Mysterious tach markings

[VNugget]

Maybe Luiz put a different tach generator in and now the tach indicates reverse...his home brew markings could be in percentages of 100% RPM.

Hmm... so far the best idea... but why would he stop at 9?

 

[A Squared]

Hmm... so far the best idea... but why would he stop at 9?

"our amplifiers go all the way up to 11"

 

[ISaidRightTurns]

If this were a problem, wouldn't you think he would also repaint the green arc/red lines? They appear to be painted on the outside of the instrument and non hazardous.

 

[FN FAL]

"our amplifiers go all the way up to 11"

Hehehehe :laugh:

 

[ISaidRightTurns]

I think this is what FNFAL is referencing....​

The tale of the backwards tachometer
The modern way to make a tachometer is to put something electronic in the engine that generates electrical pulses as the engine turns and ship these pulses via a wire to a unit mounted in the instrument panel that counts them. Of course, this approach is much too modern to use on airplanes. The airplanes most of us are familiar with have a gadget in the cockpit with a disk inside which gets turned in proportion to the speed of the engine, and the rate of turn of this disk is registered on a needle displayed on the gadget's face. Typically, this disk is turned by a cable, and the other end of the cable is mechanically driven off the accessory case of the engine. Basically, the engine twists one end of the cable which makes the other end twist the disk inside the tachometer. The M14P uses a similar scheme, except that instead of using a mechanical cable, it uses a 3-phase electrical signal. A "tach generator", which is 3-phase generator, is driven by the engine, a 3-wire electrical cable carries the signal to the tachometer unit in the cockpit, and inside this unit is a 3-phase motor. The 3-phase motor turns at the same rate as the tach generator, and the motor turns the disk that moves the needle. Some might complain that this is the heaviest possible way to make a tachometer. Some might complain that the drive shaft for the tach generator tends to get wobbly, the generator tends to vibrate against the engine mount, and other such mechanical things go wrong. All these complaints may be true, but they are the routine kind of complaints about typical aircraft technology. And, anyway, a failed tachometer in flight is not a first-class problem. I've had a more amusing kind of failure. First of all, why did they use a 3-phase generator and a 3-phase motor? Well, if you had an ordinary 2-phase generator, there would be an ambiguity about which way the motor would turn. Whatever way it happened to start going when you fired up the engine, that would be the way it would continue going. With a 3-phase system, the direction of the motor is completely determined by the direction of the generator. Here are the symptoms of the failure I had. I would start the engine, and the tachometer would read 0. It would stay that way for some unpredictable period of time. Usually, it would eventually start to indicate an r.p.m. that would bear some relationship to the engine r.p.m. Sometimes, the tachometer would start working perfectly normally. But sometimes it would do something pretty strange. At idle, the r.p.m. would read 90%. As I brought up the power on the engine, the r.p.m. would read less and less. At full power it would read 15%. Given the narrative above about the 3-phase and 2-phase systems, I guess it's pretty obvious what had happened. One of the 3-wires had disconnected itself from the tach generator, so sometimes the motor in the tachometer would get started going in one direction and sometimes it would get started going in the other. The fun thing about all this was that, as soon as the tachometer got off 0, I would be OK. You see, it didn't take me very long to learn how to interpret the backwards indications of the tachometer needle. You idle at 90%, take off at 15%, and cruise at 45%. What's the big deal?


Blatently stolen from:
http://home.att.net/~sethfuller/fredabramson.html

 

[VNugget]

I think we may have a winner! Thanks!

 

[VNugget]

P.S. Really cool article, too. If ya can't rent'em, you can read about 'em, I guess.