A return to model building and flying. Part 47
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Most of this was written before the tragic loss of my father, So it may seem a pit sporadic at times, but I will get back into some sort of routine eventually.
The Buster engine tank setup continues to give me grief. Using an OS.25FX and a Brodak 3oz metal tank (I wonder what that is in real money?.... 88.7cc it turns out.), it was giving very unpredictable runs with exhaust pressure or not.
Dropping the tank by about 10mm eased the cutting out inverted problem, but the engine is very vague to try and set. The run consists of a very rich takeoff and a very lean and much to fast for comfort middle and end of run. All of this with a seeming magical ability of the engine to run on empty tank full of fumes.Believe it or not powered landings are more common than deadstick. This resulted in landing the model at the end of one flight and wondering why after it stopped it was balanced on the wheels with the tail in a flying attitude. It was only when it nosed forward and the sods of earth flew into the air that I realised, the engine was still running.....!
It's was time for ..................................!
The cunning plan, "No it doesn't involve a potato Baldric," was to utilise the existing RC carb to restrict the top end performance, then if the engine leaned out there is a limit to how fast it could rev.
This was a partial success in that I ended up with the throttle half closed and it would still get through he schedule but the great difference in mixture between tank full and empty still exists. I am getting a little tired of constantly having to modify existing tanks to uniflow to get round this problem, and to be honest if doesn't always work with peaky engines; that covers most Schnuerle ported engines which only seem to be happy at high revs.
I'm coming to the conclusion that to use a smaller engine and just let it rip, using just the choke to limit the top end, may be a better approach; it's certainly simpler.
This is how I did it. The strange bends in the pushrod are due to the O.S. practice of making actuating levers that are so close to the carb body that a strait pushrod fouls both the mounting bolts and the port casting in the crankcase. The final kink is to keep the electric plug leg in a place where I can easily adjust it on the field.
The screw lock is made from a 13 amp mains plug leg, with one end drilled to take la long backplate bolt. You may have to use something else if you happen to live in Europe or the US, and wanted to do the same thing.
The carb aperture can be set to a known reference diameter by using a drill shank; in this case 4.5 mm. Useful when it comes to making a fixed aperture venturi, as it gives some idea of where to start.
The tank run at half throttle is endless on an 88cc tank, I have replaced it with a smaller one, only to find it too small. Ever find that something just doesn't want to play....!
Not much, due to the weather, almost constant rain and high wind for most of January, plus I had other more pressing things to worry about.
2nd of January and money was extracted from my bank account. 14th props arrived. Now I know it can be done, whether it's worth the hassle is another matter.
After playing with the MDS 48 for a few flights. the throttle response seems to have quite a lag that I can't quite get right. After a lot of good humoured badgering from the RC fixed wing and heli fliers to try using 30% Nitro; I will add a this point that as the engine has a quite high compression rating I didn't expect it to have much effect, I capitulated; I must confess I was curious to see the results myself.
The result was an engine that I could not really set the main needle, it was so woolly it could be set all over the place. Set lean, it would still run but at reduced revs with an increase in the mechanical clatter, obviously pre-ignition. in the air it had a similar performance to running on 5% Nitro at half throttle, whilst consuming fuel at a prodigious rate.
A useful experiment, even if only proved that my assumptions were correct.
I have come to the conclusion that the there is not much advantage over the SC 40 I was using before, so The old SC engine is now back in the Hellcat. I just wish I could find out why the MkIII SC 40 has such a weather dependant throttle response, as it's certainly the most powerful 40 I have, but for competition use, this behaviour rules it out.
No Progress on the model