Tas

[Keith.Williams.]

The short answer to your question is yes it does.

The longer answer goes something like this:

The relationship between IAS and TAS depends on air density. The ASI always gives the same indication whenever it experiences a given dynamic pressure. So climbing at constant IAS means climbing at constant dynamic pressure.

Dynamic pressure is 1/2RhoVsquared where Rho is air density and V is TAS. As altitude increases, the air density decreases. So to get the same dynamic pressure (and hence the same IAS) we must fly at a greater TAS. It is for this reason that the TAS at any given IAS increases as altitude increases.

But air density is affected by both static pressure and temperature. As altitude increases (up to the tropopause) , both the static pressure and the temperature decrease. The decreasing pressure allows the air to expand, thereby causing its density to decrease. But the decreasing temperature causes the air to contract, thereby increasing its density. In any normal atmosphere the reducing pressure is the dominant factor, so density decreases as altitude increases. The overall effect of the reducing temperature is to reduce the rate of density decrease, which in turn reduces the rate of TAS increase at any given IAS.

Above the troposphere the temperature is constant, but static pressure and hence density continue to decrease. So the TAS at any given IAS continues to increase.

The real aerodynamic advantage comes from the fact that the drag (and hence thrust required) is constant at any given IAS (if we ignore compressibility effects). So as we go higher we pay the same price in terms of thrust, but get a higher TAS for our money.

A second benefit comes from the fact that the specific fuel consumption of jet engines is lowest at low ambient temperatures and when operating between about 85% and 95% RPM. at low altitudes the high density air would produce too much thrust when operating in this optimum RPM range, so we cannot get best SFC when cruising at low altitude.

But as we climb higher, the reducing density reduces the thrust, while the drag remains constant. If we climb high enough we can reach an altitude at which the drag (and hence thrust required) at the speed giving best TAS to Drag ratio, is equal to the thrust in the 85% to 95% RPM band. At this point we are at our optimum altitude and at our maximum range speed VMRC.