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Old Nov 10, 2012, 06:41 PM
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Originally Posted by ciurpita View Post
why does a flap, or any trailing edge device creating a concave airfoil shape, cause an increase in the max CL?
In the table I posted yesterday the one labeled "plain flap" is also called a "camber flap". It's just modifying the airfoil camber. As with an airfoil with too much aft camber dropping the flap very far causes separation and high drag. Generally you get lift and drag increasing linearly up to some angle, about 30 degrees IIRC, and larger angles of flap deflection cause the drag to increase much faster than the lift because of separation.

A droop nose is also a camber changing device.

Properly shaped slots anywhere on the wing are stall delaying devices. That's why the various slotted flaps can be deflected to larger angles and produce larger lift increments.

Sorry I don't have time to explain all the columns in that table. However you know most of the notation so can figure a lot of it out on your own. Just remember that delta is the algebraic symbol for rate of change


Quote:
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?
Yep, that's why full size pilots are trained to turn with the rudder when close to the ground and slow

--Norm
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Old Nov 10, 2012, 06:51 PM
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Greg,

I think it's possible to give a somewhat general answer to the question of downward aileron deflection near stall (at least if the flow remains somewhat 2 dimensional over the section in question).

Looking at a bunch of 2D airfoil data with and without flap deflection, they all seem to have roughly the same features:

-Sections tend to stall at a lower AOA with a flap deflected.

-Sections tend to stall a a higher Cl with a flap deflected.

From looking at the Cl vs alpha curves, it appears that deflecting an aileron downward when the section is already near stall will very likely cause the section to stall. However, the lift coefficient of the stalled/deflected section is still likely to be slightly higher than the lift coefficient of the unstalled/undeflected section. The slight increase in lift is offset by a huge increase in drag. If not countered by rudder deflection this drag will cause yaw that through dihedral effect will cause the airplane to roll opposite the commanded direction.

Keep in mind that even though deflecting the flap will increase Cl_max, if you're already at an AOA where the undeflected section is about to stall, you're very likely above the AOA for Cl_max of the deflected section (increasing Cl_max through flap deflection won't prevent a stall).
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Old Nov 10, 2012, 07:00 PM
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Originally Posted by nmasters View Post
>So apparently my understanding of lift is wrong?
Yep
>Can someone enlighten me
Nope

Fluid dynamics is the hardest problem in classical physics.
Great answer. You're not suggesting there might be more going on than simply pushing air down are you?
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Old Nov 10, 2012, 07:02 PM
A witty saying proves nothing.
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Quote:
Originally Posted by ciurpita View Post
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

More camber means more lift. Cambered airfoils have a higher max Cl, because it increases the airfoil circulation. Flap deflection to adds camber, and therefore increases the max Cl.

Moving the aileron down also increases the AoA of that part of the wing - draw a line through the leading edge and trailing edge to see the AoA increase. If the wing is near stall AoA, this increase in AoA can stall the aileron portion of the wing. Stall means increased drag (that wing tip moves back), and stall means a loss of lift (that wing tip moves down). The result is an incipient spin.

Flap downward deflection (like an aileron too) doesn't increase the stall AoA and may actually decrease it, it only increases the lift at the AoA. Ailerons work OK until the increase in AoA of the downward going aileron moves that portion of wing past it's stall AoA.

Kevin
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Old Nov 11, 2012, 03:22 AM
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That's also why many full size planes use spoilers for roll control at slow speeds and high AOA. It leaves a greater portion of the leading and trailing edges free for lift increasing devices, and prevents unintended asymmetrical stalls, by effectively using "intended" asymmetrical stalls for roll. Ailerons work better at high speed and low AOA, so most liners have both ailerons, flaperons and spoilers.
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Old Nov 12, 2012, 03:53 PM
greg
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Originally Posted by tacx View Post
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?
not sure this question was ever really addressed.

in all other cases, the flaps or slats(?) change the airfoil shape into one that is concave while maintaining a unique trailing edge. But the split flap creates an airfoil with two trailing edges.

are split-flaps more like a spoiler, which decrease lift and increase drag, than flaps which increase the lift curve and max CL?

greg
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Old Nov 12, 2012, 04:00 PM
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They work just like sticking yer hand out the window-
they add lift and lotsa drag.
Lift is just pressure difference -and these things really increase the difference.
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Old Nov 12, 2012, 05:50 PM
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Quote:
Originally Posted by ciurpita View Post
not sure this question was ever really addressed.

in all other cases, the flaps or slats(?) change the airfoil shape into one that is concave while maintaining a unique trailing edge. But the split flap creates an airfoil with two trailing edges.

are split-flaps more like a spoiler, which decrease lift and increase drag, than flaps which increase the lift curve and max CL?

greg
I would tend to think that they are Like a Drag Break while not Decreasing lift.
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Old Nov 12, 2012, 06:57 PM
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Spli flaps work much like plain flaps. They increase the camber of the wing, therefore increasing the maximum Cl.

Perhaps surprisingly, they produce slightly less drag than a plain flap for the same Cl increase. This can be a good or bad thing, depending on your application.

There are lots of reports on split flap wind tunnel testing on the NASA server, and good information in "Theory of Wing Sections" A&D.

http://ntrs.nasa.gov/archive/nasa/ca...1993091743.pdf

Kevin
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Old Nov 13, 2012, 01:34 AM
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Quote:
Originally Posted by kcaldwel View Post
Spli flaps work much like plain flaps. They increase the camber of the wing, therefore increasing the maximum Cl.

Perhaps surprisingly, they produce slightly less drag than a plain flap for the same Cl increase. This can be a good or bad thing, depending on your application.

There are lots of reports on split flap wind tunnel testing on the NASA server, and good information in "Theory of Wing Sections" A&D.

http://ntrs.nasa.gov/archive/nasa/ca...1993091743.pdf

Kevin
Yep. Surprising it is .
Thanks Kevin
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Old Nov 14, 2012, 01:43 PM
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Quote:
Originally Posted by ciurpita View Post
....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
I've seen more than a few guys crank in a bunch of aileron while flying "slow and low" and after the model dumps into the ground they figure that the radio got glitched or that the model "tip stalled" like it's something mysitical. When in reality they forced the tip stall and resulting cartwheel towards the down travel aileron by using too much control while flying too near the stall.

We can still use SOME aileron even when flying slowly and near the stall. But it's best done in small amounts and along with a bit of coordinated rudder.
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Old Nov 14, 2012, 03:26 PM
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Actually, inboard flaps help in that situation, because they add aerodynamical washout to the wing. The central portion is at an higher AOA, so a stall is more likely to start inboard first. And generous aileron differential helps too. But for truly "walking pace" stol designs probably spoilers would be best.
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