By Tommy H. Thomason

Sunday, February 22, 2015

Swept-Wing Tip Skid

Bob Sikkel called my attention to a detail on the McDonnell F3H-2 Demon that I hadn't noticed before, a post-mounted blivet out on the underside of the wingtip. I found excellent pictures of it in Don Hinton's walk-around photos of the F3H-2 at the National Museum of Naval Aviation in Pensacola.

Note that the "saucer" of the skid itself is clearly intended to be readily replaceable.

It's a definite oddity, since I don't remember anything like it on other Navy swept-wing jets. I was familiar with the XF3H-1 originally having a much longer version of it.

However, that skid disappeared from the XF3Hs when the wing was changed to move the ailerons inboard.

Nevertheless, a very short tip skid turns out to have been incorporated on the ill-fated production F3H-1.

And it is present on the F3H-2 wingtip in almost every picture of it with sufficient resolution to discern it. It is identified in a F3H maintenance-manual illustration as the "wing tip skid". (Thanks to a Royal, a volunteer at the Emil Buehler Library at the National Naval Aviation Museum, for finding that for me.)

So what was that all about? The only other evidence of a similar device is on a circa 1946 Douglas predesign three-view of its D571-1, the predecessor to the F4D Skyray.
I haven't seen any evidence of one on either the F4D mockup or XF4D prototypes.

The Navy was nervous about the approach-speed handling qualities of a swept-wing carrier-based airplane so they contracted with Bell for a swept-wing conversion of its P-63, designated the L-39, in 1946. (Also see
There's no evidence of tip skids on it or any mention in the flight test evaluation of the need for them.

If there was a concern at the Bureau of Aeronautics about swept-wing tip contact with the deck/runway, it didn't reach Vought or was ignored in 1945 when it was proposing what became the F7U Cutlass. There is no evidence of a tip skid in any of the proposal documents or the mockup or the XF7Us themselves.

Similarly, the earliest swept-wing studies at Grumman in 1947 don't feature tip skids and they weren't on the XF10F mockup, which was constructed about the same time as the McDonnell proposals for the F3H in 1948.

My guess is that the tip skid was required on the F3H because of the minimal ground clearance of the tip when the tail skid made contact, even when the wings were level.
(Yes, that is a Mk 7 on the fuselage pylon.)

There was no assurance that the wings would be level in a tip back situation, particularly in the event of a barricade arrestment.

Hence the need for a tip skid on the F3H.


cjw_1954 said...

Kinzey in the digital Detail and Scale publication on the Demon calls this feature a "vortex degenerator".It looks a bit sturdy to be one of these so I'm inclined (as usual) to concur with you.
Any thoughts?

Tailspin said...

Bob Sikkel had questioned that description as well, which is why he called it to my attention. It turns out that Bert was told that was what it was by a volunteer at the National Naval Aviation Museum, who claimed to have looked it up in the maintenance manual but didn't provide him with a copy of the page. When I asked, a different volunteer sent me a page from the maintenance manual that identifies it as a skid. Bert will revise his F3H ebook if he hasn't already, which is one of the advantages of that media.

jimmbbo said...

A detail often overlooked by pilots transitioning from straight wing airplanes into those with swept wings is that on takeoff rotation or landing flare, the distance between the wingtip and the ground decreases... Toss in a wing-low crosswind correction, and a wingtip strike is much more likely than on a straight wing aircraft.