Mosquito wheel tyre removal

I cannot help but i do love you’re updates

Brake bladder

An old brake bladder was sectioned to show a thick layer of ‘insulating’ rubber adjacent to the brake segments. A postwar NOS brake bladder showed a different evolution, being a reinforced fabric (asbestos?) layer close to the surface, machined down to a tolerance. A NOS brake segment from a postwar brake unit showed Mintex and a friction material product code, for what inevitably would be an asbestos containing material.

Does anybody know if rubber brake bladders are still made? I cannot see a 50 year old NOS brake bladder being suitable where original rubber components were given a 5 year shelf life…
Are there any modern materials used in this application? Eaton Air Flex?

Does anybody recognise the Mintex product code and have technical information on its characteristics? What modern, non asbestos containing friction materials are used in current aerospace braking applications?

The Dunlop product catalogue for a mainwheel for Bristol 170 had an original inspection period for mainwheels on every 150 landings. This was amended to ‘upon every brake inspection’. The section on brake inspection said ‘generally on 150 landings but depends on frequency of landings and severity of use’. Probably reflecting on RNZAF supply trips into Vietnam. So a good guideline derived from the OEM for the inspection of Mosquito magnesium mainwheels would be 150 landings for a thorough inspection. All modern fleet trucks and some historic aircraft are equipped with inertia loggers that can indicate rough use. So probably an inspection after any hard landing event, if you have a pilot called Captain Schettino.

The brake units are light. Too light. Magnesium? A little bit was cut off from a ‘goner’ and the oxy torch confirmed magnesium. As you do, water was thrown on burning magnesium to see if it would cause a more intense flame. It did. A lovely hydrogen smell emitted, reminding me of the hydrogen gas I used to generate reacting hydrochloric acid with steel wool as a 10 year old, to make flaming, exploding garbage bag Zeppelins, while all this was still considered unremarkable childhood play, not the mandatory 25 year jail sentence of today…

A little research in the Moorabbin Air Museum archives unearthered a Dunlop technical manual which confirmed the brake unit body was magnesium. I figure you have to add magnesium brake units to your magnesium component inspection list. These brake units do not form part of the support geometry of the wheel. If a crack were to develop in a brake unit the rolling function of the wheel would not be impaired. But flaking bits of magnesium brake unit casing could get trapped between the unit and the drum during braking, building up heat before igniting, causing an out of control, flaming wheel Mosquito to career into a tractor towing a trailer of corn cobs, causing a conflagration of pop corn sending flaming, expanding corn kernels into the eyes of gathering crowds pouring out from a nearby legal convention.

Brakes

Doing a few brake units, after immersing them in water to keep the asbestos dust sticking in my hair. The brake segments are on a metal backing retained by a clip, retained by a spring. Each segment is relatively easy to remove, revealing the rubber brake ‘bladder’ underneath. In operation, compressed air is directed into the bladder, pushing the segments outward to bear against the brake drum. When the air is released, the brake segments re-seat under the force of the springs drawing them back. It is a simple design. The only negative is if the ‘parking brake’ is activated while drums are still hot, causing too much heat transfer to the rubber bladder over time.

The nightmare scenario is landing your Mosquito at the King Charles Coronation Airshow, having a wheel shatter and ground looping into the celebrity dias…when you do look at the cracks that have occurred, in every circumstance on these two wheels, the cast iron brake band remains unaffected, and the main substance of the wheel performing the function of supporting the aircraft remains in place. It is unlikely that the wheel will shatter into lots of pieces. What might occur is that the retaining ring comes apart, causing a tyre blowout. No doubt this is unpleasant, but the dias would probably be OK. I sense the wheel would have been telegraphing its unhappiness through micro cracks a while beforehand. These telltales could be compensated for/hidden by the brake drum and integrity of the rest of the wheel for quite some time.

I don’t think that replacing the original magnesium wheels with newly cast aluminium wheels is the answer. There is a lot of weight in the wheels and the aluminium wheels would be 30% heavier, requiring reworking of the hydraulics, centre of gravity and undercarriage design factors. I think that newly and properly cast magnesium wheels, subject to routine inspection and backed by a data logger, would be a sound method for keeping a Mosquito rolling over the next 100 years.

Cracks in the usual places

In the weakest part of the wheel, the fluted section, is the greatest weakness, a recess for seating the steel ‘noggin’ on the steel retaining band holding the split rim together. On both wheels, cracks propagated from this point, previously hidden under paint. In service, with the retaining band in place, these micro cracks would be hard to see, so perhaps a little spray of lubricant and chalk dust could show them up.

Visual inspection is worth doing

A set of original main wheels adopted for a hard life of postwar agricultural use was sent off to the sand blaster to clean up and have a closer look at cracks. To cut a long story short all cracks appeared in the weakest (least thick) part of the wheel casting AND these parts were visible from the outside. In other words simple visual inspection by a pilot or mechanic is a sound method of checking the condition of wheels. Under this logic, a crack is most likely to appear in the fluted section of wheel, which is readily visible. Upon further reflection, the fluted design acts to increase the surface area, which helps to dissipate braking heat. The thinner, fluted section will get hotter than the bulk of the wheel, so is also subject to more cycles of heat stress. Most of the landing stress is taken up by a steel T50 tube running through the wheel to the meaty spokes, connecting at either extremity to the rubber shock absorbing struts. The bead of the tyre is however retained on the rim, the weakest part, so the dynamic loads on the weakest part are coping with the dramatic spreading of the tyre upon landing and tendency to twist while turning. These particular wheels were used on an agricultural cultivator, so probably did hundreds of hours of constant work, twisting around corners at the end of the paddock. My only thought is to have a simple inertia logger on an aircraft, to track rough landings, peak loads and hours of use, linked to routine visual inspection.

The marks on the spokes are from fettling the original casting, made by Kelsey in Canada.

Off to the foundry – all the patterns necessary for complete tailwheel assembly.

Out of Time

Working in the Moorabbin Air Museum Archives I came across this missive relating to Bristol Freighter Dunlop main wheels failing at 1,200 hours. I went outside to the Freighter on display and the mainwheels seem to be Goodyear UK, so I wonder if there was a change from Dunlop to Goodyear wheels for this reason in the past. In any case, the Goodyear wheels appear to be cast magnesium. In reference to Mosquito aircraft logbooks, few accumulated 1,200 hours, so I figure that no original data was established for cast magnesium (Dunlop design, often cast by Kelsey of Canada) Mosquito main wheels, but the Freighter information provides a reasonable basis for establishing a major inspection scheme for the current day. Certainly a loaded Freighter working every day is a ‘worst case scenario’ in comparison to an unloaded modern Mosquito working once per month. Is there any other information out there on the service life of cast magnesium undercarriage wheels that anybody may be aware of?

Tail wagging

Another nice pattern for tail wheel assembly. Main body holding rubber dampers. All the different patterns show why you just can’t 3D print casting patterns without appreciating the job of the foundry man, or direct cast off an original component.

These patterns are not cheap, but I figure they could help keep 10 tailwheels in the air over the next two decades or so, so the pattern cost can be amortized to a tenth, making an individual casting more affordable.

It essentially locks up risk capital for decade, with the simple hope that lower cost casting propositions will eventually lead to its return. There are many better and smarter things to do with money, but no dollar bill ever directly blew the top off a Gestapo prison.

Ed, Magnesium was and still is, easier to cast than Aluminium and in war time was not in as high a demand as Aluminium, so was easier to get hold of.

Helicopters STILL to this day use Magnesium for Main Gearbox castings. The bigger the casting, the more likely it is Mg and not Al (with helicopters anyway). I only know of one helicopter manufacturer that uses Al for their big gearbox castings and they’ve got the casting of Al down so good that they cast all the oil lines internally within the casting with no machining necessary afterwards! Crafty Italians!!!

Just been cleaning up a Hayes Industries 27 inch USAAF wheel made in April 43.

To the rear it is market as Lynite – the alloy used in its construction.

A tale of tailwheels

Here we go with tailwheel casting pattern, all magnesium.
The pattern maker said that according to the drawings the machining would pop the side of the magnesium barrel. Surely not! Not the chaps from Salisbury Hall! Poor design, hurrumph! But sure enough, when you look at the original casting, the original machining does pop through the surface, highlighted in the blue texta. The chaps put in a steel liner insert, that fixed it ! A steel liner in a magnesium body was never meant to last electrolytically…it would certainly help as a splint if you got a crack in the magnesium.

When you think about the risk of a tailwheel, right at the end of the hydraulic run, failing to lock down, you understand why the tailwheel was sometimes isolated from the hydraulic circuit and left down at all times in service, according to some old pilots. The little hydraulic ram that lifts the tailwheel unit has to do a lot of work to lift it, which is why they probably went for lighter magnesium for a massive assembly.

Now if you told the chaps that the tailwheel would never need to retract in service, and you wanted the tailwheel assembly to last for 50 years, might they have chosen aluminium for the casting……luckily the shrinkage factors for magnesium and aluminium are pretty close, so maybe you could do both materials out of the same pattern…

The pattern maker has original plans, but draws the logic of the pattern out on a piece of wood : how the pattern breaks apart and how the foundry man will set up his sand molds. So the piece of plywood drawing goes off to the foundry as well. But you can’t email the piece of wood, you have to hold it in your hands as you ride your horse to the foundry.

Let me know what you want to do with them!

Wheel should be a magnesium alloy. Remember P&P I still have your landing lamps!

Please feed them occasionally !

Ed, unless originality is a must, I would ditch the decagonal bead and go with a standard bead/tyre combo. If the wheel isn’t driven or braked, there won’t be any relative rotation between tyre and rim, unless perhaps if the bearing were to seize in the wheel!
White tell-tale markings on the tyre will let you know if it is moving.
Bigger wheels don’t need it as they have plenty of square inches pushing on the bead to ensure there is no slippage!

Thanks for the update 🙂

Cheers
Graham

Graham, it’s a good point, re no seized bearings mean no tyre slip. I have learnt to wait though before I move on from something without clearly understanding the ‘why’. The reasons are always surprising and the product of the thought of many well trained men integrated with a flying test program and feedback from many contemporary users. It will be something weird out of left field like 68% more electrical contact between conductive tyre and wheel….

This P-38 wheel below has both a casting number and a Dow metal material identification on it, showing Heat Treated Dow H magnesium was used, making life easy.
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Because I have just gone through an election I have been conditioned by politico speech to avoid the plain truth. So, “due to market forces I wish to reposition my stance on my previous comment which was made in the context of circumstances that have now changed”, specifically the metal in the Bendix wheel is Dow H-HT, not Dow H as I previously stated. All this because I am spending a lot of time reading through old engineering books and staring at old wheels. Dow H-HT is actually a very sexy magnesium alloy with interesting herbs and spices like Nickel and Copper. It’s ultimate strength and elasticity is better than the DTD alloy used in Mosquito wheels. I had a hunch that the Bendix wheels had to be made of something interesting because they are quite an optimized design, very little wasted metal. The Mosquito wheel is kind of a heavy boned fish wife next to the Bendix svelte young thing. Dunlop seemed to chose a simple alloy and loaded plenty of redundancy into the design. I wonder how much Bendix wheels were designed for organized tarmac and Dunlop wheels for grass and mud fields.

Wheel should be a magnesium alloy. Remember P&P I still have your landing lamps!

Ed, unless originality is a must, I would ditch the decagonal bead and go with a standard bead/tyre combo. If the wheel isn’t driven or braked, there won’t be any relative rotation between tyre and rim, unless perhaps if the bearing were to seize in the wheel!
White tell-tale markings on the tyre will let you know if it is moving.
Bigger wheels don’t need it as they have plenty of square inches pushing on the bead to ensure there is no slippage!

Thanks for the update 🙂

Cheers
Graham

Corrhextion

What I know see with the Marstrand tyre, referencing the wheel, is that the inner bead is hexagonal

To avoid a hex I must correct that the bead is decagonal rather than hexagonal. I figure you don’t see this type of bead much and maybe it is a consequence of designing a small tyre to fit within an airframe and large mass or high landing speed of airframe. The streamline tyre in the picture is not a Mosquito tyre, but this seems to be a Dunlop innovation.

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