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A Nominal Low Pitch Perfect Propon a post war Cessna 140 can only be 85% efficient, limited, a High Pitch Perfect Prop on a Fast Modern Homebuilt, a Lancair, Glassair, a fast 220 MPH, extra HP, fast RV, can be a bit over 90% efficient. Fail to understand, do a poor design, and you’ll likely unnecessarily lose 5% to 10% more!
Prop companies, using old outmoded 2D Math, don’t,can’t know their real efficiency! Thinking they’re being honest, they’ll tell you their efficiency is higher than it really is, because only with precisely correct Betz Logic, and precise Goldstein, Theodorsen 3D Math,do we find that the RADIAL FLOW FEEDING THE VIOLENT TIP VORTEX IS THE LARGEST LOSSof all, which 2D math never even sees, is incapable of seeing!
I have an old metal prop on a Classic Luscombe that should be 85%, but is only 75%. But bad, the horror is that there is a nominal 10% interference loss, extra loss reacting with a non clean, too many intersections airframe, only 67%, worse at unfavorable higher power. If we were to tape the cooling ducts, glide a propless plane, charge cooling to the engine, not the airframe, it can have nominally the 58% overall that Gus Raspet showed on a Bellanca Cruisair in the 50’s. Combine a nominal 60% result with a nominal 30% thermal efficiency engine, and you’re at a nominal 18% overall thermal efficiency, burning 5.5 times more fuel than you would need at 100%! .75 x .9 x .9 x .3 = 18.2%
This is the kind of insight you learn in the book!
Fortunately, on a modern, clean composite homebuilt, or a clean amazingly well designed RV you can hardly find an interference loss, only the cooling loss, and that can be pretty good with the best ducted cooling. Those engines that dump red hot gas, are pretty terrible thermally, unfortunately.
An 85% to 90% correct prop is losing 2 to 3 times more loss than a comparable 95% wing. A nominal 20/1 L/D wing is 1/20 D/L, 5% loss, 95% efficient. Planes are not easily 20/1, but a wing by itself is easily 20/1. Realize a perfect untwisted metal prop, clean airfoil, comparable aspect ratio, used as a wing in a wind tunnel is easily better than 95% efficient, but as a Rotating wing, an efficiency disaster that needs fixing – with a perfect tapered tip, it will always have 2 to 3 times more loss, done poorly 4 to 5 times more loss. No reason for that after 1948! Realize a 60/1 L/D sailplane, 1/60 is 1.666% loss, 98.333% efficient for the whole plane, fantastic!!!!
Realize, Understand the Bottom Line, the Truth of all this. Simply, with Betz Logic, Goldstein Theodorsen essentially exact math, we can always get 85% efficiency for a nominal 100 MPH postwar Cessna, a tad over 90% efficiency for a clean fast modern composite homebuilt going somewhat over 200 MPH, at not excess RPM. Additional Insight, if you have the option of gearing for a lower RPM, larger diameter, high pitch prop for a high advance ratio near a ~.63 optimum Lambda, Advance Ratio, clearly explained in the book, we can always get a math optimum 90% efficient prop. Only with BGT Classic 3D math can you design and get that assured optimum result. 2D math is neither correct nor accurate enough to nail optimum design, determine really correct efficiency! As everyone learns, the entire Industry will adapt Classic, essentially exact Betz Logic, Goldstein Theodorsen Math. To not do that is to fail to understand the problem. It's simply easier, quicker, to design perfect props exactly correctly in a few minutes. BGT have been waiting for 60 years for us to wake up that the propeller problem got exactly solved in 1948!
Let me tell you a funny! A good prop company that had a basically good prop that wouldn’t pass vibration test, had to make a skinny tip to pass vibration, left a little blob of extra drag area out there at max lever arm, too bad, almost got it right, made their very best prop! Extra Drag Area out there at Max Lever Arm just increases the Torque Demand, extra, unnecessary induced loss, less Thrust!
The real World: The props we fly unnecessarily lose nominally 5% to 10%, more, excess loss, a few, close in the few, rare best cases, because we never knew how to design perfect props, too bad, since we just didn’t know it all got solved in 1948!