Cold War era aircraft RPM indicator / tachometer repair

[ScottBouch]

Hi all,

Just thought I'd share this... It's not strictly radio, but an interesting piece of high quality vintage technology all the same!

It contains a three phase motor who's speed is frequency dependent, driven by a tacho-generator on the engine (this is for a jet engine).

There is a permanent magnet on the end of the motor shaft that sits inside a copper bucket shape. as the magnet turns, it induces eddie currents into the copper, which build up their own magnetic fields, and the bucket get's dragged around, working against two small hair springs. As the motor turns faster, the pointer moves up the scale.

It's made nicer as it also has an additional smaller pointer geared up to clearly indicate 10'ths of the main scale.

Anyway, here's my project progress so far:

http://www.scottbouch.com/6A-3251-KT...-repair-01.htm


Cheers, Scott.

[Herald1360]

I guess at jet engine rpm a flexible cable drive direct to the magnet would be "interesting".

[ScottBouch]

For example, the RR Avon would go up to about 8000 rpm - similar ball park to a car engine really... It's only micro gas turbines in drones and models that whizz up to insane speeds.

I've also seen piston engined aircraft use the same 3 phase tacho-generator / tacho-indicator approach back in the 40's / 50's.

I guess it's just easier to route electrical cables to the cockpit!

Cheers, Scott

[Herald1360]

Car ones generally ran slower, driven by a right angle gear drive off the distributor shaft which runs at half engine rpm anyway.

The expense and reliability of the aircraft type system wouldn't be justifiable for a car accessory.

[Radio Wrangler]

The central shaft in a gas turbine is a bit inaccessible, so there is a bevel geared shaft drive outwards to power all the ancillaries which are mounted around the waist of the engine, coupled by chains of gears and driven off of a second bevel gear on the shaft from a bearing housing.

Gas turbine RPM are too fast for pumps etc, so the gear ratios out to the ancillary groups are geared down in speed. I've worked on large engines where some shafts were running around 34000 rpm. It's difficult to remember because speeds were usually expressed and gauged in percent. percent of speed ued for 100% rated power.

David

[G6Tanuki]

Originally Posted by Herald1360

Car ones generally ran slower, driven by a right angle gear drive off the distributor shaft which runs at half engine rpm anyway.

The expense and reliability of the aircraft type system wouldn't be justifiable for a car accessory.

And car-engines only usually operate over something like a 6:1 rev-ratio between idle and peak, which is a lot less than a jet engine.

[Some Jaguars used as a tachometer a small permanent-magnet DC generator connected to the rear of one camshaft, driving a moving-coil voltmeter calibrated in RPM]

The jet-engine electrical approach is intrinsically more-amenable to providing a RPM-signal to other avionics that might be ''interested" in knowing engine-RPM. It's not just the dials-on-the-flight-deck that need to know what RPM the engines are doing!

[Radio Wrangler]

Most jet engines have more than one RPM value to report. Higher performance pure jets often had more than one shaft. The Olympuses which I once worked on while at Rolls had three coaxial shafts coupling three different compressor stages in the cold end to three separate turbine stages in the hot end. Turbofan and turbo prop engines usually have a shaft in the core engine and a separate turbine, gearbox and shaft to drive the fan/prop.

Roughly, a jet engine still produces 10% of rated thrust at idle, so jet aircraft need serious air brakes to land them. You don't want less oomph on the engines because you want them going well enough to be able to bring power up quickly enough just in case.

A typical start-up routine is to spin a turbine engine up to 10% of rated power revs on an electric motor or with compressed air and the ignitors going, before you let fuel in. Below that they don't make enough power to be reliably self-sustaining. The thrust versus revs curve is a curve of a high numerical power, and it gets steep rather abruptly, so the difference in revs between idle and full take-off power is a surprisingly small ratio. Much less than car engines operate over. The sound effects loudness also follows the power curve, also, most passengers see airliner turbofan input stages turning only slowly, but further in, the engine core is running at an appreciable percentage revs.

So if you look at a concorde panel photo, there are 12 rev counters. N1, N2, and N3 for each engine, and these contribute to an analogue electronic fuel control system, so they have to be good.

[turretslug]

Originally Posted by Radio Wrangler

A typical start-up routine is to spin a turbine engine up to 10% of rated power revs on an electric motor or with compressed air and the ignitors going, before you let fuel in. Below that they don't make enough power to be reliably self-sustaining. The thrust versus revs curve is a curve of a high numerical power, and it gets steep rather abruptly, so the difference in revs between idle and full take-off power is a surprisingly small ratio. Much less than car engines operate over. The sound effects loudness also follows the power curve, also, most passengers see airliner turbofan input stages turning only slowly, but further in, the engine core is running at an appreciable percentage revs.

Prior apologies for being thoroughly off-topic, but I love that rapidly ramping-up "bite" followed by stabilising of a gas-turbine start.

There were some quite exquisite mechanisms cooked up for aviation displays before electronics, penetron CRTs and then flat-panel displays took the jewellery aspect out of things. Be wary of anything of around WW2 era with "blobby", by-now-yellowy paint applied to pointers and numbering though as it could well have still-potent radioactive traces in it- just because it doesn't glow now doesn't mean it is safe (the loss of glow is more down to deterioration of the phosphorescent component of the paint, rather than half-lives). The paint can come off as insignificant-looking but hazardous dust inside the instrument.

[ScottBouch]

I've recently been working with industrial Olympus engines from the mid 70's, attached is the startup sequence typical of a hydraulic starter.

The long period at the start is the purge cycle, where the engine is used to blow fresh air through the power turbine and exhaust stack, clearing any unwanted potentially explosive atmospheres (especially important if it's a gas-fuelled engine).

In this Olympus drawing, N1 is the speed of the HP shaft, N2 is the LP shaft, and N3 in this instance is not an engine shaft, but the industrial power turbine that drives the external generator or gas compressor (via a gearbox). N3 on aircraft engines such as the RB211, Trent etc, is the speed of the third turbine, which drives the fan via the very central shaft.

Industrial RB211's and Trents don't need the fan on the front, so have just 2 shafts through the engine, and Power Turbine behind wit shaft going rearwards.

Talking of methods of reading speeds, the industrial engines usually use a magnetic pickup, facing a phonic wheel, this generates a single sine wave. The industrial Avon and Olympus has them externally mounted via a gearbox and ancillary drive shaft. The RB211 has them internally mounted, reading directly form the shafts.

New Technologies:

When I was at RR (about 4 years ago), I took part in a project to replace these shaft driven phonic wheel probes on the Avon with a new kind of speed measuring device.. It was mounted to the front casing, had two magnets that induced eddie currents into the tips of the first row of blades, these eddie currents set up their own magnetic fields which were picked up by coils wrapped around the magnets. It was a very interesting project, the trouble with the Avon is that the blades are pin mounted and drift closer / further away form each other during operation. This resulted in the unit having a microprocessor and software to decide on the engine speed from these coils, the company who developed it in conjunction with RR got the hardware and software SIL rated, so it was good stuff.. This was / is the first microprocessor based engine modification for an Avon!

Cheers, Scott.