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4 Answers

Difference between Power and Thrust (Fixed Pitch Prop, Piston)

Asked by: 691 views Aerodynamics

Hi, Having some trouble understanding the difference between Power and Thrust with regard to a fixed pitch propeller, piston engine aircraft.

I know the formulas and definitions of each but I still cant understand the practical aspect.

In particular, At minimum Drag speed we require the least amount of Thrust to overcome Drag, If I need to make the least amount of Thrust should Power not also be at a minimum?

When flying slower than min Drag speed the Drag increases so I need more Thrust to overcome Drag so I would need to apply more engine Power to generate that extra Thrust?

Yet at a point that is slower than minimum Drag it is actually the point of minimum Power required even though more Thrust is required... Why and How?

I realise what I have explained above is incorrect going by the books but thats my current understanding of the relationship between Power and Thrust, more thrust demands more power, less thrust then less power.

If possible please explain without the use of formulas and/or use another practical everyday principle that I could apply so I can better understand.

Many Thanks!

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4 Answers

  1. Av8r1998 on Aug 14, 2017

    OK. I’m not a CFI, but I am an engineer. Hard to explain without math, but I’ll give it a try.
    What’s the first thing you learned about in flight school? The 4 “forces” of flight, specifically Lift, Drag, Thrust and Weight. First thing I’ll explain is that Weight is not a force, gravity is a force. Weight is the application of gravity to a mass. That’s why all objects on earth in a vacuum will fall at the same rate.

    Now to your question – “Thrust” is the force that moves an airplane forward in flight. Power is the rate of change in that force, or to put it another way, its the rate of change in FORWARD energy. That sounds a little counterintuitive… a plane is straight, level, unaccelerated flight is still producing power, but not accelerating. That’s because the only energy being transferred is used to overcome drag and produce lift. In this condition Thrust = Drag and Lift = Weight. If you’re in cruise flight and reduce throttle you will either A) descend or B) Slow down. Thrust has been reduced, so if you maintain level flight you must slow down, or if you maintain speed you must descend.

    To simplify (grossly oversimplify) Thrust is produced by power.
    Hope that makes sense.

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  2. Av8r1998 on Aug 14, 2017

    Again, Grossly oversimplified, but look at your tach in a climbout. You’re (in a single) at full throttle (not power) at Vy, but in a light trainer like a Cherokee or C-172, only going 76 knots, and your tach (Best measure of power that you have in a fixed pitch piston single) is only showing about 2450 rpm.

    As soon as you push the nose level, you have to reduce throttle or you’ll overspeed the prop. That’s the “math” part. Thrust = Power * Velocity. Has “power” changed? No. Velocity has, because induced drag has been reduced (and parasite drag hasn’t caught up yet). Therefore so has thrust.. Does this make any sense?

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  3. Brian on Sep 23, 2017

    The difference in definitions is:

    Power: Remains relatively constant, there is the caveat of propeller efficiency here, through the aircraft’s flight regime.

    Thrust: Varies widely through the flight regime. Starting with a high value at lower speeds, and rapidly decreasing with the crafts increase in speed.

    These statements remain true for Propeller driven, Fixed Wing, Airplanes. For Jet and other Aircraft, power and thrust relationships differ.

    For this reason, when studying Propeller Airplane’s, learn to think in terms of Power versus Drag. Thrust is only useful when working with the Vx, best climb angle = Trust available – Thrust required (or excess thrust), speed of a Propeller driven Airplane. So learn the power required curve to understand this animal, and realize its relationship to drag, speed, and angle of attack.

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  4. Brian on Sep 23, 2017


    I said, “Power: Remains relatively constant, there is the caveat of propeller efficiency here, through the aircraft’s flight regime.”

    I should have said that the “Slope of the power curve remains relatively constant for a propeller driven airplane.” It moves up and down dramatically on the y-axis between piston, turbo-piston, and turbine powered propellers. The slope remains constant though, effected mainly by the efficiency of the propeller.

    This same Slope distinction is true for Thrust curve.

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