rsieminski

eg. : O.S. .32 SX Non-Ringed w/Muffler
Output: 1.2 BHP at 18,000 RPM
Practical RPM Range: 2,000 - 22,000 RPM
Recommended Props: 10x6, 10.5x6, 11x6

The peak output is at 18K. Does this mean you should spin a prop and shoot for this RPM? Does it make sense to spin a prop @ 10 or 11k? It seems this would miss the peak of the power band? Even if the prop unloads by 10%--16.2K would be the number to shoot for. Why would the MFG recommend a prop that spins much lower than peak rpm? Any ideas or theories on this.
Thanks,
--Rick

DerFly

It's a bit like keeping up with the Jones', every manufacture wants to show his engine in the best light!

So sure, x.x hp at xx,000 rpm is great, but is it practical? How long will the engine last running at xx,000 rpm? How long before you lose your flying field because of the noise of running engines at xx,000 rpm?

It comes down to the choice of the user and my choice, fit a bigger engine, you can always use the throttle!

rsieminski

What? I understand about the bigger engine. But using the engine for max output is not abuse. I think the sx was originally designed as a heli engine, for higher rpm. My question is should you prop, with the intention of reaching the peak rpm. I know the MFG's all fudge the BHP a bit. From my very limited experience, throttle response seems better if I prop the engine closer to the MFG posted max rpm. ex. OS .15 LA , mousse pipe, 7x6 @ 16.3K ---throttle is crisp same setup but 8X5 @ 13.5K, retuned --throttle is less sensitive
OS says 17K is max BHP. So is it better to prop close to the peak output listed?

w8ye

My Feedback: (16)

There are a lot of variables involved in selecting a prop. For a larger model you would want more diameter and less pitch. On a small model you would want the smaller diameter higher pitch prop.

You should prop the 32SX for a higher rpm but 18,000 static is dreaming a little too much. Even 16,200 is a little out of practicality for an rpm read on the ground. For one thing to get there, the prop will have to be so small as to be very inefficient. The other reasons are what DerFly says.

Keep in mind that the engine unloads somewhat once in the air like you said. So a relistic goal would be in the high 14's or low 15's on the ground when you keep in mind what DerFly is talking about versus effeciency of the prop itself and the unloading of the engine in the air. This 18,000 should more likely be a never exceed speed.

On the other hand, if you don't get the rpm's up at least in the 14's, the 32SX would be just another .32.

If you want a small engine this weight and size, there are other alternatives like a LEO .37 PRO from www.justengines.unseen.org or a Megatech .37 and put a larger carb in it. The Thunder Tiger .36 has a big following too. This is considering what the OS32SX costs.

Good luck

Jim

rsieminski

So in general, the absolute max rpm is usually listed in their peak bs, err I mean BHP. It is a costly engine, but I've already got it. This was really a general rpm question? Larger props are more effecient. But in general using any engine as the example, is it more effecient to run towards the higher end of it's rpm limit? Do you loose effeciency by proping for 60% of the MFG's max listed? The end result would be to suck the most power out of the engine, w/o sacrificing longevity.

Flypaper 2

They should list the maximum torque in there adds. the best rpm is somewhere between max torque and max hp. If you could an old magazine with engine profile with torque hp graph you would see ideal prop is halfway between the two.

w8ye

My Feedback: (16)

In most cases you would be correct. But that deal of staying somewhere between torque and HP always applies. But some of the Jett engines have torque and HP only about a 100 rpm apart at around 15,000.

Jim

downunder-RCU

I'll agree that torque@revs would give a far better idea of the useability of an engine but the manufacturers all know that everyone's idea of how good an engine is has been formed from things like the old horse power race with cars where if one car had 300HP and it's competitor had 280HP then customers are more likely to go for the 300HP model.

I prefer to prop closer to the max torque figure because I'm not interested in all out speed of the plane. Of course, it's a bit of a guess where max torque comes in but even the manufacturer's figures for peak HP aren't worth anything either. Do any of them tell you how much nitro they used to get the figure they claim?

DerFly made a good point by asking how long it would last at those revs. To a great extent all engines have a life that can be measured by the number of revs it completes. Call it 100 million for argument's sake. So running it at close to the max HP revs will use up those revs much quicker than running closer to the max torque.

Any engine is at it's most efficient when running at peak torque because torque depends on volumetric efficiency. But a never exceed figure is a bit harder to come up with even though there's nothing to be gained from running faster than the peak HP revs. Even your OS says 22K is within the practical range, which basically means they're saying 22K won't cause any damage. I'd suspect it could happily run at 30K (with enough oil in the fuel).

rsieminski

Ahhhhhhhh#@!#$@! The MFG's are not making this easy, when they really could! Just post real life data. Instead they give HP X Fudge factor. They all don't seem to use the same fudge factor.
So when plotted, each engine will have a HP curve and a torque curve. And they are all different. This brings me back to buying 10 props to find which one works the best. And this "peak rpm" might as well be an arbitrary #.
I have a novel idea: Why don't they post their prop/rpm data --you know they have run dozens of props with each motor in their testing, and know exactly what to expect. Even the Honda has quarter mile data on the accord, how fast it'll stop from 60, etc.
Is there any way to find this mystical torque data out?

Jazzy

Hehehe... The journey for the information you are seeking will take you to the ends of the world and the depths of the oceans.

Mfg specs are generally not much use. I'm sorry to hear you spent so much on an engine when the same performance could have cost so much less.

The only way to know for sure what prop is going to work best for the engine is to experiment with different props, plugs, and airframes. My experience has told me that, for general sport flying, propping close to max torque while maintaining a healthy RPM provides the best all-round performance. Want more speed? Smaller diameter and more pitch. Want to hover? Larger diameter and a lot less pitch.

There was a pretty heavy discussion some time ago regarding RPMs and Torque. The max torque for an engine is usually several thousand RPM below max rated RPM. Some engines, as stated above, have very closely related max torque and max rpm.
Torque and RPM, (I forget the equation), = HP. Too much RPM and you sacrifice torque. Too much torque sacrifices RPM. Both instances however, may yield the same HP figures.

In essence, it all depends on what you intend to use the engine for.

Best of luck in your search for the Holy Grail.

w8ye

My Feedback: (16)

In the last couple years there have been several articles in Model Airplane News by a couple different authors about this very subject.

There's a formula:
HP=Torque times RPM divided by 5252.

This formula is run through all the torque values at the different rpms to figure the horsepower curve.

A pretty good measure for the laymen type is to use some fish scales behind the model with different props to see how much pull it has. This would be static thrust. But this has it's limitations because you have to stay within the needs of your airplane's design for a certain minimum speed (Pitch times rpm)

Jim

jaka

Hello!
Forget about hp ratings and torque curves if you are into sport engines like the OS .32...this is of no importance!!!
What is important is what plane you are going to use the engine in and what prop you are going to use for that plane...let me explain! The OS .32 engine is just a sport engine not a pylonracing engine and as such could swing a lot of different prop seizes ..for a low winged rather fast sport plane with a span of 120-130cm a 9x5 or 9x6 APC would be agood choise. For a bi-plane low wingloding and with a span of 120-130cm a 10x6, or 11x4 APC might work best.
Seeking max rpm is not what one should look for...instead one should look for what propeller does perform the best with that specific airplane.
In international pylonracing (6,5cc engines)we use carbonfiber props (made by ourselves mostly)that are 7" diameter and appr 7-8" in pitch...these propellers spinn appr 30000rpm + and gives our plane a top speed about 375km/h.
In my Kyosho Catlina flying boat I use 2 OS LA .2,5cc engines ( .15) and 8x5 APC props... this prop combination gives best performance and the engines rew appr 12500rpm ,slightly rich and performs beautifully with crisp throttleresponse, low noise and adequat thrust for watertakeoffs.
So do not prop for max power just trie different props and see for yourself what propeller works best for just your plane and engine.

Regards!
Jan K

Craig-RCU

I've had really good results by propping 10% below the maufacturers peak hp rpm ratings. I have a 1/12 scale Me-163 Komet with an O.S. 15fp on it. Initially, I used the recommended prop size of 8x4 (Master). I don't remember the exact rpms I got with that prop but it was around 13k ish. The thrust was not enough to lift the 163's 1.5 lb weight when I pointed its nose up to adjust the mixture. Then, I tried a Master 7x6 which tached at 14.5 k. During the same nose high mixture adjustment, I could definitely tell that thrust now exceeded weight by a good amount.

I also underprop my O.S. 25fp. Manufacturer prop recomenations are a 9x5 and I am running a 8x5 at around 14k rpms. This is giving the 2.75 lb plane that it is mounted in unlimited vertical too. One guy thought I had a .40 on it by the way it performed.

My theory as to why engine manufacturers recomend large props that won't allow you to get near peak hp rpms is that they feel that those who don't know much about engine tuning (most hobbyists) shouldn't be stressing their engines to the limit anyway. So, the conservative prop recomendations are sort of a safty factor. Also, as they say, "the candle that burns twice as bright burns half as long." Manufacturers don't want to have to deal with longevity issues with their products with these same inexperienced hobbyists. Me, I understand that I won't get the longevity from these engines that I'd get turning larger props at lower rpms, and I'm happy with that trade off. Noise is a factor too I'm sure.