stunthanger.com
General control line discussion => Open Forum => Topic started by: Tim Wescott on October 15, 2011, 02:56:42 PM
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In another thread, Brett Buck said:
The one and only trick, which almost everybody knows, is to shape the tank so you can get through the 4-leaf with about 30 seconds worth of fuel, but without uncovering the pickup.
I know there's some magic there, but I don't know what it is. I need to make an unusually short and wide tank for a model I'm getting ready (I've got a hair less than two inches of length, and I need a bit over 2 ounces as near as I can figure), so I'm wondering what I need to pay attention to.
So -- how does one shape one's tank to do this, particularly a short one? Do you just need to make a "double wedge" style? Or is there some other magic? A pointer to someone's web treatise would be just fine.
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In another thread, Brett Buck said:
I know there's some magic there, but I don't know what it is. I need to make an unusually short and wide tank for a model I'm getting ready (I've got a hair less than two inches of length, and I need a bit over 2 ounces as near as I can figure), so I'm wondering what I need to pay attention to.
So -- how does one shape one's tank to do this, particularly a short one? Do you just need to make a "double wedge" style? Or is there some other magic? A pointer to someone's web treatise would be just fine.
This one is the chinese tank, but I don't believe that you have to believe that the workers should control the means of production to make it work.
http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=257207&mesg_id=257207&listing_type=search#257241
Brett
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OK. Long and skinny and taller in back than in front -- I understand that.
But my problem is that the space I have is short and fat. I can't build a tank that's 110mm long -- I have more like 2 inches barely, and I really should either aim for 1 3/4, or figure out how to make a recess to accommodate my landing gear. Fortunately this is for an LA 25 or an FP 20, so I think I only need a bit more than two ounces -- but that still ends up with a short, fat tank.
So, can I just make a house-shaped tank, per this pdf? I'm concerned that the pitch of the "roof" is too shallow, and I may need to make a "double wedge" tank -- but I don't know at what point I should be worrying about that, if it's even a worry at all, and if it is what angle of wedge I should be looking for. I also don't know just what the downside is to making the whole thing taller.
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Would I be better if I more or less copied this, only made it uniflow?
http://www.brodak.com/shop_productdetail.php?ProductID=6420
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I either build a square tank with an internal hopper or build an external hopper, but the internal version allows greater volume availability. The pickup tube is inside the hopper, the uniflow outside.
(posted from the seats of the arena at the spousal unit's horse show - who knew they had WiFi?)
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Sounds like a Flight Streak application. Here is a view looking down the inside of a tank that may fit. You have seen it work. It is constant cross-section. Inside height is 1 1/8", inside width is 2", wedge is 60 degrees. Capacity calculation is left to the student as an exercise, but I think it's a tad more than an ounce per inch of length. Put the pickup tube at the back corner, the vent tube at the apex of the wedge about halfway back in the tank.
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Wow, Spencer shrunk! (Did I get the name right, Howard?)
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That's ol' Spencer.
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Howard:
This is, indeed, for a Frite Streak. Very little room between engine and wing (even after I lengthened the nose a bit) So, you're talking about basically a 40 sized tank that someone accidentally whacked off to half size, then patched up the ends? Like in the picture below? I think I have my "height" crossed with your "width", but basically a tall, thin tank (WRT the local acceleration vector) is better than a short, fat one?
Randy:
Internal hopper? How does one achieve that in such a way that the fuel flows into it when one wants, and doesn't just flow right back out again when it wants and you don't?
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Tim, I sent you an email. pattiandruss
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Like in the picture below?
Yep, but with the vent and pickup tubes lying along the wedge as in the Chinese tank. They can emerge from the tank however is convenient, of course.
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Tim,
I run a diagonal plate across the rear, outside corner. About a 1/16" vertical gap front and back and a couple of hole punched through. the fuel goes in but can't slosh back out at an inconvenient moment.
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I agree with Randy's previous mentioning of SQUARE tanks. I had to make one for a model and it worked great. I did have to use muffler pressure to make it cut off more cleanly. I had a short "length, and it is about 1-1/4" tall and the square section gives a touch more fuel capacity.
Big Bear
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Consider using a plastic type tank mounted on the inboard side of the fuse. Works for me on my P-38 and on at least one of my Carrier profile ships. Dan R. has used that setup on his 'Cheater Slow" slow combat plane several years ago. It was so effective the plane got ruled out of existance.
Joe
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Joe, that's what I'll do if I must, or maybe a square metal tank with a Randy Powell internal baffle (I can paint a metal tank). But I really want to keep that whole side of the aircraft clean.
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Are you going for looks or for flying??? VD~
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Are you going for looks or for flying??? VD~
Yes, exactly.
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I need to make an unusually short and wide tank for a model.............
Try a Tom Dixon style front feed hopper tank, they are the widest by design and can be quite short.
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Are you going for looks or for flying??? VD~
I go for both.
Larry, Buttafucco Stunt Team
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I go for both.
Me too. I suppose that since my flying is crappy I should aim for just performance, and to heck with looks. But I can't do that.
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Me too. I suppose that since my flying is crappy I should aim for just performance, and to heck with looks. But I can't do that.
Looks are everything to me, even if the thing flew terribly. Bad flying but a good looking model, works for me.
Static?
I can't fly the whole pattern anyway. Closest I get to the ground is about 10 feet.
I'm working on a tank also. Not the easiest thing to design and build correctly. Especially for an unusual application.
CB
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Closest I get to the ground is about 10 feet.
Sooo ......... how do you land? <=
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Tim,
As you're looking at a 3-ounce tank to put in a spot more appropriate for a 1-ounce tank you will have to make some compromises.
From my view of things, by far the best solution is to buy a Hayes 3-ounce tank, plumb it per the instructions and mount tank to the left side of the fuselage. Of course it looks weird. So does a Flite Streak. You'll get used to it...
Details: Muffler pressure is suggested. Typically these tanks must be mounted 1/4" above centerline of the motor. When filling tank notice there will be a small air bubble in forward-facing "tongue" of tank; tip model forward if expecting to fill tank completely.
As I recall this model features a fuselage you fixed or built anew or robbed from another. Each time you look at that plastic tank hung on the "wrong" side of the fuselage, make a mental note: "Plan ahead, plan ahead, what was I thinking in leaving so little room for the tank."
Dan
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Great advice Dan!
Joe
PS How about sending me your bio information and picture for insertion into the CLW magazine. For some strange reason many in other parts of the nation have asked me especially for your info.
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I mentioned I need a custom made tank.
Exactly what is the relationship between the venturi level, which cannot be changed, and the tank, plus the tube pick-up for fuel?
Can a tank have any shape but have the back end of the fuel tube the height of the venturi inside the tank, but exit anyplace?
Sorry if my question is written poorly.
CB
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I mentioned I need a custom made tank.
Exactly what is the relationship between the venturi level, which cannot be changed, and the tank, plus the tube pick-up for fuel?
Can a tank have any shape but have the back end of the fuel tube the height of the venturi inside the tank, but exit anyplace?
For the purposes of feeding fuel when mostly full, the tank can have any shape as long as the uniflow vent is the right position with respect to the venturi.
For the purposes of feeding fuel at the end of a flight, and cutting off reliably, you need a clunk tank (such as Dan is advocating) or a solid tank with some specific features to make sure that the last bit of fuel gets sucked up reliably (such as the rest of the gang is helping with).
I haven't had great luck with plastic tanks, but one of the advantages is that if you run uniflow you can adjust the uniflow height without moving the tank. With a metal tank you generally tie everything together, so moving the uniflow means, perforce, moving the whole dang tank.
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As you're looking at a 3-ounce tank to put in a spot more appropriate for a 1-ounce tank you will have to make some compromises.
What?!?
From my view of things, by far the best solution is to buy a Hayes 3-ounce tank, plumb it per the instructions and mount tank to the left side of the fuselage. Of course it looks weird. So does a Flite Streak. You'll get used to it...
Details: Muffler pressure is suggested. Typically these tanks must be mounted 1/4" above centerline of the motor. When filling tank notice there will be a small air bubble in forward-facing "tongue" of tank; tip model forward if expecting to fill tank completely.
Actually I think I've found room to cram a 2.2oz metal tank in there, and even for it to have some limited up/down adjustment. If my calculations are correct* that should be enough to do the pattern on an LA 25. The tank is kinda wide (not to mention tall), so there'll be some head change as the plane spins up. I'll be running muffler pressure.
And, if worse comes to worst, my "I'm just practicing my decal application skills" nose art will end up covered by an ugly old Hayes tank.
As I recall this model features a fuselage you fixed or built anew or robbed from another. Each time you look at that plastic tank hung on the "wrong" side of the fuselage, make a mental note: "Plan ahead, plan ahead, what was I thinking in leaving so little room for the tank."
Yea, well (blush). I was thinking "it's a built plane, how could there be problems with it?"
* OMG! It's an engineer relying on calculations! Run!
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The relationship between tank and venturi seems only to be that if the tank centerline is lined up nicely with the venturi you will at least be close enough for test flying. Yeah, yeah, sometimes this is perfect; far more often adjustments must be made.
While a 25LA could be made to run the required 6 1/2 minutes it takes to fly a full pattern on 2.2 ounces of fuel, one must wonder who would like the manner in which such an engine flies the model.
Yes, I am aware that the Hayes 3-ounce tank does not offer uni-flo venting, and so of course one cannot alter the engine run by twisting a tube, running vent higher or lower in tank. On the other hand, over the last few years there have been a great many times when NW folk just assumed I was using uni-flo venting in conjuction with O.S. 20FPs when it was standard plumbing done per Hayes instructions. I can't explain why, but this particular tank is a perfect match for your 25LA...
Dan
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While a 25LA could be made to run the required 6 1/2 minutes it takes to fly a full pattern on 2.2 ounces of fuel, one must wonder who would like the manner in which such an engine flies the model.
I think I vastly screwed up my calculations on the tank size -- 2.2 ounces just doesn't seem to be enough, unless, like you say, I cripple my LA25. I was basing it all on the time it took to run through 1.75 ounces of fuel on the ground, not thinking about how much more fuel it'd use in the air, or about whether the ground run did a better job of using the last of the fuel in the tank.
Tsk tsk. Dumb me.
Yes, I am aware that the Hayes 3-ounce tank does not offer uni-flo venting, and so of course one cannot alter the engine run by twisting a tube, running vent higher or lower in tank. On the other hand, over the last few years there have been a great many times when NW folk just assumed I was using uni-flo venting in conjuction with O.S. 20FPs when it was standard plumbing done per Hayes instructions. I can't explain why, but this particular tank is a perfect match for your 25LA...
It certainly seems like a 20FP or a 25LA is fairly insensitive to head changes as long as you're using muffler pressure. I'm obviously no expert, but what experience I've got seems to show that. It's just -- plastic. Ugh.
I'm going to make up a 3 ounce metal tank (I'm going to have the most impressive collection of spare tanks, by the way!) and try it. If that doesn't work it'll be a Hayes. You have my permission to tweak me about my extra effort if you see me with the Hayes tank when next we meet. I'm trying all this out on my Skyray so I won't mar my 20 point reasonably nice finish with all the mounting experiments on the Flight Streak (nothing could mar the finish of that Skyray; if anything poking a bunch of holes in the nose will just add more visual interest).
Maybe I can fit the nose art behind the engine.
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Tim
I would recommend recessing the tank into the profile body per the directions I wrote that are in the thread: Jack Sheeks Designs, Reply # 20 posted on Oct. 07, 2011 by Joseph Lijoi.
They are lengthy so I will not repeat them here.
Clancy
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Tim
I would recommend recessing the tank into the profile body per the directions I wrote that are in the thread: Jack Sheeks Designs, Reply # 20 posted on Oct. 07, 2011 by Joseph Lijoi.
The nose is way too short for that, and kinda -- painted.
I did consider whacking out the space between the motor mounts and putting an extra sump in the tank; given that the tank is only 1-3/4 inches long that would only gain me 1/2 ounce or so.
I'm gonna try a 3-ounce metal tank and if that doesn't work I'm gonna follow Dan's advise.
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"It certainly seems like a 20FP or a 25LA is fairly insensitive to head changes as long as you're using muffler pressure. I'm obviously no expert, but what experience I've got seems to show that. It's just -- plastic. Ugh.
I'm going to make up a 3 ounce metal tank (I'm going to have the most impressive collection of spare tanks, by the way!) and try it. If that doesn't work it'll be a Hayes. You have my permission to tweak me about my extra effort if you see me with the Hayes tank when next we meet. I'm trying all this out on my Skyray so I won't mar my 20 point reasonably nice finish with all the mounting experiments on the Flight Streak (nothing could mar the finish of that Skyray; if anything poking a bunch of holes in the nose will just add more visual interest).
Maybe I can fit the nose art behind the engine.
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Tim,
Sure hope you didn't get that idea from anything I wrote. Truly it is a mystery to me how one could think muffler pressure could possibly overcome a misplaced tank. For what it's worth, on my 20FP-powered models using the E-2030 muffler and muffler pressure, when I am real, real close tanks are shimmed in 1/64" increments. Same deal with the Wimpact series of model, 25FPs on pipe, no pressure other than atmospheric, uni-flo tank--tank shimmed in 1/64" increments, but only when very close to optimum.
Dan
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Tim,
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Tim,
Sure hope you didn't get that idea from anything I wrote. Truly it is a mystery to me how one could think muffler pressure could possibly overcome a misplaced tank. For what it's worth, on my 20FP-powered models using the E-2030 muffler and muffler pressure, when I am real, real close tanks are shimmed in 1/64" increments. Same deal with the Wimpact series of model, 25FPs on pipe, no pressure other than atmospheric, uni-flo tank--tank shimmed in 1/64" increments, but only when very close to optimum.
Dan
I must have a lead ear, because on my Skyray the tank actively wanders around and I haven't noticed overt differences in the engine run.
Of course, it's only been a month since I smartened up and started using fuel filters, which has made a world of difference (yes, I know, Bad Dobby, etc.). Between filters and getting into the habit of running the engines for a couple of minutes on the ground for warm up, then flying "for real", perhaps It'll start becoming apparent.
(The tank is made with feet through which I can screw it to the fuselage sides, with adjustment slots and all. So as long as I can maintain a good reference I have infinite adjustment range. I hadn't considered until your post that using a built-in vertical reference off of which one shims would allow for more repeatability -- I may have to set a couple of pegs into the fuse below the tank just for that, or take some other similar measure)
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Tim, I agree with DD. I had to shorten the nose of a Ringmaster because of soft mounts. I glued and screwed a 1/4 inch firewall to the old mounts and mounted a Fox 25 (Baffle) inverted. Everything was great till I tried fitting a tank. Like you, I found insufficient room for a commercially available tank. I had a 2 oz. Sullivan Poly tank and tried mounting it to the inside of the fuse. It works great. I run unpressurized with only one vent, pointing forward, and can fly the entire OTS pattern with no problem. The 3 oz. should word just as well if you need a little more fuel. One of the benefits I've found with this tank mounting, is ease of n/v setting. Just set at full scream and release, as soon a centrifical force builds the fuel head pressure increases and the engine goes to a nice 4-2-4 stunt setting. Try it, you'll like it even if it looks a bit funny. Good Luck, Al Burczycki
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Tim,
There are any number of things which might cause you to misunderstimate size of required tank. But a ground run using flying fuel, flying prop and a reasonable needle setting (around 11,500 with an APC 9-4) will get you pretty close.
As to setting tank height I can understand where the details of such has so far escaped you. And you might be testing tank height incorrectly.
A lot of folk simply fly level upright and then inverted to see which way to move the tank. And that's fine, although if the run is good enough to keep the model flying the first time I do this test is pretty much the last time.
The real test comes in maneuvers. Inside and outside loops (big ones at first) will tell you quite a lot as to tank position, as will horizontal 8s. The final test is to fly in a bit of wind, especially with small-bore engines on 400-inch profiles. The wind pretty much turns the model into a kite; this in turn results in more speed and if the tank is even the slightest bit off you will soon be roaring in one direction, slower in the other.
This can catch even the experienced out. I had fitted a new tank to my OTS model (Zilch X, 600-inch model, 25FP, tongue muffler, no pressure, APC 9-4), new tank as I wanted to try this model in Classic. In practice it was bitchen following minor shimming of tank. But at next contest there was some "interesting" wind 1st round and the thing just howled in outsides. Dropping the tank 1/32" cured it of all evil tendencies. And, yes, I confirmed this at next practice session, waiting for the wind to pick up prior to flying this model.
You are correct in establishing a reference for the tank. In my installations there are hooked rods top and bottom of the tank. These serve to hold the tank in place with large "O" rings but the neat trick is the assortment of shims between these rods and tank. In order to change tank position but not have to fiddle with clamping pressure of tank to fuselage one, for example, removes a 1/32" shim from top stack and then adds it to the bottom stack. Easy, consistent and repeatable.
As a final point, use of a clunk tank makes testing pretty easy without doing a whole flight. Get 1 ounce of fuel into the tank, fly a few loops, make required changes.
Dan
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As to setting tank height I can understand where the details of such has so far escaped you. And you might be testing tank height incorrectly.
A lot of folk simply fly level upright and then inverted to see which way to move the tank. And that's fine, although if the run is good enough to keep the model flying the first time I do this test is pretty much the last time.
The real test comes in maneuvers. Inside and outside loops (big ones at first) will tell you quite a lot as to tank position, as will horizontal 8s. The final test is to fly in a bit of wind, especially with small-bore engines on 400-inch profiles. The wind pretty much turns the model into a kite; this in turn results in more speed and if the tank is even the slightest bit off you will soon be roaring in one direction, slower in the other.
Wow. A light bulb just lit (low wattage, but still).
I had all the evidence I needed to know this (except the wind part), and never put 2 + 2 together to get 4. I'd fly upright and inverted and think "oh, this is working pretty well". Then I'd notice in maneuvers that it was different one from the other "gee, I need to adjust that tank!". But I never took the next step to "I need to go do something special to check this out".