Vmo vs. Cruise Speed

[JoeFriday]

I've been looking on the web at a bunch of aircraft I'll NEVER be able to afford (unless this lottery ticket pays off), and I've found several instances where a turboprop or light Jet has a listed cruise speed of say 300+ knots, but a Vmo of 250. How can cruise speed exceed Maximum speed?

[HighFLyer]

Vmo is probably indicated airspeed, while cruise is true airspeed. For example, as your altitude increases, you indicated airspeed will remain the same, or possibly decrease as the air becomes less dense. Your true airspeed will increase because of reduced drag. Of course this won't be very noticable in a 152 or such, but the airplanes your probably talking about will be affected by this.

[JoeFriday]

Makes sense, but I'm not quite there. For the sake of discussion, lets say we're close to sea level on a standard day in a Citation X. It's spec'd for 500 KTAS in the upper FL's, but has a Vmo of 350 KIAS. Does that mean the speed must be held down until we climb way up?
--
Al

[skid]

Yes. Indicated airspeed is what dictates operations, true airspeed is of no consequence to the airframe. At flight levels indicated airspeed may be very low for a fairly high true airspeed. That is why some turbocharged recips run hot at high alt. Even though true airspeed is high, the density of the air might make similar airflow through cowling as droning along at 100 knots @1000 msl would, only high power settings are being used up high. True airspeed is merely a correction for density alt and the airframe doesn't know the difference.

[JoeFriday]

So to get the rated TAS that the manufacturers are touting, you've got to get out of the weeds... Guess I'd better learn to fly above 3000 ft:-)

Thanks for the explanation gents!--Al

[HighFLyer]

3000 feet-I get nose bleeds that high!

[djschaut]

Sgt. Friday,

When you ask if you have to hold the speed down until you climb all the way up (to the flight levels,) it's really not an issue at level flight or during an ascent. At sea level, the Citation will probably not cruise at 350kts indicated, anyway. Furthermore, you wouldn't want it to--it's much more cost-efficient to fly at the flight levels. The only time you'd have to hold the speed down would be on the descent. With a turbine, you don't have to worry about shock-cooling, and you can come down as fast as the airframe allows. On the climbout the plane would be somewhere around 200kts indicated (depending on your rate of climb,) and the true airspeed would be increasing as you climb. When you level out, your true airspeed should be close to your GPS groundspeed, and your indicated airspeed will likely be much lower than 350kts. I don't have a POH for the Citation. Any difference between the true airspeed and ground speed would be due to a headwind or tailwind.

For a recip, the higher you go the harder it is to cool--especially turbocharged engines. The turbocharger is powered by the engine exhaust, at about 1650 degrees (EGT). It's red-hot. There is an altitude at which the air is so thin that the indicated airspeed will no longer provide enough cooling or manifold pressure for the engine. There is no more power to be had. This is called the 'critical altitude.' It can be helped by increasing prop-speed (rpms). Although some recips are certified to ceilings of FL250 or higher, their engines gradually experience loss of manifold pressure and the climb rate decreases. It goes without saying that any turbocharged engine regularly flown at the flight levels--where cooling is inefficient--may not make it to TBO unless managed carefully. The thin air exacts a toll on non-turbine aircraft. The high temps cause metal fatigue. An exception would be the larger Lycomings 560s not flown at maximum power. The larger engine mass tolerates the thin air better, whereas the smaller turbocharged engines, such as the Continental TSIO-360, suffer greatly at those altitudes. They're over-boosted and run too hot. Manufacturers like to make claims about true airspeeds and certified ceilings for their planes, but for many of them, flight at those altitudes is not practical. Mooney was good for that on its older models. For the most part, it's best to have pressurization at the flight levels, anyway.

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DJSchaut

[JoeFriday]

Thanks to you both for taking the time to explain this topic. I really appreciate the detailed responses - I learned a lot.--Al