Forum: Technology

Can anyone provide any information on military wing types and styles? Technical data would be cool if not understood for a semester (next semester is my fluids class). What are the airfoils on warplanes classified as, supercritical?

Are wing twists actually used for things and do they change as they wing gets farther out?

How do the stealth aircraft actually fly with the weird wing shapes that are required for stealth?

I know that you can make smooth curves insanely stealthy (B-2), but (and I know this may be classified) how to the wings on a stealth plane differ from those on say an eagle, other than the basic gap in years (I'm talking stealth to nonstealth)?

Why is such a large wing important? Not all of that wing is producing a significant amount of lift.

Have they ever tried more adaptive control surface than they have now?

Thank you ahead of time.

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Since I'm not an aeronautical engineer, I'll try to answer as best as I can. Anyone is welcome to correct me.

Modern fighters usually have "custum-made" airfoils, meaning the designers (or the CAD program for that matter) usually design a specific airfoil that suits best to the specific airplane that is being designed. The F-22 is an exception, as it uses a NACA series airfoil, the NACA-6. (Source and more info, More information).

As for the stealth vs. non-stealh, I don't think there's much of a difference. The B-2 and the F-117 can't fly without the assistance of the FCC (Flight Control Computer), just like the F-16.

A large wing is important since surface area is important. The lift equation is

L = CL * R * V^2 * S

where L = Lift, CL = Lift Coefficient, R = Density of air, V= Velocity squared and S = Total surface area of all producing lift surfaces (Main wings, horizontal stabs, canard etc.).

Very well put DeepSpace, nice job, correct-a-moondoe.

I think i saw something that NASA is trying to do with adaptive control surfaces. basically they have taken the surface actuators out of the aircraft and applied flexible control surfaces. these are controled by adding a layer of pezioelectric material (it moves by applying electric current) to the control surface. the FCC then actuates the control surface by interpreting the pilot's inputs and applying a certain value of current to the pezioelectric material. it has the promise of drastically cutting weight of the aircraft and making hydrualic systems a thing of the past. its only drawdown though is that once electrical current is no longer available to the control surface, it will not move.

Yeah, NASA is has been doing what's called Biologicly Inspired Systems. That may not be the exact name, but the concept is the same. They are trying to find out why seagulls wings begin curving under. That did a test and they found the results amazing. It had amazing lift and drag characteristics. They are also working on morhphing aircraft where the wing changes to fit the optimal performance envelope at the time. I don't really know my terms yet so be forgiving. They managed to get a C-17 to take off in the length of a KMart parking lot. They used a system to keep the flow from departing... I think. Allenperos knows a fair deal more than I do and this was posted in another forum, but I'd kinda want a scientific discussion about the science of it Smile

. I'm not to up on anything that NASA is doing outside of the project I'm working on (it's NASA funded not a NASA project).

Anyway, is it really viable to have a variable wing geometry aircraft? I know the B1-B, F-14, and F-111 all had it, but I also hear they were a maintenance nightmare. Is it possible to have VWG not be a problem with maintenance?

You guys have one on me, "pezioelectric material"? Flight Control surfaces without electrically controlled hydraulically actuated actuators? I don't think they would be survivable.

Surface actuators? You mean outside or just inside the fuselage skin? We'll never do away with hydraulic actuators because of ruggedness, dependability, maintainability, and conventionality. Bilogically Inspired Systems, in spacecraft, futuristic ones, way ahead of our time. I don't even think the shuttle has it. Deep space travel, where the likely hood of impact with FOD is remote; a possibility. Hey Crazyal, have any references to this technology??? A textbook perhaps? Don't bother if it's off the web.

Well allenperos, simply stated, piezoelectric material is any matter that when bended or deformed, will generate an electric current. Conversely, when an electric current is applied to a piezoelectric substance, it will bend or deform.

The Quartz crystal in your wrist watch is a piezoelectric substance. It vibrates at certain frequency when electric current is applied to it. Solid state microphones are another example. When you talk in to the microphone, it vibrates to your voice, deforming the piezoeletric material, generating a small electric current that is amplified and sent out over the air.

That said, what NASA is doing now is applying piezoelectric strips to the tail of an F-18 to compensate for the buffeting the LEX's cause on the tail. Their hope is to deform the tail in such a way as to cancel out the effects of the buffeting. Being able to bend or deform a control surface with out the need of a hinge assembly would be a true breakthough in aeronautical engineering. NASA forsees in the future, aircraft that do not need hinged control surfaces. aircraft with piezoelectric material embedded in the airfoils shape themselves according to pilot input, AOA, speed, wing loading, and g-forces. With that, hydraulic and even electric motors driving hinged control surfaces will be a thing of the past.

This is not a new idea. If you look at the first powered aircraft, the Wight Flyer, it used airfoil deformation for control in the roll axis. It is how birds, bats and insects fly. True it would not be as survivable as a hydraulic system today, but in the future, layers upon layers could be used in a wing section producing unparalelled redundancy, as long as electric power is available. This is true also for today's FBW and FBL control systems and they are as common place as the wire and pully system was in the 30's.

Info can be found here:

Lordofbunnies said

Quote:

Yeah, NASA is has been doing what's called Biologicly Inspired Systems. That may not be the exact name, but the concept is the same. They are trying to find out why seagulls wings begin curving under.

I think I understand what you are talking about lordofbunnies. This has been tried before also. The B-70 had winglets that would fold down at high altitude and high speed to direct the supersonic shockwave in such a way that it enhanced the lift properties of the airfoil and added to the total thrust. Now a seagull isn't going to have to worry about that, but if the airflow under the wing is directed in such a that it is trapped between the seagull's body and a semi-rigid surface such as water, the seagull can glide all day with minimal body motion. Ground effect i believe it is called. the russians have done extensive research into the subject. I think we are now seeing it's true potential. as for variable geometry wings, they were a means to an end. the most efficient airfoil for supersonic/transonic flight is the delta wing. the most efficient airfoil for slow speed is the long straight wing. instead of having one or the other, why not move the wing to the setting that best fits the profile that you want to fly at. nowadays i think that do to CAD and relaxed stability, variable geometry is on the way out. they are very maintenance intensive true, but you can design an all for one airfoil on a computer now and test it in software instead of having to build it. that is why you see the latest crop of european fighters all having delta wings with canards.

This brings me to another question I've had for a while. Why are there so many different wing planforms? Is there a "best" planform, or is it a specifc mission thing? I mean there are multiple different planforms out there right now that do the same thing. There's the semitraditional wing and horizontal stabilizers in the normal place on the Raptor and there's the Eurofighter delta with canards? Speaking on planform designs, there's also the X-29/Su-47 swept forward wing that is amazingly UNstable. The Russian also seem to enjoy adding a set of canards on normal planform aircraft. Is one of these planforms better for their intended mission? Does the delta wing planform give the European the warm fuzzies because they seem to use it quite a bit. What are the inherant properites of each of the basic planforms

Very interesting posts, I simply was unaware of this technology, thank you for the references. LordofBunnies, I think I should be the one in my Post Nuke Brief stating, "Please Bear with me....I'm still learning". Crazyal, thanks for the analogy of the quatrz crystal watch and piezoelectric material. Wow, flight controls incorporated onto the airfoils themselves!

Here's something else, I've heard that the C-17 has an Oswald efficiency number greater than 1. That is supposedly above-optimal performance for a wing? First, how is this possible? I've heard its all the winglets, but there must be more than that. Second, do any other planes have this sort of performance?

Other things, wing planforms and their performance. Do the different wing planforms make a major difference on their performance, or do modern computer controls make up for where some will give up performance? I know right now there are the standard set up, front canards + standard, delta + canards, scissor wing, blended wing body, forward swept wing, and variable wing geometry. I may have forgotten something. Anyway, are there different performance characteristics based solely on the planform?

Bunnies - Oswald Efficiency Factor, named after the aerodynamicist, W. Bailey Oswald discovered this Cd related aerodynamic topic in 1932 in a report he submitted to NACA No. 408. The equation is:

Coefficient of L, Sqd/3.14*e(AR), where:

e=known aerodynamic quantities, obtained from the data
AR=aspect ratio, span/chord

The rest of the equation you already know.

The equation is abbreviated as Cd,i - where Cd,i is an expanded version of the "induced drag" curve including parasite drag from given alpha units, where Cd,0 is known as the "zero-lift" coefficient. This value, is correctly known as parasite drag @ "zero lift". (I wish we had the ability to draw graphs), but it is well illustrated as the "drag polar" and every aircraft has one. It has three curves; a hyperbola representing "total drag" over the curves of parasite drag which somewhat parallels the total drag curve increasing in value with an increase in EAS and induced drag paralleling the total drag curve also from the opposite direction decreasing with an increase in EAS.

As you already know, induced drag varies inversely as the square of the EAS, and parasite drag increases proportional to the square of the EAS.

For example; induced drag = 3K Lbs, EAS = 400 Knts (~C-17), if the EAS is reduced to 200 Knts, the induced drag will quadruple because of high alpha. On the other hand, if the parasite drag = 0.8 K Lbs, and EAS = 200 Knts, if we double the EAS, the parasite drag will increase to the square of the airspeed, to a value much > 0.8 K Lbs.

As far as "efficiency number greater than 1" is concerned, I'm not really sure how to explain this, however, the performance of the C-17 has much to do with the four powerplants each producing 40 K Lbs of thrust each, and with the empty weight of the aircraft being less than that, the aircraft, along with winglets, anhedrialized wings, and high lift devices, it can be reach a very high "effiecency factor". The C-141 has similiar properties.

As far as planform is concerned, a sweptback wing is optimal for low mach cruise, as would any modern aircraft, civilian or military. The C-17 has such characteritics as does the C-141 or in the case of the F-16, both sub and supersonic cruise. I hope I have answered your questions. Rolling Eyes

Salute!

NASA or DARPA flew an Aardvaark a long time ago that had an "adaptive" wing, not simply "variable geometry swing wings". Think they used hydraulicas to move flexible sheathing in order to change the camber. You can prolly find it someplace on the 'net.

Secondly, the twists can really help things at some portions of the envelope - like when landing. The F-102 had a block with "Case 10" wings, if I remember. The 106 had a similar leading edge. Looked like they replaced the sharp leading edge with a pronounced curve. You could tell the difference when landing, but I never felt much difference at normal maneuvering speed. Later, McAir added a fancy curve to the outer panel of the Eagle. It didn't do anything, so they took it off.

Don't know of any supercritical wings that made it to production.

Early supersonic jets had a "diamond" airfoil, like the VooDoo that I flew. Tink the Zipper also had a diamond. It was really made for supersonic flight to exploit the various pressures behind the oblique shock waves. So we had one at leading edge, then two different ones resulting from the top and bottom facets. We also learned that if you had enough power you could get lift outta a flat plate, heh heh.

The famous Spit and Jug had the elliptical planform to help with induced drag and spanwise flow, I believe.

And, of course, the Viper has the variable camber resulting from its LEF system.

Notches in the wings create aerodynamic "fences" to cut down on spanwise flow. The Ruskies hadn't figured this out and kept putting real fences on their wings, which added parasite drag. So look at the SLUF, Phantom, 106 and others to see the "notch".

and there's lots more experiments since Orville and Wilbur.

later...

GUMS - More on the VooDoo, please? Do you know in Vietnam, Lts Antonio Reyna or a Richard VanBibber? The F/A-22 has supercritical wings.

Gums wrote:

Notches in the wings create aerodynamic "fences" to cut down on spanwise flow. The Ruskies hadn't figured this out and kept putting real fences on their wings, which added parasite drag. So look at the SLUF, Phantom, 106 and others to see the "notch".

Minor correction. The notches don't change the amount of spanwise flow so much as they re-energize the thicker boundary layer that results from it to keep the flow attached.

The problem is as the air flows spanwise (a result of swept wings and friction) the boundary layer gets thicker. (Friction cauess air to move slower along the surface and it builds up with distance...that is the BL). Since it's on the back side of the wing (beyond the peak thickness), it's in what we call an adverse pressure gradient (pressure going up in the flow direction). This enhances flow separation (higher drag, wing stall...), especially at high AOA.

The "notches" fix that.

First, a simplifed reminder on wingtip vortices. They are created because the is more pressure below the wing than above (otherwise there is no lift). At the wingtips, air "leaks" (or is pushed, there are a variety of mechanisms here) outward from the bottom of the wings and then seeks the lower pressure region above the wing and thus swirls around the wing tip. The more AOA and the more lift being generated, the stronger the vortices (because the pressure differential from bttom to top is greater) This is what causes the familiar wingtip vortex.

Now, think of the the portion of the wing notch in front of the wing as a wingtip. At AOA, vortices are shed off that notch. The "swirl" reaches beyond the boundary layer and into the -, and then swirls back down to the upper wing surface. When it does the latter, it brings some of the the higher energy air from the - into the slower moving boundary layer. That adds anergy to the BL and helps keep it attached ("stick") to the surface of the wing. This allows the aicraft to achieve higher AOA before the wing stalls.

Works better with pictures...hope that wasn't too hard to visualize!

Forum: Technology

Military Airfoils