Dave's Planet

So I've been fighting to get my brand new Magnum XL 15A to run and finally figured out that having the motor on a power pod mounted above the wings and having the fuel tank in the bottom belly of the plane was just too far away. The engine took off like a bat as soon as I brought the tank up closer to the engine. But I couldn't fly like that, so I asked the good folks at Hobby Haven what to do about it. They suggested a Perry Pump, so I looked them up on the web and read all about them. I was convinced it would do the trick but it seemed like an awfully lot of work to solve such a simple problem. The basic problem was I wasn't getting enough pressure off the muffler port to drive the fuel back up that high. So I decided to try something simpler first, I took the rear section off the muffler, made a thin aluminum disk out of sheet metal that fig snugly inside the muffler up snug against where the rear of the muffler started to curve, just before the exhaust pipe. I didn't need to do a perfect job of trimming this disk with the trimming sheers, it wasn't meant to be totally airtight. I routed out a hole in the center to run the screw through that held the muffler together from stem to stern, again a rough job, not trying to be airtight. Then I bent up the BOTTOM (so the fuel didn't collect) of the disk a bit 'till it looked about the same opening, maybe smaller, as the exhaust port. The idea was first to try to make the air flow around a few more corners, there was still roughly the same amount of opening as the exhaust pipe, but now with the air having to take an extra turn (and that air is moving fast with lots of momentum) there was substantially more pressure in the main part of the muffler, enough to drive the fuel back up to the motor.

All I needed to get the fuel from the low mounted tank up to the motor was just a little more pressure in the muffler pushing on the pressure port, and making the fast moving air flow around a few extra corners did the trick. If that hadn't done it then I would have bent the corner back down a little more restricting the airflow until it did work. Obviously I'm also increasing pressure slightly at the engine exhaust so I'm robbing the engine of a little more power than the bare muffler, but it was well worth the easy solution to a relativly easy problem.

Another thing that helps a lot is to add a check valve so that fuel only flows up to the engine, it doesn't fall back down as pressure drops between strokes.

This worked nicely to solve my problem but if you try it yourself you do so at your own risk.

If you liked this you might also like Some software I wrote to graph wind speed minute by minute using National Weather Service stations. I'm not running the software right now so you can see the date/times on the graph are out of date. If you need any help at all or customization just ask, I'll probably do it.

If you found this to be useful, all I ask is that you give me a link from your web page so others can find it too.

Dave's Planet

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