Jim.Thompson

I have a 26 cc converted 2 stroke industrial engine, fitted to a new plane.
I have it fitted with a 17 x 6 propeller. (Wood Turnigy brand).
Running on 98 octave gas with Stihl Ultra oil at around 33: 1 mix ratio.
I have read 8600 RPM using an optical tacho while the plane is tethered on the ground.
This seems a bit high, but I am relatively inexperienced with small two stroke gas engines around this size.

Is this engine under propped? Would it be better to try it with a 17 x 8 prop?

Jim.

mgnostic

It really depends on the particular engine. My 26 cc Zenoah is happy with an 18 x 6 prop. My converted Craftsman happily pulled a 18-10 prop. In fact it seemed to like a heavier plastic prop. It was still runing strong after 15 years when I sold the N-17 it was mounted on. Rather than saying this or that size of prop I would suggest propping to top out around 7500 rpm. With a converted rather than a purpose built engine you may just have to experiment to see where it seems happy.

Jim.Thompson

Thanks for the reply.
That all makes sense to me.
I will try a larger one.

Jim.Thompson

Hi Matt,

I have fitted an 18 x 6 propeller and tuned the engine up. It peaks at around 7500 RPM, which is what you suggest.
I will now do a static thrust test (when an assistant turns up for safety) and post back.
Thanks again.

Jim.

Jim.Thompson

The static thrust test resulted in 6.0 kg thrust.
The exercise was a success!
Ready now for flight testing in a couple of weeks.

mgnostic

Just curious, what will that engine be pulling around?

1967brutus

For the prop and rpm that sounds about right, but for a 26 cc twostroke, it's a fair bit on the low side, I get similar performance from a 26 cc fourstroke not known to be a power monster.

Normally, these industrial 26 cc engines should have no issues with 10K, and in general they peak power higher up in the RPM range.

I would strongly suggest to repeat that pull test with the earlier fitted 17 x 6 prop as well, my propcalculator suggests your engine delivers almost 20% more HP on it, it will most likely have about 1 kilo MORE static pull, and airspeed will be better as well, which is a factor when towing gliders.

These numbers suggest that your engine with the 18 x 6 is running way below its powerband and performance-wise prefers to rev higher. An educated guess would suggest best performance around 10K (based on general experience AKA gutfeelz)

Of course, that will come at the cost of higher fuel consumption (not really important) and an increased noise level (possibly very important). No idea what the local regulations are, so that can be a consideration.

Jim.Thompson

Do you mean, what will it be towing?
Or, do you mean, what aircraft is it powering?

"pulling around"???

I understand the logic Bert. However, what prompted me to try a larger diameter prop was the fact that I had trouble tuning the high needle setting at max throttle.
My local professional engine rebuilder and small engine mechanic suggested this was the reason; it was not loading up sufficiently.
This man has been my willing and patient guide throughout every one of my two stroke conversions, and small engine challenges.
I have learned most of what I know about them due to him.
Accordingly, I listen to everything he says.

However, based on your very cogent reasoning above, I will go back to the 17 x 6 propeller and see if I can tune it, if I am able to do so, I will do another static thrust test.
More later.

Thanks again,

Jim.

Jim.Thompson

Do you mean, what will it be towing?
Or, do you mean, what aircraft is it powering?

"pulling around"???

I understand the logic Bert. However, what prompted me to try a larger diameter prop was the fact that I had trouble tuning the high needle setting at max throttle.
My local professional engine rebuilder and small engine mechanic suggested this was the reason; it was not loading up sufficiently.

However, based on your very cogent reasoning above, I will go back to the 17 x 6 propeller and see if I can tune it, if I am able to do so, I will do another static thrust test.

Thanks again,

Bert.

Jim.Thompson

Bingo!
You are on the money here - again Bert!
I fitted the 17 x 6 propeller again. This time I saw the following figures:

1. RPM around 8600 - 8700.
2. Static thrust was around 10.5 kg!................Bouncing around a bit, but certainly over 10 kg!

The aircraft is under 5 kg. (I will weigh again when next I fit the wings.).

Wow!

Many thanks for your wisdom!

Jim.

1967brutus

I did NOT expect a 10 kg pull... But if so, the better...

mgnostic

I meant powering, lol. Most aircraft being pulled through the air by their propeller. Unless of course, it is a pusher.

Jim.Thompson

This aircraft shown in the attachment.