theodanuk

In my OS 40 LA manual, the details on fitting the engine to a model show specifically that the engine must be in a suitable position to ensure the pressure lines are correct etc. (see attached image hopefully)

But on an aircraft where you invert the engine, how does all this still work? I have an inverted engine on my Prangster and have not had any problems with it, but I am just confused as to why it works.

Can anyone help with this?

CGRetired

My Feedback: (1)

Even with an inverted engine, the position of the tank in relation to the carb is the goal of how you want to set this up. Pay more attention to the position of the tank and crab centerlines as shown in the diagram an you should be ok, even with inverted engines.

How do you like the Prangster? When I was working over in Harlow, I bought one and built it with one of my friends that I was working with. It flew great. One fun point about that plane.. when it flew inverted, all the dihedral came out of the wings.. ha..looked wierd.

CGr.

Flying freak

Not really sure what you mean, Its the same exact thing but the engine is upside down the tubes to the same places.. just make sure the carb is at the same level as the middle of your fuel tank and you will do just fine...

Steven

Nathan King

If you're talking about why the gas doesn't just fall out the carb it's because there is a pressure differential that causes suction; the fuel is still pulled into the engine even without the help of gravity.

The venturi narrows causing the air to speed up as it passes through and therefore pressure to lower. Also, when the crank opens to let fuel in there is low pressure in the crankcase because the piston is increasing the volume inside when traveling up. The air/fuel in the carb rushes in to attempt to neutralize the pressure. How do you think the fuel gets into the cylinder from the crankcase when right side up? The piston travels down increasing pressure in the crankcase and decreasing pressure in the cylinder by shrinking/enlarging their volumes respectively. When the two areas open up via a port in the sleeve the fuel/air again rushes up into the lower pressure area, despite the best attempts of gravity.

opjose

While there are a lot of posted warnings about the position of the centerline versus the carb, this is not quite as important as it seems to be made out to be.

Yes it helps prevent siphoning, and hydrolock, but in actual practice even a 1-2" variation has little to no affect on the engine performance once it is started, on a properly plumbed plane.

If it did we would not be able to fly uplines and downlines with the ease we do now... even on unpumped engines...

Then there are the tail draggers that effectively lower the tank position when they are sitting with the tail wheel on the runway, but raise it in level flight, etc...

So while it IS best to do as the diagram says, if you cannot do so exactly, don't worry, it is not THAT problematic...

flyX

Just think of bikecycle tire hand pump, if you still can't imagine what Nathan is referning to of how the fuel and air mixture
get's VACUME.


The venteri is sort of like an airfoil.....( imagine a garden hose nozzle.)
That's why there's lift. The air has to travel a greater distance on the top side of the airfoil, therefore the speed of the air flow increase.
The air hits the leading leading edge and meets at the tailing edge at the same amount of time.


basically a tune pipe is line a venteri..it creates vacume on the exhaust side.

Missileman

And the truth is, alot of the airplanes we fly just don't offer much choice on where to put the fuel tank. Try to put it just above the carb as shown or centered but don't be overly concerned.

2slow2matter

My Feedback: (1)

vacuum
venturi

flyX

I thought it say 2slow2matter
Send me a self address envelope , now that you won the spelling B contest.
I 'll send you some inverted glow plugs.
it dosn't retain.lol

2slow2matter

My Feedback: (1)

Good
I need some more of those, my LHS ran out and I hate to pay shipping on them. I've been "getting by" with the upright glow plugs, but can't wait to get my hands back on some inverted ones....

theodanuk

The Prangster . I love it.. It flys really well and being a novice I have pranged it a few times but no damage has been done to any of the airframe.. the worst damage was when i landed heavily and the exhaust was ripped off the OS40 engine (had to get a new engine or replace the carb so got a new engine) .. but even then, this aircraft was not damaged...

Prangster is a great aircraft to learn to fly with from my opinion. It is my first nitro aircraft, after I started learning by using the aerobird xl.....

theodanuk

Thanks to all the posters in my thread here. All the comments have really helped me understand how it all works and have given me a lot of confidence in my future builds and my Prangster setup....... I hope to go out again tonight after work.. if the weather makes a slight improvement.

Mikeis45

My Feedback: (1)

I have a new OS .46 that I’ve run about 3-4 tanks of gas. I am having problems keeping it idling. I went through all the instructions about set up. I ended up turning out the idle air bleed screw 4 turns and that seems to do the trick. In the book it tells you that if you tilt the nose of the plane up at idle and the RPM’s increase then you should back the screw out- that’s how I got it to stay idling but it still increase RPM’s when tilted up and I noticed that if I tilt it down it decreases the RPM’s…Why is that? What’s changing ? Is it simply gravity of the fuel being pushed to the carburetor causing a rich condition and then the lack of pressure or gravity when tilted up causing a lean condition? The book doesn’t say why it just says it is.

opjose

It is perfectly NORMAL for a the RPM's to increase when you point the nose up, and decrease when you point the nose down.

What you want to be sure of is that when you point the nose up the engine will NOT cut out, nor if you point the nose down that the engine will not eventually stop.

Since the engine leans a bit as the fuel level drops, it is best to perform the nose up test with about 1/3 of a tank of fuel.

Ideally you would do the same with the nose down test, but the problem is that when there is less than a certain amount of fuel in the tank a nose down test can cause the clunk to become uncovered. In turn it is easy to misinterpret this as a problem with the needles.

You are trying to get the engine into the range where it does not cut out, either nose up, or nose down, even though the RPM's may change. ( btw: this applies to idle too! ).

Once you have the engine set this way, richen it about 3-4 clicks to compensate for the normal leaning out in flight that occurs, and you'll be all set.

BTW: what causes this is primarily the weight of the fuel. Uniflow fuel plumbing setups help reduce this, and a pump will eliminate it.