Pitch Moment from Flap Deployment

[CAP1]

Originally posted by OOZ662
The flap gives a sudden increase in lift, making the aircraft rise along it's plane of travel.

when you extend your flaps, i think it's for the first 10 degrees(might be 20), it increases the camber of the wing, thus increasing your lift. once you go past that, they also add significant drag, along with some lift.
 so basicly assume you're in level flight trimmed for 200ktias, and for some reason you hit your flap button, setting out 10 drgrees.....your nose will pitch up as you've just increased your lift, and would now need to re-trim for level flight in that configuration.

hope this helps some

[CAP1]

Originally posted by Viking
One thing I find strange is that from what I've read the 109 did a marked nose-down movement when flaps were deployed; because this moved Cl a bit back, but Cg remained the same. I've heard that this is not something unique to the 109, but rather common for many light planes today.

cessna 150, 152, 172, 182, and 206,  as far as i know all pitch nose up when flaps are extended initally.....at least the 172's i fly do.

[Benny Moore]

The same is true of the Cessna 152 I flew.  As for the P-38, I think it would probably a pitch up, due to the Fowlers being less draggy.

[CAP1]

Originally posted by Benny Moore
The same is true of the Cessna 152 I flew.  As for the P-38, I think it would probably a pitch up, due to the Fowlers being less draggy.

yes, i agree.....and if i'm not mistaken, the cessna flaps are a type of fowlers too?

[Stoney74]

My Grumman AA-1B has a significant nose down pitch moment when flaps are lowered.  But, the AA-1 is a low wing plane that is extremely pitch sensitive.  But there are various forces at work.

[Serenity]

Hmmm. Ive never given this any though before. The planes I fly in real life always pitch SEVERLY down with flap deployment. Granted, they are all low-wing aircraft. Could this contribute to it as well?

[Keiler]

Yes it does contribute.
Low wing flap drag is at an offset, below the thrust vector of the engine/propeller. Thus it will produce not only more netto drag, but also induces a tilting moment around the pitch axis downwards. In order to fly straight it might be necessary to trim the plane nose-up, unless the gained lift from the lowered flap cancels out the pitching moment by itself.

Matt

[hitech]

Serenity: If you read my post above, that was posted almost a year ago you will see it talk about low vs high wing.



Keiler: Although your conclusion is correct your reason is incorrect.

Low wing flap drag is at an offset, below the thrust vector of the engine/propeller. Thus

Should read.

Low wing flap drag is at an offset, below the center of gravity of the airplane. Thus

[Keiler]

I should be more precise when writing my thoughts down

My thoughts were:

In case the COG is directly inside the lift vector (given horizontal flight), the only governing factor is the offset between thrust vector and lateral drag vector. Given constant thrust and an offset between lateral drag- and thrust vector, changing drag due to flaps will change the force equilibrium, which in turn induces a moment on the pitch axis. If the COG is still inside the lift vector with flaps down, the increased lift will not superpose on the pitching moment. But since we have constant thrust and increased drag, the plane will pitch up or down, depending wether its a high or low wing plane. If the lift vector changes relative to COG, it might increase or decrease the pitching moment.

Does that make more sense to you? Dont get me wrong, I want to get that straight

I am just a plain-jane mechanical engineer with some increased knowledge on fluid dynamics, but by no means I am an aeronautical engineer .



Regards,
Matt

[hitech]

Not sure if we are agreeing or not.

I am referring to the vertical CG, I.E. the wing is above or below the CG, not behind or in front as it relates to  normal stability calculations and lift.

Yes the engine also will create a pitching torque if above or below the CG, but that torque will not change with flaps, only the sum of all torques will.

The engine is irrelevant to this discussion because the exact same effect would happen in a glider.

Or to put it another way, since our assumption is that we start from a steady state flight, the sum of all torques must already be zero, and we are only changing one, the flaps, we can ignore the engine.

HiTech

[Keiler]

Next time I wont hipshot any discussion on that topic with you.
Of course the engine is no factor in this case.. slaps forehead.

regards,
Matt