I did a search and there is a lot of information on what prop to use with what engine but I am curious about something. I notice the recommended prop sizes by the various manufacturers of say a .46 and a .61 are almost the same. It seems to me an engine with 50% more displacement and almost identical design elements should be spinning a lot more prop. What am I missing? I have a lot of props to choose from but want to make good choices.

I think the prop recommendations by the engine manufacturers try to cover a lot of different conditions.
If you run the same prop on a 46 and a 61 you will get very different results. The 61 should turn the prop at a much higher rpm than the 46. Some areas have very tight noise regulations. A good bit of the noise our models make is prop noise. Running a big prop on a 46 will reduce rpm, which reduces exhaust noise and prop noise.

Your prop collection looks a lot like mine. I have practically every 12, 13, and 14 inch size that Master Airscrew and APC make; plus a good handful of 10 and 11 inchers.

After trying a bunch of props, I really only use 3 sizes:
.61 two stroke = 12x6
.70 four stroke = 13x6
.91 four stroke = 14x6
My planes are a trainer and some sport planes. The 6" pitch seems to be a decent compromise between speed and tyhrust for the way I like to fly.

In the end, you just have to keep swapping props until you find the one that suits you. Everyone else can point you to the one they like (see above) but you might hate it.

Prop sizes are usually recommended sizes for a given engine and are to be taken with a grain of salt for the application. You may want something with more pull but less speed. Then something with less pitch than a 'recommended' prop may be your choice. Or, more top end speed, with less pulling power.

Think of a prop as a transmission in a car. The lower gears will give you more pullint power, but much less speed for a given RPM. Then, the higher in gears you go, the more speed for a given engine RPM but less pulling power.

Pick a prop in the recommended range, try it out. If it does what you want, stick with it. If not, and you want more pull... lower pitch. More speed... higher pitch. All of this for a given diameter prop, of course. Things will change as you change the diameter of the prop, but with the general same results.

Bottom line, experiment and pick what performance you want and go with that.

CGr

Works for me. I am an old school student of use the biggest engine you can and keep the rpms low to avoid engine wear based on my automotive experience but in relationship to R/C planes I may be all wet. I do however see reports of persons trying small high speed props and end up shearing off the crank pin. I like the idea of a large engine grunting along getting the job done (just recreational flying no 3D or all that jazz) turning out torque to pull the plane and a big prop to grab the wind. thank You for the input. [8D]Jeff

Yeah, I closely monitor my RPM. for instance, my OS 1.20 AX engines never ever are allowed to go above 10k. I turn a 15-10 prop with no problems and that pulls the plane along nicely. Peak RPM around 9800 with that prop. I also may use a 16-8 if I want a little more pull. That peaks just at 10k.

The smaller engines.. never above 11K or so. And, I always run them on the rich side of lean. I'd rather clean the oil off of the wing than flakes of metal. They fly just fine with that RPM range, with the proper prop, that is.

CGr.

G'day Dick,
Well said, it seems most people on here MUST have their engines running at maximum plus a 1000 RPM, before they think the particular engine is any good, most engines 2 stroke or 4 stroke have an ideal RPM, & the engine should be propped accordingly.
Bravo Dick, me old mate.
Hope everyone had a wonderful Christmas, & will have a flying new year.
Sorry to hear about you weather over there, it's 4.55pm & 36 celcius here now, you better get on that plane & get over here real quick.

Works for me. I am an old school student of use the biggest engine you can and keep the rpms low to avoid engine wear based on my automotive experience but in relationship to R/C planes I may be all wet. I do however see reports of persons trying small high speed props and end up shearing off the crank pin. I like the idea of a large engine grunting along getting the job done (just recreational flying no 3D or all that jazz) turning out torque to pull the plane and a big prop to grab the wind. thank You for the input. [8D]Jeff

Thanks, Allan. Yeah, it's been raining for the past couple of days. Then we had the wind, and the cold, well, it's just not 39c here by any stretch of the imagination.

Yep, I was taught that noise does not equate to power. Making them scream is just not the way to try to sqeak power out of them. Oh a person could do that, alright, but as one person noted with his Evo, he blew it up running it to lean which usually equates to higher RPMs and and unsafe engine.

We will certainly try to get that plane going.. perhaps if Bruce reads this, he can get going on that..

Have a Happy, Safe, and Prosperous New Year, my friend!!

Dick.

It is obvious that .46 and .61 engines don't use the same size prop. Clearly you are having fun experimenting with props. Don't stop now?

On a .46 I start with a 10 x 7. On a .61 I start with a 11 x 7. With a few flights and a handful of props you will arrive at the correct size prop for your setup.

Bill

Yea I will of course go with the recomended size first but my concern was towards why there isn't a bigger difference . An inch of difference when you go up %50 in the cubes? [] Here's a photo for some perspective. They don't have the same profile prop but you get the idea.

Jeff. What is the difference in pitch? The second number? That can make a big difference in the power requirements.

Just checked Tower specs for a TT .46 and a .61 Max prop size respectively is and 11X6(.46) and 12X7(.61). It just doesn't seem like much difference for half again as much engine.

I realize that.. but the difference can be significant. You don't, definitely don't.. want to go up to a 14 inch prop. The 13-4 or 13-6 may work out, but it becomes a matter of experimentation. I don't recall what I was using on my OS 61 then my Super Tigre 75, but they were the same prop, as I recall. But, not the same as I was using on my OS 46 or 50.

I use an APC 15-10 on my OS 1.20 AX but this engine has much more power than even a 90. Much more.

I know this doesn't quite answer your question, but the answer is not specific to your engine/plane setup. Every one is different and will need a different prop.. and add to that, your altitude and weather conditions.. and all sorts of things begin to change.

CGr.

Oh I wholeheartedly agree that each application is different and it should be a matter of matching engine power range, intended application, and probably weight as well as airodynamic drag but who can really say how to compute all these factors on the ground. I am a firm believer in "try everything and decide what you like from there" You should have seen my gunpowder selection when I used to handload. Would have been a similar type photo to my prop collection photo.

Several interesting questions.

Long life on an engine has more to do with staying on the rich side and operating in a clean environment than RPM. If anything, low RPM is harder on the engine than higher. If you insist on lower RPM operation, use additional oil. Low RPM operation also makes the needle setting much more critical.

As to the prop size difference between 40 and 60. An 11" prop has 21% greater area than the 10", while at the same rpm has a 10% greater tip speed. When you increase speed by 10% it takes about 30% more power. Multiply the two factors (1.33 x 1.21 = 1.6). Not an exact equation and certainly not the actual equation, but close enough to give an idea of why.

Aircraft fly on horsepower. The rate of climb is directly related to hp, while the speed is related to a cubic function. The trick is to find a prop that allows the engine to develop the power while effectively transferring that power to thrust. At best a prop is only 85% efficient in transferring power, and can be lower. It is usually best to set up an engine for sport flying to operate at a RPM two to three thousand below where it is rated for peak power. This allows the engine to unload and operate closer to peak power.

Thanks everybody for all the help on this[sm=thumbup.gif].
I do have a lot of experience with automotive racing engines and I don't want you to think I am a total maroon . I was taught that engine stresses increase at the square to RPM so I was thinking that if I could keep my revs low enough , that cheap .61 I bought would give me a decent amount of service life before becoming a paperweight. I am still coming to terms with an engine turning 15,000 RPM without doing a grenade imitation[:@]. All that with an unsupported crankpin is just awesome and todays engines are really a marvel to me.

My all time favorite was IMR 4350 for rifles (25 caliber) and Bullseye for my good old Colt 1911A1 Gold Cup. However, I too had a large collection of cans of powder. I also had just about every Federal primer available. Quite a collection.

Back on topic... Yep, I really cringe when I see someone under-prop an engine at the field, then claim it's so much better and faster with that noise maker turning 15K+. I've never seen one go to lunch, but I imagine it ain't pretty.. especially at that RPM. So, I tend to work with the lower RPM range to get performance, even though it's not in the power range stated in engine specs.. which I feel is somewhat overstated.

CGr.

Big difference in automotive vs model is the pressure oil system of the auto engine. Although the rpm on the model engine is much higher, oddly enough the piston velocity is roughly the same. For truly jaw dropping performance, look at F1 car engines. They run around 20,000 rpm and make incredible power.
http://www.f1complete.com/content/view/1283/389/

Here is an actual example of engine failure. Mostly due to an engine modification known as "a knife edge rod". It's suposed to lower the internal drag, but what it is prone to do is concentrate loads on a notch and propagate a crack. I didn't buy any more engines from that tuner. This was on an engine that ran at around 26-27K in the air. The 6.5 indicates that it is a .40 sized racing engine. This is the only engine that I've had grenade in the air in over 40++ years.

Long life on an engine has more to do with staying on the rich side and operating in a clean environment than RPM. If anything, low RPM is harder on the engine than higher. If you insist on lower RPM operation, use additional oil. Low RPM operation also makes the needle setting much more critical.

jeffie8696,
First off, sorry for jumping in on your thread. I am still pretty new to this hobby, and trying to learn as moch as possible, mostly from reading alot on here.
HighPlains,
Can you please explain why low RPM operation is harder on an engine? Does it hurt the engine to fly around at half or even lower throttle? I like to do a few touch and goes, and also practice flying low and slow. Please believe me, I am not second guessing you at all. Just trying to learn, and I have never heard that before. I have two OS 46s, and run them on the rich side, put I too am starting to experiment with different props. Was planning on a max of about 12 to 13,000 rpm on those, but sounds like that might be too high?
Thanks,
Pete

I believe (could be wrong) that he was referring to overpropping where the max rpm is too low, and the main needle position is overly critical, same as running a car in 3rd or 4th gear continiously, lugging it.

IMR 4895 would be the powder I chose if I had to only choose one. Works from .223 to even the .30 range and isn't too sensitive to low recoil mild loadings for practice. Don't get me started on the reloading scene, I have been out of it for a while and I may not be able to stop babbling. Oh and Bullseye rocks, I don't care how old the formula is!

Bingo, bingo! Couldn't help myself. I suggest more oil because the rod bearing loads are fairly high in a lugging mode, while the oil film is thinner. And for the record, the RPM's that the retired Coastie uses are not lugging, but he is going to be way down in power. He makes up for it with a larger engine, giving enough torque to turn a bigger prop.

Mesa, I too like to fly at lower power setting part of the time. This mode of operation is where a ring'ed engine is best. On the other hand if you enjoy boring holes though the sky at higher speeds, the AAC, ABC, ABN style of engine works better.

On that size of engine (35-45), I usually target 12-13K. You can often improve the performance of any model by doing a lot of prop experiments. If you enjoy the vertical possibilities, the larger diameter and lower pitch adds climbing ability. I usually drop pitch first, so instead of a 10x6, a 10x5 might add quite a bit more performance. While you lose some speed due to the lower pitch, you may gain more than enough rpm to fly even faster due to the increased rpm and power. Or you might go from the 10x6 to 11x5 and find the additional climb and lower landing speeds meet your needs.

There are really no hard fast rules here, every model and engine combination requires a half dozen to dozen flights with different props sizes to find the one that works best. Even when you get it down to one or two that work well, changing prop brands in the same sizes may improve things. Or not. Kind of like a trip to the eye doctor when he has you comparing lenses. Also the conditions make a fairly big difference. What works well at sea level might not be so great when you take a trip to the mountains. Hot days Vs cold days, etc.

The main thing is to experiment. And that's just the engine, throw in the engine thrust line adjustments, models balance and throws, and general trimming it may take 30 flights just to get an acceptable flying model. Anyone who has picked this as a hobby will have a totally open ended commitment to new things to learn.

I used 4895 but prefered 4350. The main reason was that at 55.4 grains the 25.06 casing was filled to the base of the bullet. Made for very consistent shot to shot. I had a .66" group at 200 yards, 10 shots. That was a good day.

I also had a custom 257 Ackley Improved that I really enjoyed shooting. It was a wildcat that took the 257 Roberts and fire formed it in the chamber. That too shot very well with 4350.

CGr.

Man did WE get off topic, on my own thread even. [sm=lol.gif]