Steppin Up O.S.
 

I have a plane on which I'm flying an OS 61. For the longest time, I was going with the old school, "11x7 is the prop for a 61". That was fine one my pattern birds, and when I didn't know any better, but when I put it on a prop jet, say, an F18, I noticed a severe drop in performance. The plane struggled to get off the ground, and the only way to get any noticeable speed out of it was to "struggle" it up to the stratosphere and dive it. That was with a stock muffler. Then a friend of mine gave me a pipe. Ok, so the pipe was for a 95AX, but it did help just a little. Or maybe that was just suggested in my mind, lol. Then, someone said that maybe I was under propped, and suggested a 12X10. I tried a 12x10, and noticed a significant increase in performance. Now it wasn't super fast, but it was at least acceptable for me. It didn't struggle on takeoff like before, and I could get some pretty fast passes on straightaways, and when diving, JEEEEZ!

Now keep in mind, this prop jet sits pretty low, so my prop length options are limited.

Well whaddya know, I lucked up on an O.S. 95AX. Great. So here I have an engine now that my pipe is made for. When looking at the specs, the smallest prop is a 13x8, so I figure a 12x10 wouldn't be much different. However, other guys at my field (and in forums) are running 12x7/12x8's on the 95AXs, and they say a 12x10 is too much pitch. I'm thinking, if the 61 is swingin it without any apparent problems, a 95 should swing it ok as well.

So my question is, am I looking the right direction? Just go ahead with the 12x10 on the 95? Yes, I know the obvious answer is "try it out, see what happens." But I want to know I'm at least thinking right. I really don't want to go with a bigger prop because of ground clearance. One might say to not worry about it if you land/takeoff right, but we all know real life doesn't work that way. The biggest thing is that I have a few 12x10s already, and I don't want to waste money or more importantly, time on new prop(s) if they're not really correct or going to work better. And I'm concerned about the lighter pitch being harder on the engine.

Anyone with experience on this?

As I always suggest , I take a handful of props in the appropriate range and test fly them to see which one flys the plane the best. I think you should mount the 12X10 and get a tach reading to establish a baseline. I would be interested to know what the 61 Is and what tach reading you were getting that it didn't work on your setup. I use an 11X7 on my Enya 60X with great results.

Recommended Props: 14x8, 15x6-8, 16x6-8

Yeh the 12x10 my very well be a little light, a 12x10 three blade may be much better.

Good advice, thanx. Though with the 61, it wasn't so much that it didnt work, it just didn't work well. With the 11x7, id have to take up just about the whole 400' runway, and make a reallly shallow climb out. With the 12x10, it only takes about half the same runway, and climbs with way more authority. And, keep in mind, this is with a pipe for a 95 on it. Now that I have the engine that the pipe is made for, the engine/pipe combo will be correct, and I'm sure I'll get better results. Just wasn't sure if the 12x10 would underprop it. Again, I can't go much bigger, if at all because of clearance issues. Im going to try the 12x10 on the 95 this weekend and just "go for it". Or maybe try a 12x12 (a corkscrew, I know). Gonna tach it like you suggest before flight, see what happens. I'll try to post back results early next week. I welcome any other suggestions!

I must ask, is the 95 AX a real wimp of an engine, running 60 sized props and being happy about it just doesn't make sense. Thanks, Dave

Sounds to me like the 90 size pipe just happened to be the right length on the .61 for the high pitch prop. Then it dug in and scooped some air on the downhill runs. If the pipe was shorter, then it might work better on the .60 size props. It is usually a good idea to check rpm with no pipe and then with the pipe to see if the pipe is helping, or is just a bad muffler which could be holding you back, as it seemed to do on the 11-7 .61. Normally it will pick up a thousand rpm or whatever when moving, and can then hit the pipe-which will then give many more rpms in turn.

The high pitch props will make your plane land very fast and maybe not land at all. I knew a guy who had a prop jet and had to cut the engine before landing everytime otherwise he needed a mile long runway.

A big factor (one of many) with pipes is what type it is. Is this a Quiet pipe or a Muffled pipe? Big difference in tuning and performance. If it is a Quiet pipe, I don't think it will do a whole lot of difference between the 61 and the 95 size engines.

What happened to your 61 that it got such a big drop in performance? Just the airframe change? Have you checked the piston liner for peeling or scrapes? I run a OS 61FX with a Quiet pipe and an APC 11x7 with good performance. But I keep a chrome plated piston/liner set from ASP as backup for the inevitable nickel plate peeling.

The 95 will be more of a torquer than a high rpm engine due to the bigger bore but smaller intake ports. The propjet airframe really needs a high rpm engine like the 61. The 95 will require a BIG prop load to stay in the proper torque band. So a 3-blade might have to be looked into because of ground clearance and pitch constraints. I would consider a 11x10 3-blade.

I agree.

Ok, maybe I'm not explaining this clearly. I need a small prop for clearance issues. The 61 works fine with the 12x10, it just was sluggish and saggy with the 11x7. The 61 has the wrong pipe on it. The 95 is no wimp from what i have seen and flown. All I was wondering was that now that I have the correct engine/pipe combo, meaning the pipe is made for a 95, NOT a 61, and now I have a 95, what i could expect using the prop that gives me group d clearance. That's all. Basically, i want the plane faster but still need a smaller prop to clear the ground. In that way, it makes perfect sense to be happy about it.

In this hobby we experiment, its how we advance. How do you think we got prop jets in the first place? I'm sure there was someone back then that said, "I don't see the point in making a jet and then just putting a prop on it and being happy about it." But it worked, still works, and has it's place and usefulness.

Well whaddya know? Ran the 95AX with a 12X10 and pipe over the weekend. Had the prop clearance I needed. The plane easily rotated at only 1/2 the runway and climbed with authority, unlike the sluggish climb with the
.61​​​​​. On the straight levels, the plane was significantly faster and didn't fly with that nose up attitude. As it was pretty hot out, I ran her kinda fat, so she wasn't quite on the pipe yet, pretty rich, but that just tells me she's got more to give. Not only that, on landing, she slowed down normally, touched down on the runway and stopped before the end like normal, first try. And I am happy about it!

Good deal. Thanks for the update.