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Old Nov 09, 2012, 08:24 AM
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United States, MI, Macomb
Joined Apr 2009
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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
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Old Nov 09, 2012, 08:45 AM
Detroit 2-stroke junkie
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Lift is caused by deflection of air under the wing
Aka: AOA


Discuss!
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Old Nov 09, 2012, 09:28 AM
Grad student in aeronautics
United States, GA, Atlanta
Joined Oct 2010
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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).
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Old Nov 09, 2012, 10:34 AM
internet gadfly
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Colorado
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Quote:
Originally Posted by tacx View Post

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.
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Old Nov 09, 2012, 12:19 PM
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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
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Old Nov 09, 2012, 12:24 PM
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All very interesting.

Thanks
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Old Nov 09, 2012, 03:04 PM
internet gadfly
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http://www.rcgroups.com/forums/showt...un#post9045024
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Old Nov 09, 2012, 03:13 PM
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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.
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Old Nov 09, 2012, 03:15 PM
greg
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i'm sure someone can explain the purpose of flaps.

hopefully that explanation can also include the different types of flaps
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Old Nov 09, 2012, 04:17 PM
internet gadfly
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The attached table is from "Personal Aircraft Drag Reduction" by Bruce Carmichael
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Old Nov 09, 2012, 10:00 PM
greg
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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
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Old Nov 10, 2012, 07:51 AM
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Quote:
Originally Posted by ciurpita View Post
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.
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Old Nov 10, 2012, 10:22 AM
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The basic effect of flap deflection is to increase a wing's lift coefficient at a given angle of attack. One benefit of flap deflection is that (within limits) increasing deflection also increases a wing's maximum lift coefficient. This means that for a given weight, an airplane can takeoff or land at a lower speed.

In almost all cases, the increase in lift coefficient comes with an increase in drag coefficient (for a given AOA). For landing, planes often use significant flap deflection because drag is not of particular concern. For takeoff, where the goal is typically not just to get airborne, but to climb and accelerate, less deflection is normally used to achieve a compromise between runway required and the rate of energy addition during climb out.

Most of my practical experience with flaps is full-scale.

To illustrate the compromise associated with takeoff flap settings: In the F-18A-D, you set the flaps to "half" for the catapult shot. Setting the flaps to half (rather than "full") gives you better energy addition (particularly in the case where you lose an engine during the cat shot), but increases the amount you will settle in front of the carrier. In the F-18E/F you set the flaps to full (there are certain configurations where the sink is too high with the flaps at half). Even for two airplanes as similar as the Hornet and Super Hornet, the compromise between takeoff performance and energy addition leads to different catapult flap settings (both airplanes use half flaps for field takeoffs).

When landing on a carrier, flaps are generally your friend. Less landing speed means you won't break the wire you catch (definitely good). The additional drag means that the engines will be at a higher RPM which gives faster throttle response (also a good thing). The additional drag can be a problem if you've lost an engine, so you use half flaps for a single-engine approach in both the Hornet and Super Hornet (when single-engine, the ship will speed up to reduce your relative speed).

Reducing AOA for landing on a carrier is not necessarily a good thing. If you see a picture of an F-8 in the landing configuration you might notice that the entire wing is rotated (think of it as a full-chord flap). This made it very difficult to land on the boat (I've heard... It was well before my time). It turns out that engine thrust is far more effective at changing your flight path when it's not quite aligned with your flight path (as is the case when you have a few degrees of AOA).

There is no flaps "up" position in the Hornet/Super Hornet, only "auto". In this position you turn over control of the (single-slotted) trailing edge flaps and leading edge flaps to the flight control computers. Most of the time this means that the flaps are pretty close to up. The computers will adjust them slightly in cruise for minimum drag. As you increase AOA, the leading edge flaps will be the first to deflect. Once you get to high angles of attack (beyond about 30 deg), the trailing edge flaps will start deflecting significantly.

My RC experience with flaps is mostly with DLG flaperons. These have two primary uses. First, they allow you to reduce the drag in (more or less) level flight across a range of airspeeds. Second, they give you the ability to manage energy (by adjusting drag) once you've decided to land.

Hope this provides perspective on some of the ways flaps are used.
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Old Nov 10, 2012, 10:37 AM
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United States, AZ, Gilbert
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Class dismissed!

Nice overview.
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Old Nov 10, 2012, 05:24 PM
greg
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somerset, nj
Joined Feb 2005
371 Posts
why does a flap, or any trailing edge device creating a concave airfoil shape, cause an increase in the max CL?

when near stall, would moving a aileron down induce a stall, or like a flap, increase lift as well as increase the max lift coefficient, avoiding a stall?

greg
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