View Full Version : 112v DC

27th Jun 2007, 15:30
I've "done" aircraft electrics/electronics from WWII surplus in the '50s to Concorde to helicopters to Mirages.... and somehow I've never come across 112V DC.
Only just discovered the Valiant used it, and the Trident.
Does anybody here know the hows and whys and wherefores, and when it first came into use, and when it faded out?
Is it more or less UK-only, or do other examples exist?
Just curiosity (.... and wondering how I missed it all those years :rolleyes: ).

27th Jun 2007, 20:20
Salut Christiaan -
That "strange voltage" 112V DC was used in the DH Comet and SE Caravelle.
I have no operational experience with these types, but I did fly them a few times (just providing my body as co-pilot for ferry flights) long ago... in the 1970s. The 112V DC requirements was a nightmare for ground handlers.
The RR Avon engines fitted on the Comet and Caravelle had 112V DC electric starters. Both types had a large bank of multiple lead-acid 28V DC batteries in the belly, and the F/E had a selector for "Start 112V DC" or "Normal 28V DC"...
The batteries were switched by this selector to be connected in "series" to obtain 112V DC, for start purpose, then after engine start, the selector was moved back to normal (or parallel) to supply the aircraft 28V DC for most of the electrical requirements, and into inverters to produce 112V AC for various avionics.
United Airlines operated 20 SE210-6Rs in the USA, and while at their stations they had a ground transformer/rectifier to change the regular 115V AC ground power units, these airplanes often carried a portable unit when flying to other destinations.
I had old manuals from Comet 4C (Argentina) and Caravelle (United) but they probably ended in trash somewhere, so I can only inform you with what I remember from them.
I ferried a Comet 4C from ABQ to ORD (ex Mexicana) as F/O around 1975, and the battery bay showed a lot of corrosion (due to the lead-acid batteries). I think the FAA had approved to install big truck 28V DC batteries, for the ferry flight, as the aircraft batteries were dead and beyond their useful life. Other than rather "non standard", both Caravelles and Comets were very nice planes to fly.
They had 4 hydraulic systems - two were engine driven pumps, two were driven by 28V DC pumps - for gear, brakes and flight controls - I think they were named "red", "blue", "green" and "yellow"... Funny is, in case hydraulic system reservoirs were running dry, the AOM mentioned that "coffee or strained tea could be used in case of emergency"... makes me laugh (and milk was NOT recommended) - I actually quote the Hawker Siddeley Comet 4C manual.
Oh, après tout, ma petite MGA "Twin Cam" '58 (ma première auto) avait 2 batteries 6 volts, connectées en série pour produire les 12 volts. :8
Happy contrails

28th Jun 2007, 03:52
As far as I can recall the Bristol Britannia also used 112vDC for starting.
Ground Power Units had two plugs; one for 28vDC and one for 112vDC similar to Comet GPU except Comet needed more amps than Britannia so GPU was beefier.

3rd Jul 2007, 16:41
As I recall, all Comet variants had it as did all Caravelles excluding the last varient I believe. The Bristol Britannia had it, as well as 200volts, frequency wild system for powering the hydraulic pumps The GPU for both the Comet and the Brit had the same output capacity but the Brit took more amps as the very large prop took some turning, especially on a cold day!

Other military bomber types using 112volt were the Vulcan, Mk 1 and 1A only and the Victors Mk1 only.

The use of 112 volts was to start both jet and turbo prop engines thus reducing the load that a 28volt system would have to bear doing the same job. It also powered 115v single phase systems via a rotary invertor.

Batteries were a nightmare to change, especially the Brit which had two banks of 4 which in the civil version you could switch over via a mighty big switch in the cockpit but on the military version you had to physically change the back row to the front and move the front row to the back if you wanted to change the power source. All for the sake of not fitting the big switch!! Happy days.

3rd Jul 2007, 17:44
Thanks all for starting to clear up the 112V DC "mystery" (mystery for me, that was).

Still curious where and when it started, and when it died out (design-wise, I mean .... GPU-wise it would have died 30 or so years later, when the last aircraft using it went out of service).

In my case I started my "career" on Concorde, which uses a small turbine on the accessories gearbox, driven by compressed air from a ground unit, then for the other engine in the same nacelle by a crossfeed of HP air. So no electric starter motors, and no 112V DC.

3rd Jul 2007, 21:56
And we wonder why the British aviation industry died out :confused:

3rd Jul 2007, 22:26
And we wonder why the British aviation industry died outCan' really see the connection. Should we STILL be using 112V DC?

4th Jul 2007, 12:11
Since the Comet 1 preceded all other British and French built Jets of the era, thats the starting point, say early 50's but probably on the drawing board in the late 40's.

I may be wrong but I believe there is a Caravelle still flying in Africa. Or, to put it correctly, was flying last year but still probably capable of flight even though its currently parked. So the 112volt era is probably not quite dead yet.

The ex RAF Britainnia parked at Kemble is live and can taxi and is 28/112v GPU supported. There is a Comet 4 at Bruntingthorpe but whether that is live and can taxi I dont know.

In respect of design, the advent of 400Hz and the control of it did for 112 volt sysems in terms of weight, system compexity and design and build of electrical components.

As to putting it back? Unless there is a return to at least electrically operated landing gear (the Valient Bomber of the 50's), flaps and powered nose wheel steering (to dispense with tow tractors), then its difficult to see a major electrical system redesign to justify it all. But then who would have thought you could build a high content 'plastic' passenger aircraft even 10 years ago! Okay, someone was thinking about it but it wasn't me!

4th Jul 2007, 15:45
Can' really see the connection. Should we STILL be using 112V DC?

No, just a sarcastic comment about how the British aviation industry either was too late, too early, too heavy or just out of sync with the rest of the world :(

4th Jul 2007, 21:28
Interesting article about the electrics in the Boeing Airfix kit (aka as 787).
1.45MW (rather than 1.45 milliwatt, ah those journalists) installed power, with four 250kVA starter-generators.
So electrics may be making a comeback (note to javelin :) ).
But I'm sure this is already being discussed elsewhere... let's stick to the 112V DC here. I'll try to collate all the info, just for myself... no wikipedia aspirations.
For an instant I thought: "Ah, anything with an Avon", but they must have come in different flavours... IIRC the Canberra used gas cartridge starters.

5th Jul 2007, 14:12
Two thoughts
Firstly, the Boeing Airfix kit. Interesting article but I hope that (a) it takes well less than 250Kva from the APU to start an engine as, currently, GPU's only put out 180Kva continuously with not a sensationally long peak load time. Like the big prop, its all about getting the big fans at the front moving fast enough. (b) If the engine needs more than 250Kva and the APU is u/s, wot then, but I suppose this has all been thought of.....

Secondly, 112 volts and the Canberra. Cartridge and IPN starters for sure. BUT there has been talk of a DC start system but based on 28V DC,
so no joy there

In the light of developments and the way the 787 is planned electrically, its 28/200volts from now on in...unless someone else knows something different........

7th Jul 2007, 17:18
From the Flight article:The bulk of the power comes from two 250kVA generators mounted to each engine, which also act as starter units. “That’s half a megawatt in each engine,” says Sinnett. “The units are designed to be activated with one or both, and with both we start quicker.”I don't know the ratio between generating capacity and power absorbed during starting.
But you're right.... gets "interesting"....

16th Sep 2007, 11:35
There is a Comet 4 at Bruntingthorpe but whether that is live and can taxi I dont know.

That Comet is "Canopus". The last of the airworthy comets and yes, last I heard (or at least read about) she can and often does taxi, including regular high speed runs. There are also a dedicated band of men and women (http://user.itl.net/~colonial/comet/latest.html) desperately trying to have her ready to fly in 2008 at the the fiftieth anniversary of regular transatlantic jet flight. Apparently, the only real hurdle to her receiving an airworthyness certificate is the refusal of BAE Systems to release the remaining design drawings.

This may have changed however recently and I'd certainly be interested to hear any more recent news from Bruntingthorpe.

16th Sep 2007, 17:05
Apparently, the only real hurdle to her receiving an airworthyness certificate is the refusal of BAE Systems to release the remaining design drawings.

Now, isn't this just typical.
One wonders, just what is BAe afraid of...someone building a Comet in their garage...:uhoh::ugh:

16th Sep 2007, 17:29
One wonders, just what is BAe afraid of...someone building a Comet in their garage...A Comet maybe not, but what about a Nimrod?

17th Sep 2007, 01:49
A Comet maybe not, but what about a Nimrod?

I suspect that the Nimrod systems have been so changed as to not resemble the Comet in many ways.

13th Oct 2007, 23:07
Yes, Canopus XS235 or G-CPDA is alive and well.
However there are no current plans to restore her to full airworthy condition for the previously mentioned reasons.
Recently, because of the rarity of 112v DC, we had a devil of a job to source a replacement GPU but have now got a Vernon Rolls Royce petrol one. Initially rotating an Avon engine on this system needs around 900 amps of current.
PS If anyone out there has got any spare Comet 4C nose wheel steering arm bolts (that have to be taken in and out for towing) I would be glad to hear from you.