Vmo vs Vne

[uwochris]

Hey guys,

I know that Vne applies to piston a/c while Vmo applies to turbine a/c. They are both technically "limitation" speeds designed to prevent airframe damage from excessive dynamic pressures. I am also somewhat familiar with Mmo (also a limitation speed), which applies to a/c flying near the speed of sound where excessive shock wave drag can cause damage.

What I do not understand is why does Vmo vary with altitude, while Vne does not? It seems logical that if Vne is around 170 CAS at sea level, then it should also be 170 CAS at a higher alitude since the dynamic pressures are exactly the same (keep in mind that we are talking CAS, not IAS or TAS). But according to some books I have read (ex- Turbine Pilots Manual), Vmo actually changes with altitude. In some books it states Vmo increases with altitude (measured in IAS), while in other books, it states it decreases.

Can anyone please clarify this? I'm thinking that my definitions of Vne and Vmo are off-base.

Thanks in advance!

 

[A Squared]

Hey guys,

I know that Vne applies to piston a/c while Vmo applies to turbine a/c. They are both technically "limitation" speeds designed to prevent airframe damage from excessive dynamic pressures. I am also somewhat familiar with Mmo (also a limitation speed), which applies to a/c flying near the speed of sound where excessive shock wave drag can cause damage.

What I do not understand is why does Vmo vary with altitude, while Vne does not? It seems logical that if Vne is around 170 CAS at sea level, then it should also be 170 CAS at a higher alitude since the dynamic pressures are exactly the same (keep in mind that we are talking CAS, not IAS or TAS). But according to some books I have read (ex- Turbine Pilots Manual), Vmo actually changes with altitude. In some books it states Vmo increases with altitude (measured in IAS), while in other books, it states it decreases.

Can anyone please clarify this? I'm thinking that my definitions of Vne and Vmo are off-base.

Thanks in advance!

It's because Vmo (and Vne, for that matter) is not strictly to prevent overload by dynamic pressure. The limit on the velocity could be airframe overload, or it could be some other factor. Control flutter is one factor. The behavior of control flutter is driven by the actual velocity of the air over the surface, rather than the dynamic pressure of the air flowing over the control, hence TAS is relevant rather than IAS. You will occasionally run into an airplane with a Vne which changes with altitude. The DC-6 does, as does the V-tailed Bonanzas (at least some models)

 

[MauleSkinner]

I don't know the answers, but I can help define the question

The Hawker 800 has a Vmo of 335KIAS from sea level to 12,000 feet. At 12,000 feet, this equates to about .625 Mach and, in a standard atmosphere, about 404 KTAS.

Vmo then decreases by 1 knot per 680 feet to 310 KIAS at 29,000 feet. At 29,000 feet, this equates to our Mmo of .8 Mach and, again in a standard atmosphere, about 497 KTAS.

Obviously we're not dealing with constant TAS or Mach numbers for the decrease. Something else must factor in there somehow.

Above 29,000, the barber pole continues to decrease in to maintain the .8 Mach limit.

Another thing to keep in mind that the piston equivalent of Vmo in turbines isn't Vne...at least from a structural standpoint, it's Vno. Operations between Vno and Vne are authorized only in "smooth air", and there is no equivalent speed range in a turbine. My understanding (and I may be wrong on this) is that when a Bonanza or 210 is converted to turboprop, the Vmo ends up being in the neighborhood of the piston airplane's Vno.

Additionally, the BE-58P has a Vne of 235 KIAS, which doesn't change with altitude, and a Vno of 196 KIAS, which decreases 4 knots per thousand feet above 16,000.

I realize that I'm just adding to the confusion, but maybe someone can use these examples to help clarify the issue.

Fly safe!

David

 

[mar]

Short on time

I am also somewhat familiar with Mmo (also a limitation speed), which applies to a/c flying near the speed of sound where excessive shock wave drag can cause damage.

Hi Chris.

I don't have the time or resources to research your specific questions right now. I think Asquared and MauleSkinner have given you good answers.

But I just wanted to make a quick clarification on the highlighted statement above...

...the upper speed limit of jet transports is based on something called Mcrit (Critical Mach Number).

At that speed (let's say M0.92) the airplane begins to experience shock waves as it approaches the speed of sound.

The shock waves cause an *ENORMOUS* increase in drag (as you noted) and really play havoc with the flight controls.

So the limitation isn't so much a *structural* one (to avoid physical damage) but an aerodynamic problem.

Great question as usual, keeps me sharp.
Cheers.

[Editted 'cause I don't know what I'm talking about]

 

[BushwickBill]

CJ1 has a Vne that decreases with altitude.

 

[GVFlyer]

Hi Chris.

I don't have the time or resources to research your specific questions right now. I think Asquared and MauleSkinner have given you good answers.

But I just wanted to make a quick clarification on the highlighted statement above...

...the upper speed limit of jet transports is based on something called Mcrit (Critical Mach Number).

At that speed (let's say M0.92) the airplane begins to experience shock waves as it approaches the speed of sound.

The shock waves cause an *ENORMOUS* increase in drag (as you noted) and really play havoc with the flight controls.

So the limitation isn't so much a *structural* one (to avoid physical damage) but an aerodynamic problem.

Great question as usual, keeps me sharp.
Cheers.

[Editted 'cause I don't know what I'm talking about]

I also don't have time for a detailed response as I have to go fly.

Mach Crit is simply the speed at which the speed over the top of a subsonic wing goes supersonic. As the streamlined air flows around a wing the airflow accelerates. At some aircraft Mach number less than Mach 1.0, the flow will reach the speed of sound. There will be a consequent drag rise associated with the Mach shock wave. Depending on the sweep of the wing this shock wave formation will also have an effect on the development of Mach Tuck as the aerodynamic center of pressure moves aft and outboard. Neither of these phonomena are limiting although in poorly designed wings the increase in static pressure behind the shock wave can cause boundary layer separation and Force Divergence.

Also associated with drag rise are buffet, trim and stability changes, and a decrease in control effectiveness. If the buffet is severe and prolonged, structural damage may occur. This would be limiting as would wing drop resulting from the wings shocking unevenly.

In any event, FAR Part 25 Airworthiness Standards for Transport Category Aircraft establishes speed limits in two sections: Design Airpeeds and Aeroelastic Stability Requirements.

Fundamentally, limits may be established for many reasons: structural limits, instability, longevity, flutter, control reversal or other Mach effects. For flutter, you have to back off 15% from the speed at which it is encountered to establish Mmo. Otherwise,the margin at which Mc is limited by compressibility effects must not be less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of any automatic systems.

In the case of the GV/G550, Mmo was established when rudder Cl Beta went positive (fundamentally a rudder control reversal) at Mach 0.955. Backing off M 0.07, Voila! - Mmo M0.885. Vno at 340 KIAS was established for longevity so that the company could give you a 20 year warranty on primary and secondary structures. I've seen 430 KIAS in the teens with the 48 inch fans churning through the thick air.


GV

 

[A Squared]

Additionally, the BE-58P has a Vne of 235 KIAS, which doesn't change with altitude, and a Vno of 196 KIAS, which decreases 4 knots per thousand feet above 16,000.

The Vno decreases approximately with the IAS, 4 kt/1000 ft vs. 3.7 Kt / 1000 ft in the standard atmosphere at this altitude.

The DC-6 has both a Vno and a Vne which decrease 5 Kt per 1000/ft above 17,000 ft, which is closer to a constant TAS than the Hawker, but less close than the Baron

 

[scoreboard]

Hey guys,


What I do not understand is why does Vmo vary with altitude, while Vne does not? Can anyone please clarify this? I'm thinking that my definitions of Vne and Vmo are off-base.

Thanks in advance!

Simply,

VMO: relates to Mach (flutter, design load, Turbulence, whatever limit here) , hence, since Mach is temperature dependant, ie the higher you go the colder it gets, therefore it changes, well the barber pole moves, but the Ind Mach Number remains constant (usually). Jets use VMO cause we go high where it's cold.

VNE/O: nautical airspeed, speed remains constant, even though mach number changes. Used for lower altitude aircraft, props and such.

 

[A Squared]

VNE/O: nautical airspeed, speed remains constant, even though mach number changes. Used for lower altitude aircraft, props and such.

Sigh ..... Doncha just love it when somone comes into a discussion and posts information which has already been shown to be incorrect? I always wonder what exactly is going through the minds of such people. Did you not read the information already posted? Or, do you think I and others were just making up crap when we mentioned airplanes with a Vno and/or Vne which decrease with altitude?