Engine basics > Four strokes only
OS 40 Surpass assemble
Dan McEntee:
--- Quote from: Brett Buck on December 18, 2023, 12:22:04 PM --- That's something to try, Igor Panchenko was the first guy to explore that. But I am curious how you came to that conclusion, since the only real success with 4-strokes has been with ~4"or so of pitch and 10,000 rpm, going all the way back to the mid 80's and Ted Fancher.
Brett
--- End quote ---
I never got any consistency out of my Saito .56 until I subscribed to the Bob Reeves set up and some one else's recommendation to forget uniflow and use standard vented tanks on muffler pressure. I messed with it for a season and better part of another then hung it on the wall. Mine has the smallest UHP venturi , and I drilled through one side and tapped for a long nylon screw, a 10-32 I think, might be 8-32. Run the engine at a high setting and close down on the screw to restrict the venturi. When the RPM drops turn the needle in until the RPM comes back up, then repeat the process until the RPM drops off after going over 8,000 RPM or so with a 13-7 prop. On my 70 some odd ounce Top Flite Score, this worked out from the first flight. Before, I either didn't have enough fuel for the patter or the lap times screwed me into the ground. I learned to play with the restrictor screw a bit more and found a sweat spot where I could short tank it to between 3.5 and 3.75 ounces for a pattern in warm weather, and then just fill it for cooler weather. I never had a bad run, plenty of line tension, and on a heavy model that big was still able to fly a 5.3 lap time. I let Gilbert Berringer fly my Score at Oshkosh, after asking him about what size venturis he used and was told 6mm. He machines his own props with a special pitch ,but I think he used low RPM also. He gave me two thumbs up on it after flying it. I have picked up my fair share of trophies with it, but what I valued more is good comments from people and pilots I respect on how good the engine runs were, and some had never heard one run like that one did. My tach reading is usually between 7,500 and 8,000 RPM depending on the day, and I bounce back and forth among 13-6 and 13-7 props. The most notable thing about it is my engine still looks like new, just the muffler gets crusty looking. I take that as a good sign that the engine doesn't get hot other than the muffler. Berringer asked me about how much time on the engine and when I told him 2 seasons with out being cleaned up, he was impressed. Fuel is Powermmaster YS-20/20. The model is in bad need of a recover since the china kote is coming off, but after that I will bring it back out and maybe build a specific model for Saito .56/.62 engines. I experiments with this just out of curiosity because I like engines. I never won the NATS with it but was very satisfied and impressed with my results, but I wasn't moved enough to sell all my Supertigres and other engines!! I find it as just another acceptable option for a stunt model power plant that we have available to us.
Type at you later,
Dan McEntee
Russell Graves:
--- Quote from: Brett Buck on December 18, 2023, 12:22:04 PM --- That's something to try, Igor Panchenko was the first guy to explore that. But I am curious how you came to that conclusion, since the only real success with 4-strokes has been with ~4"or so of pitch and 10,000 rpm, going all the way back to the mid 80's and Ted Fancher.
Brett
--- End quote ---
Hi Brett, I came to that conclusion several different ways. Some RC guys at the field fly 3D style big pattern planes, hovering the plane vertically like a helicopter. They are using big four stroke motors running what sounds like maybe 3000 rpm at that moment, and big 21 x 12 props. It's interesting to see the low end power of a four stroke to hover the big heavy plane. I was thinking about that combination for stunt. Then I asked someone locally who flies a Shark, what prop he's using and he said 12.4 x 5.5 three blade. He didn't know what rpm his electric motor was turning, but to fly with a 5.5 pitch it must be around 8500 rpm. I also saw someone flying an F2B plane with the Discovery Retro 68 and he's using 13.8 x 6, again down near 8000 rpm. I really like the smooth flying of this engine speed on both planes. Then I was reading a bunch of articles here on stunthanger and I think I saw some people writing about using 6, 7, or even 8 pitch props on a four stroke, which means they have to be running down between 7000 - 7500 rpm. Then I saw some tourque curves for some four strokes, and yes the tourque curve is strongest between 7000 - 9000 rpm, and it goes down as the rpm goes above that. And finally I recall the stories you shared on the other thread about the Saito 82 causing weird plane vibration problems in some people's planes. I know they were running the four stroke with a high rpm with a 4" pitch. So with all that in mind, the evidence keeps pointing me to low rpm and high pitch is the way to go.
There was also some discussion about the benefit of a four stroke having 1/2 the "pulses" of ignition that a two stroke has, and somehow that is supposed to be an advantage from some gyroscopic reasons that I don't fully understand. I am curious to see if I feel any difference between the Saito 62 and the Discovery Retro 68 flying the same rpm and maybe even the same prop. If this is true about the "pulses" or whatever they are called, then there would be even fewer of them at 8k rpm than at 10k rpm.
To test all of this, I have a Saito 62, Saito 82, OS FS-40 Surpass (not for F2B) and one other four stroke for F2B that I won't mention at this time. I have an Impact plane as a flying test bed. It weighs 75 ounces so it will be a great test for the engine's pulling power. The 62 is my first test. I need to get it broken in and installed in the plane and start working with it. I've been slow to get it in the air for many reasons. I figure in late January I should have the time to start working again on this project. I plan to write a post as an update on my adventures with all of it. I'm learning a lot and having fun with it so far. I can't wait to get it in the air where the real testing begins.
Russell
spare_parts:
The 3D RC guys are hovering throttled back with very low pitch props. This allows max HP at full throttle. Typically, full throttle is only used to punch out of a hover. Flown level WOT, it will overspeed. For example a Saito 82 would run an APC14x4W.
If the prop requires the engines to be throttled to limit RPM (torque), you obviously aren't using full potential.
Engine power curve and prop thrust need to be matched to the airframe and desired performance. So it's quite possible for a given aircraft that a prop which winds out the HP just won't be the best prop. As prop data is difficult to find, experimentation with props is the best way to determine what works. IMO props matching the airframe well, should perform anywhere in a four strokes RPM range from peak torque to peak HP. I think what's given up in HP with larger diameter/pitch is gained in prop efficiency. I've been meaning to test this for a long time......
Running a four stroke at peak torque means it needs to be quite rich. Make sure, not to lean too far or you'll be testing the double prop nut's ability to retain the prop. Throttling helps reduce this problem.
Dennis Toth:
Russell,
I like the low rpm/higher pitch with the 4strokes. There was some discussion about a progressive pitch that being low at the hub, high through the middle and slightly lower at the last 15% at the tips. Some have done just high out to the last 15 -20% then lowered the pitch 5 - 7%. I like wide blades but have also tried long skinny one too. You have to match the ship.
Best, DennisT
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