sarboy w****r

Hi All,

There are a number of instruments which have a mechanical-type display of numbers, e.g. the pressure settings (mb) dial on an altimeter and the altitude drums on an altimeter, or the DME distance on an HSI. Old-style devices use mechanical counters, whereas newer instruments use LEDs (or EFIS!). I'm interested in how the mechanical devices work. Many of the counters look like this:



Each dial is about 20mm in diameter, as each number (0 - 9) is about 6mm high.

These are really simple for when the device just counts in a decimal fashion, i.e. each dial moves 1/10th of a revolution when the preceding dial has moved a full revolution. For a course display (degrees from 000 - 359), it's slightly different. From what I can see, the units dial moves as per normal, which then drives the next dial one step per revolution of the units dial, but this next dial has two digits on it (00, 01,…. 33, 34, 35) and then rolls over to 00 again, i.e. there are 36 separate numbers, each having 2 digits.

The problem I have is that if each number is about 6mm high and there are 36 numbers around a dial then the total circumference of the dial is 216mm (36 x 6). This means the diameter of the dial is approx 68.75mm (diameter = circumference / pi). This means that the centre of the numbers (i.e. centre of the dial) has to be at least ~35mm (the radius) away from the edge of the instrument so that the dial doesn't extend beyond the edge of the instrument. However, from the instruments I've seen, the numbers are much closer to the edge of the device than this. For example:



From the dimensions I have for this particular instrument, I believe the centre of the course numbers (352) is below the outside of the instrument case by about 20-25mm, yet the numbers are bigger than would be possible if they were mounted on the circumference of a dial that fits in this space.

So my question is, have you ever seen how these dials work?

An alternative would be to use a belt (like a metal/rubber/silicon/mylar timing belt) on which the numbers are printed on the outside of the belt (and so the belt could be bent internally to fit whatever space is available), but this seems like an unusually complicated solution, but I could be wrong….

Any insights into how these work (or even better, a picture of the internals of one) would be very gratefully received.

Many thanks.

BOAC

sarboy - firstly I cannot offer any help, but I felt it would be useful to let you know your question had been looked at and to raise it to the top again. I admit the puzzle had never surfaced in my head before, but now, like you, I hope an instrument expert can come along and tell us how 'the impossible' is done. Of course, with 3 drums, only 0-3 and 0-6 would be needed on drums 1 and 2, but 3..................?

There has to be a 'head-smacking' very simple answer

Notso Fantastic

40 years I was flying airliners and I never noticed this! When you wind the course counter, I don't think the hundreds counter turns as well every 10 degrees....but I can't see the line between the hundreds and tens.......

sarboy w****r

Thanks very much - I've acquired a smashed up HSI and will be dismantling it when it arrives to find out how this works.

If there are 3 drums then the LHS drum needs to display 0-3, the remaining two need to display 0-9. The middle one actually displays 0-9 for the first 300 degrees being displayed, then 0-5 for the next 60, then goes back to 0-9. Hence there are 36 separate states to display and hence the left-hand 2 dials are linked together as it makes no sense to have a separate dial for the hundreds unit to revolve at 1/36 of the speed of the units dial when the tens dial is also having to rotate at this speed too.

BOAC

- quite right - I think I had 'one of those moments'

Interested in the 'dismantling' when you have done.

Tinstaafl

Please take pictures.

jeroen79

This one uses a single tape for the 10's and 100's:
Secret Project revealed - TACAN HSI Instrument for FSX - Cockpit Building - Mutley's Hangar Forums

sarboy w****r

Hi Jeroen,

Thanks very much, that turned out to be exactly how it was done in the HSI I acquired today. The range dial was different, and instead of LED 7-segment displays was actually 3 dials on which 0-9 numerals were printed. These 3 dials were actually driven by 3 separate motors instead of a geneva wheel arrangement as pictured in my initial post - I'm not sure why this is the case, but perhaps when the aircraft is performing under significant G then this makes it more reliable.

Kind regards,

SBW

jeroen79

My guess why the range digits drums are operated independantly is that this way they can be set to any value as fast as possible.
For example, if the range were to change from 999 to 000 (like when the DME is lost) and the drums are linked then they would all have to wind all the way down, making the lowest digit spin 100 times.
With independant drums each drum would need to rotate no more than one turn.

sarboy w****r

Ahhh, that makes perfect sense, thank you!