A return to model building and flying. Part 72


July 2011

A Change In The Weather


Finally! the weather has decided to give us a few breaks. at least enough to get some meaningful information out of the test flights I am making. As it's much far too close to my clubs open carrier event to sort out all the problems I have in the time scale available, the decision has been made to just fly in Basic Carrier. Even so the supposedly simple job of having two models available has become the usual epic journey.

I had inherited two useful engines form my father after he passed away, an Irvine 40 in relatively new condition, and an original front induction Austrian built HP40 in a similar condition, even lacking my late fathers trademark burnt Castor and blobs of Epoxy finish. :) The latter I had not used because of the lack of silencer to fit the very narrow exhaust port.

The Irvine I had modified and fitted in the Hellcat proved to be wasted journey to the field number one. I had split the spigot sleeve to allow some play for fitting. Unfortunately the gap let too much air bleed by, causing the engine to run on even with a fully closed throttle. So yet more work turning another sleeve, being very careful to get the fits right and an O-ring just to make sure. The next test flight was all I had hoped for with a well behave engine; only wish the same could be said for the model which always has had the slow flight characteristics of a pregnant whale. At least now I had one reliable model to use.

Yet another day dawned, this time with with the  Bearcat fitted with my old SC40, which I know performs reliably, if unspectacularly. Only trouble was, I couldn't take off? the grass had not been mown and because of the full line rake I have built in, plus the silly way the wheel covers retain the wheels, the crabbing around was causing one of the wheels to be braked by a cover. Yet another wasted day!

So back to the workshop, with bigger wheels retained by collets, toe in twisted into the UC legs, and the wheel covers refitted. On the next test flight all went according to plan with the UC mod's working well but not happy with the SC 40 which I rattles like mad thing, but still starts first flick? it needs some serious looking at. I also kept thinking of the power of the HP40, and also the hook release is very unreliable, yet another thing that needs rethinking.

One night I was wandering round the flat after picking up an old Irvine semi resonant pipe when I realised it was about the same size fixing as the HP40, and indeed after a bit of filing of the fixing holes it looked like it was made for the job. Unfortunately it needed a bit of nose modification to the Bearcat for the HP to fit, the crankcase is quite wide, meaning a bit of wood had to be shaved off. Again my alloy plate mountings proved their worth, as I only had to alter the fixing holes slightly not drill the fuselage nose with yet more holes.

irvine pipe

The only other mod I had made to the engine was to fit a rear needle valve. What I had taken for a throttle stop screw on the original carb, was not, it was just a retaining screw. by turning down the end of this screw making it longer and of a smaller diameter, I was actually able to make it work as a stop screw even if it sacrificed a bit of the foll openness of the choke. The exhaust worked a treat, with the engine coming on song, if a little noisily, and delivering quite a lot of power. attempts to add an after muffler seemed to choke the engine to death and caused starting problems. Attempting to fly the Bearcat with said engine were a catastrophe as it was full power or very erratic slow running in flight, my carb mod had obviously been a waste of time; so it was back to the workshop.

Back home I spent some time modifying an old MDS carb to fit the HP40. I know these to work well, and actually exceeded my expectations on a test stand, idling steadily with a good pickup. So thus encouraged, it was refitted into the Bearcat.

mds carb

The hook release was modified and after a few failed ideas, ended up very much like the original Hellcat design, so I yet again reinvented the wheel, but this time knowing why it is designed as it is. Sometimes you have to take a lot of short routes to realise why the long path was there in the first place :) The next flight testing was going well until the engine started to behave a bit erratically when throttled back, and was becoming increasingly difficult to start. This all finally terminated with an engine cut out in flight. The cause was a bit obvious on landing: my rather over engineered solution to the lack of availability of a blanking plug for the carb, a turned down old needle valve assembly as I don't posses any metric fine threaded screws or taps and dies, turned out to be my undoing by going AWOL in flight. It now sports half the same assembly bunged up with car filler so that I can just drill it out if need be and return it to it's original use if necessary. Crude but effective.

This again was not a total success, the engine ticks over reliably and picks up to full chat OK on the ground, but after one lap in flight behaves as if it has crankcase full of fuel for a few laps until it clears it's throat?  Fast flight is a dream, no problems there. but any length of time running at a low throttle setting takes for ever to clear the crankcase; not an ideal situation when a instant responce is needed, and with no adjustment left on the midrange needle I'm stumped? Quit why it behaves itself on the ground but not in the air I don't have time to analyse? Well I could alter the taper on the mid range needle? but that's a bit above and beyound the call of duty, and would mean a lot of trial and error.

I have strong suspicion that it's the pipe that is causing the problem as the this behaviour has been present with the original HP carb and the MDS.  The tank arrangement has worked fine with three other engines so I think that can be ruled out. I have no standard silencer that will fit the HP, time is running out and the petrol bill is mounting with all these aborted trips to the field.  I need a quick fix to make a reliable model by whatever means. The only engine I have left is a new MkI SC 40A which has only had a couple of tankfuls of fuel through it,  I am pretty sure it will work but it won't be a winner as it's very heavy compared to other engines I have installed in the same airframe, though it should be better then the the old rattly one I tested successfully in the same model. Some ballast will be needed to keep the CG in a sensible place. "More lead Igor."


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