B-36 electrical system Q.

[John Mackesy]

I've been reading 'MAGNESIUM OVERCAST - The Story of the Convair B-36', by
Dennis R Jenkins, and have been finding it absolutely fascinating. Plenty of
hard-core techno stuff, esp. powerplant, a subject near and dear to my
heart. What doesn't seem to be covered is the B-36's electrical system.
Judging from the electronic equipment fitted, I would assume this to be 28V
DC and 115V 'wild frequency' AC (B-36 seems a bit early for CSD's). My
questions are:

Was there a DC and an AC generator on every engine?
Single phase or 3-phase AC?
Were rotary inverters used?
How was the aircraft's electrical system organized?

John Mackesy

[[email protected]]

"John Mackesy" > wrote:

>I've been reading 'MAGNESIUM OVERCAST - The Story of the Convair B-36', by
>Dennis R Jenkins, and have been finding it absolutely fascinating. Plenty of
>hard-core techno stuff, esp. powerplant, a subject near and dear to my
>heart. What doesn't seem to be covered is the B-36's electrical system.
>Judging from the electronic equipment fitted, I would assume this to be 28V
>DC and 115V 'wild frequency' AC (B-36 seems a bit early for CSD's). My
>questions are:
>
>Was there a DC and an AC generator on every engine?
>Single phase or 3-phase AC?
>Were rotary inverters used?
>How was the aircraft's electrical system organized?
>
>John Mackesy
>
>
>
Watching this space with keen interest...
--

-Gord.

[Paul Hirose]

I believe the B-36 was the first production aircraft to generate its
power at 115 V 400 Hz. Engines 2, 3, 4, and 5 had alternators with
constant speed drives. If load permitted, normal procedure was to have
three on line, paralleled, and equally sharing the load, with a fourth
excited and standing by. Under high load all four would be paralleled.

The bus tie arrangement was peculiar. Conceptually, each bus formed
one side of a square. (It was depicted that way on the AC control
panel.) The four bus tie contactors were at the corners of the square.
For example, contactors tied bus 2 to its neighbors 3 and 5.

By contrast, all the planes I've worked on had an AC system arranged
like a fork with four tines. Each tine (generator bus) was connected
to the fork body (tie bus) via a bus tie contactor.

Either way you end up with four BTCs and power supply redundancy. But
the B-36 arrangement was like nothing I've seen elsewhere. It does
have the advantage that all four buses remain in parallel after any
single bus tie contactor fails open.

To supply 28 VDC the B-36 had seven transformer-rectifier units
powered by 115 VAC. They used selenium plate rectifiers. A central TR
unit test panel allowed you to check loads and voltages.

Essentially, the power went into one big DC bus. There weren't DC
sub-buses fed through relays like other planes I've seen. Turning the
battery switch on applied its power to the whole DC system. The only
exceptions were the alarm bell, IFF destructor, and parking brakes.
Those items were hot regardless of the battery switch position.

Some B-36s had a feature which automatically disconnected the battery
if AC was lost. This conserved its precious power while you turned off
non-essential DC loads, whereupon you threw the switch to the
emergency position to turn the battery on again.

For emergency AC power, a manual rotary switch selected any alternator
of the four and routed its output to one TR unit and an instrument
power transformer. This bypassed the contactors and forced those units
to accept whatever the alternator was putting out.

Some B-36s also had an inverter so backup instrument AC could come
from the battery too.

John Mackesy wrote:
>
> B-36 seems a bit early for CSD's

The alternator constant speed drives gave much trouble, especially in
the early years. Sometimes crews took off without a full set of four
good ones.

SAC pressed very hard to get B-36s over Washington for Truman's
inauguration in 1949. (Ever seen the "B-36s over the Capitol" photo?)
Some of the planes landed with only one alternator operating.

Later that year a B-36 lost all AC power at night. The crew spotted an
airfield (Westover AFB, Mass.) and circled it until they could make a
daylight landing. The fire department had to snuff the engines with
foam because the electrical engine controls were inoperative. (I've
read "foam" but I wonder if they actually used CO2.)

The alternator was the first thing you encountered after entering the
engine cooling air inlet at the wing leading edge. In Myers Jacobsen's
B-36 book, one former electrician describes going in there before
engine start so he could make some checks under power. First there was
a lot of coughing and shaking as the engine cranked, then a fierce
draft from the engine cooling fan right behind the alternator. He
could hardly breathe as he took readings with his meter. There was
nothing between him and the whirling blades. Even by the relaxed
safety standards of the 1950s it was pretty crazy.

--

Paul Hirose >