Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Propeller and thrust

  1. May 7, 2016 #1
    Does the weight of a propeller have any effect on the thrust provided? For example, if there are are two propellers of the same size same speed but different weight, would the one with more weight have a greater or lesser thrust? also would effect do the number of blades(faces) have on the way a propeller works? for example there are propellers which are 2 faced, 3 faced and so on...
  2. jcsd
  3. May 7, 2016 #2
    I don't see why the weight of a propeller would have any effect. Energy starts with the motor, goes through the propellor, and then pushes the air, producing thrust. A heavier propeller would require more energy to start turning, but (ignoring friction) it's greater momentum would then cause it to continue moving air after the engine had stopped. It's a needless store of energy that would just make the plane heavier.
  4. May 7, 2016 #3


    User Avatar
    Science Advisor
    Gold Member

    For two otherwise identical propellers of different weights the maximum safe rotational speed for the heavier one would be lower than for the lighter one .

    Propeller thrust is a function of rotational speed . So maximum achievable thrust for the heavier propeller would be less than for the lighter propeller .
  5. May 7, 2016 #4
    I wish to learn more about propeller designs, do you any any nice resources? Also I saw a man on youtube who had built a hoverbike. It could lift him uptona feet and a half. How powerful do you suppose his motor was?
  6. May 7, 2016 #5


    User Avatar
    Science Advisor

    The number of blades is selected to balance the propeller in all dimensions at all speeds.
    The blades must slice the volume of air efficiently without a blade cutting through air disturbed by the previous blade.
    So the RPM of the propeller will be inversely proportional to the number of blades.

    One blade with a counter balance will be very fast and so not need a reduction gearbox. Two blades is simple and better balanced than one. Three blades will be well balanced, but will need some reduction box. More than three blades will be well balanced, but slower rotating and so can have stronger blades.
  7. May 12, 2016 #6


    User Avatar

    No. The weight (mass, actually) will influence the transient response (spool-up and spool-down time) and gyroscopic loads during rapid maneuvering. But steady-state thrust will be unaffected.

    The blade count is driven primarily by the need to carry power. The bigger (i.e. - more powerful) the engine, the more blades you need to convert that power to thrust. This is because the blades have structural limits, so the available power has to be divided between them.

    According to idealized propeller theory, you lose aerodynamic efficiency every time you add a blade. So propellers typically have the minimum number of blades required to carry the power of the engine (this is a bit simplified, weight and structural loads play a role in blade selection as well).

    Not sure what your background is, but if you have the math skills, here is a good starting point: https://www.amazon.com/Elements-Aerofoil-Airscrew-Cambridge-Classics/dp/052127494X.
    Last edited by a moderator: May 7, 2017
  8. May 12, 2016 #7


    User Avatar
    Homework Helper

    Blade count is often increased because of ground clearance issues during takeoffs and landings.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted