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On 10/29/2015 3:22 PM, James R. Osborn wrote:
Hi guys,
So I am going to be building an RV-10. I sourced Bobby’s spare 20B / RD-1C combo and my plan is to work on the motor first, kind of bass-ackward but that is what I am doing. My plan is to build a motor/mount test stand and work everything out on the ground before I ever put it in an airframe. I plan to set it up NA using the existing side port configuration. I don’t know if it is ported - I am going to attempt to determine this by inspection without cracking it open and doing a dynamic compression test to evaluate the health of the motor. If it is necessary to entirely rebuild the motor, I would probably go PP. So I am thinking the target HP as it is would be in the 275 HP range and probably 325 HP if it ends up being a PP. How do these numbers sound to you all?
I will be looking for a way to dyno it, hopefully borrow one to get some data. But I was also thinking about just getting an appropriate fixed pitch prop and working through the bugs based on static run ups. I like the idea of a three blade prop. I like the idea of optimizing for cruise at 75%, say 5800 rpm which would put full throttle at 7733 rpm on the 20B. At 5800 rpm, the prop would be spinning 2035 rpm and at full throttle 2713 rpm (using the 2.85 ratio RD-1C PSRU). Do all these numbers sound about right?
If so, then how do I choose a prop that will result in 2713 rpm static run up at full throttle, but be tuned for efficiency at 2035 rpm cruise? I am assuming at these reasonable RPMs that it will be a larger swing and the three blade makes sense - what do you think? Is there a convenient way to find props out there that will work at the target full throttle HP of 275 or 325? Also if you guys have any good resources to learn about how to figure these things out (books, web pages, online calculators, etc.) that would be great!
Another option might be to go with something like the IVOPROP Magnum, either ground adjustable or electric inflight adjustable. What do you guys think of these? Would the adjustability really help me adapt to however my power plant turns out? Would the extra complexity be worth it? For the ground adjustable one, I am thinking the process would be: 1. select the “correct” swing (how?) and go for three blades, 2) use ground adjustability and work up to THE pitch while I tune the motor until I achieve full throttle static run up around 7700 RPM. Will this result in a reasonably efficient setting for 75% cruise? Am I thinking about this clearly? All opinions are welcome.
There’s a lot for you to chew on!
— James
Sounds like a great project; I'm looking forward to seeing another 3 rotor fly.
Couple of thoughts.
First, the rpm numbers vs HP only apply at sea level, if you're normally aspirated. At normal cruise altitudes of 7-8k ft, 75% power is the same rpm as full power at sea level. So, 75% cruise prop rpm will be roughly the same as full power sea level prop rpm.
Second, the best way to buy a fixed pitch prop is to find a reputable prop maker & give him honest performance numbers for the plane (hp, cruise & max expected *prop* rpm, max useable prop diameter (let him recommend the number of blades), expected cruise altitude and speed).
Third (because I can't count), my experience with Ivo's is at least a decade old, but my experience with them is that they didn't do well at all at RV speeds. They didn't have enough twist to perform well above ~160 mph. That problem existed even when the prop was pitched for high speed. (The blades also move in the hub if run on a Lyc, but hopefully that won't be a problem for you.)
Last, if you're anywhere near the south-central US, come on down to Slobovia Outernational (MS71), near Jackson MS, the weekend of Nov 7 for our annual Pumpkin Drop. We're having a mini-rotary gathering in conjunction with the P-Drop. Shoot me an email at ceengland7@gmail.com for details.
Good luck with your project,
Charlie
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