Engine Stalling
 

What does it mean when my engine stalls when the nose of the plane is pointed down? During my pre-flight check, the engine runs fine when the plane is level and when the nose of the plane is pointed upward. If the nose is pointed down, air bubbles appear in the fuel line and the engine quickly dies.

RE: Engine Stalling
 

I would think that the clunk is staying near the back of the fuel tank, and when pointed nose down, it can't suck any gas. This is a good thing, and is normal. You don't want your clunk line to bend in half and go towards the front of the tank, because it will probably stay there.

RE: Engine Stalling
 

It might mean that your fuel clunk is in the back of the tank and when you point the plane down the clunk is no longer in fuel and is sucking air. Just a thought on my part, Good Luck, Dave

RE: Engine Stalling
 

The clunk not picking up fuel with the nose down isn't a problem unless you're doing some serious aerobatics. You'll never be nose down and quickly decelerating in flight, so you won't have a fuel problem.

RE: Engine Stalling
 

If the tank is full when you do the nose down test, then this should not occur.

If the tank is about 1/2 or less, it will occur... one reason you DO NOT want to do aerobatics on less than 1/2 a tank!

Nose down and quickly accelerating or decellerating makes NO DIFFERENCE, the fuel flows foward either way, exposing the clunk.

RE: Engine Stalling
 

I noticed the engine sputtering when doing some loops. Do I have another problem?

RE: Engine Stalling
 

Is there any experienced pilot that can check the tank installation and fine tune the engine for you?

RE: Engine Stalling
 

If holding the plane stationary and pointing the nose down while at throttle, the fuel in the tank (if not completely full) will rush to the front of the tank exposing the clunk to air. If you hold it in this position for any period of time, the engine will likely start to sputter and die. This doesn't happen in flight because when nose down the inertia of accelaration pushes the fuel to the back of the tank, where the clunk will still be able to draw fuel. Think of swinging a bucket of water in a complete loop above your head.

RE: Engine Stalling
 

My instructor did tune the engine before flight. The fuel tank is secured in place with a wooden brace (it cannot move fore/aft). My instructor was not sure why the engine would sputter after a single loop. He told me to take a closer look for anything irregular when I got home.

RE: Engine Stalling
 

OK, then I recommend you reading this excellent article written by MinnFlyer, if you have not done it yet, before you take a closer look for anything irregular.

http://www.rcuniverse.com/magazine/a...rticle_id=1068

Regards!

(Edited to add link)

RE: Engine Stalling
 

Absolute rubbish!

There is a nice video of fuel in a visible tank posted on these forums that completely disproves this myth, which unfortunately is often repeated even by very experienced flyers.

It is simply UNTRUE.

RE: Engine Stalling
 

My guess is that the feed pipe to the clunk is a little bit too long. I set mine so that the open end of the clunk weight is maybe 1/4 inch from the end of the tank when you hold the plane nose up.
When the plane is horizontal, the bend in the clunk pipe brings the weight slightly further forward which is about where it will be when pointed nose down. A fuel level below that will of course cause air (or pressurising exhaust gas) to be drawn into the tube. However, if you are maintaining positive G in a loop, then the fuel should be in the bottom of the tank.
The fuel will only go to the back of the tank when pointed nose down if the aircraft is being accelerated longitudinally at greater than 1G. If the engine stutters, the acceleration ceases and the fuel sloshes forwards compounding the problem. A big fuel filter like the Saito F1 can act as an accumulator giving you a few seconds breathing space.
One source of bubbles in the fuel can be a crack in the clunk pipe; this normally occurs just at the inside end of the metal outlet pipe and can be as a result of the sharp edge of the tube. I rub mine down with emery but I still get cracks occasionally. The thing is, they only reveal themselves when the crack is exposed out of the fuel, so you tend to get half a tank used then a dead stick.
If the tank is too high (normally the outlet pipe is level with the needle valve), when you loop, the pressure to the needle valve increases caused by the increased head in the tank and this can cause a rich cut in extremis.
Finally, fuel tends to go off. It can absorb water vapour when the bottle "breathes" at dusk and the nitromethane can evaporate in hot weather. Rotten fuel is evidenced in 2 strokes by a difficulty in getting a stable idle, needing a rich needle setting at full throttle and a tendency for the engine to die after take off as the engine leans due to rpm increase.

'Hope there's something in there

RE: Engine Stalling
 

I can tell you it behaves this way in my nexstar. I'm curious now. I always just assumed basic physics works this way. Why doesn't my engine stall in a prolonged dive but it will die if i hold it nose down stationary when the tank is half full?

RE: Engine Stalling
 

Here is the post I think you are referring to : http://www.rcuniverse.com/forum/m_8843327/tm.htm
The video: http://richter-lackierung.de/aktuali...amefuntana.wmv

I'm posting this before I have checked which video that is, but if memory serves me correct it's the one.

RE: Engine Stalling
 

Yup that's the one.

The bottom line is that swinging a bucket produces a centrifugal effect ( no it's not a "force" per-se ).

But flying a plane with the nose pointed down, even at full throttle does not attain nor sustain the 2G acceleration required to hold the fuel in the back of the tank as people assume.

As the video shows the clunk becomes uncovered in downlines.

The video was produced after I got into an arguement about the physics with a few people who refused to believe it.

My Funtana was getting fuel starved in prolonged downlines....

The only think keeping your plane running when the nose is pointed down, ( at less than half a tank of fuel or so ), is the fuel that remains in the lines.

The combined effects of throttling back, and the prop unloading, drops fuel consumption dramatically, keeping the engine running... however do this too long with the throttle too high and you'll get a deadstick every time.

RE: Engine Stalling
 

You are ASSUMING it behaves this way, but the assumption is not correct.

Your Nextar behaves just like the video. Have a look.


Physics says that to maintain the fuel in the back of the tank, you need about 1G of acceleration. So if the nose is pointed down, and the plane is accelerating at 1G, the acceleration is canceled and the fuel is free floating... to make it "stick" to the back, you need 2G's acceleration.

Do the math and you'll find that none of our planes can even begin to approach this rate of change, nor the resultant velocities involved.

Don't confuse G loading with the acceleration needed to maintain a 1G inverted environment within the fuel tank.

The video makes all of this much easier to follow.


Your engine WILL stall in a prolonged dive, if you let it, particularly if you keep the throttle up, once the clunk becomes uncovered.

To simulate what happens on your plane in a dive, run your plane up while it is level, then pull the engine off to idle. This loads up fuel into the carb and lines making the engine go rich as well... then put it nose down. Eventually it will lean out and shut off, but it will take a while.... then double the amount of time it took, because in flight your engine is unloaded due to the air flowing past the prop.

Your Nexstar doesn't SEEM to die, because you are not holding it nose down in flight long enough for it to die.


However on my aerobatic planes, I've had this problem doing extremely long and slow downlines... blenders, etc... which prompted the original discussion in an even earlier thread.

RE: Engine Stalling
 

I guess the guys at my LHS were incorrect then as they said it should behave the way I initially described when I called them to ask why my engine would stall when I'd hold it nose down. In the video it almost seems in a dive the plane stopped accelerating, allowing the fuel to move toward the front of the tank. So I'm guessing in a powered dive situation with accleration the fuel would stay at the back of the tank? Also, I've seen some planes pop crow and go into a very long extended dive without stalling. I wonder how they achieve this if the inertia isn't enough to keep the fuel at the back of the tank?

RE: Engine Stalling
 

The German company that posted this video, offers several solutions to the dilemma thru the tanks they fabricate.
One of those is to wrap the clunk with some absorbing pad, able to retain enough fuel for short times as to avoid air entering the tube.

It may help in this discussion to consider that the engine sucks small volume of fuel, and that it can be alive for several seconds only with the volume contained within the pick up tube, hose and filter.

A 0.40 engine can fly a model for 15 minutes with less than 8 ounces of fuel at full to medium throttle.
As 15 minutes are 900 seconds, the average fuel usage is 8/900 = 0.0089 ounces / second = 0.26 cc / second.
At idle, it must be much less than that.

Also, I have seen that any middle size bubble, pass thru the spray bar fast enough (due to its low density and friction) as not to kill the engine, if there is enough vacuum in the carburetor ventury (high rpms).

One main cause of flame out during static nose down or inverted flight, is build up of fuel in the crankcase by enrichment of the mix due to gravity.
During a dive in, the flow of air forces the propeller to turn, regardless of the right mix entering the engine.
I have never had a dead stick during a dive in, but many during inverted flight, when the propeller is loaded and the right mix is critical.

RE: Engine Stalling
 

I guess my plane is fine if the engine maintains a steady rpm when the plane is level or pointed nose up.

RE: Engine Stalling
 

being little miss pedantic here:- The acceleration needs to be in excess of 1G for the fuel to go to the back of the tank. 2G is in excess of 1G, but it doesn't need to be that high.
Any plane that has not reached its top speed is capable of accelerating, so if it's pointing straight down, it will have an acceleration in excess of 1G if the engine is full blast (1G from gravity plus the bit that comes from the engine). Of course, it won't be able to maintain that for long because (a) it will attain its top speed and (b) it will meet Momma Earth.
Any model capable of prop hanging has a thrust to weight ratio in excess of 1, even when the prop is not at its most efficient, so it could achieve a 2G downwards acceleration.

RE: Engine Stalling
 

Yes, but say at 1.3 g, the fuel is so relatively "light" that it will be sloshing around to the point of not getting to the clunk anyway.

No it doesn't as you can clearly see from the video.

Even when the pilot flips the nose straight down and at full throttle the fuel flows forward because the net acceleration downward does NOT offset the acceleration due to gravity, and the fuel sloshes forward uncovering the tank.

Our planes DO NOT reach nor maintain the requiste over 1.xG ( 2G for illustrative purposes ) acceleration required.

Nope, and there is the problem!

That Funtana ( and mine which did exactly the same thing ) illustrates this point. The RC plane cannot accelerate downward fast enought to hold the fuel at the back... so it sloshes forward.

RE: Engine Stalling
 

My ST 90 did this. Man it was frustrating! I was thinking a brass tube was cracked and it was sucking air in front of the tank. Every time, no matter if the tank was completely full, if I pointed the nose down past a certain angle, it died.

What is wierd is that my engine runs amazingly, as long as it didn't point down. One person mentioned that the crank case could be filling up with fuel and this fix requires a minor carb adjustment.

I purposely ran the ST rich, being a ringed engine. Is the fix to lean both top and bottom ends up?

RE: Engine Stalling
 

Hi!
If you have set your engine properly and have a good tank set-up all airplanes can take a prolonged dive without problem at full power.
The reason this works is the fuel in the fuel line acts as a buffer.

-It would be interesting to see for how long most of you can dive your planes without engine running out of fuel (or ground coming up and get you...), guess none of you can do it for more than 4-5seconds..! This our engines can easily cope with.

RE: Engine Stalling
 

Watch the video...fuel will not go to the back of the tank in a dive.....fuel moves forward and un covers the clunk...but the engine runs on what is in the lines and the engine....it's one of the reasons you want to run a little rich on the ground

RE: Engine Stalling
 

jaka,

Hello.

This sure seems like voodoo. But something has to be happening to keep them running in a dive- or- starve them in a dive. The video is definitley eye opening. I've never paid attention, but I must not put my aircraft in a nose down manuever for more than a few seconds. I'm still puzzled though.

I'm starting to think I was running too rich. Does this sound correct?

RE: Engine Stalling
 

I didn't see that the video disproved Newton's Laws of Motion.
It was clear that in decelerating lines, the fuel sloshed forwards and in accelerating lines, it sloshed backwards.
Frankly that would be obvious to anyone who understands mechanics.

If you have problems with fuel starvation and you are using tank pressure then I would recommend the following (not necessarily all together)
Wider bore fuel lines
A longer length of fuel line (coiled once if necessary, coiled twice and it's too long)
A bigger fuel filter to act as an accumulator, Saito F1's are nice (but pricey).
A 1/2 oz or 1oz auxiliary tank next to the carb .
A Perry VP20 or VP 30 pump.
A baffled tank.

I fitted one of the Perry pumps to an Irvine system I had because I was getting poor pick-up on opening the throttle. Fabulous! It was like revving a motorbike afterwards.

RE: Engine Stalling
 

Hi!
As I said ...Engines runs due to the remaining fuel in the fuel line...when engine has sucked it up it stops...but it takes time! Several seconds ...and most of us don't dive our planes straight down for several seconds. I have never in my 35 years flying R/C experienced an engine stopping during straight down dives!

RE: Engine Stalling
 

Could you not just use a small header tank the same as the heli guys use, or use more flexible tubing, like helis use, they stay in all sorts of position for long periods of time.