Didn't bring the right box of props to the field, so did not get a chance to get the performance up to the point I would be comfortable flying it inverted. (Not enough thrust pulling up into maneuvers. 52' lines, Zinger 10x4) So the upright/inverted fixes remain to be tested next. If I have the engine-to-tank height set correctly, I will finish working on the new cowling. I can't cut the holes in it until I know if I have to shim the engine up or down a bit. So far, it is a riveted aluminum shell with balsa nosebowl and one lite-ply former.
Dave, I haven't responded to this thread since I didn't know the answer, since I have never run the 25FP upright/inverted myself (although a lot of people have done it with success - but I don't know where the tank ended up!).
By the way, someone had previously posted this link:
http://www.ohio-superstar.com/dave-brown-product-line/fiber-filled-motor-mounts and they are definitely in operation as of a few months ago. I got a lifetime supply of Lectra-Lite wheels in May.
I get the prop issue, but what about 52' lines? If the engine is running remotely right, virtually no airplane you put it in will want 52' lines. I have seen 25FP Noblers that would have had reasonable lap times on 72' lines! The typical issue we have had with properly-running 25FPs is that it is still just too much power for the airplanes we have used. 52' lines are not in the plan, it will screw you into the ground if it is running correctly and you have the best available prop (APC 9-4 or 10-4).
That makes me think that there is some other issue, specifically, that you are trying to run the engine *far too slowly* for it to run correctly without massive modifications (that greatly reduce the power). Running too slowly, and assuming a stock engine (which is where I would start, at least), it will be *extremely sensitive* to almost any fuel-flow related factor - needle, tank height, phase of moon.
The reason is the stock venturi is right only if you run it about the way it was intended, that is, a medium-fast 2-stroke, maybe 12,000-13,000 in the air. It's far too large for running around at 9000 RPM in a 4-stroke. With a 4" pitch prop of any variety, and 52' lines, 12,000 rpm is going to end up going about 3.5 seconds a lap or quicker. If the lap time is in any way reasonable with a 10-4 on 52' lines, it has to be running very slowly. Of course, you don't want a Zinger of any variety, but when you get your 9-4 and 10-4 APC it will likely want to go even faster.
So I am a little curious/concerned. I would certainly start on 60-ish foot lines, or longer. I fly the Skyray/20FP on .015x60 or .015x62, for example, at about 4.5 second laps. While I could legally fly it on .012 stranded, I would be scared of that, because the pull, while less than a "regular" airplane, is pretty substantial. To set the engine, start it, then "peak" the engine, that is, lean it out until it is 2-stroking, and slowly search out the point of maximum RPM, as lean as it will go without sagging. Then back off until you get a distinct drop in RPM, usually 3-6 clicks of the needle. It should still be running in a very strong 2-stroke. With an APC 9-4, that might be upwards of 13,000 RPM. but don't go by the RPM, go by the sound. Fly it, do the maneuvers, and if it at any point appears to "sag" lean in the maneuvers (like the top of the vertical
, then with no other changes, arbitrarily back of off a few clicks richer before the next flight, try again, until it doesn't sag at anywhere in the flight. If at any point it goes into a 4-stroke, same thing, except turn it up two clicks leaner, try again. If it does both in the same flight, then we need to do something different (probably get a slightly smaller venturi).
Brett
Topic: Tank Height for an Upright OS 25FP? (Read 4141 times)
