Bad inverted run

Hi

Comming out of wingover (inverted), the engine looses power but regains it again in inverted level flight. It's picking up power after 1/2 a lap and at full power again at 1 to 1 1/2 laps.

The engine is rear exhaust mounted on the side with two preasure lines, one each side of the muffler. The pressure feeds crossover to to the opposite inlet pipes on the fuel tank. So in upright flight, the upper muffler feed line goes to the lower overflow pipe on the tank. The lower muffler feed line goes to the tanks upper inlet pipe. I did it this way to try to ensure I have one feed line uncovered by fuel in the tank or oil in the muffler at all times.

The fuel tank is a No.1 aeroflyte metal stunt tank mounted exactly in the centre between the engine bearers.

Any ideas how I can improve engine run?

What engine, prop brand & size, glowplug brand and type, fuel (nitro, oil type and %), venturi size, NVA spraybar diameter? Somebody's going to ask, anyway. Steve

Old setup and new problem, or new setup, always had this problem?

My Streak was the former, uniflow vented without pressure and developed the same problem. Marvin Denny helped me troubleshoot the problem to a defective tank. Standard vented it and all my troubles were gone. Would be an easy thing to check.

Tools

Unless you just want to mess around with this pressure set-up, I would suggest you use a standard uniflow set-up with (or without) muffler pressure to the uniflow vent. This is a proven method. It really doesnt matter if fuel is over the pressure inlet.

FWIW

The majority of the messagers here won't know anything about your fuel tank (is it Ozzie or UK?) We do have a couple of each of those who do visit here and might have more specific suggestions. Almost all model engines do exhibit some preferences for offset tank locations whether mounted upright, inverted, or sideways.

On profiles, a sideways mount also must contend with increased outboard distance to the fuel pickup on some of the wider "profile" fuel tanks we have used in the USA. The famous, or some would say, "infamous" old Fox stunt 29/35 engines may "burp" in turns in that direction, but for those who experience this symptom, it is momentary only.

Going from an atmospheric vented tank to a Uniflow, the offset on the tank becomes more pronounced, in my experience.

.

Kiwi

>Hi
>
>Comming out of wingover (inverted), the engine looses power
>but regains it again in inverted level flight. It's picking
>up power after 1/2 a lap and at full power again at 1 to 1
>1/2 laps.
>
>The engine is rear exhaust mounted on the side with two
>preasure lines, one each side of the muffler. The pressure
>feeds crossover to to the opposite inlet pipes on the fuel
>tank. So in upright flight, the upper muffler feed line
>goes to the lower overflow pipe on the tank. The lower
>muffler feed line goes to the tanks upper inlet pipe. I did
>it this way to try to ensure I have one feed line uncovered
>by fuel in the tank or oil in the muffler at all times.(snip)

I, too, would like a few more particulars, but one thing I would like to know is what happens when you go back upright? Are lap times equal, upright and inverted? Does it do this only when going inverted, or when it goes back to upright as well?

I am wondering if you are fooling yourself with this double pressure tap thing. Since this does not seem to be a problem with others (myself included) and you are the first person ever whom I have heard suggesting this, it appears to be an answer to a problem that does not exist. We ran muffler pressure for years (to a uniflow tank) without any problems in the pressure system. (Well, once in a great while something might plug up, but this was easily diagnosed and corrected. Even when suspected, it usually was not the case.)

When you say the engine loses power, is it going rich and losing power, or is it going overlean? If it is going overlean, I might suspect the very pressure system you are using as being the culprit rather than the savior. It may be that oil is actually flowing into the bottom line and then as you flip the system over, things change, affecting the pressure for just a bit. If you have a single vent, this is venting all the time and there won't be any oil accumulation to worry about.

As an easy check, put a cap on one of the vents after you fill the tank and run your muffler pressure from the muffler to the other vent only. Check upright and inverted lap times to make sure they are even and see if the problem persists.

Hi

The engine is a Taipan .15 (2.5 cc) gold head, Rear exhaust, 3 transfer ports.
Venturi 6.9 mm ID spray bar is hard to measure without dismantling, my guess is 3.3 mm Dia.

Don't have any photos of the setup but include a drawing of engine and tank in current configuration with the tank inset into the profile fuselage. The muffler outlets are not shown but approximatly half way along each side. I did the double pressure lines because I had a concern about oil laying in either side of the muffler though in flight, it should throw to the end.

I think the engine sounds rich in the inverted pullout. I have not timed inverted laps though it subjectively seems to regain full speed. I do get a little dizzy after a few inverted laps. Have timed upright laps at 110 kmph. Prop is a ballanced master airscrew 8 x 4. After 8 or so inverted laps I go to a wingover and pullout upright without any disruption to engine performance.

The problem only occures when pulling out inverted. After a 1/2 lap inverted, the engine has recovered enough to wingover from inverted and powers up the climb well.

As for the tank, both the inlet and overflow tubes crossover inside the tank. Everthing looks symetrical to me.

Hi Ken,
Thought you might be running the Taipan - For others information, this is a very high performance engine, not unlike a Cox Conquest or rear ex Supertigre. It is designed to run in the 20-25K range (I used them in Goodyear racing), and yes Leonard, that is a "tuned muffler" - a neat 1/4 wave expansion chamber, remove the muffler & the engine goes slower! So my thoughts are:
- It is way overpropped on an 8/4, a 7/4 Taipan or other stiff narrow blade prop would help.
- It should be running on crankcase pressure or a bladder (they worked OK in combat this way) - IF it has the standard CL venturi insert.
- The tank is non-uniflow, but on pressure it wouldn't matter. It may be possible, or even desirable, to install a much smaller venturi to operate on suction/uniflow. I dont really know if this would work either, considering that the porting, particularly the shaft timing is intended for 20K+ this is an engine that wants to run fast.

One question: does it do inside/outside loops OK at reasonably matched speeds, with no funny stuff?

Hi David

Yes, well I've been a little scared to prop this motor correctly.
I had used a polished prop some time ago and ripped the fuse off the wing in flight. Now the fuse is more like a half fuse than a profile and has so far stayed attached to the wing.

It's on a sports trainer with a 1/2 inch flapped wing and does around 3.5 sec laps. That's fast enough for me at the moment so I have deliberatly overproped. Maybe an 8x3 would allow it to rev better whithout going too fast for me. We neasures the revs on ground at 13.5 K but in flight, it must be doing at least 18K to get 110 kmph.

In anycase, I'd like to understand why the motor performs differently in the inverted pullout to the upright plullout.

Oh I forgot, the outside loops seem a little wider and slower than the inside ones but not by much.

Seems to me like it's likely going lean on the outside to inverted, no doubt aggrivated by the low rpm, and an overly large venturi. Why not on the inside? The outside of the tank may be perfectly centered, but is the pickup tube perfectly centered inside the tank??? It may also be simply due to the fuel hose going around one side of the engine. I have heard of folks who'd run the fuel feed hose completely around the engine, to get some kind of symetry. I can't say that I see the logic, but I haven't had that sort of problem, either. I vote to pull the venturi and spraybar apart, measure the bore of the venturi and OD of the spraybar, and check against Leonard's venturi chart. Muffler pressure is barely significant in terms of fuel feed, so I believe you need a smaller venturi or a pressure system. Steve

LAST EDITED ON Mar-29-04 AT 10:21 PM (CDT)
 
Hi Ken,
Hmmm... well, I reckon that if the engine was running faster it might be more stable,hence more easily able to deal with mixture variations - an 8/3 could be an improvement.

It might be time for compromise with the fuel tank position - can you try raising the tank a little? at that rpm & venturi size, a little might go a long way... how about 1.5mm?

BTW is it a steel or ABC engine variant ?

Edit: standard prop size for this engine in F1C FF was 7/3.5 cox copy in fibreglass, thinned as much as your bravery would allow!

>Hi
>
>The engine is a Taipan .15 (2.5 cc) gold head, Rear exhaust,
>3 transfer ports.
>Venturi 6.9 mm ID spray bar is hard to measure without
>dismantling, my guess is 3.3 mm Dia.
>
If the venturi is 6.9 mm, then it is indeed a big one no matter how large the needle (and if your measurements are correct, the needle is small.) So fuel draw could be a big part of your problem. Tank height will then also become very critical, and moving it just slightly one way or another could make a difference. I would opt for going to a smaller venturi to improve the fuel draw, and then with some tank height adjustments you may get there.