Fuel pressure gauge

[Paul_Sengupta]

The Bulldog POH actually has a graph that gives a direct Fuel flow in UK gal/h vs fuel pressure (thats shown on the gauge as PSI).
In your picture paul, its showing 2.4, which equates to 7.6 gal/hr = 34.5 ltr/hr.

Sounds about right. It's up at 11500ft and I can't lean at that height as much as I can down low. 90 knots IAS down low is normally leaned to just under the 2. Perhaps on the 2 with two on board.

[Skyhawk-N]

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[Rod1]

Can be useful in spotting the onset of fuel vaporization. Recommended (possibly mandatory?) on some LAA aircraft. It is an excellent way of using all the fuel from a tank, provided you switch as soon as the pressure starts to drop.

Rod1

[anglianav8r]

My Rans S6 taildragger has fuelpressure gauge, whereas my previous tricycle geared version didn't.
The difference seems to be that the diaphragm pump is working hard pulling the fuel uphill to the engine, from the fuselage tank, whilst on the ground. My old aircraft had wing tanks and fuel flow was gravity aided.
I have to pump the fuel line to 0.2bar prior to start, which is achieved by a primer bulb. I intend to replace the bulb monstrosity with a 12v pump for use on the ground and in the circuit.

[Awful Charlie]

I think it is part of certification that a low wing (or strictly, a low tank) has to have a fuel pressure gauge to show the pressure delivered to the float bowl (with a carburettor) or the injector metering unit (with fuel injection), and as pointed out, for an injected engine, the pressure in the fuel divider can be used to get a good approximation of the fuel flow.

It wouldn't surprise me to find with high wing tanks, a carburettor setup does not need a fuel pump at all, as the fuel pressure difference between a full tank and nearly empty tank is accommodated for by the float bowl, whereas a fuel injected engine might well have a fuel pump to deliver a constant pressure to the metering unit

[Paul_Sengupta]

Just to make it clear here, the fuel pressure gauge in the initial post isn't the one most people are talking about here. The one most people are talking about is the small one connected between the fuel pump and the carb/throttle body/metering thingie to show what the fuel pressure is being delivered from the tank to the engine.

The one talked about in the initial post was the one that connects to the fuel injection system and can be marked in pressure, flow or percentage power (assuming leaned to the value in the POH), and gives an indication of fuel used.

As Skyhawk N says...

Those fuel flow gauges are just pressure gauges with calibrated gallons/hr markings on it.

And as Ridders points out, if it's not marked in gallons/hr, then there should be a graph in the POH to equate the pressure readings with fuel flow.

The Bulldog POH actually has a graph that gives a direct Fuel flow in UK gal/h vs fuel pressure (thats shown on the gauge as PSI).

[hatzflyer]

I dont see how a fuel pressure gauge can measure flow. If you remove the fuel line from the carb and turn on the pump the flow will be at its greatest but as the pump is going to atmosphere the pressure will be at its lowest.( virtually nil because there is no resistance).

Conversly a partially blocked hose will show an increase in pressure with a reduction in flow.

[Keef]

I had the same question, Hatz. The Arrow (injected) has a fuel flow gauge which goes up and down with throttle and mixture settings. Concorde has a fuel pressure gauge that sits pretty steady at 4 (whatever the units are) when the engine's running, and goes up to 5 with the electric pump on. Mixture and throttle settings make no detectable difference.

The Arrow POH says this:

The fuel flow portion of the manifold fuel flow gauge is connected to the flow divider and monitors fuel pressure. This instrument converts fuel pressure to an indication of fuel flow in gallons per hour and percentage of rated horsepower.

So there you go.

[hatzflyer]

That is a fuel flow gauge.
Any electrical gauge is basically an accurate amp meter or volt meter .
What it is connected to determines what it reads. A pressure transducer alters the current (as you prob know very well ) and drives the gauge.
Using pressure to detirmine flow is a poor way of doing it. it is not overly accurate at best and absolutely bluudy useless if you introduce a resistance (blockage)in the fuel line.
Mind you we are talking Yanks here !

The one in the jodel is pressure pure and simple it lets you know you have a good head of fuel. It can give a hint of potential fuel punp failure if it delines slightly over a period of time it would be worth looking at the pump.
Chances are the pump will fail and then the guage will drop to zero so in reality its as much use as a garden ornament.
Which is why we all keep some fuel in the front tank and run the back one to minimum and let gravity do the work on landing and take off don't we ?

[Keef]

I think the lines are a bit blurred with the leccy example, in that an ammeter could be seen as a voltmeter, measuring the voltage across a known resistance to determine the current flowing through that resistance. That's how my AVO does it, anyway.

I suppose you could acquire a little rotating vane type device to put in the fuel line and use that, but the differential pressure measurement isn't all bad - if the pipe comes off the far end and the fuel pours out, the differential will increase and the device may still read correctly. You'd know you had a problem, anyway.

[Paul_Sengupta]

I think if you have known hole sizes in the fuel injectors, then the greater the pressure, the more fuel you're going to be spraying out. I believe the indication would be higher if you had a blocked injector.

Just did a google and came up with this:

http://www.bobtait.com.au/forum/general-knowledge-agk/1596-blocked-fuel-injector

the fuel flow meter is really just a fuel pressure meter calibrated to indicate flow. While the system is working normally, an increased fuel pressure would be accompanied by an increased fuel flow. However, a blocked injector causes the pressure to increase, but the actual flow decreases. The instrument gives a false indication of increased flow because of the increased pressure.

[Dutch experiment]

From time to time I fly aeroplanes that have a fuel pressure gauge, and while it is interesting to see the needle move I'm not entirely sure what use it is when compared to say fuel flow.

Ian

And if you know what it tells you, your still wondering during the run up check how to interpretate it........well I do
Considering a classic carburetor engine: The fuel pressure is to check if your engine fuel pump is oke. You suppose to see a change during your run up check (in my case a little increase in pressure). If you also have an electrical pump you might see a pressure difference when you switch it on/off and get a indication of the quality of your pump. This is mainly an useful check if you fly low wing aircraft, because gravity feeding at high wing suppose to be enough even during take off. Mind you a high fuel pressure is just as bad as a low pressure.
For a fuel injected engine the usefulness of the fuel pressure is different. You have mechanical fuel pumps or electrical driven pumps (UL power and car conversions) and in any case a fuel regulator. The fuel pressure gives you an indication of the quality of the fuel regulator. If the needle moves a lot, it's time to replace the regulator because your system only works well with a constant fuel pressure.

Ton

[Paul_Sengupta]

That's not the "fuel pressure" gauge he's talking about. The one Ian's talking about is the one that indicates the fuel flow. Sometimes marked in fuel flow, sometimes marked in pressure. Occasionally it's marked in % power.

This one has the markings in GPH but then has pressure for the top and bottom markers. You can see it is the same gauge as in Ian's photo on page 1, just marked up as fuel flow rather than fuel pressure:

http://skywagoncity.com/wp-content/uploads/SW00151.jpg

[Dark-Wing-Duck]

Mine tells me that my backup electric fuel pump is working as it should before I start the engine, something for which a fuel flow sensor would be useless