Prop Question- 3 blade vs 2- Catto vs Sensenich

[elippse]

Hopefully, very soon. Vari-prop will soon make a three-blade CS of my design, and I have a prop I loaned to a fellow in Wichita to try on his RV-6 so I can get parasite drag data. He should get the prop tomorrow, and, if the weather stays good, maybe he'll have some comparitive test data this weekend. This was my first prop, and was not yet an optimized design, but I have it modeled im my equations, and so can use it for data gathering. He has contacted a fixed-pitch prop maker about making him a three-blade if all goes well!

[elippse]

One other thing I failed to mention. Along with Jeff Lo's composite-over-wood prop being thicker than his metal prop and having three blades rather than two, it was also several inches less in diameter! Obviously we have a breach in the space-time continuum that is reversing all of our mathematics and well known facts! Where will this end? Someone should contact George Noory or Art Bell at "Coast-to-Coast AM" and tell them to get the word out so that this trend may be reversed. Global warming will have to take a back seat to this new threat!

[Daver]

I've just started the wings on my RV9A but I've been thinking about and trying to learn as much as possible about engines, props, intruments for the day when I have to make these decisions.

This is very complicated stuff.

I think I'll just sacrifice a goat, chant some funny latin words and burn some incense in hopes the airplane gods will grace me with good judgement!

Dave (the confused)

[roadrunner20]

Catto 3-Blade (66x76) 4 way gps, 2660 rpm, a few weeks ago gave me 198mph @ 8500msl.

Winds aloft will be calm here Saturday, I'll record some new numbers.

[aadamson]

Originally Posted by elippse

One other thing I failed to mention. Along with Jeff Lo's composite-over-wood prop being thicker than his metal prop and having three blades rather than two, it was also several inches less in diameter! Obviously we have a breach in the space-time continuum that is reversing all of our mathematics and well known facts! Where will this end? Someone should contact George Noory or Art Bell at "Coast-to-Coast AM" and tell them to get the word out so that this trend may be reversed. Global warming will have to take a back seat to this new threat!

Paul,

What ever happened with this data? Not that I need an RV prop, but I would like to see your props make the mainstream. They are indeed awesome as exhibited by the performance that i've seen on your personal airplane and the planes and Reno that fly your airfoils.

Let us know,

[hevansrv7a]

Originally Posted by elippse

My 15% THICK, THREE blade design on the Jeff Lo-Chris Ferguson Reno biplane "Miss Gianna went 4.3% faster than their best-of-two THIN MacCauley metal props at the same rpm. That's like having 13.6% more power. ....

Assuming for the sake of discussion that the McCauley was 85% efficient, that would make your prop 96.56% efficient. If that's not reasonable, then what do you suppose the two efficiencies are? If the McCauley was not at 85%, is that no longer a valid rule of thumb? I am having trouble understanding this data in context of what the community uses as operating assumptions.

[elippse]

I would guess that because of the high tip speed these props run at the high rpm they turn that their efficiencies are probably more in the 75% to 80% range. That would put mine more like 85% to 91%. Keep in mind that when a long prop is operating with the outer portion of the blade at high subsonic Mach, the much higher drag is multiplied through the long radius to reflect a high torque load to the engine. At M0.85 the profile drag Cd is about eight times greater than at M0.65. Since the prop has little-to-no lift near the tip, it can only have drag, and that will be proportional to the blade area. So wide tip-chord at high Mach equals big loss! My data seems to indicate that the three-blade on my Lancair has a cruise efficiency of about 90%-92%. Some have said it is theoretically impossible to exceed 90% efficiency, but some recent thought holds that it could go as high as 95% maximum efficiency. I'll leave that to the aeronautical theoreticians to battle over. Those hypotheses will have to wait until someone actually runs some rigorous testing on some kind of advanced blade profile. The nice thing about the Reno results is that there isn't too much you can argue with over the race results!

[gmcjetpilot]

Originally Posted by Daver

I've just started the wings on my RV9A but I've been thinking about and trying to learn as much as possible about engines, props, instruments for the day when I have to make these decisions.

This is very complicated stuff.

I think I'll just sacrifice a goat, chant some funny latin words and burn some incense in hopes the airplane gods will grace me with good judgement! Dave (the confused)

Enjoy the Ride Dave, we all have been there and that is what home building is all about. Not only do you end up with a beautiful plane but you get learn a lot. It is not complicated, just take small bites. The fog will lift. Some old golden rules:

Keep it simple
Build it per plans
Keep it light weight
Airplanes are compramises, if its better in way way, its a loss in others.

[hevansrv7a]

OK, the high tip speed explanation makes good sense without any rigorous testing. Does your design have big advantages at, for instance, 2700 rpm with a 72-74" prop? Thanks.

[PerfTech]

And if you think about it, what relationship exactly does hp have with blades? As long as the engine can turn the blades to the rpm you want, say 2700-2800, then what more can it do? The blade only knows it is going 2800 rpm, it doesn't know what hp is pushing it. So hp is more related to blade pitch, length, width and number (only so much as you want it to spin at a given rpm).[/quote]

ACTUALLY IT HAS EVERYTHING TO DO WITH IT!

The true reason for efficiency loss as you add blades is this and this alone. You are adding more frontal area (leading edge of blade) and also increasing the total wetted surface area. These two items account for the only differences in efficiency. This extra drag consumes a considerable amount of power that could be used to create thrust and power the plane. For instance, were you to keep adding blades you would soon need to start taking out pitch in order to maintain the RPM. Eventually you would have a propeller with no pitch and many blades that would consume all available power just to spin it through the air and create zero thrust. Keep in mind the leading edge tip speeds are approaching 650 mph on each blade and that requires considerable power to push through the air. Regards All, Allan