I have a new GP P-51 Mustang, OS .25 engine. Haven't flow this yet. Do these engines Idle ok inverted. Or do you turn the plane upside down, so the engine is right side up then start it. Turn right side up and fly full throttle till out of gas. I've put a rudder in mine and would like to do some Hamer Heads etc. where idling the engine is desired. Don't want the throttle to be just a kill switch.

Thanks

Art

If the engine is broken in, and you set the idle mixture correctly and don't expect too low of an idle, yes, it's doable. It's no different than running any other engine inverted, which is done all the time. You might find it idles better if you use an idle-bar plug, or a hotter plug, but you might not. It is worth playing with plugs a little though.

As for starting, I've seen guys do both, start them upright, or flip it over to start. Your choice. If you have a good cradle, starting it upside down so that the engine is upright and the plug lighter is easier to get to is probably easier and safer though.

Thanks

Art

Idle is no problem whilst inverted. Starting is, if you allow fuel to puddle in the head.
An engine that sits too long in readiness will be harder to start, so it is best to start with cylinder upright. You still have to set the right idle mixture though [>:]

Be careful not to start it with an electric starter flooded (if starting inverted). Also if the tank height is positioned properly it should run fine upright or inverted. The problem with an inverted engine usually is fuel syphoning into the engine because the carb is lower than the tank. In this day when most start thier engines with electric starters, as opposed to by hand, the likelyhood of damage from a flooded engine is much more likely.

While flooding is a problem, I've never seen siphoning be a problem on an inverted 2-stroke. When inverted, the spraybar is below the port in the crankshaft, so when a siphon starts, it just pours the contents of the tank out through the spray bar, and then down, out of the carb and on to the ground (or into the cowl). I've actually seen this done. Except for all the fuel on the ground, it was pretty funny. "I swear I filled the tank!". I had a side-mounted engine that was also prone to this if I wasn't careful. It wouldn't flood the engine, but it would empty the tank (half way, in this case) if you got a siphon started and didn't notice and didn't get the engine started.

With an upright engine, the siphon can fill the crankcase rather quickly, but siphons are less common.

But in general, you are correct, it is much easier to flood and hydro-lock an inverted 2-stroke, and that can cause problems.

I've been flying enough, I'm aware of all the hydraulic problems, tank issues. But haven't run too many RC engines upside down. One of the guys at our field has an OS 1.08 in a Spit that runs flawlessly upside down withough ignition. More concerned about the idle needle set up and wether it is effective or not. Not prepared to add on board ignition on a 25 size model. What Nitro do they use in combat or does it matter. I thought I might fly a few combat planes and then try and muster up some interest?? Our old control line planes we just started up side down. No thrortle though. Haven't flown many small engine planes for a while.

Keep in mind that the GP mustang and corsair really aren't much in the way of combat planes. They are really sport-scale planes that happen to be legal for some classes of combat. I see you are in Canada, and they play by differnet rules there vs in the US now. However, in general, the GP kits aren't very comptitive and they are very fragile. They are fast on a .25 with mousse can though.

Tank position is important. You may not have a lot of vertical adjustment in the mustang though. Your engine does not care weather it is upside down or not when running if the tank is in the right spot. If it is high your engine will go lean inverted. If it is low your engine will go rich inverted.
Also syphoning is an issue if the throttle barrel is closed. It is just important to be aware of this as freely flowing fuel to your engine is never a good thing. Again if the tank is positioned properly it will not syphon.

With the engine upside down, it is harder to get fuel into the engine than to keep it out. Fuel will flow downstream and drip out of the carb like Kirk already pointed out. Syphoning will flood the upright engine, not the inverted one. To avoid that, the plane in readiness should be kept slightly nose-up, so the carb is higher than the tank top.
The real problem is puddling of the oil that has collected inside the engine. With the engine at rest, the oil will accumulate in the lowest point, which happens to be the plug. Even if you can get the plug wire to glow, there will be a slug of oil inside the plug body that prevents the plug element from lighting the fresh charge. Once the engine is running, the oil is expelled better, and plug blocking will no longer occur.

Clamp a hemostat on the line to prevent leakage.