Blade Twist & Angle of Attack

Every station on a blade sees a different airflow. The aircraft moves forward at V, the same for the whole blade, but each station also moves round the circle at 2πnr, which grows in direct proportion to radius. The air therefore arrives at the blade along the resultant of those two, at the helix angle φ measured from the plane of rotation. Near the root φ is large because the rotational speed is small; out at the tip φ is small because the rotational speed dominates.

Angle of attack is the difference between the blade angle built into the section and that helix angle, α = β − φ. If the blade angle were the same everywhere, the falling helix angle would drive the angle of attack up towards the tip and down towards the root, so most of the blade would be working at the wrong angle and part of it could be stalled while another part produced nothing. Twist, or washout, is the fix: the blade angle is made large at the root and small at the tip so that α stays roughly constant and the whole blade works.

The same geometry defines pitch and slip. Geometric pitch is how far the blade would advance in one revolution if it went through the air like a screw through wood, 2πr tanβ. Effective pitch is how far the aircraft actually advances in one revolution, V/n. The shortfall is slip, and it exists because the blade must meet the air at a positive angle of attack to make thrust at all. Note also that blade angle is quoted at a stated station, conventionally 75% of the radius, precisely because it changes along the blade.

• Try this: Switch to the flat blade and move the station from root to tip. Where does the angle of attack stall, and where does it go negative?
• Try this: Hold RPM and increase forward speed on a fixed-pitch blade. Why does the angle of attack fall away, and what does that do to thrust in the cruise?
• Try this: Set forward speed to zero, as on a static run-up. What is the slip, and why is that figure not a sign of anything being wrong?