Flat airfoils: slow vs. high speed, small vs. large

[stjobs]

I'm trying to think - flat airfoils are quite effective on RC models, but never employed on full scale aircraft. What is the upper size limit to using a flat airfoil? Also, what specifically allows the model to fly with a flat airfoil? My initial thoughts are that air has a much higher viscosity proportionally to a small model plane, thus enabling it to "cut" through a denser medium. A large plane is moving through air which is much less viscous proportionally to the wing area, and thus must have a cambered airfoil to produce lift. Am I on the right track?

[Ollie]

No.

[stjobs]

Instead of just saying "no," care to elaborate?

[macr0t0r]

Heh, airfoils do two things:
1: improve pitch stability.
2: reduce drag from the created lift.

For the smaller aircraft, as long as the material is thick enough, your pitch stability should be sufficient. And seriously, we really don't care about drag in our little 3D models.

As you get bigger, you will need a thicker wing for pitch stability and structural integrity. Soon, the "rectangle-airfoil" will become too heavy and too draggy to be useful, so it is better to shape it to a standard airfoil.

The above is greatly over-simplified and is missing some details that I'm too lazy to go into, but it will at least give you the gist of the idea.

I saw a 46" 3D-flyer with a flat-wing that did pretty well. It was using 6mm Depron wings with carbon-fiber reinforcement. However, he couldn't go too fast without the plane departing up or down.

As for me, even my 24" has an airfoil. <shrug>

[Sparky Paul]

The flat plate foil bashes thru the air.
A cambered foil romances the air to move out of its way.
The flat plate won't support the angles of attack a cambered foil will.
In essence, the flat plate takes more power to work with, and has a smaller operating range.

[raptor22]

Flat airfoils work so wello on small wings because very low RN airfoils tend to be very thin, and with little camber when a wide speed range is needed. So, it gets tot he point where there is relatively little difference between a flat plate and a low-enough RN arifoil.

--Alex

[50+AirYears]

About the only successful flat plate airfoils I ever saw were on some control line sport and combat ships. They could be very touchy and unstable. They can also be somewhat effective on small free flight hand launched gliders.

[Sparky Paul]

Electric foamy 3Ds are using flat plates quite effectively.

[raptor22]

Plus, P.R.A.T. It is fast and a decent size, so it isn't even operating at those very low rn's

--Alex

[Purdue Aero Man]

I'm trying to think - flat airfoils are quite effective on RC models, but never employed on full scale aircraft. What is the upper size limit to using a flat airfoil? Also, what specifically allows the model to fly with a flat airfoil? My initial thoughts are that air has a much higher viscosity proportionally to a small model plane, thus enabling it to "cut" through a denser medium. A large plane is moving through air which is much less viscous proportionally to the wing area, and thus must have a cambered airfoil to produce lift. Am I on the right track?

http://perso.wanadoo.fr/warbirds_en_normandie/1900-1935/Images/Fokker%20Albatros/Caudron%20arriere.jpg

just one example of flat airfoils being used on full scale aircraft. The reason they are not used very often anymore? Efficiency at the Re that most airplanes fly at today. And don't be too hard on Ollie. He's a good guy.

[raptor22]

True on Ollie.

But, that's undercambered, not flat. They were that way before because good enough building tech was lacking and they really did not understand the difference it could make anyway.

--Alex

[Purdue Aero Man]

NO NO NO. Look at the tail feathers.

[JMP_blackfoot]

Electric foamy 3Ds are using flat plates quite effectively.
Maybe it's because they spend most of their time sitting in one place with their nose up ? :D
It makes sense that a flat airfoil must be as efficient as any a zero airspeed. :rolleyes:

[flying4fun]

From what i have learned, the way flat plate airfoils fly is by varying the angle of attack (the angle of the wing compared to the oncoming airflow). If you could pause a 3d plane with a flat airfoil in mid-flight, and examine it very closely, you would see that the wing is not completely parallell to the incoming airflow. If it (the model) were in level flight, the wing would actually be at a positive (wing leading edge pointed towards the sky) angle of attack. Now, a "real" airfoil (one with camber) produces lift in a different way. An airfoil with camber produces lift by deflecting air downwards at the trailing edge, which produces an upward force on the wing (Newtons 3rd law). So airfoils with camber do not need to have a positive angle of attack to stay in level flight, only to climb (increasing the angle of attack on a wing in increasing amounts will keep on producing more lift and more drag. But at a certain angle of attack (different on every wing) the drag will overcome the lift produced, and the wing will stall). Anyone is free to correct me on any part of this, im just sharing what i have come to know. Hope this helps.

Alex

[macr0t0r]

A bit simplified, but pretty true. A fully-symmetrical wing will still require an angle of attack, though. However, these planes are so light, that neither the airfoil nor flat wing require much angle of attack to support flight.

Here is another over-simplified way to look at things: the more abruptly you change the course of air, the more turbulence (drag) will result. So, an airfoil will "curve" the air downwards while the flat-plate will "slap" it down. Therefore, airfoils reduce drag.

The other thing airfoils do is reduce pitch-sensitivity. An airfoil can change pitch to increase lift while minimally altering the moment. A flat-plate, however, can have a "grabbing" effect when pitching at higher speeds. Tell a fellow 3D flat-winger to do a high-speed low pass! He better have his CG forward if he tries it.

[tommy321]

The picture in post #10 reminded me of a historical side-bar in my Aerodynamics text book from university.

Basically, it said that the reason that all early aircraft had cambered-flat-plate-airfoils (as shown in the image post #10) was because of the wind-tunnels used at the time. The wind tunnels were small low speed wind-tunnels which would probably only fit the average park flyer sized model. At the low Reynolds numbers that these wind tunnels operated at (small, slow), a cambered flat plate works great. Why add the weight of a fully shaped airfoil if you don’t have to? So the Wright brothers and their compatriots scaled up these flat plate airfoils to make their full size aircraft. This is why almost every WW1 fighter airfoil you see is a very thin, slightly cambered flat plate.

This continued until the German aerodynamicist Prandtl realized that at the speeds that real airplanes fly, a fully enclosed, airfoil shape would work better. He did this I believe mostly through mathematics because large fast wind tunnels were still not available. The first successful implementation of an “airfoil” as we’d call it was the Fokker DR-1 Triplane. If you look at the airfoil for this airplane, it looks just like the Clark-Y airfoil that we all know and love :D. This gave the aircraft its stunning performance for a couple of reasons. First, the cambered airfoil worked really well for full size aircraft, it produced more lift with less drag. Secondly, all of the wire bracing could be hidden inside the wing structure now that the wing was more than just a couple inches thick. This reduced the drag caused by the flying wires in most aircraft of the time.

I always thought it was interesting how full scale airplanes were first designed by scaling up what worked well for models. It took more than 10 years for people to realize that what works well for small slow models doesn’t necessarily translate into large fast aircraft.

Cheers,

Tom

P.S. This came from Aerodynamics by John D. Anderson. If you have the opportunity to read any of his text books, I highly recommend it. They’re very informative, and present more than just the math equations that many text books limit themselves to.