perhaps a related industry. In any case, a million thanks for that post. I'll just respond briefly:

Your remarks on wind tunnels led to about an hour of fascinating Googling and wiki-ing. I now feel a lot more comfortable with the theory behind what's going on. Also, I learned at our meeting yesterday that we actually have two tunnels on campus; one is new and subsonic, which we'll be using for the small scale model (which we'll start constructing a week from today, squee!) while the other is rather older, and is supersonic. I don't know whether the supersonic tunnel is blowdown or continuous flow, but from your post it seems likely to be blowdown. This is consistent with the masters theses I read, all of which either use the subsonic tunnel, or had test times under a minute.

Re dynamic systems: I think I followed most of what you were saying. The [imath]\dot{x}[/imath] stands for a time derivative in your notations, right? It will take me a few more readthroughs, and probably some library time, to fully appreciate what you're talking about, but it superficially makes sense now.

I really appreciate it, actually. Thanks!

On sensors: I was relieved when you said they are usually selected later in the design process. We discussed sensors a bit yesterday, and apparently we're going to go with a fairly minimal set right now. Airspeed, GPS, AoA, and altimeter are all we're looking at for the moment. Oh, and a video camera in the nose.

A bunch of design decisions were made in my absence from the previous 2 meetings, so here's a quick update:

• Flight plan: we're looking at flying in a racetrack pattern, as I mentioned earlier. The straightaways are going to be roughly 40 miles, and we're looking at having the plane make the U-turns in a 5 mile radius. It seems that we're aiming to have the plane make the turns at speed, which I'm told translates into pulling 10 Gs.
• Airframe: we're going with an F104-ish design, but greatly simplified. The structures guys were talking about a Haack body; wiki tells me this is a low-drag airframe. We're going to be using carbon fiber for the skin and foam for the innards.
• Propulsion: We've selected our propulsion method, too: there will be no rocketry, because we're going with either a single or twin JetCat P200s. Our president is in contact with JetCat, trying to persuade them to donate the engine, which is pretty sweet.

We're going to start fabricating the small scale model a week from today with the first of our dues money. I'll put a more in-depth update on my blog when I get a bit of free time (likely later tonight). Thanks everyone for the input so far, I'll keep you all updated!
Kewangji wrote:Someone told me I need to stop being so arrogant. Like I'd care about their plebeian opinions.

blag

gorcee
Posts: 1501
Joined: Sun Jul 13, 2008 3:14 am UTC

### Re: Civilian Avionics

nehpest wrote:
gorcee wrote:Lemme drop some knowledge on ya.

I was beginning to suspect you work in aerospace, or perhaps a related industry. In any case, a million thanks for that post. I'll just respond briefly:

Your remarks on wind tunnels led to about an hour of fascinating Googling and wiki-ing. I now feel a lot more comfortable with the theory behind what's going on. Also, I learned at our meeting yesterday that we actually have two tunnels on campus; one is new and subsonic, which we'll be using for the small scale model (which we'll start constructing a week from today, squee!) while the other is rather older, and is supersonic. I don't know whether the supersonic tunnel is blowdown or continuous flow, but from your post it seems likely to be blowdown. This is consistent with the masters theses I read, all of which either use the subsonic tunnel, or had test times under a minute.

Re dynamic systems: I think I followed most of what you were saying. The [imath]\dot{x}[/imath] stands for a time derivative in your notations, right? It will take me a few more readthroughs, and probably some library time, to fully appreciate what you're talking about, but it superficially makes sense now.

I really appreciate it, actually. Thanks!

On sensors: I was relieved when you said they are usually selected later in the design process. We discussed sensors a bit yesterday, and apparently we're going to go with a fairly minimal set right now. Airspeed, GPS, AoA, and altimeter are all we're looking at for the moment. Oh, and a video camera in the nose.

A bunch of design decisions were made in my absence from the previous 2 meetings, so here's a quick update:

• Flight plan: we're looking at flying in a racetrack pattern, as I mentioned earlier. The straightaways are going to be roughly 40 miles, and we're looking at having the plane make the U-turns in a 5 mile radius. It seems that we're aiming to have the plane make the turns at speed, which I'm told translates into pulling 10 Gs.
• Airframe: we're going with an F104-ish design, but greatly simplified. The structures guys were talking about a Haack body; wiki tells me this is a low-drag airframe. We're going to be using carbon fiber for the skin and foam for the innards.
• Propulsion: We've selected our propulsion method, too: there will be no rocketry, because we're going with either a single or twin JetCat P200s. Our president is in contact with JetCat, trying to persuade them to donate the engine, which is pretty sweet.

We're going to start fabricating the small scale model a week from today with the first of our dues money. I'll put a more in-depth update on my blog when I get a bit of free time (likely later tonight). Thanks everyone for the input so far, I'll keep you all updated!

Excellent. Composites are actually fairly easy to work with on the prototyping level. I don't know what you have in the way of facilities, but where I went, RPI, we had a composites lab that was fairly accessible.

My work has me about 75% in the aerospace industry. My company specializes in controls and navigation, although our research is fairly diverse. Some of my current/recent work has me working on control systems for suppression of aeroelastic phenomenon, diagnostics/prognostics of sensor/actuator faults, fatigue life prediction of jet engine turbines, and health monitoring/condition based maintenance scheduling of land-based vehicles. The majority of my work comes from NASA/DoD, and it's largely R&D-oriented, so it just so happens that the things you're asking about are more or less things that I deal with every day.

A Haack body type maps the cross-sectional area of the aircraft to a cigar shape. It turns out that it doesn't matter a whole lot what the actual cross-sectional shape is (the cross section being taken in a plane orthogonal to the longitudinal axis of the vehicle), as long as the area, if drawn as a perfect circle and extracted longitudinally, would look like a cigar. Look at a bottom-up view of any large jetliner. The fuselage appears to get skinnier where the wings are. This is to try to maintain that shape. Specifically, this is done to reduce wave drag at near-transonic speeds.

52 lb of thrust isn't a whole lot. If you have two engines, then you'll be pushing 104 lbs, and if you want to get supersonic I'm guessing that your vehicle is going to have a span of not much more than 2-3 ft. maybe up to 5'? I have a hard time envisioning anything much bigger than that being able to power through that compressibility region with only 104 lb of thrust.

nehpest
Posts: 518
Joined: Fri Jun 12, 2009 9:25 pm UTC

### Re: Civilian Avionics

gorcee wrote:
Big ol' snip
Excellent. Composites are actually fairly easy to work with on the prototyping level. I don't know what you have in the way of facilities, but where I went, RPI, we had a composites lab that was fairly accessible.

My understanding is that we'll be using a 3d printer for the nose cone, but that at least for the prototype we'll be fabricating by hand. In fact, that's what we're doing on Friday - sanding carbon fiber into the proper shapes.

My work has me about 75% in the aerospace industry. My company specializes in controls and navigation, although our research is fairly diverse. Some of my current/recent work has me working on control systems for suppression of aeroelastic phenomenon, diagnostics/prognostics of sensor/actuator faults, fatigue life prediction of jet engine turbines, and health monitoring/condition based maintenance scheduling of land-based vehicles. The majority of my work comes from NASA/DoD, and it's largely R&D-oriented, so it just so happens that the things you're asking about are more or less things that I deal with every day.

That's a happy coincidence for me, and it sounds like a fascinating line of work. Also coincidentally, I signed up to come up with the maintenance/ops plan for the plane on Thursday, which gives me a hand in determining our constraints - I'd like to continue picking your brain if you don't mind!

52 lb of thrust isn't a whole lot. If you have two engines, then you'll be pushing 104 lbs, and if you want to get supersonic I'm guessing that your vehicle is going to have a span of not much more than 2-3 ft. maybe up to 5'? I have a hard time envisioning anything much bigger than that being able to power through that compressibility region with only 104 lb of thrust.

We're planning on roughly a 3 foot wingspan, and an upper weight limit of 100 pounds, or 80 if we can manage it. The fuel is supposedly going to account for ~20lbs of that. With one turbine, we'd be looking at a thrust-to-weight ratio of between .50 and .65, depending on our final weight. While this compares favorably to the Concorde, it pales in comparison to the other aircraft on the list. If we were to go with twin engines, our T/W would nearly double, since the turbines only weigh 5 lbs apiece.

Also, for anybody who is interested, I updated the blag with a bit on this... more to come, as always.
Kewangji wrote:Someone told me I need to stop being so arrogant. Like I'd care about their plebeian opinions.

blag

gorcee
Posts: 1501
Joined: Sun Jul 13, 2008 3:14 am UTC

### Re: Civilian Avionics

nehpest wrote:That's a happy coincidence for me, and it sounds like a fascinating line of work. Also coincidentally, I signed up to come up with the maintenance/ops plan for the plane on Thursday, which gives me a hand in determining our constraints - I'd like to continue picking your brain if you don't mind!

Maintenance isn't the first thing I would worry about for a prototype vehicle. It probably won't be flying super-often, so designing for a lifespan of 40 hours is reasonable. You probably won't have to worry about the traditional fatigue issues in your turbines, although I can't really speak for what might come up, because I don't know what the engine is made of, etc.

At this scale, I would worry about two (three) things: the ability for the engine to actually run at supersonic speeds, the ability of the airframe to handle compressibility loads, and the third, which is important, but tangential, the static stability of the aircraft. Without a controller on board, you'll need full 3DOF static stability. That's as much for the aero guys to work out as not, but that also becomes a control systems problem.

Regarding loading, since this is a test vehicle, I would push to include some accelerometers/strain gauges on the wing and tail surfaces. Every flight structure has a flutter velocity, and you're going to encounter that somewhere in your flight envelope. One of the tests you'll need to do is ground vibration testing (GVT), which involves putting the model on a shake table and computing its vibrational modes. You'll need these sensors anyways to compute the damping ratios, etc. during the GVT, so you may as well include them in the actual design.

Aeroelasticity and aeroservoelasticity are going to be your biggest enemies with this. Free-play induced LCO is going to kill you unless you have a plan for handling it. Carbon fiber-wrapped foam is fairly stiff, but the forces at compressibility are vicious. A quick calculation says that at Mach 0.8, at sea level, you're looking at dynamic pressures of around 947 psf. The problem with foam, structurally, is that it exhibits plastic deformation somewhat quickly. So you'll want to do low speed wind tunnel tests first to try to determine the aeroelastic modes to try to determine what kind of loading you might see in flight.

Last, regarding your engine. I'm not entirely positive these 50 lb thrust engines are going to really do it for you. It's just not a matter of thrust v. weight, even though that's a number that gets used a lot. It's a thrust vs. wing area issue.

Take the F-104, for instance. This had a zero lift drag coefficient of about 0.017, and an induced drag coefficient of 0.048. Now, remember, drag coefficients for aircraft are based on the wing planform area, not the frontal cross-sectional area. Why is the wing area important? Recall that the drag equation is

$D=\frac{1}{2}\rho U^2 S C_D.$

S is your wing planform area. Your thrust has to directly counteract this drag. Our drag coefficient is determined by design, and the density of air only really changes with altitude (and that difference is not huge in your operating envelope). So for a given airspeed, we have to consider thrust vs. wing area (wing area is also involved in lift, which is why thrust v. weight is used, but for our comparisons, let's assume a suitable C_L is obtained). the F-104 put out 15,600 pounds of afterburning thrust (10,000 lb dry), and had a wing area of 196 ft^2. It generated 9244 lb of drag at Mach 0.7

Assume you used a perfect scale model of that aircraft, you're looking at a wing area of about 3.7 ft^2. At Mach 0.7, you'd be generating 174 lb of drag, which is more than what 3 of those engines would put out. So you'll have to substantially reduce your drag coefficient to achieve even Mach 0.7.

brötchen
Posts: 112
Joined: Mon Aug 31, 2009 1:45 pm UTC

### Re: Civilian Avionics

Would it be feasible to retrofit the engines with an afterburner? while I'm not very knowledgeable in the area but it seems to me that an afterburner is a fairly light piece of equipment which could improve your thrust to wight ratio quiet a bit (but also wastes a metric f#&k-ton of fuel). is it an requirement that the flight is level to achieve your goal of braking some sort of record? it would seem to me that you would need less thrust if you flew slightly downward while attempting to reach supersonic speeds for example, if I'm not missing some thing, your thrust to wight ratio would be increased by
0.5 if your flight path was a 30° decline.

edit: after looking at the wikipedia page about the sr-71 (which can reach speeds of mach 3.2+) i found that it has a thrust to whiegt ratio of just 0.382 so thrust might not be your main problem anyway (althoug i dont know if your engines will develop their full thrust beyond mach 1)

excuse my bad englisch, I'm not a native speaker
Last edited by brötchen on Sun Feb 13, 2011 3:43 pm UTC, edited 1 time in total.

gorcee
Posts: 1501
Joined: Sun Jul 13, 2008 3:14 am UTC

### Re: Civilian Avionics

brötchen wrote:Would it be feasible to retrofit the engines with an afterburner? while I'm not very knowledgeable in the area but it seems to me that an afterburner is a fairly light piece of equipment which could improve your thrust to wight ratio quiet a bit (but also wastes a metric f#&k-ton of fuel). is it an requirement that the flight is level to achieve your goal of braking some sort of record? its would seem to me that you would need less thrust if you flew slightly downward while attempting to reach supersonic speeds for example, if I'm not missing some thing, your thrust to wight ratio would be increased by
0.5 if your flight path was a 30° decline.

excuse my bad englisch, I'm not a native speaker

The power gain would be offset at least in part by the weight gained by building an afterburner nozzle. And the heating of this element would require some other type of support structure capable of withstanding the heat.

EdgarJPublius
Official Propagandi.... Nifty Poster Guy
Posts: 3726
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Location: where the wind takes me

### Re: Civilian Avionics

An afterburner is a good idea if you go for a jet over a rocket. The massive increase in available thrust without much increase in mass and volume of the engine makes reaching supersonic speeds much easier. You do have to watch fuel consumption with an afterburner though, it'll provide nearly double the thrust, but will eat more than twice as much fuel.

If you use rockets though, then it doesn't matter

I recommend reading up on Critical Mach Number (the speed at which airflow over part of the wing becomes supersonic, which is less than the speed when the aircraft becomes supersonic because air flows over the wing faster than the aircraft moves through the air), and Mach Tuck A combination of conditions that can occur when the critical mach number is reached (shockwaves form over the wing surface, sapping lift by stalling part of the wing, moving the center-of-pressure backwards and reducing the effectiveness of separate controll surfaces). That collectively lead to a nose-down attitude.

It's a good idea to counter this with airfoils designed with the critical mach number in mind, having some way to counter the shifting center of pressure (generally by shifting fuel around to adjust the center of mass and the trim, but can also be accomplished with aerodynamic controls such as the 'speed-bump flaps' on the P-38) and having full-moving elevator controls.

I know some of this has been mentioned already and many of y'all are already aware of this stuff, but I thought it would be a good idea to make some more explicit statements and give the 'proper' name to some these ideas.
Roosevelt wrote:
I wrote:Does Space Teddy Roosevelt wrestle Space Bears and fight the Space Spanish-American War with his band of Space-volunteers the Space Rough Riders?

Yes.

-still unaware of the origin and meaning of his own user-title

2385a4
Posts: 8
Joined: Mon Jan 17, 2011 8:58 am UTC

### Re: Civilian Avionics

Three small points no-one else seems to have mentioned yet:

1. It's no good saying that your minimum ceiling is 10,000 ft AMSL. You need to work out your target ceiling to a slightly finer resolution because it affects your choice of electronics. High altitude operation affects the reliability of electronics, primarily due to the increasing intensity of cosmic rays as altitude increases. Even at 10,000 feet, the highly damaging proton flux is around 50 times greater than at sea level, and still increasing exponentially. By 30,000 ft, it's about 1,000 times stronger. That's a lot of random bit-flips. (Other problems include: temperature extremes; reduced efficiency of cooling fans; low humidity and triboelectrification; low pressure sparking; outgassing of electrolytic caps; and failure of air-bearings in hard drives. And in any aircraft, naturally high frequency vibration is also a big problem.)

Consequently electronics are available in "aerospace" ratings, which are radiation hardened but extremely expensive. If money is no object you might just get aerospace rated stuff anyway, but on a budget, you may like to calculate the probability of a soft or hard error per unit time at you target altitude, and see what you need to harden and what you can get away with. Some non-aerospace systems might be OK if you make them dual redundant. If you do get standard aerospace rated systems, you may find that dictating your voltage backplane (28 V, I seem to recall?) and communications bus protocols and cabling.

Presumably the FAA will have rules on this, but I don't know them.

This is particularly the case with a remote controlled supersonic aircraft that you plan to fly over land in a populated area. If the control system fails, it could "land" anywhere within about a 90 mile radius, and if there's property or people there, that's a disaster. So you require extremely high reliability from your control system; certainly well over 99%, indeed well over 99.9% (Or, perhaps, a moderately reliable self-destruct; I did read an interview with Nick Sabatini of the FAA, who was involved in defining new rules for UAVs. He was saying that if the applicant couldn't guarantee that they could control the aircraft at all times, then they had to be able to destroy it.)

2. You'll also want to read this:
http://en.wikipedia.org/wiki/DO-178B

3. Incidentally, 99% reliability is not the same thing as wanting to hit 99 flights before failure. If you system has 99% reliability per flight, the chance of completing your 99th flight without failure is only 37%. To have 50:50 odds of 99 flights before failure, you need 99.3% reliability. For 99% chance of 99 flights without failure, you need 99.99% reliability. Achieving 99.99% reliability on a novel complex system is extremely difficult. You will need a design with lots of redundancy, fail safety, and plenty of subassembly testing.
Cheers.
Beulah.

p1t1o
Posts: 956
Joined: Wed Nov 10, 2010 4:32 pm UTC
Location: London, UK

### Re: Civilian Avionics

With travel distances of 40+miles and heights of n thousands of feet and whatever safety margin, is the aircraft going to require some degree of "robotic" autonomy should the communications with ground fail? If not then I'd be curious as to what sort of communications hardware is necessary.

gorcee
Posts: 1501
Joined: Sun Jul 13, 2008 3:14 am UTC

### Re: Civilian Avionics

2385a4 wrote:Three small points no-one else seems to have mentioned yet:

1. It's no good saying that your minimum ceiling is 10,000 ft AMSL. You need to work out your target ceiling to a slightly finer resolution because it affects your choice of electronics. High altitude operation affects the reliability of electronics, primarily due to the increasing intensity of cosmic rays as altitude increases. Even at 10,000 feet, the highly damaging proton flux is around 50 times greater than at sea level, and still increasing exponentially. By 30,000 ft, it's about 1,000 times stronger. That's a lot of random bit-flips. (Other problems include: temperature extremes; reduced efficiency of cooling fans; low humidity and triboelectrification; low pressure sparking; outgassing of electrolytic caps; and failure of air-bearings in hard drives. And in any aircraft, naturally high frequency vibration is also a big problem.)

Consequently electronics are available in "aerospace" ratings, which are radiation hardened but extremely expensive. If money is no object you might just get aerospace rated stuff anyway, but on a budget, you may like to calculate the probability of a soft or hard error per unit time at you target altitude, and see what you need to harden and what you can get away with. Some non-aerospace systems might be OK if you make them dual redundant. If you do get standard aerospace rated systems, you may find that dictating your voltage backplane (28 V, I seem to recall?) and communications bus protocols and cabling.

Presumably the FAA will have rules on this, but I don't know them.

This is particularly the case with a remote controlled supersonic aircraft that you plan to fly over land in a populated area. If the control system fails, it could "land" anywhere within about a 90 mile radius, and if there's property or people there, that's a disaster. So you require extremely high reliability from your control system; certainly well over 99%, indeed well over 99.9% (Or, perhaps, a moderately reliable self-destruct; I did read an interview with Nick Sabatini of the FAA, who was involved in defining new rules for UAVs. He was saying that if the applicant couldn't guarantee that they could control the aircraft at all times, then they had to be able to destroy it.)

2. You'll also want to read this:
http://en.wikipedia.org/wiki/DO-178B

3. Incidentally, 99% reliability is not the same thing as wanting to hit 99 flights before failure. If you system has 99% reliability per flight, the chance of completing your 99th flight without failure is only 37%. To have 50:50 odds of 99 flights before failure, you need 99.3% reliability. For 99% chance of 99 flights without failure, you need 99.99% reliability. Achieving 99.99% reliability on a novel complex system is extremely difficult. You will need a design with lots of redundancy, fail safety, and plenty of subassembly testing.

1.) They're looking at an aircraft with a 3ft wingspan. I'd be shocked if it can carry fuel to reach 10k ft, let alone 30k.

2.) They're looking at an aircraft with a 3ft wingspan made out of carbon fiber wrapped foam. The vehicle will probably degrade structurally long before the 99th flight.

nehpest
Posts: 518
Joined: Fri Jun 12, 2009 9:25 pm UTC

### Re: Civilian Avionics

gorcee wrote:Maintenance isn't the first thing I would worry about for a prototype vehicle. It probably won't be flying super-often, so designing for a lifespan of 40 hours is reasonable. You probably won't have to worry about the traditional fatigue issues in your turbines, although I can't really speak for what might come up, because I don't know what the engine is made of, etc.

Oh, definitely agreed. No, this "plan" is really a skeleton document that we're putting together as part of our "I can has outside funding" package. There isn't much in it that is particularly groundbreaking, and almost everything in it is subject to change. WRT the turbine, JetCat USA (our supplier) has a three-year limited warranty that covers turbine replacement every 25 flight hours. I (VERY optimistically) provided for a complete airframe replacement after 50 flight hours or 100 flights, whichever comes first.

At this scale, I would worry about two (three) things: the ability for the engine to actually run at supersonic speeds, the ability of the airframe to handle compressibility loads, and the third, which is important, but tangential, the static stability of the aircraft. Without a controller on board, you'll need full 3DOF static stability. That's as much for the aero guys to work out as not, but that also becomes a control systems problem.

The club president seems confident that the engine can run at supersonic speeds; I remember something being said about exhaust velocity, but I'm sure there's more to his assurance than just that. As for compressibility, I think we'll get the definitive answer on that round about the end of April, which is when we've tentatively scheduled our first wind tunnel tests on the scale model. I don't know much about the last item, but I believe the aero guys have some ideas for it, as I heard them discussing it before the last meeting.

Regarding loading, since this is a test vehicle, I would push to include some accelerometers/strain gauges on the wing and tail surfaces. Every flight structure has a flutter velocity, and you're going to encounter that somewhere in your flight envelope. One of the tests you'll need to do is ground vibration testing (GVT), which involves putting the model on a shake table and computing its vibrational modes. You'll need these sensors anyways to compute the damping ratios, etc. during the GVT, so you may as well include them in the actual design.

This is valuable advice, and I will pass it along! I seem to remember accelerometers being small and cheap, so it shouldn't be a hard fight.

Aeroelasticity and aeroservoelasticity are going to be your biggest enemies with this. Free-play induced LCO is going to kill you unless you have a plan for handling it. Carbon fiber-wrapped foam is fairly stiff, but the forces at compressibility are vicious. A quick calculation says that at Mach 0.8, at sea level, you're looking at dynamic pressures of around 947 psf. The problem with foam, structurally, is that it exhibits plastic deformation somewhat quickly. So you'll want to do low speed wind tunnel tests first to try to determine the aeroelastic modes to try to determine what kind of loading you might see in flight.

After some Googling, I conclude that I lack the knowledge to respond intelligently to the LCO bit, assuming you mean limit cycle oscillations. I haven't covered dynamical systems in a formal class yet, so all I know is what I've picked up by wiki-ing. This, combined with the next portion of your post...

Last, regarding your engine. I'm not entirely positive these 50 lb thrust engines are going to really do it for you. It's just not a matter of thrust v. weight, even though that's a number that gets used a lot. It's a thrust vs. wing area issue.

Take the F-104, for instance. This had a zero lift drag coefficient of about 0.017, and an induced drag coefficient of 0.048. Now, remember, drag coefficients for aircraft are based on the wing planform area, not the frontal cross-sectional area. Why is the wing area important? Recall that the drag equation is

$D=\frac{1}{2}\rho U^2 S C_D.$

S is your wing planform area. Your thrust has to directly counteract this drag. Our drag coefficient is determined by design, and the density of air only really changes with altitude (and that difference is not huge in your operating envelope). So for a given airspeed, we have to consider thrust vs. wing area (wing area is also involved in lift, which is why thrust v. weight is used, but for our comparisons, let's assume a suitable C_L is obtained). the F-104 put out 15,600 pounds of afterburning thrust (10,000 lb dry), and had a wing area of 196 ft^2. It generated 9244 lb of drag at Mach 0.7

Assume you used a perfect scale model of that aircraft, you're looking at a wing area of about 3.7 ft^2. At Mach 0.7, you'd be generating 174 lb of drag, which is more than what 3 of those engines would put out. So you'll have to substantially reduce your drag coefficient to achieve even Mach 0.7.

...is somewhat disheartening, to be sure. The numbers you provided for drag seem correct; I don't know if we have any plans for reducing [imath]C_D[/imath], and in fact I only know that we plan to measure it in a few weeks. I'll bring this up on Thursday and see what kind of response I get.

brötchen wrote:Would it be feasible to retrofit the engines with an afterburner? while I'm not very knowledgeable in the area but it seems to me that an afterburner is a fairly light piece of equipment which could improve your thrust to wight ratio quiet a bit (but also wastes a metric f#&k-ton of fuel). is it an requirement that the flight is level to achieve your goal of braking some sort of record? it would seem to me that you would need less thrust if you flew slightly downward while attempting to reach supersonic speeds for example, if I'm not missing some thing, your thrust to wight ratio would be increased by 0.5 if your flight path was a 30° decline.

edit: after looking at the wikipedia page about the sr-71 (which can reach speeds of mach 3.2+) i found that it has a thrust to whiegt ratio of just 0.382 so thrust might not be your main problem anyway (althoug i dont know if your engines will develop their full thrust beyond mach 1)

excuse my bad englisch, I'm not a native speaker

The engineering details of afterburning (as laid out by gorcee below you) aside, I would hazard to guess that fitting the turbine with a 'burner would probably void our warranty. Since we're not a cash-heavy organization, we would like to get as much free repair work out of our suppliers as possible. Also, to respond specifically to the fuel issue, we're anticipating about 20 pounds of fuel in the final design; this is roughly 3 gallons. With a fuel consumption of 24 oz/minute at full power, that gives us 16 minutes at full power. A more reasonable estimate accounting for time to get to and from altitude and a safety margin, and we're looking at more on the order of 5-8 minutes at full power. An afterburner would cut this down to 2-4 minutes.

EdgarJPublius wrote:An afterburner is a good idea if you go for a jet over a rocket. The massive increase in available thrust without much increase in mass and volume of the engine makes reaching supersonic speeds much easier. You do have to watch fuel consumption with an afterburner though, it'll provide nearly double the thrust, but will eat more than twice as much fuel.

If you use rockets though, then it doesn't matter

I recommend reading up on Critical Mach Number (the speed at which airflow over part of the wing becomes supersonic, which is less than the speed when the aircraft becomes supersonic because air flows over the wing faster than the aircraft moves through the air), and Mach Tuck A combination of conditions that can occur when the critical mach number is reached (shockwaves form over the wing surface, sapping lift by stalling part of the wing, moving the center-of-pressure backwards and reducing the effectiveness of separate control surfaces). That collectively lead to a nose-down attitude.

It's a good idea to counter this with airfoils designed with the critical mach number in mind, having some way to counter the shifting center of pressure (generally by shifting fuel around to adjust the center of mass and the trim, but can also be accomplished with aerodynamic controls such as the 'speed-bump flaps' on the P-38) and having full-moving elevator controls.

I know some of this has been mentioned already and many of y'all are already aware of this stuff, but I thought it would be a good idea to make some more explicit statements and give the 'proper' name to some these ideas.

Thanks for the proper terminology I'm sure the aero guys are aware of these ideas (at least, I'd hope so by this time). I didn't know about the P-38's special flaps, tho - that was an interesting topic to read about. As for shifting the fuel around, that might actually be something we will incorporate - some of the team members are looking at ways to keep the plane balanced as we burn through the fuel, so it may be a relatively small incremental effort to be able to shift fuel around this way. I'll pass the idea along!

2385a4 wrote:Three small points no-one else seems to have mentioned yet:

1. It's no good saying that your minimum ceiling is 10,000 ft AMSL. You need to work out your target ceiling to a slightly finer resolution because it affects your choice of electronics. High altitude operation affects the reliability of electronics, primarily due to the increasing intensity of cosmic rays as altitude increases. Even at 10,000 feet, the highly damaging proton flux is around 50 times greater than at sea level, and still increasing exponentially. By 30,000 ft, it's about 1,000 times stronger. That's a lot of random bit-flips. (Other problems include: temperature extremes; reduced efficiency of cooling fans; low humidity and triboelectrification; low pressure sparking; outgassing of electrolytic caps; and failure of air-bearings in hard drives. And in any aircraft, naturally high frequency vibration is also a big problem.)

The 10k ft figure was a mis-remembered detail on my part - the floor of the HASSC is 5,000 ft AMSL. I sincerely doubt we're going to try to go much higher than that since we'll need all the fuel we can get for accelerating through Mach. As for the proton flux, I know our turbine and its associated electronics are generally intended for serious model plane enthusiasts, so I'd suspect they can handle the flux at that altitude. It'll be a good question to ask them, though.

Also, re: hardened equipment: I doubt our budget will accommodate that, but I'll pass along the idea.

This is particularly the case with a remote controlled supersonic aircraft that you plan to fly over land in a populated area. If the control system fails, it could "land" anywhere within about a 90 mile radius, and if there's property or people there, that's a disaster. So you require extremely high reliability from your control system; certainly well over 99%, indeed well over 99.9% (Or, perhaps, a moderately reliable self-destruct; I did read an interview with Nick Sabatini of the FAA, who was involved in defining new rules for UAVs. He was saying that if the applicant couldn't guarantee that they could control the aircraft at all times, then they had to be able to destroy it.)

We'll be flying in a region of California's High Altitude Supersonic Corridor (HASSC, mentioned above somewhere); specifically, the only thing near our test area is Edwards AFB and China Lake Naval Weapons Station. The club president is in contact with Edwards, and they seem enthusiastic about having us out there. Regulatory questions are above my paygrade, but I'll pass the concern along.

2. You'll also want to read this:
http://en.wikipedia.org/wiki/DO-178B

Interesting! I suspect the aero guys have touched on this in their coursework, but I'll try to remember it if/when the time comes for us to actually code things.

3. Incidentally, 99% reliability is not the same thing as wanting to hit 99 flights before failure. If you system has 99% reliability per flight, the chance of completing your 99th flight without failure is only 37%. To have 50:50 odds of 99 flights before failure, you need 99.3% reliability. For 99% chance of 99 flights without failure, you need 99.99% reliability. Achieving 99.99% reliability on a novel complex system is extremely difficult. You will need a design with lots of redundancy, fail safety, and plenty of subassembly testing.

Ouch, right. That was a fairly elementary mistake on my end; thanks for catching it. I mis-stated our goal; we really are looking for 99 flights before catastrophic failure, but realistically we know we're going to have to replace pretty much everything well before that. Our newly-written ops guide calls for a complete airframe rebuild after 100 flights or 50 flight hours, whichever comes first; unless we decide to go for mid-air refueling, 100 flights is going to come much, much sooner than the 50 hours. We'll see what the rest of the team says to my reliability "estimates".

p1t1o wrote:With travel distances of 40+miles and heights of n thousands of feet and whatever safety margin, is the aircraft going to require some degree of "robotic" autonomy should the communications with ground fail? If not then I'd be curious as to what sort of communications hardware is necessary.

We're anticipating a main ground station and two backup stations located near the extreme edges of the flight path. The UAV should be within easy range of at least 2 of these stations at all times. As for autonomy, it, like most things, is something of an open question at this point.

gorcee wrote:1.) They're looking at an aircraft with a 3ft wingspan. I'd be shocked if it can carry fuel to reach 10k ft, let alone 30k.

2.) They're looking at an aircraft with a 3ft wingspan made out of carbon fiber wrapped foam. The vehicle will probably degrade structurally long before the 99th flight.

1. Absolutely true. 30k is way outside our design range.

2. Sad, but probably true. So you're saying I might have to adjust the maintenance schedule?
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### Re: Civilian Avionics

nehpest wrote:The club president seems confident that the engine can run at supersonic speeds; I remember something being said about exhaust velocity, but I'm sure there's more to his assurance than just that. As for compressibility, I think we'll get the definitive answer on that round about the end of April, which is when we've tentatively scheduled our first wind tunnel tests on the scale model. I don't know much about the last item, but I believe the aero guys have some ideas for it, as I heard them discussing it before the last meeting.

The challenge in supersonic operation isn't the exhaust velocity, it's the inlet velocity. Flying at supersonic (and even transonic) speeds will cause shockwaves to form in your inlets. These need to be controlled, otherwise you can get compressor stall, and your engine will cease to operate.

This problem is one of the reasons that older supersonic jet engines (ie, MiG-21) had a cone on their inlet. The shape of this cone ensured that the shockwaves would help compress the air going into the inlet, rather than having all sorts of funky shock reflections playing havoc on the airflow over the stators. Modern inlets have complex geometry to accomplish the same thing without needing inlet cones (there's a technical term for them that I can't recall at the moment).

It's not impossible, but it needs to be considered.

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### Re: Civilian Avionics

gorcee wrote:(there's a technical term for them that I can't recall at the moment).

You get the intakes with "shock cones", as described. You also get "2D intakes" which are the wedge-shaped ones seen on many supersonic craft, these feature movable or stationary ramps to manage shockwaves at different velocities, slowing and compressing incoming supersonic air. The simplest (also lightest and least efficient) is a "pitot inlet" which essentially resembles a normal intake, although designed with placement of shockwaves in mind.

The more complex designs with moveable surfaces are more appropriate for craft that need to maximise their efficiency at widely differing mach numbers, although its always a tradeoff.

nehpest
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### Re: Civilian Avionics

gorcee wrote:The challenge in supersonic operation isn't the exhaust velocity, it's the inlet velocity. Flying at supersonic (and even transonic) speeds will cause shockwaves to form in your inlets. These need to be controlled, otherwise you can get compressor stall, and your engine will cease to operate.

This problem is one of the reasons that older supersonic jet engines (ie, MiG-21) had a cone on their inlet. The shape of this cone ensured that the shockwaves would help compress the air going into the inlet, rather than having all sorts of funky shock reflections playing havoc on the airflow over the stators. Modern inlets have complex geometry to accomplish the same thing without needing inlet cones (there's a technical term for them that I can't recall at the moment).

It's not impossible, but it needs to be considered.

I called our turbine supplier today to ask about inlet speeds and other data for the engine we're considering; unfortunately, "Bob" (presumably an engineer) wasn't available this afternoon, so I'm going to try again tomorrow.

Also, I shared your drag calculations above at our meeting today. It's a good thing you pointed this out, because 1) your results are an order of magnitude smaller than what our aerodynamics guy was getting, which is good because he knew his data was wrong, and 2) nobody had actually done this calculation. A couple of people assumed we'd be fine because they thought drag was quadratic in area, but alas it is not so.

We're looking at decreasing CD as our top option, possibly doubling up the engine, and possibly decreasing wing area a skosh. We've got a bunch of people cranking through X-plane designs in Solidworks to find one that will work best for our flight plan. Those of us who don't know how to use it are going to be trained next weekend.

p1t1o wrote:You get the intakes with "shock cones", as described. You also get "2D intakes" which are the wedge-shaped ones seen on many supersonic craft, these feature movable or stationary ramps to manage shockwaves at different velocities, slowing and compressing incoming supersonic air. The simplest (also lightest and least efficient) is a "pitot inlet" which essentially resembles a normal intake, although designed with placement of shockwaves in mind.

The more complex designs with moveable surfaces are more appropriate for craft that need to maximise their efficiency at widely differing mach numbers, although its always a tradeoff.

A week ago I'd've said that moveable surfaces are unlikely on our craft, but after today's meeting it looks like we're open to just about anything to improve our transonic performance. I'll post something to the club's Google group regarding variable geometry, as the actual details are way beyond my Solidworks skills

EDIT: In fact, we've reopened the questions of whether we will use rockets to assist getting past Mach, and whether we'll go for some kind of RATO setup.

If you know any other technical terms I should Google, I'd gladly listen to them!
Kewangji wrote:Someone told me I need to stop being so arrogant. Like I'd care about their plebeian opinions.

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### Re: Civilian Avionics

Ooh, how about launching from a light aircraft? Towed behind?

If you fancy a google sesh, try these:
area ruling
Douglas X-3 Stiletto
Lifting body

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### Re: Civilian Avionics

Roosevelt wrote:
I wrote:Does Space Teddy Roosevelt wrestle Space Bears and fight the Space Spanish-American War with his band of Space-volunteers the Space Rough Riders?

Yes.

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nehpest
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### Re: Civilian Avionics

p1t1o wrote:Ooh, how about launching from a light aircraft? Towed behind?

If you fancy a google sesh, try these:
area ruling
Douglas X-3 Stiletto
Lifting body

I came across area ruling's wiki page recently; it seems pretty straightforward.

The X3 was an interesting read, but according to the wiki it couldn't break Mach in level flight; this seems to be a feature of underpowered vehicles, as our current design may be >_< The lessons about inertial coupling are something we'll have to take into account a bit later in the design, since we now intend to maneuver at speed.

As for lifting bodies, we're investigating that. As team members churn through X-plane designs, we'll inevitably test the X-24 and the -38, both of which employed lifting bodies. Even if we don't go ahead with either of those designs, we may incorporate it into our final design if it helps

Incidentally, if anybody here knows about high-thrust hobbyist turbines, I'm all ears! I've found only one so far with a greater max thrust than the P200; it's the Titan from a Netherlands-based company called AMT. Does anyone have familiarity with it?
Kewangji wrote:Someone told me I need to stop being so arrogant. Like I'd care about their plebeian opinions.

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### Re: Civilian Avionics

If you use an off-the-shelf hobby turbine, I would highly recommend looking into adding an afterburner to it. An afterburner is basically simple and light but can nearly double your available thrust, making mach speeds much easier to obtain.
Roosevelt wrote:
I wrote:Does Space Teddy Roosevelt wrestle Space Bears and fight the Space Spanish-American War with his band of Space-volunteers the Space Rough Riders?

Yes.

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### Re: Civilian Avionics

EdgarJPublius wrote:If you use an off-the-shelf hobby turbine, I would highly recommend looking into adding an afterburner to it. An afterburner is basically simple and light but can nearly double your available thrust, making mach speeds much easier to obtain.

Except for the part where building the afterburner nozzle will require materials capable of withstanding the heat, which are not entirely compatible with the construction method of the rest of the frame; or, would require a support structure of different, heavier materials than the carbon fiber-covered foam they intend on using. Although you can definitely construct a titanium tail empennage that contains the engine/afterburning element, and it wouldn't be a huge weight increase, that's kind of expensive.

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### Re: Civilian Avionics

You should just be able to use a flange on the after-burner to connect it to the end of the turbine. Nothing especially heavy or complicated about that and the afterburner could just hang out past the end of the fuselage, minimizing thermal issues as well.
The weight issue really isn't, even if you completely over engineer the afterburner+attachment+support, you won't have a weight increase that comes near the increase in available thrust.

Thermal issues I'm less sure about, you should be able to support the afterburner such that the hottest portions are beyond the fuselage, and depending on what specific material you're using for the fuselage, many carbon+foam composites actually have very good thermal properties, in the end, you may have to make the afterburner itself out of something expensive such as titanium or a high-temperature aluminum alloy, but it doesn't look like your budget will be much of a limit here, and the actual amount of material we're talking about to make an afterburner out of is small.
Roosevelt wrote:
I wrote:Does Space Teddy Roosevelt wrestle Space Bears and fight the Space Spanish-American War with his band of Space-volunteers the Space Rough Riders?

Yes.

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### Re: Civilian Avionics

EdgarJPublius wrote:You should just be able to use a flange on the after-burner to connect it to the end of the turbine. Nothing especially heavy or complicated about that and the afterburner could just hang out past the end of the fuselage, minimizing thermal issues as well.
The weight issue really isn't, even if you completely over engineer the afterburner+attachment+support, you won't have a weight increase that comes near the increase in available thrust.

Have you actually ever built an afterburner? If not, this may be applicable.

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### Re: Civilian Avionics

EdgarJPublius wrote:You should just be able to use a flange on the after-burner to connect it to the end of the turbine. Nothing especially heavy or complicated about that and the afterburner could just hang out past the end of the fuselage, minimizing thermal issues as well.
The weight issue really isn't, even if you completely over engineer the afterburner+attachment+support, you won't have a weight increase that comes near the increase in available thrust.

Thermal issues I'm less sure about, you should be able to support the afterburner such that the hottest portions are beyond the fuselage, and depending on what specific material you're using for the fuselage, many carbon+foam composites actually have very good thermal properties, in the end, you may have to make the afterburner itself out of something expensive such as titanium or a high-temperature aluminum alloy, but it doesn't look like your budget will be much of a limit here, and the actual amount of material we're talking about to make an afterburner out of is small.

An afterburner is much more than just "dump fuel into the exhaust to make fire." An actual afterburner is effectively a ramjet, where the inlet air comes aft of the turbine (hence the name) and is already traveling at supersonic (more or less) speeds. Actual afterburner design requires a chamber capable of withstanding that heat. Although the engine gets hot, it is effectively cooled by engine bypass air. Such is not the case with an afterburner. The hot exhaust gas is heated even more, and then we have shock waves and all sorts of good stuff happening that has to be designed for. Even a small amount of a heat-resistant alloy is very expensive when you consider the cost of machining the parts.

So yeah, you could hang the afterburner out past the fuselage, but then you're looking at an increase in parasite drag. Mind you, the design team is already trying to reduce parasite drag by a factor of 4 over their benchmark design.

Right now, the design team is trying to design an airplane around an engine. This doesn't work (very often) in practice. Instead, they should design the aircraft, and then choose the required components based on their design needs. But, it is a learning experience on the design process.

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### Re: Civilian Avionics

I'm not sure that parasitic drag is necessarily the problem here, the F-104 planform is already relatively low drag and highly suited to super-sonic flight, if you find that drag is too high for your engine, then I'd tend towards increasing thrust over attempting drastic drag reduction.
It's unlikely that any hobby turbine will have enough thrust out of the box for super-sonic flight, so your options basically come down to additional engines (either multiple jets or rockets+jets) which will be heavy and complicated (wheeeeee, throttle syncing!), or an afterburner which will be lighter, relatively simpler and can be accommodated easily in a single engine design.

Remember also that until recently, level supersonic flight was almost exclusively the realm of afterburning jets and rocket planes. Other than exotic engines such as the SR-71's 'turbo-ram-jets' and a collection of rocket-planes, you're basically left with the English Electric Lightning, the TSR-2 and the Tu-128 (all twin engined) until the late eighties/early nineties with the introduction of late fourth generation (4.5 gen) fighters with super-cruise such as the Rafale (most of these are also twin engined, I think the only single engined supercruise capable aircraft maybe an experimental F-16 variant and the up-engined Gripen with the GE F414)
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### Re: Civilian Avionics

EdgarJPublius wrote:You should just be able to use a flange on the after-burner to connect it to the end of the turbine. Nothing especially heavy or complicated about that

Well, there's no flange on the turbine exhaust nozzle, so you're going to have to make a flange for it and weld the flange to the nozzle. They're going to have to be the same or similar material, so you need to machine and weld two fairly thin objects made of (probably) exotic metal without warping either one. That's non-trivial for a group of students. The flange also needs paths for the fuel and sensors/controls.

Then, you're going to have to design the afterburner flange. It has to be able to thermally expand along with the other flange without breaking. You have to seal the joint with something that will survive the 1100F exhaust gas.

EdgarJPublius wrote:Thermal issues I'm less sure about... carbon+foam composites... titanium... high-temperature aluminum alloy

As noted above, the exhaust gas temperature is 1100F or 1500R. This means that for a 50% stagnation temperature rise (which is small), you'll end up with gas of about 1800F-1900F. That's well above the maximum recommended use temperature of any of the materials you mention. Here's the maximum recommended use temperatures for several metals, as stated in the Aeronautical Vestpocket Handbook, Pratt & Whitney P/N 79500:

Al Alloys: 600F
Ti Alloys: 1000F
Carbon Steel: 1000F
Stainless Steel: 1500F

The only metals that will survive these temps are Ni or Co superalloys, which are expensive, heavy, and difficult to machine. You'll probably also want to coat them in thermal barrier coating.

Other issues:

Aerodynamic design: You're going to have to design fuel manifolds and flame holders that won't cause excessive total pressure loss. Everything downstream of the flame will probably have to be film cooled. If you can't find an existing design that is perfectly scalable to your needs, you'll have to design your own.

Controls: Adding an afterburner will change the pressure/mass flow relationship of the original turbine, which may mean changing its controls. The afterburner will need a fuel pump and controls of its own.
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### Re: Civilian Avionics

I'm going to ask a total newbie question.
Will an off the shelf turbine even work at transsonic or supersonic speeds?
You discussed a lot about how aerodynamics work differently at those speeds, and how you have to take that into account when designing the plane itself, but you seem to assume the engines will work the same as at low speed.
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### Re: Civilian Avionics

idobox wrote:I'm going to ask a total newbie question.
Will an off the shelf turbine even work at transsonic or supersonic speeds?
You discussed a lot about how aerodynamics work differently at those speeds, and how you have to take that into account when designing the plane itself, but you seem to assume the engines will work the same as at low speed.

This is discussed a bit above, but it's worth expanding upon. At the inlet, the engine's compressor needs to see subsonic flow. The supersonic outside flow can be slowed down to subsonic through one of two ways:

(Note: I use "stagnation pressure" a lot in the following explanation. Stagnation pressure or total pressure is the pressure that a gas would obtain if slowed down to zero velocity isentropically. It can be thought of as a measure of low entropy or as a capacity to do work. When stagnation pressure decreases (is lost) the flow has increased in entropy and decreased in the amount of work it can perform.)

1. A shock wave diffuser. The air comes in, hits a shockwave, and decelerates to subsonic speeds within a few mean free paths. This causes a loss of some stagnation pressure. The faster the flow is just before reaching the shock, the more stagnation pressure it loses.

In its simplest form, it is the "Pitot Inlet" described above. A shockwave normal to the flow of the air in a duct in which the air initially travels at full freestream velocity. This causes the most pressure loss of any inlet, but is the simplest. Because of the relatively low mach numbers we're talking about (making for smaller losses), it may be the best choice.

A more complicated inlet is the oblique shock diffuser, which uses one or more shockwaves at angles to the flow in addition to a normal shock to decelerate the air. These angled shock waves are caused by placing angled wedges in the inlet. The multiple shock wave are used to decrease the loss of total pressure.

2. A slow, gradual (isentropic) transition from supersonic to sonic to subsonic using a converging-diverging nozzle. The benefit is a very low stagnation pressure loss, but it come at the cost of flexibility.

The nozzle will allow perfectly isentropic flow at only one mach number. Going above this mach number results in stagnation pressure loss. Going below that design mach number reduce the inlet to the Pitot inlet described above. In fact, in order to get isentropic flow through the inlet at the design mach number, one must first accelerate the plane past that design speed far enough for the inlet to "start", then slow down to exactly the design speed. Dipping back below the design mach number at all will result in the nozzle "unstarting", which puts you back where you started.

Moving on to the outlet:
This is a bit simpler. To produce thrust, the jet must have an exhaust velocity higher than the plane's airspeed. The thrust is equal to mass flow rate*(exhaust speed-airspeed)
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p1t1o
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### Re: Civilian Avionics

Surprised no-one mentioned that it doesn't matter how light your afterburner setup is, the extra fuel weight and volume required will be significant, making the multiple engine/afterburner choice already more cloudy.

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### Re: Civilian Avionics

jmorgan3 wrote:This is discussed a bit above, but it's worth expanding upon. At the inlet, the engine's compressor needs to see subsonic flow. The supersonic outside flow can be slowed down to subsonic through one of two ways:

I remember seeing words like "Pitot inlet", I just had no idea what it was. Thanks for the explanation.
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### Re: Civilian Avionics

Caution: Following are large PDF files:

Basically all the information you need to build an Afterburner for a hobby turbine, they use Stainless Steel which seemed to work. Only problem was the exhaust nozzle but the documents contain recommendations for improvements to their design.
The main cost items were pressure and temperature sensors, about half their budget. You should be able to reduce that cost a bit, but I would recommend finding a sponsor willing to donate high cost parts such as these (in general, it's easier to find sponsors willing to donate parts than willing to donate money)
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gorcee
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### Re: Civilian Avionics

Caution: Following are large PDF files:

Basically all the information you need to build an Afterburner for a hobby turbine, they use Stainless Steel which seemed to work. Only problem was the exhaust nozzle but the documents contain recommendations for improvements to their design.
The main cost items were pressure and temperature sensors, about half their budget. You should be able to reduce that cost a bit, but I would recommend finding a sponsor willing to donate high cost parts such as these (in general, it's easier to find sponsors willing to donate parts than willing to donate money)

And these same documents suggest that the melting point of stainless steel is 500 degrees K below the maximum gas temperature. And actually, you want to avoid that number by quite a bit (there are all sorts of things that can happen pre-melting point, depending on the alloy). The DANTE team avoided the issue due to the effects of bypass air cooling, but this is on a static test bed with a much smaller engine.

Even if you could equip the engine that the OP linked with a suitable afterburner similar to the DANTE design, which obtained a 50% thrust increase, that's still not even enough to get a twin engine vehicle to Mach 0.7. The numbers we pushed earlier suggested that you would need something like 3.5x the max thrust of that engine to hit Mach 0.7 with a low-drag body type. That's pre-transonic. Throw some 50% thrust-boosting afterburners onto a twin engine bird, and you'll get to Mach 0.85. Maybe.

They need a bigger engine. Which means a bigger airframe. And they might still need an afterburner, but with a larger airframe, they'll have a lot easier of a time engineering the materials to withstand the necessary thermal/structural stresses.

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### Re: Civilian Avionics

gorcee wrote:And these same documents suggest that the melting point of stainless steel is 500 degrees K below the maximum gas temperature. And actually, you want to avoid that number by quite a bit (there are all sorts of things that can happen pre-melting point, depending on the alloy). The DANTE team avoided the issue due to the effects of bypass air cooling, but this is on a static test bed with a much smaller engine.

You can cool a large, moving engine with bypass air just as easily. There's no reason to act like the temperatures achieved inside the afterburner form some magical hard-limit on the materials you can use, cooling is probably the easiest part of the whole design, I'd be much more worried about the injector and nozzle designs.

They need a bigger engine. Which means a bigger airframe. And they might still need an afterburner, but with a larger airframe, they'll have a lot easier of a time engineering the materials to withstand the necessary thermal/structural stresses.

I've not and never have disputed that a bigger engine is necessary, honestly I've been using the Titan as the basis for my assumptions, not the P200, and a more powerful engine is preferable.
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### Re: Civilian Avionics

EdgarJPublius wrote:There's no reason to act like the temperatures achieved inside the afterburner form some magical hard-limit on the materials you can use

Actually, that's exactly what they do. And the design decision is affected by life-cycle analysis as well. You might be able to get by with a less heat-resistant material if you only need to run it a handful of times to prove something. So it depends on your operating design parameters as much as anything.

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### Re: Civilian Avionics

gorcee wrote:
EdgarJPublius wrote:There's no reason to act like the temperatures achieved inside the afterburner form some magical hard-limit on the materials you can use

Actually, that's exactly what they do.

Only if, for some reason, you forget or ignore that it's possible to cool the thing
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gorcee
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### Re: Civilian Avionics

EdgarJPublius wrote:
gorcee wrote:
EdgarJPublius wrote:There's no reason to act like the temperatures achieved inside the afterburner form some magical hard-limit on the materials you can use

Actually, that's exactly what they do.

Only if, for some reason, you forget or ignore that it's possible to cool the thing

Cooling or not, temperature affects things like corrosion rates, fatigue crack formation and propagation, free-play tolerances (for moving parts), and so on.

If the temperature didn't affect the material choice, then what's preventing me from using papier mache? Or copper? Or cardboard tubes?

ryanjsull
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### Re: Civilian Avionics

I have a bit of experience from a few internships at one of the 3 big jet engine manufacturers where I worked on compressor aero, and did a bit of structures work on a turbine vane and an afterburner flameholder. I also have experience working at a graduate lab studying combustion. From experience, I can tell you that titanium is really only used upstream of the combustor. Afterwords, it is all high temp superalloy, much of which is coated in ceramic. The truth is that cooling as well as material choice is of utmost importance. Changing the metal temperature by a few hundred Kelvin can make the difference between thermal mechanical fatigue destroying a part in 10 cycles vs a million cycles. As always with engineering, there are tradeoffs. For example if you made the material thicker, you might be able to get away with longer uncooled bursts as you'd have more mass to heat, but this would severely impact the weight of the afterburner.

Also, looking at the example senior design project posted reminds me that you'd need to make a variable area C-D nozzle to effectively capture the energy added to the gas during the combustion process. This alone would tricky to do without prior experience and would absolutely need to be load bearing, cooled, and adjusted by a controller. Also, because while the afterburner is in operation you would be choking the flow, the afterburning combustion chamber would be pressurized. A failure here could cause the entire airplane to explode.

These are just some of the structural difficulties associated with afterburner design. The main aerodynamic issue would be that most of the data for afterburner design is export controlled, and therefore not easily accessible. I know this because I work at a combustion laboratory where they work on afterburner projects. When the students working on these projects present their work, they make foreign nationals leave and usually call it something like "work on bluff body stabilized flames". Lastly, powerful combustion instabilities capable of tearing the engine apart can easily be produced if you are not careful about the design and running conditions.

tl;dr: in the great scheme of things, designing an afterburner for a bought jet engine would likely be a large project and could wind up taking longer than the rest of the project if you don't know what you are doing. Failure wouldn't be tough to accomplish and it would likely be catastrophic. There would be few resources available for afterburner design because of ITAR restrictions on data, so you'd need somebody who already has experience with afterburner design on your team. Therefore, buying a disposable solid rocket motor would likely be cheaper, and would suit this project better if all you are trying to do is a few flybys at supersonic speeds. That being said, it would be an awesome side project to design an afterburner if you understand that there will be a large commitment to time, resources, and the willingness to try a few times before success.

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### Re: Civilian Avionics

Caution: Following are large PDF files:

Basically all the information you need to build an Afterburner for a hobby turbine, they use Stainless Steel which seemed to work. Only problem was the exhaust nozzle but the documents contain recommendations for improvements to their design.
The main cost items were pressure and temperature sensors, about half their budget. You should be able to reduce that cost a bit, but I would recommend finding a sponsor willing to donate high cost parts such as these (in general, it's easier to find sponsors willing to donate parts than willing to donate money)

So you are backing up your statement that
EdgarJPublius wrote:An afterburner is basically simple and light but can nearly double your available thrust.

by linking to a an afterburner which required a team of 8 people two semesters to design, was made out of heavy materials, and was never verified to have increased thrust by even 50%?
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### Re: Civilian Avionics

gorcee wrote:
EdgarJPublius wrote:
gorcee wrote:
EdgarJPublius wrote:There's no reason to act like the temperatures achieved inside the afterburner form some magical hard-limit on the materials you can use

Actually, that's exactly what they do.

Only if, for some reason, you forget or ignore that it's possible to cool the thing

Cooling or not, temperature affects things like corrosion rates, fatigue crack formation and propagation, free-play tolerances (for moving parts), and so on.

If the temperature didn't affect the material choice, then what's preventing me from using papier mache? Or copper? Or cardboard tubes?

Don't strawman my position, I never said or even implied that material choice and temperature were separate issues, only that temperature does not present a hard limit on material choice.
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### Re: Civilian Avionics

Small point, the P200 and the Titan engines, they are both turbojets rather than turbofans right? If that is the case then there will be a lack of pressurised cooling air available for cooling anyway - I doubt scooping in outside air will be preferable. I could be wrong.

ryanjsull
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### Re: Civilian Avionics

p1t1o wrote:Small point, the P200 and the Titan engines, they are both turbojets rather than turbofans right? If that is the case then there will be a lack of pressurised cooling air available for cooling anyway - I doubt scooping in outside air will be preferable. I could be wrong.

Pressurized cooling air typically comes from bleeds in the high pressure compressor... this would be very tough to accomplish on such a tiny scale. This is done because the bypass air goes through the augmenter, and the augmenter would be at the same pressure as the bypass air (it wouldn't be pressurized when compared to the augmenter combustion chamber). For those who care: augmenter is the term for a combustion chamber at the end of a turbofan while afterburner is the term used for a combustion chamber at the end of a turbojets.