by Tommy H. Thomason

Friday, September 30, 2011

F-111B Aft Main Landing Gear Door

It turns out that there's an F-111 main landing gear maintenance trainer at the Chanute Air Museum in Rantoul, Illinois with the original aft main landing gear door and the Curator there, Mark Hanson, was kind enough to take pictures of it. I've posted a couple of them, one heavily annotated, HERE. (Note: I subsequently revised the annotated picture because I had incorrectly guessed what the hidden portion of the aft main landing gear door leading edge idler looked like.) I'm still working on a more accurate illustration of the mechanism than this one, which turned out to be a pretty good guess as to the moving parts:
Since then, I've renamed Link A as the aft main landing gear door bellcrank and Link B as the aft main landing gear door leading edge idler.

TF-1/WF-1 Forward Fuselage Change from S2F-1

Click HERE for my guess at the differences between the forward fuselage of the S2F-1 and the TF-1/WF-1.

Saturday, September 10, 2011

F-4 Flap/Stabilizer Change

Because the original design was intended for a somewhat lighter aircraft, the F4H (F-4) Phantom II wing underwent some subtle changes in development to provide more lift to go with the increase in gross weight. The first of these were the addition of the inboard leading edge flap and boundary layer control to the sixth F4H. (See HERE and HERE.) Note that I've added information from Craig Kaston about the  earliest changes to the stabilator. Also see:

As on all airplanes that don't have the highest level of pitch stability augmentation, the F4H stabilator created down load in most flight conditions. The original stabilator had a symmetrical airfoil.

Because the F4H was short coupled, as carrier-based airplanes tended to be for compact parking on deck, an early modification of the original stabilator added notable negative camber to increase the download available. (Note the zig-zag line at about 20% chord where the cambered leading edge was added on this interim stabilator.)
This was the sixth Phantom aboard during early at-sea trials. Note the cutout in the bottom of the rudder due to the stabilator trailing-edge-up travel being provided at the time.

The Phantom's gross weight continued to increase over time with upgrades, fixes, additional equipment, etc. It was clear that with the next round of improvements planned for the F-4J, another lift increase would be required. Step one was obvious, drooping the ailerons when the flaps were lowered: 16.5 degrees was determined to be adequate from wind tunnel test.

As expected, the droop resulted in more nose-down moment with the flaps extended, too much as it turned out. The problem was most obvious with the increase in nose wheel liftoff speed and the inadequate pitch control power after catapult launch and during bolters. Additional wind tunnel tests established that elimination of the inboard leading edge flap (added during development as noted above for increased lift) reduced the nose-down moment at nose-wheel liftoff speeds and turned out not to affect lift at approach speeds.

Although eliminating the inboard leading edge flap was beneficial, it was inadequate. Further wind tunnel tests indicated that the stabilator was stalled at nose wheel liftoff speed. As McDonnell engineer Bill Weber remembers it, "We tried a matrix of planform and area changes to correct the problem and finally determined that adding a fixed leading slat would delay the stall and could fix the problem. (Note: These wind tunnel tests were conducted without a simulated jet exhaust - later tests with jet effects indicated that the slat did not delay the stall and the improved stabilator power was probably due to an increase in effective area.)"

Even before the wind tunnel tests of the slotted stabilizer were accomplished, McDonnell program management decided to fabricate a slotted stabilator by adding a fixed slat to the leading edge of the existing one and flight test it. Bill Weber again: "As a consequence flight tests of the slotted stabilator took place very shortly after we had wind tunnel results. In any event the flight tests demonstrated that the problem had been fixed. We used the same configuration changes to fix a similar problem on the F- 4E which didn't have aileron droop but had a more forward C.G. due to the installation of the gun."

Although created for the F-4J, which first flew in June 1966, the drooped aileron and associated changes (the slotted stabilator and the elimination of the inboard leading edge flap) were of benefit to the F-4B as well. The package of changes was incorporated with Block 26 production, the first of which was BuNo 152995 that first flew in March 1966, and retrofitted to most of the surviving earlier F-4Bs over time.

Retrofit involved the development, qualification, and approval of a different package of drawings and other documents for use by the Naval Air Rework Facilities before crash-damaged and overhaul-due F-4Bs would get these changes. I've read that "by late 1971 or early 1972 it would have been rare to see a F-4B without these modifications, except perhaps in RAGs or Marine reserve units." For sure the slotted stabilator was present on F-4Ns coming off the Bee Line at North Island. The first F-4N flew in June 1972.

This is an F-4N, which is an upgraded F-4B. Note the drooped ailerons and fixed inboard leading edge along with other detail changes to the original B configuration.

Thursday, September 8, 2011

World War II ASW Schemes versus the Norfolk Scheme

In January 1943, the U.S. Navy directed that its combat airplanes be painted in a new, complex camouflage scheme that utilized counter-shading and counter-shadowing. (Sometimes referred to as the tri-color scheme, it actually involved more than three shades.) It took some time for the Navy airplane manufacturers to switch over to the new requirement, no doubt in part because they had to figure out exactly how to implement it and then get Navy approval for their design.

In the meantime, the Navy had to repaint airplanes that had already been delivered and for a time, those being delivered. It appears that a goodly number of these repaints were done at Norfolk, Virginia to a standardized and somewhat different scheme than the ones that the manufacturers would come up with. It is characterized by the Sea Blue upper surface color extending almost straight down on the side of the fuselage to the leading and trailing edges of the wing and relatively less Sea Blue on the side of the fuselage than the eventual production schemes. In this example, the TBM? on the left has the factory paint scheme and the TBF? on the right is the Norfolk scheme.

Click HERE for my prior discussion of it and more examples.

ASW Schemes I and II were implemented by the Commander Aircraft, Atlantic in July 1943 but not covered by a Navy specification until June 1944. In a recent discussion on one of the modeling websites of these schemes, I began to wonder if I hadn't mistaken at least one picture of an aircraft with an ASW scheme for an example of what I term the Norfolk scheme.

ASW Scheme I: For use in areas where the prevailing weather was clear or clear with broken clouds (the southern United States seaboard, Gulf of Mexico, Caribbean, and South America). This was a topside of nonspecular Dark Gull Gray, sides of nonspecular Light Gull Gray, and bottom of Gloss White. The side surfaces in the shadow of the wings and horizontal tail were to be painted nonspecular Insignia White. In a gray-scale picture, it looks very much like the blue tri-color scheme.

ASW Scheme II: For use in areas where the prevailing weather was overcast of heavily clouded (the middle and northern United States seaboard and the North Atlantic). This was a topside of nonspecular Dark Gull Gray, sides of nonspecular Insignia White, and bottom of Gloss White. The difference between it and Scheme I was the use of White rather than Light Gull Gray on the sides.

In both Scheme I and II, the leading edge and inside of the cowlings, propeller domes, and propeller blades (out to the inner edge of the cowling opening) were to be painted nonspecular Insignia White.

However, the examples are somewhat confusing. This is a picture of SBDs from VMS-3, which was based in the Virgin Islands (rough duty).
It would seem that these airplanes should be in Scheme I (Light Gull Gray on the sides) since they're based in the South Atlantic. Instead, the sides appear to be white, which is Scheme II. One theory for this discrepancy is that the anticipated basing of the airplanes was not the same as their actual assignment. Note that the extension of the top color directly down to the wings is very similar to the Norfolk scheme in this example as well as the two that follow.

This is an example of Scheme II provided by Steven (Modeldad) Eisenman:
The white sides are pretty unmistakeable.

This is an example of Scheme I (Light Gull Gray sides) provided by Steven:
Note the white propeller dome in both cases as well as the early rocket launchers; a hole made by a rocket was an effective way to keep a surfaced submarine from submerging to the relative safety of the depths.

The picture that got my attention was this one:
It was also cited as an example of the ASW Scheme I. However, based on the fact that it has a circle and star national insignia, (and does not have the white-painted propeller dome and inner blades) my guess is that it's an example of the Norfolk tri-color scheme, since the start of the transition to the star and bar insignia predated the multi-color ASW schemes:
n.b. A two-tone white ASW Scheme was reportedly used on the TBFs, if not the F4Fs, of VC-9 when it deployed aboard Bogue in early 1943 to the North Atlantic. However, I haven't found a picture that clearly shows no grey on the Avengers. This one, provided by Steven Eisenman, of TBFs on Card that was possibly taken in July 1943 is the best evidence of that scheme on TBF that I've seen so far.