On the middle of January I bench ran the .61 Aviastar again, with a prop 12.5X5 (Master Airscrew Classic). This prop is used in RC for scale models. It is not the prop that I intend to use with the Aviastar for CL, yet, but nearer the size of the one that I plan to use.
The numbers that I got were:
9600 rpm for 4-2 cycles and 11300 rpm in the max speed (without muffler, Fox long plug)
9700 rpm for 4-2 cycles and 11600 rpm in the max speed (without muffler, Enya 3 plug)
9400 rpm for 4-2 cycles and 11000 rpm in the max (with tongue muffler, Enya 3 plug)
As Pete Cunha opined before for the other props, they are not so bad.
In spite of the lack of compression, it restarted easily when hot. The only thing that I had to do in this condition was to bring the fuel, by gravity, until the entry of the Venturi, by disconnecting the fuel line, turning the fuel line down and connecting it again, after fuel arrived. With this, after flipping the prop two or three times, it restarted. When hot it didn’t have enough compression (vacuum) to suck the fuel only by choking the Venturi and hand flipping the propeller, so the necessity to do something more to suck the fuel. This, sometimes, happens with ABC engines in this situation. As it is an engine for stunt, not for speed, this doesn’t bother any much.
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One other thing that I observed in these last runs is that in one of them the engine ran away when I turned it to up side (70° more or less). Even after turning the engine back to level and opening the needle valve, more three or four turns, its speed didn’t slow down. The only way to stop with the run away was to pinch the fuel line, stop the engine, close back the needle to the original aperture and restart. After then, it restarted normally, in 4-2 T and it didn’t happen again, but it can happen and in flight, that would be worse, so I am considering sending its head to Bob Z to do the mod that he offered.
The compression of my .61 Aviastar, when cold, compared to a typical .61 ABC, giving a number, is about 40% of it. When it arrived was with something between 20 and 30%. It raised a little, after bench running the engine for three sections, but it only hit this value, that I would say is a little below 40%. With 70% I would be satisfied. The growth (10% to 20%) of compression was almost all due to the unburned castor oil between cylinder and piston when the engine stops wet.
So, at first, I intend to order from Brodak another cylinder and piston, with a better tight fit, to solve the problem of compression and, then, I will send the head to Bob Z to do the mod. I expect that with the mod the compression tends to diminish a little, since the volume of the combustion chamber will grow a bit, but nothing to worry about, if the engine has a good compression.
Since January 04, I am exchanging e-mails with Brodak Mnfc, telling about the poor compression of the engine and about my interest in ordering another cylinder and piston assembly for it. My unique condition was that it had a good tight fit. Brodak Mnfc. is always telling me that they would be sending another one, with the fit that I was asking. They only asked me to wait sometime until they have it to ship. They also let me know by enter lines that they would not bill me for this, although I have already said that I would pay for it and for its postage. The last e-mail I received from them was on January 28, telling me that they would be with it in a few days. So, I am waiting for the confirmation of the shipment and the amount to be paid, if necessary.
Luiz