Whats the best Prop size for an O.S 40LA?

I'm flying a "Magic Extra 300"

i'd say you wanna put an 11-5 on that

if you want to tend more to speed then put a 10-9 on it or a 10-11

if you want to be doing sort of things like prop hanging and stuff like that then get a 12-3 on it but if you want imbetween then as said above, 11-5

So as a rule of thumb a shorter prop with large pitch is going to give you more speed as where a long prop with a small amount of pitch will give you a large amount of "Torque" or hanging power?

Thank you for the reply.

I forgot to mention, the prop that is on there now (Flew with today) is a graupner 9.5x5.

yeh you've just about got it

as a rule of thumb, lets say a 40 will swing an 11-5 as said earlier

the reason for downsizing to a 10-9 is so not to overload the engine, an 11-9 would probaly put stress on the engine but give more speed

and as the pitch is smaller you can put a big diameter prop on because a lower pitch is less strain on the engine so theres more rev's allowing for a bigger prop

so yeh you've about go it there

a size .40 engine can take a much bigger prop than a 9.5-5

how does it feel at the moment?

if it feels underpowered then fit a bigger prop, try and get a tachometer to read the rev's on the engine, if you are getting any higher than aout 14,000 then put a bigger prop on as if engines dont have the right load on them then they can lose life span

too fast and they wear out too quickly, too slow and they get overheated and burn up

i would put an 11-5 regardless of what the current performance is like, you'll get more economy out of your engine therefore more flight time

It flew pretty well, but it did feel a bit underpowered when I would climb out.

I'm not looking to scream across the sky at 120Mph, or to be able to hang it in mid air for minutes at a time...... but I would like to be able to climb out in a hurry off the runway...... or into a stall turn, etc.

Thanks again for your reply.

yeh, an 11-5 will definatly be a good choice then, just imbetween so theres plenty of thrust available and with some speed too

A 40La is a weak engine. To keep the revs up and give you good all around power may I suggeat a 10-6 prop. A 11-5 is too big a prop for that engine IMO.

I'm turning an APC 10x5 on my OS .40LA. It pushes the revs higher, so you have to be a bit more careful about lean runs, but the climb is quite good. I didn't try an 11inch prop in part because the 10" ones are cutting grass enough as it is.

As you can see, there are a lot of options. Don't be afraid to try something different. I do think a 9.5x5 is a bit small for that engine, but as you already know, it's not killing anything.

Also, different brand props at the same size can be very different, so sometimes changing brands in the same size will see a performance improvement.

A 10 X 9, or 10 X 11 would also be way too much prop for that engine.

To answer your question, yes, larger diameter/smaller pitch = torque, smaller diameter/larger pitch = speed.

I agree with the 10 X 5 or 10 X 6, but I think the 11 X 5 might also be an option.

(10 X 6 is the usual prop for a decent 40 size engine.)

By the way, as a general rule of thumb, when changing between diameter and pitch, increasing one by an inch usually requires decreasing the other.

(ie. 9X7 = 10X6 = 11X5) But like all rules of thumb, there's room for interpretation.
Dennis-

I'm not looking to scream across the sky at 120Mph
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thats good coz the extra will flutter itself to death!


buuzzzzzzzzzzz snap!

So, let me ask you all this...... lets say I go from a 9.5x5 to a 10x6. I have increased both prop diameter and pitch. What will the effect be on the planes performance? I understand Larger Diameter/smaller pitch = torque, and Smaller Diameter with more pitch equals speed.......... but what happens when you increase diameter and increase pitch????

Am I going to get both more speed and torque?

Thanks for all of your replies.

So what will allow me to fly slower, a lower pitch or a bigger diameter?

So what will allow me to fly slower, a lower pitch or a bigger diameter?
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both, i would suggest a 11x 4 to start with.
this will give you good throttling and thrust with lots of propwash over the surfaces to enable a nice prophang

Sorry to ask but what exactly more torque means in terms of flying?

When people talk about getting more "torque" out of the engine, what they usually mean is thrust, which basically means the ability to climb.

In a perfect theory kind of way, a plane with a really large diameter, really low pitch prop will fly level at a nice, slow rate. Point the nose straight up, and it will fly striaght up at the exact same speed.

A plane with a really small diameter, really high pitch prop will go like heck in a striaght line, but will slow way down when climbing and turning, and might not be able to climb much at all. It will have trouble even getting off the ground with out a long takeoff roll and a smooth runway.

Now, the question about "what happens if you go up in pitch and diameter both". The answer is "It depends". All the stuff about going up diameter and down pitch or vice versa assumes the engine will produce the same RPM with both props. But that's not really the case. Engines aren't perfect, linear devices, they have all kinds of limits and non-linear things going on, so all the prop stuff is really estimation and trial and error.

Generally, a "smaller" prop will let the engine operate to a higher RPM than a "bigger" one. You want a prop that causes the engine to operate in it's designed power band, the range of RPM at full throttle that the engine produces good power and doesn't over or under rev, and produces lots of torque (real torque on the crankshaft this time).

If you put on a prop that is too small, the engine will try to over-rev, overheat, and maybe break itself. If you put a prop on that is too big, it will "lug", and not be able to turn it, it will run poorly, overheat, and die, maybe even damage itself. The good news is that you really have to work at it with most modern engines to actually damage it.

To complicate things, different engines are built differently, and have differnet HP and torque curves, meaning they handle the same props differently.

For example, the intake, and exhaust timing, the compression of the engine, the bore-to-stroke ratio, and stuff like the bearings or bushings all affect the max RPM an engine can acheive. So, if a given prop is already "*****g out" the RPM of an engine, going smaller won't result in more RPM, just a poor running engine that doesn't move enough air to fly your plane and shortens it's lifetime.

A real life example from flying combat might help with the confusion. Lets take a Magnum .15XL and an OS .15LA engine. In Open A class, we can turn any prop we want, and most guys typically put a 7x4 on a .15 engine to produce lots of RPM, and gobs of thrust for tight turns and steap climbs. The Mag .15 produces a noticeably higher amount of power with the 7x4 prop. But, in SSC class, the rules mandiate an 8x3 prop. It so happens that the OS LA engine likes to turn at a lower RPM than the Magnum, and can handle the 8x3 "better", so all of a sudden, the OS LA engine acutally pulls about the same, or slightly better than the Magnum. The Magnum can't reach it's peak RPM with the 8x3, while the OS is likely over-reving a tad with the 7x4.

So, with a 7x4, the Mag is more powerful, with an 8x3, the OS wins.

Another example. In B class combat I run Magnum .28XL engines. I run a mousse can tuned exhaust and a 9x4 prop. The engines turn over 18k this way, and I get a lot of climb and speed. The funny thing is that if I change from a 9x4 to a 9x6, I loose climb as expected, but I also find that the airspeed of the plane GOES DOWN. Why? Because the engine can't turn the same RPM with the bigger prop. The Mag .28 doesn't produce enough torque to turn a large prop, but it loves to turn up to high RPMs on smaller props.

I've also left out a lot of other issues. One is different airframes will make the same engine/prop combination fly differently, and you may find the same engine in differnet airframes likes different props. There are others.

I hope this hasn't confused you too much. The point to take away is that prop selection is an art, and takes trial and error to get the "perfect" prop for you. There are many factors, such as engine design elements, that affect how a particular engine/prop/airframe combination works out.

The good news is that 99% of the time, you don't need "perfect" you just need "good enough". You can always start with the prop the engine manufacturer recommends, and you can check around to see what other folks in your area are doing and go from there. If the plane flys well enough, you're good to go.

Don't be afraid to experiment. You won't damange anything if you make small changes. If you do put on a prop that is a little bit too big or too small, the plane will fly poorly, but you won't hurt the engine at all, so make small changes as you experiment. You may surprise yourself with the results. Sometimes a differnt size or brand of prop can really "wake up" a sluggish airplane.

Ps, for sport flying, you have no business turning north of 18K . Most .40 and smaller two strokes shouldn't be much over 16k, and many produce the most torque at around 12-14k, or even lower. The highest HP readings are usually up over 16k. Most 4 stokes should be kept under 10k, with the best torque around 8. (You YS guys, keep it to yourself , there are, of course, exceptions).

The peak HP and torque for an engine rarely occur at the same RPM. Most planes fly better at the peak torque number, which is usually at a lower RPM with a larger prop.

btw, this is why the HP readings listed by engine manufacturers are mostly BS, they put on really small props to wind the engine way up and get those numbers. But there isn't a much thrust being produced at that point.

Anyone have torque and HP curves available for some engine?

thank guys, I'm using a 10x6 on an LA.40 now, and going to try 11x4 to see what happen.

Thanks for all of the replies. I will pick up a couple of props to play with this weekend.

Probably a 10x6, 10x5, and 11x5.

:rainfro:

I use an 11 X 4 APC on all my .40 engines & an 11 X 5 APC on my .46's. Good climb power & slower landing speeds. MAX H.

I fly my TH40 with either a 10x6 or 11x5 with good results. I've also use a n 11.5x4 that works well.