Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Stoney74 on November 23, 2006, 08:07:36 PM
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This spilled over from a different thread and I thought the extra exposure may help.
What determines that the pitch moment from flap deployment will be up or down?
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physics
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The flap gives a sudden increase in lift, making the aircraft rise along it's plane of travel.
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Some thoughts:
In clean configuration the wing profile is optimized for certain incidence angle which depends of the used wing profile. For symmetrical designs it can be a couple of degrees and asymmetrical designs may need none at all. FW190 has incidence angle of two degrees and an asymmetrical profile which causes the wing incidence angle to be negative in high speed and the a/c flys in a slightly nose down attitude.
Ok, when the flaps are lowered the wing profile incidence angle changes so that if the actual angle is, say, 2 degrees and it increases when flaps are lowered, it causes a pitch down moment until the aircraft's speed has lowered to a level that is "sensible" with the effective incidence angle which is now more than 2 degrees because of flaps. And the profile change usually moves the COL aft of COG which teds to lift the plane's ***.
Of course if more flaps can be lowered you can reach a point when the flow no more can support the wing no matter how thick the wing appers to airflow and the a/c stalls.
I understand it so that the flaps are used to change the wing profile artificially to a thicker one which supports more lift at slower speed. Many wing designs are able to provide lift if AoA could be increased but the problem is that the profile is so thin that it causes the airflow to separate from top of wing at AoAs of more than ~16 degrees.
I don't think the flap placing itself can explain the pitch down momentum as the flaps are always (?!) behind the COG and despite that some a/c pitch up.
So I'm arguing that the pitch up is caused by two things: geometrical incidence angle change and the movement of center of lift aft from center of gravity.
I hope that confuses you.
-C+
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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.
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The Air Tractor 802 aircraft described in this incident report has it's flap operation described as "Extending the wing flaps resulted in a conventional nose-up pitching moment.".
http://www.atsb.gov.au/publications/investigation_reports/1998/AAIR/aair199800640.aspx
So, at least some people regard the nose pitching up during flap extension as conventional. The Air Tractor web page is at: http://www.airtractor.com/
Regards,
Malta
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As the flaps are extended the centre of pressure (Lift) moves in relation to the cord of the wing (distance from leading to trailing edge) Generally (very) the CoP is close to the Centre of Gravity. So how the plane pitches depends on the the relationship of the CoP to the CoG.
e.g You extend the flaps one notch. If the CoP remains in front of the CoG the nose pitches up. Then you move the flaps another notch. CoP moves behind the CoG the nose pitches down.
Something like that. Sorry that was 20 years ago and I think I was thinking of the girls hockey team at the time. :)
Happy to be corrected.
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Stoney74: Your asking fairly complex question, i'll out line for you most of the forces at work.
Lift:
When lowering flaps assuming no aoa change do to other forces lift will increases as flaps start to be lowered.
This lift has 2 effects, If the Center of pressure of the lift does not move the extra lift will create a pitching down force do to th cl /cg relationship. But do to the increased lift the plane will also start moving up,and do to that upward movement the AOA on the tail will change causing a pitch up force.
Drag:
Drag will typically increase as you start to lower flaps. If you have a Hi wing plane this extra drag will produce a pitch up torque. On a low wing plane it will produce a pitch down torque.
Center of lift i.e. pressure.
Depending on the flap type the the CP will move. If it moves back, it will create a pitch down torque, if it moves forward a pitch up torque.
Lift moment. As airflows over and under a wing, more drag is created on one side of the wing than the other. This different in drag of top vs bottom of the wing creates a pitching torque. How much torque and direction can change with different flap types.
These are the basic forces at work, sum up all the torques , what ever direction the sum is, determine what direction the plane will rotate.
HiTech
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My experience with the RC-planes is that it's trim change is quite difficult to know without actually testing it. One glider pitched up when the flaps were opened with V-tail, while the same plane with T-tail became almost neutral so some times the flaps seem to affect on flow around tail. Generally the mid and low wing planes seem to usually pitch down and the high wing planes seem to usually pitch up. I can program the trim change by mixers in the transmitter so in practice there is no problem.
gripen
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Ok, sounds good... Thanks for the explanation--I've wondered about this for some time now. Is the discrepancy between what happens in the game and what is described in some of the POH's merely a side-effect of the way the flight model works then?
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it depends. some airplanes pitch nose up, others nose down and others depending on the speed and flaps setting don't really do much.
Most light airplanes seem to pitch up. heavier airplanes have tended to pitch down.
I think part of the extra balloon that comes with AH is the combat trim. I haven't thought too much about it because i'll either just roll in some elevator trim to compensate or simply push the stick forward. all fixed.
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I've been doing some continued thought on this...
Lowering flaps dramatically increases the camber of the wing. Typically, small changes in camber increase the wing section pitching moment dramatically. Unfortunately, the NACA Report 824 doesn't show flapped pitching moments for the 23012 airfoil (which closely resembles the Republic S-3 airofil of the P-47). But, for almost every airfoil section shown in the report, the pitching moment for the flapped section is roughly 600-800% of the non-flapped pitching moment at the same Reynolds number.
The only reason I'm stuck on this is the reference in the P-47 POH stating "lower your wing flaps on final approach and trim to relieve the resulting nose heaviness". I thought the nose up pitching in the game might have been a speed issue (i.e. lowering flaps at greater speeds than typical approach and landing) but it creates a large nose up moment even under 200 mph. I've also tried it with and without combat trim, with no appreciable difference.
I'll keep doing some reading, but I'm perplexed none-the-less.
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Originally posted by Stoney74
Ok, sounds good... Thanks for the explanation--I've wondered about this for some time now. Is the discrepancy between what happens in the game and what is described in some of the POH's merely a side-effect of the way the flight model works then?
It would just be a detail that wasn't properly filled in. If you have a list of planes that diverge in this characteristic from what its pilot manual describes, please give it to me and I can change those. Thanks.
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How are you going to do that? Do you plan on changing the general flight model to work around that, or changing the lift and weight points on the aircraft themselves? Or are you going to simply add in arbitrary modifiers to the behavior?
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Originally posted by Pyro
It would just be a detail that wasn't properly filled in. If you have a list of planes that diverge in this characteristic from what its pilot manual describes, please give it to me and I can change those. Thanks.
I don't have a voluminous list of POH's, but I have 3: The P-47, P-51, and P-38. Only the Jug and Pony POH's make mention of a "nose heavy" pitching moment when flaps are lowered. The P-38 doesn't say--merely states "re-trim" the aircraft when flaps are lowered.
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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
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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.:D
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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.
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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?
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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.
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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?
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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
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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
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I should be more precise when writing my thoughts down :D
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 :D
I am just a plain-jane mechanical engineer with some increased knowledge on fluid dynamics, but by no means I am an aeronautical engineer .
:confused:
Regards,
Matt
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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
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:aok
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