by Tommy H. Thomason

Tuesday, September 2, 2014

F-111 Auxiliary Flaps

One of the fixes required of the F-111B for Navy acceptance was restoration of the requisite over-the-nose visibility for carrier landings. It had been accounted for, of course, in the original design.
Yes, that is an Essex-class carrier.  The A3D Skywarrior, aka the Whale, was of a similar size/weight and routinely deployed on Essex-class carriers.

However, as the overly optimistic empty-weight prediction began to be exceeded, the angle of attack on approach increased and the carrier deck began to be less and less visible to the F-111B pilot. There are three basic ways to restore the required sight picture without going faster on approach: lowering the nose/raising the cockpit, reducing the weight, and increasing lift. All were employed (weight reduction being the hardest to achieve), with the production airplanes after the first two to have a raised cockpit.

Most of the wing-lift changes were introduced in production with F-111A #12 and F-111B #4. (An F-111B span, five-segment flap version of the new wing was evaluated on F-111A #4.) The most obvious was the so-called rotating glove on the inboard fixed section of the wing, shown here on F-111A #4 while it was "armed" with dummy Phoenixes during aerodynamic evaluation of the new wing.

One last lift change was also reportedly introduced on the first two production F-111Bs, the six and seventh built. This was the auxiliary flap, which was located inboard of the existing flaps.

These little flaps were intended to squeeze the last bit of lift out of the existing wing planform. They were electrically driven and controlled by the flap handle. If the pilot selected more than 28 degrees of flap and the wings were swept 16 degrees or less and the wing-sweep handle was set at 16 degrees or less, then the auxiliary flaps would lower. (The 16-degree sweep limitation was driven by the need for clearance of the inboard edge of the auxiliary flap from the side of the fuselage.)

These were reportedly incorporated on the USAF  F-111s (its function is described in the F-111A flight manual) and FB-111s as well as the Australian F-111C. The only thing is, I've never seen a picture of them extended on any F-111. I'm not sure what the wing sweep or flap deflection is in these pictures of 152714 and 5, but for sure I don't see obvious auxiliary flaps inboard of the regular flaps.

The only hint of the auxiliary flap I've seen on the other F-111s that reportedly had it is in this picture of an Australian F-111C taken from below.

The auxiliary-flap function was reportedly disabled fairly early as being a maintenance burden and not being very effective from a lift standpoint.

Monday, September 1, 2014

F-111B Envelope Expansion Wing

One of the remaining mysteries to me about the F-111B program was the presence of the four-segment flap wing (i.e. the F-111A wing) on F-111Bs #1 and 4. I didn't even notice it in the picture of #1 on the cover of my F-111B monograph until someone eventually pointed it out.

Note that the wings have the ferry tips so that the span is that of the F-111B.

This is the F-111B early five-segment flap wing.

Tim Lent pointed out that F-111B #4 also had a four-segment flap wing at one point.

(You can take my word for it - a lightened version of a high-resolution scan of a pretty good photograph shows that the fifth flap segment is not present.)

The reason for Tim's search for evidence of this wing on #4 was the desire to identify the third F-111B in this photo, the one with its back to the camera.
 I was pretty sure that it was #4 because it appeared to have the rotating glove flap on the fixed portion of the wing and did not have the pod on the fin tip. However, Tim noted that it had a four-segment flap with the ferry tip.

My speculation is that this set of wings was specifically instrumented for high-speed envelope expansion and the use of the ferry tips allowed it to be used by both General Dynamics on an F-111A and Grumman on an F-111B. In fact, #4 was used for flutter/envelope expansion in late 1966 and early 1967. After its crash, #1 was used for flutter/envelope expansion at Edwards in the fall of 1967, which is likely when the monograph cover picture was taken.