Understanding flaps - RC Groups
Nov 09, 2012, 09:24 AM
Registered User
Question

# Understanding flaps

Hi All,
I'm going to ask this question in two forums, so please don't have a hissy fit.

It's my understanding that lift on an airfoil (wing) is a direct result of the top contour of the airfoil. When using standard flaps or flaperons on a plane I can see that the top profile of the airfoil changes. It gets longer which is what gives it more lift. When the air has to travel a longer distance to get past the wing it creates more lift on the top.

I have acquired my first SPLIT flaps plane. A Spitfire. On this plane the flaps are deployed under the trailing edge of the wing. The top of the wing profile is not changed?

So apparently my understanding of lift is wrong?

Can someone enlighten me

Thanks
Tom
 Nov 09, 2012, 09:45 AM Detroit 2-stroke junkie Lift is caused by deflection of air under the wing Aka: AOA Discuss!
 Nov 09, 2012, 10:28 AM Grad student in aeronautics This NACA smoke tunnel test should answer your questions. I suggest watching the whole video: http://www.youtube.com/watch?v=3_WgkVQWtno The split flaps begin here: http://www.youtube.com/watch?v=3_Wgk...ailpage#t=433s You can judge drag by how much separation you see. Separated flow is indicated by the cloudy looking regions (as the smoke is being mixed). You can judge lift by how many streamlines are diverted to go over the top. This is indicative of circulation. Notice that when an airfoil reaches the stall angle of attack, if the angle of attack increases the streamlines actually shift so as to have less circulation (less streamlines going over the top). Last edited by DPATE; Nov 09, 2012 at 01:27 PM.
Nov 09, 2012, 11:34 AM
Quote:
 Originally Posted by tacx So apparently my understanding of lift is wrong?
Yep

Quote:
 Can someone enlighten me
Nope

Fluid dynamics is the hardest problem in classical physics. The length of surfaces story that you've heard was BS made up by Theodore von Karman to get rid of a newspaper reporter.
 Nov 09, 2012, 01:19 PM Registered User So much ado about so simple a problem Lift happens anytime there is a pressure differential- no matter how you do it and you can produce it using a flat plate or a ball or a cylinder . or a piece of cloth. HOW it occurs on each is different but the same net effect - there is always a difference in pressure. Please ---- no text book quotes -most are simply long winded obfuscations
 Nov 09, 2012, 01:24 PM Registered User All very interesting. Thanks
 Nov 09, 2012, 04:04 PM internet gadfly
 Nov 09, 2012, 04:13 PM Registered User Also, flaps is a generic name for many different types of lift augmentation devices. The simplest ones increase the wing camber, and in a small measure the angle of the wing relative to the fuselage. Other incerase the wing area by extending rearwards, and then proceed to deflect downwards to increase the camber. Split flaps like those used in the Spitfire are a tradeoff between different needs. They lower the stall speed slightly by increasing the wing camber, increase wing drag making it simpler to control the speed using the engine only, and the fixed upper portion of the wing mitigates the wing's pitching moment, maintaining the tail's effectiveness at slow speed.
 Nov 09, 2012, 04:15 PM greg i'm sure someone can explain the purpose of flaps. hopefully that explanation can also include the different types of flaps
Nov 09, 2012, 05:17 PM
The attached table is from "Personal Aircraft Drag Reduction" by Bruce Carmichael

### Images

 Nov 09, 2012, 11:00 PM greg i've never flown a plane with flaps which is why i hoped someone else would provide an explanation. My experience is with spoilers on a glider, which only affect drag and the glide slope to control the landing point. any airfoil has a particular angle of attack that maximizes the lift to drag ratio. Ideally the plane will be flown at this AOA which along with the area of the wing and the aircraft weight determine the best airspeed. A smaller wing would allow a high airspeed with best L/D. Flying both above or below this best airspeed results in a lower L/D which can be used to control the glide path. But there are limits to how slow you can fly before stalling and the L/D (drag) may not change as must as desired. Trailing edge flaps significantly change the airfoil shape and its characteristics, as well as effectively increasing the AOA without changing the pitch of the aircraft . With increased AOA, the airspeed will slow. But by making the airfoil more concave (reflexed) the lift curve can be significantly increased, increasing the maximum lift coefficient, reducing the stall speed and allowing the aircraft to be flown much slower. Extending the AOA also allows the aircraft to flown further from its optimal L/D, further increasing drag and lowering the L/D. See the wikipedia description of flaps. Without changing pitch, extending flaps can significantly reduce airspeed and lower the L/D to increase the glide slope (visibility can be an important factor as well as landing planes with high L/D). its not clear to me how much of the above explanation of flaps applies to split flaps where the trailing edges do not converge. While they may increase the lift curve and reduce the stall speed, perhaps they also have characteristics similar to spoilers which simply reduce L/D providing control of the L/D. greg
Nov 10, 2012, 08:51 AM
Registered User
Quote:
 Originally Posted by ciurpita i've never flown a plane with flaps which is why i hoped someone else would provide an explanation. My experience is with spoilers on a glider, which only affect drag and the glide slope to control the landing point. any airfoil has a particular angle of attack that maximizes the lift to drag ratio. Ideally the plane will be flown at this AOA which along with the area of the wing and the aircraft weight determine the best airspeed. A smaller wing would allow a high airspeed with best L/D. Flying both above or below this best airspeed results in a lower L/D which can be used to control the glide path. But there are limits to how slow you can fly before stalling and the L/D (drag) may not change as must as desired. Trailing edge flaps significantly change the airfoil shape and its characteristics, as well as effectively increasing the AOA without changing the pitch of the aircraft . With increased AOA, the airspeed will slow. But by making the airfoil more concave (reflexed) the lift curve can be significantly increased, increasing the maximum lift coefficient, reducing the stall speed and allowing the aircraft to be flown much slower. Extending the AOA also allows the aircraft to flown further from its optimal L/D, further increasing drag and lowering the L/D. See the wikipedia description of flaps. Without changing pitch, extending flaps can significantly reduce airspeed and lower the L/D to increase the glide slope (visibility can be an important factor as well as landing planes with high L/D). its not clear to me how much of the above explanation of flaps applies to split flaps where the trailing edges do not converge. While they may increase the lift curve and reduce the stall speed, perhaps they also have characteristics similar to spoilers which simply reduce L/D providing control of the L/D. greg
Flaps work the same way on a powered model- as on your glider AND the model will still fly even when stalled out- but under better control.
why?
most of the wing stalls ( the flapped portion) before the outpanel.
Notice that the model is still flying at a cruise attitude- even tho the flaps are deployed
Ideally, the plane is always pointed in exactly the same direction it is heading
Depending on the flap type and size and angle of deployment - the amount of lift and drag will be such that controllability on all axis is still good.
If you really want to see how this works (beats trying to calculate it), buy one of the new tiny indoor super light models -( the VAPOR is perfect) and in a large room (small meeting room is fine) and practice flying at different attitudes
You will quickly find that power must increase as you add AOA- just like using flaps- otherwise the angle of descent will increase. And no fears of damage thru a crash -they weigh 17 grams and are incredibly stable due to low aspect ratio and minimal wing loading.
This beats any other method of seeing-up close and personal- how it all works, that I have found in 60 years of flying.