"Flying Wing Specific" Aerodynamics Questions - RC Groups
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Oct 16, 2004, 09:33 AM
Fly safe, fly often!
Buzz_Man's Avatar

"Flying Wing Specific" Aerodynamics Questions

Can anyone explain to me - in basic, simple terms, how a flat delta wing flys?

I understand about a "typical" wing - flat bottom, bump on top, where air pressure creates lift and so forth - got it.

But what about my 42" flat delta wing? What makes it fly? It DOES fly, but I don't understand the aerodynamics. I think it has something to do with power and angle of attack, but that's as far as I can go with it.

Also, is there some sort of formula or physics that explains why more wings aren't flat? Is it possible to have large flat wings? I've heard one of the disadvantages of flat wings is their lack of glide ratio and too-fast diving capability. Mine seems to wander around nice and slow with the nose up if I want it to, but then, I drop it down and punch it and zoom, off she goes! I just don't understand why there aren't more flat wings - they're cool!


(gee, I hope this doesn't get tossed to another forum )
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Oct 16, 2004, 10:43 AM
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EdBall Flies!'s Avatar
A certain wingloading if light enough will make any flat plate fly maybe? Good question too. I know 3D plane (those flat ones thrive on light weight) but I really doubt if too high a wingloading will make a model fly properly. I try a Coroplast delta shape before and compared the handling of a proper shaped coroplast (2layer) and a flat delta. Both have the same wing area but the two layer one is of course heavier. The Flat one does fly but stall at higher speed and tends to give me control gitters. The airfoiled shape one stalls at slower speed and handles my input better I think.
Oct 16, 2004, 12:52 PM
Registered User
Ping's Avatar
Originally Posted by Buzz_Man
Can anyone explain to me - in basic, simple terms, how a flat delta wing flys?
The flying brick theory explains this perfectly. Strap a Speed 400 capable of delivering a very impressive amount of thrust, to a very light say 12oz. brick.
If you have ever flown one of these flat delta's you know they "hardly" fly and are very much a handful at lower speeds. My guess and experience with them would be that your theory of angle of attack is indeed correct. The airspeed traveling over the bottom of plate is traveling much faster then the spoiled airflow over the top of the plate. Which would explain the reflex you have to apply to the elevons to even consider these things flyable.

During a high angle of attack stall the airflow on the bottom of the plate is spoiled due to the high angle of attack. This latter type of stall will likely produce a spiral of death resulting in balsa (or foam) shards embedded into the ground.

During a low angle of attack stall, the airspeed over the top of the wing approaches that of the bottom of the wing. At this point though your not so much flying as having a ballistic tajectory.
A low angle of attack condition is correctible assuming enough forward momentum is avalible to re-achieve the "sweet spot" AOA.

It's not hard to add a spar, and two sheets of balsa to make a more airfoil shape. I strongly suggest anyone building a flat plate delta wing consider this in their design or building phase.
Last edited by Ping; Oct 16, 2004 at 12:54 PM.
Oct 16, 2004, 01:05 PM
Registered User
Tony Oliver's Avatar
Try starting with the premise that a symmetrical section is a flat plate too.The section will only produce lift when the front to back line is positive to the airflow - even a tiny amount works hence your findings - fast and low angle, slowhigh angle with appropriate power to overcome the drag. The only difference is that it has a fairing around it which smooths the airflow over a wider range of angle of attack before airflow separates and dragbuilds up/ lift reduces. A semi-symmetrical section is a flat plate with a hump in - this time it will produce lift with the convex side uppermost but will be essentially a one-speed section with a narrow range of attack at which it will 'work'. To extend the envelope you can put a similar type of fairing as the symmetrical one, around the humped plate (try drawing one with about 5% on top and the same below for roughly 10% stations along the chord). This time you will find more lift over a wider range and the zero lift angle will be a negative figure. This follows for flat bottom sections too.

Oct 16, 2004, 04:28 PM
Delta planforms typically have a lot of leading edge sweep angle. This results in a vortex on the upper surface with the axis of the vortex pretty much parallel to the centerline, This vortex is responsible for the delta wings ability to operate at a very high angle of attack before stalling. They have poor lift-to-drag ratios, so are rarely seen on gliders, but on high-powered aircraft that are intended for high speeds,such as jet fighters, the Concorde,Xb-71,etc, this design works very well. The delta planform packs a lot of wing area into a compact package, and it allows a very favorable area-to-structural weight-ratio, making it a good choice for the sport R/C powered model. If you want to make a large area wing out of one piece of a thin light-weight material, such as sheet foam or corrugated cardboard without having to add any stiffeners, the delta is a good choice. Just keep in mind that although the drag at low angle-of-attack(high-speed condition) is fairly low, enabling very high speeds, the drag builds rapidly as AOA is increased so you will want to avoid trying to pull-up rapidly for loops,etc. Also remember that at low speeds you'll need plenty of thrust to handle the high drag encountered there.
Oct 16, 2004, 05:23 PM
Striving to Rip the Bring
raptor22's Avatar
First off, we need to start of with how a plane flies. It has nothing to do with curved on top, flat on bottom. At all.

In basic terms, the wing pumps the air downwards mostly from wash over the top of the wings, as well as a little from the bottom. The downwash produces lift through two means: newtonian and bournoullian. Both are "correct", and exactly proportional to the lift at less than transonic speeds.

Newtonian theory places the lift moving the air donwards and the "every reaction has an equal and opposite reacion". Bournoullian produces lift through pressure differences.

The reason the air is accelerated (and the air DOES NOT meet at the trailing edge, hence my comment that it has nothing to do with camber) is because it has to be accelerated because it is changing directions, basically, when the airoil is at a lifting AOA and it must be pulled down to prevent a vaccuma nd fillt he space behind the wing.

Oct 16, 2004, 05:27 PM
Striving to Rip the Bring
raptor22's Avatar
Flat plate wings work pretty much as well as airfoiled sections at low RN (small slow planes), but decay at higher RN. This is because low RN airfoils are very thin with little camber to prevent seperation, and increasingly approach a flat plate shape anyway.

Oct 16, 2004, 06:34 PM
Fly safe, fly often!
Buzz_Man's Avatar
This thread is definitely a keeper - just saved it on my hard drive, printed it out, put it on my PDA and backed it up on CD.

THANKS EVERYONE!!! Honestly, I'm not really sure I understand all this - maybe if I read it over four or five more times it'll begin to sink in.

Basically, I guess you can get anything to "fly" given enough thrust and proper control surfaces.
Oct 16, 2004, 08:54 PM
Registered Disabuser
Wertygrog's Avatar
so what you are saying raptor is that the reynolds number helps to dictate the most efficient airfoil/planform i think.....hmmmmm......than why do full-scale fighter jets have such slim wings? Just to increase overall speed? And how does under-cambered wings (such as slow stick) fit into the picture?

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