Welcome Guest. Sign in or Signup

2 Answers

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

Asked by: 351 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!

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

2 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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

  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?

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.