Let's say you had two cylinders 2" diameter and 3" long, and you attached one on each side of your plane. Assume that these cylinders have a drag coefficient of 1.0. 2" X 3" X 2 X 1.0 / 144 = 0.083 sq. ft. Now let's say your RV has an equivalent parasite drag area of 2.2 sq. ft. When you add those cylinders your EPDA goes up to 2.283 sq.ft. a 3.8% increase. That's going to slow you down 1.2%, so if your plane would have gone 200 mph TAS before adding the cylinders, you are now down to 197.5 mph TAS. Let's say you have a CS prop on your plane that the manufacturer says has 84% efficiency. That means that 84% of your engine's power is converted into thrust, so a 200 hp engine will put out 168 thrust horsepower. But when you attached those cylinders on your plane the increased drag was the same as adding to the thrust requirement from the engine and prop. so basically you reduced your thrust horsepower 3.8%, leaving you with 161.9 hp. That would be the same as if you had a prop that was only 80.9% efficient. Do you know where I'm going with this? Are you one of those guys who competes in the SARL races? Would you like to reduce your speed 1.2%? 'Doesn't sound like much, does it? So if your propeller is one that has 84% thrust conversion efficiency, but the blade roots are this God-awful cylindical shape that you see on so many CS props, you got shafted! Think how much worse it would be if it was a three-blade prop. That's why multi-blade props got this reputation as being not as good in cruise. It had absolutely nothing to do with any so-called tip loss; it was all in the root shape. But there is a silver lining to this cloud. Some enterprising individual among you could make up a streamlined carbon fiber boot or glove to attach to the blade root as they did on the P-51 and B-29. If it's done with the correct amount of twist and airfoil shape, you could end up a prop that's more efficient than originally, and almost as efficient as mine! Actually, the latter was too much of a stretch, but what the Hades. Prop shops around the country know how to attach these glove sections to a blade, and you might talk to one of them.
), but have also had some intersting conversations with Paul about 3 and 4 blade FP props, during which we discussed the root shape.
