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Author Topic: Tank Question  (Read 1591 times)

Offline Tim Wescott

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Tank Question
« on: July 27, 2011, 03:32:19 PM »
I have a more or less bog standard metal tank on the Waiex.  Five sided, uniflow, pickup is at the back outside corner of the tank.

In the beginner's pattern, in the overhead 8 the engine is consistently hiccuping.  I can have one of these events, and get up to 25 more laps out of the plane, so there ought to be plenty of fuel.  I did a test today, and if I do the overhead 8 at the beginning of the flight the plane flies just fine.  It just doesn't like doing so at the end.

What should I be looking for?  Are there any easy things to try?  The one thing that I can think of is that it does seem to lose fuel when it's heading downward, and the tank is mounted dead straight.  Is it very likely that this problem could be a result of not cocking the tank out at the back?  If so, about how much should I cock it to really make a difference?

'Tanks'
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Offline Mark Scarborough

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Re: Tank Question
« Reply #1 on: July 27, 2011, 04:21:00 PM »
Tim
two things, first, I have taken to building my profile tanks with a wider aft crossection than the front, usually 1/8 to 3/16 wider at the back, this helps.
the other thing I found with my Gee Bee profile, if I have the leadouts to far back, then in the overheads the excessive yaw will lead to "bubbles" getting in the pickup tube because the fuel is not at the back of the tank, this coupled with the gravity acting directly upon the fuel to move it away from the pickup seemed to give me grief. I had exactly the same problem, by moving the leadouts forward, I eliminated the problem. Now that I am getting closer to a commited correct fuel load on the Avenger,( not so many " oh crap I scared myself so go level a couple laps" extra laps) I find that the same thing is happening there. SO I am going to start working my leadouts aft on the Avenger now as well.
For the Waix, since its built, existing and functional, You could also make a 1/4" shim to put at the back of the tank. However I would look seriously at the yaw angle in flight. as you are flying, sight down the leading edge of the model, you should not be able to see either the leading edge, or the trailing edge if its square to the lines., ok well in reality, if you fly with your handle by your side instead of centered on your nose, then you would see a bit of the leading edge ( meaning the front of the leading edge as opposed to the profile of the leading edge) were you to sight down the lines then you should only be able to see the exact airfoil and no leading or trailing edge material in flight.
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Offline Tim Wescott

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Re: Tank Question
« Reply #2 on: July 27, 2011, 04:30:50 PM »
When I look at that thing in flight I can't see wing root at all (it's a constant chord wing).  I guess that's an advantage of making an "easily built" wing!  But I'll take another look -- everything else seems to be going well, revisiting the leadout placement may be a good idea.

I'll cock the back of the tank out.  It'll be easy because I've got balsa pads glued to the tank so it won't scratch up the paint -- I can just replace the rear one.  I've taken to making tapered tanks, too -- but the Waiex tank came before that change.
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Offline Tim Wescott

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Re: Tank Question
« Reply #3 on: July 30, 2011, 04:19:47 PM »
Thanks Mark.  The tank did, indeed, need to be shimmed out in back.  I tried that today, and the motor ran flawlessly through the overheads.

Why would you want to make this adjustment with the leadouts?  Wouldn't it be better to shim the tank?  Or is there just not room to do so on the planes you were mentioning?
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Offline George Waters

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Re: Tank Question
« Reply #4 on: July 30, 2011, 07:25:03 PM »
Tim  You could look at the wheels in flight to see if they are lined up as a way to see if youare yawed in or out. I fyou have it flying straight ahead it will help with overhead line tension as well help with the fuel starvation problem. I usually cant the tank out a bit if I can as it also stops that annoying on off on again at the end of the flight.  George
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Offline Tim Wescott

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Re: Tank Question
« Reply #5 on: July 30, 2011, 07:27:09 PM »
This plane has excellent line tension in the overheads.
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Offline Steve Helmick

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Re: Tank Question
« Reply #6 on: July 30, 2011, 09:16:53 PM »
Tim...Mark was asking about the LO's position because if they're too far aft, it could cause that sort of result, plus protracted burping at the end of the engine run. Correcting the LO position could cure the run issues, but also maybe not. If not, then shim the tank's hindquarters outboard. You should be able to sight down the LE, or flap hingeline to give you guidance on the LO position. I don't put that much faith in sighting on the wheels/LG struts, because 1) they're too close together 2) they may never have been aligned perfectly, and 3) do take somewhat of a beating in doing their jobs.

Good to hear that the tank shim fixed your OH8 problem. If you can do a good pair of OH8's, it should be much appreciated by your judges. It's a maneuver that looks pretty easy, but in fact is NOT.  H^^ Steve
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Offline Tim Wescott

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Re: Tank Question
« Reply #7 on: July 31, 2011, 07:58:27 PM »
Quote from: Steve Helmick on July 30, 2011, 09:16:53 PM
Correcting the LO position could cure the run issues
Perhaps I was misreading what Mark said, but I wouldn't want to change the LO position if the plane was flying right but the engine were wrong -- because I wouldn't want to screw up the line tension in the overheads just to get a good engine run if I could have both.
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Offline Mark Scarborough

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Re: Tank Question
« Reply #8 on: July 31, 2011, 10:33:35 PM »
Tim, as a general rule, moving the leadouts forward ( to a point) will improve the overhead tension.
as a question, you state the overhead tension is great, however, do you really know its as good as it can be?
that said, my suggestion was based upon observation with two of my airframes that exhibit this tendancy and what the REAL problem was.
In my situation, the problem wasnt with the tank construction, it was with the trim, the airplane was flying nose out just enough to cause some minor uncovering of the pickup in the overheads.
dont accept that you think the tension is as good as it can be, explore , experiment, and try things. you dont know till you try. I moved my leadouts forward about an 1/8 and it fixed my problem. Are my leadouts far enough forward,, ? not sure yet, give me another 50 flights...
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Offline Jim Thomerson

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Re: Tank Question
« Reply #9 on: July 31, 2011, 11:43:03 PM »
I've never actually done this, but it seems like a neat idea,  Put a vertical paint line on the wing tip, and a vertical paint line above and below on the fuselage, so that you can see the wing  yaw (or not) by seeing how the lines line up. 
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Offline Tim Wescott

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Re: Tank Question
« Reply #10 on: August 01, 2011, 10:45:56 AM »
Quote from: Mark Scarborough on July 31, 2011, 10:33:35 PM
as a question, you state the overhead tension is great, however, do you really know its as good as it can be?
OK.  I see what you mean.  Yes -- that's a good point, and I should experiment with it, even though I think it's OK.
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