Vmo vs Maneuvering Speed Question

[Wink207 78]

I am in the habit of flights 2-3 hrs in length with the Lear 35 and a good deal of that time I am away from the sim working on other things. These flights are typically at FL410 and above. Twice in the last 6 months or so I have returned to the sim just in time to avoid the aircraft crashing to earth (water). After much thought I believe I have suffered high altitude stalls. As one travels along the route of flight the Wx (ASN) changes, the temperature in particular, as I fly in the Caribbean most of the winter and on up to the lower half of the US, and the engine thrust changes. I believe what has happened is the thrust changes at that control setting and a stall results.

 

Ok, so here's the question.

 

On the kneeboard Vmo is given as .75 Mach. Maneuvering speed is given as 195-245 kts IAS depending on weight. If you're pretty heavy, let's say TO weight at 18,000 lb, you can easily arrive at FL400 + around 17000 lb. Barber pole is at around .85 Mach. So to have a reasonable margin to Maneuvering (stall) I need to fly at around .80 Mach as IAS at .75 Mach, FL410, T= -33 C, is around 225 kts, which could be right at or below Maneuvering Speed, risking a high altitude stall. Haven't looked at it at higher yet but as we all know this aircraft can climb!

 

Would it be considered poor airmanship to exceed Vmo to have a margin against high altitude stall?

[ThomasAH 61]

You should not exceed Vmo, maybe only in emergencies. But you can fly at Vmo if you monitor the speed closely.

If you need to exceed Vmo to avoid stalls at a certain weight at a certain altitude, you should not fly at this altitude until you have burned enough fuel.

[skelsey 765]

Welcome to Coffin Corner!

 

I am not familiar with the Lear, but this is a problem common to high-altitude operations in general. As you have noted, the higher you fly the smaller the margin between the high-speed buffet limit (Vmo) and the stalling speed. Eventually if you keep climbing you will reach the point at which if you fly any faster you will overspeed and go in to high-speed buffet, but if you slow down you will stall.

 

As noted above, the answer is to fly lower (which will widen the margins), especially if you are expecting turbulence or other weather conditions that might cause the airspeed to exceed the limits.

 

Of interest, the U2 is particularly notable for its 'coffin corner' -- at the typical altitudes/mission profiles flown, there are at most 10 knots between Vmo and stalling speed, meaning that extremely close attention to speed control was/is necessary when flying those things.

[scottb613 1,186]

Hi Folks,

 

Just to clarify the definition a bit - Maneuvering Speed (Va) has nothing to do with stalling - Manuevering Speed is that speed where you can still make full deflection of the controls without damaging the aircraft...

 

Regards,
Scott

[FDEdev 955]

Hm,  V_A is IMO closely related to the stall speed as V_A is the speed at which the plane stalls before structural damage occurs.

But even more important is the fact that V_A is only applicable for 1 axis at a time. e.g. when pulling out of a diving turn you can easily overstress a plane despite being well below V_A.