AD Skyraider Modeling Notes

I was excited to read about Aero Research's new product, Modelers' Guide to the Skyraider by Jay Sherlock.

20 April 2022 Update: Jay has now published a second edition that includes some of the material that I mention below that was missing from the original one. It costs $21.95 with shipping to the United States. For a description and to order, see http://www.aeroresearchcds.com/book_shelf.htm

I am very pleased with the content based on a quick read through. Jay has done a great job of delineating the differences between the several variants of the Skyraider and the plastic kits that represent it. However, it is not comprehensive, particularly with respect to the interior and photo coverage of details. For that, I recommend that you supplement it with the excellent Walk Around A-1 Skyraider by Ed Barthelmes and Richard S. Dann.

 Another excellent source of AD Skyraider configuration information, photographs and maintenance/pilot's manual illustrations is Steve Ginter's Douglas AD/A-1 Part One, Naval Fighters Number 98: http://www.ginterbooks.com/NAVAL/NF98.htm

Note that the following provides information and illustrations that were missing in Sherlock's first edition:

In his kit reviews, Jay mentions the non-uniform pylon spacing but doesn't illustrate it.

The stick on one of the pylons is the strut to brace the wing when it is folded.

He also mentions the presence and absence of nose flaps, but unless I missed it, again you're on your own. There were six slightly curved segments that were hinged on the circumference of the cowl ring so they could be folded back along the interior of the cowl. When they closed off the cowl ring, they shortened the time needed for the engine to warm up, as indicated by the oil temperature. I also think they kept the engine cylinders from over-cooling when dive bombing.

There was a cutout in the upper inner edge of the flaps at the two and ten o'clock position, presumably to clear the magnetos on the nose case when the nose flaps moved between the open and closed positions, and a small triangular gap at the inboard side of the other flap corners.

Dave Cantrell provided the following photo of an AD cowl with the nose flaps open.

Note the "pin" indicator for nose-flap position sticking out of the cowl at 11 o'clock looking aft. There was a single cockpit spring-loaded switch for both cowl and nose flaps - open/off/close. They automatically went full open after touchdown via a landing gear squat switch. The cowl and nose flaps could be closed after shutdown with the cockpit switch overriding the squat switch. However, they all went full open the next time electrical power was applied, as on engine start. The nose flaps closed after the cowl flaps were fully closed and opened fully before the cowl flaps started to open. The nose flap indicator on the cowl ring stuck out the most when the nose flaps were fully closed; I think there was a color band on it to indicate that they were fully closed. You could, of course, tell when they were fully open because the cowl flaps, which you could see, started to open then. Cowl-flap position was predicated on cylinder head temperature but basic guidelines were full open for ground operation, takeoff and go-around; full closed for cruise and let down; full open or partially open for climb depending on the cylinder-head temperature. I don't know how much the nose flaps were used other than after shutdown and probably dive bombing, but they appear to have been removed or disabled on non-Navy applications.

He writes that the 1/72 Airfix A-1J kit has a number of shape errors in the tail. I'm not sure what he's referring to but one oddity that is actually not an error is the offset and shape of the vertical fin. Airfix went to a lot of trouble to depict it. Douglas drawings of the AD (SACs and company display models) show the vertical fin angled to the left, in some instances specified as three degrees.

It's not easy to see on the airplane. I just had the chance to literally look over the AD Skyraider at the National Naval Aviation Museum at Pensacola. It's pretty obvious when you know where to look but doesn't show up well in a photograph:

Note that the static source probe (it's not a pitot; that was under the right outboard wing) is actually offset to the left of the centerline and the spine of of the fuselage curves back to the right. The appearance of an airfoil lifting to the right is exaggerated by the rudder being slightly displaced as if left rudder was applied.

Why the asymmetry? High power at low speed and a high angle of attack (e.g. during a wave off) requires some right stick and a lot of right rudder. The torque rolls the airplane to the left, requiring right stick; right rudder also helps keep the right wing down. (If the airplane is on the ground, more torque results in yaw to the left due to the higher load on the left main gear wheel.) More importantly, P factor yaws the airplane to the left. (These power-induced trim changes are offset to a small extent by the swirl effect of the prop wash, which pushes the tail to the right but also rolls the airplane to the right.)

Two other design conditions of interest for the vertical fin are minimum-trim drag in cruise flight and not requiring excessive rudder trim changes in the low-power, high-speed dive.

The fin being angled to the left is, in effect, automatically applied right rudder during a wave off when the air accelerated through the propeller disc hits the fin. However, the XBT2D-1/AD-1 SAC drawings and the AD-1 display model drawing clearly show the fin with an asymmetric airfoil that would lift to the right (left rudder) as a function of airspeed/prop wash, although it is still angled to the left (right rudder). That the fin has an asymmetric airfoil is not so obvious in the AD-5/6 display model drawings or later SACs.

My guess is that the airfoil shape provided lift as a function of speed and therefore offset the lift provided by the angle of attack of the fin, so little trim was required in cruise (medium speed and prop wash) and/or a dive (high speed and low prop wash). Or maybe it was the other way round. Or the draftsman made a mistake and the fin is not only angled to the left, it has an airfoil shape lifting left. Or the AD had an asymmetric airfoil early on but not later.

I haven't seen anything definitive on the Skyraider's sonobuoy/searchlight pod used for ASW. This is my best guess so far (the AD-5 stores pylon is shown; the AD-4 pylon was smaller and the stores attach points were farther aft):

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.

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: http://tailhooktopics.blogspot.com/2012/11/f-4-stabilator.html

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.

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.

Early F4H Redux

It turns out that there was a change to the tailhook and the deletion of a fairing behind the tailhook that I hadn't noticed. There was also a change to the stabilator that was brought to my attention by Craig Kaston. I've updated the second early F4H entry, HERE, with a drawing and a couple of pictures that illustrate the changes to the tailhook and the deletion of the fairing. I've updated the first early F4H entry, HERE, to illustrate the stabilator change.

F9F-6 vs. 8 Cougar Redux

I've updated this entry, HERE, in response to a question about the difference in the length between the F9F-6 and the -8 as provided on the interweb—and also in Steve Ginter's monograph Number 66, Grumman F9F-6/7/8 Cougar Part One—of 16 inches versus the eight inches that I show. I also updated the planform view of the F9F-5/6/8 to reflect the eight-inch increase in length of the -8 compared to the -6.