By Tommy H. Thomason

Tuesday, May 14, 2013

The Last Flight of Vought's XF7U-1 Cutlass BuNo 122472

Once again, I've come across something that I would have liked to include in one of my books, in this case the crash report for XF7U-1 BuNo 122472 on 28 September 1949 during takeoff at Vought's temporary flight test facility at Ardmore, Oklahoma.

The first XF7U-1 in January 1949:

When I was writing my history of the F7U-1 (see, I only had a few cryptic statements about the circumstances of the crash, which the pilot, Paul Thayer, survived with only cuts and bruises.

I had assumed that the crash was caused by the load asymmetry (there was a drop tank loaded on the right inboard wing) combined with a new yaw control concept and that it was the first flight in this configuration. I speculated on how that caused the crash. That took some doing because you'd expect a load on the right wing to result in the right wing tip hitting the runway, not the left, if there wasn't enough control power at takeoff speed to keep the heavy wing up.

As it turns out, however, the load was on the right wing and was even greater than in the reports I relied on, a 250-gallon drop tank not 150.  Not only that, this was Thayer's third flight that day in this configuration, which was a big surprise. The only difference on the takeoff this time was a gusty crosswind from the right, not down the runway as before, and he had the canopy open instead of closed. As with the previous two takeoffs, the leading edge slats were closed, which was the preferred configuration for field takeoffs at the time.

Thayer lifted off holding left rudder as I had assumed because of the drag and weight of the drop tank on the right side. (It was even more necessary with a crosswind from the right that I didn't know about.) He then moved the gear handle to the up position, so the gear doors opened and the landing gear began to retract. As the landing gear was coming up, the airplane began to slowly yaw to the left. Thayer pushed right rudder but the left yaw didn't stop and the left wing began to drop. Right stick and more right rudder didn't stop the left roll and yaw so he pulled the throttles off, not wanting to keep trying to fly an airplane that he was not in control of.

The left wingtip touched first just off the left side of the runway, with the airplane slewing around to the left about 100 degrees as it slid across the grass, wheels up, for about 750 feet. During the slide, the forward fuselage broke off just ahead of the engine inlets and wound up lying on its right side at right angles to the fuselage ahead of the left wing.

Vought couldn't find anything wrong with the control system, engines, or anything else for that matter. Although the XF7U was instrumented, it was by means of a photo-observer panel, which means that a picture was taken of a separate set of instruments every five seconds. In other words, the engineers were looking at relatively infrequent snapshots of a rapidly deteriorating situation. Their best guess was that Thayer was too slow and not aggressive enough in his initial response to the left yaw that he had caused with the left rudder required on the takeoff roll. It didn't help that while the landing gear was coming up, there was a significant reduction in directional stability. Or that the wind was gusty.

Their conclusion: "The airplane would be expected to yaw and roll to the left when stalled at high powers with slats closed. The accident, therefore, is explainable on the basis of an early onset of a stall."

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