TD .09 with MVVS throttle and custom made muffler

On 25% Omega fuel and a Cox 7 X 3.5, this throttled TD .09 does 4K up to 14K with good acceleration.

For the longest time, conventional wisdom had it that small engines run too cool based on the percentage of fin area to displacement as compared to larger engines, as in .40s and 60s. As Cox engines like it that way and as Norvels have massive cooling fins and they run very cool in comparison and do just fine, this is a case where figures can lie.

No doubt the math is correct but I always had a feeling that air cooled engines, while they CAN run hot and this can hurt performance, running cool, too cool or cooler does no harm and can even help.
Note the engine sagging a bit till the ice cube is applied. As the ice cube makes it's way down to the cylinder fins, the rpms pick up and the engine sounds smoother. When the ice is taken away, the engine starts to sag again.

Yes, a muffler causes an engine to run hotter but if this engine ran too cool in the first place, the muffler supposedly should have helped.

It's been argued that I have a loose cylinder to piston fit and the extra cooling on the cylinder tightens this up giving more/smoother power. In fact, the opposite is the case here. The cylinder was obtained from Cox with the ports cut but without final honing to size. A homemade lap was used to create a very tight, tapered fit such that it mimics what you find with an ABC engine. As the engine wears in, power does increase as expected. This was common in the old days of getting a tight fit that you broke in to a just right, perfect running fit.

Yes, it's actually running kinda crappy but this one is still a work in progress. Will try lapping the piston/cylinder in place using coarse rubbing compound to take out some of the tightness. The idea is to sneak up on that perfect running fit.

My conclusion is that small engines are generally designed correctly and DO NOT run too cool. As evidenced by Cox's RC .05 and the RC .09 and my own version here, adding a muffler makes them run hot. Without the muffler, they're just about right.

No doubt that's why Norvels run great even with those massive cooling fins.

BTW, considerable testing in the air with a variety of props has proven to me that Cox props are THE most efficient props ever designed. None can match and only one comes close. Wish they or someone would come up with the sizes we need all the way up to .90s and beyond.

Category:

Howto & Style

Tags:

• RC
• Engine
• Airplane
• Cox
• TD
• .09

[GermanBeez]

aww, why do you castrate your TD like that? it has to breathe freely, without restrictions through rc throttles and mufflers...it's like a bird in a cage...

[hopeso]

@GermanBeez Well, because I fly RC. RC means radio CONTROL. Imagine if your Airbus had two settings, WFO and dead stick.

What you propose goes back to WW1 where the Sopwiths and Fokkers had no throttles. It was hell bent for leather or switch off to land. They controlled their descent by switching, or pulsing the engine, on and off. They got pretty good at it but we have glow engines that don't allow for that sort of thing.

Search for, Diesel Norvel .074 pulling my MiniSport

[crookedtool]

Nice job, it sounds like it has nice response. Is there any easy way to add a throttle to an engine like this? Such as a short tube w/ ball valve attached to the exhaust port with JB Weld.

Any suggestions?

[hopeso]

Yes, that's a good idea to try. Most Cox engines respond well to exhaust throttling and your idea is superior to an exhaust sleeve as is commonly used.

[hopeso]

How to do that, however, calls for some machine tools. A lathe is required to make the exhaust collector/muffler header. From there, you put your ball or rotary valve at the muffler's outlet.

The issue there, though, is that you can't use muffler pressure to the tank. If you did, at idle, fuel delivery to the engine would be excessive.

[GermanBeez]

@hopeso lol ok, point taken. i was just thinking that this engine was also commonly used in c/l flying, and if there wasnt a special third line attached for throttle control, it would also be flown at full blow until it was out of fuel.

[valic000]

Very interesting testings and theories..I suscribe to your channel.Thanks for clips!

[proanti1]

that is a cox .09, nearly all of them are torottled, and if they are not... you can use a norvel big mig .09 carb. there no no "exhaust port", there are 2 slits on either side. you can get exhaust sleves that already have the throttle system attached if you were to go that route. i custom made a sleeve out of brass on my lathe for an .010 and soldered a small brass tube on it that directed the exhaust through a little servo actuated brass ball valve inside the plane, then exhausted out the back.

[crookedtool]

Oh, I see now. Two slots on the sides.. that's tough. What about an air intake valve? Would that control the throttle as well as an exhaust throttle?

[crookedtool]

I don't know exactly what the exhaust port looks like on there, but I assume its just a little hole. My idea is to just fit the metal tube/valve directly to the hole and use JB Weld to hold it in place. Wouldn't that work about the same? I'm only interested in controlling the throttle.