What does this signify in an engine?

Hi All,
following on from my other post, what might it mean if an engine that has an interruption in it's fuel flow *really* struggles to get the RPM back up again once the fuel is flowing freely again? This is on a rich 2-stroke setting.

Some details:
I resolved a terrible engine run on my OS 10 FP by tightening the bolts that had worked loose in my mounting plates (the old loose-engine gremlin shows itself in wierd ways) and found a leak in the fuel tube that only opened under certain flight forces.

Result is a lovely clean engine run, as I remembered, EXCEPT for a sudden sag/near-cutout at the very end of inside loops.

So I started the engine on the ground, and as the fuel ran out and large bubbles got into the line the engine slowed down, just as I tilted the model to get the pickup in the fuel again. The fuel began to flow freely, but the RPM stayed stuck in the depths and took several seconds to start to build up at all.

So, my suspicion is that the "cough" in the inside loops is due to the pickup getting uncovered and the engine not recovering as soon as the fuel is available again.

Could poor compression do this? I know an oversize venturi could, but I have the correct OS one and NVA.

Any guidance much appreciated.
Best regards
Jonathan

The answer may be that you're simply running it too lean to begin with.

Randy Ryan <><
AMA 8500
SAM 36
BO all my own M's

What size prop have you got on it?

Warren Leadbeatter
Port Stephens - Australia
AUS-14782

That's a sign that the engine has over heated which makes it lose compression, that's how abc/abn engines work. More of a problem on smaller engines that need closer tolerances on the piston fit. If the enigne's not too loose already, a couple of runs like that will get it there.


Giffy

Over lean needle settings cause overheating.

Randy Ryan <><
AMA 8500
SAM 36
BO all my own M's

Hi Guys,

the engine is running a rich 2-stroke on the ground (which is where I can observe the sag close-to)

I'll try today with a really rich setting and see if it behaves differently.

This engine has had a hard life. It was my first working engine, so was kind of put through the mill in my childhood days. I think it's had a lot of lean runs! However, it was always run on 20% all-castor fuel so it should have been reasonably well protected.


What's the best way to check the compression to see if it's been toasted? It does seem rather loose when warmed up, but it's so subjective... Is there a good was to judge compression?

Jon

EDIT:
I tried with a richer setting and it worked better, but seemed to teeter on more wierd behaviour. My fuel has got a (very) small amount of grit in it and I need to fit a filter to the fuel feed on this model as I'm sure some of the inconsistancies is due to gunge in the NVA.
Mind you, I'm getting one-flip starts...

If the bubbles caused a sag without first an increase in RPM, most likely you are running too lean.

Some engines get harder to needle as they get over the hill.

Check for loose head bolts or backplate screws. Check that fuel tank is close to engine, fuel tubing correct size, and up the nitro content a little.

Point model nose up and choke engine to see if you can draw fuel. This will check for air leaks around needle and fuel line. You might also try pointing the engine straight up before launch to check fuel draw.

Is this an FP-S or a converted RC engine? Perhaps they sent the wrong venturi? How was fuel draw before?

Good luck.

George

Hi George,

If I point the plane up on the ground it runs fine. It seems to be a flight thing. Last time this happened to me it turned out the engine was completely worn out, and the big end of the conrod was cracked and opening up slightly on load.

The FP is a conversion from R/C, and curiously when the venturi arrived there was a note on it saying "OS venturis now 8mm". I had terrible problems to start with with just this because I was sent the 15 size NVA. Fitting the correct 10 sized NVA solved the problem completely back then.

I have a feeling some of the wierdness which doesn't always follow changes I make to the needle are due to muck in the lines. I need to add a filter, clean it out and put in a few test flights with small needle changes before I get too excited.

Oh, and check the tank again for leaks...
Cheers
Jonathan

EDIT
I just checked the Tower Hobbies site and it says that the 10 FP's venturi should be 7mm outside and 6mm inside. I will check mine tonight...

Jonathan,

It's a 10 FP, right? Should be ABN, unless it is one of the early FPs with an iron piston in a steel sleeve. Plated brass sleeves expand when running, so they do feel looser right after a run. If it is iron in steel, 20% to 25% castor should keep the compression pretty nice for a long time. You mentioned it is an old, well-used engine - perhaps wear is taking its toll.

Still, it is a small engine, and they are usually trickier to set. That's because they don't have a big reserve of power - there's no substitute for cubic inches when you just want more power.

ABSOLUTELY put a filter in the fuel line to the engine, and keep it clean! Reverse flush the NVA, first, by squirting fuel through from the needle valve side, too.

The FP series has a wide RPM range in 2-cycle, so your rich-two setting should have worked. If the tank has no leaks, and nitro and castor are about right for the engine, the RPM loss and bogging down you're seeing may have to do with prop load. As many mentioned, too lean means too hot. The FPs can run a lot more RPM than we're used to for a stunt run.

We can let such engines run in their 'happy' RPM range on a lower pitch prop (presuming diameter itself isn't too big.) How many times have you seen that kind of run called 'running in a lower gear?' Since the engines can be happy at higher RPM, we can get the same airspeeds/lap times.

Every engine has an RPM range on each different prop. Since the FPs can run well at pretty high RPM, a prop that's too much load considering the speed, weight and drag of the model, can prevent the engine from reaching a 'happy' RPM for cooling, fuel draw, and enough power to meet maneuvering loads. The engine may hit its max RPM or torque for that prop load and just run hotter, not faster.

Flying straight paths, without maneuvering drag loads, may work fine, but, when you do loops, wing lift may have to go to about 10g - ten times as much as for level flight. Induced drag is associated with lift, and isn't much for level flight. It increases as the square of the increase in lift. So, the induced drag in a round loop may be 100 times as much as in level flight - still not a whole lot, just a lot more than in level flight. That's the drag load the prop has to deal with. If your set-up is marginal for the conditions, you may hit the margin.

Oh, by the way, are you running muffler pressure to the tank? Stock FP-S (or converted RC) engines in all sizes I've seen had relatively large choke diameters. The instructions mention, even encourage, muffler pressure to assure a dependable fuel supply to the engine. The bigger hole goes with the need for 'breathing' at the higher RPM usage the engines were designed for. Large chokes don't draw fuel well at medium RPMs...

Best of luck, and I wish you a happy model with its little engine singing in the right key...

...Another quick thought:

Are you using a uniflow tank? Rectangular tanks might just uncover the fuel pickup towards the end of a flight, but usually not early in the flight time... A wedge tank, or plastic bottle with flopper pickup, at least keeps the pickup in the fuel almost all the way to the end of the flight.

Uniflow tanks on suction are more critical about height relative to the engine centerline, and this can vary between samples of the same make and size of engine. When 'off' they run differently in inside and outside g conditions; upright vs inverted, insides vs outsides... That's why we most often make tank height adjustable these days.

Great post Lou, thanks. I'll definitely try connecting for muffler pressure on the uniflow vent. It helped when I had the undersized NVA, so there must be something in it. I just figured it was a bit of a kludge! Also makes refueling a little more complicated.

I'm running a 7x6 prop instead of the 7x4 because I've none left! I guess that can't help, so I'll get the correct one.

Also, I followed the advice of riching up the mixture to the point where it was very rich on the ground, and lumbered into the air, and when I got it right I got a switch from rich 2 to leaner 2 on manoeuvres. Suddenly it was behaving really well (I'd never run this engine in anything other than just off flat-out).

The only issue was that now near the end of the tank it would falter, even die at the vertical intersection of the horizontal 8s.

This is wierd, because it really seemed to be the vertical part, yet the wingover was perfect. Also lazy 8s caused no problems, so I'm starting to suspect the fuel pickup may not be at the back of the tank (long time since I built it) However, if I fix it, won't I get the same problem when going vertically *downwards*? I can cant the tank out at the back to help, but it always amazes me that stunt tanks work at all when near empty...

Best regards
Jonathan

Jonathan,

The tanks feed well down into the total volume because 'centrifugal force' is usually somewhere between 2 and 5 g. ...Outward, of course...

So, the fuel surface in the tank - except for possible foaming from vibration - is perpendicular to the combined effect of 1 g straight down, and, say, 3 g outward. That's the 'local gravity' the fuel 'feels.' It would be about 3.16 g aimed out 3 and down 1, if CF at the model is 3 g.

In maneuvers, the gravity vector is still 1.0, straight toward the ground. Basic CF, from flying around the flight circle, is about what it was in low level flight, or a bit less. Maneuvering g is at right angles to the wing. Better picture of that: it is in the direction the wing is lifting. The model feels that g as added weight aimed out from the center of the curve path. Loops can run to 10 g, and, per Bill Netzeband, corners may reach well over 20 g. Fortunately those corners don't last long...

Yes, it is amazing that we can do all this, but let's not argue with that reality?

Hi Lou,
thanks for that. I now have the opportunity to take a look inside the tank, as the engine cut out in the middle of the 8 and I saved it once, but twice was tempting fate too much. The Akromaster is now in 5 (fairly big) pieces. Rats!

Best regards
Jonathan

That goes for people to. Just ask my wife

When my engines do this it's usually a tiny air leak in the fuel line.