Need a little help here.

[Andrew Saunders]

I need a little help here guys. Yesterday i went flying at the field and ran into a problem with my engine. The engine is the Blue LA 46.  On the first run I adjusted the engine on the ground to 9500 RPM's. When it was in the air, The RPM's dropped. The second run I adjusted it to  10 RPM's and it dropped again once in the air. The fuel is new. Sig half castor half synthetic. Brand new Glowplug even tried changing it out and same results. What would cause the drop in RPM's once in the air? Tank too high?

[Dave_Trible]

I would ask if this airplane/engine setup has been flown much before or is this new?  A good uniflow tank setup will do exactly what you describe as you get air velocity down the uniflow line making a little more pressure on the fuel.  I wonder if you were using muffler blockage (er...pressure) before and forgot to put the fuel tubing on and now running open air?  Otherwise I'd check for a loose spray bar or air leakage there around the Venturi or maybe a pinhole in the fuel line.  Then maybe a leak test for the tank.  These things usually cause the engine going lean rather than rich but sometimes just a going over everything cures it.

Dave

[Andrew Saunders]

Its been flown  before . Over a year old. Flew it at the nats last year. The tank is a uniflow.

[Andrew Saunders]

Ill defeintely check it out for leaks. Thanks Dave.

[Paul Taylor]

From my experience a air leak would make it lean out.
What prop are you using?
Weight of the plane?

Could it be getting hot?
Maybe a plug going bad?

Did you change anything?

[Brett Buck]

I need a little help here guys. Yesterday i went flying at the field and ran into a problem with my engine. The engine is the Blue LA 46.  On the first run I adjusted the engine on the ground to 9500 RPM's. When it was in the air, The RPM's dropped. The second run I adjusted it to  10 RPM's and it dropped again once in the air. The fuel is new. Sig half castor half synthetic. Brand new Glowplug even tried changing it out and same results. What would cause the drop in RPM's once in the air? Tank too high?

      How do you know the inflight RPM?

      Brett

[Andrew Saunders]

Brett the last time i flew the plane which was last weekend i was able to get a 5.2 second lap at 9500 RPM's. This time, when in flight, you can hear the difference after it took off and my lap time went to 6.5 seconds. Way too slow. Im gonns change out the fuel lines tomorrow and check the NVA and the tank for leaks.

[Andrew Saunders]

PAul im using the MAster airscrew 11x5 like i did all of lastyear. I changed the plug twice . Check the engine temps and they were fairly cool. I think ill check for leaks and go from there. I have plenty of time till the NATs

[Wynn Robins]

clean out the spray bar too.....  they gunk up.

go to muffler pressure - LAs love it

[Dan McEntee]

  By the term "RPM drop" we can't tell if it's slowing down rich or slowing down lean. I'm guessing it's going rich from your description. An engine should go just a bit rich in the air if all the fuel system is right and tight due to centrifugal force. usually takes at least a whole lap to settle in.It's possible that you had some sort of restriction previously that has cleared out, and now you have a lot more fuel in the mixture. I would change your fuel lines like you plan (check the pick up line in the tank if possible), then fly the model again. Check the tank height to make sure it isn't too high, but I doubt that is the problem. Get another experienced ear to go with you to help determine if it's going rich or lean and that will help you go in the correct direction, and change only one thing at a time.If you change a bunch of stuff and the problem goes away, you won't know what fixed it. To me, the big thing is to figure out of it's going rich or lean, and you should be able to tell the difference. Since it's doing it once it is up to speed in flight, I'm betting on it going rich.
   type at you later,
  Dan McEntee

[Andrew Saunders]

Hey dan its going rich. IIM getting allot more oil on the wing  and the rpms drop big time once in the air as soon as its up it drops. Even before the complete first lap.

[t michael jennings]

Andrew,

Sure sounds like a leak in the fuel system.

Pressure test the tank then replace the fuel tubing.  That would be a good start.

Then check if the needle valve is leaking.


T Michael Jennings
Knoxville, TN

[Andrew Saunders]

Well Guys I must first say thank you for all your help. I really appreciate it being im still learning . The problem has been solved. I tested my fuel line and there was no leaks. I checked the Super Tigre NVA and it was good. I tested the tank, and there it is. A leak at the back of the tank. So i got out my bernzomatic small butane torch and soldered it shut. Capped off the fuel feed and the over flow. Took my syringe  connected it to the uniflow, dropped the ntank in water and put pressure to it, no leaks. The real test will be tomorrow at the flying field. Very excited. I hope it works.

[Randy Cuberly]

In a uniflo tank system the air is drawn into the tank through the fuel before fuel can be drawn out by the vacuum inside the engine...you know that right!  When sitting on the ground with the engine running your hole in the tank was covered by the fuel so the uniflo "restriction was working exactly as it should.  After going airborne centrifugal force pushes the fuel to the outside of the tank which undoubtedly uncovered the hole in the tank and the air flows freely through that hole and the uniflo restriction is up set...Viola engine runs richer!

You knew all that right...this is a common cause of this kind of problem...Like the old professor said...You should have known!!!

Randy Cuberly

[Andrew Saunders]

Would a leak in the tank make the engine harder to start then usual?

[Dan McEntee]

  A leak in the tank will draw air  when you choke the engine, and lessen the effect of the crank case vacuum to pull fuel, so yes, it may make it hard to start. It again is a "sound" thing. if it gets just enough fuel to burp like it's running out a prime, that sort of thing. But if it leaked that bad, you would think maybe it would drip while the model is sitting there with a full tank? MAke sure you report back with a flight test result.
  Type at you later,
   Dan McEntee

[Andrew Saunders]

Well here's the progress report. I pressure tested the tank two more times before i left the house. No leaks. Got to the field and the first two flights Still went rich on take off. I was very frustrated Until fellow Club member Eric Taylor Showed up. Well the problem then was my tach is a piece of Junk and was way off. The engine like the true 9,500 Rpms to 10,000 rps. It also Likes the 11x4 APC sport prop. Runs great now. THANK YOU ERIC!!!!  Plus he sat me down and took an hour to explain some of the maneuvers to me and how it should work and gave allot of advice. Exactly wht i have been needing.