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Thread: Vmo vs MMo
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Old 18th Jan 2006, 13:08
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Old Smokey
 
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Originally Posted by Macgyver
Hey Mates,
Can someone please explain the difference between Vmo and Mmo? I'm thinking that Vmo is a limitation speed based on excessive dynamic pressure from flying at high speeds, while Mmo is a limitation based on shockwave damage from flying at high speeds. Is this correct?
Also, I do not understand why Vmo changes with altitude. I know for piston a/c, Vne does not change with altitude, so it seems logical to me that Vmo would be a constant CAS, while only the TAS would change with altitude.
Thanks.
Vmo is a limitation imposed, as correctly stated by you, to limit excessive Dynamic pressure at higher speeds. As Vmo is expressed as a CAS, and not EAS (which is a direct indication of Dynamic Pressure), the CAS associated with a constant EAS will increase with increasing altitude. A Vmo of, say, 330 CAS (EAS) at Sea Level, would be equivalent to 353 Kt CAS at 30,000 feet. Some manufacturers opt for the variable 'barber pole', some opt for the simpler constant CAS. For the example given, 330 CAS at 30,000 feet is only 311 Kt EAS, a much lower dynamic pressure.

I would not go so far as to say that Mmo is imposed to prevent shockwave damage, but rather to avoid, or contain within reasonable limits the CONTROL problems of excessive Mach number.

wondering, at first glance, this seems like a bird strike limitation. Umm, birds don't fly above 10,000 feet do they?

Regards,

Old Smokey
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