Engine stops in negative G

[Philflyer]

I am practicing flying the schedule for the B certificate but having problems. Every time I come out of the bottom part of the inverted loop into the vertical and push the throttle open my engine cuts. The motor is mounted upright with a square SLEC tank with the standard exhaust pressure connection. The set up has been totally reliable until now and flies inverted perfectly. Anyone any ideas?

[Biggles' Elder Brother - Moderator]

Does it throttle OK? There could be two reasons:

 

1. the negative G - in which case you've got a fuel feed problem - engine might be a touch lean, or your clunk doesn't get down. But the fact that it runs inverted OK tends to argue against that.

 

2. The bottom end needle is not quite right and the bottom end is a bit lean - the inverted scenario just being an aggravating factor.

 

You don't say what the engine is?

 

BEB

 

PS Someone beat me to moving this thread!

Edited By Biggles' Elder Brother - Moderator on 18/07/2011 10:28:19

[Martin Harris - Moderator]

Sounds as though it's (negative) G related - the fuel will be several times heavier in a bunt than in steady inverted flight. If the tank is mounted reasonably high in the model then when inverted it may be excessively low causing lean running.

 

Is it happening as you open the throttle? You could try richening it slightly on the idle screw. If it happens after the engine has throttled up then try richening the main needle.

 

P.S. Someone beat me to posting a reply! 

Edited By Martin Harris on 18/07/2011 10:32:23

[Philflyer]

Thanks both. Yes it is happening as I open the throttle, on the flat inverted part of the loop while pushing down elevator to keep the loop round. The engine is an old, much loved and ultra reliable MDS .40 (don't laugh), one of the last, with the silver head. It has been slightly modded with a steel bushed small end (after it wore the original one oval) and a J'En silencer. It has a new needle valve assembly and has been set up well but I will try an extra 1/8 turn on the idle setting. It is not sensitive on the fuel settings. The airframe is a new DB Mascot (a lovely kit) and the tank just fits in the square hole provided in the formers; so no room for manoeuvre. The fuel feed height is within 5mm vertical alignment with the spraybar as you would expect from a Boddo design. Am I waiting too long to open the throttle?

[Biggles' Elder Brother - Moderator]

On the "am I waiting too long to open the throttle" - from a purely aerbatic standpoint I'd actually say "yes". I go with the giudeline start opening the throttle gradually at about the 4 o'clock postion - but that's firstly to maintain speed through the bottom of the bunt and secondly to avoid any big torque reaction by having to open it really smartly from the bottom! The plain fact is the engine should still throotle OK anyway - whenever you open it!

 

Let us know if an extra 1/8 turn on the idle needle does the trick!

 

BEB

[Philflyer]

Will do - if it ever stops raining!

[Myron Beaumont]

Obvious I know but can the clunk reach the "top" of the tank easily OK I ask myself

[Martin Harris - Moderator]

Should do as it runs OK inverted, we're told.

[Philflyer]

Good point. I have assumed it can reach the top because I can hear it rattle but I will take the tank out again and check visually. If anyone has any more suggestions I will try them all. It is embarrassing and more than a little scary when the engine cuts in this particular situation i.e. dead stick inverted with not much airspeed and usually downwind.

[Biggles' Elder Brother - Moderator]

Downwind? You would usually enter a bunt going down wind so the upward leg, where you want the "go" is into wind!

 

Not that it helps with your engine problem!

 

BEB

[Bob Cotsford]

Can the clunk reach the front of the tank or does it stay at the back of the tank in a nose-down attitude, as in the first half of the bunt. Might be that half a bunt is the time it takes for the engine to emtpy the feed pipe and start sucking air?

[ken anderson.]

hello philip.......apart from the advice you allready have been given........try tuning your engine with only 3/4 tank of fuel...then open it one click(the throttle/needle valve)then fill the tank and have a go.......

 

ken anderson ne..1. advice dept.

[kc]

Maybe the clunk is just long enough to reach the back of the tank when vertical and sucks itself to the tank wall, cutting off fuel, but when inverted it is a bit on the diagonal so the clunk clears the back of tank. Shorten the clunk tube by a few millimetres. Also an old bit of fuel tube might have stretched a little.

[PatMc]

It might not be anything to do with high G manoeuvres simply the transition from tick-over to full throttle causing the carb to lean the engine too much before the revs can pick up.

With a half empty tank, try running the motor at tick-over for 15 - 20 secs then fully open the throttle. If it doesn't cut repeat with a full tank. If it now cuts it's probably a leak in the tank pressure somewhere - either the tube between exhaust & engine, the filler vent isn't properly blanked off or the silencer to engine joint is leaking.

If it cuts in both cases try opening the throttle very gradually. If this improves or cures the pick-up then the slow running jet is probably too lean. However MDS have a tendency to leak air through the front bearing after a bit if wear so fixing the problem might not be economical.

 

BTW you can prove whether negative G has any bearing by half rolling inverted at the height you would start the bunt & doing an inverted bunt

 

 

 

[Philflyer]

Thanks everyone. I will try the suggested tips one by one and post which works. Could be a while with the weather as it is.

[Brian Parker]

Noticed this thread.

It reminded me of Beatrice ‘Tilly’ Shilling OBE.

‘Tilly’ developed what was officially known as the R.A.E. ‘restrictor’ and thus solved the problem of the early SU carburettor equipped Merlin engine cutting out under negative ‘g’.

‘Tilly’ and her team travelled around the WW2 bases fitting the restrictor. This restrictor was universally (and unofficially) affectionately known as ‘Miss Shilling’s orifice’.

Tilly apparently was quite a character, she lapped Brooklands at over 100mph on her racing Norton and worked on the Blue Streak missile, but her attitude to the ‘Establishment’ prevented her reaching high office.

Tilly died in 1990.

[Philflyer]

Well, the weather improved yesterday and I managed to install some of the improvements offered by you helpful lot. After removing the tank to check the clunk I discovered that the mid point of the tank was nowhere near in line with the spraybar as I had thought but 12 mm lower. I had installed my engine higher than shown on the plan to give easy access (see my photos) then forgot to move the tank to allow for it. Felt a right plonker. I cut out as much of the former as I dared from above the tank which moved the tank up 8 mm. Not perfect but easier and quicker than moving the engine. Out at the club field I reset the mixture with half a tank of fuel and made sure it ran ok holding it vertical. All good, so I filled up and took to the air. It was all great during the inverted figure 8s, throttling perfect so I tried the bunt. Deadstick again. Landed without incident and had a good think. What's the first thing I used to attack if I had acceleration problems with petrol cars in my youth? Usually the ignition and of course the plugs. So I changed the glow plug and tried again. After starting there was not even a slight drop in revs after disconnecting the glow battery, a good sign. This time, instead of holding the throttle at idle during the first (vertical down) part of the loop as I had been doing, I gradually opened it so I was at about 3/4 by the time the Mascot was inverted. Perfect! It raced up and around as if it was an OS FX. So pleased I did it 3 more times. Problem solved. Thanks guys for all the helpful suggestions. I am sure the successful solution was not just one idea but a combination. I will know better next time!>>