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 drops well down on large goose-neck hinges 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".

F4H (F-4) Phantom Main Landing Gear Shrink Mechanism

Airplane designers sometimes want to make the landing gear shorter when it is retracted, i.e. compress the shock strut from its extended position when it is in the air, for various reasons. Here are a couple of examples of nose gears that "shrink":

http://tailhooktopics.blogspot.com/2019/07/mcdonnell-f2h-banshee-nose-landing-gear.html

https://tailhooktopics.blogspot.com/2019/04/mcdonnell-f3h-demon-nose-landing-gear.html

The main landing gear on the F4H Phantom is another example but one that is even less obvious. Note that the tricky part of providing a shrink capability is that the lower portion of the strut must be allowed to extend and compress freely when the gear is extended.

This Jean-Marc Moulin picture shows the basic elements of the F4H main landing gear shrink mechanism.

Note that there are three components to the mechanism: the upper shrink link, the lower shrink link, and the idler that connects them, all located on the rear outboard side of the main landing gear shock strut.  The lower shrink link is attached to the lower part of the shock strut and can slide freely through the idler until the link reaches a stop on its upper end (the scissors or some other feature within the strut probably establishes the length of lower strut travel but the stop is necessary for the shrink function). That allows the shock strut to extend about 17-18 inches when the wheel is not on the ground. Also note the location of the pivot point of the upper shrink link, which is outboard of the pivot point of the main landing gear strut itself.

When the landing gear is retracted, these three components, combined with the location of the upper shrink link pivot point, cause the shock strut to compress significantly.

Because the upper shrink link pivot point is outboard of the strut's pivot point, it is in effect shortened that many inches relative to the shock strut when the landing gear is retracted. That pulls the idler up against the strut, which causes the lower shrink link to pull the lower part of the shock strut on which the wheel is mounted back up into the upper part.

This Howard Mason picture shows the components of the mechanism on the left main gear from the rear.

 Howard Mason

McDonnell F2H Banshee Nose Landing Gear

McDonnell redesigned the nose landing gear of the F2H-1/2 Banshee for the heavier F2H-3/4 so that the strut could be extended for a catapult launch at a higher initial angle of attack.

The -1/2 nose gear was a trailing-arm configuration with no separate scissors to keep the upper and lower sections of the shock strut aligned. The -3/4 had a separate scissors with the nose gear fork swiveling on a bearing mounted on the front of the shock strut.

The nose wheels were the same diameter. The redesign resulted in it being located farther forward relative to the nose wheel well.

Note that the nose wheel axle is located directly under the shock strut on the -3/4 Banshee rather than significantly behind it as on the -1/2 and the nose landing gear doors are splayed out on the -1/2 and vertical on the -3/4.

The shimmy damper was relocated to the lower side of the shock strut and a shrink strut was added to compress the shock strut as the landing gear retracted.

Most of this detail was hidden by the nose gear doors when the airplane was on the ground.

North American FJ-4/4B Main Landing Gear

A question has been asked about the accuracy of the FJ-4 main landing gear in model kits.

The FJ-4 main landing gear was different from the FJ-2 and FJ-3's. It was a so-called trailing arm configuration, which separated the landing gear strut from the shock absorption function. My guess is that this was done in this instance to reduce the width of the landing gear when viewed from the front, because the FJ-4 wing was thinner to increase the airplane's critical Mach number (there are other benefits - see John Eaton's comment below).

This is the best picture I've found so far of the configuration:

Note that the piston of the shock strut extends above it and shock strut is attached to the one-piece trailing arm forward of the wheel's axle. Roughly speaking, a line drawn between the trailing arm pivot point (see below) and the wheel axle should be parallel to the ground.

Static-display FJ-4s may have the shock strut fully collapsed as in this picture:

Photo by Fotios Rouch

What's worse, the display FJ-4B might not have the correct wheel hub:

Bill Spidle Photos

 The actual main landing gear hub/wheel appears to be identical to the F4D's:
https://tailspintopics.blogspot.com/2015/07/its-not-that-easy-to-avoid-error.html

The main landing gear is angled aft, probably because of the desired location for the mounting structure in the wing versus the required location of the wheel relative to the airplane cg. The trailing arm is not accurately depicted - it actually has a kink in it between the gear strut and the wheel as shown above.

Note that the tire diameter is given as 26", which is the green circle.

The kink in the trailing arm is required to allow full extension of the shock absorber.

The main landing gear wheel doesn't appear to have changed during its service life, always being the "forged" configuration. The nose landing gear wheel does seem to be different in some instances.

Note that the "link" extending aft of the shock strut is attached to the swiveling section of the nose landing gear and "rides" a ramp in the wheel well to mechanically rotate the wheel so it lies flat in the wheel well.

Proper "sit" is important to a model's appearance on its landing gear. Note however, that it will vary with the weight of the airplane (with and without full fuel and external stores). If the airplane's brakes are applied while it is being towed or pushed back, the nose gear strut may also compress or extend slightly, changing the true static sit.

Note that I've corrected the original drawing for height vs width using the water lines versus fuselage stations and also sloppy drafting. The dashed line is the location of the ground with respect to the overall heights given above the canopy and the vertical fin. (For 1/48, the box is 9" x 13.5")

Grumman F9F-8 Upper Control Surface Color

Every so often, there is a color-scheme or markings query that calls into question how sure one can be about the answer.

Today's example is the color of the upper surface of the flight controls of an F9F-8 Cougar in the Gray/White scheme. At least early on, new production aircraft had gray rudders (the scheme would eventually require white rudders but an F9F-8 with a white rudder is pretty rare). The whole upper surface of the horizontal tail was white, rather than just the elevators. The upper surface of the flaps was white as was the small trimmer on the trailing edge of the left wing near its tip. However, the roll-control spoilers forward of the flaps were initially gray, although the aileron equivalent was usually white like the elevators in order to minimize damage from the thermal effect from the detonation of a near-by nuclear bomb. The roll-control spoilers were subsequently painted white.

Note that the color scheme subsequently approved for use by Navy overhaul and repair facilities might not be exactly the same as the one that Grumman signed up for in a production contract.

And then there's this picture of four F9F-8Bs, circa 1957:

Note that two have white horizontal stabilizers, one has gray, and one is something else (possibly spares that were already painted gloss sea blue). The two with non-white horizontal stabilizers have white spoilers.

As a general rule, however,  F9F-8s in Navy fighter squadrons with a single-letter tail code do not have white spoilers while those with two-letter tail codes do (the change to two-letter codes was decided upon in mid-1956 but was probably not fully complied with for at least a few months). There are exceptions. Also, with the exception of the above picture, I haven't noticed any Navy F9F-8s that have the upper surface of the horizontal stabilizers painted.

Comments, corrections, and additions welcome.

McDonnell F3H Demon Nose Landing Gear

Most landing gears consist of an oleo or cylinder mounted to aircraft structure and a piston on which the wheel is mounted. The piston functions as a shock absorber. The two are connected with a mechanism that does not allow the piston to twist within the oleo. Often that is a torque link or scissors.

The scissors on the F3H nose landing gear is more complicated than usual because it also provides the means to shrink the unit to fit in a shorter wheel well. Instead of two links, there are three. Two shrink struts, one on each side of the scissors, pull the piston up as the nose landing gear is retracted. Note that the right-hand shrink strut, but not the left-hand,  has a bend in it to provide clearance with the landing gear strut and the lower end of the middle link extends beyond its connection to the lower link.

The shrink link is angled slightly downward and does not move as the piston absorbs the shock of landing. The middle and lower links keep the oleo and the piston aligned.

However, as the landing gear is retracted, the location of the upper mounting point of the shrink struts ahead of the mounting point of the landing gear strut causes the shrink link to be rotated upward to be parallel to and lie against the oleo. Since the pivot point between the shrink link and the middle link has moved up, the middle link moves up, pulling the lower link and the piston with it, shortening the nose landing gear assembly.

The lower section of the middle link that extends beyond the pivot point either keeps the scissors from over-centering or takes some of the load off the pivot point, possibly both.

Grumman S2F/WF/TF Engine Nacelle Detail

Every once in a while someone asks a simple question about an airplane I happen to know quite a lot about and I don't know the answer. In this case, it was the engine exhaust system on the Grumman S2F/WF/TF (S-2/E-1/C-1). The upper exhausts are obvious (this is a picture of an early S2F-1):

The question basically was, did the exhaust gas from all of the cylinders come out here or were there other exhaust stacks on the nacelle. You'd think the answer would have been included in the S2F monograph that I wrote with Bob Kowalski (see http://tommythomason.com/books/grumman-s2f/). It wasn't.

It turns out that in addition to the two troughs on the top of the nacelle there was a third, virtually identical one located on the bottom of the nacelle:

It ended at the forward edge of the oil cooler door. Note the convex stiffener running longitudinally down the center of the trough. The oil cooler door is open in the following picture:

My guess is that there were also two pipes located here that were the exhaust system for the lower four cylinders of the Wright engine.

This is an illustration of some of the engine nacelle panels:

Item 11 is the panel that includes the lower trough. Item 10 is the oil cooler exit and door. Item 4 is one of the two upper troughs (5 is an insulation blanket).

Interestingly, this trough is represented on the ancient Hasegawa 1/72nd S2F kit, another example of its high level of accuracy particularly compared to the much newer 1/48th kit from a different manufacturer...