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Author Topic: Belcrank size  (Read 2307 times)

Offline Shorts,David

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Belcrank size
« on: October 21, 2019, 10:22:31 PM »
Hi, I didn't see any answer to this question though I'm sure it's been brought up before.
What size plane should have a 3 inch vs 4 inch belcrank. Also, why doesn't anyone make a five inch belcrank if bigger is better.
My daughters plane is 44" wingspan, but thick root, about 600 sq inches. It's a profile p-51.
David


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Offline Dave Hull

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Re: Belcrank size
« Reply #1 on: October 22, 2019, 12:26:09 AM »
Like most engineering, picking a bellcrank size is a compromise. But to make an attempt to itemize some requirements, choices and consequences:

1. It has to be strong enough to carry worst case flight loads from the lines plus a factor of safety. Since flight loads are approximated by the aircraft speed and weight, it is easy to calculate these loads. It means that the crank has to survive this load; that the support bearing has to survive and not bind up; that the bolt or support rod (axle) has to survive this, too. And the airframe has to be strong enough locally to react and distribute the load. To bound this, the rules have added a safety factor that comes in the form of a pull test. Note that the above is valid whether you are talking about a 2, 3 or 4 or even larger bellcrank.

2. The larger bellcrank (assuming equal strength of the different sizes) can be used to change the control ratios, allowing you to use a larger handle spacing and keep the ratio closer to 1:1.

3. A second important reason is that the actual control surface deflection loads on the components are reduced. This is most easily described as having more leverage. There is an upper limit that can occur with simple control surfaces, high speeds where even with all of the centrifugal flight loads applied thru one line, you still may not be able to overcome the aero loads. Unlikely youll experience this, but imagine the situation would be more likely with a light airplane with huge control surfaces. The aero forces are what they are, but if you have more leverage in your linkage, the loads in the components are reduced. If all you use on a larger bellcrank is the wider line spacing and not a larger pushrod radius, then what you have gained is less torque on your wrist for a given deflection. If you also use a larger radius to the pushrod, and you match this with your flap horn and also on the elevator, then the pushrods see less load. Less load, less friction, less wear.

4. So why not put huge bellcranks into every plane? First, they would be hard to fit in most airplanes. You have to cut away a lot more structure to get them in, and provide clearance for the increased line translation. So ridiculously big is Ridiculous. (Porterism.) If you merely have a large bellcrank, you are going to want large handle spacing to match. Just the opposite problem has plagued 1/2A promoters forever. A guy badmouths a known good 1/2A stunter as just too squirrely to fly. So you volunteer to check it out. Turns out he is flying an 11 oz. airplane with a 2 bellcrank and using a standard E-Z Just handle (huge spacing) and to top it off it has kinks in the cable. But everyone knows 1/2As are squirrelly. Not really. Just some guys that mismatch their control systems.

5.The larger bellcrank (and perhaps larger, beefier attachment hardware) is going to weigh more. Why save grams of balsa weight and then put in a larger bellcrank than you need? On the other hand, if you custom make a bellcrank, you can make it to match the weight of your plane, not some generic PAMPA Sled that weighs 65+ oz. to meet the strength requirement listed in item 1 above. Buying a super-duty crank just so you can get 4 spacing isnt efficient.

6. Putting a huge bellcrank into a wing with short semi-span can cause a significant bend angle in the leadouts if you try to place them close together. On the other hand, placing the guides far apart may not work well in your wing if the tip is small. Widely spaced leadout exits have detractors that describe yaw disturbances introduced as control loads are introduced. Problems would be exaggerated if using solid leadouts.

7. If you use larger throws on the bellcrank and horns, then the running clearances between the pushrods and these components wont cause as much control surface slop as if you had a small bellcrank radius and horn radii.

From the limited information given for your particular application, it would seem you should be looking at either a 3 or 4 bellcrank. If the 4 fits without cutting important structure Id do that. It sounds like we are talking about an R/C 3-D conversion plane, so no need to cut thru an I-Beam spar or thru wing-mounts for landing gear.

This is a generalized discussion, but might help you decide.

Dave

Offline Ken Culbertson

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Re: Belcrank size
« Reply #2 on: October 22, 2019, 08:04:01 AM »
My experience has been to use the largest one that will fit.  I have been at this for 60 years and I still have planes with 3" bellcranks that I have no trouble flying.  I accidentally put one into a 70oz PA ship recently and once I got the handle spacing correct I can't the the difference.  "Hitting the Wall" is more of an issue with the older more nose heavy designs of the 70's.  Today's ships have a more aft CG and require far less control movement to make decent corners.

The 4" makes it easier to get a more aft leadout position without cutting the center out of as many ribs.  Also keep in mind that the distance to the pushrod connection and the position of the flap/elevator holes has just as much to do with the control response as does the length of the bellcrank arms.

Ken
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Offline Tim Wescott

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Re: Belcrank size
« Reply #3 on: October 22, 2019, 10:10:04 AM »
My daughters plane is 44" wingspan, but thick root, about 600 sq inches. It's a profile p-51.

That's an aspect ratio of 3 1/4, which is way too small.  You've either made an error in your calculations, or you have much bigger problems than bellcrank size.

But -- as big as will reasonably fit.  I've been putting 4" bellcranks into my 700 sq-in stunters, but if I get time to build another one before I die it'll have a bigger bellcrank.
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Offline Ken Culbertson

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Re: Belcrank size
« Reply #4 on: October 22, 2019, 11:40:15 AM »

My daughters plane is 44" wingspan, but thick root, about 600 sq inches. It's a profile p-51.
David
Did you by any chance mean "Chord" instead of "Root"  44" span and 600 is somewhere around 13 1/2" assuming no taper.  That is about the same root chord of the average PA ship and would most likely sprout a 4" crank.  If it is strength that worry's you, a 3" is actually stronger than a 4" if made the same.  It is all about making it easier on you at the handle.  I am one of those weird people that actually likes being able to feel the pressure on a corner.

Ken
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Offline Shorts,David

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Re: Belcrank size
« Reply #5 on: October 22, 2019, 01:54:37 PM »
Yes, it is an r/c 3d conversion given to my daughter after the last raffle. I am currious about the ultra low aspect ratio and performance, however in my daughter's hands performance will not be a factor for some time.
There is room for any belcrank ever created. Since she has small hands and will use a small handle, it sounds like i'll be okay using a 3" belcrank.
Thanks for the explanation.
David

Offline Ken Culbertson

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Re: Belcrank size
« Reply #6 on: October 22, 2019, 02:28:24 PM »
Yes, it is an r/c 3d conversion given to my daughter after the last raffle. I am currious about the ultra low aspect ratio and performance, however in my daughter's hands performance will not be a factor for some time.
There is room for any belcrank ever created. Since she has small hands and will use a small handle, it sounds like i'll be okay using a 3" belcrank.
Thanks for the explanation.
David
Wish her luck.  My oldest daughter was almost hooked on sailplanes (which is my lesser passion) when she was in her early teens until her "friends" hit her with the "ewe - boy stuff" and she quit.  That is not the stigma it used to be.  Hope she stays with it.

Ken

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Offline Dave Hull

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Re: Belcrank size
« Reply #7 on: October 22, 2019, 08:51:51 PM »
Just a follow-up comment, now that you have confirmed the 3-D R/C nature of this endeavor:

A fellow club member had a fat wing, very low aspect ratio 3-D R/C profile plane. He put a Chinese 60 on it. A big hunk of engine. With big muffler. R/C ported. No way that's going to work. Wrong. He flew it ridiculously slow, and the high thrust (even running slow) overcame the very high induced drag in the crazy sharp corners. He could send out for a  burger during the first pass on the reverse wingover and eat it on the second pass. It was super entertaining to watch. Our "professional judges" kept pointing out that when you fly that slow, they can count each tiny bobble, so it was never going to score well, just because. Regardless, it was his favorite plane....

Hope yours turns out to be a favorite, too.

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Offline Motorman

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Re: Belcrank size
« Reply #8 on: October 23, 2019, 09:37:53 AM »
I've got a 3" in my 1/2a pathfinder.
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Offline Paul Smith

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Re: Belcrank size
« Reply #9 on: October 23, 2019, 04:41:33 PM »
Hi, I didn't see any answer to this question though I'm sure it's been brought up before.
What size plane should have a 3 inch vs 4 inch belcrank. Also, why doesn't anyone make a five inch belcrank if bigger is better.
My daughters plane is 44" wingspan, but thick root, about 600 sq inches. It's a profile p-51.
David

I've flown for over 60 years with 2" and 3" bellcranks.  They work just fine.  There might be some advantage to bigger bellcranks, but any advantage will be discounted by more weight and the need for more clearance room inside the model.  I've gone as small as 1" and 1.5" on speed and racing models. 
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Offline Brett Buck

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Re: Belcrank size
« Reply #10 on: October 23, 2019, 05:16:23 PM »
Hi, I didn't see any answer to this question though I'm sure it's been brought up before.
What size plane should have a 3 inch vs 4 inch belcrank. Also, why doesn't anyone make a five inch belcrank if bigger is better.
My daughters plane is 44" wingspan, but thick root, about 600 sq inches. It's a profile p-51.
David

   Since this is a converted RC 3D airplane, it's a particularly good case for a larger bellcrank. These airplanes have huge control surfaces, so lots of hinge moment, and you need to overcome that with more available torque. 
 
    I am sure it could be made to work with a lot of different control configurations but there's no real downside to using a 4" or larger crank.

     Brett

Offline Howard Rush

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Re: Belcrank size
« Reply #11 on: October 26, 2019, 03:13:39 PM »
Hi, I didn't see any answer to this question though I'm sure it's been brought up before.
What size plane should have a 3 inch vs 4 inch belcrank. Also, why doesn't anyone make a five inch belcrank if bigger is better.

Good questions.  Brett's answer above emphasizes what I think is the most important consideration.  You are trying to deflect the control surfaces accurately, but with a weird, nonlinear spring between you and the airplane.  Here's a discussion: https://stunthanger.com/smf/open-forum/question-on-the-netzeband-wall/ .  The greater the ratio of leadout movement to control surface movement, the less effect the spring will have, hence the more accurately you can fly the airplane.  The bigger the bellcrank and the greater the angle it travels, the greater that ratio is.  There is a point of diminishing return.  When I can calculate or measure control surface hinge moments, I can determine where the point of diminishing return is, and I'll be able to answer the first question. 

I'll also be able to answer the question about why nobody makes a 5" bellcrank.  The answer is either: a) you don't need it, or b) you need it, but nobody realizes it.  I suspect it's the latter.

My Impact has a 4" bellcrank that moves +/- 80 degrees and has an Igor mechanism that increases the ratio of leadout movement to flap movement even more at large flap deflections.  This gives a handle spacing of about 5.6".  It flies OK, but I might put a 5" bellcrank in the next one. 
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Offline Shorts,David

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Re: Belcrank size
« Reply #12 on: October 27, 2019, 10:51:28 AM »
Thanks again to everybody. So many of these seemingly trivial details have so much complex theory behind them that I am slowly struggling to grasp. I suppose this is the real "Trivial Pursuit." I'm also glad as (I think) Keith Trostle put it, "The guy who wiggles the handle best usually ends up on top." Of course that statement has limits, but while I work on my theory, I'll continue working on my wiggling.

David

Offline Brett Buck

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Re: Belcrank size
« Reply #13 on: October 28, 2019, 10:22:18 PM »
Thanks again to everybody. So many of these seemingly trivial details have so much complex theory behind them

  The larger bellcrank isn't that complex. It's like a "cheater bar" on a socket wrench, you only have so much force, so to get more torque, you make the cheater bar longer.

     Brett

Offline Shorts,David

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Re: Belcrank size
« Reply #14 on: October 29, 2019, 08:53:07 PM »
I've got the four inch in my under-construction Junar. Maybe i'll try something even bigger for the plane I have planned down the road.

Offline Tim Wescott

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Re: Belcrank size
« Reply #15 on: October 30, 2019, 09:56:57 AM »
I've got the four inch in my under-construction Junar. Maybe i'll try something even bigger for the plane I have planned down the road.

Just keep in mind that it's really easy to get fixated on one aspect of a design and to lose sight of everything else.  Unless you're intentionally experimenting, I'd recommend that to the best extent possible you look over the shoulders of the guys who are winning the top contests or their closest hangers-on, and go with that.

The only exception to the above advise is where they're doing some super-duper modification (like the active timers, or Igor's logarithmic flap linkage) that are difficult to make work right and are only benefit if you're That Good.
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Offline Air Ministry .

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Re: Belcrank size
« Reply #16 on: November 01, 2019, 05:41:30 PM »
Quote
What size plane should have a 3 inch vs 4 inch belcrank. Also, why doesn't anyone make a five inch belcrank if bigger is better.

Low & behold , a FIVE INCH Bellcrank .



« Last Edit: November 02, 2019, 09:44:18 PM by Air Ministry . »


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