Engine experts:

I'm a returning control line guy.  I flew control line in the 50's with a fox .35 on profile models.  I went into r/c in 1960. 
I've been flying control line again for 2 years.  I've got a nagging fuel delivery (I think) problem.
My setup:
Sig Banshee with r/c throttle control
Evolution .40NT r/c engine
APC 11x3 prop
OS #8 plug
Cool power 10%
Muffler pressure on a 4oz clunk tank mounted on engine centerline in side view

I tune the engine to a rich 2 cycle.  When I fly maneuvers, inside maneuvers are generally ok.  engine might lean out a small amount on the down line part.  When I fly negative g maneuvers the engine goes waylean.  Not so lean as it will quit but I think that since it quickly goes back to the rich 2 cycle when I pull out to level flight helps here.  I will never be a competitive flyer but trying to fly decent looking maneuvers is made more difficult with the way the engine runs.  I have noticed that if I throttle the engine back to about 3/4 it makes very little difference in my lap time but the leaning out problem is improved. I also have a Brodak Accentor with the same setup except the engine is inverted....same problem.  Also, inverted laps run the same as upright laps

I've run the evolution engines in my r/c models for years and never had this problem.  I know that control line is not the same but I'm stumped.

Any advice would be appreciated.

Steve

Thanks for the reply Motorman
I'll try lowering the tank and going richer yet.  I've got the props you suggest so I can add them into the mix.  the idea to lower the tank is pretty straight forward on the Banshee and I think makes sense as the fuel head should be a bit greater in negative g's.  One question comes to mind though.  The location of the clunk moves up or down based on the + vs -  g loading.  Not an issue with a metal tank.  Should I switch to a metal wedge? what about modifying the clunk to a uniflow plumbing configuration?  I've never used a uniflow but read that a lot of guys swear by them.

As for the Accentor, the Evo .40 seems to have plenty of power.  Flies nice through overhead maneuvers.  If I have to go too much richer to deal with my problem I don't know if that will screw up the overhead performance.

I've attached a picture of the Accentor for critique.

Steve

Welcome back.  I am not much of an engine man so I only know what works for me and I just keep doing it.  Uniflow works great for me in a metal tank both on and off muffler pressure.  I have never been happy with Uniflow in a clunk.  Simple muffler pressure.  One thing you can check if you are using simple muffler pressure is to make sure that the pressure tube is always out of the fuel.  I run mine to the forward top corner so that it never gets into the fuel.  I have also had an issue when the fuel tube is too strong and the clunk weight falls more on up or down that the opposite or too long and it catches on the back of the tank in transition.  IMHO, nothing can beat a properly set up clunk.

Accentor - Nice!

Ken

Ken

All
There is a story  regarding the r/c engine.  When I got back into control line 2 years ago I was concerned about getting dizzy as a 70 year old guy.  I decided to control the throttle by radio so I could end the flight at will.  Dizzy never became a problem with 5 sec laps.  What I did learn is that the throttle adds another dimension to sport flying (did I mention that I am not a competition flyer?).  For example doing touch and go's, doing a touch and go and doing a (name your maneuver) immediately after the go part and then doing another touch and go immediately after the maneuver.  Being able to taxi back to the pots is also fun.  Anyway I like the throttle.  My flying buddies are also putting a throttle on their planes (both glow and electric). To me, this is all about having fun.

Ken, right now the pressure (vent) is straight up so it will be in the fuel for at least half of the run.  My tank is round so the clunk movement will cause a change in fuel head in maneuvers but it should be the same with + or - g's.  My clunks are free and do not touch the end of the tank.

Tim, the carb has an o ring seal and I always make double darn sure it's not sucking air there.

I'm still struggling with why there is such a big difference between + and - g maneuvers.

Thanks for the nice comments on the Accentor.  I did an r/c version of the Accentorthat you might like too.  I did a John Player Special livery like the old Lotus Formula 1 race car. I did the graphics in gold leaf. at the time Brodak was selling the kits for $69 so I bought two.

Steve

Ken
I understand the concept of a submerged pressure line making bubbles but I don't see how the bubbles would sink to get to the pick up tube.  That being said, a  uniflow has the vent tube closer to the pick up and it too makes bubbles.

Steve

I agree with Tim and Motorman, about lowering the fuel tank. Would also suggest that CL stunting is a never-ending experiment...always keep trying new props as they come available to you. Props are a major factor in maximizing the performance. APC's are excellent, and they have LOTS of sizes, but they are not always available at the LHS.

Also have doubts about the throttle being a potential source of inconsistency, but the results don't seem to show that. If it did, I'd try putting a good layer of some sort of grease over all the seams to prevent air leaks. I wouldn't be concerned with upright/inverted laps being the same as much as insides/outsides. A speed difference in the maneuvers WILL throw your timing off and the result won't be good. Lower the tank until you get the insides/outsides the same. Horizontal 8's are the way to fine tune that. If you can physically limit your throttle to 3/4 and that helps, that'd be a start, IMO.

Both your planes are real pretty!  Steve

 

Ken,
EBay has a uniflow clunk, do the search for "Custom made Stainless steel clunk pick up for tank for Model Airplane Engine". Look at the pictures and you can see the whole set up. You could buy the one show or make one by soldering a piece of copper tubine along side the clunk as shown in the picture.


Best,   DennisT

A conventional clunk tank should work fine, as long as the vent line (that lets air or exhaust into the tank) is near the inboard edge of the tank, that is, the side the ullage resides once it takes off. Any other vent should be blocked off for flight. This will give you a classic "suction" tank that leans out as the flight progresses - which is generally a small effect if the venturi is the right size.

    I would suggest starting with muffler pressure attached to the vent, but you can try it with an open vent, too.

    Since you have a throttle, you should be able to play the throttle VS the mixture to get it right speed and acceptable mixture at the same time. *if it was me* I would seek out a throttle position that permit the engine to run in a rich-medium 2-stroke in level flight, and then it should lean out in the maneuvers.

    Part of the issue is that the engine you have is likely to be *far too powerful* for the airplane. Using an 11-3 prop should help it be more inefficient so that's a good starting point, and being able to effectively run a very small venturi (by closing the throttle) should give you a method of reducing the available power to closer to what you need.

     I don't have a lot of experience with RC throttle settings, but what little I have done suggests you need to adjust it very carefully over the full range of throttle positions, even if you only run it in a fixed throttle setting in flight. That means tweaking the air bleed for an idle that is more-or-less consistent with your full-throttle mixture. The other thing I (and many others found) is that it will just change power slightly up and down at what appears to be random, eitehr from the barrel shifting slightly, or varying air leaks, or something else. Usually, on 2-strokes, it's livable, and much more significant on 4-strokes.

     Brett

The Evo .40 has a decent 2 needle carb.  It has good mid range response The idea of running it a little rich with the throttle barrel partly closed is something I can experiment with.  I've been adjusting the mixture at wide open throttle and then reducing the throttle after take off.  I think I'll try setting the mixture with the throttle partly closed and use the throttle channel end point adjustment to set the desired barrel opening. The Evolution engines are pretty powerful for their size and the .40 on both the Banshee and Accentor  is more than they need.  I'm trying to deal with that with throttle position and prop.  I'd try a 12x3 but I can't find anyone that makes that combination.

Another question...is there a difference in the rich to lean problem with an upright mounted engine vs a side mounted one?

Steve

The Evo .40 has a decent 2 needle carb.  It has good mid range response The idea of running it a little rich with the throttle barrel partly closed is something I can experiment with.  I've been adjusting the mixture at wide open throttle and then reducing the throttle after take off.  I think I'll try setting the mixture with the throttle partly closed and use the throttle channel end point adjustment to set the desired barrel opening. The Evolution engines are pretty powerful for their size and the .40 on both the Banshee and Accentor  is more than they need.  I'm trying to deal with that with throttle position and prop.  I'd try a 12x3 but I can't find anyone that makes that combination.

Another question...is there a difference in the rich to lean problem with an upright mounted engine vs a side mounted one?

   Yes, that is extremely common, but there are versions of the problem that cannot be corrected or improved by moving the tank around, it's about how the gas flows in the engine, and how it responds when you maneuver it around. The infamous "Fox Burp" on profiles is a classic example (that is almost 70 years old).  There is no real rhyme or reason to it, but in a lot of cases, baffle-piston engines frequently have problems when mounted on profiles, and schneurles have more problems with the cylinder mounted upright or inverted. But it depends heavily on which particular engine we are talking about.

   There is little/no experience base for an RC-carb equipped Evolution 40 on CL airplanes, so you are treading some new ground.

    Brett

 

This is essentially what the Bob Reeves set up for running a four stroke engine does, but he uses a nylon screw to set the throat opening in a fixed venturi instead of a moving throttle barrel.

     I would add that the Berringer-style "screw throttle", which Bob Reeves adapted to remove a lot of the precision machining very effectively, give you about the right kind of adjustability you need for almost any stunt application. If someone was ambitious, one might want to set about trying to take the barrel out of a standard OS RC carb and replaceing it with an adjustable screw. I think it would probably work just as well on any of the 46 and smaller 2-strokes, the limitation for larger engines is that the little OS aluminum needle valve body may have too much flow restriction for ideal in-flight response on the larger engines that suck 7-8-9 ounces per flight. It will certainly still needle but might not run correctly, going too lean at points in the pattern.

   Otherwise, it gives what you want - fuel is injected in the middle of the stream instead of along the wall, and it has a bluff body to stir up the flow and promote better mixing.

    Brett