Ive got a LT40 ARF with an O.S 46fx on it. Being the beginner in glow that I am, I snapped it on its last flight. I had a 11x6 which is what O.S recommends. Would a 11x5 work ok? I have one and thought "well might as well use it" I can solo so Im at least that far along. Also have a 11x7.5 but I thought that was a little much, or is it?

What did you snap? The plane or the prop? 11x5 should work OK. I use 11x7.5's on .61's.

the prop is what I snapped. It was wood. I like the way the wood looks but am thinkin of going to composite when I get a chance to get back to LHS. Will there be a "quite noticeable" difference between 11x6 to the 11x5?

Try an APC 11x6. Doesn't get much better than that...

But what about 11x5 vs. 11x6 (no matter if wood or comp.) Thats what Im really interested in. 11x5 is not one of the "recommended" props OS printed in its manual. Is it not enough pitch for the motor, and will it not perform as well on take off?

An 11-5 will be just fine. I can hang my LT40 on the prop for a couple seconds with it.

It depends. In theory, the 11x6 will give you more striaght line speed, but less climb than an 11x5. The 11x5 should give you a shorter takeoff roll and better climb.

However, a lot depends on the torque and RPM abilities of the engine. It's possible that the 11x5 could give you better climb and a higher straight line speed at the same time, since it would load the engine less, and you'd have a higher RPM. This might or might not be a good thing for you. On the other hand, if the 11x5 is too light of a load on the engine, you will produce a lot of noise and heat and not much else.

The engine manufacturers are usually very conservitive with their prop ranges, you're far better off trying it and seeing for yourself. You won't hurt the .46FX with an 11x5.

I'd say an 11x5 on a .46 FX would be fine. A 12x4 would work too.

Why do you feel you need an 11x5 on a LT40?

The lower the pitch the more torque but the less speed and an airplane like the LT40 produces lift with speed. The lift generated off the wing makes it fly, not the engine. The engine just pulls it forward so that the wind over the wings will create the lift to make it fly. Take away that speed and you produce less lift.

An 11x5 will fly it but your not going to benefit from it, at least not on an LT40. It'll just take more rpms to make it fly resulting in more fuel use.

Remember it's a trainer were talking about here, not a 4lb fun fly that you want to hang on the prop.

WreckRman2 said
and
Yes, it's true that you don't NEED that prop on a trainer. But you also don't need the airspeed a 6" pitch gives you on a trainer, these aren't racers either

In my experience, most guys do better with a slower moving trainer that can climb away from the ground at a low airspeed than a faster moving plane. And the LT-40 and other good trainers are perfectly capible of flying at the slower speeds, they don't need that high airspeeds to stay in the air. So give the beginner more time to react by flying slower. You can fly slower with a 6" pitch by throttling down. But they you have to throttle up to climb away from the ground. Not a big deal, of course, but if you can slow the plane and maintain the climbing ability, why wouldn't you?

A 12x4 should rev about the same as an 11x6, so the fuel useage would be simular. I agree than an 11x5 might be more RPM than you'd want, with a higher fuel draw and more noise. But I honestly don't think it will be that much higher fuel draw or noise.

On the whole, you might not even notice a difference. You might decide you like the 11x6 better than the 11x5. Or vice versa. Or you might find a totally different prop that suites you and your combination of engine, airframe, air density, fuel etc etc. Heck, just compare a MAS and an APC in the same size on the same plane, there's a noticeable difference right there.

But don't be afraid to try other sizes that aren't listed in the manual. And don't be afraid to try other brands, they perform differently.

btw,
12,000rpm at 6" pitch -> 68mph
12,000rpm at 5" pitch -> 57mph

An LT-40 is no where near stall at 57mph, so you certainly don't need the extra lift that 11mph might give you, but 11mph could be that extra split second between crashing or not

(ob disclaimer, airspeed based on pitch and rpm are ballpark guesses only. Real life is way more complicated)

I built the LT kit and installed the .46Fx about 2 years ago. I also went thru the wondering about the best prop. It won't make an appreciable or detectable difference to you one way or tuther. I even ran mine with a .32Fx and could not see much difference in training ability. Page 14 of the owners manual says to use 10.5 x 6 or 11x6,7, or 8 on low end and 12x7 up to 12x9 forf sport flying and 11x7,8 or 9 for acrobatic work. I like the 11x6 for instructing purposes or general flying but any will work. You don't need the 5 pitch props for anything with the LT unless you are pulling gliders into the air.

If I were to change from an 11x6 I'd go with an 11x7 before I'd use an 11x5. Again being a trainer the faster you pull it the quicker it produces lift and besides we never fly a trainer at full throttle so top end speed isn't what were looking for. I'd lay money that an LT40 will climb faster with an 11x7 over an 11x5. Why? Because a flat bottom trainer produces more lift at higher speeds. When instructing new pilots with a trainer such as the LT40 you should teach them that increasing the throttle makes it climb not giving it up elevator. In other words more speed and an 11x6, 11x7 will get it up to speed faster making it climb faster.

Good point... some people use the LT40 to tow gliders and when doing so the extra torque of an 11x5 would be useful. It's just like pulling a heavy trailer with your truck. If your climbing a hill you keep it in low gear for more torque or pulling power.

WreckRman2 said

and
So, umm, according to that, a higher airspeed is important for more lift, but since it's a trainer we dont' fly at higher airspeeds. Ok, gotcha.

Ok, seriously, of course more airspeed is more lift. And yes, you don't want to teach someone that back stick makes you go up.

Weither an LT-40 would climb better on a 11x5 or 11x7 depends more on the engine and brand of prop. But an OS .46FX on a 6-7lb trainer, I'd take that bet .

Is there anything wrong, IMHO with an 11x6 or 11x7? Nope, nothing wrong with it. Is a 5" pitch necessary? Nope, it's not, never said it was.

What is "better"? It Depends.

Do you fly off of pavement, or a nearly unimproved back lot? Do you have all day to climb out, or do you have 60' of trees at the end of the runway? etc etc etc. Lots of factors.

All I was saying was that there's no reason not to try an 11x5 if you have one. You might like it. You might not. You won't hurt anything by trying it. Go ahead and try an 11x7, and see what happens, you might like that better.

However, I will say that my experience teaching has been that guys get in to more trouble early on with learning to think ahead of the plane rather than reacting to it. Slowing the plane down makes it easier on a beginner. It makes landing a little easier as well.

The only way that I will agree that an 11x5 will fly a trainer slower is if both are flying at full throttle. The only time you need anything close to full throttle on a trainer is for takeoff, climbing and if your in trouble. Other than that a trainer will putt around at half throttle or less for normal flying. If you want it to fly slower then back off the throttle. The plane isn't going to stall anytime sooner just because you using a lower/higher pitch prop. The LT40 will glide in at idle or deadstick no matter what prop so I don't see how it makes it easier to land.

When I said I'd bet an 11x7 would climb faster I'm not talking about straight up vertical, I mean flying level and giving it full throttle and letting it climb on it's own. Of course an 11x5 will pull better going straight up because it has more pulling power but again were talking about an LT40 and most people aren't looking for unlimited vertical on an LT40.

Wow. There are some "interesting" arguments in here.

Whether you go with an 11x6 or 11x5 makes little difference, and should be based on how you want your airplane to perform. Buy each, and find out which you prefer. In general, the 11x5 will give you better acceleration, which when training can be important at times. The 11x6 should give you just a hair more speed, but not necessarily enough to be noticeable.

Try them both, and see what you like. Stay away from an 11x7, as that is over-propping the engine in my experience.

Also note that different brands of props will perform slightly differently. Try some different brands as well, and see what you like.

Looks like we're debating to become the next Mayor. LOL

Very true statement... An 11x6 Top Flite is a big difference from an 11x6 Master Airscrew and/or an 11x6 APC.

I have found that the APC is the best of those three and being it is a composite prop it can take a few dings when those landings are perfect. Sure beats changing a wood prop every bad landing when trying to learn.

Ok, I agree with most everything you just said . Certainly you can fly at whatever airspeed you wish with the proper throttle and elevator trim. And I never said it allowed you to fly slower.

What I said was that you can fly slower and still have something in reserve for "OH *****!" with out waiting for the engine to respond. I will agree that 99% of the time this isn't needed though. And the vast majority of the time, it won't make any difference at all when it comes to learning to fly.

However, throttle response on most engines isn't linear, and there is a limit to the lowest speed you can idle. If you are flying from a short field with obstructions, a 7" pitch at 1,200 rpm vs 5" at 1,200 rpm can make a difference. Yes, I know you can slow down with the 7" prop by just feeding in a bit more elevator a bit earlier. And there are dozens of other tricks experienced pilots can use to bleed speed and hit a small field. But it's easier for many guys to with the lower pitch prop. Not all the time, but not never.

Yeah it does .

But I think we both know that in many ways we are splitting hairs a bit.

If you're ever in MD, let me know, I'll fly with you anytime I'm certainly not taking this personally, and I hope no one else is.

If you need to slow down for a short field landing you should be prepairing for this way before you line up with the runway. In other words you should bleed off the airspeed before even trying to land. A little elevator will slow it down yes but a good instructor should teach the pilot to slow your airspeed on the downwind approach allowing the airplane to settle on it's own as you turn base and on to final approach. Then the only elevator movement needed is used to flare just before touching down.

I see way to many guys force a plane down because they turn onto final then dive for the runway... by the time they get down they are way to fast and end up crash landing.

Speaking of wood props.... Way back when I was learning to fly, I was told all beginners should use wood props, since the prop will give before causing any damage to the engine when you nose over. That had to be way up there on the BOGUS meter, definatly in the top-10 bit of bad advice I got along the way . Has anyone else heard that one?

At the time, APC props didn't exist. I don't remember what the other plastic props were on the market at the time, I think the black MAS were.

For durability, you can't beat the black MAS props. I think APC generally pulls better though. (in most applications).

Wreckman,
Yep, that's the ideal way to deal with a short field. But I've been to fields where there were obstructions or no-fly zones that made that impractical.

Heck, at my current field, you can't run a normal base leg if the wind is from the left-to-right, you have to cut it short and do a circle approach because of the no-fly areas (roads) near the field.

Where I learned to fly, up in RI, we had a small field that was right next to an old factory building. If you took off to the right, you had to turn right after liftoff to avoid hitting it. If the wind was reversed, it made landing a real trick. Most guys took off and landed directly away from the pits, with base being right accross in front of the pits, turning away to land. It was "intersting" and being able to dump airspeed in a hurry was important.

Actually there is some truth to that however giving the small diameter of the prop and the low rpms of the engine during landings the chances of it bending the crankshaft are greatly reduced.

If you take for example a lawnmower swinging a 22" blade cutting the grass at full throttle then you hit a stump in the yard. 80% of the time you just bent the crank. Why? Because your running it at full throttle and two because your swinging a 22" blade. The momentum of that blade swinging causes the crank to bend. With a small glo motor swinging an 11"prop at idle just doesn't have the momentum to bend it like the lawnmower does.