4200 RPM PROPS? BYW. L.MCCOLLUM EAA 96936 It has become a common conception (misconception?) that props should turn 2700 rpm or slower. The reason always stated for this is to get high efficiency. I started to question this belief when attempting to compute the difference in efficiency between two props . . . one designed for 2700 rpm and one for 4200 rpm. This was done for a previous article titled Flying Truck Engines. I found as most of you already knew that efficiency as defined by advance ratio was constant with tip speed. Let's look for a minute at what a prop is there for. A prop has just one job — to take horsepower from a turning shaft and turn it into thrust to move the plane through the air. So you see, it isn't part of the prop's job to act as a handhold while pushing the plane into the hangar. If we had a prop that was 100% efficient it would develop 375 lbs. of thrust for each horsepower in the shaft at one mile per hour. We will use this one mile per hour point since static thrust is hard to determine mathematically (at zero mph thrust should go to infinity). So at 100% efficiency your 100 horsepower engine would develop 37,500 lbs. of pull on the engine mount. How would you like to design for that force? If you remember some of the static force measurements often quoted, the prop is operating at only 2% or 3% efficiency. Yet, because it is beyond nearly all our capabilities to measure the prop at speed, this static measure

is often used as a figure of merit. In order to raise this thrust or efficiency at low speed, the prop needs lots of area. The most obvious way of getting this area is to make the blades long. Then the prop must turn slowly to keep the tips below Mach 1. Another way to accomplish this desired larger area would be to increase the blade width. Before we go off the deep end to get high static thrust, let's explore the 94 MARCH 1997

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