[Milsurplus] 115 VAC 400 Hz tolerance, and the B-36

[Mike Hanz]

On 1/14/2011 8:48 AM, Ray Fantini wrote:
> I would speculate that's why DC to AC inverters was used because until constant speed drives were developed there was no way to control AC frequency or phase of multiple generators in a large aircraft until the post war forties or the fifties, goes back to the B-36 again.

I'm sure that was a factor, Ray.  The 28vdc bus in the B-29 and other 
combat aircraft was distributed all over the airframe, as were multiple 
junction boxes.  As the avionics evolved (as it did rapidly the last two 
years of the war) it was a simple thing to add another MG-149  
(http://aafradio.org/NASM/Enola_cockpit_026a.jpg ) or even larger 
inverter to the power infrastructure, close to the device requiring the 
power.  The Navy appears to have had a more forward thinking approach 
with their NEA-* ("Navy Electrical Alternator" perhaps?) series of 
engine driven alternators, which did have its own bus in some of the 
larger aircraft, but there was no attempt to worry about frequency 
control so much.  It appears that was a problem that was pushed off on 
the avionics designers, leading to a power frequency range spec of up to 
1,600Hz! :-)   Their only mistake was to choose 800Hz as a standard 
frequency because of inductive load effects that needed lots of 
capacitors to correct.  The 400Hz standard was a better tradeoff in many 
aspects, which is why we still have it today, I suppose.  The Navy 
reluctantly began acquiring the MG-149 for their 400 Hz requirements 
toward the end of the war - there is a blurb about it in the Airborne 
Electrical Maintenance Notes document.  I will say that the Navy 800-1 
and USAAF MG-149 are not all that stable from a frequency standpoint, at 
least the ones I have here.  They do hop about in frequency a bit if you 
have varying loads....I notice the APA-10 panadapter baseline hunts up 
and down slightly with the centrifugal regulator corrections.


> Trivia: the early versions of the B-36 used an ARC-8 (BC-348/ART-13) and later used AN/ARC-21. I am still looking for pictures, parts  or any information on the huge mysterious "drum" radio. Think the last time I brought this up the opinion was it was extinct.

I still see AN/ARC-21 modules on ebay from time to time, even acquired a 
couple with some vague notion of potentially retrofitting my AN/ARC-65 
(bottom of http://aafradio.org/flightdeck/vhf.htm ) back to ARC-21 
condition, but haven't found all of them.  As KK5F mentioned, there are 
a couple of other list members who have them as well, one an original 
ARC-21, IIRC.  The ARC-65 case has a nomenclature plate inside that 
states that it is an ARC-21 part, reflective of the fact that most (but 
not all) of the ARC-65s were converted from an ARC-21 condition.  There 
are two different power supplies for either set - a weighty triple 
dynamotor assembly and a later 400Hz AC replacement.  The AC supply is 
an incredibly small box that is cleaner from a noise and maintenance 
standpoint, but relating back to the subject line, requires a 21 amp 
115vac 400Hz power source in the transmit mode (37 amps while tuning!!) 
in addition to 20 amps of 28vdc, a daunting task even in this day and 
age.  The power connectors are unique enough to demand a *very* hefty 
price tag, even from Bill Perry which is where I found mine.  The latest 
control head for the ARC-65 has an AME (AM Equivalent) position to 
simulate AM transmission (carrier plus one sideband.)  I have the 400 
pound manuals for both the ARC-21 and ARC-65 and can provide details if 
you can tell me what in particular you are interested in.  There are a 
few snippets of interest in the link above (and the 
http://aafradio.org/flightdeck/arc-65.htm sublink) that may satisfy part 
of your curiosity, including a few photos of its components.

73,
Mike   KC4TOS