Mach Number Effects

Some model aircraft have propellers operating in the high transonic region. The principal message here is that propeller pitch must be reduced in the arere where Mach number is high. As background you can read the article on Mach Number. Look at the graphs of airfoil performance and below for Mach number distribution vs. radius for an example F2C propeller. You will see that at high subsonic Mach numbers the drag rise is very rapid and thin airfoils delay the Mach number at which this drag rise occurs. The message is clear: It is pointless to use an airfoil at Mach numbers beyond the "drag divergence Mach number" You are better off with a propeller of smaller diameter. Operating the airfoil at lower angles of attack can help delay the drag rise.

The other significant change, however, has a large effect on propeller pitch. The lift curve slope changes. The angle of attack to get a given lift coefficient goes down with Mach number. The simple theory, which the program that will be used to produce the results here uses is accurate for thin airfoils and underestimates the effect for thick airfoils. You have no business using thick airfoils at these Mach number, however. Look at the results below, measured in a small wind tunnel that more closely approaches model aircraft Reynold numbers then larger tunnels used later.

As an example of what these data mean, if you wanted to operate an airfoil at 7⁰ above the zero lift angle, then at Mach number 0.7 you should operate it at an angle of attack of 5⁰. This effect is marked at the propeller tips, generally producing a desired pitch that decreases with radial station as opposed to increasing, as a low Mach number would require.

The graph below shows a typical F2C pitch distribution computed with and without Mach number correction. No special tip correction was used to compensate for the inflow around the tip

The general conclusing is to use thin sections of the tip Mach number is high, reduce the pitch at the tips and reduce diameter if there is any danger of drag divergence.