ExpoCrank

LAST EDITED ON Jun-12-06 AT 10:34 PM (CST)
 
Ran across this and remembered some questions about it a while back. Maybe it is still useful....

Can you give us a little bit of a working description?

LAST EDITED ON Jun-13-06 AT 07:47 AM (CST)
 
>Ran across this and remembered some questions about it a
>while back. Maybe it is still useful....

Let me look through my old correspondence. That is a design by a gentleman who sent me the drawings for my products column back in Stunt News years ago. Fred Bachkle (sorry I cannot remember the exact speling of the last name) designed that particular one and sent me a mock up to show how it works.

Since the BC pivots in the normal manner but also "arcs" due to the inward point of attachment, the push rod movement is slight at the beginning and progressively gets more push rod movement as more input is entered. Very slight control surface movements are possible while still maintaining full throw. I have yet to build one and test it but, IIRC, Larry Cunningham has used them. Possibly others. (??)

The theory is nice but fails in application. Glen, one of our club member in the CC has a couple of models with this b/c installed. Great workmanship, smooth operation but only moderatley flyable. Jumps in the corners and requires more pressure when more control input is needed.
Since we only use about five to ten percent control movement during a flite, this tranlsates to nearly a linear movement on the models control system.
The Expo crank is/was a good idea but does not work. RJ

You are better off with an exponential handle. See the "Ultimate 1/2A Handle" by RSM. The leadouts from the handle are entrapped by two plates. There is a center plate that controls the line spacing. John Wright has a handle where the center plate is a disk. That provides a linear handle output that translates to mild expo when you consider the reduced line spacing at deflection at the bellcrank. On the RSM handle, the plate is rectangular. That provides an increase in line spacing with deflection and that is amplified at the bellcrank.

Since the center plates are plywood, they can be reshaped to provide exactly the performance you desire. Make them pointy, and they revert to a standard handle performance. Round the ends and you have one like John's. Or leave them rectangular to get the maximum possible effect.

I find the expo effect unnecessary on flapped models, but valuable on unflapped ones.

Netzeband and Baron used an expo bellcrank in the Humbug. According to the article is was required for maximum performance. Without it, the best CG location would result in uncontrollable bobbles coming out of maneuvers.

Larry Cunningham mentioned that the first time he tried the Expocrank, it felt all wrong. Then he tried flying it differently - essentially banging the controls in the turns. And then it worked extremely well.

"As soon as I tried it in my Shogun (bashed SV-11) and found that the Expocrank took some getting used to! Yes, it did affect the feel of the controls considerably. At first, I felt that the ship was very sluggish, and even wondered if it would do a loop, because it was so insensitive around neutral. Ultimately, I relaxed and simply started SLAMMING the controls. Wow! I found that I could easily fly better than ever with the Expocrank, it simply took getting used to. The really qualifty feature was just how easily it was to snap a sharp corner and return to level flight without a bobble. Immediately, the experience convinced me to use it exclusively, and I continue to do so."

http://www.egpworld.com/forum/default.aspx?f=2&m=9584&g=9633#m9633

What is the relationship between the distance between the anchor bolt, the pivot bolt, and the amount of expontential movement of the pushrod? Shorter distance = less expotentail difference (if the anchor bolt and the pivot bolt were co-located there would be no expotential movement)? and a longer piviot bolt to anchor bolt arm = greater expotential movement?

Just talked to Larry Cunningham. He'll be out of the hospital tommorrow
and will answer your questions.

Robert

My theoretical bellcrank is a circular bellcrank with a gear to make a rack and pinon set up with the rack on the pushrod. Linear system at the bellcrank. Does one also do a rack and pinon at the flaps and elevator? All theoretical of course.

In the early 1980's there was a gentleman in Phoenix AZ that built combat ships (a flying wing type), with expo-bellcranks. His name was Ken (I can't remember his last name). I flew his airplanes several times and they flew pretty well but fnankly I couldn't tell any outstanding quality imparted to the airplane by the expo-crank. They were stable and maneuvered well, but so did everyone else's that we compared them to...It all begged the question: So What, it works but it's very complex and somewhat fragile, why bother. A well trimmed airplane works pretty well with a standard bellcrank system and a poorly trimmed airpane doesn't and an expo crank doesn't contain any "magic bullet" to make a poorly trimmed airplane work any better.
I agree with Whitely here...it simply isn't worth the effort.

Depends on who you talk to. Larry said he flew much better with it, a matter of coming out of the squares without bobbles. Exponential is seen as something necessary in RC aerobatics these days, as well. Allows you to pull hard corners, yet maintain fine control in the middle. It might not simply be a matter of installing the bellcrank - you might have to retrim the plane to make full use of it (maybe move the CG back, perhaps) and train with it for a while (it'll feel very different, from what I've heard).

I have long since have given up trying to convince anyone to even try the Expocrank, and a similar comment applies to linearization of control systems. What I have come to realize is that humans are supremely adaptable, when it comes to controls on a stunter.

If someone claimed the Expocrank was some sort of "magic bullet", it certainly was not me! At the same time, I can predict with utter certainty that if someone won the Nats with an Expocrank, it would quickly become de rigueur overnight, just like Ted's handle design! Tell me I'm wrong. It's simple human nature (and common sense) to try to copy what succeeds.

As it stands, the Expocrank's idea is even contrary to what least one top flyer has expressed: a preference for an even further increase of sensitivity around neutral. And people who have developed the fine reflexes to fly competitive stunt (over decades) are not inclined to explore new motor skill training exercises to try out something as radical as exponential controls.

There is another aspect to using the Expocrank, which is often unappreciated. If it is not set up with adequate control horn radii (that is, controls are not slowed, which implies a larger bellcrank movement), the effect is small and not terribly significant.

For those not fully dissuaded from the Expocrank yet, or you simply want to see an analysis of Expocrank vs conventional bellcrank control rod movements, feel free to email me and I will send you a PDF copy of my ancient SN article The Expocrank Advantage.

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"The greatest threat to freedom is the absence of criticism." -Wole Soyinka

Glad to see you are back! Hope all came out OK, I missed the final details of your illness if they were published.

Anyway, I believe that the expo control system is mostly beneficial for non-flapped systems, as the flaps smooth out the neutral while delivering extra lift in the turns. Not extra turning ability, they actually defeat that. But you can fly a more sensitive elevator setting with flaps than without.

I am not actually a good enough pilot with reliable enough aircraft to do a formal evaluation of the systems, but I prefer expo on non flapped, and regular on flapped models.

Live long and prosper!

One other problem Larry, the handle neutral has to be adjusted to the bellcrank and the bellcrank has to be exactly neutral when the plane is flying level. Otherwise you will get varying degrees of control "boost" when you move the handle. See Art Adamisin's comment on using a Y bellcrank in the Mar-Apr Stunt News. It haa a similar effect, but again, the handle neutral, the bellcrank neutral, and the plane neutral all have to be matched up.

Phil,

Although I agree with you in principle, I did not find the neutral alignment to be a particular problem, primarily because the Expocrank is most INSENSITIVE around neutral. The "boost" effect doesn't kick in until there is significant deflection of the controls. As I reported in my article, when I first flew the Shogun ship with the Expocrank, the controls felt sluggish to me, and I wondered if it was going to be able to loop. After I adjusted to the Expocrank, I found I could easily "slam" the controls hard for corners, and then return to a clean neutral without a bobble.

Perhaps there is a setup issue, as I said, the Expocrank needs to be setup so that the control throw is large, to get its effects, which again, implies "slow" controls. The handle line spacing is then used to get the desired feel back, wider = faster, of course.

The Expocrank is not for everyone, and naturally if you go to all the trouble of installing one in your ship and DON'T like it, it's a hassle to change it out, which discourages serious efforts to try one.

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"It is easy to be brave from a safe distance." -Aesop

Larry,
The "Magic Bullet" comment was mine and not meant in any way to indicate that you had suggested it was.
I read your SN article many times when it came out and was impressed by the technology. Not enough however after my previous experience with it to invest the time and energy to explore it.
Hard, Flat corners in my opinion are the product of very stiff controls, particularly the relationship between the bellcrank and flaps and the handle and bellcrank. In my opinion the Fancher handle is successful because it eliminates any real "stretch" between the lines and handle. The lines of course stretch significantly and that's why I (and lots of others) prefer solid lines. Not because of perceived lessening of "drag", but because they stretch far less than stranded. Not that the drag isn't a factor but just that it's probably less important in trimming than the "Spring" in the control system. The entire control system acts as a spring mass with different time constants between each elements. Shortening those time constants as much as possible by making everything "stiffer" is, in my opinion, probably more important than limiting the control movement in any particular position as done with an exponential system. Ideal CG location is also very critical in hard flat corners. A little too far in either direction increases the potential for a bobble (believe me, I'm the bobble king and have done a lot of experimentation to eliminate it). I've discovered the easiest way to ensure bobbles is to have weak (springy) flap horns and springy cables on your handle.
I'm probably only telling you things you already know and none of it has the intention of criticizing your work with exponential controls.
I just wanted to comment on your discussion about "Hard, Flat Corners".

No worries, Randy, I didn't take anything written here as anything but constructive.

As for the magic bullet, I know what it is: practice! ;->

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"History is a vast early warning system." -Norman Cousins