Spitfire wheel well clarification

[Peter Roberts]

Can anyone confirm this area of detail on Spitfires please? I note that later Spitfires (eg Mk VIII and Mk IX) have slanting walls on the well where the wheel retracts. This is also present on Spitfires back to the Mk VC.

 

I have been looking at lots of photos of earlier Marks, I and II, and other Mk V's, A and B, but photos are not definitive to determine if walls were also slanting on these aircraft.

 

My own thoughts are that the well walls on these aircraft are perpendicular to the lower surface of the wing, but had to change to sloped/slanted with the change in geometry of the undercarriage needed for the C wing (?) Is this correct?

 

Is anyone able to access this area on a Mk I, II, or a VB, to confirm this one way or another? Or perhaps a definitive photo(s)?

 

TIA

 

PR

 

 

Edited January 29, 2020 by Peter Roberts

[Work In Progress]

I continue to take the view I took at the start of the month. 

They all slant. If there is a minor variation in the angle of that slant between Vb and Vc - and I see no evidence that there is - then it is so trivial as to be of no practical significance for modelling purposes.

 

This is an unrestored IIa and to me the slant rearwards is clearly evident. (Panel line in the left lower corner is the rear of the D box, the rear of the aircraft is to the right and up  in a 2 o'clock direction.

 

Edited January 31, 2020 by Work In Progress
added pic

[Peter Roberts]

Thank you - my apologies, 'old age moment'

 

Thank you for the reminder and info (again)

 

PR

[Peter Roberts]

I can now add a little to this thread, having had the chance to look at the wheel wells of a MkVc and a couple of Mk I’s

 

The wheel wells between the two are different.

 

On the Mk I, the wheel well, the area where the wheel retracts, slopes up from the lower surface and back towards the rear (trailing edge).

 

On the Mk Vc, the wheel well slopes up from the lower surface and out towards the wing tips. This geometry has also carried through to the later Marks.

 

I haven’t seen a Mk II (yet), or a Mk Va or Vb. WIP has posted that the Vb has the same wheel well as the Vc. 

 

So the question is, when did the change in geometry happen? With the Mk V?

Edited May 27, 2022 by Peter Roberts

[Graham Boak]

The change in wheel position in the well changed with the introduction of the Universal (C) wing.  On this wing the skew angle of the undercarriage was altered, moving the axle position forward when retracted.  This allowed the wheel to lie flat in the well as opposed to needing a bulge in the upper wing surface..  This was linked to a stronger upper wing at the root, and it seems likely that it was these changes that produced the effect you see.  However there were also previous mods aimed at strengthening the wing root which I know no details of, so these may also have had some effect.

 

I'm afraid I don't understand quite what you are seeing, nor (if I read it right) how this could come about.  There were no change to the wing's basic dimensions in this area.  However the change to the Universal wing would seem to be the logical time for major alterations to this area.

[Scimitar F1]

I got permission to climb under a MK1 at Duxford - the wheel well was definitely slanted.

[Graham Boak]

The wing gets thinner (reduces in depth) as you move outboard.  It also thins as you move aft from the main spar.  Is this what you mean by "slanted"?  However the Spitfire wing is not deep enough to have any significant plate surface between the upper and lower wing surfaces.  So no "wheel well" in the sense of an individual unit dropped into the wing structure.  Just a hole in the lower surface with walls around it.  

 

Which would suggest that (allowing for the odd few thou of differing skin thicknesses) there'd be no significant difference between the Marks. But if any difference did occur then the redesign to the Universal wing is the one most likely to change things.  However this change did not affect the external wing dimensions.. The undercarriage had to fit in the same limited space.

[gingerbob]

11 hours ago, Graham Boak said:

with walls around it

 

I think these are what the slant [of them] is about.  I agree that it seems logical that the change would occur with the 'c' wing geometry, not before.

[Scimitar F1]

It may be logical but it is not correct. When I was building the Airfix Mk1 in 2020 I wanted to check this out. I was at Duxford and they had a Mk 1 so I asked if I could get some detail photos that they agreed to kindly.

 

So under the wing I went and had a look for myself. And yes the walls of the wheel well are not perpendicular to the wing lower surface. They are at an angle with the join with the upper surface behind the join to the lower surface. 

[Peter Roberts]

20 hours ago, Graham Boak said:

The change in wheel position in the well changed with the introduction of the Universal (C) wing.  On this wing the skew angle of the undercarriage was altered, moving the axle position forward when retracted.  This allowed the wheel to lie flat in the well as opposed to needing a bulge in the upper wing surface..  This was linked to a stronger upper wing at the root, and it seems likely that it was these changes that produced the effect you see.  However there were also previous mods aimed at strengthening the wing root which I know no details of, so these may also have had some effect.

 

I'm afraid I don't understand quite what you are seeing, nor (if I read it right) how this could come about.  There were no change to the wing's basic dimensions in this area.  However the change to the Universal wing would seem to be the logical time for major alterations to this area.

It is a bit tricky to explain and I may not have done a good job here Graham. 

 

The walls in the wheel well are not perpendicular between the wing surfaces. For the Mk I they slope up and back towards the trailing edge. To put it another way, if I was to cut a hole in the upper wing where the walls meet it, that hole would be above the hole in the lower wing, but slightly toward the trailing edge.

 

For the Mk Vc, the wall in the wheel well slopes up, and towards the wing tip. If I was to cut a hole in the upper wing where the wall meets it, the hole would be above the lower hole, but now positioned slightly out board, towards the wing tip.

 

3 hours ago, Scimitar F1 said:

It may be logical but it is not correct. When I was building the Airfix Mk1 in 2020 I wanted to check this out. I was at Duxford and they had a Mk 1 so I asked if I could get some detail photos that they agreed to kindly.

 

So under the wing I went and had a look for myself. And yes the walls of the wheel well are not perpendicular to the wing lower surface. They are at an angle with the join with the upper surface behind the join to the lower surface. 

 

Yes, they all have angled. Or sloping walls, but the direction of the slope or slant is different between the Mk I and the Mk Vc, as I have tried to explain (though perhaps not very well). I agree with gingerbob, the logical thought here is that this happened with the change in geometry of the undercarriage for the ‘c’ and later wings (as Graham has also pointed out above), which required a change in the undercarriage doors and may have required a change in geometry for the well walls also.

 

However, WIP has posted above that the Mk Vb and Mk Vc have the same wheel wells - ???

[Peter Roberts]

Sorry to keep banging on about this but it has been a thorn in my side. I finally saw a Vb wheel well, on video, and it confirmed (to my eyes) that it was the same as the Mk I and Mk II. I suspect the Va had the same wheel well too as it was an ‘a’ wing.

 

This would mean that the change in Spitfire wheel well geometry came with the ‘c’ wing, and I’ll wager is also the same on the ‘e’ wing. I have thought this was due to the change in the geometry of the undercarriage leg, but could it also have inherently increased the strength of the wing - that is, sloping the well walls out towards the wingtips instead of to the rear? I recall Edgar mentioning a modification that strengthened the wings, but he couldn’t find any details. The early wings (‘a’ and ‘b’) were strengthened with strakes on the upper wing surfaces, above the wheel wells, but clearly the later ‘c’ and ‘e’ wings didn’t need these additions, possibly because of the changes to the wheel wells? I have zip evidence for this - just an observation, so take it for what you may consider its worth. Just putting it out there for comment

 

 

Edited December 2, 2022 by Peter Roberts

[epozar]

Modifications to F Mk V

 

"507 strengthen wheel axle (VB & VC only) 27-1-42"

 

it may be that some Vb were also affected

Edited December 3, 2022 by epozar

[Graham Boak]

There were a number of mods to improve the inner wing strength on the A/B, but I've not seen them explained.  I suspect they involved thickening the upper wing skin, but this will only have been a matter of some thou or low tens of thous.  The wing was redesigned for the Universal (c) wing, the obvious relevant feature being the smooth surface without the bump to cope with the retracted tyre, and the forward rake of the undercarriage with the wheel now fitting flush into the wing and a bulge required on the wheel door around the leg.  I have always understood that this implied a thicker top skin.  (With more power and more weight in an expanding flight envelope, the problem was to return when dive-bombing in France.)

 

 I suppose it is possible that changing the rake of the leg whilst maintaining the original pick-up point outboard meant that it simply wasn't possible to fit the wheel back into exactly the same hole, so that this required some adjustment to the position of the hole in the lower wing, and hence changes to the walls.  Why they should have sloped in the first place I don't know either, unless it was to pick up on some strong point on the upper wing/ribs that meant sloping the wall to meet.

[Peter Roberts]

The sloping walls have been a puzzle to me too Graham - the walls are sloped on the 'a' and 'b' wings too, but in a different direction. Yet looking up at the lower wing it appears that the wheel would retract into the opening irrespective of the walls (unless they sloped inwards!).

 

Another aero-engineer has suggested to me that the wheel wells probably didn't have too much to do with the strength of the wing, so it may well be as you have said, a thicker wing skinning. Pity we don't have Edgar scrutinizing the NA vaults, assuming the details are there.

[Nigel Bunker]

Oh how I miss Edgar when Spitfire questions arise.

[Troy Smith]

On 31/01/2020 at 17:02, Work In Progress said:

This is an unrestored IIa and to me the slant rearwards is clearly evident. (Panel line in the left lower corner is the rear of the D box, the rear of the aircraft is to the right and up  in a 2 o'clock direction.

 

Is that actually the ex 74 Sq Bob survivor  Mk.I hanging up in Chicago?    Note the well is the wing underside colour, a point that still gets debated, even though no-one has ever shown convincing proof that there were anything else,  while very photo that does show the well shows the underside colour....

(Even though the BBMF flight think they are grey-green....) 

 

 

[Zephyr91]

Interesting discussion.   I have just been perusing my newly acquired second-hand copy of Spitfire The History (Reprint 1989) says he proudly 🤫.    Whilst it doesn't explain the structural mods it does seem to have some sketches which show in plan view the structural changes between A, B and C wing.  See pages 156 and 157.   Unfortunately for this discussion it barely touches on the undercarriage bay except that I think it reflects the upper and lower positions of the bay for the B wing.   So it is possible to see the fore/aft "slope" reflected by the discussion above - if I've understood the discussion and I'm looking at the drawing correctly.    Can someone else have a look and see if I'm on the right track?

 

The changes shown to the ribs and local structure outboard of the undercarriage bay would seem to be consistent with that described by @Peter Roberts and @Graham Boak et al above.   One could imagine adding thickness/strength to the internal structure whilst simplifying the load paths (i've worked with, but am not myself, a Structures Engineer so apologies if I've mis-used terminology), at the same time as thickening the upper skins.   I am happy to be corrected, but I don't see that changing the bay wall slope direction would change the overall strength much in its own right.   Rather any changes here would be for easing wheel retraction  problems if it was really necessary (remembering that ANY changes  are always resisted by the "Manufacturing guys" 😉 because it fouls up their schedules) (Another time in the bar might be appropriate to "ask me how I know" 😝) . 

 

HTH

 

Rob

[Dave Swindell]

The slope of the wheel well walls and the change of angle has nothing to do with wing strength and everything to do with geometry. 

The Spitfire undercarriage leg is retracted and extended by swiveling the leg on a simple straight pivot pin, but the positioning of that pin and its angles in relation to the wing are critical to get the undercarriage from being neatly stowed in the wing to where it's needed on the ground.

When you look at a Spitfire with the fuselage horizontal, the wing main spar is vertical, and the undercarriage pivot pin is bolted to the rear of this spar. 

To enable the leg and wheel to retract into the wing behind the spar, the pin is angled inwards pointing aft in plan view (90 degrees to the leg cut-out in the wing underside)

To enable the leg and wheel to angle forward when extended and give a wheel contact point forward of the spar (and aircraft centre of gravity, so it's not a nose sitter), the pin also needs to angle down aft of the spar, and it's this angle that affects the side walls, because it means that as the wheel enters the well and moves upward, it is also moving aft the further into the well it moves.

There are options here, you can make the opening in the bottom of the wing big enough to cover the plan view of the wheel at any position in the well as it retracts, or you can give the wheel a small clearance where the wheel enters the wing, and have the sides follow the wheel path as it moves upwards, outwards and backwards in the wing. To follow this accurately would result in a complex curve, but as the curve is very gentle, Supermarine have simplified this into a straight sided "tube" which is angled aft and slightly outboard so that the wheel has enough clearance all round as it moves into the wing. 

The undercarriage geometry didn't change from the initial MkI through to the MkVb, but when the Universal "C" wing was being developed, it was recognised that engine development would lead to heavier engines being fitted with a centre of gravity further forward, and to compensate for this, the wheel contact point with the ground would also have to move forward to maintain stabilty whilst taxiing, taking off and landing.

The required movement of the wheel forward (4" IIRC) was achieved by angling the leg further forward, and consequently the pivot pin had to angle further downwards to match the leg angle change. This was achieved quite simply by fitting a wedge plate between the spar and pivot pin to give the required change in angle.

However, changing the pivot pin so it angled further down when viewed in profile meant that if the wheel passed through the hole in the underside wing skin as it retracted, it also had to move slightly further aft and outboard in the wing as it moved up in the well. To maintain the same clearance between the wheel and the well sidewalls they had to be repositioned to angle further aft and outboard as well. This change in undercarriage geometry was retained for all marks of Spitfire with the "C", "D (PR) and "E" wings.

The other effect that angling the pivot pin further downwards had was to position the whole undercarriage leg and wheel slightly lower down in the well. This enabled a flush skin to be fitted above the wheel well, eliminating the small blister previously required to give the wheel adequate vertical clearance, and also enabled internal stiffening of the skin where previously external stiffeners had been introduced to prevent skin buckling. With the leg now sitting slightly lower the previous flat leg door would no longer fit flush with the lower wing, and a slightly dished/bulged door had to be introduced to fair the leg into the wing underside. 

[Zephyr91]

Excellent description andmakes sense to me.   Thanks for the effort there.

 

[gingerbob]

5 hours ago, Dave Swindell said:

This change in undercarriage geometry was retained for all marks of Spitfire with the "C", "D (PR) and "E" wings.

 

Actually, I think the PR.IV had the earlier geometry, and the newer didn't come until the PR.XI.  Also, the change was originally intended for the Mk.III, though was not on the first prototype.  This, of course, had the Merlin XX, so already a slightly longer and heavier engine. 

 

Sorry, good summation, just figured I'd point out a couple of fine points. (point out a point?)

 

bob

 

p.s. The figure I have in my head is that the tyre moved 2 inches forward.  I'm getting less confident in those things stored in my head, though....

[Dave Swindell]

32 minutes ago, gingerbob said:

 

Actually, I think the PR.IV had the earlier geometry, and the newer didn't come until the PR.XI.  Also, the change was originally intended for the Mk.III, though was not on the first prototype.  This, of course, had the Merlin XX, so already a slightly longer and heavier engine. 

 

Sorry, good summation, just figured I'd point out a couple of fine points. (point out a point?)

 

bob

 

p.s. The figure I have in my head is that the tyre moved 2 inches forward.  I'm getting less confident in those things stored in my head, though....

I think you might be right re the PRIV, Bob, it did cross my mind after I'd posted. I'm away for the W/E so no refs to check timeline, I was thinking of the MkIII for the engine-U/C change link but wasn't positive enough to state this. the Vc would be the first to go into production with the mod though. 

2" doesn't sound enough but you could be right, Edgar is probably looking down at us in despair, "didn't they listen to anything I said?"

[Troy Smith]

20 hours ago, gingerbob said:

The figure I have in my head is that the tyre moved 2 inches forward. 

 

19 hours ago, Dave Swindell said:

2" doesn't sound enough but you could be right

from previous discussions, I think the C introduced an extra  2 degrees of forward rake compared to the A/B wing. 

https://www.britmodeller.com/forums/index.php?/topic/235052956-spitfire-v-b-vs-c-wing-and-uc-details-and-prop-variants/