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I’m a great fan of the ‘KISS’ principle (Keep It Simple Stupid!) so the standard Rotorway cabin air mixer pretty much meets with my approval - it just needs a little refinement. From my experience of car ventilation I have learned that the smaller bore the ducting the higher pressure is needed to overcome any restriction and move air along. The Rotorway air collector beneath the radiator will only collect air a relatively low pressure - as the cooling fan rotates. The silver duct hose supplied is cheap and nasty and with all the bends and folds along it’s (quite significant) length will offer a fair resistance to smooth air flow.
The guys at Southern Helicopters told me that they couldn’t find a suitable fan to boost the air flow a little without adding too much bulk or weight so I took a look around.
This was my first find at a recent Exhibition - a tiny 12 Volt blower. It rotates at over 18,000 rpm and shifts an awful lot of air. Size, weight (268 gm) and power are OK but because it’s a centrifugal design, fitting it in the air flow is not straight forward.
This is probably a better candidate. A 50mm diameter axial fan that will fit in-line with the duct hose. At 22 cfm it’s pretty powerful for it’s size and very light - only 170 grammes and draws only 350 ma - but it’s very expensive!!
I made a filter from a piece of Brass mesh. It’s quite a delicate little fan so I felt it needed protection from sucked-in grass and leaves.
The drain pipe ‘T ‘ supplied by RW for conversion into the warm/cold mixer valve. has a worst case cross sectional area of just over 2 square inches. Aside from the idea of a piece of sewage plumbing forming a part of my helicopter, I felt that this was too much restriction in such a low pressure ventilation system so I made a new ‘T’ piece from 2 inch diameter 16 gauge aluminium tube. This gives a cross sectional area of almost 3 square inches - every little helps!
First improvement was to mill a flat on the butterfly spindles.
Rotorway’s idea of a cable clamp is to poke the cable under the hose clip and tighten it!! Yuk!! I silver-soldered a stainless bolt to a good quality hose clip and used a rubber lined ‘P’ clip to secure the cable outer. This allows the cable length and direction to be adjusted without loosening the hose clip. I don’t think the RW supplied hose clips are good enough quality for this mod.
Here’s the finished assembly. The RW duct hose is a little larger than 2 inches inside diameter so I cut a couple of rubber rings from a bicycle inner tube to stretch over the aluminium T and match the diameters.. You can see the P clips securing the cable outer sheaths to the hose clips. I’m looking for a piece of thin wall rubber tube to join the fan housing to the T.. A short length of 2 inch radiator hose does the job but it’s a little bulky and inelegant..
I was pleasantly surprised to find the weight of my aluminium ‘T’ at 164 gm to be less than Rotrways plastic one at 205 gm. A popular English phrase for this would be - ‘A Result’
Of course, my big challenge now is to convince my CAA inspector that this is a safe and well-engineered minor modification that has no mechanical failure or reliability issues - or I could be reverting back to the sewage pipe!!
CABIN WARM AIR COLLECTOR
Rotorway have designed a wedge-shaped sheet aluminium air collector box that sits under the radiator and ducts warm air along a length of flexible ducting to the ‘T’ piece above.
From the folded template it was clear that it wouldn’t fit as prescribed - by mounting in a hole through the side of the GRP radiator shroud. But it didn’t really matter; the basic shape was OK although the 2 inch hose connector spigot design could be improved. Rotorway suggest a strip of rolled aluminium, snipped to a ring of points at one end rather like the top of a Kings Crown. The points are folded down alternately, one inside and the next outside of a hole in the collector box. Hmm...
I began my version of this component with a 2 inch length of 2 inch diameter aluminium tube and a billet of aluminium with a 2 inch hole bored through it.
I annealed the tube (by heating it with a gas torch until it was hot enough to char a piece of softwood then letting it cool naturally) then inserted it into the billet with about 3/8 of an inch protruding through. I then gradually beat the end over to form a ‘Top Hat’.
Here it is on the outside of the collector box,
drilled and riveted it on the inside.
and sealed on the inside open seams with a smear of Polyurethane adhesive.
Here’s the finished box with trimmed and beaded outlet tube and rubber seals all round. I’ve drilled the flanges in three places and I’ll fix it to the radiator with nylon ties like the one pictured here. If you’re not familiar with these handy gadgets, they work along the lines of cable ties except that one end has a moulded-in square base and the ratchet end is a separate square moulding. They are designed for fixing cooling fans to radiators by pushing the tie through the fins of the rad core and tightening up on rubber pads. Four of these will hold a heavy fan securely onto a radiator so they’ll have no problem with our light alloy box.
Here it is mounted in position.
I lined the hole with rubber ‘U’ channel
And here’s the T showing the mounting brackets. The lower, aluminium one has two rivnuts and is fixed through the vertical panel in the passengers footwell. The 2 inch diameter rubber hose is glued to the ‘T’ with Epoxy adhesive and the fan is secured with a lightweight hose clip. I silver-soldered a second mounting plate to the hose clip.
The installed valve assembly. I invested in some better quality silicone duct hose which has a much smoother bore and will offer less restriction to air movement,
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