Well no luck with Brodak .40, different prop. etc. Our resident expert flyer thinks that it has a strange resonant noise that can be felt when holding the
plane. We told our friend to put an electric motor in it!!!!!!( He is headed in that direction anyway) Thanks for all the replies.
Is this unflow or suction, and does it have muffler pressure or open intake to the tank.
If suction and open: the answer is obvious - at some point the fuel pressure drops too low (which is much more common on a profile) and it can't get enough fuel. Adding castor probably makes this worse, since the ability to suck fuel depends on several items, only one of which is generally considered. Everybody more-or-less understands the effect of the venturi diameter, but most miss completely the fact that the fuel viscosity also has a huge effect - the thicker the fuel, the more difficult it is to draw through the needle/spraybar/fuel lines. One of the things David and I have found over the recent years when we have been experimenting with the fuel system is that even the 75 with its tiny venturi can have occasional "funnies" that can be changed or eliminated by using oversize fuel lines, reduced-viscosity fuel, and/or counterboring the spraybar. The average flow rate is of course higher on the 75 since it's burning *8.5* ounces a flight, but the real problem is not the average flow rate, it's the peaks. If you watch the fuel line you can't see it and it looks like a continuous flow, but in reality it goes in spurts, and that raised the peak velocity in the lines.
Even the regular fuel line makes some difference but the biggest difference by far was made by counterboring the spraybar. We were trying to understand a few things one day, and we ended up comparing the PA spraybar to the Jett spraybar, and noticed that the cross-sectional area on the Jett was about 2x-3x as large. David took a PA and bored it out - which was a complete failure since it removed the needle valve seat! On the next try, he bore it almost to the seat. Then like magic, all the occasional random blips of 2-stroking were gone. Same thing happens when running low-vis oil and a standard spraybar.
I would hasten to point out that it ran pretty darn well even with the stock spraybar and normal fuel tubing - well enough to win the WC and several NATs. It just ran better and better as restrictions in the fuel system were removed.
I tried the larger line, larger feed line in the tank on the Jett 61, but it didn't make much difference, probably because the peak flow was much lower due to the longer intake timing, less suction, and of course, 6.7 ounces instead of 8.5. However, it was markedly different depending on the oil content of the fuel. I wasn't sure if it was really just the viscosity or some combustion or cooling effect from the oil. So I tried an experiment - I took some copper fuel tubing and wrapped it tightly around the header, and then JB-Welded it in place. I ran the fuel from the tank, to the copper coil, and then from the other end of the coil to the filter and spraybar. This created a fuel pre-heater much like the regenerative cooling for a rocket engine, except intended to heat the fuel instead of cool the engine. Nor surprisningly this made for a massive needle change, and bit of lag when starting, but the engine ran even better than normal. Same fuel, no heater, ran much different even when the needle was reset to get the same speed. It greatly reduced the boost in the corners, and the engine ran substantially smoother, almost to the point it was *too* smooth for anything but perfect conditions. It also fell apart in a few flights - jb-weld is OK for fixing leaks in the back end of the pipe, but right off the header seems to be a little too hot.
If uniflow and pressure: I don't know why, but I have had something very similar to what you describe several times happen to me, both on the ST46 and 30 years later on the Skyray/20FP. It would go about halfway, right after the square 8, and take off. I figured it might have something to do with heating up the fuel and causing it to foam. but that's a little bit if a stretch. It didn't matter where I put the vent outlet inside the tank. I ran many flights with uniflow and pipe pressure on the PA61 and never had an issue, but it does happen occasionally.
Suction and pressure - I would be surprised of this worked less well than suction and no pressure and for sure the mechanism would be different. In this case, maybe the needle opening has to be so small to handle the extra pressure that it starts vibrating like the ST needle used to,. I can't run the Jett on pressure (suction or uniflow) because the needle opening is so tiny with some needles that it was impossible to adjust.
Uniflow and open: no idea.
In any case I would suggest just trying it in a different configuration than it is now, and see if the problem changes, which might give you a clue. Another alternative is to put a 1/2 ounce of stick-on weight *on the tank* (clean well first!) - if it is a resonance issue with the tank, that will certainly change. We found a problem on the original/prototype "Doctor" that responded much differently when we tacked some weight to the tank.
I am actually amazed we don't have more problems with resonance than we do.
Brett