Pete55199

I copied this post over from the ARF forum, thought it might fit here better.


Hi everyone, this is my first post here on RC Universe.

It's been a rainy day here in Alabama, thanks to Tropical Storm Fay, and classes at Auburn have been canceled for the day, so I had some spare time on my hands. First off, I have a Hangar 9 P-51 Mustang PTS, which I fly as often as I can. It's powered by an O.S. .46FX with an 11x6 prop mounted. I started looking at the motor this morning, and wondered, "What would happen if that thing had a ram air scoop on it?" That question immediately led to a resounding "Well let's find out!" I got out a spare sheet of balsa, some epoxy, and a spare piece of MonoKote and went to work. Here's what I ended up with:

http://s138.photobucket.com/albums/q...r%20Induction/

Looks like it turned out ok, but I've never seen this done before, and have no idea if it'll make any difference or not. I'm certain that I'll have to richen up the mixture to avoid burning up the engine. But I was wondering, has anyone ever tried this before? If so, what kind of results did you get? It's raining outside right now, so I won't get a chance to test it out today. When I get that chance though, I'll be sure to write in with some results.

Thanks for reading!

~Pete

jeffie8696

Post before and after tach numbers please please!!

jeffie8696

Post before and after tach numbers please please!!

da Rock

You know, you can post photos right in each post. Don't need photobucket, and your readers see the picture with the text. It's much more convenient for everyone.

JustErik

Or he could just post the image code.



Some folks, myself included, like to store photos (or other data) at a central on-line location like photobucket or whatever. It makes it easier to access them from various locations. Also, it frees up resources here at good old RCU.

To keep this on topic, I too would be interested in seeing the results from this experiment.

Stew99

Unfortunately, when you post photos offline I can't see them because our network blocks access to personal storage sites. I'm sure there are others with the same problem.

GRANT ED

I seam to remember someone else trying this a while back. I think it didn't make any difference but let us know how you go. You might get a better result.

jeffie8696

Is There any reason it wouldn't work? I did a web search and found that supercharging a 2 stroke doesn't work but I don't see why a slight increase in intake charge density would not result from this. It may only be slight but I hope it is enough to verify.

da Rock

You read that supercharging a 2 stroke doesn't work, and the same reasons apply to increasing the available air at the venturi.

Supercharging is really just getting more gas (fuel and air mixed in the proper ratio) into the combustion. And it actually does work in 2 strokes but only in a limited way. Tuned pipes do exactly that, increase the amount of gas in the combustion chamber. But they work with the gas after it's become gas (been mixed).

Your increased air isn't needed. What's needed is to get more gas through the front of the engine.

da Rock

Supercharging in full scale works because the engine encounters less and less air as the airplane's altitude increases. We don't need to find more air for the carb with our models.

And atmospheric pressure is what makes our intakes work. That pressure is a much greater force than a bit of airflow prepping can provide.

Keep in mind that our intakes are closed for a great amount of each rotation. Flow stops but your ram air won't.

Jezmo

There is a darn good reason why some of the free storage sites get blocked at the gateway. They sometimes offer up more than just the photo you are trying to view. (Read drive-by downloads of crapware.) I have them blocked on the networks which I administrate for just that reason.

Bob Mitchell

Speaking in general here about IC. IC engines are effectively air pumps, and the amount of power generated is proportional to the amount of air one can move through the combustion chambers. It's easier to move fuel through IC engines than it is air, so they are air limited, rather than fuel limited (in general). That's why power adders are designed to push more air (oxygen) through the engine, ie. superchargers, turbochargers, Nitrous, etc.

da Rock

The gas mentioned is the combination of the air and atomized fuel from the spray bar. And that gas has to be moved from the venturi where it's created, into the crankshaft opening that only opens intermittently. Our simple 2cycles don't move air by itself anywhere in our engines. They move gas. They have to move a fuel/air combination from the point of combination, and that move starts right where atmospheric pressure is acting to both combine the two components and to provide the muscle to move the combination into the intermittently opening shaft induction port.

Our engines aren't general IC engines. They're very specifically configured ones that have a very, very simple carb slap up against the "internal" part of our IC engine. Bigger IC engines really do have places where air can be pushed around before it gets into the engine. With out little engines, the area needed for carburation is more than adequate to move more air than is usually needed. The design of our engines actually limits what can be "rammed down the engine's throat". Our engines usually benefit most from the other end of the cycle. Tuned pipes have worked with model engines almost from the start. Why? Because the front of the engine can quite easily provide more than it's being asked to provide, and simply does so when the tuned pipe creates the demand. But try to force feed our engines and from the carb on back, there isn't the same easily provided increase in capability.

da Rock

BTW, if it was simply a case of getting more air into the carb, a larger opening in the carb would work too.

That actually does work, but only to a very limited extent. Very limited. And the limitation isn't on providing additional fuel flow, because that is even easier to do. Our needle valves aren't anywhere near their maximum opening. And we can easily provide appreciably more pressure to the fuel to provide that a much greater rates.

jeffie8696

I have had good luck polishing the inside of the crankshaft and crankcase to get the flow moving so I don't see why a little extra air pressure wouldn't help.

da Rock

If you feel it will, have at it. Best thing about our hobby is that we can do just about anything we wish and test to see the results.

If you really want to get a measurable benefit from trying to hop-up our 2cycles, remove the corners that the intake gas flow sees going into the cylinder. The rest of the engine really doesn't have much that hasn't been optimised or matched to all the other dimensions and such.

As for why "a little extra air pressure wouldn't help"........ You're very often not actually providing anything that's needed and can actually be used effectively. Very often, modifying the intake unbalances a layout that's been optimised. Working with the intake side of our designs to get more power more often than not simply makes the engines harder to handle. Keep in mind that the intake function is really about air blowing through a hole. The system in operation in that hole is really pulsing and not the highspeed thing a lot of people think. It's actually blocked for more time than it's open. What happens then can actually be disrupted by "supercharging".

If you want to actually have some predictable effect on the intake side, start recording the density altitude of each flying session and go from there. You'll notice that changes from day to day give more significant results than whatever you're changing up front. Of course, you'll absolutely need to be accurate about your testing and that means sticking with the same prop, fuel, test strategy. With almost all our engines, our efforts at the front of the things CAN help, but it won't be much unless more is done throughout the whole system.

It was a workable strategy back in the old days to have different ID venturis that would be swapped to match conditions. The conditions of the day mattered, and what you were also doing to the rest of the engine mattered too. And some guys worked it all from working out the density altitude first thing.

Hey, it's kewl to do this stuff. Have at it. It's a lot of fun trying to make this stuff better.

BTW, it would be a very good idea to see if you can find "2-STROKE GLOW ENGINES FOR R/C AIRCRATE" by Dave Gierke. I think it's one of the Model Airplane News publishings. There are a couple of decent 2-stroke books around.

da Rock

BTW, while you're working with the nose of the engine, there is a trick that works when it's needed.

Have you noticed fuel residue baked on the front of the cylinder? If you have, the trick might be useful. Of course, you might also have bad seals and a leaking front bearing.

Watch the intake area closely while running the engine. Watch closely in the intake area itself at both idle and WOT and in between. Do you see any raw fuel spitting out? It does that when the induction port closes off. The gas has just built up some momentum heading into the engine and BONK the hole closes and you get kickback. First thing to do is insure that it's not simply a bad needle setting. Check for too rich needle and too rich idle needle.

Theory says the venturi can be lengthened and it'll solve the problem. Actually, the safe theory says the venturi is too short and leaves the rest up to us.

If this was happening, then you've also got a situation where a ram air intake just might solve that problem. It also might simply blow the rebounding gas out and away somewhere else.

Another trick that is going to possibly be useful.............. Usually, when what you've done with the intake side of the setup starts to work, you'll find that the needle setting will become more critical. The needle setting range will narrow. And if you're messing with a throttled engine, you'll probably also make the engine less dependable on the low end. Intake improvements to the high end usually work against the low end. Unfortunately, when you've gone too far, the needle will also be super critical.

VeeAte

4 stroke will take to forced induction a lot better than 2 stroke.

Something to do with induction and exhaust ports both being open for a small period during the intake stroke.

This looks cool. I forget when I found the picture or who it is that made it. But even if it didnt work well it sure looks the goods.

JoeAirPort

My Feedback: (41)

Yeah it does look cool. Wonder of it works?

VeeAte

I am pretty sure I found these pics on here some years ago.

I will do a search later tonight and see if I can find it....

JohnW

My Feedback: (6)

I recognize those 4C supercharger pics, and I believe it showed promise. Interesting project to keep one entertained; however, there are several production 4c motors that use supercharging, so not a feasible project for most peeps. If you want a supercharged 4c, just buy an engine with the manufacture designed supercharger. On the OPs modification, like others have posted, you will see considerably more gain by installing a tuned pipe and altering the exhaust port timings. I’d doubt any measurable gain would be seen by the ram setup in the pics, but await any results.

jeffie8696

So are the RB Innovations superchargers that Tower sells not for 2 stroke engines?

Gary L.

That would be a great conversation starter..

jeffie8696

Wonder how one could be adapted to a typical airplane engine. Wouldn't want it hanging off the sie in the airflow. Perhaps behind the engine with a geared shaft running off the prop driver.

VeeAte

They are for 2 strokes. Designed for buggies etc. So I doubt they would give much boost on anything bigger than say a .26 engine. You could overdrive it with different sized pulleys.
And I did some research on those superchargers a few years ago and there wasnt any real evidence of a significant gain in power. Lots of statements from various customers etc, but thats all. Tho with the advent of more and more engine and chassis dyno's coming on the market there may be some proper technical data on their worth.

I reckon if they were all that great they would be in pylon racing years ago.

Still a very cool concept.

I dont think having it hanging out in the airstream would cause too much hassle. Some planes with small cowls fly fine with half the cylinder block hanging in the breeze.

If I had the time tho I would have one simply for cool factor. hehehe