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Integration of pressure distribution of each blade get net force(thrust) which is allways pointing forward.So there is no drag force at any propeller ,that tells that size of propeller doesnt change total drag force.

So plane with propeller of 1m diameter will has same total drag force as plane with propeller of 2.5m diameter? (if both planes are same and both propellers have same thrust force,and both propellers are fixed at tail so wash not blow at plane body,causing aditional skin friction and pressure drag)

boat analogy:

Why all speeds boats use small propellers,instead big ones?I allways think reason was big propellers increase boat overall hydrodynamic drag,but when I better think propeller cant cause drag when producing thrust!

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    $\begingroup$ This is a great question. Propellers definitely have drag when they are not spinning. There may be localized spots on the prop that are actually dragging rather than pulling under power depending on prop rpm and AOA. Prop wash turbulence interacting with wing and fuselage airflow may also be considered. $\endgroup$
    – Robert DiGiovanni
    Aug 31, 2020 at 17:18
  • $\begingroup$ @RobertDiGiovanni Why all speeds boats use small propellers,instead big ones?I allways think reason was big propellers have more drag,but when I better think propeller cant cause drag when producing thrust... $\endgroup$
    – member2017
    Sep 2, 2020 at 10:02
  • $\begingroup$ @RobertDiGiovanni, and propellers windmilling (spinning, but the engine not producing any power) create even more drag than propellers not spinning. $\endgroup$
    – Jan Hudec
    Sep 4, 2020 at 5:47
  • $\begingroup$ @member2017, boats use small propellers, because water being denser the induced power is low while the parasite power is high. $\endgroup$
    – Jan Hudec
    Sep 4, 2020 at 5:49

2 Answers 2

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the drag force that a propeller generates while under power is expressed as a torque applied to the engine's crankshaft and arises because of skin friction on the propeller blade surfaces. as long as the prop is generating net thrust it is not generating drag in a direction opposite to its direction of travel.

Prudent design of the prop blades guarantees that no portion of the blade is ever generating backwards thrust for all possible combinations of airspeed, RPM, and blade pitch.

A propeller that is not generating thrust is generating drag which does act against the airplane's direction of travel.

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I don't really like the sentence « there is no drag force at any propeller ». In fact, there is a drag force which could be overwhelmed leading to only net thrust (by projecting integrated force on flight direction).

Under your hypothesis: same net thrust, same plane and no interactions between aircraft and engine.. Yes you could consider the size of your propeller does not affect general drag.

Nevertheless, this is an interesting question and the distinction between drag and thrust could be headache, specially with integrated propulsion system. It is sometimes more convenient to think about power and energy getting in and out the system than forces applied to it. See Power Balance Method which was developed and presented by M. Drela.

Finally, note that even under your conditions (and as mentioned by @Robert DiGiovanni), there are other parameters than drag, limiting propeller (or more generally an engine) diameter:

  • Ground clearance: for practical and safety reason

  • Rotational speed: which reducing as prop diameter increase (for aerodynamic performance purpose i.e. shock waves at blade tip).1

  • Weight: which change the total lift needed by the aircraft

  • Noise constraints

  • Etc..

1 Moreover, for a gas turbine carrying the prop, reducing this speed involves turbine speed reduction (and so its efficiency) OR using a bigger gearbox (increasing the gear ratio) leading to an heavier and more complexe system (more energy to diffuse).

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