Performance and Vmca

[powdermonkey]

Hi all,
can anyone shed a little light on this?
Why does Vmca decrease with increasing pressure altitude ( from the qbank, I guess they mean a higher FL )Is it for the same reasons as Vy decreases with Alt? ie the change in TAS with increasing alt, doesn't happen as quickly as the increasing difference between TAS and IAS with increasing alt, leading to a reduction in Vy as an IAS.

Any info would be great!
Thanks
TF

[Capt Pit Bull]

Don't take this as gospel as its been a while since I taught perf.

Isn't this as simple as saying engine thrust decreases with altitude, and therefore the asymetric control problem decreases, and therefore a given control input will retain directional control down to a lower IAS?

Or is something more complex to do with the IAS/TAS relationship and geometry being asked for?
CPB

[powdermonkey]

Hi CPB. question was as simple as the way I put it, with no criteria added, so you may well be right. Sounds like it's the same as for Vmcg when at high temp high pressure alt, when an engine goes, asymmetric problems are reduced. I'll go with that...I 'll at least rember it for the exam!! If anyone knows of a different reason............?
Thanks
TF

[Luc Lion]

I view it as Capt Pit Bull.
For a given thrust assimetry, Vmca will measure the control aerodynamic effectiveness and will evolve with the air density and and the square of TAS,
thus roughly staying constant in IAS (for low mach numbers).
However Vmca will increase with the thrust assimetry, and thus will decrease as thrust decreases with altitude.

Here is a page which describes it well for light twins
Flying your twin safely (http://www.copanational.org/non-members/safety/2000/safetyPPDec00.htm)

Luc

[powdermonkey]

Thanks guys,
Very happy with that answer.
Thank you for taking the time:ok:
PM