Spitfire Fuel Tank Senders

[Supermarine]

Apologies for this posting but I'd like to request assistance in repairing electrical components as fitted to the World War 2 Spitfire Fighter Aircraft.

Our company manufactures parts for the restoration of and ongoing support of the worlds airworthy Spitfires.

I saw a post on the forum regarding Guy Martins TV programme on the restoration of the Spitfire, many of our parts were used in that restoration.

The part in question is a wire wound variable resistor used in the fuel tank senders on the aircraft which are now becoming warn as the wiper arm wears through the wire would resistor(basically a glorified Scalextric trigger!)

One of the "bodges" that has been used in the past is to apply solder to the break in the resistor and then to adjust the gauge to give some indication that reflects the amount of fuel in the tanks, not a good idea on an aircraft!!

If you look at the attached pictures you can see and example of the senders minus the sweep arm, one can be seen to have a blob of solder on the windings shorting out a break in the wire wound resistor, the other is in serviceable condition.

To give you an idea of scale the units are 64mm overall diameter and the resistor winding is 30mm.

Does any member of this form know where we can get these units rewound so can repair the fuel sender unit and get it back in the air.

The units are for airworthy aircraft so the work must be of the highest quality.

Any help would be much appreciated.

Regards

Andy

[paulsherwin]

Please stay on topic in this thread. Wider discussions of vintage aircraft issues are OT here and any offending posts will be deleted.

Is it reasonable to rewind these, if so 'wires.co.uk' will have a suitable resistance wire. You could send them a sample to get an equivalent (maybe free given the cause). One way of ensuring the windings don't short is to wind with a coated wire and then rub off the working bit.

[woodchips]

Weld the broken ends together, solder won't work, use a spot welder.

Or remove the resistance tracks and turn them over so the other side is against the wiper, good for another 50 years then.

Can't imagine some wire wound resistor tracks to suit would be hard to find? Is it possible to pull one out, photograph it, and make some accurate dimensions please?

What is the working angle of the wiper? Can't be more than 20-30 degrees? Wings aren't that thick unless the cockpit tank I suppose.

[Biggles]

As you say, the work would have to be of the highest quality to satisfy the airworthiness inspectors. I would have thought that one potential problem with an aftermarket "fix" may be that of a bad contact between wiper and track producing sparks which could ignite aviation spirit vapour present in the fuel tank. Saying that, presumably this system has been deemed safe for the past seventy odd years!
Alan.

[Dickie]

Without wishing to state the obvious, has the OP tried any of the specialist automotive suppliers, e.g. a cursory Google produced:

http://www.tristarrradiator.com/index.html

[high_vacuum_house]

Good evening,
If the wirewound resistor is just in series with an ammeter and a fixed resistor then if you know the resistance you could possibly use a standard off the shelf potentiometer or take the track out from one and fit that into the original housing.

Maybe a motor rewind company could assist in winding. Similarish construction to a variac.

[elanman99]

Andy

Wirewound potentiometers of about the right diameter (your 30mm measurement) are still manufactured and might provide a donor replacement track for your sender. A lot will depend on how 'dismantle-able' the sender (and the new pot) is. All of the senders I have seen are wound with a coated wire which has the coating removed in the wiped area. I cannot imagine any kind of solder or welded repair being a satisfactory solution as it would be very difficult to preserve good contact with the wiper.

I would think the chances of finding a new potentiometer that has a wound element of the right resistance and wattage (although as it looks to be used as a pot rather than a rheostat) might not be easy but whether it could be persuaded to fit in your housing is another matter.

Another thing to bear in mind is that if the 'track' has worn through than I would think that the wiper too is well worn so replacing that might need to be investigated.

I presume this sender has gearing or something like an Archimedian screw to enable it to use more than a small portion of the track?

I am not conversant with the rules or conventions of restoring old aircraft but if the main concern (after safety requirements) is retaining the period appearance then a complete brand new high quality wirewound 'servo' potentiometer could be installed within the old housing and a robustly designed interface circuit could drive the existing contents gauge.

Another thing that I for one, have become more and more aware of, is that if you have a particular problem, so probably do lots of other people! As Dickie pointed out a websearch might lead you to a ready made solution.

Ian

[Oldtestgear]

There are a few specialist companies that might be able to produce these with the appropriate certification and the all important paperwork required for aerospace applications.

I used to work for this company in the 1980s whick has changed ownership & premises since but the website suggests they have the required background:

Precision Varionics Ltd
307 The Commercial Centre
Andover Hampshire UK
SP11 6RU

Tel 01264 334522
Fax 0131 440 3949
E-Mail sales-tech2013@varionics.co.uk

Might be worth asking.

Phil

[Herald1360]

Originally Posted by Biggles

I would have thought that one potential problem with an aftermarket "fix" may be that of a bad contact between wiper and track producing sparks which could ignite aviation spirit vapour present in the fuel tank. Saying that, presumably this system has been deemed safe for the past seventy odd years!
Alan.

The odd spark is quite safe- there's not enough air in the tank to permit the petrol to burn, just an extremely rich petrol vapour.

The same technology (though of significantly lower quality of construction) was used in the petrol tank of pretty much every car as well.

[Radio Wrangler]

It might also be worth contacting some of the potentiometer making firms like Bourns, Penny & Giles... Is Colvern still on the go? They might be interested in helping for a bit of publicity.

David (Designing Mode S/ADSB transponders is the day-job)

[David G4EBT]

Maybe it's just a trick of the light, or the camera angle, but to my eyes - particularly image #1 - the former onto which the resistance wire is wound seems to be progressively thicker when viewed in an anti-clockwise direction. Ordinarily, wire-wound pots are of course linear, but if the former thickness is tapered in width (or height,) the thicker the former, the greater the length of wire per turn, and hence the greater the resistance per turn. If the fuel take is an irregular size - if for example, it's bowl shaper rather than having vertical sides, then lower down the tank, every inch of depth will have progressively less fuel, so a linear pot with an actuator arm which lowers as the fuel level lowers will not give an accurate indication of the fuel level, unless of course the meter is graduated in a non-linear fashion.

Or so it seems to me, sat here flying nothing more hazardous then a desk

Given that this is a safety critical component, it seems to me that it's beyond the scope of a DIY repair and is one for firms who specialise in such work, several of which have been mentioned.

[dominicbeesley]

I know Col (retired) had a go at making some a while back I can't remember if he ever got them "perfect"...https://www.vintage-radio.net/forum/...ad.php?t=60050

[David G4EBT]

Originally Posted by dominicbeesley

I know Col (retired) had a go at making some a while back I can't remember if he ever got them "perfect"...https://www.vintage-radio.net/forum/...ad.php?t=60050

I think he managed to re-wind the car fuel gauge sender, though it's one thing if a car has an inaccurate fuel gauge and comes to a standstill - quite another if it's a Spitfire!

AFIK, despite him attempting to fabricate a replacement for the peculiar pot on the AVO CT160, it came to nothing, but I won't elaborate on that as it's not relevant to this thread.

[Anthony Thomas]

Many automotive fuel gauges have some form of linearity adjustment so a rewind could usualy be accomodated into the calibrating of the gauge prior to fitment, I would think that this is the case on most fuel gauges on this aircrafts era. I do not know if the gauge is moving iron (damped) or bimetal which can take up to a minute to obtain a consistant reading, regardless both gauge types have linearity adjustments to calibrate the dial to the "major calibration points" present on most gauges.

Since this is a fuel related device and submerged in a fuel tank then I would start my enquiries at Burlen Fuel Systems and that they have also rebuilt aircraft carburettors in the past, they have in the past had lots of equipment and facilities to repair similar devices returning them to their former condition.

Tony

[Supermarine]

Many thanks for all your help I will chase up the leads given, sorry for breaking any forum rules with a rash cry for help in supporting our Nation's aviation heritage!
Once again thanks everyone.