Old 16th Apr 2014, 09:37
  #86 (permalink)  
HazelNuts39
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Originally Posted by Vilas
My question is if Vmca is below VS then how is it demonstrated? It cannot be flown.
Just to illustrate a point I made earlier:



Vmc is normally determined by flight test at minimum weight. At 5 degrees of bank Vmc reduces with increasing weight. That change is small so for practical purposes it may me ignored, which is conservative. So then Vmc is constant with weight.

Stall speed increases with increasing weight. At some weight Vs will be equal to Vmc, and at higher weights it is greater than Vmc.

If the cross-over weight is below the lowest weight that can be achieved with the test airplane, then Vmc is obviously not limiting and the regulator will permit a value determined by analysis to be used instead.

Originally Posted by 737Jock
So please explain me then why a manufacturer would put on a fin that is so big that VMCA is below Vs?
Firstly, you have to consider all weights and flap settings. Secondly, there are other considerations for sizing the fin, for example the decrab maneuver when landing with crosswind.
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