Propeller efficiency

[elippse]

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.

[Geico266]

Are there any test results in better efficiency after adding the blade cuffs to the P-51? Were the efficency improvement confirmed?

Interesting thread!

[Snowflake]

On a P-51, where the blade roots are (for the most part) out in the free air (the front of a P-51 is pretty darn narrow relative to the blades) I could see how this would help. On an RV, where a fair bit of the increased "thrust" would just blow into the engine intakes, i'm not so sure.

Remember that both the chord and the angle of attack on the blade at the root need to increase as you approach the spinner, in order to maintain efficiency. I suspect even the root fairings on the P-51 propellor are a compromise.

[pierre smith]

....that the root fairings blend into the rest of the prop?

Interesting,

[Alan Carroll]

Originally Posted by elippse

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.

Paul,

It makes sense that the relatively round blade roots would be contributing drag rather than thrust. However, I would have thought this was already factored into the overall efficiency of the prop? How is the efficiency actually measured?

[rocketbob]

The blade cuffs on the Mustang were added to get more air into the doghouse for better cooling during ground ops in hot conditions. Lower than 15K the blade cuffs actually slow the airplane down and above that altitude is when they help with cruise speeds. Its pretty rare nowadays to see the cuffs installed on a flying Mustang.

[Bob Axsom]

One cannot help but respect your accomplishments with propeller design for Reno racers, especially biplanes, but I will stand off to the side and observe on this one. The interaction of several things at the front end of an RV mean changing one thing affects others so changes often need to be made in other areas to establish a new optimum. If I were going to start down that trail I would think of trying a larger diameter spinner to cover the exposed prop shank drag.

Bob Axsom

[pierre smith]

...my -10 is quite a bit faster than a buddy's -10, even though he has a James cowl and plenum. He has a three-bladed MT with big, exposed, round
roots, while my -10 with the blended Hartzell two-blade has airfoil shaped roots.

Interesting,

[larrynew]

http://www.rswproducts.com/index.php/p-51-blade-cuffs

[rvmills]

Originally Posted by pierre smith

...my -10 is quite a bit faster than a buddy's -10, even though he has a James cowl and plenum. He has a three-bladed MT with big, exposed, round
roots, while my -10 with the blended Hartzell two-blade has airfoil shaped roots.

Interesting,

Pierre,

In talks with Mark (F1Boss) and Tom (Mr. Fastest F1), your BA Hartzell is faster than my Hartzell D-twist and the MT 3-blade (in that order). Tom said that his prop change was the most measureable speed mod he's done (Tom, hope I'm quoting you correctly!)

I've been saving my pennies for a Hartzell BA like yours (wanna trade? ), but have also had some intersting conversations with Paul about 3 and 4 blade FP props, during which we discussed the root shape.

Trying a cool Ellipse prop is very inviting, as rolling up with a 4 blade just sounds sexy...and if its faster... . However, I'm a bit intimidated by the swap from CS to FP (in terms of other work that goes with that...crankshaft, etc). Alternatively, I could put Hartzell BA blades right in my current hub, or do a complete hub and blade swap...and in either case, still use my spinner.

Either COA takes a commitment in time and $$, and as Tom, Mark and Bob Ax will tell ya, you never know till you do it and test it.

Paul, as others have said, you have some fascinating ideas on props and wingtips (hope you got my e-mail reply on your triangular tip recommendation). Fun discussion here!

Cheers,
Bob