I am flying a SS with a OS .91fx 2 stroke. What are others using for props and what engine head temperature should I look for?

I have a 15-6 on right now and I am overheating I think. I have the cowl nostril cut all the way open and the air flow area and path look great. I am going to experiment with props again...already did but more for performance. I think I might be over or under proped for how I fly. I know I fly with full throttle a lot latley. I am wondering if I might also just be running the engine too hard. I could also just be too lean. I think I am pretty rich by my tuning opinion but that is what a temp gauge is for.

So more than prop size a target temp would be what I need to know. I have read other threads about what range of props guys are using.

Cooling on the ground is going to be very different than the cooling that cowled engine will see in the air.

And unless you can record the head temps in the air, whatever temp is still in the head after you land isn't going to tell you anything. Unless you land with the engine running flat out and get to the site of the landing VERY quickly.

Does the engine sag in the air?
There is little reason a 91 on a 7.25pound airplane would get hot, unless you're trying to hover or harrier or such.

If the 91 is mounted at 9o'clock, try closing off the cowl opening on the 3o'clock side. Or put a diverter baffle in it like this one in my Decathlon. (I don't happen to have a clear picture of the same diverter in my Skybolt or I'd have attached it.)

Overheating is very often lack of hot air exhaust area or exhaust location. Next most is allowing cool air a path that skirts the engine on it's way to the exhaust opening. It will simply block the hot air around the engine from going anywhere. And the engine cooks.

I'm running 15x6s on my 91s on a H9 P47 Thunderbolt, and a Sukhoi. They're heavier than my Skybolt. The engines haven't shown any problems with the 100degree weather we've had recently. I also happen to run a 15x6 three blade on the P47 now, and even that "not as efficient, way too much prop" hasn't heated the 91 up any more than before. Yeah, a 15x6 THREE blade. And it turns a bit more rpm than most of the two bladers I've tested with it. Yeah, not supposed to happen, but you know what...... there are lots of things the experts know that doesn't happen the way they say.

As for your maybe being just too lean. Good possibility.

Until you hear the word on heat temps, try clicking a couple clicks rich next flight. And if you don't see exhaust for sure, the next flight, click a couple more rich. You won't lose 5% of the power available, and that'll still leave way more than needed for that engine/airplane combo. And if you were flying too lean, you'll actually be getting MORE power. It won't be as high tech, but it will answer your question about maybe being too lean. Lot's of ways to do things.

As for you running the engine too hard. Not probably. The airplane is lighter than anything the 91 was designed for. Lots lighter.

BTW, see the prop on the Decathlon?

It's a 12x5, right? Well, it started life as a 13x5. But when I swapped the OS55AX into that airplane in place of the OS46AX, the engine acted like it needed a bit more blade area to work a 5"pitch and not run too cool. So I made that 12x5 out of a 13x5 and the engine seemed to run smoother. And the airplane benefitted from the lesser pitch on the approach. So with that prop it gets the lesser pitch and the engine gets a slightly better resistance to work against. So it runs warm enough now throught it's envelope. And with the baffle, it was running too cool. hmmmmmmmmmm Should I have pulled the baffle? [sm=wink_smile.gif]

Yes. As he said.

You can "overload" an engine and merely have it sag, but not overheat if it is properly tuned, but not quite prop'd properly.


On a different plane we found my GMS .76 had it's head temp rising to 400 degrees!!! I was fighting to get it in tune.

I remembered that someone here posted that the head temps should be around 290 degrees F.

I richened the engine out to the point that the temp dropped to about that, and my troubles were gone.

It still seemed a bit rich to me on the ground, but this proved perfect in the air.


I was running a large prop too...