by Tommy H. Thomason

Friday, July 22, 2016

Grumman F8F Bearcat Vertical Tail

Although there are numerous detail differences between the -1 and -2 (see http://tailspintopics.blogspot.com/2010/12/f8f-bearcat-1-vs-2.html), the tail (and now the windscreen, see prior post) is the most obvious.

Note that the -1 is an early-production F8F provided to NACA for flight test and does not have the turnover structure behind the pilot's headrest; the -2 is the -2P photo reconnaissance variant.

For years I've used this Grumman drawing from the F8F SAC to illustrate the difference between the -1 and -2 vertical tail.
Upon closer examination recently, I was chagrined to discover that the difference in height depicted (as opposed to dimensioned) was incorrect. Instead of the -2 tail being 12 inches taller, it scaled to be only about eight inches taller. (As a result, the 1/72 Monogram kit's tail is not quite halfway in height between the -1 and -2 but a little less that halfway.) There was also no difference between the trailing edge of the rudder and the trim tab of the -2 versus the -1, which is now obvious to me in pictures.

The larger tail was not just the result of the -2 having a somewhat higher horsepower R-2800. The -1 Bearcat was in need of more directional stability at small side-slip angles and had marginal directional control during a waveoff. BuAer therefore requested NACA to do an evaluation of vertical tail modifications. These are sketches of the original vertical tail and two variations that were flight tested.

This is the test F8F-1 with the configuration 2 vertical tail:
Note the kludge to increase the chord of the rudder and the lack of the dorsal-fin fairing.

NACA recommended configuration 3 (larger rudder and trim tab) with the addition of a small dorsal-fin fairing like the F8F-1's.

I had assumed that Grumman just added 12 inches to the bottom of the existing F8F-1 vertical fin to save engineering and tooling cost and retained the -1 aft-spar attach structure. However, doing that made the leading-edge angle wrong compared to pretty good pictures taken from the side. It then occurred to me that Grumman probably wouldn't want to change or kludge the location of the attachment of the vertical fin's front spar, which meant that the -2's leading edge had to come down at a steeper angle.

This is the resulting pretty-good approximation of the difference between the -1 and -2 vertical tails (the dorsal fairing is a not so pretty approximation). Note that the angle of the -2's trailing edge changed below the trim tab to be steeper so that it terminated in the same place as the -1's.


3 comments:

  1. T. Thanks for this, seems even slight changes are not that simple under the sheet metal. Nice detective work Dr. Holmes! One question, in the NACA sketches there is a notation of a spring on the left cable. I assume they are talking about the left rudder cable. I am guessing that on a wave off applying right rudder to the max would stretch the spring on the left cable allowing more right deflection once the spring pressure was overcome? A total guess on my part, cannot figure this out... Thanks, Pat D

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    1. According to the NACA report, "The rudder control system of F8F-1 airplane 94873 included a spring on the left rudder cable which had been installed to reduce the tendency of the rudder forces to lighten in right sideslip with tail configurations 2 and 3."

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  2. Got it. They needed to heavy up the rudder force. Another one of those small items needed to make 'em fly right. Thanks again~! P.

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