Hi, I have set my engine up on the ground fine but the performance in the air is allot different.

When I tip the nose up (on the ground) the RPM increases but holds. When I put the nose level again, the RPM drops and she runs a little rich (which I can live with). If I lean up the main needle to get max RPM then hold the nose she cuts indicating too lean. This issue with the main needle isn't so bad, it is still OK as flat put RPM isn't a problem.

However with the slow running idle, I've adjusted it so that the transition is pretty quick when on the ground, but when I get it airborne there is a 3 second delay, a bit of a splutter and then she goes flat out.

Any of you guys got any ideas??. The engine is ringed, 2 needle carb. 40 size running on a 10x4 prop.

You have just described the dreaded "Balsa Effect".

When an otherwise perfectly adjusted engine is placed within 50-75 cm of a balsa object (model plane), AND this object is placed at a distance greater than it's gliding distance from the field...this "BALSA EFFECT" is at it's maximum......It has been shown to cause all sorts of dreadful effects. Many a Master Builder and Expert mechanic have been created in this fashion...

<GGGGG>

In all seriousness....

Make sure you're airplane's fuel system is a good approximation of your test stand set up. Ie: Fuel tank c/l on a level with the engine'e needle valve. Fuel lines as short as possible, tank adequately padded to prvent foaming, Needle set for 2-300 rpm below peak on the ground, etc, et al.

As for the "transition delay"....understand that the load the engine sees in flight is different than that on the ground. In flight, the prop is unloaded somewhat (the blades unstall) and the mixture needs to be "pre-adjusted" on the ground to compensate. It's almost like the engine is over-running the throttle opening. Maybe DAR or someone has a better insight into remedying this issue. Could be as simple as a little more/less lean on the low end. Easier to fix when you're standing right there to see it.

Good Luck.

Can't wait to visit the UK again too!!!

Thanks for taking the time to respond.

I've come across some interesting explanations to my problems.

As for the main needle, leaning out allot with the nose up, one guy suggested the tank pressure is not enough. Funnily enough the silencer I have on is not manufacturer supplied and is an irvine. This is slightly bigger and so the pressure wont be as high.....hows about that one??. But as I say the main needle I can live with.

As for the slow running, My old man agrees with you in terms of the engine unloading in the air. I will continue to lean up the idle needle. I was worried about going too far but have been told that I will know I've gone too lean when the engine dies!!! when I throttle up quickly.

I am struggling with telling the difference between a SLOBBERING pickup and a HESITANT pickup. Some people have talked about hesitant pickup as being too lean but I cant see this being the case with mine.

Only one thing for it perseverance!!

I will also try the much acclaimed PINCH test.

When you adjust the model's carburetor with the nose pointing upwards, you are anticipating two problems. The first is the reduced pressure head of the fuel in the tank as the fuel burns off over time. The second is the engine "unloading" when airborne. Actually, the second issue is not a problem, because when a two-stroke unloads, it actually needs less fuel, not more. But this is an R/C myth, so we shall humor it.

What is happening with your engine's idle and transition is that you have NOT anticipated the leaning effect of burning off fuel as the flight progresses when it comes to the low speed adjustment.

If you adjust the low speed needle to its perfect idle and transition mixture when the model's tank is full and sitting on the ground, it will become leaner as the flight progresses. Richen up the low speed a bit and see if it improves things.

One must keep in mind that we are dealing with averages when we adjust our suction fed glow engines. We never want the engine to run too lean, even if it means that the throttle is a little sluggish in the beginning of the flight. Good flying.

Hi, again, thanks for the advice. You have thrown me by telling me to go in the opposite way with the idle needle (Rich). But what you say makes sense.

One question, is it possible to have the slow running idle set too lean and the engine keeps going??? What would you expect the syptoms to be??.

When I open up the engine the syptoms are best described by it seeming to take a deep breath and then go flat out, absolutley no mid range.

That sounds like too lean.

Too rich, the engine goes, "ba-ba-ba-ba-ba-ba-baaaaaaaaaa!" It's kind of a stuttering, burbling, gurgling, or slobbering sound.

The "splutter, then goes flat out" description sounds like you might be a tad on the rich side where the Lo needle is concerned.


Your question as to how lean an engine should be at idle is a toughie....

MOST 2C engines I've ever encountered in 25 years seem to like as lean an idle mixture as they can get. Whether they will transition from that lean idle setting to a blazing top end with any speed/smoothness is altogether something else. It's usually a balancing act. Not all carburetors are created equally. One problem with the "too-rich" low end is that it allows fuel to collect in the engine crankcase which is then thrown at the engine rapidly when the throttle is advanced. Instant smoke, gasp, gargle....you get the picture.

It sounds like your real close in your setting....just need a bit of fine tuning.

For fun....

Set the high speed as you normally would. With the engine at a normal operating temp...allow it to idle for 15-20 seconds. Smoothly, but briskly open the throttle to full.

What happens?

1) Sputter...smoky exhaust...stumbling acceleration? Too Rich.

Lean the LO needle slightly....1/8th turn....repeat the test.

2) Engine seems to almost "die" momentarily...then races ahead.


Engine too lean on the Lo needle....richen slightly. 1/8 turn.


Patience, Patience and more Patience. Sounds like you're close...just a little one way or the other and you should get it.

Good Luck!

Hi guys, After looking into this subject and your kind responses, my thought process now is :-

1) concentrate on ground setting first

a) get her going and have a flight, warm up so to speak
b) refuel - start her up and take her to idle and try the pinch test

if she accelerates allot and holds and then dies- shes rich so lean her off at the idle screw
If she dies pretty quickly - shes lean so richen up the idle screw
If she accelerates a little and then dies pretty quick I'm about right.

2) Take her up and see what happens.

a) if the pickup is still poor in the air in comparison to on the ground, I will take the point that the engine is "unloading" and so I will need to richen up the idle screw.

b) if shes fine then I will be over the moon and I can practice hovering until I get too cocky and trash the plane at my toes!!!!!!!!!!!!!!!!!!!!!

Rather than go straight to step 2, the field where I fly isn't great on dead sticks so I am airing on the side of caution.

My bet is in theory from what I've gathered is that I will need to set up the idle on the pretty rich side whilst on the ground, as I am convinced she is leaning out allot in the air. The pick up is not a slobbering one but a big hesitation( almost no noise), deep breath and then flat out. An engine cant accelerate without fuel so I don't think shes getting enough.

Hey as you say patience and perseverance but I learned allot from you guys.

I'll try at the weekend

Artisan,

Unloading and lean runs are NOT an R/C myth.

When the load is reduced on the engine; whether from unloading in flight, or from using a smaller prop, RPM increases.
Unless your engine is already propped at peak horsepower (like a muffler equipped, sport .60 on an 11x7 prop...), every rise in RPM is also a rise in horsepower, even if torque is in a decline.

To produce more horsepower the engine needs more fuel.
Even though faster airflow through the carburettor increases suction, the main needle needs to be opened further to maintain the correct air/fuel ratio, as RPM increases and horsepower is too.

This is why it is imperative to enrich the main needle, with the nose up, by 100-200 RPM (more with the plane horizontal).

Extraslim,

Make sure you have the shortest possible fuel line from the tank to the carburettor (or to the needle). You didn't bother to tell us what make of engine you are using, but a 2 blade 10x4 is too small for your engine. It may not over-rev, but your plane will sound like you are driving in first gear all the time.

Long fuel lines do cause transition to be hesitated, as the plane accelerates. From the descriptions you wrote, it hesitates on pickup, not sputters. Fuel going to the engine must also overcome acceleration forces. Maybe the immediate initial acceleration from your low pitched prop is a part of the reason for the hesitation.

I suggest to enrich the idle mixture 1/8 of a turn AND to change to a 10x5, 11x5, or a 10x6 prop.

Tell us about your results.

Sincerely,

Hi Guys, the engine is a HP 40, autrian made. I have flown these engines for many years on my fathers planes and they are a superb peice of engineering. (I was always too young to fiddle as Dad did it for me) They have a .25 size case with 40 horses. Excellent power to weight ration. Mecoa are now producing them with a high price tag to boot.

I must admit I thought about more of a funfly prop. (what advantages would I get with a APC 11x4 typoe prop?) I will add that to my list of things to try.

Thanks for the interest and as soon as the weather picks up and I can get out I will let you guys know the results.

Extraslim,

With an 11x4 you will still be under-propping.

If you are destined for fun-fly, an 11.5x4, or a 12x3.75 would serve you better.

Do make sure you make the fuel lines as short as possible and the tank as close as possible to the engine. The HP is not known to be a very good sucker.

If the plane is a tail-wheel fun-fly, adjust the idle mixture at runway attitude; three point stance with the nose up. Also make it a bit richer for this purpose.

There is a usable range of adjustments, not just a single point.

Sincerely,

Hi, I have had to move the engine a little forward in the mount to get it to balance, and the fuel lines are as short as possible. The tank is wedged as far forward as possible and the distance to the carb certainly isnt long at all.

I am also on the hunt for another silencer, as I have mentioned the irvine silencer is bigger than the one supplied and so the pressure to the tank will be down slightly.

With all your help I'm sure I'll get there in the end. I have a trouble shooting list now.

The only question is in what order I will try them. I think I'll try

1) Pinch test with runway attitude - get it set up at that on the ground
2) then see what is does in the air
3) if the transition is still hesitant in the air I have learnt that it is more than likely that the engine is leaning out and so the idle screw needs richening and retrying ( I have been screwing the idle in hence leaning to try,and combat the "in air" antics.

Cant wait for the rain to stop!!!!!!!!!!!!! UK weather sucks as you would say.

Hey Slim..

You're really tapping this group for all it's worth hey? <GGG>

You're a good man Charlie Brown. Keep asking questions! There's more to learn than any person could gather in 5 lifetimes!

Here's a handy "rule of thumb" propeller load comparison formula that I've used for years....works well.

** Prop Diameter (in,cm) x 2 + Pitch (in,cm)

Example: A 10-6 prop would yield a load of:

10(2) + 6 = 26

an 11-4 prop would yield...

11(2) + 4 = 26.

Both props approximately load the engine the same (flight performance would be noticeably different).


It's a handy method of determining if you're increasing/decreasing the physical load on the engine.

Have fun!!!

Guys Hi,

The result is that I have had to wind the idel screw in 2 and a half turns, to all but a 1/4 away from fully wound in!!, it was so rich it was behaving as if lean. I also found an air leak around the carb seal which wasnt helping. Solved with bath sealant!.

One word though, the pinch test in this instance seemed to mislead me in fixing this. With the idel set up great as per the pich test, the pickup was still very rich.....interesting eh??

The engine is now performing very well on a 10x4. I have tried a 10x6 and 11x4 but just cant seem to get the RPM or the vertical performance. I'm happy with what I've got.

All is well and thanks for the feedback.