As it came up in the other thread, I thought I would break this out of it, because it is more general that Motorman's tank thread.

   Almost all 2-stroke engines are designed to run at maximum power, that is, peaked out in a 2-stroke. That means all of them. Almost all of them will run in a 4-stroke if you make them rich enough. That doesn't mean that this can be made to work in flight. Running in a 4-stroke entails *drastic* fall off in power compared to the design condition (peaked out lean). Even a Fox 35. You can use this effect to some degree in some cases, and in all cases, running far below the peak design power also greatly reduces the effective fuel draw.

   Ideally, if you wanted to run an engine far below the peak, you would want *a throttle*, instead trying to run it insanely rich. That is exactly how RC people do it. Stunt people generally use fixed venturis - that is a throttle that is permanently set to only one power level. The smaller throttle opening increases fuel suction over "wide open but rich".

   There is absolutely no guarantee that any particular 2-stroke engine will reliably run far off-peak. What stunt people call "runaway" runs is just the engine working it's way to a point at which stable operation is possible. It speeds up until it is running at a stable point. That is not really a defect - that is a faulty understanding of how engines work.

    The infamous "Small engine experiment" started when it dawned on me why you could needle a 40FP to just about any speed you wanted on the ground, but as soon as you took off or did a manever, it sped up until it was going way too fast, even with 4" of pitch. In fact, what it was providing was almost exactly twice the power necessary to go the speed we wanted. So, why not get one that runs the same, but it half the size - a 20FP. Instantly perfect, flew Twisters and Banshees exactly the speed we wanted with it backed off just enough to get some "action" in the maneuvers.

    All the subsequent experiments showed very similar effects, you didn't try to force the power down by setting the needle so rich it was misfiring every other rev, you let it run as designed and then make small tweaks to get a beneficial in-flight setting. - with *all the stock parts*.

   There are two ways main ways to screw this plan up. First and most popular is to *not use the stock parts*. Putting in a erzatz 4 mm spraybar is 30% off the power. It will run better needled way rich that way, but only because the overall power is down so much. This means it *won't* fly a Twister or Banshee with any sort of authority - not enough power, or you are so gutted out just to get it to a decent level slight speed that it is dead in the maneuvers. Of course, now the engine is also permanently damaged so that you can't easily put it back.

   You can't make engines "better" by drilling holes in them.

    The other way is to put too big an engine on too small an airplane. Run as intended, and as it has to run, a 20FP is *far far far* too much power for a Ringmaster, Akromaster, Flying Clown, etc. We know exactly what it does in that conditions - 3.8 seconds a lap with an APC 9-4. Now you might be able to think of workarounds like much-lower-pitched props, but I challenged everyone to go look for 9-2.8 props and see how many you find. In fact, the 9-4 APC is intended to slow it down by running a smaller diameter that it could handle, to keep the revs up by being inefficient.

   These tiny "19-sized" airplanes were designed around things like a McCoy 19, Fox 15 (of Hurl Fame). A 20FP has astronomically more power than that, of course it is too big. You can just get away with a *15FP* if you are careful, if you are not careful, it, too, will be a complete bomb.

   Almost every engine we tried in the small engine experiment ran very well, when run as intended, with minimal or no modifications. The problem occurred with the larger engines, like the ST-32 - FAR FAR too much power for the sorts of airplanes we were using. It did work well with the throttle reduced by a large amount to get the speed in control, and allow a leaner setting near the peak for the throttle setting.

   You *have to* run these engines in a medium 2-stroke or faster in level flight, and count on the thermal effects to make up power in the maneuvers. It *will not work*, as a general proposition running in a 4-stroke, it will almost always "run away" - that is, run as intended - at some point in the flight.

    A third problem is that when you go to peak out your engine lean, a lot of old-timers will run up to you and try to screw it richer while screaming that "you are going to BURN THAT ENGINE UP!" because if you did it on a Fox, that was a real possibility. I have run at least 3-400 runs in a row where it was absolutely peaked out lean, including one run of about 5 minutes on gasoline and Wesson oil for an experiment. Runs as new. Same thing with break-in - I have taken brand-new engines out of the box, and the very first time it ran was an official flight at a contest. The only problem with that is that you have no idea how long it might run, or rather, it will be so rich by the end that it won't keep running.

   Brett

Hi Brett, I will assume you are correct, however if an engine setting is repeatable, and if it has the right amount of power, I'd be satisfied. Of course my la 25 and Mccoy 35 both run best as you described.  Okay, and my st.51. 

Quote from: Shorts,David on September 01, 2026, 04:16:41 PMHi Brett, I will assume you are correct, however if an engine setting is repeatable, and if it has the right amount of power, I'd be satisfied. Of course my la 25 and Mccoy 35 both run best as you described.  Okay, and my st.51. 

  I can assure you that you will not get a repeatable setting and the right amount of power with the sort of small modern engines I am describing, unless you do it more-or-less as described.

   Also relevant to this thread - the engines that *can* run reliably in a 4-stroke in flight have to have other methods to make it possible. For instance, the stock venturi/spraybar on the 20FP is *larger* (more choke area) than the venturi that David Fitzgerald used on the PA75 to win the world championship, and it also has tuned exhaust to keep the RPM and boost/brake in a favorable range. If you tried to put a venturi with proportional choke area on the 75 you would have 4-5 HP before it was stable to run, and it would be going 3 seconds/lap .

     Brett

Have you tried using FAI fuel in these smaller engines to help match power levels to smaller planes? 

Quote from: Motorman on September 02, 2026, 05:17:43 AMHave you tried using FAI fuel in these smaller engines to help match power levels to smaller planes? 

   I never tried that since it is so hard to find any. I did try 5%, and the needle was far too touchy and didn't make much difference in the power.

    Brett

Quote from: Brett Buck on September 01, 2026, 05:21:41 PM... For instance, the stock venturi/spraybar on the 20FP is *larger* (more choke area) than the venturi that David Fitzgerald used on the PA75 to win the world championship, and it also has tuned exhaust to keep the RPM and boost/brake in a favorable range. If you tried to put a venturi with proportional choke area on the 75 you would have 4-5 HP before it was stable to run, and it would be going 3 seconds/lap .

Well, I have a suggestion if he ever finds himself mysteriously low on line tension.

Quote from: Tim Wescott on September 02, 2026, 12:51:24 PMWell, I have a suggestion if he ever finds himself mysteriously low on line tension.

   It's a little big bigger now - but nowhere near 75/20ths times as big. Plus he is doing electric now.

  There's a reason that almost all good-running systems fall in a very narrow range of choke areas (.0175-.024 square inches) - because that gives you want you need in terms of power - real power by the engineering definition, not the "stunt power" , whatever that means. The power requirements of any competitive models in the last 70-ish years is in the range of maybe .35 to .6 hp. Almost any engine can do that, the trick is getting it to give the right maneuver response when the steady-state power is in that range.

     Brett

Just for chuckles I converted HP to Watts, and applied the "7W per ounce" rule from electric -- and the answers were 37 to 63 ounces.  So apparently slime engines and electric systems read the same physics books.

If the "slime" range is a bit low it's probably because that "7W/oz" rule is for power roughly calculated at the battery.  I don't know what the battery-to-shaft power efficiency is of a modern electric setup, but I have a notion that it's distinctly less than 100%.

Quote from: Tim Wescott on September 02, 2026, 07:21:55 PMJust for chuckles I converted HP to Watts, and applied the "7W per ounce" rule from electric -- and the answers were 37 to 63 ounces.  So apparently slime engines and electric systems read the same physics books.

If the "slime" range is a bit low it's probably because that "7W/oz" rule is for power roughly calculated at the battery.  I don't know what the battery-to-shaft power efficiency is of a modern electric setup, but I have a notion that it's distinctly less than 100%.

  There's really nothing too mysterious about it in the big picture. The devil is in the details, as always.

   You probably missed it, but in the Stuka Stunt days we had a very very long thread where Ted and I posted some results from what I thought was *very careful* ground testing, which were internally consistent to within a few percent - HP, torque, Static Thrust, simulated loads at various airspeeds, predictions of static thrust from different props, etc including backing out the level flight drag.  I may not be the sharpest tool in the shed, but I do know how to do engineering tests and check for BS results. It was also completely consistent the theory and with measurements that Randy Smith (and probably Scott Bair) took years earlier.

    The result - and endless circular argument about what "power" actually means, how "torque flies airplanes", and basically why were were all stupid eggheads who don't even know the basic "wisdom" of stunt. The culmination of this grand event was Ron Burn ( who was a really nice guy but typical stunt stubborn) showing up with a Wildcat/ST60 at Golden State. Usual Ron Burn construction, a work of art. He's over there flying it with a 13-6, it is going maybe 5.7 second laps, and never getting above an altitude of maybe 55 feet - despite the 70 foot lines. David and I go over, helpfully, to suggest he needs to crank it up about 500 RPM. But nope, we hear a lecture about how "torque flies airplanes" because Al Rabe says so, so instead, he gets a *14-6* that goes even slower. Besides, Aldrich always flew 7.0-7.5 second laps as long as he had the right prop. Official flight comes up, wingover only makes it to about 45 feet, and the tension is recovered just in time to permit an inverted pancake crash.

   I gave up at that point, people love their bullshit far more than they like reality. I relent (as above) when it seems relevant, but talking about "power" is something I avoid as much as I avoid saying "Fox 35".

    Brett
 

Quote from: Brett Buck on September 02, 2026, 10:47:28 PMThe culmination of this grand event was Ron Burn ( who was a really nice guy but typical stunt stubborn) showing up with a Wildcat/ST60 at Golden State. Usual Ron Burn construction, a work of art. He's over there flying it with a 13-6, it is going maybe 5.7 second laps, and never getting above an altitude of maybe 55 feet ------- Official flight comes up, wingover only makes it to about 45 feet, and the tension is recovered just in time to permit an inverted pancake crash.

      Brett

Hi Brett,

I had the pleasure to meet Ron Burn and had many long conversations with him regarding his construction techniques which he always willingly shared.

He attended a contest at Whittier Narrows.  I cannot remember the date.  He had that famous Wildcat.  I launched that Wildcat.  I do not remember if it was for a practice flight or an official.  I was reluctant to release the model because of the noticeable low RPM.  He insisted on the launch.  After several laps, he started what appeared to be an attempt to be a wingover.  The lines went slack long before it reached anything near the top of the circle.  If I remember right, it crashed almost nose first, totally destroying that masterpiece.

He has at least two other models of similar quality.  Some derivative of the Italian Macchi Folgore series and a Japanese Zero.  I never saw him fly either of these models.

Keith
 

Quote from: Trostle on September 02, 2026, 11:29:10 PMI had the pleasure to meet Ron Burn and had many long conversations with him regarding his construction techniques which he always willingly shared.

He attended a contest at Whittier Narrows.  I cannot remember the date.  He had that famous Wildcat.  I launched that Wildcat.  I do not remember if it was for a practice flight or an official.  I was reluctant to release the model because of the noticeable low RPM.  He insisted on the launch.  After several laps, he started what appeared to be an attempt to be a wingover.  The lines went slack long before it reached anything near the top of the circle.  If I remember right, it crashed almost nose first, totally destroying that masterpiece.

He has at least two other models of similar quality.  Some derivative of the Italian Macchi Folgore series and a Japanese Zero.  I never saw him fly either of these models.

   He was a nice guy and a great modeler, and subject to one of the greatest rip-offs in NATs history when his airplane got shoved to the back row in appearance judging just because it had a scale flat finish, complete with "not-flush" rivets. Just typically "stunt stubborn", one of Al's disciples.

    I think you are describing the same airplane at around the same time, he was absolutely convinced all he needed was a bigger prop, not more speed, and while he didn't wipe it out at the GSSC, it was clear that if he didn't do something different, it was going to get destroyed at some point.  He had gotten through parts of flights successfully in practice, but he went with the aforementioned 14-6 for his official flight, with entirely predictable results.

    I was very naive at the time, so was Dave. This was the first time I saw directly that giving advice was dangerous, because people would frequently go off and do the exact opposite, or at least, anything other than what you suggested, just so show they knew better. You can even use it tactically if you are subtle about it, and it is guilt-free because you actually gave good advice - but with full knowledge that whatever you suggested was the very last thing anyone would do.

     Brett

Quote from: Tim Wescott on September 02, 2026, 07:21:55 PMIf the "slime" range is a bit low it's probably because that "7W/oz" rule is for power roughly calculated at the battery.  I don't know what the battery-to-shaft power efficiency is of a modern electric setup, but I have a notion that it's distinctly less than 100%.

   It's probably not that bad - but you left out the other part, the prop efficiency. Which in general is very low for model sized props at the RPM we run them. 50% would be about the peak for modern 4-5" pitch props.

   As per yet another thread - you can use extreme inefficiency (in level flight) to help you burn off power and give you speed control. Until you really do run out of power.

    Brett

I found your Post 1 very interesting Brett and I think I have absorbed it correctly. You say using the needle valve to reduce ground power of an over-sized engine is not correct procedure, which makes sense. I also see a couple of times (your ST32 example and PA75 comment later on), if I have understood, you say/infer that reducing choke area say with smaller venturis, is acceptable.

So as an example, if I have an LA 46 (no smaller engines available) and I would like to use it in a Thunderbird or Nobler, is it "acceptable" practice to reduce the choke size so that power can be reduced while still setting the needle valve correctly (as in close to peaked out) ?

I know from reading on this site that many people do just that but is that practice a sound one compared to obtaining the "correct size" (smaller) engine ?

Quote from: redout on Yesterday at 05:09:04 PMSo as an example, if I have an LA 46 (no smaller engines available) and I would like to use it in a Thunderbird or Nobler, is it "acceptable" practice to reduce the choke size so that power can be reduced while still setting the needle valve correctly (as in close to peaked out) ?

I know from reading on this site that many people do just that but is that practice a sound one compared to obtaining the "correct size" (smaller) engine ?

  First and foremost, it is just advice and an explanation, not a law, so anything is hypothetically "acceptable", I am not the venturi cops!

    Of course, using a smaller venturi so you don't have to try to needle the power down is an obvious solution. The alternative method that everyone keeps trying, sometimes by accident, is using a much larger spraybar to the same effect, usually the accursed ST or ST clone. As noted, that knocks the power down by about 30% on the 20FP and makes it incapable of flying the airplanes I recommended. A fact that has gotten me so damn much grief and angry accusations of being a "anti-American con man" or something similar about once a week for the last 30 years.

   But note well - *you cannot make an engine better by drilling a hole in it* which is what you have to do to put in an erzatz spraybar.

    The entire point of the small engine experiment was to try to find a system that would sub in for a Fox 35 or similar junky engine from the dim mists of time, AND VERY IMPORTANTLY, find something that the less-than-expert fliers could reliably get to work with either no, or very minimal,  modifications.. This follows from many years of people carving up engines using some "secret hop-up tricks" that someone thought the knew, and destroying irreplacable parts.  This even goes back to at least ST46 days, where some reworkers, as their very first act, *drilled a hole through the venturi boss* to put a "through the middle" venturi. That effectively destroys an irreplaceable part. They aren't making any 20/25FP or 25LA crankcases any more, drilling a hole in it is an irreversible process.

   Add to that to people who, clearly, can't be trusted to even use the right screwdriver and destroying more parts - like muffler screws.

     So any idea that people should be taking their engines apart - at all, for any reason (other than maybe getting dirt in it) - tends to defeat the purpose. Because the instant people get it in their heads that taking it apart is required, the gates are open, and apparently even rather sharp people including world champions can't always manage it -  and now everything is fair game, get the grinder and a handfull of head gaskets.

    That the alternatives provided vastly superior in-flight performance was almost a lucky side effect.

     But of course, yes, you can match the power to the airplane to a certain extent by changing the venturi. That is exactly what we do with real stunt engines, and I have a big box of different types and sizes of venturis, which I have experimented with to get favorable power with favorable props. Note that every venturi I have ever run on the RO-Jett 61 is substantially smaller than the one that comes on a 46LA. Of course it can pull much more power out of the smaller venturi, even more so on the 75.

     So, you can adjust it that way, and it makes sense *if you have the discipline to carefully test and carefully evaluate the results* and know what you are looking for. And the discipline to avoid going off on tangents with all the other "helpful advice" to add a bunch of head gaskets, grind the ports with a Dremel tool or as we have seen repeatedly here, how to not destroy the screws using a Craftman Phillips-head screwdriver, etc.

   I would also caution that one of the *huge* problems I see with venturi experiments is to make wild, grossly excessive changes, from one that is way too small, another that is way too big, and makes it nearly impossible to make sense of the results. A difference of .005" in the venturi bore diameter is about the right kind of change and is distinct enough to know it changed and how, but not too big to put in a completely different regime that you can't make sense of.
 
     Lot of people have gotten their Nobler/46LA to "work" on some level. How well, that's a matter of debate, but at least flyable. Read Dennis Nunes' post about his Gladiator model where he did literally everything I don't recommend, and how long it took him to sort it out - and its' still really touchy, although very good when he hits the needle.   That was exactly what I was trying to spare everyone.


    Brett

Thanks Brett. I've never been tempted to drill or grind anything as I don't have the knowledge nor even the need to do it. For casual fun flying, I'm sure a stock LA 46 is not the limiting factor compared to the (or rather this  ) pilot's ability. I also didn't realise how incrementally small changes to the venturi size are so significant. Good to know.

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