Keith,

I purchased a 120SP last year and have run a gallon of Omega 10% through it on the bench, breaking in per the attached instructions. It idles nicely within the recommended 1400-1600 rpm range and I'm currently getting 5500-5600 rpm at full throttle with a Master Airscrew 18x12 Classic prop. Here's the puzzling part. I switched to a custom 17.5" 3 blade prop with adjustable pitch. I set the pitch @ 12, expecting to loose around 500 rpm. (I didn't have a 20x12 2 blade for comparison purposes, but expected the custom 3 blade to reach near the same rpm as has been advertized with the 20x12.) I can only turn it at 4400 rpm! I switched to a Zinger 18x12 3 blade, thinking that the custom prop might be a little heavy. I can only turn the Zinger at 4300 rpm. Am I missing something in the engine setup? For comparison purposes, what rpm should be expected from the 120SP turning the recommended APC 15.75x13 3 blade prop? It just seems like the engine should spin these props a bit faster.

My plane is heavy for a .60 size because I added all the goodies (i.e. rotating retracts, retractable tailwheel, glass and paint, etc.) plus the extra weight of the 120SP. It weighs in at near 13 lbs. I really want to use a 3 blade prop but, at this point, I'm worried about generating enough airspeed to safely fly it. Any assistance would be greatly appreciated.

Sincerely,
Joe

Hi, It seems to me that you are all over the map with props. In my understanding, when changing from 3 blade to 2 blade you add 1'' to the prop (15x12 4 blade to 16x12 3 blade to 17x12 2 blade). also the weight of the custom 3 blade may be much more than a standard 3 blade. Have tried 15% Omega? please don't quote me on this I would would like to see how thing turn out

Actually, I don't think I'm "all over the map" with props. The 120SP is supposed to turn an APC 20x12 at around 5150 rpm. I wasn't able to get a 20x12 so I opted for an 18x12 (another size recommended for 120SP break-in). I've read on many sites that when going from a 2 blade to a 3 blade prop, you need to downsize diameter by 1-2" (depending on which site you look). So, if the 120SP is supposed to spin a 20x12 prop at 5150, then it makes sense to me that it should spin an 18x12 3 blade prop at a simular rpm. As I stated earlier, I expected to loose a few hundred rpm when going from an 18x12 2 blade to an 18x12 3 blade prop, but not 1100-1200. I wanted to get advice from Keith, since he invented the engine.

Hi Joe

I think all the numbers you are getting are about right. Your engine seems to be giving a fairly healthy power output.

The discrepancies in the numbers are due to difference in prop design which means that a 20x12 of one design will often spin at a different speed to a 20x12 of another design.

In particular APC props are thinner and tend to spin faster than other props for a given power. The 20x12 APC in particular has very thin blades and spins a lot faster than a normal profile prop. This probably accounts for the main discrepancy. If you put on a standard 20x12 2 blade you would expect to be down in the mid 4000s.

The standard prop power calc we use indicates that on the SP for a given design of prop (i.e. same blade shape, thickness etc) if you went from a 2 to a 3 blade prop at the same diameter you would expect to loose about 1100RPM.

Cutting the prop diameter by 1/2inch would gain you about 200RPM. So if you had managed to find an identical form 17.5x12 3 blade Master airscrew you would expect to see around 4600. The fact that you are 200RPM lower than this can only be due to differences in the detail design of the prop, and is well within the sort of variations we typically see.

The weight (i.e. mass) of the prop will not affect the max RPM, merely the spool up speed which should generally be pretty quick anyhow.

Its actually a hot topic this one! We are starting to think that using larger props on the SP range and loading them down to the mid 4000s might be a good way to run them, as long as you have enough pitch in hand for your forward speed. The following thread covers a similar topice:-


http://www.rcuniverse.com/forum/fb.asp?m=8635635


Anyhow hope this helps

Regards

Keith

Thanks Keith,

Actually, my custom prop is that same prop that Josh is running on his plane. I'm getting about the same rpm with my 120SP and 12" of pitch that he is getting with his 90SP and 10" of pitch. I had talked to Josh about it a while back and had deduced that the power difference in the two engines was evidenced in our tests of his prop design. What you say tends to coincide with our findings.

Here's the $64,000 question. After seeing Josh's video, do you believe Josh's prop set at 12" pitch will fly my 13 lb. monster? I believe it will, but I'm just worried about generating enough forward speed. But I think I will try a big 2-blade first.