Philip Aerodynamics
20th Jun 2005, 13:34
A basic question for you Dave or any other Principle of flight instructor. Please remind me why the glide distance is greater with a coarse prop.
Send Clowns
20th Jun 2005, 14:24
A complete answer would require some complicated diagrams, unfortunately I do not have any source or the time to draw, so cannot really give that detail.
However in simple terms it is because the energy must come from somewhere to turn the prop. This is in the total reaction over the "wing form" of the blade from the net airflow - inflow of air and the prop already turning. Total reaction has a component in the direction of airflow - drag on the aircraft. At fully course the total reaction points less aft, so less drag on the aircraft. If you cannot see that total reaction is less aft, remember that the prop has negative angel of attack as compared to its angle of attack when providing thrust.
Another way to think is that the prop will turn more slowly when coarse (think of the limiting case of coarsening, a feathered prop, which does not turn). The slower turn requires less energy. The ideal is to stop the prop, and I know someone (an extremely experienced instructor, 50-odd years of patter by now, thought he should try what he had taught so often) who has made the airfield after a donkey stopped only because he managed to stop the prop. Sailors who race cruising yachts put the motor in gear to stop the prop - gets about 1/2 knot extra (which is a lot at 6 kts!) for the same reason.
So finally a coarse prop turns more slowly and creates less drag, improving lift:drag ratio. Glide ratio is identical to lift:drag ration.