Flap Deployment vs Angle of Attack

[Hangtime]

Oh lord... I really shouldn't do this.  

I've spent most of my adult life building and flying airplanes. I'm also a soaring fanatic.. and I eat, live, sleep and breathe airfoils. Without pointing any fingers; allow me to shed some light.. if I can.

When the airfoil is flapped; its inherent AoA is changed. Try this.. draw a typical airfoil on a piece of paper. Draw a straight line thru the leading edge to the trailing edge. For simplicity, and without using techincal lingo so all the boys and girls out there can get the picture we'll call this the 'waterline' of the wing. An aircraft always trys to 'fly' the 'waterline' level.

Next; flap the airfoil. Draw a line from the leading edge to the trailing edge. Guess what.. the 'waterline' of the wing changed.. and the nose just pitched DOWN.

Why do HP aircraft have slats? So when they deploy flap; the slats lower; preserving the mean chord level; killing pitchdown; and the additional undercamber on the airfoil develops more lift at low speed without massive trim changes being required.. in short it keeps that fast airfoil controllable and producing lift at very low speeds. As I recall the first combat AC to have these were the Me262.

Flaps produce additional lift only at small deflection angles. Spill a hair too much flap; and yah just get a massive amount of drag.. no additional lift because the airflow over the airfoil has become 'unattached'. With flap as an added help to turn radius only SMALL amounts are benefical and even small deployments will affect trim IMMEDIATELY resulting in some pitch down... full flap deployment is futile for the purpose of improving turn radius.

My competition sailplanes always mix some up elev trim in with flap.. getting the porportions right depends a lot on AC speed at deployment. The faster the plane is traveling; the more noticeable the pitchdown. Note that the pitchdown effect can be moderated somewhat by tailplane design.. T-tails help some with pitchdown due to the stabs placement well above the center of effort of the pitch radius; dampening pitchown.. but NOT eliminating it.  

Hang

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PALE HORSES
"I looked, and behold; a Pale Horse, and it's riders name was Death, and Hell followed with him" Rev 6.8

[ra]

Hang,

The 109 and the George both had slats.

--ra--

[juzz]

So did the Bf110, Swordfish, Avenger(oops slots not slats) Helldiver, Me163B had slots too, etc...

I think he must mean full-length slats, ie: the other plane's slats are only near the wingtip, not covering the inner "flapped" area of the wing(actually, the ailerons on the Bf109 and 110 are used as flaps too). The Me262 slats are part of the entire leading edge of the wing.

[Hangtime]

Juzz!! thank you.. that is correct. The slots were fairly common.. and intended to reduce tipstall, and increase low speed stability at high AoA. Slots are un-articulated as opposed to slats which are articulated; just like flaps.

The Germans really were VERY much into soaring and refined and advanced the art by lightyears all thru the twenties and thirties; for the most part government sponsored and funded. Most of the better airfoils in use today have their roots in this era or german dominance in wing efficiency. And slats coupled with transitional airfoils as a means to control camber were first used in this enviornment.
Further, almost all of the best early war LW pilots learned to fly in gliders..  

The full length slats used on the 262 were nothing short of brilliant. They were not even controlled by the pilot.. they were actuated by a negative air pressure 'air-dam' that levered them out as airflow became unattached. An absolutly astounding development at the time. Without them, the 262 would not have been able to get airborne at all... at least not from the runways available at the time.

The concept created quite a stir in the US after the war and as the depth of engineering utilized by the 262 was examined in detail, testing at Langley's wind tunnel was done on the slat system and the entire 262's wing camber changing system was quickly adopted by development engineers here. With the advent of the swept wing (another German idea) and as materials and construction capabilites improved, the camber control system found its way on to the F86 Sabre used in Korea. And we all know how fine a job that plane did in it's role as air superioty fighter in a sea of lighter and more nimble adverarys.

Hang

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PALE HORSES
"I looked, and behold; a Pale Horse, and it's riders name was Death, and Hell followed with him" Rev 6.8

[Duckwing6]

low wing airplanes will pitch down (e.g. require nose up trim ) and high wing airplanes will pitch up (e.g. nose down trim required) These are my observations as a pilot -> gliders, and General Aviation light aircraft up to 2 tons (bout 20 types all in all)

AMEN
DW6