Nov 10, 2012, 06:41 PM
Joined Aug 2006
2,264 Posts
Quote:
 Originally Posted by ciurpita 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
 Nov 10, 2012, 06:51 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,077 Posts 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). Last edited by ShoeDLG; Nov 10, 2012 at 07:09 PM.
Nov 10, 2012, 07:00 PM
Registered User
Germany, BW, Stuttgart
Joined Mar 2012
1,077 Posts
Quote:
 Originally Posted by nmasters >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?
Nov 10, 2012, 07:02 PM
naive realism
Joined Jan 2007
3,836 Posts
Quote:
 Originally Posted by ciurpita 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

# Images

 Nov 11, 2012, 03:22 AM Registered User Joined Oct 2004 3,155 Posts 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.
Nov 12, 2012, 03:53 PM
greg
somerset, nj
Joined Feb 2005
372 Posts
Quote:
 Originally Posted by tacx 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
 Nov 12, 2012, 04:00 PM Registered User United States, UT, Salt Lake City Joined Oct 2007 8,597 Posts 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.
Nov 12, 2012, 05:50 PM
t=1007606
Joined Dec 2006
500 Posts
Quote:
 Originally Posted by ciurpita 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.
Nov 12, 2012, 06:57 PM
naive realism
Joined Jan 2007
3,836 Posts
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

# Images

Nov 13, 2012, 01:34 AM
t=1007606
Joined Dec 2006
500 Posts
Quote:
 Originally Posted by kcaldwel 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
Nov 14, 2012, 01:43 PM
B for Bruce
Joined Oct 2002
12,655 Posts
Quote:
 Originally Posted by ciurpita ....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.
 Nov 14, 2012, 03:26 PM Registered User Joined Oct 2004 3,155 Posts 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.
 Jan 11, 2015, 10:14 PM Registered User Joined Feb 2014 276 Posts what is better for a P51, to help it land, since it can stall easily, split flaps or plain flaps?
 Jan 11, 2015, 10:36 PM Official Boat Bum United States, NV, Reno Joined Mar 2000 8,088 Posts If it's a P-51 it should have plain flaps or single slotted flaps, not split. That's for "scale" purposes. A P-51 with split flaps is not gonna look like a P-51. P-51's don't "stall easy" they just are not as forgiving as they are quite maneuverable even close to the stall. Without flying a stable approach and without having a smooth hand on the controls it's easy to aggravate an incipient stall into a spin entry when flying a higher wing loading, faster, more maneuverable machine. Many warbirds like the P-51 and similar tend to fall into this category. When you fly them correctly they can be quite docile.
Jan 12, 2015, 12:20 AM
Registered User
United Kingdom, England, Southampton
Joined Oct 2013
2,800 Posts
Quote:
 Originally Posted by tacx 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
Yes it is.