Sikorsky ASW HO3S-3

Sikorsky's S-55 design rectified an increasingly significant shortcoming of his helicopters up to that point, which was the lack of relatively indiscriminate loading of cargo and/or passengers because the single-rotor helicopter could not accommodate a wide center-of-gravity range.  As a result, he lost a Navy competition for a plane-guard helicopter—a mission that he had pioneered, promoted and provided the first helicopters for—to Piasecki, who proposed one with his tandem-rotor configuration that had a wide cg range.

In the S-55, he located the passenger/cargo cabin directly under the rotor and the fuel tanks directly under the cabin, minimizing cg travel due to loading/unloading passengers/cargo and fuel burn. The engine, which had been located directly under the rotor to simplify the drive train, was moved to the  nose. The cockpit, which had been in the nose, was relocated above and forward of the cabin. The Army awarded him a development contract for it on behalf of the U.S. Air Force as the YH-19. First flight was accomplished on 7 November 1949.

A military service has always been quick to adapt a successful helicopter design to its own mission requirements whenever one is generally suitable, since helicopter development generally had and has a low priority relative to other demands for funds. And so it was that the Navy, needing an ASW dipping-sonar test and evaluation helicopter until Bell's HSL ASW helicopter (see http://tommythomason.com/books/Bell-HSL/) completed development, procured the S-55 as its HO4S-1 in April 1950. The 600 hp takeoff rating of the -1's P&W R-1340 proved inadequate for the hover performance required for the dipping-sonar mission so it was replaced for HO4S-3 production with the Wright R-1300 with an 800 hp takeoff rating, although at least initially it was limited to 700 hp by the helicopter's drive system.

As a result of the delays in the development of the Bell HSL that was intended to be the Navy's operational ASW dipping-sonar helicopter, the Navy procured the HO4S-3 as a placeholder until either the Bell HSL or the backup program it had initiated with Sikosky for the HSS (now better known as the H-34) could be deployed.

The multi-tank fuel system in the belly of the fuselage allowed for an opening in the bottom of the fuselage for deployment of the sonar.

Note the large oval-shaped fairing on the belly around the opening. My guess is that this covered up the fuel lines that interconnected the tanks and I would appreciate receiving a more informed explanation.

The bottom of the sonar dome could extend beyond the bottom of the fuselage when not fully retracted. (Note that this HO3S does not have a rescue hoist.)

The configuration of the sonar dome changed over time the one utilized by the HO3S looked like this.

The "ball" was flat on one side, with the antenna surface protected by a wire screen.

The cable reel was housed in the cabin. The enclosure design also changed over time so this may just be representative.

The sonar operator's station looked something like this (there may have been a second seat beside it in some cases):

He could control the depth of the sonar and determine the distance and relative bearing of a contact.

The HO3S-3 could be armed with a single Mk 43 homing torpedo that was mounted on and dropped from a variable-angle rack on the left hand side of the fuselage.

This is the best picture I have of the rack:

This is the HSS rack, which appears to be very similar (note that the forward sway brace has been "cut off" by the draftsman so as not to hide details behind it).

The "lever" extending aft of the rack pulled out the pin from the torpedo that armed it and/or started its turbine.

The addition of the 375-lb torpedo generally limited the operational use of the HO3S to hunter-killer tactics, with the three-man crew of the hunter (pilot, copilot, and sonar operator) reduced to two (pilot and "bombardier") when the torpedo was carried.

Note that the Air Force and Army H-19s were subsequently modified with a different tail boom, bigger vertical fin, and much smaller horizontal stabilizer with no anhedral.

Angling the tail boom down reduced the propensity of main rotor blade strikes on it. As far as I know, no Navy or Marine S-55s received this mod.

Information on and illustrations of the Royal Canadian Navy HO3S-3 can be found here: http://jproc.ca/rrp/rrp3/ho4s3.html

Grumman F6F Hellcat Belly Tank

Another interesting question - what are the different F6F 150-gallon belly tanks?

I don't have a complete answer but I do have some information. First, the F6F belly tank had to be nonstandard because of the location of the oil cooler air outlet.

It was located pretty close to the desired center of gravity, which is where you want the disposable load to be if at all possible so the cg doesn't change much when you use fuel, drop bombs, etc.

When the Navy decided the F6F needed more fuel, Grumman developed an attachment approach evaluated on an early F6F (note the wheel covers) that provided clearance with the oil cooler air outlet.

It was refined to this (note that it has the later rectangular pylon):

The two retaining straps kept the tank from falling off and allowed it to be jettisoned (note the two holes on either side of the oil cooler air outlet above that allow them to be attached to standard bomb shackles). The pylon on the aft end of the tank contained the plumbing that transferred the fuel in the tank to the airplane's fuel system and along with the two sway braces, provided the separation between the tank and the bottom of the fuselage.

The leading and trailing edges of the early production pylon was curved instead of straight as on the subsequent one.

Note the rectangular profile of the later pylon fairing on 69Z versus the early streamlined one on 68Z. The latter is white to match the bottom color of the early F6F camouflage. The tanks were obviously interchangeable.

The early tank appears to have invariably had a vertical flange, separating the left and right halves of the tank.

The later tank had a vertical flange initially as shown in the illustration above and then was subsequently produced with a horizontal flange.

The tank was also changed from aluminum to steel at some point, possibly in conjunction with the change in the pylon fairing shape, because of the relative availability of steel versus aluminum.

In April 1945, a Service Change was issued to enable the F6F to carry the new "universal" 150-gallon tank. The retaining straps were basically the same, the forward sway braces were interconnected, a fuselage mounted strut was added to hold the aft part of the tank away from the fuselage, and a flexible hose was added to transfer the fuel from the tank to the original fitting on the bottom of the fuselage.

Douglas XB-42/43 Main Landing Gear

Sometimes the question is so interesting that I can't stop myself from trying to answer it. In this case it was the configuration and operation of the Douglas XB-42/43 main landing gear. For background on the program, see https://oldmachinepress.com/2017/08/05/douglas-xb-42-mixmaster-attack-bomber/

The interesting question was how the fuselage-mounted landing gear went from being extended with the wheel outboard of the strut (see picture above) to retracted with the wheel inboard of the strut, which was covered by a bulge along the underside of the wing when retracted. My guess is that the retracted arrangement was desired to minimize the internal space required for a wheel well. Note that this picture is of the propeller-driven XB-42, which had a slightly different gear door arrangement than the jet-propelled XB-43's.

I mocked up the strut and wheel using a rotating-head toothbrush, a paper clip, and a toothpick for the angle of rotation.

The answer was that it could be done with a single axis of rotation, angled at about 45 degrees to the strut (the axis of rotation might also be angled a bit laterally but determining that exactly would have been a even bigger time-waster).

The landing gear door arrangement was about as convoluted as I have ever seen, even after it was probably simplified for the B-43.

When retracted, the wheel is covered by three separate doors, the middle one being hinged to the upper one rather than the fuselage. The bigger door that covers the strut is hinged so it drops well down to be out of the way when the gear is retracting. Then there is a small door under the wing that appears to allow the forward side of the upper end of the strut to swing aft and a larger one forward that covers the drag link/retraction actuator after gear retraction.

Ian Shillingford created a 3D video model of the retraction:

One thing he noticed was the vertical fork on the strut: "it is now

obvious that that fork attaches to the rear wing spar when the

undercarriage is extended to transfer the weight away from the strut

hinge and onto the spar".