# Propeller design

I see on internet, that aircraft propellers most effective blades angle only 15 degrees on every blade. Why propellers not use 45 degrees angle on propeller's blade that give more thrust on the aircraft rather than 15 degrees?

• Because of the stall angle... there are many resources that explain this. Sep 13, 2017 at 14:33
• Google "virtual wind direction" Sep 13, 2017 at 15:24
• This question may be better suited to SE Aviation.
– Fred
Sep 13, 2017 at 17:11
• A propeller is simply a vertical wing and as such is subject to all the principles as a wing, such as stall angle. And I agree with @Fred; this is more aptly suited on the Aviation community. Sep 13, 2017 at 20:38
• Note that a "propeller" with many blades (anything from 20 up to say 100) so the air is constrained to go through the passage between two blades, and running inside a duct so there is no leakage at the blade tips, often does have a blade angle greater than 15 degrees - but such things are called fans not propellers, and as the start of this comment implies, they don't operate in the same airflow conditions as conventional propellers. Sep 14, 2017 at 8:50

## 1 Answer

Just think of the propeller as a mean to thrust air in one direction. The higher the angle of incidence, the more the propeller is pushing the air (up to about 45°, then the force perpendicular to the rotational direction starts to take over).

Now consider the fact that air is inertial mass. So accelerating it means building up pressure behind the propeller. Now when the angle of incidence becomes too big, so it thrusts too much air, the pressure "behind" the propeller becomes to high. The energy needed for the air to flow against that pressure is extracted from the air, thus the flow starts to separate from the propeller blade and the blade begins to stall. That means that it is losing most or even all of its aerodynamic properties and no or only little air is being thrusted.

This critical angle of incidence depends on many factors, like the rotational speed, density of air, propeller profile, relative axial speed of the propeller to the air flow, etc....