Friday, July 1, 2022

McDonnell F4H Total Air Temperature Sensor

 In my monograph on the birth of the McDonnell F4H-1 Phantom II (see, I only made a passing reference to its Central Air Data Computer (CADC) and provided no details on the requisite Total Air Temperature sensor. However, since the subject of its location has come up, herewith is a discussion of the requirement and specifically, its presence on the U.S. Navy F-4s.

As airspeed increases, a simple outside air temperature (OAT) sensor begins to read high due to air compression and friction. Up to about 300 knots the increase is negligible. However, at higher speeds it is increasingly important to correct for it in calculations that require an accurate measurement of OAT. The result was the development of the Total Air Temperature sensor.

Moreover, the proliferation of aircraft systems that relied on basic air data resulted in the desire for a CADC that would serve as a single source for it.

The CADC in the F4H was one of the first applications, if not the first.

The first flight of the F4H was expedited in order to beat its rival, the Chance Vought F8U-3 into the air. That is probably why it had a simplified engine inlet ramp and no TAT sensor. One was subsequently added, though, under the nose.

Note that the sensor is mounted on a post so that its inlet is beyond the boundary layer air.

On the second F4H, the sensor was mounted on the forward-facing nose landing gear door.

For various reasons, however, the sensor was not installed on all of the early F4Hs, most notably on No. 6, the carrier-suitability F4H, where it was probably displaced by the angle-of-attack lights and high-speed flight was not a factor.

The sensor was subsequently relocated to the leading edge of the vertical fin at some point during Block b production (BuNo 145307-145317):

Note that it's not nearly as prominent as depicted in the illustration above from the February 1961 Plane Captain's Handbook and in this location, no standoff is required.

It was subsequently relocated from the vertical fin to just under the left ECS inlet fairing. The reason give for the change was: "Prevent total temperature sensor from being damaged by precipitation."

Almost half of the total F-4Bs produced were delivered with the fin-mounted TAT. According to Peter Greengrass, my go-to F-4 subject matter expert, "It was relocated by ASC-139; production effectivity was Block 16 (BuNo 151399), which first flew in July 1963".

As a result, early F-4Bs (and USAF Cs were delivered with the fin-mounted TAT sensor. For example, VF-102 went aboard Independence in January 1962 with F4H-1s with the fin-mounted TAT:


Other examples provided by Peter include F-4B BuNo 149429 in May 1965:

F-4B BuNo 149406 marked up as the F-110A:

And the first F-4C, 63-7407, photographed at Lambert Field in August 1963:

The RF-4B has the TAT located on the forward-facing nose landing gear door.

1 comment:

  1. Interesting information! I have a picture of 63-7407 with the under ECS intake TAT probe. I wonder if they changed it over or did it have both?