M

Mike Matthews

I'm New Here
I?m building an RV-8A and have gotten to the point of making my engine purchase. I have decided to go with Van?s engine/ prop package; however I noticed that they have a 72? and a 74? Hartzell prop for the tri-gear models. Van?s claims that no testing has been done to compare performance and yet they are the same price. Are there benefits to one verses the other? I am a new pilot and a first time builder so this is all relatively new to me.
 
At 8500' on a standard day at 200 mph TAS when flying at rated 2700 rpm, the tip speed of the 74" prop will be M0.85 whereas the 72" will be at M0.83. The higher tip Mach will result in more tip loss and less speed/efficiency. The longer prop will give slightly more static thrust and climb rate, all other things such as pitch and airfoil being the same.
 
My opinion would be go shorter. It will give you another inch ground clearance. Less rocks and debris.
 
A related question, though I'm far from this stage: I'd heard that a key advantage of 3-bladed props vs. 2-bladed props was that the 3-bladed props are shorter, so you get more ground clearance, and slower tip speeds and less noise. However, the rep at Van's told me that the 3-bladed props they recommend are the exact same length as the 2-bladed props! So what's the advantage of 3 blades?
 
another point of view

Tony Bingalis used to write in his EAA technical column concerning propellers: "Keep you propeller as long as possible, as long as possible" Greater efficiency, and another thing would be that you have a little extra blade length to work with when it comes time to overhaul the blades.

steve ciha
 
"A related question, though I'm far from this stage: I'd heard that a key advantage of 3-bladed props vs. 2-bladed props was that the 3-bladed props are shorter, so you get more ground clearance, and slower tip speeds and less noise. However, the rep at Van's told me that the 3-bladed props they recommend are the exact same length as the 2-bladed props! So what's the advantage of 3 blades?"


Detectable vibration in the cockpit is less. Yes there is less noise with a 3 blade. Generally, a 3 blade is going to be slower compared with a 2 blade. There is less drag associated with a two blade prop,so more of the HP is transmitted into thrust.

steve ciha
 
"Generally, a 3 blade is going to be slower compared with a 2 blade. There is less drag associated with a two blade prop, so more of the HP is transmitted into thrust."

And Tom Aberle's Reno biplane racer didn't pick up 20 mph when he went from a two-blade to a three-blade, or pick up another 12 mph when he went from a three-blade to a four-blade!

I've heard this kind of thing over and over, but the source of it never gives the reason for the "less drag" with a two-blade than with a three-blade. They often come up with some nebulous "tip loss" or "blade interference" reason but again without an explanation. As I've noted on other postings, a well designed three-blade has as good, if not slightly better speed than a two-blade if it is kept shorter to reduce the high-Mach tip loss. A bad design propeller has terrible aerodynamics in the root region and the tip region which definitely does have more drag. But it's not the result of more blades! Look at all of those turbo-prop airliners and cargo planes with their 6 and 8 blade props! If you take this "more blades, more drag" thingy to its ultimate projection, then all of those huge airplanes carrying millions of people all over the world with their big 18-24 blade, fixed-pitch propellers must have horrible efficiency! (The fan! Get it?). Eventually the prop makers will improve their aerodynamics in the root and tip and produce higher efficiency, lower noise, multi-blade props. The only thing holding them back is economics; it costs a bunch to test and certify a new blade design. Why should they when people keep on buying the same old thing?
 
i have had one of each 2 and 3 blade props

"And Tom Aberle's Reno biplane racer didn't pick up 20 mph when he went from a two-blade to a three-blade, or pick up another 12 mph when he went from a three-blade to a four-blade!"

Tom probably did pick up 20 mph. I would not know. However i do know this, Mike, the original poster will not have the same Horsepower that Tom does while he races at Reno, He will have about 200 HP. If you have more HP than 2 blades can handle, go to 3, or 4 or whatever, just like the WWII fighters did. But if you have 200 HP then 2 blades will do it. If all it took were more blades to go faster, then i would put about 200 of them on my plane and really go!

steve ciha
 
"Tom probably did pick up 20 mph. I would not know. However i do know this, Mike, the original poster will not have the same Horsepower that Tom does while he races at Reno, He will have about 200 HP. If you have more HP than 2 blades can handle, go to 3, or 4 or whatever, just like the WWII fighters did. But if you have 200 HP then 2 blades will do it. If all it took were more blades to go faster, then i would put about 200 of them on my plane and really go!"
steve ciha

Steve, you're missing my point. Properly-designed multi-blade props have no more loss than does a two-blade prop. By taking advantage of the greater amount of air being pumped at an even smaller diameter, a multi-blade prop can be even slightly more efficient than a two blade, and will give more static thrust and better climb. Certainly, higher HP engines require more and fatter blades to keep diameter/rpm in a reasonable range. But the three and four blade props on Tom's IO-360 plane were less than 60" in diameter! Jim Smith's 150HP RV-6 with its three-blade "bow tie" prop goes 192 mph TAS at 7000' dalt at 2740 rpm. My Lancair has only a 125 HP O-235, yet it's takeoff and climb with my three-blade is better than a two-blade, yet I get 214 mph TAS at 1000' dalt and 200 mph TAS at 10,000 dalt. I'm just trying to show that a multi-blade prop that doesn't have as good a cruise speed as a two-blade is because it has a poor blade design, not because it has more blades! It has been very difficult for me to get across that these popularly-held notions of prop diameter, number of blades, and blade thickness have often been in opposition to the truth! Long-held, incorrect popular notions die hard! Another of these is that the curved-divergent submerged duct, the "NACA duct", is the best in every application! I designed some much smaller inlets to replace NACA ducts on some Reno racers that gave much more air for intercoolers and cabin ventilation.
 
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