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Vmca, minimum control speeds and flap.

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Vmca, minimum control speeds and flap.

Old 18th Apr 2002, 06:44
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Vmca, minimum control speeds and flap.

Aircraft X has a Vmca speed predicated on the usual conditions, one of which is takeoff flap (for argument's sake 10deg)

Keeping all other things constant, flap settings less than or more than 10 deg flap will cause the mimimum control speed to rise.

Why?
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Old 18th Apr 2002, 21:04
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To understand this its important youīre familiar with the term
P-Factor...which is the asymmetrical thrust caused by the descending right blade of the propeller creating more thurst than the left one (asymmetry gets greater w angle of attack)

Imagine a situation where the RIGHT engine is failed....The left engine is producing thrust, and the RIGHT blade of the LEFT engine (closer to the long. axis) is producing more thrust than the LEFT blade.

Now if the LEFT (critical) engine fails...then the right engine will be producing thrust and (at high AOAs) the RIGHT blade (the one farther away from the long axis) of the RIGHT engine will be producing the greatest amount of thrust...

Now...the more flaps you have, the less angle of attack you will need to create the same amount of lift, if the angle of attack decreases, then P-Factor decreases, and the yawing moment decreases too... thatīs why the amount of airflow the rudder will need to counteract the yawing moment will be less.

The opposite takes place in airplanes with counterotating engines. The more flaps you have the higher VMCA will be...draw you a pic and think about it.

Cheers
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Old 20th Apr 2002, 14:58
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CS,

If your purpose in asking this question is related to the ATPL exams, you need to be a little bit careful with this subject.

The POF exam often has questions along the lines of "Which aircraft will have the greatest rolling or yawing tendency following an engine failure?

a. Twin jet.
b. Twin prop.
c. Twin contra-rotating prop.
d. Twin counter rotating prop.

The props will have greater rolling and yawing tendencies than the jet, because of the asymetiric propwash over the wings. Flaps increase CL and CD at any given angle of attack, so they will also increase the asymmetric lift and drag following single engine failure.

But if you look at the CAP 698 figures 4.8 and 4.9, you will see that VMCG for the MRJT 1 does not change. The small table for 5 degree flap is identical to that for 15 degree flap. If flap setting does not alter VMCG, then it (probably) won't alter VMCA. So if you are asked how does flap setting affect any of the control speeds, it is probably safest to assume they are referring to jets and pick the "no effect" option. It would also be wise to make a written comment to the effect that increasing flap will increase control speed for props but the CAP shows no change for jets.

I realise that the real world might well be very different.

Last edited by Keith.Williams.; 20th Apr 2002 at 15:09.
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Old 23rd Apr 2002, 01:18
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I'm studying this myself at the moment, so thought I'd make a observation:

In addition to SW's comments about P-factor increasing Vmca with decreasing flap, also consider assymetric lift.

More flap will mean greater disparity in lift between dead engine side (low speed airflow) and live engine side (high speed airflow due slipstream). So presumably, at higher flap settings the aircraft will be more inclined to roll, giving a higher min control speed.

However full flap would cause significantly more drag on the live engine side, tending to assist the rudder in reducing yawing moment. So I'm not sure about your claim that any change in flap from 10° increases Vmca.

cheers,
O8
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Old 24th Aug 2006, 03:30
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Originally Posted by sudden Winds
Now...the more flaps you have, the less angle of attack you will need to create the same amount of lift, if the angle of attack decreases, then P-Factor decreases, and the yawing moment decreases too... thatīs why the amount of airflow the rudder will need to counteract the yawing moment will be less.
The opposite takes place in airplanes with counterotating engines. The more flaps you have the higher VMCA will be...draw you a pic and think about it.
Cheers
I understand the first part but can someone please explain why it would be opposite on counter rotating? The counter rotating suffers from P factor too, maybe Im not drawing this pic right.
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Old 24th Aug 2006, 11:31
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I can't comment on a prop, but there is NO reason why Vmca will either rise or drop with change of flap for a jet type at least; it depends on the aircraft (we have aircraft where it happens both ways).
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