My avistar has a air bleed screw. I followed the instrutions and the engine does not stall at any angles so I think it is adjusted correctly. But I would like to know the real purpose for it.

An air bleed is the low end mixture control for your engine. It functions similar to a carburetor that has two needles, one for hi speed, and one for low speed. The biggest difference between a needle setting and an air bleed setting is the direction they are turned to achieve the desired results. In other words, to lean a two needle carb, you would turn turn the lo end clockwise to lean, and an air bleed screw would be turned counter clock wise to lean. When an air bleed is opened, the principle is more air, less fuel, therefore lean. Hope that helps_bob

That would answer it pretty well, but here's another one. Why do the instructions say to move the plane 15 deg down and up to see if it stalls? What does this have to do with the idle?


I'm pretty good with nitro engines, and can tune almost any nitro RC car. My rc car idles the same upside down as right side up as long as there's fuel in the line.

Normally if a setting is too rich, the engine will slow down and die with the nose down. Let the engine idle for 15 to 20 seconds on the ground. Advance the throttle and see how it acts. If you're familiar with nitro engines, you know the drill. The nose down thing has to do with something called, "tank head" Maybe you don't notice any changes in your car because of the different amounts of fuel a plane carries, and the size of the tank compared to a car. How is your plane idling etc?

I've not run nitro cars, so I don't know if you have the same issues. In an airplane, you get a several factors that change your mixture setting when you leave the ground.

The prop "unloads" and turns to a higher RPM than it does with the plane sitting still on the ground. It's a big effect.

The G forces frequently cause the engine to lean out a little. Espeically when you are climbing, nose up, the engine has to now draw fuel "uphill" against gravity. I don't think you get that situation in a car for as long as you do in a plane, though I could be wrong.

As the fuel level drops in the tank, the engine will lean out a little.

I will say this for sure, if you have your needle set a hair too lean, your engine will run fine sitting on the ground. But if you pick up the plane and point it striaght up, the RPM will drop a hair. If the needle is set right, you will not hear any RPM drop.

So, good practice is to run the engine to throttle, set the needle for a hair short of peak power, then point the airplane vertically. If you hear the RPM go down, richen 1-2 clicks and try again.

Some guys set the needle in the first place by pumping only a half tank, then holding the plane vertically, and peaking the RPM.

The nose down thing is to make sure you aren't too rich, that the engine won't flood and quit if the fuel system starts a syphon from the tank in to the carb in a dive.

Head pressure is basically the same reason people use water towers to get water in to your house. The higher the water level, the harder the water come out of the facuet, all powered by gravity. The same is true for a fuel tank, the fuel tries to run "downhill" into or out of the engine, changing the mixture slightly.

(btw, as a general habit, I try not to use the word "stall" in reference to the engine. The engine stops or quits or you go deadstick. The airplane "stalls" then falls out of the sky. That's just me though)

To make tuning even more difficult.

I usually tune my engines so that pointing the nose up at full throttle results in a slight increase in RPM. That way I can be fairly sure of never running lean.

I don't try to squeeze every last ounce of power out of my engines like some I've seen at our fields. I prefer having my engines last longer.
Dennis-

Good point. An increase in RPM as the nose goes up is fine. A decrease is bad. No change is ok, but you are close to being too lean.

Thanks guys, I'll see how the plane acts right after work. I wish the instructions would say something about this, but at least I know now. All I need now is to get one of them retired trainers to teach me to fly

Weekend is comming up, maybe it won't rain.