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General control line discussion => Open Forum => Topic started by: David M Johnson on April 26, 2012, 05:01:32 PM
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I was looking at some of the Brazilian designs and noticed some of the had the fuel tank recessed into and even sticking out the inboard side of the fuselage. without getting into a heated debate what are the advantages? I could see possibly structure issues on impact with the ground but what fuel feed advantages would this design do.
David Johnson
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I was looking at some of the Brazilian designs and noticed some of the had the fuel tank recessed into and even sticking out the inboard side of the fuselage. without getting into a heated debate what are the advantages? I could see possibly structure issues on impact with the ground but what fuel feed advantages would this design do.
David Johnson
You mean on profiles? It reduces the fuel pressure loss from centrifugal force.
Brett
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I've seen the tank placed on the inboard side on profiles. It gives you more room for a larger / longer tank when it's not jammed right up against the back of the engine. Ringmasters and Magicians are served well with that layout, as are other "short nose moment" designs.
Brian
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Here are some photos. Of what I'm talking about.
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Here are some photos. Of what I'm talking about.
That's what I thought. This increases the fuel delivery pressure over putting the tank up against the outboard side of the fuselage.
Brett
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The better centered the tank behind the engine, the less fuel pressure varies from centrifugal force..
To convince yourself of this, imagine the fuel tank mounted on the outboard wing tip. The weight of the long fuel column would greatly reduce fuel pressure as the centrifugal force of circular flight kicks in. Line tension increases would see the engine leaning.
Now consider the opposite case, with the fuel tank mounted on the inboard wing tip. As centrifugal force takes effect, the weight of the long fuel column increases fuel pressure. Line tension increases would see the engine richening.
Continue the thought experiment by sliding the fuel tank toward the engine; there is a point (somewhere near the centroid of the engine) where effects of centrifugal force on fuel pressure are minimized. While ~1/2" of fuselage thickness might seem to be an insignificant adjustment distance, I assure you it is not.
Those are horizontal forces with respect to the model in flight. Vertical forces must also be dealt with - "height" adjustment of your fuel tank may be needed to get similar engine runs upright and inverted.
I've always experienced better engine runs on my profile ships with the fuel tank recessed into the fuselage. Ted Fancher's Imitation profile design comes to mind as one of the more prominent examples of centering the fuel tank.
L.
"Logic merely enables one to be wrong with authority." -Doctor Who
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50 years of tanks on the outboard side of a profile fuselage, and we've been doing it all wrong! Why didn't someone discover this before?
Floyd
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Floyd, it's not ALL wrong. It's just very convenient, and it often works tolerably well.
As for 50 years, I got my tip from Bill Melton, who had been doing it long before he ever met me.
Working tolerably well is one thing, but remember how profile ships have {earned} bad reputations for poor engine runs compared to full fuselage designs. Lots of people gripe about it but accept it as the nature of the beast without trying to correct it. Building a more substantial profile nose (e.g. using long engine mount beams, plywood doublers that extend to the high point of the wing, using aircraft grade plywood instead of Lite Ply, recessing the fuel tank) will fix many complaints.
The problem with putting in the time and effort to make a profile fuselage work very well is that nearly the same investment can produce a quality full fuselage.. ;->
L.
"You may find that a bit difficult without your space helmet.." -Siri, iPhone app with attitude
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50 years of tanks on the outboard side of a profile fuselage, and we've been doing it all wrong! Why didn't someone discover this before?
My dad explained the pros and cons of where the tank goes to me in about 1968, and I got the impression that it was common knowledge in the 40's.
Brett
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And then there is the balloon tank in a box version. ;D
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Floyd, it's not ALL wrong. It's just very convenient, and it often works tolerably well.
As for 50 years, I got my tip from Bill Melton, who had been doing it long before he ever met me.
Working tolerably well is one thing, but remember how profile ships have {earned} bad reputations for poor engine runs compared to full fuselage designs. Lots of people gripe about it but accept it as the nature of the beast without trying to correct it. Building a more substantial profile nose (e.g. using long engine mount beams, plywood doublers that extend to the high point of the wing, using aircraft grade plywood instead of Lite Ply, recessing the fuel tank) will fix many complaints.
There are some down sides to burying the tank in the fuselage - mostly, it takes away material that would otherwise serve to reduce the other big problem, i.e. vibration. That was/is a big problem with the original "Doctor" - tank was in a good spot, recessed, but it had engine run issues from the vibration. As you say, fixable, but eventually you get to the point that you might just as well build a real fuselage. Something like an All-American fuselage is no more difficult than a profile, the parts count is probably lower.
Certainly, a good profile front end that centers the tank and handles the vibration well is much more complicated than you would likely put into a $1.95 kit.
BTW, the simplest very effective solution is found on the MUSTUNT I plans - build a platform to put the engine upright (or with a minor mod, inverted). Correct fuel system geometry and handles the vibration in the direction you care about a lot better. I think it violates the definition of "profile" in the AMA rule book but I don't think that definition applies to stunt.
Brett
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"Certainly, a good profile front end that centers the tank and handles the vibration well is much more complicated than you would likely put into a $1.95 kit."
Boy, howdy, I guess! A $1.95 kit would probably not even have enough wood to build the fuselage.. ;->
I'm not convinced that mounting the engine upright or inverted is necessarily a panacea for profile models. I've seen many profiles with engines mounted at 9 o'clock that run extremely well. I think it is most important to have a very rigid nose - and you can't have resonances that induce vibration problems. Building a 90 degree transition into the nose for upright engine mounting must be well done.
(That said, I certainly respect the Mustunt design series; my friend Gary Marchand had a couple which worked well. The Mustunt design provides a properly centered fuel tank position, which is our thread subject.)
Other profile engine maladies can be helped simply by using a decent engine mounting plate, say 1/8" aluminum or composite sheet, to smoothly distribute forces over a larger area. The worst setup I witnessed was a Fox .35 on a Flite Streak, with multiple small washers between the engine {for angle offset} and its profile fuselage "beams", short lengths of basswood. This configuration vibrated so much it was felt at the handle. The engine run was particularly rotten and erratic, and the simple old Perfect wedge suction tank made it all the worse.
Collections of small improvements frequently add up, but not always.
L.
"Design flaws travel in groups." -Fifth Law of Design
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"Certainly, a good profile front end that centers the tank and handles the vibration well is much more complicated than you would likely put into a $1.95 kit."
Boy, howdy, I guess! A $1.95 kit would probably not even have enough wood to build the fuselage.. ;->
Not today But when this idea came along, $1.95 was about the price of a Ringmaster - the most notoriously poor profile engine mount design there is.
Brett
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Whatever you do with the tank mount, make sure you have a solid front end and a solid mount for your engine/profile setup.
The number 1 reason for bad motor runs on profiles is the construction/execution , be it a weak, floppy, or poor designed, or one that vibrates a lot without damping. Or one that has a fuel tank that is not mounted solidly.
Randy
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BTW, the simplest very effective solution is found on the MUSTUNT I plans - build a platform to put the engine upright (or with a minor mod, inverted). Correct fuel system geometry and handles the vibration in the direction you care about a lot better. I think it violates the definition of "profile" in the AMA rule book but I don't think that definition applies to stunt.
Read the rule book. Upright and inverted engines are specifically allowed, and what you can do with the mounting provisions for them is spelled out. Basically, per the rule book if the extra width from the motor mounting goes away by the time you're 25% of the wing chord back from the leading edge, you're golden (but -- read the rule).
There's no specific category for profile stunt in the AMA rule book -- I think that all the profile events are local. NW Profile is much more permissive than the AMA rulebook as far as what you can do to the nose -- it calls out a fuselage width of no more than 3/4" from the trailing edge back, and that the engine must be fully exposed "from lug to plug", and that's about it. That's a lot more permissive than the AMA definition, and certainly gives one a lot of latitude for locating tanks, &c.
I should be more pragmatic, but -- I like to keep the inside of the airplane clean, because that's what I look at when I'm flying!! A custom cut-out in the fuselage, filled with a custom-made tank, is cool -- but if I did that, then certain highly opinionated Experts from the North here on the West Coast would tell me that I should just stick a Hayes tank on the inside nose and be happy.
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Read the rule book.
!
Brett
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So, for a profile with a 20-ish OS motor (the FP 20, LA 25, and 25S all have the same mounting hole locations, and I suspect the FP 25 does, too), the bearers need to be about 1-3/32" apart. If you leave yourself some wiggle-room for adjusting the tank up and down (which the newer OS motors don't seem to need -- but let's pretend), then you'd build a tank that's about 15/16" tall, or maybe 0.9". If you make it 3" long (which should fit on a Skyray), you'd need to make it 2" wide -- so it should fit quite well.
But my question is -- what do folks then do about reinforcing the humongous hole that they've gone and made in their fuselage structure? Making room for the tank means that you're opening up some serious windows in plywood that you would really like to still be there, to keep your fuse rigid while retaining lightness. And all those holes means that when you do something like run into the pavement at 50mph, your fuselage nose is more likely to break.
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You mean on profiles? It reduces the fuel pressure loss from centrifugal force.
Brett
Hello David,
I am one of those brasilian that use in one profile Pathfinder those kind of thank. Because the reasons well explained here, it help to mantain the plane velocity inside manouvers riching as the centrifugal goes up.
To compensate the structural hole, I made same carbon reinforcement.
Best regards,
Jorge de Azevedo
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But my question is -- what do folks then do about reinforcing the humongous hole that they've gone and made in their fuselage structure? Making room for the tank means that you're opening up some serious windows in plywood that you would really like to still be there, to keep your fuse rigid while retaining lightness. And all those holes means that when you do something like run into the pavement at 50mph, your fuselage nose is more likely to break.
Lightweight models that fly well tend to be mutually exclusive with heavy models that can endure crashes.
As for the humungous hole where the fuel tank recesses into the fuselage, you first run your maple 3/8" x 1/2" motor mounts completely back to the intersection with the wing airfoil.
You can construct somewhat of a crutch with vertical pieces both fore and aft on the fuel tank cutout hole. See illustration below. While that works well, my latest approach (which uses a 3/8" thick profile fuselage core) is to simply cut (pin route) the crutch from 3/8" plywood. See 2nd illustration. Combined with 3/32" aircraft birch plywood doublers extending aft to the high point of the wing, it builds a very strong profile nose.
My experience with crashing hard is that the outboard wing often breaks off. If the profile fuselage fails, it is on the aft portion, behind the flap line. My laminated nose was not broken, even when my engine suffered a bent crankshaft.. Your experience may vary.
Soo... Don't crash! ;->
L.
"It's a poor sort of memory that only works backward." -Lewis Carroll
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David,
See yourself the Pathfinder with recessed tank flying and pay attention to the engine functioning.
http://www.youtube.com/watch?v=BorHJgRYWs8
Jorge
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Jorge,
Very nice looking model and quality video. Fine motor run. I also like the yellow biplane!
Well done.
L.
"Our knowledge is a little island in a great ocean of non-knowledge." -Isaac Singer
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Thank you all especially Jorge. As you probably could tell the plane is a Cessna AgWagon and the nose will have on the left side a structure tying into the wing which will help the vibration issues the crutch goes all the way to the wing as well. For power I'm using a OS Max s 35. the plans came from Flying Models October 1979. I stretched the fuse a couple of inches is all. I just love ag aircraft and I had a relatively fresh OS to throw at the project.
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Lightweight models that fly well tend to be mutually exclusive with heavy models that can endure crashes.
As for the humungous hole where the fuel tank recesses into the fuselage, you first run your maple 3/8" x 1/2" motor mounts completely back to the intersection with the wing airfoil.
My light Skyray has proven *much* more crashproof than the heavy stock version. The stock wing just disintegrated on the first crash. The light wing had endured at least 30 nose-in hits.
I think your nose construction is generally OK for very smooth-running engines. But unfortunately I have seen several made about like that, both with the tank compartment open on the "bottom"/inboard side, and closed/covered on the bottom, and they all had apparent vibration-related issues. I think losing most of the doubler on even one side really hurts the stiffness. Taking off the second one doesn't make much difference. IF you do this, for any engine that runs anything other than sewing-machine smooth, would be to change to 5/8 or even 3/4" deep motor mounts. Drill holes in them until they weigh the same as 1/2" stock.
Brett
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50 years of tanks on the outboard side of a profile fuselage, and we've been doing it all wrong! Why didn't someone discover this before?
Floyd
It was by a man by the name of Dick Mathis. Look at his Excaliber designs. H^^
There has been others since then also.
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(True, it's been many decades since I flew profiles powered by heavy vibrators..) When the nose turns into a 1" blur, those short basswood motor mount beams and 1/2" Lite Ply doublers that barely touch the wing LE could have something to do with it!
Ultimately, all that really counts is what works. If lightening long motor mounts by cross drilling the aft portion works well, by all means it should done. I find 3/8" aircraft birch plywood quite rigid. And I enjoy the accuracy and convenience of alignment of a single piece plywood engine crutch.
Losing "most of a doubler", even on one side, is assumed to greatly compromise stiffness? Maybe I'm fostering a mistake - I was just considering tank cutout holes on both doublers, to gain a bit more lateral tank adjustment. (Again, my doublers are 3/32" aircraft birch plywood.)
Virtually all profiles I have built (since 1987) had longer nose moments, say ~9" from spinner back to wing LE. By contrast my experience with vibrating engines was on stock built Flite Streaks {in high school during the 1960s, I built four}, running my worn Fox .35 and the ubiquitous 10-6 Top Flite nylon prop - smaller, shorter coupled models. Perhaps I accidentally avoided resonances; combined with my silky smooth OS .35/.40 and ST .51 engines, I had luck.
I do know once I started recessing fuel tanks in the fuselage, engine runs improved. That, combined with uniflo venting, proper stunt fuel, etc. has greatly enhanced my experience with profile ships.
L.
"You've no idea of what a poor opinion I have of myself, and how little I deserve it." - W.S. Gilbert
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Yes, I completely agree,
To be successful with recessed tanks in profiles, is necessary reinforce all airplane nose; use low vibration engine; good balanced propellers and everything else to promote smooth run. Or make as Martin Quartin that located his Ringmaster's tank inboard and didn't make any hole at all.
Jorge
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(True, it's been many decades since I flew profiles powered by heavy vibrators..) When the nose turns into a 1" blur, those short basswood motor mount beams and 1/2" Lite Ply doublers that barely touch the wing LE could have something to do with it!
Ultimately, all that really counts is what works. If lightening long motor mounts by cross drilling the aft portion works well, by all means it should done. I find 3/8" aircraft birch plywood quite rigid. And I enjoy the accuracy and convenience of alignment of a single piece plywood engine crutch.
Losing "most of a doubler", even on one side, is assumed to greatly compromise stiffness? Maybe I'm fostering a mistake - I was just considering tank cutout holes on both doublers, to gain a bit more lateral tank adjustment. (Again, my doublers are 3/32" aircraft birch plywood.)
Once you get rid of the first one, the second is pretty superfluous, thats for sure. I don't know why that was left in the cases I have seen it, it appears to be dead weight, since the only help it provides is what it takes to bend it. When you have both sides, it has to compress one side and pull some strain in tension on the other.
I would suggest exactly the same as you have it, except wider.
Brett
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(Not even CF veil will rescue me?).. perhaps I can dispense with my flimsy doublers altogether.
Save some weight. ;->
If a little is good, then lots is surely lots better.
L.
"Everyone has a fair turn to be as great as he pleases." -Jeremy Collier
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David
I think this was all covered in my article "SIG Banshee Modifications per Jack Sheeks - Mods I made to Mr. Ugly" in the January 2012 issue of Control Line World magazine.
Clancy
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!
Brett
You've been schooled! LL~
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How about the outright heresy of a plastic clunk tank on a profile? Particularly mounted inboard, with the engine in the 3 o'clock position.. ;->
(speaking of things that work pretty well)
L.
"If my kid held a hamburger like they do in TV commercials, I'd slap him blind."
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How about the outright heresy of a plastic clunk tank on a profile?
Like this works pretty well:
Brett
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Whatever works. I'm currently trying to find a decent way to optionally use either a metal or a plastic clunk tank in a full fuselage model.. (I need to solve packaging issues while keeping a smaller cowl.)
The setup shown on the Skyray is similar to what I saw working so well on Grady Widener's profiles in Tucson.
L.
"In comic strips, the person on the left always speaks first." -George Carlin
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Hi.
To put it short & simple, moving fuel tank left can have 2 benefits:
-It makes engine run richer when the model tends to speed up in wind.
-It makes engine run leaner in overhead maneuvres, more speed is good up there.
BUT, if a little is good, a lot more is not necessarely better. You must find the golden middle.
I have ended up in a setup where pick-up tube (inside tank) ends 9-11mm more outside than needle valve CL (side-mounted engine). It seems to work well for me.
Lauri
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Whatever works. I'm currently trying to find a decent way to optionally use either a metal or a plastic clunk tank in a full fuselage model.. (I need to solve packaging issues while keeping a smaller cowl.)
The setup shown on the Skyray is similar to what I saw working so well on Grady Widener's profiles in Tucson
It's a bone-stock 20FP with a bone-stock SS-4 tank, set up with three tubes in the expected places. Can't get any more plain-vanilla than that. You *do* need to put it "wide side to fuselage" as shown; it won't work the other way. For the same reason we started this thread, the fuel pressure change is too much.
It's pretty easy to make a tank compartment that will fit either tank - make the floor 1 5/8" (or whatever the clunk tank needs) from the bearers, and make sure it's 6" long. For the clunk, adjust the uniflow to get even runs, for the conventional tank shim it in the abundant available space.
Brett