SC.40 - what prop for a trainer??

Wrong.

Assuming that an SC .40 is a .40 size glow engine (sorry, I'm not familiar with an "SC" engine) your best bet in a prop will likely be an 11x5 or an 11x4. An 11x8 usually is too much prop for most .40 size glow engine.

Tejas Pedro

"What, me worry? I'm voting for Kinky!"

How does it fly on the props you've used? My not-so-seasoned opinion is

10-6. I'm going to have to find out waht a SC 40 is brb, OK, that engine appears to be a ball bearing, ABC engine and should be powerfull enough to swing a bigger prop. That brown bolly 10.5X6.5 might be good, or an 11X5 mk

Rule of thumb in this area is that most .40 engines work best with the 10/6. Frank Schwartz

Depending upon the size (parasite drag) of your trainer, an 11x5 or

11.5 X 5 (Bolly) will be optimum with that engine for training.

Why?

You _don't_ want speed when training. You do_want sound idle, good transition, responsive throttle, good acceleration of a trainer's draggy airframe and decent climb, plus prop braking assisted descent.

Acceleration and deceleration (braking) with a 5" pitch both = the perfect compromise for training with most draggy yet high lift coefficient trainer airframes/wings, and an 11" diameter in conjunction with that pitch blade, will allow that engine to operate in the band between its peak torque and peak power curves.

One of the things that is enjoyable about today's modeling scene is the huge selection of oddball sized props. To a senior modeler, such as myself, the

11x5 would have been the perfect prop "back in the days" of my instructing ventures.

While the 10x6 is fine, it does not offer the braking of the 11x5. Nor will it get the model off of the ground as quickly. But, the 10x6 is a good prop that will not overload a good ball bearing .40. Neither will the 11x5. I don't like running 11x6 props on .40s that are meant to rev. Just a personal thing. I can imagine the engine overheating on hot days with such a prop.

When utilizing a plain bearing .40, I would use a 10x5 or an 11x4. Most of these engines are ported so mildly as to be a bit overloaded with the other size props we mentioned for use with a .40 ball bearing engine.

Yes, there are exceptions to every rules. Two-strokes live longer lives when permitted to rev up a bit. Save the lugging for Diesels and four-strokes.

Ed Cregger

I agree with Ed on his choices, but I would lean to the 11x4 if you have prop clearance. The 11 inch prop will idle better with more centrifugal force because of the longer length, and the 4 pitch will fly slower with better climb. The "steep pitch" gives you more speed in level flight at the same carb setting. Good Luck, Harry

It never ceases to amaze me how many folks install a 10x6 on a .40 and stop right there. Experience has taught me that prop size experimentation can have a big payoff. For the typical so-called ".40 size" trainer airframe I've found that an 11x5 or an 11x4 is almost always a better performer than a 10x6. Those sizes accellerate the airplane better, help to keep the top speed down and they don't overload any decent .40, not even an OS LA.

I've got a bit of a prop collection, and any time I begin flying a new airplane I like to try all of the sizes that the engine can handle, because I never know which one's going to provide the performance I'm seeking until it's in the air. Static thrust and RPM are sketchy guidelines at best, the only way to know what prop is most suitable is to fly the plane.

Tejas Pedro

wow, thanks very much all, some very interesting information, and certainly very useful indeed!

it appears, then, that the 11x8's in my flight box should be sent back in exchange for 11x4's and more 11x6's - can anyone recommend brands that can offer lower noise when in use?

As some of the above replies note, I have also noticed more 'pull' with a larger prop, but this is combined with a lower top end speed - something that really benefits me as a learner as i need to get the basics down first before gunning for the finish line!

Also, I can concur with whomever suggested that the plane would appear to slow down quicker with a lager prop, again, another feature i feel thats beneficial as a learner.

many tahnks all, looking forwards to replies regarding the brands of props that could bring my db-level down.

thanks again!

Without a doubt APC's. APC props also seem to provide greater thrust than equivalent dia/pitch props, particularly when compared to Master Airscrew brand.

I do, however, confess to having a small collection of Rev Up wood props that are second to none. Anybody got some Rev Up wood to sell?

Tejas Pedro

"What, me worry? I'm voting for Kinky!"