|
|
So I finally got a dynamic pressure test done on this 20B. Here is what I got:
Front: 105-125-105
Center: 105-101-87
Rear: 130-125-120
This is in PSI and was done with all spark plugs out except the rotor being tested. That one got one spark plug and the compression fitting with pressure transducer attached in the other. I used the RD-1C installed, but not engaged - I bolted up the flex plate without the adapter and bolted up the RD-1C with a starter to be able to crank it over. So the redrive was not slowing things down when I cranked it over. Then I recorded the pressure wave on a laptop and looked at three bumps in a row. I did this a few times for each rotor with a pause in between and took the highest readings of any three successive peaks to estimate the health of the three faces on each rotor.
My understanding is that higher numbers are better and the same numbers across all faces in the motor is better. I am unclear what an unacceptable low number might be. The actual values you get are dependent on a number of factors including how fast you are cranking (how well charged your battery is, how beefy the cables are, what kind of porting, other losses like the PSRU if it were engaged, etc.) and if the motor is broken in or not. I don’t know if this motor was broken in, or if any seals were replaced. It looks clean inside and there is evidence of porting, but I don’t know exactly what (this is a side port configuration motor).
When I lost a side seal on my 13B REW in my RX-7, I got something like 118-120-50 on the rotor that had the problem. So that was clearly not good. When I rebuilt that motor that time, I had the plates ground as they were beyond the wear limit and I used all new side and corner seals and springs. After that rebuild, I got uniformly 120 to 130 on all faces which I think is very good.
So, what do you guys think of my 20B numbers? Is that low face on the Center rotor bad enough to warrant tearing the motor completely apart? If I decide to do that, I might very well go PP. Though Bobby at least thinks that is a bad idea due to noise and tuning difficulties with the PP. Or I could just live with it with these numbers, go side port, and break it in and start checking this periodically to see if there is any improvement or further deterioration.
Thoughts or opinions?
— James
> On Oct 29, 2015, at 1:22 PM, James R. Osborn <rxcited@gmail.com> 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
>
|
|