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ILS27R
24th Apr 2002, 08:56
Hi all,

Why have the speeds Vmo and Mmo replaced Vno and Mno? What governs the Vmo or Mmo speeds for an aeroplane?

Many thanks,

ILS27R

Genghis the Engineer
24th Apr 2002, 09:11
"NO" or normal operating limits are below "NE" or never exceed limits, which are the same as "MO" or maximum operating - the difference here is terminology.

What determines the maximum operating speed?

Firstly structural considerations are used to determine a theoretical Vd/Md - the design limit. This is the limit to which flight testing is allowed to take the a/c.

If (as often happens) a handling or flutter deficiency is found below that, then a lower limit of "Vdf", or flight design limit is laid down.

Finally , Vne/Mne/Vmo/Mmo is determined as a margin below Vdf. The actual margin is a function of the certification standard - for light aircraft codes it's simply 10%, for "higher" codes like part 25 it's rather more complex.

The use of Vno/Mno is about 2-tier limits. Basically you are given a limit which you should not NORMALLY go beyond, but if you do you know that up to Vne/Mne there is no risk and you need not report it. If you go beyond that there is a risk of structural damage and/or unacceptable handling characteristics being met. Some organisations prefer 2-tier limits, some do not.

G

ILS27R
24th Apr 2002, 09:32
Thanks Genghis;)

ILS27R
24th Apr 2002, 10:08
As I understand, it is safe to travel through the VMO region (in still air) therefore why have a VMO pointer, wouln't it be better to have a VNO pointer? Also what variable drives this VMO pointer? Is it mainly driven by OAT?

Crossunder
25th Apr 2002, 12:50
Hm. I thought it went like this: JAR 23 aircraft have Vno / Vne, while JAR 25 aircraft have Vmo / Mmo..?

Larger aircraft have a greater margin to Vd (design dive speed, where no excess flutter will occur etc; JAR 25-335) ... Because these are often jet engine powered; an upset in attitude (pitch down) is more difficult to arrest, and speed build-up more rapid, whereas a JAR 23 a/c will most probably have a propeller (more drag) and speed build-up is slower (pull to flight idle, and problem solved), allowing the pilot to correct the upset in a timely manner even though the margin to Vd is 10% (?).
Either way, I think the difference between Vno and Vmo is only in the safety margin. Large a/c = greater margin, and it is not safe to exceed Vmo anymore than it is to excee Vne.

Am I completely off the mark? Anyone?;)