Classic Designs > Classic Planes

4" Bellcranks, are they necessary in Classics??

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Jim Pollock:
Guys,

I have used  4" Bellcranks in .35 sized airplanes that were originally flown with
3" Bellcranks.   I think that 3.5" is really as large of a bellcrank that is needed for a 50-56" span classic.  I think the more important value
in a bellcrank is that amount of control throw built into it.


 

Dennis Adamisin:
Jim:
A lot of the classic plans show very short horns, sometimes on inner holes of bellcrank... mechanically they are a disaster!  Thus I think one of the best things you can do for most classic designs is to give them a modern control system.  For me that includes using the 4" cranks with the 15/16" (outermost) throw to a 15/16" elevator horn, flaps usually slower.  If the classic bird did not have room for a 4" crank then..... I guess I would readjust everything so the outputs came out the same.

Dennis Adamisin:
Jim P:
Thanks.  I believe in the early 1960's you will also find the name of a Mr. Gerry Cipra, who I believe earned the trophy as a JUNIOR.  And yes, he WAS that good!

I won Junior in 1969 with my Hawker Typhoon, in 1972 I flew my Fouga Magister in Senior & the Walker Cup.  Al was then, and still is the leading proponent of semi scale CLPA, we spent a lot of time chewing over this and that.  I still think the Sea Fury was the single most impressive CLPA model ever, but his new Bearcat looks pretty good too!

My tag line just lets me trash talk a little..!  8)

OK back on topic & CASE IN POINT: I am looking over Sheeks Swinger plan and the one thing I MUST change will be the controls.  The plans show the Pushrod going from #2 hole in the bellcrank to the middle hole of the flap.  Weak mechanically but not unheard of.  The killer is that it shows the elevator driven off the innermost hole off the flap, to the middle hole of the elevator; meaning the elevator travel will be 1/2 the flap travel - don't think Jack built his that way, and I for sure will not!  I will stuff a 4" bellcrank into it, reversed so front line is up (per Al!), and run a rod from the outer hole of the crank to the outer hole of the elevator.  I will then run a second rod from #2 hole in the crank to the outermost hole in the flap.  This should result in a 2:3 flap to elevator, and some pretty light bearing loads at all points.  However, I will also probably have to put a "hump" on the top of the fuselage to cover the longer horn!

Dennis Adamisin:
John & Tom:
You are right, I can get the correct ratios, but to do so inside the low Swinger fuselage means using a 1/2" horn on the flaps, with corresponding short output off the bellcrank.  However, I get real anal about maximizing the mechanical advantage and reducing bushing loads.  Thus I intend to use the 3/4" output from the bellcrank to a 1" flap horn.  Unfortunately that is about 1/4" ABOVE the top of the fuselage, thus the "radome" bump, which should look right at home on the jet anyway.  You know the Type 13 radar used on the second production batch of the F111b... - HONEST!!!  n~

Dennis Adamisin:
BTW, just checked out the plans for Sheeks Ryan SC and Bob Millers "extended Gee Bee"  Both show the elevator pushrod set up to give LESS elevator than flap throw.  This cannot be right (can it???)

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