Elevator position when applying flaps

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Elevator position when applying flaps

Postby FlyingTiger » Thu Mar 29, 2012 8:47 pm

Hi, all. I just got a 1400mm P-40 from Nitroplanes. I have always wanted one of these because of the guy in my avatar photo, my dad in Karachi in 1944. Anyway, my question is when programming my radio to apply flaps, how does the elevator come in to play? Flaps down/elevator down? Or, flaps down/elevator up? Also, how much elevator for flaps at 50%, 100%?
Thanks for the advice.
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Re: Elevator position when applying flaps

Postby FlyingTiger » Thu Mar 29, 2012 9:03 pm

I think I found an answer but please, everybody chime in and give me your thoughts.

Pitching Moments

Pitching moments makes the aircraft rotate around its centre of gravity either nose up or nose down. Any single change in configuration usually requires an offsetting adjustment of trim. The use of flaps is a change in aircraft configuration as is the landing gear. In a C-182RG the simultaneous retraction of landing gear and 10-degrees; of flaps have offsetting pitch changes so that no trim adjustment is required. A nice piece of engineering as is the 1:1 relationship that exists between notches of flaps and a full turn of trim in many Cessnas.

Just adding flaps will cause an aircraft to pitch up or down depending on how the change in lift and drag created is positioned around the centre of gravity. Flaps lowered on high-wing usually cause a nose-up pitching moment as the camber changes the lift around the centre of pressure. The resulting drag also causes a nose-up pitch. Low-wing aircraft flaps causes drag that pitches the nose down. Only the aggregate of pressures of lift and drag determines the direction of pitch change.

Flap extension also affects the airflow over the horizontal tail surfaces and thereby affects its lift. This lift is normally a downward force and the flow from the flaps can have a greater effect than either the camber or drag. The extension of the gear can, at various points of the extension cause either up or down pitch changes. This gear effect tends to be more marked in low-wing aircraft than in high-wing. The last configuration changes that can have effects are in power or propeller changes.
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Re: Elevator position when applying flaps

Postby quarry44 » Thu Mar 29, 2012 9:07 pm

I had a dhc-2 beaver,in which the flaps made very little difference to pitch.However,a super chipmunk showed a marked nose up attitude,so down elevator needed to be applied.The only plane where I needed up elevator was a 3D type,which didn't really need the flaps anyway.My$0.02.
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Re: Elevator position when applying flaps

Postby jbcoop » Fri Mar 30, 2012 5:22 am

Each aircraft type is different. Just go out and fly your plane, set the flap then trim the elevator until it is balanced. Note how much (and what direction) you had to trim and when you get home you can programme that into your flap/elevator pairing.
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Re: Elevator position when applying flaps

Postby RCModelReviews » Fri Mar 30, 2012 9:22 am

jbcoop is correct -- each plane is different but it's been my experience that when flap is added, high-wing planes tend to pitch nose-up and low-wing planes tend to pitch nose-down.

I think this is because of the fact that the flaps create drag which causes the model to try and rotate around its CG. Since the CG of a high-wing plane is below the wing, this causes the drag of the flaps to lift the nose. Likewise, when the drag of the flaps kicks in on a low-wing plane, it tends to drag the nose down.

There are some exceptions to this but it's a fairly reasonable starting point.
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Re: Elevator position when applying flaps

Postby ergocentric » Sat Mar 31, 2012 2:04 pm

I learned last year that it also depends on horizontal stab incedence
stab could be neutral lift or push down
note this zenair ch701 appears to have the h.stab mounted upside-down
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