[ARC5] Navy LM Use

[Mike Morrow]

> ...bring up the question again of why the USN included an LM
> frequency meter with essentially every large aircraft radio
> installation. 

I don't believe this is a valid observation.  No WWII-vintage
USN transmitter or receiver system had the capability to be
set on frequency accurately with the indications on the radio
set itself until the ATC.  Most liaison transmitting systems
of the era have a CFI input terminal that goes to the LM-*,
the LM was integral to frequency adjustment.

An accurate assessment of USN installations would simply be
that *ALL* aircraft with a radio operator (and liaison system)
always carried the LM-*.  Aircraft without a radio operator
did not.  The only exception might be the large single-engine
aircraft like the SBD, TBD, and the like, which had radio
operator/gunners.  I don't know if an LM was always standard
in such aircraft. 

> This continued postwar, with the Navy version of the C-119 having
> an LM installed.

That's an obscure citation...there were only about 90 total R4Q-1 and
R4Q-2 aircraft built.  But nonetheless, an LM-* would indeed be on board,
just like a SCR-211-* would be on board the C-119* that didn't have
something like an AN/ARC-21 or later liaison set.

> Of course the USAAF had the BC-221 and theoretically, at least,
> the unit of issue was one with every B-17 and B-24 and perhaps
> other aircraft as well...

The statement I made above for the USN use of the LM-* applies to
the USAAF and use of the SCR-211-*.  All aircraft with liaison
sets and radio operators had the SCR-211-* on board.  USAAF radio
sets were no more or less accurately set and maintained than their
USN equivalents.

> - but it is rarely seen in photographs.

Which doesn't mean it wasn't there.  It most definitely was.

> So why did the USN place such emphasis on having an LM freq
> meter on board?  Were their radios more prone to getting off
> frequency or was there some operational need...

There is *no* difference between the USN use of the LM-* and the USAAF
use of the SCR-221-* that exists to be explained.

The real question about frequency meter use applies to both USN and USAAF
installations.  The very first liaison transmitter that had a built-in
accurate calibration frequency indicator (CFI) was the ATC.  Without
*any* external frequency standard, the ATC (and T-47/ART-13) could be
operator set at 5 or 10 kHz intervals, depending on frequency.  The USAAF's
T-47A/ART-13 was even better...it could be set to 1 kHz accuracy for the
full 2 to 18.1 MHz range without use of any external frequency standard.
Once the liaison transmitter is set, the liaison receiver is easily
netted to the transmitter. (The USAAF did a better job of this with
the NORMAL-MONITOR netting switch that was part of the AN/ARC-8.)

There was little need for a frequency meter for liaison sets with
the T-47 or T-47A.  There was none needed for the crystal-controlled
commend sets like the USN AN/ARC-1 and -12 or the USAAF AN/ARC-3.
Yet often a LM-* or SCR-221-* was still carried.

The LM-* was even less important for the USN AN/ARC-25, which is the
combo of the AN/ART-13 and the AN/ARR-15.  In this system, the AN/ARR-15
receiver can easily be set to 1 kHz accuracy without external calibration.
Yet, the LM-* often appeared even where the AN/ARC-25 was installed.

In USAAF aircraft, the SCR-211-* disappeared as AN/ARC-21 HF sets replaced
the AN/ARC-8.  In USN aircraft, the LM-* disappeared as the AN/ARC-25 was
replaced by the AN/ARC-38.

Mike / KK5F