Tacho Meter

[luckykathryn]

Sorry but the search facility is disabled at the moment so this question may have already been asked before.

Usually when I'm hiring a plane it is on the basis of brakes off to brakes on.

Now I'm being offered the hire of a plane on the basis of per Tacho Hr.

The school is saying that I should get abt 1.10-1.15hrs flying time per tacho hr. Which sound great to me.

What I don't understand is how the tacho meter works. I assumed (obviously wrongly) that as soon as the engine started the tacho meter started as it is combined with the RMP gauge.

Does the meter only start to move when the RMP is increased abv a certain level i.e. 1000 rpm? Or is it linked to the amount of power that the engine produces i.e. manifold pressure.

A simple answer would be great.

Thanks

[Tinstaafl]

The hour meter counter part of the tacho is calibrated to clock over 1 'hour' in a real hour at a certain RPM - typically cruise RPM.

If the revs are less than calibrated then the counter ticks over proportionally slower. If revs are greater then it ticks over faster. Similar to an electricity meter.

In most flights you will average a lower RPM (for the whole of the flight - start up to shut down) than the calibrated RPM. End result is that the counter doesn't tick over an hour in a real hour but typically .8 to .9. Shorter flights at low power settings gain more than longer flights in the cruise or at high power settings.

[luckykathryn]

Thanks Tinstaafl.

There had to be a simple answer.

[witchdoctor]

It's agreat way to pay for rentals, but a rotten way to be paid.

Tinstaafl pretty much has it about right. Go easy on the revs (about 2200 on a C150 is a good cheap cruise - about 85kts and 5hrs enduance) and you'll get plenty of time for your wad.