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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

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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.

Peter Kämpf
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  • @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? –  Sep 13 '20 at 14:13
  • 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. – John K Sep 13 '20 at 15:08
  • @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. – Peter Kämpf Sep 13 '20 at 18:28
  • @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? –  Sep 13 '20 at 18:34
  • @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). – Peter Kämpf Sep 13 '20 at 18:44
  • @PeterKämpf Does than manufacture tell as for what speed is aircraft propeller design?(in boats no..) –  Sep 13 '20 at 18:53
  • @Сократ: You should be able to tell from the advance ratio and the RPM. Look at the graph at the bottom of the linked answer. See the efficiency over speed curve. Where it has zero at high speed the slip is zero. Go back to maximum efficiency: That is the slip you need for the prop to work properly (pun intended). Combine with advance ratio times RPM (in sec.) to find your optimum flight speed. The manufacturer doesn't know your prop speed, you do! – Peter Kämpf Sep 13 '20 at 18:57
  • @PeterKämpf in advance ratio in denomitator is nD, what this term represent,this is not tangential/circumference speed? –  Sep 14 '20 at 05:39
  • @Сократ No, advance ratio is the ratio between distance traveled in one revolution if the prop has no slip (like screw in solid material) and the prop diameter. Slip is caused by the angle of attack of the prop blade relative to local airflow. – Peter Kämpf Sep 14 '20 at 11:09
  • @PeterKämpf But this information seems not useful,wouldnt be more useful if we know ratio between freestream airspeed and tengential velocity at blade tip? In my book write V/n is aerodynamic pitch,so advance ratio is aerodynmic pitch nomalized by propeller diameter,doesnt that mean that advance ratio include propeller slip? –  Sep 14 '20 at 16:41
  • @Сократ Slip is variable. Low load = little slip and vice versa. Would you like to have a variable advance ratio? Why don't you stick to what has been defined? If you double-guess everything you will miss the clever conclusions others have made and only repeat their errors. – Peter Kämpf Sep 14 '20 at 17:29
  • @PeterKämpf I think you didnt understand me,I just want to ask what advance ratio tells as,becuase to me " seems" not useful..if formula will be V / (nD x 3.14) than denominator become tangentil velocity of blade tip and to me it has more sense.But formula dont has π number in denominator... –  Sep 14 '20 at 17:42
  • @PeterKämpf Can you explain why advance ratio refer to no-slip condition,isnt V in numerator real aircraft speed which is changing?I still dont understand this part? –  Sep 15 '20 at 18:52
  • @Сократ Because it is defined that way, plain and simple. Again, don't double-guess everything. Read the answers, follow the links and do some more research when you still have doubts. But don't try to redefine all technical terms. – Peter Kämpf Sep 15 '20 at 20:35
  • @PeterKämpf But why I cant find this defintion? Here is defined V as true aircraft airspeed : V is the freestream fluid velocity, typically the true airspeed of the aircraft or the water speed of the vessel https://en.wikipedia.org/wiki/Advance_ratio –  Sep 15 '20 at 21:14
  • @Сократ All the times I was using the term "advance ratio" I was actually talking of "propeller pitch". Sorry for the confusion. – Peter Kämpf Sep 17 '20 at 05:10