Vmo vs. Mmo

[uwochris]

Hey guys,

Is there a difference between these 2 speeds that apply to high speed a/c (Vmo vs. Mmo)? Or, are they the same (i.e. does the red barber pole on the ASI represent Vmo, Mmo, or both?). Also, I saw a diagram somewhere that showed that Vmo increases with altitude, while Mmo decreases (in terms of IAS)- does anyone know why there is a difference (Turbine Pilot's Manual)?

Another thing from the Turbine Pilot's Manual that I found odd was that Vmo (in terms of IAS) decreases with altitude, while Vne does not. If both of these speeds represent limitations from excessive dynamic pressure, why should they differ between altitudes?

Thanks in advance,

Chris.

 

[JetPilot500]

Hey guys,

Is there a difference between these 2 speeds that apply to high speed a/c (Vmo vs. Mmo)? Or, are they the same (i.e. does the red barber pole on the ASI represent Vmo, Mmo, or both?). Also, I saw a diagram somewhere that showed that Vmo increases with altitude, while Mmo decreases (in terms of IAS)- does anyone know why there is a difference (Turbine Pilot's Manual)?

Another thing from the Turbine Pilot's Manual that I found odd was that Vmo (in terms of IAS) decreases with altitude, while Vne does not. If both of these speeds represent limitations from excessive dynamic pressure, why should they differ between altitudes?

Thanks in advance,

Chris.

VMO = Maximum Indicated Airspeed
MMO = Maximum Mach Speed

When you are at lower altitudes, VMO is more limiting than MMO. As you get higher MMO becomes more limiting. For example: If you are climbing at 300 KIAS and your VMO is 370 Knots (MMO is Mach .87), you are well below your VMO Speed. However, as you continue to climb at 300 knots, your Mach speed continues to increase because of 2 factors. First, because at a constant IAS your TAS will increase with altitude (higher TAS = higher Mach speed). Second, (keep in mind that Mach speed is based on temperature) as the temperature drops in the climb, a given Mach speed occurs at a lower TAS. This also means that Mach .87 will occur at lower Indicated speeds as you climb. Therefore, eventually, your MMO becomes more limiting than VMO. Your barber pole will adjust accordingly on the Airspeed indicator. So in our example, when your MMO of Mach .87 becomes less that 370 KIAS, your barber pole will keep moving to a lower IAS.

Unlike a piston aircraft, VNE does not exist on a turbine aircraft.

Hope this helps,
JetPilot500

 

[Foxcow]

Good info!

 

[typhoonpilot]

I'm taking a WAG without much reference here so bare with me.

Is there a difference between these 2 speeds that apply to high speed a/c (Vmo vs. Mmo)? Or, are they the same (i.e. does the red barber pole on the ASI represent Vmo, Mmo, or both?).

There is a difference, they are not the same, but the red barber pole does represent both ( it just doesn't do it at the same time ). It switches from Vmo to Mmo at a certain altitude/temperature.

As you know, Mach number is dependent on temperaature. As you climb, the temperature decreases. At sea level the speed of sound is about 661 knots, at or above 36,000 feet ( in a standard atmosphere ) it is 573.8 knots.

The only time that an airspeed indicator is correct is at sea level on a standard day. That is what it is calibrated for. Once you move from that it no longer indicates correctly. That is why we have so many kinds of airspeed, IAS, CAS, EAS, TAS, etc.

If you are flying an aircraft with a Vmo of 347 knots and an Mmo of .84 Mach it is not possible to get remotely close to .84 Mach while at sea level ( .84 X 661 = 555 knots ). Conversely, if you are at 36,000 feet it wouldn't be possible to have an indicated airspeed of 347 knots due to the effects of air density and compressibility. You tend to have a much lower reading. Depending on temperature, your indicated airspeed at .84 Mach might only be 280 knots.

Clear as mud so far, right ? So you takeoff in your shiny jet aircraft with a combined airspeed-Mach indicator. Initially you are referencing IAS in the climb ( and like a good boy you stay below 250 until out of 10,000 feet, right ? ). Passing 10,000' you let the big dog hunt ( Piedmont saying ) and increase to your normal climb speed of 310. All the while the Barber pole is sitting at 347. As you climb through the mid to high 20s you transition to climb referencing Mach and continue climbing at about .74 Mach. It is about this time that the Barber pole starts creeping downward to a figure less than 347 and representing .84 Mach. As you level off at cruise altitude your indicated airspeed is at 280 and the indicated Mach is .76. The Barber pole is sitting on an indicated airspeed of 295. ( note, not real world speeds, just making an example ).

Not sure about your other two questions as I don't have the Turbine Pilot's Manual charts to understand what they are getting at. I don't know of any airplane that has both a Vne and a Vmo. It is usually light GA aircraft or otherwsie slow airplanes that have Vne, while it is bigger and faster aircraft that use Vmo.


Typhoonpilot

 

[airbaker]

Typhoonpilot's response is the most correct regarding the a/s indicator "sharing" the barberpole for both VMO and MMO. While the barberpole for Vmo represents more of a dynamic pressure absolute, you'd never get there up in the flight levels before exceeding Mmo/critical Mach, and the resultant shock wave separation.