Technote : Vickers Potts Oil Cooler.
Technote is an informal record of a part restoration where corrections or additions by others assists the record.
Brass finned engine oil cooler design used on late 20’s to mid 30’s aircraft : Bristol Bulldog II (Jupiter engine 1929-1935), Hawker Hart-Hind-Demon (Kestrel engine 1930 – 1936) and retrofitted to aircraft in warm climates. Same design concept with larger surface area used in Anson (Cheetah engine 1935 – 52)
Designed 1927 (Vickers dwg/Pat1968)
Design opinion :
An iconic 1930’s ‘silver biplanes’ manufacture, but can’t understand why it is made of brass. A heavy item to stick to an aircraft, why not make it out of aluminium or even steel ? (Specific gravities : alum 2.85, steel 7.8, brass 8.5). Thermal conductivity of brass very low. Your saucepan has a copper bottom to slowly gain heat and slowly lose heat – why not use aluminium which would rapidly transfer oil heat into the airstream ? Modern oil coolers/heat exchangers/air conditioners use aluminium radiators for the most efficient heat exchange, later aircraft used aluminium. So the VP oil cooler is a heavy item which is a poor heat exchanger, what was the sales pitch ? Can’t be corrosion resistance, fair enough for brass water system radiator, but irrelevant for oil system, where even oil tank was duralumin.
Design is hollow plate type heat exchanger subject to 25 psi oil pressure test, 15 psi service load ( RAAF Fitters IIE 1943, pg 163), in fact a pressure vessel with a fragile geometry. Rivets everywhere to hold pressure vessel sides together. Best guess is that at the time of design aluminium sheet was considered too weak (alum sheet tensile strength 12 tons psi [Design of Aeroplanes, Judge 1917, pg 177] only rising to 25 tons psi with metallurgical improvements nearly 20 years later [Materials of Aircraft Production, Hill 1934]). Use of steel (35-65 tons psi) would require new, separate processes for steel brazing, corrosion protection in respect of existing processes for brass radiator manufacturing using brass sheet (18-24 tons psi). So perhaps this is a design that developed as a sideline from brass radiator manufacturing, radiator workshop utilisation being a key design consideration.
Why do you need an oil cooler at all ? Especially if it is a poor heat exchanger ? If it protrudes into the airflow to create resistance ? Best interpretation is that the introduction of supercharging from the mid 20’s introduced higher engine inlet temperatures from compression of air charge which raised engine oil temperatures. Schneider Trophy Supermarine S6 had extensive oil cooling ducts on floats in late 20’s, late war twin stage Merlins had intercoolers to lower temperature of compressed charge, so the VP oil cooler was perhaps the first commercial answer to the new problem of heat dissipation arising from supercharging. Early supercharging to 4 psi meant not too much heat was required to be lost, allowing an inefficient heat exchanger to be viable, in fact consistent with allowing cold, treacly early formulation mineral based oils to rise to temperature as quickly as possible. Later wartime emergency supercharging, better oil formulations (scramble from cold) and the urgent need to shed engine heat made the brass VP oil cooler obsolete.
In it’s time, for the Vickers salesman in his spats and brilliantined hair, here perhaps was the pitch :a scalable ‘lego brick’ system, off the shelf, where more fins could be added to modulate heat loss. It could be retrofitted to European climate aircraft introduced to ‘colonial’ climate service. It solved the new problem of shedding heat from newly supercharged engines or closely cowled radials. It kept the boys in the radiator department busy, all good in a Depression.
By: aircraftclocks - 25th November 2013 at 06:44
AAP356 Coolant radiators and temperature regulators
Ed
I know this will be a long shot for you but AAP356, Coolant radiators and temperature regulators, may cover what you are looking for.
I say a long shot because it was issued in 1943 and is most likely focussed on american aircraft in service at the time.
There are a couple of copies at the AWM. One copy suggests that it covers american supplied equipment and the other is neutral as to source of supply.
By: Bulldogbuilder - 11th November 2013 at 07:02
Nice work. I do however think that if you are going to make some of these that you should look very closely at aluminum as opposed to brass. Brass is okay for the museum class replicas, but a lot of work to fabricate. The aluminum would be easier by many degrees.
One thought about the corrosive factor. The life span of these airplanes were such that design progress and obsolescent would far surpass the corrosive question in relation to time.
Ed (still beating out way too many parts)((working on last wing panel))
By: powerandpassion - 5th November 2013 at 11:58
Oil Cooler Dreaming
The brass fins of the oil cooler have spacing rings between them made of aluminium. Inside the brass fin itself is a matching aluminium ring, so the brass plate is sandwiched between aluminium. I am looking at machining these aluminium rings up. The cross section of the spacing ring is figure 8 shaped. Why do this ? Why machine in an extra groove in the guts ? Why not just a simple piece of tube cut to length ?
Then it hits me : Hot oil would pump through the assembly under pressure, the aluminium spacer ring, with its higher heat conductivity, would heat and swell before the brass fin. In fact the figure 8 section would heat and swell fastest at the thinnest section, effectively forcing the ‘beads’ at the top and bottom of the figure 8 to seal the brass fin against leakage. How bloody clever ! How elegant ! How simple ! These days a plumber squirts silicone gel around a seal and charges you a quid a second for this masterclass, but in those days there was no silicone, in engines or breasts ! A self sealing metal to metal contact made by folk who knew their materials and made them perform. I am slowly falling in love with my VP oil cooler, and learning to take no design feature for granted.
Another lesson is patience. There is no fully annealed B12 type brass sheet I can easily get on the market, I will have to take half hard and anneal it myself. The model railway folk are good here, the gents who make one eighth scale working steam locomotives out of brass sheet – true craftsmen.
The man doing destructive tests on solder is in hospital, so I have to wait.
The toolmakers all want a lot of money for a multistage press tool that presses and blanks out the brass shells and openings. I will have to either press out then waterjet openings or waterjet blanks with openings then press out, hoping that the distortion in the sheet does not put holes out of alignment.
There are lots of spacing rings, distance pieces to machine out, that will keep things busy until after Christmas.
I put my head under a DC-3 bonnet and there was a brass oil cooler, and when I think about it I have a seen a brass oil cooler on a P40 too. So brass oil coolers travelled through to mid thirties designs and across design schools of thought. The choice of brass must have been far more purposeful than just the fact that brass was a familiar material from radiator work. The more that I think about it the late 20’s -30’s oil cooler was really a heat modulator rather than oil cooler. In other words it was designed to allow oil to HEAT up first ( by being an inefficient heat transfer device) then to allow it to lose SOME heat at altitude. Old Jock was a fitter friend of a friend who described heating up oil outside of the aircraft in the scramble days then pouring it in just before readiness, so engines could be started from ‘cold’ and roar away from base within minutes. So you really didn’t want a highly efficient oil cooler in the first ten minutes, you wanted a good heat modulator. Ahh yes ! The Hawker Hart-Hind-Demon had a coolant radiator that the pilot raised or lowered into the airstream depending on conditions. Raise out of the airstream at start to allow the engine to heat up then lower as things settled in. So the VP Oil ‘cooler’ design was consistent with this engineering approach, except the pilot did not have to raise or lower it into the airstream, because the designers knew their materials.
Self intelligent metal to metal joints, heat modulating materials eighty years ago. Grandpa had some tricks !
By: powerandpassion - 4th October 2013 at 14:03
Destructive tests
Some destructive test findings, so you don’t have to repeat them !
Fabrication is made of two shell pressings, top shell and bottom shell (larger circumference)
Shell pressings appear to be hot dipped in high temperature (hypothesis) solder, as inner and outer surfaces are evenly coated with thin solder, and this would be impossible to accomplish after rivetting of shells together, as solder could not drain out.
(Hypothesis) Electro chemical corrosion between aluminium spacer rings & brass fins in an assembly subject to water spray would be aggressive, as fins and spacer rings are designed for metal to metal contact.Therefore dip coating is zinc rich, as zinc is relatively passive to aluminium. Central steel oil distribution pipe is not in fact in direct contact with brass fins, so another potential source of electro chemical corrosion is avoided. The original designers have been thoughtful in combining dissimilar metals prone to attack each other.
Each copper rivet has a brass (B6) spacer. Spacers are in three sizes – large, medium, small. All rivets, including short rivets that from outward appearance show the shell pressings kissing each other, have a small spacer. Spacers soldered onto bottom shell to fix in place. All rivets have a copper seating washer under top and tail. All rivets have a blob of solder over top and tail. Cross section shows this solder penetrating down the shank of the rivet, well sealed.
Each shell pressing is B12, fully annealed brass. Bottom shell with larger circumference is turned over at edge to form a bead over the top shell. Bead is flooded with solder to assist seal, cross section shows solder sneaking into the folds of the bead, well sealed. This bead is even around the circular edge of the fabrication – no crinkling, splits or deformation to this bead around the circle.
Most commercial grades of brass sheet used for modern pressing in half hard condition. Very difficult to replicate the even beading around the circumference in half hard material. Very hard to achieve with Jenny wheel. Perhaps this is why a fully annealed material was specified, in combination with a “low skilled labour” method of mechanical beading.
(Hypothesis) Order of manufacture for fin assembly :
1. Bottom shell, with rivet openings, was pressed with outer edge turned up 90 degrees, top shell pressed to snuggle in to top shell, both shells zinc dipped.
2. Copper spacers soldered to bottom shell fixed in jig with standing pins used to locate copper spacers.
3. Top shell placed over bottom shell in assembly jig, progressively riveted.
4. Circumference beaded.
5. Circumference and rivet top & tails slobbed with low temperature solder.
Have completed autocad drawings, will output as .dxf files to send to the toolmaker. Seems like a relatively straightforward pattern to CNC from a dxf file. Have found an old press tool to cannabilize to make up shell pressings. Brass sheet suppliers all scratching their heads as they only have half hard sheet on the shelf, and the last of the Mohicans still manufacturing things happy with that. May need to special order fully annealed material. Better open a beer while I think about that.
By: Whitley_Project - 29th September 2013 at 10:57
P&P I like your post a lot – I sent you a pm.
By: powerandpassion - 29th September 2013 at 03:20
Oil cooler inlet & outlet tees
Vickers Potts Oil Cooler inlet and outlet tee pieces are die cast aluminium components common to coolers used on Hart biplanes and Ansons.
Does anybody have spare tees in a box?
At this stage a low production run of tees can be accomplished via CNC milling from billet (no tooling) or single tool die cast, more work, more fun.
By: powerandpassion - 29th September 2013 at 03:12
Oil Cooler material specs
Material specs for oil cooler
Sources :
[1] Vickers drawings 1927 via Pat 1968
[2] Materials of Aircraft Construction FT Hill 1934
[3] RAAF Fitters IIE 1943
[4] Destructive test
Brass fin pressings made from B12 [1] being Brass sheet, annealed, very soft for intricate pressings [2], 26 SWG [3], 18-24 tons max stress per sq inch [2] Cross section photo below shows pure copper rivets [1,4] used to join fin halves, bottom half larger than top half, bottom half lapped over and sealed with solder. Need to do oven tests to determine if this is Grade A (180 degrees C melt point), Grade B (205 degrees C) or Grade C 225 degrees C). Brass fin copper rivet heads sealed by additional solder. Whole assembly appears to be dipped in solder.
Intermediate spacing rings made from L1 [1], being Duralumin, 17 – 25 tons max stress per sq inch [2].
Proposed modern material 2024 T3
NB – all joints between fins and spacers are metal to metal [3,4], no sealing compounds. Old coolers subject to brass ageing which will frustrate sealing as hardened metals will not seat into each other as effectively. Modern use of aged components may require modern seals for previously metal to metal contact surfaces.
Connecting tubes given as mild steel [1,3] but tungum listed in oil cooler application in Tungum advertising in Aircraft Engineering. More logical than mild steel as Tungum has lower electro chemical corrosion potential than ms. Oil Cooler assembly of ms, aluminium and brass has large differences in electro chemical potential, setting up corrosion in the long term
By: powerandpassion - 29th September 2013 at 02:42
Avro Anson VP Oil cooler
Avro Anson Vickers Potts Oil Cooler images attached.
Hart biplane cooler (Air Ministry Type A 325 – 25 psi test pressure)
Anson cooler (Air Ministry Type 802 or 826 – 90 psi test pressure)
Data source RAAF Fitters IIE 1943
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September 29, 2013 at 2:37 am