Airline Pilot Central Forums - View Single Post - Vmo indicated airspeed decrease with altitude

02-21-2010, 09:22 AM

#5

ZnCrO4

Line Holder

Joined APC: Jan 2009

Position: PTC

Posts: 73

Quote:

Originally Posted by JobHopper

First of all, Mach is already a percentage, the ratio of the speed of sound to your current speed at a given altitude. Limiting speeds are not based on a percentage of a percentage.

Second, V speeds do not change with altitude until you hit the limiting Mach speed, at whch time the Mach limit takes over. If you are looking at a chart where the Vmo is decreasing with altitude, then at some point on that graph you passed Mmo and you are actually reading Vmo base on the limiting Mach. .48 M is apparently that speed for the Beech you mentioned.

Dr. Mach discovered that air behaves the same way for a given percentage of the speed of sound, regardless of altitude. That discovery is why he got the speed named after him. Those same laws apply to all aircraft, not just high performance ones. A C-172 would have a limiting Mach if it had an engne powerful enough to go that fast.

I agree with your first comment, though I believe the second one is more aircraft based. Some aircraft do in fact have a Vmo that will decrease with alt although it decreases at a slower rate as to become higher than Mmo at some transition alt. For instance in the HS125 I believe the Vmo to Mmo is somewhere in the neighborhood of fl330 depending on the day. Vmo starts to decrease above 12k if i remember right. The reason for this I am not entirely certain although I think it may have to do with flutter.

thats all I have to add...

Reply