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What happens to air flow and propeller efficiency if it is moving with Wind perpendicular to rotation axis.

The air is blowing in a direction lying on the Planet of the tips.

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    $\begingroup$ As the far field inflow angle changes from axial to off-axial, the power falls off nearly as the cosine cubed. That works well for smallish deviations. $\endgroup$ – Phil Sweet Apr 11 '20 at 23:05
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    $\begingroup$ When talking about big deviations, you need to look at different geometries and different operating principles. Once the inflow is significantly off angle, you have stalled blades over part of the disk, and you have to start looking at drag-governed devices. The old Danish windmills operated at about 45 degrees to the wind, and ran on drag. England's Henry VIII tasked the Royal Society to build a better windmill, and the RS discovered one in a survey that ran like a propeller does, based on lift, and they build/modified about 2000 of them in a couple years. $\endgroup$ – Phil Sweet Apr 11 '20 at 23:12
  • $\begingroup$ A bit late now, but do you mean a propeller as in a helicopter or as in a generator like in a wind turbine? A propeller operating at zero advance ratio is a hard problem and can do some funny things, like create a donut of whirling air with nearly no lift resulting. A bit of crossflow can really help in some of these cases. $\endgroup$ – Phil Sweet Apr 12 '20 at 3:11
  • $\begingroup$ Do you mean a Savonius rotor? $\endgroup$ – Solar Mike Apr 12 '20 at 4:54
  • $\begingroup$ @Phil Can you link a page that explain the cosine cubed? In Example if the airflow is at 3 degrees I lose 30% of thrust (cosine 0.9, cubed 0.7)? $\endgroup$ – CoffeDeveloper Apr 12 '20 at 10:36
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If the wind is near perpendicular to the propellers axis it will make it turn and creat lift. It is called a gyrocopter. Usually, another fan on the back provides the thrust and the big near-horizontal disk creates the lift.

But if you position a propeller in the exact perpendicular wind it loses the lift and will stall.

Unless one designs a custom profile blade specifically designed for maintaining lift under wind coming from a tangent plane. But even then building directional control, authority, efficiency is a different ballpark.

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