I have a Thunder Tiger 60 Trainer with an OS60FX Engine. The manual recommends an 11x7 prop for break-in. I have followed the break in procedure. Now I have put an 12x8 prop on. I picked this one because it was in the middle of the range for a 12 inch prop. What is the correct procedure for selecting a prop to use? Do I try to reach a certain RPM?



Thanks

ogriv

To select a prop you must know first exactly what you want from the engine / prop combo. I little info to help you. Larger diamenter, less pitch props will provide you with more torque, vertical performance at lower speeds. Lower diameter higher pithc prop the opposite, less torque and more speed (consequently longues take offs). In my opinion, a prop with low pitch would suit a trainer better and make it easier to learn. I would suggest a master airscrew (harder to break than APC) 12x6. Then again, Im sure someone else around will jump in to give their insights regarding your prop.

Good luck and welcome to the hobby

So a 12x6 would give me more torque. Which would result in shorter take-off's. I imagine the plane would be more lively with the throttle at 1/2. So would a 12x5 hurt anything (It is the lowest pitch for a 12" that the manual recommends)? Now if I change from the 12x8 to the 12x5-6 will top speed be the only thing I lose. I am headed to the hobby shop in an hour to buy some props, any opinions are apprieciated.


Please forgive my ignorance

the lower pitch prop will give you more autority at lower speeds and will the plane speed lower as well. I think even your 11x7 will do better than your 12x8.
Regarding the exact prop you should buy, Id rather have someone else advising you at this point as I have never owned a TT60.

Thanks I Got Hit,

I guess My thought of "Bigger is Better" does not always apply.

ogriv,
I Got Hit has pretty much summed it up for you......I've never owned that engine either, but here's what I usually do to determine the best prop/engine/plane combo. As I Got Hit stated, first, think about what you want from the prop engine combo and then buy a few different props.....test fly the plane with all of the props, and try the same manuevers each time with the different props......then decide which to keep. The type of plane will have an effect on what the best prop to use is. (not so much in a trainer, but when you start moving to other models, it will make more of a difference.)

Thanks for the assistance,


I think I'll just buy a couple of each size recommended. Can't hurt and they are cheap enough.

That's your best beg Ogriv. I think I used a 10x6 exclusively for the first year that I was flying.....I really didn't know any better or was told any better. Eventually I started experimenting with props and use all different types now, depending on the plane.

Just FYI

I just checked the engine manual and this is what it reads:


Sport Aerobatic
61FX 12x6-8, 13x6-7 12x9-11

You just have to buy a selection and try them.

Going down in pitch with out changing diameter will usually slow the plane down and increase torque as above, but sometimes the plane will go faster and have more climb at the same time. The reason for this is that the engine will be less loaded and will pick up RPM, sometimes enough RPM to make up for the pitch difference. Weither this will happen or not is not really predictable, you just have to try it.

Also, be sure to try multiple brands of props. An APC and a MAS in a 10x6 will pull the same plane very differently, for example.

You can also frequently go a bit bigger or smaller than the manual states.

If you have a tach, that can help you keep from putting on too small or too large a prop. For a .61, I'd say you want the top RPM on the ground no higher than 16k, and if you turning any less than 10k the prop is too big. Any prop that puts the engine in that range will be "safe". Then you test as mentioned above and pick what you like best.

Great, thanks.

I just borrowed a Tach from work. I read somewhere the target rpm for this engine was 16000 rpm (Just like you stated). So I'll measure it with the 12x8 and compare it to the other sizes. I just did not want to make some dumb mistake and damage my engine. I did notice with the 12x8 it used up alot of runway to take off.


You all have been great !

16k is your max, not necessarly your target. Some planes will fly better with a bigger prop turning at a lower RPM than with a smaller prop at a higher RPM. Specifically, high drag designs like trainers and espeically bipes do better with big props turning slowly.

Alrighty, I am armed with knowledge. On my way to the hobby store. I'll let you know how it works out.

Are you just starting out? If so... just use a 12x6. It's plenty plenty powerful for anything you want to do in a trainer. But remember when you change props to retune your carb.