Classic Designs > Classic Planes
4" Bellcranks, are they necessary in Classics??
John Miller:
Ya know, Dennis, in the old days, we simply used 1 to 1 ratio for the flaps and elevators. Nowadays, with planes with a reasonable wing loading, we've found that we don't need as much flap as we used to use.
I was speaking with Gordan Delaney about this very subject, the other day. He and I both agree that most of our planes have about a 6 or 7 degree difference, at full throw. The degree readouts on my most recent plane is about 24 degrees flaps, and about 32 degrees for the elevator. It's a very good idea to use an adjustable elevator horn. Too much flap can make "swoopy" turns, as I'm sure you already know..
John
Dennis Adamisin:
Yup. Kinda hard to explain it was a model of a French trainer in Belgian Airshow Team colors. However, one guy called it a Nobler....
tom hampshire:
The above posts are all correct. BUT... It is humbly submitted that the missing part of the analysis so far is the ergonomics, meaning the hand movement necessary to generate a given control deflection. Clearly, a 3 incher will give more angular bellcrank deflection for a given hand movement than a four incher. This excess bellcrank rotation generates two undesirable effects, the first being that over rotation. Once the bellcrank is past 20 or 30 degrees, the effective moment arm of the bellcrank gets noticeably shorter. The shorter the arm, the more force is required, and the less tolerant the whole system is to minor corrections. Which leads to the major difficulty with 3 inchers, the controls are too quick. Think of it in terms of control deflection per length of leadout travel, not per the angular rotation of the bellcrank. Most of us have experience with some plane which had ultra quick controls. Try one again sometime, and see how difficult it is to fly accurately with quick controls. The slower controls make the whole setup easier to fly accurately. So most of the poster so far haven't thought of it in terms of flyability, e.g. the comfort of the pilot.
tom hampshire:
Hi Alan & Bill - I don't know why a narrow handle spacing doesn't feel right to me... only that it doesn't. If you want to slow down a set of to fast controls, usually you wind up with a front slider bar handle. Funny thing, I wandered into the team trials about 15 years ago and got drafted by Mr. Tiahrt to be pit boss. I learned that none of the entrants at that level used a handle with a slider bar. All of the handles had the control hardpoints as close to the knuckles as could be. My guess is that the excess overhang (Having the handle arms too far from your knuckles) gives excessive sensitivity around neutral. Other opinions welcome. Tom H.
Dennis Adamisin:
I think Lou's comment on long horns and lower pushrod loads really nailed the issue. H^^ Lower bearing loads = long lifed control systems.
Whether you use a 1" or 5" bellcrank, it is POSSIBLE to get the correct ergo relationship between hand movement and control surface movement, but you mght have to take some extreme measures. I just find that with my fixed spacing handle, a 4" crank works pretty darned good - so I will continue to use those cranks and the longest feasible control horns I can.
BTW I use a DIFFERENT (smaller) handle for a bird with a 3" crank and just got another handle ready for a bird wih a 2" crank. Then there's the Goldberg handles for the 1/2A's!
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