|Prop Pitch Study|
|For those who have been interested in my adventures in the mysterious world
of propellers, here's the latest. Our illustrious and very generous fellow
Cozy builder, Phillip Johnson, crated up his homebuilt 3 bladed prop and
shipped it all the way from Ottawa to So. Cal for me. He had been getting
around 2300 RPM static on the same engine and gearbox as me (EG33 1.85:1
Ross). I have tried 4 other fixed pitch props ranging from 84" pitch to 89".
All of those got less than 2000 RPM static and all topped out in high speed
cruise at about 2700 RPM (my peak power is at 2900). I was intrigued that
his prop got better static even though it had higher pitch. I needed to
know is this some facet of the prop design or does his engine produce that
much more power
It arrived yesterday. I couldn't wait to try it out, so I left work early and it bolted right on. I did a static check and after warm up I got 2065. That's about 10% more than my other props but still 10% short of Phil's
engine. Based on this I conclude that my engine is producing about 10% less power at static. This is probably due to intake and exhaust differences between my engine and Phil's. He has a home made intake and his exhaust is tuned and has a cross over where mine is just straight pipes 3 in to 1 on each side. That looked pretty good so I tried a high-speed taxi test. It had noticeably better acceleration. My normal rotation speed is 74 kias. This prop got there in 1300 ft as opposed to about 1800 ft with the other props. Sounded pretty good! So I rechecked the torque and looked things over and when for the takeoff run. I checked the static prior to brake release, still 2065. After accelerating to about 20-30 kts, I noticed that the RPM had dropped to about 1892 but the acceleration seemed good. It reached 74 kias and lifted off a fair amount shorted that my other props but after lift off the acceleration was very poor and the climb rate was even worse than my other props. I let it accelerate to about 90 for climb but the rate of climb was less than 500 fpm. I slowly coaxed it up to 1000 ft, leveled off and let it accelerate. It did but very slowly. It topped out at 163 kias, 1100 ft, 62 deg F, 167 ktas. The RPM was only 2411. That's 300 RPM less than my other props.
1) My engine's static power is down about 10%. Some of this I can improve by putting an exhaust cross over in to the system.
2) The higher static with Phil's prop is due to a)3" less diameter, b) stall of the root area in the static conditions, c) smaller chord: Phil's 4.25", Sens 5.125". However Phil's total blade area since it is 3 bladed is
actually 24% higher. So I doubt c.
3) When the 90" prop un-stalls with inflow it acts like you would expect a 90" prop to act and produces less RPM
4) Assuming I get 5% more RPM from the cross over, I still need less pitch to get at least 2300 static. I'm guessing about 73"
5) I need less pitch for high-speed cruise also. Sensenich tells I should get 100 RPM for each 2" of pitch change. If the 90" is 2400 then I need and 80" pitch. If I say the 85" pitch gave me 2700 RPM then I need an 81" pitch.
6) If I get a 73" pitch for adequate static I will over rev at the high speed end to an estimated (7" delta from 80"=increase of 350 RPM) 3250.
The variable pitch IVO is looking like the way to go. But I will get some more data and get the cross over tube installed first. I just talked to Sensenich and they think I can still get there from here. They want to reduce the diameter to 68 and the pitch to 77. In the process the chord will be reduced some. Between these they think I may get pretty close. Since I have 2 of their props and wouldn't mind having a 2 blade spare, I think I'll let them try.