Author Topic: Interesting Bellcrank (Read 18920 times)

Dennis Adamisin · « **Reply #50 on:** February 13, 2012, 08:13:34 PM »

Recently got a couple of Thomas Wilks CD's with scans of many of the old articles. I think it is a hoot to see how many times the designers write that THIS (their) design is the SILVER BULLET! I do not think that they are insincere, just enthusiastic. Over the years new people & components come and go. Meanwhile if you circle back, build one of the classics with good power you are still competitive at any contest in the world. Witness that the current World Champ used and engine that has been out of production for... 20 years?

I am currently building my 1969 Hawker Typhoon. I remember it as a GREAT airplane, wonder if the new one will affirm my memories of it? Kinda like when Billy Werwage built his newish Vulcan as a ways of going back in time to see who far(?) he had come.

I first saw Fred Bachl's drawings of his Expo-Crank back in the early 1960's, after all these years I will finally get to fly it and see if/how I like it - just cuz I want to know. I also want to try a circular bellcrank some day for the same reason.

Larry Cunningham · « **Reply #51 on:** February 13, 2012, 08:47:21 PM »

In spite of a normal human tendency to believe we have discovered the Holy Grail of features for our stunter, it almost certainly isn't. However, if you really look at the progression of construction, finishing and power techniques and features, you can see major improvements. Like nearly all real things,progress occurs in small incremental steps, and the summation of features is really what makes a prime competitive model airplane possible.

I remember the first time I flew a decent flapped stunter with a quality engine. I remember how smooth and sweet it felt; it was so refined over Fox combat engined Flight Streaks I had built and attempted to learn {outside loops!} on in high school. (The difference was astounding, but I really wish I had back one of my old Streaks for nostalgic reasons - I understand why we have such groups as the Brotherhood of the Ring!)

Witness the cumulative effect of minor improvements on any modern CL stunt ship! The simple fact we want to know about things, even less important or obscure things, is what keeps CL stunt such a rich and interesting hobby for me.

L.

"The greatest homage to truth is to use it." -Ralph Waldo Emerson

Dennis Adamisin · « **Reply #52 on:** February 14, 2012, 06:54:11 AM »

I think if there was only ONE way to build a stunter the event would lose a lot of its charm...

Igor Burger · « **Reply #53 on:** February 14, 2012, 07:04:33 AM »

I can say that my results on EC and WC are significantly better with logarithmic crank and worse with ususal 1:1 linkage. And I used it mixed, sometimes yes and sometimes not. "With" was visibly better than "without" ... but I can say that I did not feel difference myself, just results was much better :- ))) ... may be judges see the difference

KenP51 · « **Reply #54 on:** February 14, 2012, 09:20:51 AM »

Would someone clarify this a bit please?
What is the difference in a logarithmic crank and a expo crank or is different names same thing?
What is the effect on the elevator in (control effect) regards input, soft center for the amount of wrist input with increasing elevator response or is there decreasing response.
Also since we can feel our planes what does this do to feel? More feel around center, or is there, more feel at the ends?
And how do the two, feel and control effect relate to each other? Does one increase or decrease the other, or is there not really any perceivable relation?
There is enough in this thread that I think I got confused some were. Just try to visualize this.

Regards.

Larry Cunningham · « **Reply #55 on:** February 14, 2012, 11:43:06 AM »

The exponential and logarithmic bellcranks produce opposite effects. With a conventional simple bellcrank, sensitivity is maximum around neutral, and the logarithmic bellcrank will increase this effect. The exponential bellcrank has decreased sensitivity around neutral and more sensitivity at deflection.

I'll try to attach a PDF of a reprint of my old PAMPA Stunt News article, which includes plots that should clarify its effect. The plots are for the same geometry (bellcrank drive radius = .75" and swingarm drive radius = 3") as my current version.

L.

"All virtue is summed up in dealing justly." -Aristotle

KenP51 · « **Reply #56 on:** February 14, 2012, 12:54:46 PM »

Thanks Larry

Howard Rush · « **Reply #57 on:** February 14, 2012, 01:14:18 PM »

Hey, actual calculation. That's a step in the right direction. Now if somebody wants to go further and show handle torque vs. deflection, he could answer Ken's question about feel.

I have some quibbles:

1. I don't think the names "exponential" or "logarithmic" are right: the names don't describe what the mechanisms are doing. I did the figuring on the "exponential" crank (page 1 of this thread), and it came out sines and cosines without the imagination required for exponential. Correct me if this is wrong.

2. Many folks discussing this issue disregard the relationship between hinge moment and line elasticity (both from shape of the curve the lines take and the stretchiness of line material). That's probably the most important consideration in control geometry affecting how accurately you can fly your airplane. I'm trying to figure it out. It's hard to do (for me). From what I can tell so far, for stunt planes with flaps, it looks like the Slovakians' nonlinearity favors being able to fly accurate stunt. Igor's application to flaps only looks even better to me (as shown below), but for all I know Dave Fitzgerald could be using an Expocrank.

Igor Burger · « **Reply #58 on:** February 14, 2012, 01:15:26 PM »

there are actually 4 different things mentioned in this thread:

classic bellcrank with 1:1 ratio between handle, bellcrank and elevator
makes the same speed of controll surfaces aroun neutral and in deflection, and make stronger feedback in deflection than in neutral

expo crank
makes slow speed of controll surfaces around neutral and thus smaller sensitivity around neutral, and make quicker movement in deflection, while make small feedback in neutral, but stronger in deflection (compared to classic bellcrank) ... flaps to elevator ratio will be the same like in classic configuration (if horns are the same)

logarithmic crank from first post in the thread
makes higher speed of controll surfaces around neutral and thus larger sensitivity around neutral, and make slower movement in deflection, while make small stronger feedback in neutral, but smaller in deflection and thus allows sharper corners (compared to classic bellcrank) ... flaps to elevator ratio will be the same like in classic configuration (if horns are the same) ... means it is just the opposite to expo crank

logarithmic device on flaps only and 1:1 elevator
makes higher speed of flaps around neutral and slower flaps in deflection, taht makes small stronger feedback in neutral, but smaller in deflection and thus allows sharper corners (compared to classic bellcrank) ... flaps to elevator ratio is "lot flaps" in neutral and loops for smooth levels and loops and "little flaps" in corners for quick turning in corners

Igor Burger · « **Reply #59 on:** February 14, 2012, 01:17:35 PM »

Howard, I know that guy from somewhere :- )))))) .. my wife has lot of pictures with him

Howard Rush · « **Reply #60 on:** February 14, 2012, 01:22:59 PM »

Quote from: Igor Burger on February 14, 2012, 01:15:26 PM

there are actually 4 different things mentioned in this thread:

Five, if you count the bellcrank with the holes not on a straight line.

The "logarithmic device on flaps only and 1:1 elevator" is different from the rest. It actually changes the airplane dynamics with control deflection. I recently learned that an airplane with such a mechanism won the US Nats about 30 years ago.

Igor Burger · « **Reply #61 on:** February 14, 2012, 01:27:15 PM »

:- ))) ... so another proof of its function

KenP51 · « **Reply #62 on:** February 14, 2012, 02:49:05 PM »

Thanks Guys
That clears it up nicely.

Larry Cunningham · « **Reply #63 on:** February 14, 2012, 03:06:41 PM »

I have a strong desire to implement a mapping ROM, with the look up table controlling deflection of right flap, left flap, elevators, and RUDDER with an optional field for ENGINE RPM (!) vs the bellcrank transducer output, so we REALLY can play with this stuff.

(You guys left out differential flaps! Maybe we want an exponential inboard and logarithmic outboard flap.. did you think of that?)

Also we may want a "pilot feedback" field which will stiffen the bellcrank transducer according to deflection.

And we definitely need a 16-position rotary switch for Pilot Select:
0) {1-1 Conventional Bellcrank Emulation}
1) Howard Rush
2) Igor Burger
3) Brett Buck
4) Ted Fancher
5) ..et al
:
E) ..et al fin
F) .. GEORGE {auto-pilot}

Hand me that PIC16F68 - I'll get right on it. ;->

L.

"The price one pays for pursuing any profession or calling is an intimate knowledge of its ugly side." -James Baldwin

Larry Cunningham · « **Reply #64 on:** February 14, 2012, 03:23:49 PM »

Quote from: Howard Rush on February 14, 2012, 01:14:18 PM

Hey, actual calculation. That's a step in the right direction. Now if somebody wants to go further and show handle torque vs. deflection, he could answer Ken's question about feel.

I have some quibbles:

1. I don't think the names "exponential" or "logarithmic" are right: the names don't describe what the mechanisms are doing. I did the figuring on the "exponential" crank (page 1 of this thread), and it came out sines and cosines without the imagination required for exponential. Correct me if this is wrong.

Howard, I think you're right. Look at the equation in E4, the calculation of effective radius for an expocrank, from my EXPO.XLS spreadsheet. But the graph curves UP, much like an exponential function.

L.

Edit: PS - Howard, don't the Euler identities relate exponentials to sinusoids? I remember that e to the j pi = 1.. (or was it -1?) {Nothing like muddying the waters, eh? Sorry..} Euler was messing around with quarternions too I think - but he didn't fly CL Stunt.

"Be not the first by whom the new are tried, Nor yet the last to lay the old aside." -Alexander Pope

Howard Rush · « **Reply #65 on:** February 14, 2012, 04:35:46 PM »

Quote from: Larry Cunningham on February 14, 2012, 03:06:41 PM

I have a strong desire to implement a mapping ROM, with the look up table controlling deflection...

That's essentially what Keith Trostle did on his Bearcat's rudder, and it's mechanical and programmable!

KenP51 · « **Reply #66 on:** February 14, 2012, 06:16:45 PM »

Quote from: Larry Cunningham on February 14, 2012, 03:06:41 PM

To the auto pilot add a sensor to sense the down leg and ground proximty with mild zap administered to the pilot to force him to pull elevator.

You demonstrate it first.

Larry Cunningham · « **Reply #67 on:** February 14, 2012, 06:28:24 PM »

All this time I thought it was Carl Goldberg, not Rube Goldberg models.. ;->

L.

"If you don't want to work, you have to work to earn enough money so that you won't have to work." -Ogden Nash

"If you don't eat your meat, you can't have your pudding!" - Pink Floyd

Larry Cunningham · « **Reply #68 on:** February 17, 2012, 08:41:59 AM »

Well, I *do* listen, and have a strengthened version of my expo bellcrank, shown below. To insure adequate strength and durability, this one is machined from SR-71 grade titanium {I ruined several drill bits and jigsaw blades} and for added durability and appeal, it is "layered" with a single molecule thickness of 24K gold. Pretty, isn't it?

Pretty it may, but it's not going to flex and "pinch" anything!

L.

"Praise, like gold and diamonds, owes its value only to its scarcity." -Samuel Butler

Lauri Malila · « **Reply #69 on:** February 17, 2012, 01:40:39 PM »

Hi again Larry.

I think it's still not good. In order to make the thing stiffer, you are just adding unnecessary material to the bellcrank. But the basic design fault is still there. If I were you, I would do as Howard suggested and close the other end of the slot so that it becomes an oval- shaped hole. Only that way you can have harmonic load paths on both tension- and compression side of the bellcrank.
With your current shape, the compression side is problematic because the loads must go through a Z bend which eventually will flex and tend to close the slot.
If you do as I suggest, you will end up with a lighter, stiffer and more stabile crank, and you have no need to use titanium. Just a good aluminium or correct type of plastic will do. And they are a lot nicer to work with.

From cold Moscow, Lauri

Larry Cunningham · « **Reply #70 on:** February 17, 2012, 04:46:00 PM »

Lauri,

I apologize, my crude attempts at humor frequently fail. I wouldn't be making this part or anything else from titanium, nor would I be plating it with gold. I should have included a winkie ( ;-> ) to clarify that I was joking.

Your concerns may be based on assumptions that I don't share, e.g. the plastic sheet material is overly weak or flexible or that overly large forces are routinely applied to it. If it really does need to have its slot end closed off, that's easy enough to do. Or one might use a thicker sheet material, say up to 1/4". Or a tougher material like Delrin might be used. If a problem actually exists, there are multple approaches to solving it.

But remember - this is a bellcrank part, suspending a model airplane that typically weighs 3 to 4 lbs which normally experiences {in level flight} centripetal forces in the 3 g range. In typical operation (say during maneuvers at higher elevations) forces are even smaller (confirm this with line tension we experience).

That said, I bow to the School of Mechanical Overkill (tm) and present you with a Closed Slot Solution; icing on the cake is my chosen material - 6677 unobtainium, plated with hardened bearing grade whoknowsium. Beautiful, isn't it?

;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;-> ;->

Best regards,

L.

"Know thyself? If I knew myself, I'd run away." -Johann Wolfgang von Goethe

Larry Cunningham · « **Reply #71 on:** February 20, 2012, 10:14:45 AM »

If a little is good, then more is better, and most is best of all.

By increasing the thickness of my overkilled expo bellcrank, as well as molding it from a new 3D cubic matrix composite structure from elbonia (which combines the strength of carbon fiber and the natural bearing properties of nylon), just look at the masterpiece of engineering and material construction I have created!

Stand back, you rubberneckers, this ain't no car accident.. ;->

L.

"It is only one step from the sublime to the ridiculous." -Napoleon I

Mark Scarborough · « **Reply #72 on:** February 20, 2012, 01:37:28 PM »

GROAN,,
you are not being a little touch smart alec there are ya,, just by chance ?

Larry Cunningham · « **Reply #73 on:** February 20, 2012, 05:27:01 PM »

So, Mark, you didn't buy the new 3D cubic matrix composite structure from elbonia (which combines the strength of carbon fiber and the natural bearing properties of nylon) part?

L.

"Only those who will risk going too far can possibly find out how far one can go." -T.S. Eliot

Mark Scarborough · « **Reply #74 on:** February 20, 2012, 09:34:50 PM »

welllll,,
actually I think you made a small error in the balance of nylon to CF, I think instead of a 50 50 blend you would have found significant benefit in going to a 72.3 % CF to 20% Nylon with 7.7% spandex blend,, of course since this blend is only available from lower slovakiastan, you would have to order it machined ready to use,, and of course we all know then you would be violating the builder of the bellcrank rule,, so its probably mute,,

tom hampshire · « **Reply #75 on:** February 21, 2012, 06:07:57 AM »

Here's a real Rube Goldberg expo setup mounted (grafted) to an existing flying tail airplane. Its built to allow changes to the length of the swing arm, thus changing the gain of the system. The overall throw is changed by resetting the height of the stabilator horn. The output ball link and the swing arm ball link are mounted on a crosshead laminated from 3 pieces of pc board. No exotic materials as my sole source of affordable supply is the scrap box.

tom hampshire · « **Reply #76 on:** February 21, 2012, 06:09:05 AM »

next photo.

tom hampshire · « **Reply #77 on:** February 21, 2012, 06:11:29 AM »

Last photo.

Igor Burger · « **Reply #78 on:** February 21, 2012, 06:14:52 AM »

Quote from: Mark Scarborough on February 20, 2012, 09:34:50 PM

lower slovakiastan

... does it mean something for me?

Avaiojet · « **Reply #79 on:** February 21, 2012, 08:52:09 AM »

Tom,

Is that model your design?

I'm working on a twin boom.

Charles

Mark Scarborough · « **Reply #80 on:** February 21, 2012, 09:23:43 AM »

Igor,, NO,
I was being silly, It was simply the first thing that came to mind
sorry for any confusion,,

Igor Burger · « **Reply #81 on:** February 21, 2012, 09:28:52 AM »

aha

... I thought I did not get something what I should

Howard Rush · « **Reply #82 on:** February 21, 2012, 09:55:52 AM »

Quote from: tom hampshire on February 21, 2012, 06:07:57 AM

Here's a real Rube Goldberg expo setup mounted (grafted) to an existing flying tail airplane. Its built to allow changes to the length of the swing arm, thus changing the gain of the system. The overall throw is changed by resetting the height of the stabilator horn. The output ball link and the swing arm ball link are mounted on a crosshead laminated from 3 pieces of pc board. No exotic materials as my sole source of affordable supply is the scrap box.

That should give you any advantage of the "expo" without the disadvantage of lack of leverage on elastic lines. It will also tell you how important "feel" is. You won't have much.

tom hampshire · « **Reply #83 on:** February 21, 2012, 03:18:33 PM »

I dunno if it ever had any feedback worth mentioning because the stabilator axle is at 25% MAC. I'll have to fly it and see. I could move the axle if it seems to need it, but my guess is that the gain should be optimized first. Make sense?

Larry Cunningham · « **Reply #84 on:** February 21, 2012, 04:34:38 PM »

Since this ship has only a stabilator, wouldn't it be easier to install a flap horn with adjustable slider for setting gain? It's all nicely accessible. You could add a threaded link to adjust neutral.

And (allowing that there may be subtle differences), isn't the gain adjustment equivalent to adjusting line spacing at the handle?

L.

"In character, in manner, in style, in all things, the supreme excellence is simplicity." -Henry Wadsworth Longfellow

tom hampshire · « **Reply #85 on:** February 21, 2012, 05:05:09 PM »

Larry - The static adjust slider horn alone would give about the same effect as a wider handle spacing. The effect of the 'expo' linkage is to make the entire system less sensitive around neutral, with faster control deflection near the end of travel. The prior flights on this airplane, and an earlier version flying tail, both resulted in a lack of finding a good compromise between acceptable neutral stability and adequate cornering. When the CG was moved aft, or the handle spacing increased, or the stabilator horn shortened, it would reach some pretty aggressive turn rates, but at the cost of a 'hunting' level flight. Hence, some form of rising rate system is an attempt to get both neutral stability and hard turns.

Larry Cunningham · « **Reply #86 on:** February 21, 2012, 06:26:07 PM »

Tom,

I wasn't looking closely enough at your picture. What you're referring to as a "rising rate system" here is equivalent to the expocrank. The path of the drive point is a mirror of that for equivalent (same swing arm radius and neutral bellcrank drive radius) expocrank.

Lengthening the swingarm radius increases the expo effect; reducing the neutral drive radius also increases it. So, with your experimental tuning, you may find a sweet spot.

BTW - the seemingly "simpler" expocrank will exhibit more drag than the original version, due to the direction of the force vector the slot applies to the drive pin (see illustrations). In a frictionless world, they would be equivalent.

Beating hunting problems can be perplexing; by experimenting with CGs, we found that we could make a stunter (a stock Twister) hunt with the CG too far forward as well as too far aft. And sliding the line guides fully back and forth. And using different props! It was enlightening. Or discouraging, depending on how you looked at it..

I hope your system works out.

L.

"If nobody uses it, there's a reason." -Rule of Reason

John Sunderland · « **Reply #87 on:** April 19, 2012, 11:24:05 PM »

My Dad befriended Fred Bachl a few years back and bought a few of his plans. We showed them at the cub meeting in Columbus. The bellcrank became the topic of conversation over the stunt ships at the meeting real quick. Within a few weeks Bob Campbell had a working expocrank he made out of lexan for demo purposes. From there Joe Reinahrd put one in the new club trainer. The club trainer was highly modified flying wing more like the Half Fast than the Stunt wing with an RC motor mount up front and a more stunt friendly airfoil which held a custom tank neatly concealed in the LE. The thing was pretty rugged and was perfect for putting new people in the air with a hand launch out in the grass anywhere big enough. Capable of doing a recognizable pattern and any other zorping around you might want to do. But, ...I never got a warm fuzzy out of the feeling at the handle based on what the expocrank was bringing to the equation. JMHO

It was stable and easy to fly, but i never got used to the feel

Larry Cunningham · « **Reply #88 on:** April 20, 2012, 04:10:07 AM »

Hi John,

I like your term "zorping" - it pretty much describes my own flying skills. ;->

It's my suspicion if one intends to adopt an expocrank (or perhaps a logaricrank), they pretty well need to commit to using it exclusively. {Unless you're Bart Klapinski}, you probably can't instantly switch back and forth.

Here's a thought experiment for this. Suppose you had just flown your well trimmed stunt ship through a fine pattern. While you were off getting a coke, I visited your handle and offset the neutral adjustment, say about 1/2". Next flight, you'll wonder what happened to your model - suddenly it flies with this.. bias! Even after you figured out it wasn't your ship, you're likely uncomfortable while you compensate. Have you had a similar experience when some very good flier (whose pattern you just witnessed) invites you fly his model? It can feel all wrong, making you wonder how he flies it so well! ;->

I'm referring to how our muscle mechanics train into the control system trim and model flight characteristics. We adjust our reflexes, and eventually "lock" onto a setting, then get comfortable. Humans are very adaptable and adjustable but the training often takes time.

Anyone expecting to simply make a step function change to a different control system/feature and immediately feel at home and comfortable with it is likely to be disappointed. My first flight with an expocrank felt strange; I wondered if my model was even capable of a loop, it seemed so dead around neutral. Only when I snapped the handle and saw it respond did I start to appreciate it. Once adjusted to it, I decided like it could be an advantage for me. And I readily admit my own expectations may have been a large factor. Whatever advantage I felt, I'll take it! Hahah.

It is not my intent to prosthelytize about expocranks - I hope I didn't give that impression.

I hope everything is going well for you. I always admired your models (even though Lou Wolgast wonders why you didn't go MoPar..)

L.

"A work that aspires, however humbly, to the condition of art should carry its justification in every line." -Joseph Conrad

John Sunderland · « **Reply #89 on:** April 23, 2012, 09:39:15 PM »

Hi Larry!

I certainly remember your articles on this of course. I suppose you may be right. Perhaps, had I flown an expo/logari crank "exclusively" for a bit, I might have developed a better feel for it. I certainly would not be one to argue that they aren't a big plus in a real stunt ship either. But I did get to fly it a fair amount...and I flew the crap out of it and showed it no mercy just to see if it would put up a dog fight as a slow combat ship. Still a hasty conclusion on my part.