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Author Topic: Bellcrank 3" vs 4"  (Read 8065 times)

Offline Dennis Toth

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Bellcrank 3" vs 4"
« on: July 25, 2012, 10:33:33 AM »
Guys,
In my current build project for OTS I am getting to the point on selecting a bellcrank. The ship is non flapped and the 3" bellcrank fits nicely in the wing space. The ship is a 35 size around 32oz. I know for a flapped ship theres lots of geometry going on and forces but it seems that for a simple elevator to bellcrank setup it shouldn't make much difference as long as the bellcrank moment arms are in the same proportion (as long as you get the needed elevator deflection with reasonable bellcrank movement). Am I missing somethink here?

Best,       DennisT


Offline Tim Wescott

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Re: Bellcrank 3" vs 4"
« Reply #1 on: July 25, 2012, 10:42:24 AM »
Getting more line movement for the elevator movement gives the springiest part of your system (the lines) the most leverage.

Folks seem to think that's important.  I'm not that good of a flyer to know, but I can understand the mechanics, and I can regurgitate what I've read.

So if you cram a 4" bellcrank in there, with the same pushrod-hole to pivot-hole spacing as your 3" bellcrank, then you'll have to move the lines 33% more to get the same pushrod (and presumably elevator) movement.  This means you can move your lines out on the handle, and gain more mechanical advantage around your lines, which is good.

That having been said -- there's a lot of 32-ounce class stunters out there flying on 3" bellcranks that seem to do just fine.
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Offline Steve Helmick

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Re: Bellcrank 3" vs 4"
« Reply #2 on: July 25, 2012, 10:51:39 AM »
It's more likely with a 3" BC that you'll end up with narrow handle spacing of 2"-2.5". The wider the handle spacing (3"-4"), the more precise your controls, because slight hand/wrist errors are not so magnified. Yes, you could slow the controls enough to require 12" handle spacing, but I think that would be clunky at best. Like flying a stunt kite with a single handle...

Somebody I know typically uses about a 5" spacing (by my eyeball), which seems about the maximum practical. Maybe he will comment on the why, how, and results?  ??? Steve
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Offline Dennis Toth

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Re: Bellcrank 3" vs 4"
« Reply #3 on: July 25, 2012, 10:59:42 AM »
Tim,
That's the way I see it. On the last OTS ship, a S1 Ringmaster I had a Sig 3". At first I had used the outer most hole on the bellcrank and this gave way to much elevator even with the longest tail horn I could practically get on it. I had an opportunity to get into the bellcrank (read this as crashed) so on the rebuild I move to the inner hole and it was then reasonably flyable but could have gone in a little more and still had plenty of elevator deflection. I have the handle spacing pulled in to about 3 3/4". The new ship is an El Diablo. I think with the bellcrank pushrod hole set at 1/2" it should give plenty of control with a little wider handle spacing.


Best,    DennisT

Offline FLOYD CARTER

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Re: Bellcrank 3" vs 4"
« Reply #4 on: July 25, 2012, 11:44:26 AM »
Every S1 Ringmaster I have ever built (about 5) have ended up with too much elevator movement! The only practical solution is to extend the elevator horn.  Using those Nylon horns, I've had to put  ply spacer under the horn and use longer bolts.

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Offline Howard Rush

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Re: Bellcrank 3" vs 4"
« Reply #5 on: July 25, 2012, 12:32:43 PM »
I have been working on a short monograph on this subject.  It's not finished yet, and I hesitate to put down the conclusions, because some arm-waver will dispute them, then I'll need to duplicate the derivation, which nobody will look at anyhow.  In brief, 1) use the big bellcrank, and 2) nothing you do with handle spacing can overcome too little control leverage at the airplane.
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Offline Jim Thomerson

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Re: Bellcrank 3" vs 4"
« Reply #6 on: July 25, 2012, 05:42:48 PM »
I've been using hand made wooden four inch bellcranks in 1/2A up to 15 size stunt airplanes.  It is a challenge to get a 4 inch bellcrank into a 6 inch chord wing. 

Offline Howard Rush

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Re: Bellcrank 3" vs 4"
« Reply #7 on: July 25, 2012, 10:06:18 PM »
It is a challenge to get a 4 inch bellcrank into a 6 inch chord wing. 

I'll bet it flies well when you do. 
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Online Brett Buck

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Re: Bellcrank 3" vs 4"
« Reply #8 on: July 25, 2012, 10:12:00 PM »
I have been working on a short monograph on this subject.  It's not finished yet, and I hesitate to put down the conclusions, because some arm-waver will dispute them, then I'll need to duplicate the derivation, which nobody will look at anyhow.  In brief, 1) use the big bellcrank, and 2) nothing you do with handle spacing can overcome too little control leverage at the airplane.

   But, Howard, all you need is a bigger handle and then you can apply more torque, like a "cheater bar". Or you need a smaller one, and then you get a better mechanical advantage. Or something like that.

    You and your so-called "mathematical truth" and strange runes. I think it's entirely possible you might be a witch.

   Brett

Offline Douglas Ames

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Re: Bellcrank 3" vs 4"
« Reply #9 on: July 25, 2012, 10:12:38 PM »
Every S1 Ringmaster I have ever built (about 5) have ended up with too much elevator movement! The only practical solution is to extend the elevator horn.  Using those Nylon horns, I've had to put  ply spacer under the horn and use longer bolts.

Floyd
I'm currently throwing together an old Ringmaster kit. The ply horn was garbage, even the geometry was wrong so I made it 90 deg. to the elevator and 3/4" from the hinge. The bellcrank is an old Top Flite 2 1/2"

I was under the impression that 4" bellcranks are for large flapped stunters for more leverage at th BC and handle.
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Offline Dennis Toth

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Re: Bellcrank 3" vs 4"
« Reply #10 on: July 26, 2012, 08:12:02 AM »
Doug,
Don't use the 2 1/2" bellcrank in the Ringmaster (full 35 size). You need at a minimum the 3" and use the inside hole on a Brodak type bellcrank (if you use the Sig 3" drill a new hole below the existing inside hole say 3/16" down). I have one with the Sig 3" and changed from the outer hole to the inner and have a 1" tail horn and it is still to much elevator. I use a handle spacing of pulled down to 3 3/4" it works but I would go with a closer bellcrank hole and wider line spacing if I were doing it over again.

Best,       DennisT

Offline Ward Van Duzer

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Re: Bellcrank 3" vs 4"
« Reply #11 on: July 26, 2012, 08:14:54 AM »
Dennis,

FWIW...

When I worked with Walter to create the LD kit we did everything sneaky (and legal) to slow down the controls and minimize the elevator movement. Particularly on the LD! Then Hunt comes along a while back and blows Red's secret! (Red lived with, and worked in Jim's shop for years. Bob and Red were like bro's.) Red used the classic 3 inch aluminum BC and drilled a new pushrod hole as close to the pivot as he could and still get balanced movement. He was also known to have cut his own BC'c with over sized dimentions.

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Offline Howard Rush

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Re: Bellcrank 3" vs 4"
« Reply #12 on: July 26, 2012, 08:39:25 AM »
I think it's entirely possible you might be a witch.

You're the one who does the pentangles after every pattern. 
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Online Brett Buck

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Re: Bellcrank 3" vs 4"
« Reply #13 on: July 26, 2012, 08:45:20 AM »
You're the one who does the pentangles after every pattern. 

   Pentagrams are intended to WARD OFF evil, not invoke it. It has been less than entirely successful, of course.

    There is actually a reason for it, but I expect that no one will figure it out.

    Brett

Offline Steve Fitton

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Re: Bellcrank 3" vs 4"
« Reply #14 on: July 26, 2012, 08:54:08 AM »
If he weighs as much as a duck....
Steve

Online Brett Buck

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Re: Bellcrank 3" vs 4"
« Reply #15 on: July 26, 2012, 09:56:40 AM »
If he weighs as much as a duck....

   Ah!  A nerd test, somewhere between INT and ADV skill class. To win the Chapman Cup, you will have to do a lot better - like a scene-for-scene reenactment of The Crimson Permanent Assurance from memory. Promising start, however.

    Brett

Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #16 on: July 26, 2012, 10:14:12 AM »
Getting more line movement for the elevator movement gives the springiest part of your system (the lines) the most leverage.

Folks seem to think that's important.  I'm not that good of a flyer to know, but I can understand the mechanics, and I can regurgitate what I've read.

So if you cram a 4" bellcrank in there, with the same pushrod-hole to pivot-hole spacing as your 3" bellcrank, then you'll have to move the lines 33% more to get the same pushrod (and presumably elevator) movement.  This means you can move your lines out on the handle, and gain more mechanical advantage around your lines, which is good.

That having been said -- there's a lot of 32-ounce class stunters out there flying on 3" bellcranks that seem to do just fine.

ok, here is another outlook Tim,, If you have the same lines,,same airplane same weight,, you can assume the same stretch with the lines right?
so given that, if you move the lines 3" for max control deflection, versus 1 inch for max control deflection,, given the same stretch,, which will feel more connected??

ok so math to understand better,,

lets assume 1/16 of line stretch just for grins,,
so with a setup using 3" or travel , to derive the percentage of stretch,, .0625/3= 2.08 percent of line movement is stretch
with a setup using 1.5 inches of line movement for full deflection
you would be looking at .0625/1.5= 4.16 percent of the line travel,,
I think while you may not be able to feel the difference and recognize it as line stretch,, you would be able to tell that it is not manuevering as crisply,,

my two cents worth,,
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Offline Steve Fitton

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Re: Bellcrank 3" vs 4"
« Reply #17 on: July 26, 2012, 10:40:05 AM »
   Ah!  A nerd test, somewhere between INT and ADV skill class. To win the Chapman Cup, you will have to do a lot better - like a scene-for-scene reenactment of The Crimson Permanent Assurance from memory. Promising start, however.

    Brett

Here I was, just thinking about a simple test to determine if Howard is a witch, and you go and bring up a whole new skill class to complicate things!

I'm just a lowly intermediate it would seem, since its been since school that I watched Meaning of Life and I forgot what the pirate scene was called  :(
Steve

Offline Howard Rush

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Re: Bellcrank 3" vs 4"
« Reply #18 on: July 26, 2012, 10:43:32 AM »
ok, here is another outlook Tim,, If you have the same lines,,same airplane same weight,, you can assume the same stretch with the lines right?
so given that, if you move the lines 3" for max control deflection, versus 1 inch for max control deflection,, given the same stretch,, which will feel more connected??

ok so math to understand better,,

lets assume 1/16 of line stretch just for grins,,
so with a setup using 3" or travel , to derive the percentage of stretch,, .0625/3= 2.08 percent of line movement is stretch
with a setup using 1.5 inches of line movement for full deflection
you would be looking at .0625/1.5= 4.16 percent of the line travel,,
I think while you may not be able to feel the difference and recognize it as line stretch,, you would be able to tell that it is not manuevering as crisply,,

my two cents worth,,

Great explanation. 
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Offline Howard Rush

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Re: Bellcrank 3" vs 4"
« Reply #19 on: July 26, 2012, 10:44:43 AM »
Here I was, just thinking about a simple test to determine if Howard is a witch, and you go and bring up a whole new skill class to complicate things!

I'm just a lowly intermediate it would seem, since its been since school that I watched Meaning of Life and I forgot what the pirate scene was called  :(

Look, if I watch all those movies and all that TV to keep up with you guys, I'll never get anything built.  I don't even know from the Matrix.
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Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #20 on: July 26, 2012, 10:46:22 AM »
Look, if I watch all those movies and all that TV to keep up with you guys, I'll never get anything built.  I don't even know from the Matrix.

Howard,, I was wondering how you got your new plane built so fast,, and apparantly it looks as nice as the last one too,,I mean, the last one took what,, multiple years?


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Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #21 on: July 26, 2012, 10:49:21 AM »
Great explanation. 
whew,, I was waiting to get shot down cause I didnt think of some intrinsic nuance,,  ;D

I kind of went around on this with Pat,, he thinks pretty simply that you just keep narrowing the handle spacing to get the control response,, and to a point its valid,, but it does change the feel,, so I have been trying to establish a baseline controls ratio in the airplane that will allow me to get my spacing where I want it,, ( mostly cause I dont build normal airplanes,, I always have to change something,,  sick puppy that I am,, )

thats ok, the next one will be a mainstream, real life, copyed, from someone else,, design,,
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Offline Tim Wescott

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Re: Bellcrank 3" vs 4"
« Reply #22 on: July 26, 2012, 11:16:08 AM »
(Snip -- TaW)

I think while you may not be able to feel the difference and recognize it as line stretch,, you would be able to tell that it is not manuevering as crisply,,

my two cents worth,,

Yup.  The only thing that I can add is something that's come up in these forums: the line stretch isn't just the amount of stretch that you get in the cable itself in similar circumstances.  The line is effectively more springy because of drag -- it's bowed, and the more you pull on it the less it bows, and the less you pull on it the more it bows.  That all adds up to a much more compliant system than just the cables being pulled on the ground.
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Offline Tim Wescott

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Re: Bellcrank 3" vs 4"
« Reply #23 on: July 26, 2012, 11:18:10 AM »
thats ok, the next one will be a mainstream, real life, copyed, from someone else,, design,,

Yup.  That's my plan for my next one, too.  In fact, to make sure that it's really normal, I'm going to be copying from two or three "someone else's" designs.
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Offline Tim Wescott

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Re: Bellcrank 3" vs 4"
« Reply #24 on: July 26, 2012, 11:21:56 AM »
I have to note that one thing I'm taking as guidance on this thread and on past related threads is that while folks often recommend using a bigger bellcrank than what a kit or plan called out, I don't see anyone saying "oh, no, it'll work better with a smaller bellcrank".  Even the response to a 4" bellcrank in a 1/2-A plane isn't "that's dumb", it's "Wow.  Hard to fit".

Granted, going along with the consensus often means that you're a sheep happily running over a cliff with all the other sheep.  But often it means that you're a sheep happily running to greener pastures with all the other sheep.  I think the latter case holds here.
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Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #25 on: July 26, 2012, 11:34:57 AM »
Yup.  The only thing that I can add is something that's come up in these forums: the line stretch isn't just the amount of stretch that you get in the cable itself in similar circumstances.  The line is effectively more springy because of drag -- it's bowed, and the more you pull on it the less it bows, and the less you pull on it the more it bows.  That all adds up to a much more compliant system than just the cables being pulled on the ground.
That is true enough,, however I elected to ignore that point ,, my thinking being that the "bow" in the lines,, ( I cannot recall what its called, its not parabolic,, it has another name) would be fairly consistant with both systems,, but it certainly is an addative,, note also that thicker lines will have MORE arc given other factors remain constant,( weight, speed, line length,,), therefore have more compliance,,
Now,, where is the break point,, thinner lines have less drag,, so have less arc in them, but they stretch more than thicker lines given constants remain, well,,, constant,, so at some point, the reduced drag arc compliance,, and the stretch compliance should converge,, At MY piloting level, I have felt,, ( or beleived I felt) that the thicker lines feel more connected on MY airplanes,,, so that is where I go,, and now that I have moved up to larger airframes,, ( read heavier) I happily no longer have to be concerned with that decision,,
I should state that a lot of my thought process on line size was motivated by Dirt talking about using .012 lines versus .014 on his flight streaks and other smaller airplanes,, and the stretch that came from smaller lines,,
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Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #26 on: July 26, 2012, 11:35:48 AM »
Yup.  That's my plan for my next one, too.  In fact, to make sure that it's really normal, I'm going to be copying from two or three "someone else's" designs.

Oh lord,, Tim,, Tim tim,, thats not quite what I meant dude,, copy ONE design per airplane,, sigh,,
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Offline john e. holliday

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Re: Bellcrank 3" vs 4"
« Reply #27 on: July 26, 2012, 11:45:00 AM »
I have stated this before, "Try it and see if it works for you.  If it does fine.  If not file it away and it may work on the next model".   I am going back to what the plans call for on the planes.  I figure the designer knows more than I do.   H^^
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Offline Steve Helmick

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Re: Bellcrank 3" vs 4"
« Reply #28 on: July 26, 2012, 12:00:07 PM »
Could one of you CAD guys just draw out the relationship, handle to elevator? And how about making one of those cool calculator things that starts with the desired elevator deflection and ending up with line spacing at the handle? Or a spreadsheet? IMO, 15 degrees is more than enough at the elevator.

Every time I hook up the lines to the Eagle 7 and wiggle the handle, I'm shocked with how little elevator deflection there is, and how hard a corner it can make. I'm considering reducing the handle spacing more...probably will if I have to fly it in any real heat or at altitudes over 500'-1,000'. I'm also thinking about adding a little nose weight, tho it shows none of the usual indicators (glide's good).

This topic started out as a Ringmaster subject, so let's look at that...the one thing you absolutely do NOT want on a Ringmaster is too much elevator movement. The wing can be too easily overloaded, with no flaps to increase lift. Ergo, use the 4" BC...unless you can cram a 5" BC in there.   H^^ Steve
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Offline Tim Wescott

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Re: Bellcrank 3" vs 4"
« Reply #29 on: July 26, 2012, 12:19:18 PM »
This topic started out as a Ringmaster subject, so let's look at that...the one thing you absolutely do NOT want on a Ringmaster is too much elevator movement. The wing can be too easily overloaded, with no flaps to increase lift. Ergo, use the 4" BC...unless you can cram a 5" BC in there.   H^^ Steve

And my own recent experience with my first S-1 in ages is that I got it wrong, with my lines way too close together at the handle and the thing is still (as far as I can tell) stalling in the corners, yet is showing signs of too much springiness in other places.

Argh.  Sigh.  Etc.  I just do not want to tear into the wing of that thing.  Maybe I'll extend the elevator horn and put a wheel on the end of it.  Or put an idler right behind the wing to reduce the elevator pushrod motion by a factor of 2 from what comes out of the wing.  Or something.
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Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #30 on: July 26, 2012, 12:24:12 PM »
the mighty RM is a flapless airplane,so there is literally no excuse to NOT fix the problem,, since options exist to avoid cutting into the wing,, but trust me Tim, yours is not the first to have this,, my Gee Bee flew the first year with like 2.25 spacing at the handle,, until one fatefull crash opened up the wing centersection sheeting for me, and i moved the attachment on the bellcrank,,

just lenghten the elevator horn,, some is better than none,, put a skid on the tail,, and pretend you meant it that way,, or put a matching skid on top of the nose to protect the needle valve, then you have continuity of design,, y1 y1
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Offline Tim Wescott

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Re: Bellcrank 3" vs 4"
« Reply #31 on: July 26, 2012, 12:40:24 PM »
Could one of you CAD guys just draw out the relationship, handle to elevator? And how about making one of those cool calculator things that starts with the desired elevator deflection and ending up with line spacing at the handle? Or a spreadsheet? IMO, 15 degrees is more than enough at the elevator.

Well, here's the "linearized" math -- that means that it ignores the "anti-exponential" effect of bellcrank rotation and the "pro-exponential" effect of elevator rotation, and the fact that the motion is not all on one geometric plane, etc.

I can't post a spreadsheet -- the system won't accept it.  I tried.

Inputs
  • h is the handle spacing
  • b is the bellcrank spacing
  • c is the distance from bellcrank center to control rod
  • f is the distance from the flap horn hole to the hinge
  • fe is the distance from the flap horn elevator hole to the hinge
  • e is the distance from the elevator horn hole to the hinge
  • rh is the handle rotation

outputs:

  • handle to bellcrank ratio (degrees/degrees) = hb = h/b
  • bellcrank to flap ratio (degrees/degrees) = bf = c/f
  • handle to flap ratio (degrees/degrees) = hf = bf * hb = (h * c)/(b * f)
  • flap to elevator ratio (degrees/degrees) = rfe = fe/e
  • handle to elevator ratio (degrees/degrees) he = rfe * hf = (h * c * fe)/(b * f * e)

When you want to go take all the angles and whatnot into account, then it gets complicated -- which is why Howard keeps talking about posting results, but hasn't gotten around to working through the math yet.  I suspect that the above is good enough to get you into the ballpark, though.
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Offline Tim Wescott

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Re: Bellcrank 3" vs 4"
« Reply #32 on: July 26, 2012, 12:44:49 PM »
the mighty RM is a flapless airplane,so there is literally no excuse to NOT fix the problem,, since options exist to avoid cutting into the wing,, but trust me Tim, yours is not the first to have this,, my Gee Bee flew the first year with like 2.25 spacing at the handle,, until one fatefull crash opened up the wing centersection sheeting for me, and i moved the attachment on the bellcrank,,

just lenghten the elevator horn,, some is better than none,, put a skid on the tail,, and pretend you meant it that way,, or put a matching skid on top of the nose to protect the needle valve, then you have continuity of design,, y1 y1

It already has a long elevator horn, any more length and it'll be hitting the ground.  At least, with the tail skid that I have.

But even though I made the tail skid removable so I could put on a longer one if the spirit so moved me, it didn't occur to me.  It's nice that you're standing by with roll of duct tape and a raised eyebrow to keep me on the path of expediency.  I hadn't planned on flying it in OTS anyway, so I can change the LG without too many tears.

I'll try bolting on an elevator horn extension and a longer tail LG, and see if it works.  Thanks.
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Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #33 on: July 26, 2012, 12:45:49 PM »
maybe,, ballpark, but it ignores the nuances,, which can in fact add up,,
do they matter in reality, ,for Paul, Howard, and Norm,, probably so,, for you and me,, maybe,,

I think the solution is to learn Inventor,, or some other parametric modeler,, and to create the control system in 3D,,
It is my intent to do that in CAD,, however, pinning the axis of rotation on elements and having them constrained to only rotate, not move, is cumbersome,, but worth it for the knowledge,, I just want the time to do it,,

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Offline Steve Fitton

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Re: Bellcrank 3" vs 4"
« Reply #34 on: July 26, 2012, 01:00:24 PM »
And my own recent experience with my first S-1 in ages is that I got it wrong, with my lines way too close together at the handle and the thing is still (as far as I can tell) stalling in the corners, yet is showing signs of too much springiness in other places.

Argh.  Sigh.  Etc.  I just do not want to tear into the wing of that thing.  Maybe I'll extend the elevator horn and put a wheel on the end of it.  Or put an idler right behind the wing to reduce the elevator pushrod motion by a factor of 2 from what comes out of the wing.  Or something.

Why not just build a Skyray per the instructions Brett Buck wrote at great length about, and go fly?
Steve

Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #35 on: July 26, 2012, 01:04:02 PM »
Why not just build a Skyray per the instructions Brett Buck wrote at great length about, and go fly?
Steve, with all due respect,, sometimes you have an airplane that is built,, and the process of sorting it out is educational and pays dividends in the future,, not to mention it is a valid challenge all on its own,,

not to mention that even building a skyray stock,, takes time,, and its flying season here,, ( well for those who even have time for flying LOL)
we are getting Tim headed the right way, but we need to let him exercise his "engineer" too, or he will get bored,, LOL
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Offline Tim Wescott

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Re: Bellcrank 3" vs 4"
« Reply #36 on: July 26, 2012, 01:27:05 PM »
Why not just build a Skyray per the instructions Brett Buck wrote at great length about, and go fly?

Because I can't fly one of those in the Ringmaster Fly-a-Thon in October.

Which is what this plane is specifically built for -- getting it tuned up so that it can at least stagger through the pattern is a plus, but isn't actually necessary to the fly-a-thon.  In fact, for that event a 1/2 ounce tank and a good pit crew is probably the best bet.
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Offline Steve Helmick

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Re: Bellcrank 3" vs 4"
« Reply #37 on: July 26, 2012, 01:34:47 PM »
Randy proved that it only has to look like a Ringmaster (kinda) to post flights for the Ring-a-Thon. Only problem with it is that his .40VF won't start. I hope he gets that fingered out!  y1 Steve
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In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

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Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #38 on: July 26, 2012, 01:36:43 PM »
Because I can't fly one of those in the Ringmaster Fly-a-Thon in October.

Which is what this plane is specifically built for -- getting it tuned up so that it can at least stagger through the pattern is a plus, but isn't actually necessary to the fly-a-thon.  In fact, for that event a 1/2 ounce tank and a good pit crew is probably the best bet.

I could also bring along my 12 gauge "in flight throttle shut off " tool as well,, though restarts may be a challange,, VD~ S?P
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Offline Mark Scarborough

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Re: Bellcrank 3" vs 4"
« Reply #39 on: July 26, 2012, 01:37:47 PM »
Randy proved that it only has to look like a Ringmaster (kinda) to post flights for the Ring-a-Thon. Only problem with it is that his .40VF won't start. I hope he gets that fingered out!  y1 Steve

figures,, Randy with the most reliable stunt engine,, ( next to electric) ,, LOL

well he will figure something out,, hopefully before the next contest,, which I may even be able to attend,,,,
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Re: Bellcrank 3" vs 4"
« Reply #40 on: July 26, 2012, 02:36:15 PM »
Here I was, just thinking about a simple test to determine if Howard is a witch, and you go and bring up a whole new skill class to complicate things!

I'm just a lowly intermediate it would seem, since its been since school that I watched Meaning of Life and I forgot what the pirate scene was called  :(

 You're not the Messiah, you're a very naughty boy!

   Brett

   

Offline Ted Fancher

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Re: Bellcrank 3" vs 4"
« Reply #41 on: July 26, 2012, 03:03:21 PM »
I have been working on a short monograph on this subject.  It's not finished yet, and I hesitate to put down the conclusions, because some arm-waver will dispute them, then I'll need to duplicate the derivation, which nobody will look at anyhow.  In brief, 1) use the big bellcrank, and 2) nothing you do with handle spacing can overcome too little control leverage at the airplane.

Howard,

IIRC a well known several times National Champion championed the use of handles with as much as six inch spacing in order to apply the necessary input to a three inch bellcrank he preferred in his quite large aircraft which utilized big flaps in order to develop the lift necessary to fly them. Hmmmmm.

Ted

Offline Trostle

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Re: Bellcrank 3" vs 4"
« Reply #42 on: July 26, 2012, 03:22:21 PM »
Howard,

IIRC a well known several times National Champion championed the use of handles with as much as six inch spacing in order to apply the necessary input to a three inch bellcrank he preferred in his quite large aircraft which utilized big flaps in order to develop the lift necessary to fly them. Hmmmmm.

Ted

And I think that well know several times National Champion with the extra wide handle (he put extensions on his large EZ-Just handles for extra width) also trimmed his models which some would find more than a bit nose heavy.  I cannot fault success, it worked for him.

Keith

Offline Howard Rush

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Re: Bellcrank 3" vs 4"
« Reply #43 on: July 26, 2012, 03:31:06 PM »
To expand #2 above, nothing you can do with handle spacing can compensate for the varying fraction (per Mark's post 16 above) of line travel that's due to line elasticity and drag.  I'll write this up after cessation of flying weather at home.  
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Offline Howard Rush

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Re: Bellcrank 3" vs 4"
« Reply #44 on: July 26, 2012, 03:34:45 PM »
Well, here's the "linearized" math...

I offered everybody (and sent you) a free spreadsheet that has the whole 3D geometry from leadouts to elevator. 
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Offline Ted Fancher

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Re: Bellcrank 3" vs 4"
« Reply #45 on: July 26, 2012, 03:50:01 PM »
And I think that well know several times National Champion with the extra wide handle (he put extensions on his large EZ-Just handles for extra width) also trimmed his models which some would find more than a bit nose heavy.  I cannot fault success, it worked for him.

Keith

Hi Keith,

Success has little to do with the physics involved in the use of handle spacing twice that of the bellcrank spacing.

Nose heaviness has little to do with the ability to deflect the controls.  Full deflection will result in the same rate of body angle change no matter how big or small the handle is.  What does change is the amount of force at the handle is necessary to overcome the load due to deflecting the control surfaces.

 The rotation at which full deflection will be achieved will be something like half that if the handle spacing were three inches on a three inch crank.  The Netzeband wall comes to mind.  I like to think of extremes to get a feel for what will happen.  Think of flying a modern big stunter with a three inch handle spacing and a one and a half inch bellcrank...(a 3/4" arm trying to move those big flaps and elevators on a ship going nearly 60MPH!)  tough to do!

Ted

Offline billbyles

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Re: Bellcrank 3" vs 4"
« Reply #46 on: July 26, 2012, 04:50:55 PM »
That is true enough,, however I elected to ignore that point ,, my thinking being that the "bow" in the lines,, ( I cannot recall what its called, its not parabolic,, it has another name) would be fairly consistant with both systems,, but it certainly is an addative,, note also that thicker lines will have MORE arc given other factors remain constant,( weight, speed, line length,,), therefore have more compliance,,
Now,, where is the break point,, thinner lines have less drag,, so have less arc in them, but they stretch more than thicker lines given constants remain, well,,, constant,, so at some point, the reduced drag arc compliance,, and the stretch compliance should converge,, At MY piloting level, I have felt,, ( or beleived I felt) that the thicker lines feel more connected on MY airplanes,,, so that is where I go,, and now that I have moved up to larger airframes,, ( read heavier) I happily no longer have to be concerned with that decision,,
I should state that a lot of my thought process on line size was motivated by Dirt talking about using .012 lines versus .014 on his flight streaks and other smaller airplanes,, and the stretch that came from smaller lines,,

"( I cannot recall what its called, its not parabolic,, it has another name)"
How about catenary curve...
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Offline Bill Little

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Re: Bellcrank 3" vs 4"
« Reply #47 on: July 26, 2012, 05:14:04 PM »
OK, so who's going to be the first with a 5" or 6" bell crank?  Has it been done?

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Re: Bellcrank 3" vs 4"
« Reply #48 on: July 26, 2012, 05:18:42 PM »
OK, so who's going to be the first with a 5" or 6" bell crank?  Has it been done?

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Yes it has been done, also a 4 1/2 inch BC  I have several here

Randy

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Re: Bellcrank 3" vs 4"
« Reply #49 on: July 26, 2012, 05:23:13 PM »
OK, so who's going to be the first with a 5" or 6" bell crank?  Has it been done?


  Yes!   At some point the drag on the leadout guide from the larger convergence angle will start outweighing the benefits. I think all the more recent bellcranks I have made are 4.5 or 4.75" because that's the biggest one that will fit and still let me put the pivot on the spar without a lot of extra wood.

    Brett


Tags: controls  bellcrank