Light Aircraft Association

Can anyone tell me the minimum pressure that the mechanical fuel pump should output?
As I understand it, the overhaul kits are no longer available. I don't suppose anyone has any kits in their cupboard?

Cheers

Gavin

The minimum pressure output of a mechanical pump on the O-200 is 0psi as it's only producing a flow. That flow should be 51 to 55 lbs/hr multiplied by 125%. Get together with your Inspector and carry out a Fuel Flow check to verify this figure if you're concerned.

Hi Sandy, by design, any pumped system will require a resistance in order to create pressure.
In our aircraft the resistance is the float closing the needle valve. So during a fuel flow check one would of course expect near zero pressure, but in flight the closing of the needle valve causes the pump to create a pressure. As flow increases, pressure drops.
It's the same story with the oil pump, big clearances in bearings causes drop in pressure.

I will though discuss a fuel flow test as I agree this is really the meaningful measure of how effective the system is at getting fuel to the carb, regardless of the pressure.

I guess though, that someone has calculated that given a typical cruise fuel flow on an O-200, say 20Ltr/min, that has to pass through the cross sectional area of the needle to needle seat when open, then a pump with a given output will create a given pressure and this is what we use to monitor the performance of our fuel supply systems.

:?:How do you do a flow check on the mechanical pump?

Gavin, what you say about resistance at the needle seat is correct, there will be a back pressure when it's closed but the O-200 manual does not state a figure. If you want me to commit to some then I'd say 2 to 6 psi. Bear in mind that you could have 6psi built up at idle but if there's only a dribble coming from the pump then it could mean nothing when you open up for take off.

Now....a fuel flow check is to ensure that enough fuel is available at Max revs/takeoff, it's got nothing to do with cruise availablity and 51 to 55 lbs/hr is the figure given in the manual. I make that 40 litres/hr, so your 20litres for "in the cruise" would be well short at take-off.

Ideally a fuel flow check would be carried out with a separate tank plumbed directly to the carb. However, you could do it Rotax-wise and bleed off fuel into a measure, timing it as you go. If there's fuel in excess of the 125% then you're in the clear.

Please also bear in mind that the pump could be functioning perfectly well, but if there's a restriction in the suction side (we've seen it) then the pump will only cavitate and you may only get the aforementioned dribble; It's always a good idea to carry out a gravity check to the Gascolator output too.


Edit:- Would this information be far off the mark. See the foot of page 5. I'll let you do the conversion sums yourself. http://www.lightaircraftassociation.co. ... 050-M1.pdf


ok... tsk, being kind to you, I make it 1.3psi..

Hi Sandy,

Thanks once again for taking the time to look at this and respond.
Fully agree with your logic on the fuel flow.

With regards to the testing of the mechanical pump, I think the following would work, given my carb is fed from 2 lines going into a double banjo bolt at the carb inlet:
Turn up a dummy blank end fitting to the same dimensions as the end fitting from the mechanical pump. Assemble with this in place so the carb is fed only from the electric pump.
Replace the output pipe from the mechanical pump with a suitable pipe that can be securely fixed to a container such as a Jerry can.
Run engine for say 5 minutes and then measure how much fuel is in the can. With the engine running at say 1200rpm for 5 mins, then this should give a margin over how much would be pumped at full climb revs, say 2300rpm.
repeat the process for the electric auxillary pump. Once the fittings and substitute pipes are available oit becomes simple to repeat every year and compare outputs, hopefully highlighting any degradation before it becomes a problem in the air!
Can you add anything to this Sandy?

Regards

Gavin

90 Seconds or less at Max revs should give you a Litre (40litres/hour), but speak to your own Inspector about it. I don't particularly agree with your half revs method but maybe that's because I'm old school and used to standing in the draught.


Edit:- Since it's a Jodel, it's a good idea to repeat the exercise drawing from each of the fuel tanks. That should prove there are no restrictions in the line from tank to selector. The D150 I've just finished doesn't appear to have a tank crossfeed system, so it's safe enough to assume yours is the same setup.

Sandy's quite correct,

It's no use doing a critical 'test' of the pump which is intended to replicate as closely as possible full power take-off and/or climb out by a test when rev's are at nothing more than half speed. Then trusting that a X2 paper multiplication is quite O.K. !

Any element in the line from tank pick up right through to carb. feed pipe could play up at full chat. Especially if the pump itself has any partial failing (say diaphragm, or follower spring) - which means when pressed it may not perform as hoped for.

Deviating from known grounds for a/c operation purely on the basis of 'common sense' is a way strewn with the corpses of people who knew it all ! One might as well merely offer up reliance purely on a paper calculation based on makers' performance data and pipe sizes etc.

I know what I'd like to test. After all why be scared of a full power run, the 'plane does that each & every take-off under P1 control without a problem, so why not once in a while whilst tethered ?

mike hallam.

These methods sound a bit dangerous. How about if you measure the electric pump flow, and then note the fuel pressure from the electric pump when the line is reconnected. Run the engine on the mechanical pump at full power noting the pressure reading, and relating it to the electric pump pressure/flow, ie if the pressure reading on both pumps is the same, it could be logical that the flow is the same.

I prefer to use a tried and tested method, not keen on reinventing wheels, agree a totally certain method is preferable to making assumpions or basing things on calculations.
If anyone wants to come and hold the fuel line I'm happy to run at full power....

Gavin, I don't know about reinventing the wheel, but fuel sloshing about, a makeshift supply line, and someone holding a measuring bottle sounds as if you want to re-discover fire! Lets keep this in perspective, you are trying to establish if the mechanical pump will deliver sufficient fuel to keep the engine running at full power in a climbing attitude. A few fluid ounces here or there is of no consequence.

I've just been looking back over my old training notes and CAIP's.

The requirements USED to be:- "The flow rate must be at least 125% of the consumption rate of the engine at maximum take off power, OR the consumption rate plus 10 Gallons per hour, whichever is the greater"

That would have made the old O-200, 32litres plus 45.4 litres; much more than todays requirements as stated in CS-VLA.