I looked at the following site to see if I could use my 9x6 prop: http://www.rcexchange.com/html/body_props.html


What exactly do they mean by "starting prop."


I have an O.S. .40 LA. Can I use a 9x6? I noticed that I max out my rpms at about 3/4 throttle. I currently have use 10x6. Is that preventing max RPMS? Thanks

"Starting Prop" normally means the same thing as "break in" prop.

Remember that certain engine lines produce more power than others.... so there may be differences in the suggested prop tables.

Your .40LA engine is rated for an 11x5 or 11x6 prop according to OS's site.

However I find that it runs better with a 10" prop as it is not a "strong" .40 engine since it is of the "budget" variety.

I also run a .46LA with a 10x7 prop which equates to an 11x6 in terms of load on the engine, that is recommended for standard .46 sized engines.

Go by the 11x5 recommendation from O.S. and adjust accordingly. This means you can go one size up in pitch if you go one size down in diameter and get the same loading on the engine... ( and yes I'm being very loose about this to the cognizanti ).... or vice versa...

So 10x6, 11x5, 12x4 would all work.

On a lighter plane you could also use 10x7, 11x6 and 12x5's.

a 10X6 is the recommended break in prop for the .40LA. You can run the 9X6 as long as it's not in a heavy plane. I don't think it would have the power to keep a heavy plane in the air. I would recommend a 9X8, 10X6 or 11X4, maybe a 11X5, but nothing longer or steeper pitched.
Starting with the 10X6, if you go up one inch in length, go down 2 in pitch.a MAS 11X5 prop would be stretching it. It would still turn almost optimal RPMs on the ground, but it would lose speed in a sharp turn.
I like the LA engines. I use them in most of my SPADs, but they are a bushed engine, and not as strong as a bearing engine. They'll take tons of abuse and come back wagging their tail.
I have to disagree with OpJose. As far as load on the engine, a 10X7 would be the same as a 11X5, and there is no way a .40LA will spin a 12X4 more than 8,000 rpms. Remember, nitro engines get their power from RPMs. The faster the prop turns, the more power it will keep when turning and going vertical.

I have a tower trainer. I think it's about 5.5 lbs. I have a 10x6 on her now, but if I somehow break that , I only have a 9x6 until I can get the LHS. Thanks for the input.

"opjose" is a little optimistic about the power of an LA. If that engine is on a trainer, I'd run a 10 x 4 or 11 x 4 on it. It's not a strong engine.

Dr.1

Prop's for that size engine are really inexpensive. The APC 10-6 Sport Prop is only $2.49 each.

http://www3.towerhobbies.com/cgi-bin...?&I=LX1547&P=7

Hell, you can buy 10 of them in anticipation of breaking them, and have plenty of spares. I bought them in 5 prop packs from the hobby shop when I was in training and was breaking them quite regularly in the beginning. And now, after several years of flying, I find that I havn't broken one in over a year now.

Breaking props is all part of the training process. It happens to all of us, and the size prop you are talking about is very inexpensive and can easily be bought in quantities.

As far as props for your OS 40 LA engine, well, it's best to try different ones to see what one performs best in the air. What one will give you the power to take off, and fly normal patterns without having to fly at full throttle all the time. Yes, the OS 40 LA is not a power house as compared to some of the others, but it was designed as an economical engine that will give the user a good engine that starts easy and runs well, and will fly most 40 size aircraft without problems.

And remember that RPM range is not necessarily what you will get in real life. That all depends on what prop you use and, while flying, what load the engine will encounter. It may not go to full RPM at full throttle, it all depends on if it's able to turn the prop you have selected. If you maintain a reasonable RPM range, for that engine, I imagine that is around 12,000 RPM, it should do just fine. It becomes a trial and error sort of thing. Remember that the advertisement says 1.2 BHP at 15,000 RPM. You will NEVER see 15,000 RPM in a reasonable application. If you do see 15,000 RPM, your engine may not last longer than a few flights. The noise alone will probably drive you to get a prop that will operate it at a more reasonable RPM. Noise does NOT equate to power.. just noise.
CGr.

Heh, I'm not the one who is optimistic...

This is what O.S. states... not that I agree with it.

Thanks for the info. Yeah, I might just buy a ton of props from tower. It is sometimes just inconvenient to drive all the way to the hobby shop. Thanks for all of your help.

Just put a tower .46 on my tower trainer. I have a 10x8 on it for now. But, what I really want is a lot of vertical. Will 11-6 do the job. I'm willing to sacrifice a lot of speed for a lot of vertical I know there are tons of recommendations out there, but . . .

I've written about this several times before, but here goes again.

Consider the pitch of a prop as you would the transmission in a car or truck. Lower gears give you more power but at a higher RPM. Higher gears give you more speed at a lower RPM. If you are climbing a steep hill, and you have a standard transmission, you will probably shift down to get more power. An automatic transmission will do it for you. Then to go faster on the highway, you can then shift up in gears and it will go faster at the same RPM as you were using for the climb.

The pitch is the same as the gears in a transmission in the car or truck. If you want to climb that hill (go vertical), then reduce the pitch. If you want to go faster, then up the pitch. Example: Climb or go vertical = 10-4 Higher speed.. 10-8.

Do the math. I will use the 10-4 prop as an example. The 4 is 4 inches per revolution. AT 10,000 RPM, a 10 pitch will move through the air 4 inches for each rotation, or in one minute will travel 40,000 inches or .631 miles per minute or approximately 37 miles per hour. Double the pitch and, seeing as it's all linear, well, you will double the speed. But, with a given amount of power, you will take longer to get there (to 74 MPH). With the 4" pitch, you will accellerate faster, and have pulling power for a stronger pull to vertical.

Also keep in mind that some engines won't have the power to turn a 8 pitch prop at 10,000 RPM (the 40 LA may be one of those) so you will never see that RPM. This will show up as sluggish operation and a real lack of oomph when flying.

There are other drawbacks too. Decreasing the pitch will cause RPM problems with the engine. Your engine that normally would run at 10,000 RPM with a 6 pitch prop will definitely increase in RPM with the 4 pitch prop. This makes more noise and will cause problems with the engine if run that way to long. So, there is a trade off.

Bottom line: Yes, you can change the pitch (decrease it) for more vertical at the cost of speed and more noise and possible engine issues if the engine is not in good enough condition to handle the increase in RPM. Don't take the specs as gospel for engine RPM capabilities. Not to many off-the-shelf engines will really do 16,000 RPM and run reliably.. or run at all for very long for that matter. Stick with what the manufacturer recommends for prop size and experiment.

Thank you very much for taking to explain that. It all makes sense now.

My favorate prop for a trainer with an OS .40LA was the APC 10x5. I ran that engine/prop combo on a beater trainer for quite a while. (also, you can take the baffle out of the muffler for free power at the expense of a little extra noise).

CG's explination is really good, but keep in mind that the only way to pick a prop for a particular plane is to go get one and try it. Airframe drag plays a huge part of how a particular prop works on a particular engine in flight. This is why RPM measurements on the ground don't always tell the whole story.

Also, glow engines are not linear. Sometimes you can actually go faster in level flight by dropping pitch and not changing the diameter. The reason is that the lower pitch prop will load the engine less and you will get more RPM. Depending on the torque curve of the engine and other factors you might just get enough extra RPM to more than make up for the loss in pitch. Sometimes you can go faster by keeping the same pitch and going up in diameter, though that is less common. (if that happens to you, then you probably have a draggy airframe and were trying to prop it like a pylon racer).

One last thing, if you change props and don't notice an RPM change on the ground, that should tell you something. If you went from a larger prop to a smaller one, and the RPM didn't go up, you're probably redlining the engine already, and you should go back to the larger prop. If you put on a bigger prop, and the RPM didn't go down at least a little, then the first prop is almost certainly too small. At the very least, you aren't getting the most from the engine.

Thanks for adding that, Kirk. I usually preach that very thing.. only way to know is to try it and see. That's why I have a rather large box partitioned off with several props in each partition. I can grab one if I need one (if I break one.. hasn't happened in quite some time now since I perfected my landings) or if I want to change pitch or diameter for some reason.

CGr.