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Reducing Vmca with flap

After doing a quick search most responses to the question of:

"How does flap affect Vmca"

seemed to state that it was aircraft specific. Depending mainly on how it affected airflow over the rudder.

However I was thinking that if the use of flap lowers Vs, and since Vmca must be = or < 1.2 Vs, then it follows that reduced Vs means reduced Vmca with flap.

Is this line of thought correct?? :ugh:

In a word, No!!

I'm not sure where you got the VMCA-Vs relationship. The two have no connection to each other. VMCA is dependent on the amount of assymetric thrust present in an engine-fail situation vs the effectivness of the rudder, whilst Vs is dependent on the wings ability to create lift. Changing one has no direct effect on the other.

Where I think you're getting confused is that V1 cannot be less than VMCG, V2 cannot be less than 1.2 Vs, and V2 cannot be less than V1. In a light weight takeoff, you might have to use VMCG as V1 and V2, but in this case this number would be more than 1.2Vs.

Hope that helps.

Understood.

Thanks for response.

Quote:

Originally Posted by Wizofoz (Post 2907724)

There IS a regulatory requirement which links the two, but there's, as you say, no technical direct connection.

Quote:

Originally Posted by FAR25.149

(c) VMC may not exceed 1.2 VS with--
(1) Maximum available takeoff power or thrust on the engines;
(2) The most unfavorable center of gravity;
(3) The airplane trimmed for takeoff;
(4) The maximum sea level takeoff weight (or any lesser weight necessary to
show VMC);
(5) The airplane in the most critical takeoff configuration existing along
the flight path after the airplane becomes airborne, except with the landing
gear retracted;
(6) The airplane airborne and the ground effect negligible; and
(7) If applicable, the propeller of the inoperative engine--
(i) Windmilling;
(ii) In the most probable position for the specific design of the propeller
control; or
(iii) Feathered, if the airplane has an automatic feathering device
acceptable for showing compliance with the climb requirements of Sec. 25.121.

With the appropriate caveats regarding the amendment level of a given regulation

The extension of flap can affect Vmca if the aircraft has a blown wing. I can only speak for the P-3, which has a lower Vmca when you lower flap, but this is negligible and not considered practically. What happens is that the flap behind the operating engine generates more drag than the flap behind the failed engine. The higher drag on the operating engine side creates a yawing moment that opposes the moment due to the asymmetric thrust and thus the net yawing moment is reduced. This in turn means a reduced Vmca. Hope this is what you were looking for.:8