Entrepreneur7

I just bought a Modeltech Magic Extra 300s from someone. It was used so I knew it would fly well. I had a couple of the instructors at the field take it up and trim it out first since this is my first non trainer plane. Man, that thing flies like a dream.


The only thing is during the 3rd flight whenever I went inverted the power went way down. Now, the engine is actually mounted upside down, so when it's inverted the engine is right side up and losing power. How could this be? Where should I look for the problem?

I think my trainer is officially retired, that is until I put a wireless camera in it. At least that will keep it from collecting too much dust.

-Brian Renner

Dukester

My Feedback: (31)

Possibly your fuel tank may be too high. If your tank is mounted slightly high to the carb, then in normal flight, you have a little gravity assistance delivering fuel to the engine, however when inverted the engine actually has to suck the fuel uphill as the carb is now high to the tank. Muffler pressure should help with this, but depending on how high the tank is over the carb, it may not be enough.

What kind of engine and muffler do you have and where is the carb relative to the tank?

Duke

Razor-RCU

My Feedback: (50)

I think Dukester hit it.... sounds like the tank is not level with the carb- Otherwise I say your clunk is stuck to the back of the tank on the bottom, inverted means top no fuel!

Montague

Since the engine is inverted, the carb is almost certainly "low" compared to the tank when the plane is upright. So you tend to set the mixture a little on the lean side so it peaks out nicely. When you roll inverted, the carb is now above the tank as was previously mentioned, so the engine goes lean and looses power.

Just open the needle 3-4 clicks, the problem should go away.

Checking the tank and lines to make sure they hold pressure is a good idea, but I doubt you'll find much there.

Dukester

My Feedback: (31)

Do you guys think a check valve between muffler and tank to help hold pressure provide any benefit? I've never used one myself, just wondering.

Secondly, for Brian, if your tank/carb distance is extreme, you can mount the engine sideways (with exhaust ports down so muffler is on bottom). this will raise the carb position tohelp balance out the carb to tank heights whether upright of inverted. Also has the side benefit of helping to empty a flooded cylinder compared to inverted as the fuel can drain out the exhaust port if needed rather than pooling on plug.

Good luck and tell us what you end up doing!
Duke

photoniq

this is a little off topic so apologies for that. Is the extra 300s your second plane!? I was asking about second planes and everyone told me this was NOT a suitable low wing trainer.. [] I really wanted one..

Montague

It's a "Magic Extra" not an "Extra". The "Magic Extra" is actually a pretty decent 2nd plane, it's really a funfly plane with a sort-of-if-squint-a-bit-extra-style fuse. But the wing is totally NOT an Extra wing, and that makes all the differnece in the world.

I think that kit is designed for the engine to mount upright, so if it's inverted, see if you can lower the tank inside the fuse, that will help. (not sure, and I'm too lazy to go look, but I'm pretty sure the one's I've seen have had upright engines). Anyway, if it's possible to remount the engine upright, it might make things easier.

I agree with Duke, my favorate way to mount an engine is sideways, muffler down. It really makes a lot of engine tuneing easier than either upright or inverted. Inverted engines can be a bit of headache to get set right, though certainly not always.

A one-way valve might work, or it might not. Give it a try. But be prepared to mess with the high and low speed mixtures, as the extra pressure tends to make the engine run rich. What kind of engine is it? Some engines have finer needle valve adjustments than others, and the coarser ones don't like too much pressure, as they will tend to go from too rich to too lean in just a few clicks.

Entrepreneur7

Well, I don't know what happened but I took the plane out the other day and everything worked fine, inverted and all. Maybe it was just being finnicky that first day.

-Brian