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This is one of the questions in the ATP material:

What effect will an increase in altitude have upon the available equivalent shaft horsepower (ESPH) of a turboprop engine?

A) Lower air density and engine flow will cause a decrease in power
B) Higher propeller efficiency will cause an increase in usable power (ESHP) and thrust
C) Power will remain the same but propeller efficiency will decrease

I chose (C) but the correct answer was (A). Could anyone explain why? Isn't the whole purpose of a turbocharged engine to sustain engine power at high altitude (below critical altitude)? Why does lower density at high altitude cause a decrease in power?

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You have confused a turboprop engine with a turbocharged engine. They are very different and have very little in common.

A turboprop engine is powered by a turbine engine geared to a propeller. Turbine engines have less power available at higher altitudes so answer "A" is correct.

A turbocharged engine is a piston engine which is fitted with a small compressor driven by exhaust gas pressure. The compressor (turbocharger) can increase manifold pressure to restore sea level power at altitude. (Turbo-normalized) A turbo charger can also supply greater than sea level manifold pressure for more power. (turbo-supercharged)

Both types of turbocharged piston engines can maintain full rated power as they climb, but they will eventually reach a limit where full power can no longer be maintained.

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  • $\begingroup$ Okay I might have confused the turboprop with turbocharger, but doesn't the turboprop also have a compressor in the engine system? If so, can't it maintain air pressure flowing into the turbine at altitude? $\endgroup$ – lemonincider Jul 24 '17 at 2:16
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    $\begingroup$ The compressor compresses with a certain ratio, let's say 9:1. Air exits the compressor at 9 times the pressure that it entered in, at higher altitude the entry pressure is lower and so is the exit pressure. $\endgroup$ – Koyovis Jul 24 '17 at 2:52
  • $\begingroup$ @Koyovis When you say the compressor in your comment, is it limited to the compressor of turboprops or does it include that of turbochargers? $\endgroup$ – lemonincider Jul 24 '17 at 7:22
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    $\begingroup$ I was referring to the turbine compressor. The turbo charger has an arrangement where a turbine in the gas exhaust drives an intake air compressor, very similar to a turbine compressor arrangement actually. A ‘turbo-normalized’ turbocharger maintains sea level horsepower performance as it reaches higher altitudes. $\endgroup$ – Koyovis Jul 24 '17 at 8:20
  • $\begingroup$ @lemonincider, actually, turboprop and turbocharged engines are very similar and have almost everything in common. Both (and naturally-aspirated ones too) are limited by air density and their power decreases with altitude. However, all kinds of engines are also limited by other factors—maximum pressure and temperature the engine can withstand—when the density is above certain value, so below corresponding altitude, they have constant power (are “flat rated”). The only difference is the typical altitude to which different kinds are flat rated. $\endgroup$ – Jan Hudec Jul 24 '17 at 21:16
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Because the performance of a turboprop engine, like any other air cycle heat engine, is directly proportional to the density of the air moving through the engine. Denser air means more working fluid in a greater amount of energy can be obtained from it by combusting it with fuel. As you rise in altitude, the air becomes less dense and the amount of work that can be obtain from a given volumetric flow rate of air becomes less and less the higher you climb. This also lowers the equivalent shaft horse power that one can obtain from the turboprop engine, as it both reduces the shaft horse power to the propeller as well as the jadditional et thrust from the exhaust.

Manufacturers of turboprop engine sometimes compensate for this by flat rating the turboprop engine. That is, the fuel control unit for the turboprop presets the amount of output power that the engine will produce. The gas core of the turboprop may be able to produce much more power at sea level or at lower altitudes than you can obtain with a full throttle command from the cockpit. The maximum amount of power the gas core on a flat rated turboprop can produce at sea level at standard atmosphere is referred to as the engine's thermodynamic power rating. A flat rated engine will continue to produce a uniform power output as the aircraft continues to climb until the flat rated power output is equal to the thermodynamic power output of the engine at the current flight level. If the aircraft continues to climb, the engine's power output will begin to decrease.

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The turboprop engine produces less power at altitude, because the mass stream through the engine is smaller: there is less air. The turbocharged piston engine also produces less power at altitude due to less air entering the cylinder. Both have a compressor, and some effects of altitude on power generation are indeed similar, but the design conditions differ due to the great differentiator: weight.

A turboprop engine's weight increases much less as a function of max power at sea level, than that of a piston engine. The turboprop can be designed for cruise conditions, and take the surplus of power at sea level as either:

  • a bonus, so that much more power is available at sea level and power output decreases gradually with altitude;
  • a liability for the gearbox, so that power limitations at lower altitude are applied by the FADEC.

If the same approach was followed for a piston engine, the weight penalty would be much higher than for the turbine engine. So piston engines are designed for maximum power at sea level, and this power is maintained as long as possible with increasing altitude, by pumping in more air through the super/turbocharger.

Note that the turboprop with derated power and the turbocharged piston have very similar characteristics, both deliver a constant power as altitude increases, up to the critical air pressure...

The weight advantage of turboprops over piston engines was demonstrated in the 1960s by the conversion of the Cessna Skymaster into the Conroy Stolifter: the two Skymaster piston engines produced 155 hp less and weighed 117 kg more than the single TPE-311 turbine engine that replaced them.

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