V.M.C.A.

[Bob Ayling]

OK people, a query: What happens to VMCA (IAS) with an increase in altitude or temperature ? Is the change, if any, a constant across aircraft types?

I would have thought it increased, but I recently found a perf page which shows it decreasing, and I am having difficulty reconciling this, as I would have thought the reducing thrust was more than made up for by the change in density alt and TAS.

[This message has been edited by Bob Ayling (edited 18 April 2000).]

[billovitch]

If the engine loses thrust quicker than the rudder loses side force with increasing altitude, the VMCA will decrease.

PS the YMCA in HKG is very reasonable!

[4dogs]

BA,

You asked if the change is a constant across aircraft types: given that Vmca is a function of the resultant between thrust couple and rudder couple, the temperature effect on engine thrust will be one issue and the offset of the thrust line will be another. As Vmca is measured in IAS, which is (all things being equal) self-compensating for altitude and temperature, there will be no apparent change if the thrust couple remains constant. Thus, for flat rated engines, the Vmca should remain unchanged until reaching the flat rating temperature and then decrease as thrust falls off.

The apparent change will vary between aircraft as a function of their engine/airframe configuration.

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