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Could someone please help me understand the difference between horsepower and thrust on turbocharged engines? I believe I have a good understanding regarding the purpose and operation of a turbocharger, but what I don’t understand is why the overall performance goes down even when the turbocharger is capable of making max PSI below the rated “critical altitude”. For example, if my turbocharger can produce 40” from sea level to 12,000 DA, why does my climb performance decrease with altitude? 40” will produce max horsepower from sea level to 12,000, but the performance charts clearly demonstrate a loss of climb performance between sea level and 12,000’ DA, and I just don’t understand why. Thank you!

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Assuming you are talking about a turbocharged propeller driven aircraft, your question really makes it easy to explain as you have already identified the two parts of your power unit that are affected by increasing altitude (thinner air). The amount of air it can breath and the amount of thrust your prop can generate.

The turbo only takes care of the air breathing part, yes, it has as much power as at sea level because to turbocharger can compress enough of the thin air to feed it. No problem there.

But the prop is still spinning thinner air, so it can not get the same "bite" as at lower altitudes with out going to even higher RPMs. So, when your RPMs max out due to engine limitations, or even the tips going supersonic, that's it, no more thrust increase available.

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The main issue is the sea level torque is available to the engine, but as density decreases the propeller blades are forced to operate at a higher and higher blade angle (higher and higher local angle of attack) to absorb the available torque, to keep the rpm at the speed set by the governor. This takes the propeller blades out of their most efficient AOA range so that the thrust efficiency of the propeller declines somewhat as the air thins out.

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Engine/prop combinations provide approximately a constant power into the airflow. (Jet engines proved approximately a constant thrust.)

Here are some links, if you’re interested in that relationship. http://www.srmuniv.ac.in/sites/default/files/downloads

http://web.mit.edu/16.unified/www/SPRING/systems/Lab_Notes/airpower.pdf/class10-2012.pdf

http://www.dept.aoe.vt.edu/~lutze/AOE3104/climb.pdf

Power = force x velocity, so the power required from the engine to maintain level flight for a given operating condition is drag x speed. If the engine can put out that much power (with an efficiency term for loss in conversion) then you can maintain level flight. If the engine can put out more, then you can convert the excess power available into climb. If you hold drag fixed somehow but increase velocity, you have less power available to climb.

Note that the speed to get required power is your speed with respect to the fixed inertial frame of the air mass, and so is TAS not IAS.

Now your drag is a function of IAS. Your best rate of climb Vy is at a fixed IAS, so in a climb at Vy, you see the same drag no matter the altitude.

So as you go higher, the power required just to maintain level flight at Vy goes up since your TAS goes up for the same fixed IAS.

Your climb rate is going to be a function of the difference between power available from the engine (fixed, thanks to the magic of turbocharging) and power required for level flight (increasing as you go higher due to a higher TAS for the same IAS).

This leaves less excess power available to convert into the climb as you go higher.

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  • $\begingroup$ Thrust is thrust, whether it comes from a rocket, a jet, or a propeller. The propeller gives less thrust at a given RPM because the air is thinner. The difference in TAS and IAS is actually an advantage at altitude, your drag is from IAS, your GROUND SPEED is TAS. For a turbo, your power does not change (nor would it if the prop fell off the plane), but the efficiency of converting RPM to thrust drops in thinner air. Eventually thrust drops to where you can only maintain IAS needed for level flight. That's where the pilot needs more oxygen too. $\endgroup$
    – Robert DiGiovanni
    Jun 1, 2019 at 16:03
  • $\begingroup$ Robert, there are too many references to back up my assertion that props are constant power while jets are constant thrust, but here is one. srmuniv.ac.in/sites/default/files/downloads . And another, equation 6 on page 1. web.mit.edu/16.unified/www/SPRING/systems/Lab_Notes/… . And this one, page 2. dept.aoe.vt.edu/~lutze/AOE3104/climb.pdf $\endgroup$
    – MikeY
    Jun 1, 2019 at 19:41
  • $\begingroup$ @ MikeY Well, the bridge over troubled waters is constant RPM. Piston engines are unfortunately limited in RPM, but theoretically would provide more POWER (fuel burn per second) to spin the prop FASTER if RPM or SONIC BARRIER were not an issue. Ways around rpm limitation include more blades, wider blades, damn the sound barrier, full speed ahead (noise regulations and Thunderscreetch in mind). But jets pour that fuel in to get more POWER to turn their fans at higher altitudes, where greater TAS makes up for lower THRUST per gallon efficiency. But power does not move the plane. $\endgroup$
    – Robert DiGiovanni
    Jun 1, 2019 at 19:51
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    $\begingroup$ @RobertDiGiovanni: MikeY is right, fly faster and the same power means less thrust. If the propeller's solidity has been calculated with density at FL180 in mind (hint: If the critical height is FL180, the propeller should also keep up with converting that power to thrust), then density just means that blade lift coefficient goes up a bit, but thrust is not affected by the lower density. $\endgroup$
    – Peter Kämpf
    Jun 1, 2019 at 20:04
  • $\begingroup$ @Peter Kampf MikeY answer is good, except for the notion that the power/thrust relationship is somehow different than jets. It may be inaccurate to describe climbing ability on power, particularly in this case. A good turbo could go up to 60,000 feet, and if he could reach out his window and put a 25 foot 8 bladed supersonic prop on there, he WOULD get to 60,000 feet at 2500 rpm. RPM is the limiting factor with piston props, and in this case the prop craps out while the engine has more to give. Actually, it may be you can't fly faster with the same power if there is less thrust. $\endgroup$
    – Robert DiGiovanni
    Jun 1, 2019 at 23:59

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