1
$\begingroup$

Propeller blade AoA depends on tangential veloctiy caused by rotation and forward speed "head wind". If propeller has fixed pitch is flow attached on blade at every RPM / speed conditions?

For example when plane has full throttle when staying on ground at same place,forward veloctiy is very small,that mean AoA is higher,does this cause flow separation at low pressure side of blade at this RPM/speed condition..etc

Improve this question
$\endgroup$
2
$\begingroup$

No, flow separation far away from the intended operating point is common.

Witness racing planes. Before Ratier made the first useable variable-pitch propellers in 1932, high speed aircraft had abysmally low propeller efficiencies at low speed, so much that world records were set with floatplanes (which had virtually unlimited "airfields" at their disposal). Same with small racing boats: To get over the hump (transition from buoyancy to planing), drivers need to lean forward and to patiently gain speed with their high-pitch propellers providing only marginal thrust due to flow separation.

The rotating movement of propeller blades helps to accelerate the boundary layer outwards, so flow separation on propellers happens much later (at more adverse pressure gradients) than on wings. Regardless, if blade angle and flow direction at the blade don't match well enough, separation is unavoidable.

Improve this answer
$\endgroup$
16
  • $\begingroup$ @So that mean that every fixed blade propellers are design for only one narrow aircraft/boat speed range?If flow separtion happend at propeller boat,will engine RPM jump or fall down?When I accelrate outboard engine(with full throttle) at boat from zero to max speed,I never notice that RPM jumps or slow down,how they design this propellers for so wide range of speed? Propeller ventilation has nothing to do with flow separation because of too high AoA? $\endgroup$
    – user52248
    Sep 13 '20 at 14:13
  • $\begingroup$ Yes. With light airplanes, fixed pitch props are usually optimized either for the cruise case or the climb case, so called "cruise props" or "climb props", and you have to pick one and live with that.And with a constant speed prop, where the pitch at full throttle will be finer than either of the fixed pitch cases, the acceleration from a standing stop is quite a bit stronger. Both fast airplanes and STOL airplanes really need variable pitch/constant speed propellers.Planes in the middle, like a 172, can get by fine with a fixed pitch prop, well enough at any rate that it's not worth the cost. $\endgroup$
    – John K
    Sep 13 '20 at 15:08
  • $\begingroup$ @Сократ: Not only pitch but also twist will fit only one speed (better: advance ratio), so separation is not an on/off thing but happens partially. The separated area is large at low speed and gradually diminishes as speed is increased. You have no chance to notice a jump in speed. $\endgroup$
    – Peter Kämpf
    Sep 13 '20 at 18:28
  • $\begingroup$ @PeterKämpf The separated area is large at low speed and gradually diminishes as speed is increased At every fixed blade propeller this happend?Does flow separation at blade require more engine power at same propeller RPM compare to attached flow? $\endgroup$
    – user52248
    Sep 13 '20 at 18:34
  • $\begingroup$ @Сократ Yes, that happens with every fixed pitch propeller if it is optimized for high speed (coarse pitch). Yes, separation should require more power since drag is mostly tangential at low speed and will increase the needed torque for the same RPM compared to attached flow (as on a fine pitch propeller). $\endgroup$
    – Peter Kämpf
    Sep 13 '20 at 18:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy