From a  two-fin tail  (HOK)   via a three-fin tail  (HOK, HUK)  towards the four-fin empennage  (HOK, H-43A, H-43B:

Development of the tail construction for the HOK-1 and later for the H-43A and B was done over a couple of years, starting in 1953.

With Central fin, with fentral fins mounted on the tail boom, or dorsal fins, or both.

The original , 1953 ,  2-fin tail : 

HOK-1 125528 , seen here at Bradley Fld, CT during first acceptance on 28 April 1953. The "NAVY" title was later replaced by "Marines"

This "Aviation Week"  issue 11 May 1953  photo of 125528  shows the earliest tail configuration. Photo via W. Mutza

HOK-1  125531   , Kaman Photo , no date, ca1953

The initial 3-fin tail :

The following two pictures show the first stage of the three-fin tail, described as follows by Mr. Rita :  The tail configuration is one of the initial iterations using another of the same fin type with an area of 21.4 sq ft, before ultimately being replaced by the current elongated 37.2 sq ft  version 

HOK-1  129803  , Kaman Photo ;  note farrings for Radio equipment

HOK-1  129803  , Dayton, OH Sep 1954  ; photo credit MAP

The final three-fin tail, used for all HOK and HUK aircraft :

HOK-1  125530  , Kaman Photo 1020-1 , 30 Aug 1955 ; note the "Bear Paws"

HOK 125531 with T53 engine , unknown date,  Kaman Photo 1044-7, col. NIMilHist

Test Aircraft 129801 : 

HOK-1 129801 , Oct 1973 ,  seen on display at the Bradley Museum grounds, with modified tail configuration. Photo taken and provided by Mr. Sean Carroll.

The tail construction in this Oct73 photo (ventral and no dorsal fins) is explained in the following Kaman Report No.113   : 

Kaman Aircraft Corp, Pilot Report No.113 , dated 12 Aug 1958 :

First flight of standard HOK-1 129801 with prototype servo tail installed, dated 11 Aug 1958. Ventral fins installed on the tail booms (no dorsals). Wide tail and no central fins. Second flight on 12 Aug 1958. Both flights by Chief Test Pilot W.A. Newton.

The report ends : "In way of conclusion and without delving into details as to reasons, it is strongly felt that the dorsal fins should be a firm requirement in the "B"  helicopter".   It was also adopted as such for the H-43A.

This tail configuration of 129801 was also mentioned in a Report about flight tests undertaken over the period 29 July-27 Aug 1958. Other participating aircraft were HOK 125528 and H-43A 58-5524, which flew the major portion of the test flights.

HOK  129801 with Mr. Sean Carroll at Bradley Museum ground   Jun-Jul 1974 

Mr. Rita's comment : Diagonal braces like that shown in the picture were used for a number of test related purposes ranging from safety considerations  such as providing for test purposes an added measure of rollover protection for the fuselage crew compartment in the event of an incident as well as configurational assessments in the development testing of the HOK for obtaining vibratory airframe flight test data to help in defining the necessarystructural interface between the lower cockpit tray area and basic fuselage structure.

As to why it was remounted in the aircraft when given to BradleyMuseum, I can only surmise that it might have also been last used to provide comparison vibration data for a/c 129800 which was involved in Vibration reduction at the time. Usually, at the end of a contract, when an aircraft is disposed of by the Government, all sorts of bits and pieces associated with it's use, and paid for by the Government, go with it. 

Never the less, It's not the brace that ends up as the subject, but that of the Cabin Door Sill Gusset, primarily with it's configurational necessity, removal for the HUK and H-43A fuselage configurations and lastly the H-43B and it's current configuration of necessity. 

The HOK as designed has a large cabin door capability such that the extended lower tub area is the primary forward structure with minimum loads carried by the forward cockpit canopy frames and pilot's overhead roof. However; while adequate structurally, initial cockpit and pilot station vibration data indicated that an increase was required in it's flexural stiffness for minimum cockpit and crew station vibration levels. While  it's obstruction was not an issue for the  Marine's HOK  mission, it needed to be re-considered in light of the HUK and extended SAR and Plane Guard requirements. Shortly after the HOK Carrier Evaluation Program (05 May-06 Jul 1956) with two modified aircraft with wide doors,  a Kaman Engineering Change Proposal (ECP), titled "Removal of Cabin Door Sill Gusset", dated 27 Aug 1956 was approved for both a block change for subsequent HOK-1 orders( a/c s/n series 139xxx and above) and it's addition to the list of ECP's and changes approved for Model HOK-1 helicopter as part of the detail specification for the HUK helicopter.

The 4-fin tail for the H-43A :

HOK-1  N298B  (is supposed to be BuNo 129839)  at Anacostia  April 1958  , photo gained from EBay2014 auction 

H-43A  58-5524  (which is supposed to be a HOK-1, BuNo unknown - who knows, maybe the same N298B or 129839 seen at left)   -

May-June 1958 timeframe    Kaman Photo 2703-1   

The initial 4-fin tail for the H-43B :

H-43B  58-1841 , most probably 13 Dec 1958 , Kaman Photo 2791-3  ,

ventral fins only

H-43B  58-1841 , most probably 13 Dec 1958 ,

photo taken from Internet , Ron Dupas Collection      ventral fins only

H-43B  58-1841 , tiedown testing ,  Kaman Photo 2810-8 , dated  15 Dec 1958 ,  ventral fins only

Mr. Rita recalls :  Since I was directly involved in the flight test instrumentation with 1841 at the time, I can verify the configuration of both the H-43B and H-43A aircraft at the time. However, I am not sure of the exact date when the specific configurational changes were made to the H-43B, but I do know that the ventrals were an interference problem during the tie-down testing phase for personnel when entering or exiting the H-43B from the rear, particularly with fire fighting gear and other equipment like with  oscillograph data magazines. Note also in the picture the bar added between the outbboard fins to prevent running into the ventrals. Note the sticks at the top of the outboard fin which were used to determine rotor blade to fin clearances and, although for monitoring and safety considerations associated with rotor dynamic behavior during the ground test, they would ultimately provide the initial clearance data for removal of the ventrals and installation of the dorsal configuration shown in the March 25th photo, here below

H-43B 58-1841 with  dorsal and no ventral fins  and H-43A 58-1823 at Bloomfield, CT  ; Kaman Photo 3040-1  25 Mar 1959  (photo from EBay16 auction)

H-43B  58-1846     , Edwards AFB, CA   date  17 May 1960  , USAF Photo 164753AC   (coll. Ragay)  , dorsal and no ventral fins

towards the final 4-fin tail of the H-43B : 

Original Empennage Configuration (81846) and Modified Empennage Configuration (91548) during Jun-Aug 1960 , AFFTC Report TR-60-21 ,  May61

From this AFFTC REport :

The H-43B Category II/III system evalu­ation tests were conducted jointly with the Air Defense Command to provide an operational and functional evaluation of the Support Sys­tem in a typical base environment. This report covers 340:20 hours of a planned 500 hour test. The test was terminated early because of a succession of rotor blade-to-tail interference in­cidents. Testing of the secondary mission - fire suppression - was delayed due to the blade-to-tail interference problem. 

Aircraft incidents

A series of rotor blade to tail interference inci­dents have occurred. Three rotor to tail interference incidents occurred at Kaman Aircraft Corporation between September 1959 and March 1960.

The fourth incident occurred at AFFTC on 4 April 1960. One of the right rotor blades clipped the top 2 inches from the right rudder and vertical stabilizer assembly while making a nose down slope landing on a 14 degree slope.  

After the incident at the AFFTC, the following events took place:

5. On 18 April 1960. Stead AFB, Nevada re­ported an accident involving rotor-tail interference while the aircraft was hovering in a crosswind.

          (Ragay : most probably 58-1856)

6. On 29 April 1960, Stead AFB, Nevada re­ported a second accident invoking rotor-tail inter­ference while the aircraft was in aurorotative flight. 

The AFFTC recommended grounding all H-43B's on a May 1960 because of this problem. 

7. Rotor-tail interference was experienced at Kaman Aircraft Corporation on 4 May 1960 during a slope landing. 

As a result of these incidents all H-43B's were grounded on 4 May 1960 in accord­ance with T. O. 1H-43B-518. (A 14).

From a Kaman News Release :

The first production H-43B HUSKIE helicopter with a new and improved tail configuration has been delivered to the U. S. Air Force. The first aircraft went to Warren Air Force Base, Cheyenne, Wyoming. (Ragay : 59-1556 was delivered from Kaman on 06 July 1960)

The Kaman Aircraft Corporation, builders of the helicopter, said the new vertical tail surfaces are 14 inches lower than on previous models.

The new tail design eliminates two vertical surfaces located on the upper surface of the stabilizer and reduces the height of the helicopter's two rudders. Surface area is maintained by the addition of two fixed end plates located 20 inches outboard of the rudders.

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H-43B 58-1841 survived with the earliest tail configuration (dorsal and no ventral fins , because it never entered service -  since 1962 the aircraft was in use as Ground Instruction Aircraft , assigned to 3750 MSG at Sheppard AFB, TX)

 HH-43B   on display at Cresson, TX  , photo 24 Mar 2001  (coll. Ragay) 

last update 21/12/2016