manifold/fuel flow gauge

[mh53j]

Is cessna PN C662037-0109 the correct fuel flow gauge for an I0-360- A1A? The gauge works but fuel flow seems suspect.

Thanks,

Brett

[gmcjetpilot]

Originally Posted by mh53j

Is cessna PN C662037-0109 the correct fuel flow gauge for an I0-360- A1A? The gauge works but fuel flow seems suspect. Thanks, Brett

Brett the Dual MAP/FF gauge measures fuel flow indirectly with pressure if I recall correctly. Fuel flow system is actually sensing pressure, with the gauge set up to correlate pressure to flow rates. If your RV fuel system is not set up like a Cessna's, even with the same engine, the pressure may be different and FF will be off.

The advantage of Cessnas set up is its simple and does not need a fuel flow transducer. The down side it just this issue you are experiencing. An example would be of an injector ins plugged, it provides more back pressure, thus making the fuel flow appear higher.

As you probably know many or most Fuel Flow gauges use a flow transducers use some kind of impeller that spins at when fuel flows, sending a calibrated pulse based on speed of the flow. The fuel flow gauge translates the pulses to Gal/Hr of flow.

Bottom line your Fuel Flow gauge is really a PRESSURE gauge (I think). Its the same on Pipers. Before start you turn the boost pump on, move the mixture and see fuel flow, but really its pressure. You might or may need to calibrate the gauge or the pressure some how. Not sure you can even do that easily or even practically. Check with an instrument repair station. In theory it should work the same as a Cessna with adjustment for your fuel system.

[deene]

Yes, most certified fuel flow gages in small aircraft are really pressure instruments if they have analog (steam) indicators. You can usually see small markings in psi which are used in calibration.

The pressure reading comes from the fuel distribution block or spider. The theory is that the injectors are calibrated nozzles and by reading the pressure drop across them, the flow rate can be determined. The exact analog is using a shunt and measuring the voltage across it to derive current.

The down side of this approach is the nonlinear pressure vs flow ratio, leading to nonlinear gage markings and the need for the injectors to have a very accurate flow rate. Indeed a clogged injector can cause the flow indication to be inaccurately high and confusing since the cylinder with the restricted flow may have a high CHT/EGT indication.

So if you want to use the gage, make very sure it is used with the exact engine you have and you will probably want to have it calibrated.

Deene Ogden
RV8QB
EAA Tech Counselor/Flight Advisor