drewboy

gday all.

i have an os 91 fx 4 stroke to go into a spitfire

the fuel tank is well above the center line of the carb

can anyone tell me will i need a pump to regulate fuel flow or

will exahust preasure be sufficent to maintain constant fuel flow?

i think in level flight it will run rich and inverted will be lean due to gravity

regards Drew

firstplaceaviator

My Feedback: (27)

I would go with a pump.I have had the problem you described with a few planes.
On a P-51, the fuel tank was only a 1/4 inch higher than the carb.
The engine went rich every time the plane turned.
After installing a Perry VP-20 Oscillating pump, the problem went away!

I had a pylon plane where the engine should have ben mounted sideways. That would have put the carb directly in line with the tank.
However, the engine was installed vertically on the plane, putting the carb slightly higher than the tank.
Every time I made a high G turn, the fuel would get sucked out of the carb and back into the tank.
After several flights, I couldn't stand another dead stick landing! I installed a Perry P-30 pump on it.
The problem was not completely cured, but it was considerably improved, as the engine would only go lean on rare occassion in a high G turn.

Doug

alan0899

G'day Mate,
Lower the tank, save yourself money, & stuffing around with something that adds weight, & complexity.
A pump which provides excess fuel delivery, because, the tank is too high causing excess fuel delivery, is a very strange concept, if you think about it.
If your tank was too low, or too far from the motor, a pump may be an alternative, but not in this case.

mscic-RCU

My Feedback: (6)

I disagree with the last post. A pump will deliver a constant fuel volume, hence that is why it is a pump. With the tank that high, you will adjust the HSP for a full tank and as the tank empties the engine will become leaner. Lower the tank as low as possible and if it is still high, regulate the fuel flow with a pump.

drewboy

i have placed the tank as low as i can
center of tank is about 25mm above the carb.
do these pumps need crank presure to work? or will they work with exahust preasure?
the 91fx has a ventilated crank conecting to the fuel intake
thanks for your help

regards Drew.

firstplaceaviator

My Feedback: (27)

The Perry VP-20 is your best bet. It operates off of engine vibration.
So, no pressure tap is needed for it. I know that sounds like a gimmick.
But, it works very well! I installed the pump over 4 years ago.
My P-51 is still flying with no problems.
Here is a link to their site: http://www.perrypumps.com/prod02.htm

Doug

drewboy

thanks for your help
Perry vp-20 is on the shopping list

cheers Drew

flapper

Drewboy,
That wouldn't be a Top Flite spitfire would it? I am having the exact same problem with a Magnum/SC 91FS. Have got tank as low as I can and engine inverted. The engine will run at high revs but no matter what adjustment I do on the low end it just floods. Even after initial fueling with the plane sitting level on my bench the syphoning effect starts and the carby fills with fuel!
Could you let us know how you get on with the Perry pump as I may need to go the same way. I don't really want to buy a new engine with pump as the Magnum is new anyway.
Regards
Mark

drewboy

yes flapper it is a topflite spitfire.
i am getting close to covering stages.
just looking for anything that could go wrong.
the build has slowed down now, spare time and money
is hard to come by at the moment.
if you get any more info please let me know

regards Drew

flapper

Try and build as light as you can. I thought I did and I am still way too tail heavy. I moved everything as far forward as I could inside the fuselage. I lightened up the stab and fin as best I could. I have added 350 grammes to the inside upper part of the cowl but this is still not enough. I only have a Magnum .91FS fitted.
Will send a PM with some pics as I don't want to hijack thread.

Anyway, due to the small space for the tank and the fact you need it as far forward as possible, the spraybar on the carby with an inverted four stroke will be 1 to 1.5inches under the centre line of the tank. I tried having the line from the tank running upwards over the mount then down to a filler valve. This seemed to slow the syphoning a bit. Basic plumbing principle of having a the line above the max level of the tank.

Best of luck.