I have a ST 90 on a Sig 4 Star 60. I've been flying it with a 13X7 APC, however, I would like to get a little more pull.
The recommended prop range in the manual only goes as high as a 13X7 so I have been hesitant to try a 14X6. I'm not sure if this will damage the engine.
Any ideas on size would be appreciated and whether or not this will damage the engine.

The two best props I have used are the MA 14x6 and the APC 13x8. I have been running ST 90s for 15 years. You will not hurt the motor. If BAX post on this forum he will tell you to try something smaller than I run. He seems to like his Tigres to turn more RPM. Nothing against his advice he knows his stuff. I just have better luck with the two I mentioned.

David

When you say more pull what do you mean?

Do you want the hover kind of pull or do you want it to pull it through stunts with more authority?

I have an Ultra Sport 60 with a G-90 and I like the 12x8 the best.
Not real fast but plenty of pull.
13x8 was kind of lifeless. 6 pitch props are pretty slow. But pull well.

I've wanted to try a 12x9.
It should work out pretty well, but won't know until I try.

I don't have clearance to run a 14" prop on this plane.

Dave,
Excellent. I will try both.

I always ran mine with the APC 14x6 and it was good, no over heating at all. That Bax will tell you 12x6 or 12x8! He wants like 100,000 RPM on his G90. That I never ever understood that when its for sport flying, but then BAX does know a lot.

The MAS K 12x8 is perfect and the flex won't hurt you a bit. ST 90 do like RPM and will trounce the OS91 running their bigger prop recommendations. The S2 series will not flex like the K series but will be up a little in RPM. I like the scimitar MAS props now but they don't make a 12x9 or a 13x7. The ST90 is made to run but not on 4 stroke RPM numbers.

The best thing to do, in my opinion, is to buy one of a few different sizes. Spare props are always good to have, and at three or four dollars per prop, it's not an overly excessive cost. Also keep a tachometer on hand. Put the prop on that you would like to try, tune the engine for optimum operating RPM with that prop, then put it in the air and see if it meets your requirements.

Remember not to over tach any engine. Manufacturers usually put an RPM operating range but often, the high end is a tad bit optimistic. Operating that size engine at 15,000 RPM is somewhat excessive (again, in my opinion) and will make a lot of noise and will probably not give you what you want. I try to maximize my operating range to a maximum of 12,000 RPM on smaller engines and 10,000 on larger ones, then prop it accordingly, varying pitch to make my requirements for climb, speed, and pulling authority. And, I don't usually fly my planes at wide open throttle for long, usually for take off then when I want to go vertical for some maneuver. Other than that, it's usually at half or so throttle. I get good performance and fuel economy operating it like that, and try to choose props and tune to meet that.

Everyone is different, though, and I am not inviting an arguement here, just saying what I do to optimize my engine/prop combination to how I like to fly my planes.

DS.

When was the last time you bought a prop?

Dave Olson

Consider a 13/8 as said earlier

For some reason, the cost of props rises rapidly at the 14 inch point. A 13x6 is still pretty cheap, but I was amazed at how the price jumped when I started looking for a 14x6.

Brad

I run 13X6 at 5000 feet altitude. engine doesn't over heat and speed and performance is outstanding.