Bleed Effect on max power

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Unread post15 Dec 2018, 16:43

Hello dear herc crew! can some one kindly guide me about the following, please:-

As per graph of bleed effect on max power, with all bleed on the torque at sea level is lesser as compared to that for 2000 feet and 4000 feet and higher pressure altitudes. Lets say we start with 15000 thousand inch pounds, and see torque against sea level, then 4000 feet then 8000 feet; the torque keeps rising? why the effect of drawing all bleed (air conditioning, pressurization & anti-icing) is more pronounced at sea level aa compared to higher pressure altitudes?


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Unread post16 Jan 2019, 07:24

I would speculate that at sea level the engine more readily produces torque than at altitude. Thus a lower engine rpm setting at sea level produces the same torque as a high rpm setting at altitude for same prop angle of attack, meaning the engine at altitude would require more gas generation.

Thus a set bleed for larger overall gas generation at altitude would mean a smaller fraction of that larger amount would be taken out. That means that the torque loss would be a lesser fraction. Whereas at sea level, the little amount of gas generation needed to generate no-bleed torque would be completely taken by the bleed powering.

So if bleed took a constant 10 unit of power available and you need 100 units of power for 50% torque at altitude, the bleed would take 100-10 or the decrease would be 10% at 90 units of power corresponding to a corresponding small torque loss.

If you are at sea level and only need 40 units for 50% torque but bleed still takes 10 units, you are down 25% in power and corresponding higher torque losses.

Just my 2 cents but do not quote me on it until I got the actual formulas. Helicopters are big on torque so you might find more info on pressurized helicopters.

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