What exactly does a Tacho count?

[dublinpilot]

Ok..

This question has been asked before in various different ways, but the answers have always confused me even more!

So let me try to ask it my way

The tacho is in the centre of the rpm guage. The rpm guage refers to the rpm of the prop (not the engine). Therefore am I correct in saying that the tacho effectively counts revolutions of the prop?

Let me clarify this with an example.

If the prop is set at 2,400 rpm, and it continues at this setting for 1 hour, then it will make 2,400*60=144,000 revolutions in 1 hour. My (ignorant) belief is that the tacho is callibrated to record "x" number of revolutions of the prop as being 1 hour. Lets say 150,000 prop revolutions is calibrated as 1 hour. So in the case of a prop set at 2,400rpm, then 1 hour later the tacho would have recorded 144,000/150,000*1 hour = 0.96 minutes.

Is this understanding correct, or am I completely wrong? If I'm correct, then the MP in no way affects the tacho reading.

For the sake of clarity, I'm only thinking of cruse settings, and not the low rpms that might be used on the ground while taxiing.

Thanks
dp

[Circuit Basher]

dublinpilot - what you have described is exactly what I understand by a tacho gauge. My interpretation is that it is just geared from the drive cable and integrates the number of turns of the prop. It is set to increment by one full unit if the prop is run for one hour at its cruise setting (whether that be 2300 rpm or whatever). If you run at 2530 rpm (ie) at 110%, then the meter will show 1.1 hrs after an hours running.

If you want true running time to be measured, then you must use a Hobbs meter or Datcon meter, which is not driven from the propellor - it is 12V powered (see quote below for discussion of this) - these are both digital cumulative clocks that are independent of engine settings.

See the Web link / quote below for a far better explanation!

Info on Engine Instruments

Q. I have a Hobbs meter fitted to my Beech Bonanza as well as the tach time indicator on the RPM indicator. These two always seem to indicate a small difference. Why is this and which one is more reliable?

A. It is difficult to say which of the two indicators is the more accurate one as it is largely dependant on how the Hobbs meter is activated.

Firstly, let me briefly explain the tach time indication on the RPM Indicator: This is merely a geared drive from the RPM indication which in turn is engine drive via a cable. By implication, this means that the tach time is only 100% accurate at one particular engine RPM, therefore if the engine's RPM at cruise is 2300 RPM (this is where the indicator will be most accurate) and you are running the aircraft on the ground at only 1150 RPM (50% of cruise RPM) the tach time will run at half speed, i.e. indicate 30 minutes for every hour of which the engine run at this RPM. It is therefore important that the correct RPM indicator is fitted to the aircraft while on the other hand this will still imply that 100 hours of engine running time will reflect as 100 hours on the RPM tach indicator.

Now for the Hobbs indicator:
There are a number of different ways of connecting up a Hobbs meter, all of which measure a "different" time. The first and most seldom used method is connecting the Hobbs meter directly to the aircraft battery master switch which means that the Hobbs meter starts clocking time just as soon as the battery is switched on, all be it that the engine is not running. Another method of activating the Hobbs meter is via an airspeed switch or a switch on the undercarriage, which essentially measures flight time and does not record any engine running time on the ground. The third method of activating a Hobbs meter is via an oil pressure switch. As soon as the aircraft engine is started and the oil pressure exceeds a predetermined pressure the switch makes and the Hobbs meter runs.

In light of the above it can be seen that there is no clearly defined answer to your question but I would suggest that you ask your maintenance AMO to determine the method employed to drive the Hobbs meter and then you will know what time is being measured.

[FlyingForFun]

Small point, I know, but....

Engine speed and prop speed are, in almost all piston-engined aircraft, exactly the same. So it doesn't really matter which one you relate your tacho time to.

In those engines where they're not the same (e.g. Rotax engines) the prop is driven by a fixed gearbox - double the engine speed, and you double both the prop speed and the tacho speed. In the case of your 150,000 prop rpm/hr tach, and my 2.2 ratio gearbox, the tach could be described as a (150,000*2.2=)330,000 engine rpm/hr tach and it would mean exactly the same thing. In fact, it would make more sense, since my RPM guage measures engine RPM, not prop RPM.

As for MP, no, that makes no difference. So if you pay by the tacho hour, you'll be best flying at high MP and low RPM. Which is interesting, since that's not exactly an economical way to fly in terms of engine maintenance costs!

Hope that (at least kind of) makes sense!

FFF
-------------

[dublinpilot]

Thanks guys for the replies.

FFF-most of my flying is done in machines with a wobbly prop. Obviously in these cases engine rpm is not directly related to prop rpm.

I've been reading the poh, and looking at the various different prop rpm/mp settings set out there, and just trying to educate myself a bit more on how they affect things!

Thanks for confirming my understanding

dp

[FlyingForFun]

FFF-most of my flying is done in machines with a wobbly prop. Obviously in these cases engine rpm is not directly related to prop rpm

Yes, it is. The prop is still bolted straight onto the front of the engine, and will still turn at the same speed as the engine, wobbly or not.

You're flying along at, say, 2400rpm, 24". You advance the prop lever. The prop blades become finer. Because they are now finer, they don't meet so much air resistance as they turn. So they can turn faster. This results in the engine turning faster, too, since the engine turns at the same speed as the prop, even though it still generates (roughly) the same amount of power and the throttle hasn't moved.

I'm trying to think of a good analagy. The closest I can think of would be driving along a rough surface with long grass, at 20mph, in second gear. Then you reach the edge of the grass. Don't move your right foot, and keep driving. The wheels will meet less resistance now that you're on tarmac, and so they'll begin to move faster. Because they're directly connected to the engine (via a diff, a gearbox and a clutch in this case) the engine will speed up too. (Your natural reaction to this would be to ease off the gas - but I told you not to move your right foot!)

FFF
-------------

[LowNSlow]

dublinpilot as FFF says, unless you are flying a Rotax powered aeroplane or a Cessna 175 (C172 with a geared Continental O-300 engine), your engine rpm is the same as your prop rpm cos it's bolted onto the end of the crankshaft.

[IO540]

dublinpilot

I don't think there is a universal answer to your question because in different planes the readout you refer to can mean different things.

It can be a straight RPM count, running all the time the engine is running. It can be a timer activated by oil pressure, landing gear up, or even airspeed. It can be a straight engine-running-time timer which ignores RPM (that's what I have).

It doesn't really matter (because one logs the actual times anyway) unless you are being charged on the basis of the reading, in which case it's possibly a ripoff.

[Brooklands]

Sorry guys, but I've got to disagree with you on this one. The tachometer is the gauge indicating the R.P.M., and NOT the total number of revolutions an engine has done

The O.E.D agrees with me Tachometer

I'd say the the readout in the centre of the gauge is a Rev Counter, because it counts the total number of revolutions that the engine has done.

However, I agree that it seems to be common practice to refer to the gauge that measures engine speed in R.P.M as a rev counter, rather than a tachometer

Brooklands

[Boing_737]

In both cases, it is true to say that they both record "Pounds spent"


(sorry for the flippancy )

[IO540]

brooklands

I'd say the the readout in the centre of the gauge is a Rev Counter

That is true on a simple all-mechanical instrument only, driven via a cable somewhere off the engine. But not [necessarily] if the rev counter is electronic...

[Dude~]

unless you are being charged on the basis of the reading, in which case it's possibly a ripoff.

Possibly, but not in my experience. I like paying by tacho, beacuse its very precise (in increments of 0.01 of an hour (36 seconds)) and also, taxying around airports doesn't cost me the same rate as cruising does, unlike many schools which charge by hobbs, in increments of 6 minutes.

[dublinpilot]

Thanks guys.

FFF-That makes perfect sense when you explain it like that. You learn something new everyday

[Keef]

Ignoring for the moment the "correct" names of the instant RPM readout (which I understand) and the hour meter (ditto),

[b]What]/b] units exactly does the hour meter count?

I remember in my distant youth the farm tractor (no speedo etc) had an RPM gauge and an hour meter, the latter with the legend "Hours at 1800 RPM".

So: our aircraft "hour meters" count hours at what number of RPM?

[dublinpilot]

Keef,

I think each one would be calibrated differently, according to some recommonded rpm setting.

...I think...


dp