DavidS wrote:
From the blade’s point of view the relative wind is coming from the bottom left.
The relative wind could be coming from any direction, depending upon the direction that the plane is flying. As the plane is flying in the wind (if there is any that day), that wind, other than gusts, would not have an effect on the propeller, the propeller is only affected by airflow around the aircraft.
[in response to DavidS & his diagram]
Fantastic!
This also means that a huge part of the braking effect comes from the aerodynamic forces acting on the rotating prop – this works even if there is no torque from the engine acting as a brake.
I still don’t get why, at the same RPM, the number and shape of blades makes any difference to this. It does not make a (significant) difference when moving under engine power.
Pilot_DAR wrote:
The relative wind…
A huge misunderstanding, I think. DavidS means airflow relative to the propeller blade. The meteorological phenomenon called “Wind” is not what he means.
“I still don’t get why, at the same RPM, the number and shape of blades makes any difference to this. It does not make a (significant) difference when moving under engine power.”
I think it is tricky as the net effect of number of blades is non-linear as it relates to length of prop relative to prop speed for high number of blades
At slow speed & small prop radius (high reynold numbers) 2 blades should give better airbraking forces than N blades as any “L & D” vectors in the latter will have more chances to be zero…
At high speed & large prop radius (laminar flow), N blades should give N×"L & D" vectors?
Going forward, hopefully the positive camber and the optimum AOA, mean the drag versus thrust is optimum. With negative camber and non optimal AOA, theses effects of larger propeller with more area are likely more important.
I don’t think switching from a two blade prop to a three blade prop on a given engine/plane would make much difference on braking unless something else is different too – as posted above the low pitch stop being finer is I believe the reason that MT three blade props provide so much braking, the engine is being spun faster. That in my mind may dwarf the effect from the propeller drag itself. Without numbers it’s hard to say.
(typing on a tiny screen) yes I did mean the airflow relative to the blade, in fact helicopter people tend to call it “RAF” relative airflow.
Cobalt wrote:
I still don’t get why, at the same RPM, the number and shape of blades makes any difference to this. It does not make a (significant) difference when moving under engine power.
When the propeller is windmilling at a given RPM, every blade contributes the same amount of drag so a three-blade propeller would have 150% the drag of a two-blade prop.
When the engine is driving the prop the question is rather how much power the engine can produce. The propeller must be be able to absorb that power. At a given rpm and power output a three-blade prop will be in finer pitch (by design for a fixed-pitch prop or by the propeller governor for a variable-pitch prop).
One of the reasons people install three blade props is that their diameter is smaller, providing more ground clearance and less tip-speed generated noise, while absorbing the same power. I think in this circumstance the blade area does not scale with the numner of blades, and also that the blades are designed to operate efficiently at a similar angle of attack regardless of how many there may be.
Airborne_Again wrote:
When the propeller is windmilling at a given RPM, every blade contributes the same amount of drag so a three-blade propeller would have 150% the drag of a two-blade prop.
I don’t think this is the complete explanation.
The reverse statement “When the propeller is being turned by the engine at a given RPM, every blade contributes the same amount of thrust so a three-blade propeller would have 150% the thrust of a two-blade prop.” is obviously not correct, since when an aircraft is converted to a three-blade prop, the operating RPMs are similar.
