Warbird Information Exchange

On some of the older Mark Spitfires flying today they have two longitudinal stiffeners on top of each wing, why? I've never seen it on a wartime photo.

Wing strakes. It was a modification to reinforce the area above the wheel well.

It is my view that as refinements were made to the chassis/undercarriage/gear (take your pick) axles, to introduce variations of 'camber' and 'toe in' of the wheels, this had the effect of slightly changing the spatial relationship of the wheel in the well, gear up. Clearance on the internal structure became an issue and the quick fix was to slim the inner structure and compensate with the external strakes.

Later wings had inner compensation structure and 'local bulges'.

PeterA

ps.

Here is a shot of the Mk I R6915 suspended in the IWM Lambeth in London. Not a flying warbird, more a 'National Treasure', and still in its original last service 1944/45 livery.

I asked the same question of a friend of mine who works on a Mk XIX, I was lucky enough to be invited to have a look at it at.

He said that the camber change comes with the difference between grass axles and tarmac axles. He showed me in the wheel well and you can see where the wing rib is thinned to the point it is barely there.

I seem to recall the bulge in the top surface of the wing was out board of this and was to acomodate the tyre.

Regards

Ric

Exactly, very minor changes to the relationship of the axis of the wheel to the axis of the oleo operation (which is fixed), for toe in and camber issues have a marked dimensional effect at the tyre/tire.

On an individual basis you rarely find two undercarriage legs that match. I think there are at least 25 types with visible variations.

...ongoing development ...by the customer.

PeterA

Thanks.

Am I correct in assuming that the difference is in toe-in?

Does this imply that a grass axle is unsuitable for operation on tarmac and vice versa?

Christer

I think its a big difference in camber, but probably toe angle as well. The tarmac axles the XIX had fitted keep the wheels perfectly vertical to the tarmac.

As far as I can remember from the conversation, one type of axle can be used for both, but one can't. I'll have to ask (gives me an excuse to go pester my friend again).

Regards

Ric

Thanks Richard!

While you're pestering your friend, could you ask if the variants are a "modern" development or if they were developed while still in service (including THUM flight)?

Christer

Sure. I should be seeing him one night this week.

I'm not familiar with the abbreviation "THUM" though?

Ric

I am not sure, but I think THUM refers to Met or weather research flights dept. Not to be confused with THUMB, which comforts some of the starboard wingnuts on WIX since Nov. 4.

_________________
Bill Greenwood
Spitfire N308WK

Temperature and Humidity flight, THUM. Meteorology flights performed by former PR XIX aircraft in to the mid 1950's. Actually why we have Mk XIX Spitfires today.

Apart from the multitude of Spitfire legs, axle geometry variation, spline type, forward scissor link, rearward scissor link, stroke etc, is the additional factor of four different wheel sizes over the development period for 10", 10.25", 12" and 13"diameters. These in the main were to generate the installation of larger brake units. Lowering the aspect ratio of the tyre, to approx maintain the same overall diameter of the tyre, accommodated potential wheel clearance problems. Wheel/tyre and appropriate brake were not Mk specific and in many cases were capable of retrospective fit.

The Spitfire was conceived as an interceptor fighter operating from grass airfields where cross wind was not a major issue. Initially tyres were smooth. In due course block tread grass tyres were introduced.

In today's Warbird environment, with aircraft having to search for spares, rather than 'draw them from stores', some issues of compatibility and suitability have arisen. The 10.25" tyre is still available to the old original specification. 12" tyres introduced just after the big Griffon engine Mk XIV's came in to service have used an airline nose wheel tyre or the Sea Vixen tyre. These modern tyres have vastly superior strength and wear capability.

There have been issues where the early legs, with considerable toe-in, fitted to mid Mk Spitfires with the grass tyre and operated on runways, have very short tyre life. Indeed the Israeli A/F Mk IX 'Black 57' which had enormous distances to taxi out to depart, had reduced the tyre life to single figure landings and they were considering making a trolley unit to ferry the aircraft to the end of the runway. In the event they changed the axles or legs. Increased toe-in of course gave good directional stability on the take off run on grass.

The manual for XIV/XIX Spitfire does give a list of compatible and permissable combinations of different leg/wheel/tyre/brake but these are to do with torque link fore or aft rather than axle geometry. There is enough swing to control on take off with the big Griffon without compounding it with variable axle geometry side to side which could reduce, or worse, add to the effect depended how fitted, left or right.

PeterA

Bill is correct.

A Spitfire Mk XIX, PS888, made the last operational RAF (Spitfire) flight from Seletar on 1 April 1954 but the Mk XIX served the THUM flight until 10 June 1957 when PS853 made the last (Spitfire) flight from Woodvale.

See a recent thread over at Key: T.H.U.M Flight?

Christer

Ahh...

That might make asking the questions a little easier as my friend works for Rolls Royce, it's their PR XIX (PS853) I was basing my initial answer on.

Regards

Ric