Old 02-21-2010, 02:31 AM
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r1830
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Default Vmo indicated airspeed decrease with altitude

I was trying to figure out why Vmo lowers as altitude increases. This is what I have com up with. Am I on the right track? Thanks

Vmo is a speed defined by the flight envelope which is based on Load Factor and Gust Factor.
Flying above this speed and encountering a gust or increasing load factor may result in structural damage. Airspeed is presented to pilots as indicated airspeed.

In my research, it seems that Vmo/Mmo is based on a Percentage of Mach Speed.
In the limitations section of the BE1900D Manual the Airspeed Limitations Chart lists Vmo from 13,200 ft to 25,000 ft as 248-195 kts indicated airspeed. It has an asterisk next to it that says these speeds correspond to .48 Mach. This implies that the limitation is based on percent of mach speed. .48 Mach would result in an airspeed based on true airspeed and not indicated airspeed. It appears the limit airspeed is based on a true airspeed. As a result, as we increase altitude the indicated airspeed for Vmo will decrease. This makes sense for aircraft that operate at speeds approaching Mach 1. At speeds below Mach 1, airflow around curved surfaces of the aircraft could accelerate to speeds at or above mach 1 which could result in shockwaves forming in undesirable locations and affecting controllability of the aircraft.

Other limit V Speeds such as Vle (landing gear extended) and Vlo (landing gear operating) do not change with altitude. They are not based on true airspeed; instead they are based off of indicated airspeed. Indicated airspeed is essentially a measurement of the volume of air passing a point in a given amount of time. A wing needs the same “volume” of air to pass over/under it to produce a given amount of lift. As altitude increases, the density of air decreases and the aircraft has to fly at a higher true airspeed to maintain that volume of air. Vle and Vlo speeds most often are predicated on the operation of the gear doors. Some aircraft like the 747 have very large gear doors that open up for extension and/or retraction, but then close once the gear cycle is complete. In these cases Vlo will be a lower speed than Vle. Another factor for Vlo might be the amount of stress created on the downlock mechanism. As airspeed increases, parasite drag increases resulting in a greater airload on the landing gear. This load is based on a volume of air passing by, that is why these airspeeds are an indicated airspeed.
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