In general, most turbo-propeller aeroplanes operate significantly lower than… — Page 263, Lesson 321

Altitude and speed control in the cruise — that's what we're really talking about now, and I want to show you how it plays out differently depending on what's spinning up front: a jet, a turbo-propeller, or a piston engine. Let's start with the turbo-propeller aeroplane. In general, most turbo-propeller aeroplanes operate significantly lower than their jet counterparts. They seldom operate above 30,000 feet, and because of that, they never really suffer from the effects of getting close to the speed of sound. That's a key point — the compressibility problems that plague jets at high Mach numbers simply aren't a factor for turbo-props at their operating altitudes. Now, here's the interesting part. A turbo-propeller is based on the same engine design as a pure jet — it's a gas turbine at heart. So the effect of altitude on the turbo-propeller is very similar to the jet aeroplane. As altitude increases, the increasing true airspeed — that's the TAS — and the slightly decreasing specific fuel consumption help to improve the specific range. Specific range, remember, is how far you get per unit of fuel — nautical miles per kilogram, or miles per pound. So higher altitude gives you more TAS for the same fuel burn, and the engine gets a little more efficient. But — and this is the trade-off — that benefit is offset a little by the increasing power required at higher altitude. The air is thinner, so to maintain that higher TAS, the engine has to work harder. So whilst specific range does improve with altitude, above 10,000 feet it only improves by a small amount. The curve flattens out. That's why the choice of altitude for a turbo-prop may depend more on the wind considerations, and the time and fuel considerations involved in climbing to the selected altitude. You might pick a lower altitude with a better tailwind, or a higher one if the climb penalty isn't worth it. Now let's switch to the other type of propeller aeroplane — the piston engine. You'll recall that the piston engine aeroplane has a more or less fixed specific fuel consumption, even though specific fuel consumption is lowest at high manifold pressures, low rpm, and with the mixture correctly set. So the engine's fuel efficiency is essentially a constant — you can't improve it much by changing altitude. That means the only remaining variables in the specific air range formula for the piston engine aeroplane are the true airspeed and the power required. So here's the contrast I want you to hold onto. For the jet, altitude changes both the TAS and the specific fuel consumption, and the optimum altitude shifts as weight changes — that's what Figure 5.26 shows, the optimum altitude increasing as weight reduces through the flight. For the turbo-prop, altitude helps a little, but the benefit flattens above 10,000 feet. For the piston engine, the specific fuel consumption is fixed, so your only levers are true airspeed and power required. That's why altitude and speed control in the cruise matter so much — and why they matter differently for each powerplant.