Slip
Kinnerhatz has asked about the slip of my propeller design. The use of this term has become so common-place that it's accepted as a real operating condition relative to a propeller. Somehow or other, this term "slip" or "slippage" has been introduced into the propeller vocabulary as a word to explain the difference in the pitch stamped on a propeller and its actual helical path through the air. It's as if the propeller should always follow the exact same helical path regardless of load and any difference, such as the speed in a climb at a given rpm and the speed at the same rpm in straight and level flight, is attributed to this mythical "slip". To me, when something slips, such as a tire on a wet surface or a clutch, it's not a good thing, But a propeller is just a wing in rotary motion. So if you're flying at a low airspeed and your wing is at a high angle of attack, would you say that it is slipping? After all, it's not pointed in the same direction as when in cruise. Of course not! It's just that at a lower speed, with reduced dynamic pressure, the wing must be flown at a higher AOA in order to produce the same lift. So, too, a propeller. When you are climbing with reduced dynamic pressure and mass flow due to lower rotational and forward velocities, the propeller blade must also operate at a higher AOA to produce the increased thrust. It isn't slipping; it's just doing what it should do in order to produce the thrust at the higher loading. But some would like to be able to explain this difference by assigning some value of slippage between the two flight profiles. Maybe there is some empirical number that could be attached to a particular prop - "When I climb with my Q-root prop, it has 10% slippage." or something like that. I would rather stick with the prop's percentage of efficiency. If you know the efficiency vs rpm and power, you can estimate quite well how your plane will perform at any given load. Let's throw out meaningless terms like that and concentrate on real physical values.
