fuel flow/C.H.T. meters
 

Hi, We've got a Robin DR400 with a Lycoming 360 engine with one of those horrible carburettor things. I'm not that happy with just pulling out the mixture control until the R.P.M. drops, and then pushing it in a bit, so can anyone suggest a good combined C.H.T./Fuel Flow Meter that we could fit to our pride and joy?

Please avoid giving answers that are going to require us to sell the plane in order to afford it!!!

Ta.

The standard products are an EDM700 for the CHTs/EGTs and there is a version with fuel flow.

For separate fuel flow instruments there is the EDM450, or ones from Shadin.

For a proper install, the fuel flow totaliser links into your GPS for a constantly updated computation of the FOB at landing.

These instruments are fairly cheap especially if bought direct from the USA, but the installation cost can be significant - get some quotes showing the anticipated labour hours.

Personally I am not convinced multi probe EGT's are worth the money on a simple carb equiped 4 cylinder. The mixture distribution between the cylinder
is quite uneven, making it difficult to finesse the leaning procedure. My experience with leaning a PA 28 -180 first by the installed sophisticated multi probe EGT and then repeating the exercise by just leaning untill the engine ran rough and then enrichening to smooth operation got me to the same mixture knob position.

A different story for fuel injected engines, especially the bigger 6 cylinder motors. Here a multiprobe EGT is definitely necessary to properly lean the engine

If you want to spend money then IMO spend it on an electronic fuel flow instrument. properly set up these can be amazingly accurate and compensate for the utterly useless fuel quantity guages fitted in almost all light aircraft

If you think that you have , it's time to overhaul the carb!

Some thoughts for you...

CHT = Cylinder Head Temperature [indicator] is meaningless for leaning. EGT = Exhaust Gas Temperature [ndicator] much more useful. As Big Pistons says, the fuel distribution will be rather uneven between the cylinders on carb'd engines. I do, however, recommend a four cylinder scanner EGT because it allows you to see which cylinder is hotter or colder in real time. My experience (more than 2500 hours using a scanner on carb'd engines) is that often the fuel distritibution will change with power setting. You may find a power setting in a useful range, where the mixture distribution is even enough that good leaning really works well for you. One place where EGT is wonderful, is the occasion of sticking open an exhaust valve. When this happens, you'll know which one right away, and that will save expensive diagnosis, and removal of he wrong cylinders.

As for fuel flow, yes, they will work with carbs, but not as well as fuel injection. As carbs have a float bowl, and metering valve, the flow on the back side of that valve, is not necessarily representative of the instantanious fuel flow in the engine. I certainly would not attempt to lean a carb'd engine with a fuel flow indicator, other than over a rather long "trend" period. For fuel systems with no fuel pump, the fuel flow systems make me nervous, as they have very small flow holes in the transducer, and it wold take little to block them, with no pump presure to keep the flow going. We had one in a test C150 once, but I would not do it again.

Generations of pilots have managed very well with carbs for more than half a century. Yes, fuel injection offers some real benefits, but carbs still work, and will often continue to work in conditions where fuel injection becomes unreliable, as long as the carb is operated properly. This, particularly, in the context of a fuel pump or flow problem. With your wits, you can nurse a carb'd engine home more easily than an injected one after a problem has become apparent.

I hope this helps...

DAR

You raise some good points. One clarification: I did not mean to imply that an electronic fuel flow meter be used for leaning, rather that if you "have" to spend some money on an electronic gizzmo a fuel flow meter is a worth considering because it will give you accurate indications of fuel used and fuel remaining (if properly set at the start of the flight) which will be more usefull than the pathetic fuel guages. The Robin has a fuel pump but your caution about fuel flow meters in gravity feed fuel sustems is however a good point.

I am still not convinced you can consistantly get an engine to run significantly more leaned with a multiprobe EGT than by the old lean to roughness method. The engine diagnostic capabilities are real but IMO not alone worth the cost of the advanced EGT systems. I think a multi probe CHT is of more use because IMO excessively high CHT is a far more widespread problem than sticking valves, and will usually reduced cylinder life. The only other way to detect a too hot engine is oil temp, but by the time the oil temp has spiked you will have allready hurt the cylinder. CHT will also show deteriorating baffles, a common problem

Yes, I tend to agree.

Quite a lot of people have fitted an EDM to a carb engine and found that the engine goes too rough long before they reach peak EGT or anywhere past that (best SFC point).

The benefit of an accurate fuel totaliser is fantastic - a hugely useful add-on which transforms one's ability to do significant trips, in any aircraft. It's not that pricey to install; the instrument is about $1k, and then you typically have two teflon hoses at $150 the lot, a few fittings, and the $600 Flo-scan 201B transducer. Make sure the wiring to the transducer is done with shielded cable; one of UK's best known avionics shops doesn't know this... I have some installation details written up; if anybody is interested drop me an email. Basically, you implement the old Shadin STC as far as the installation details are concerned. The hose runs are critical to avoid turbulence errors. I have an old version of the Shadin Microflo (the TBM700 version) in my TB20 and it is accurate to about 1% - a good deal better than most airport pumps south of the Alps ;) ;)