Hello,
4" bellcranks seem to be the most popular for larger models, perhaps because you can simply buy them from Brodak.

Brodak sells one 4" bellcrank in which the arm length is ~0.8" (Item#BB-374) and there is only one pushrod hole and one 4" bellcrank (from SIG, Item#SH-754) in which there are two pushrod holes in the arm (~0.8" and 1.0").

If the pushrod from the bellcrank arm connects to the highest hole in the heavy duty flap horn from Brodak (1.3"), the elevator's pushrod usually connects 0.75" hole in the flaps horn to the 1.0" hole in the elevator's horn.

The pushrod from the b-crank arm and the elevator's pushrod may connect to the same flaps horn hole.
In this case, there are more choices to select the flaps to elevator's ratio but they are pre-defined by the holes position and only some ratios are practically in use while flying the pattern.

4.33" bellcrank (from Yatsenko) is also in use. There are also Yatsenko's adjustable nylon horns that allow for a very fine adjustment of the flaps to elevator's ratio. There is Igor's logarithmic flaps horn - interesting idea but it must be machined and is somewhat complex to build and setup properly.

Certain people build the customized bellcranks and horns having the geometry that suit them.

Lastly, there are choices of the handle lines spacing.
1:1 ratio between handle and b-crank is commonly used but there are some people that use slightly different ratio, custom build handles etc.

I trying to decide what to use in my large planes (one with Ro-Jett 76, one electric) in order to have the wide range of adjustments.

Yatsenko's b-crank and horns seem to provide the widest range of adjustments but some say that 4.33" b-crank must have 4.33" handle lines spacing.
This means the custom build handle as Brodak's handles I have do not provide such spacing.

Perhaps it is time for me to start building the customized handles?

Your comments and suggestions are appreciated as always.

Regards,

Matt

 

 





 

 
   

Here is how I'd look at the problem.  You are moving the control surfaces via flexible control lines, the flex coming from line elasticity and the shape the lines assume from the air blowing on them.  Thus you want to have a lot of leverage over the control surface hinge moment.  This leverage is differential leadout travel per degree of control deflection.  To get that, you need a big bellcrank that moves through a large angle.  I assume that the ratio of differential leadout travel per degree of control deflection (per degree of elevator deflection for the Igor system) should be constant over the range of travel.  To get that, you need to do a three-dimensional analysis of the control system kinematics.  You can get that constant slope of differential leadout travel per degree of control deflection either with a tilted flap control horn or a straight-up-and-down flap control horn, by the way. 

I'd pick bellcrank size to minimize the structural loss to the wing from the sweep of leadouts considering all possible leadout positions at the wing tip.  You won't have to rotate a 5" bellcrank through as big an angle as you would a 4" bellcrank to get the same differential leadout travel per degree of control deflection.  My current dog has a 4" bellcrank that rotates nearly 90 degrees in either direction.   I plan to print a 5" bellcrank for the next one. 

Handle spacing is a dependent variable.  Pick what you need for a given airplane.  You can't overcome a deficiency in leverage over control surface hinge moment in the airplane by picking a handle spacing.  I'd pick a handle spacing that feels good, per Dorin's advice above.  There's probably some physiologically optimal handle deflection, but that's not my department.  However, if you end up with a handle spacing of less than 4" or so for an airplane with flaps, you probably don't have enough differential leadout travel per degree of control deflection.   

If the response is too "dead" and you can't fix it satisfactorily by moving the CG back, then consider a custom handle.

Just as you can't fix a control leverage problem with handle spacing, you probably shouldn't try to overcome a handle constraint by moving the airplane CG. 

So how is it done with a straight-up-and-down flap control horn ?  You have dwg or pictures ?

I have a program that figures the geometry. By fiddling with stuff like bellcrank location and bellcrank output arm pivot position, you can get most any reasonable flap control horn to yield a linear, symmetrical response. I’ll send you a copy of the program when I get back to where my computer is.

I have a program similar to Howard's that I got from Larry Cunningham. I posted results and trends that I graphed from its output a few years ago. The response from one fellow was, "Huh." So I won't repeat that. However, one surprising outcome of running this program was that it appears that the most "orderly" (e.g. rods and horns perpendicular at neutral) looking setups do not produce the most linear or symmetrical outputs at the flaps and elevators. You can choose a "useful range" of flap deflections where asymmetry (differences between up- and down deflections for the same handle deflection) is within a fraction of a degree, but past that range, "up" and "down" deflections diverge more rapidly. These spreadsheets seem to me to be the only way to make control output uniform. That having been said though, we fly our planes without a lot of trouble in that regard anyway. I think we compensate for asymmetrical controls almost automatically. Just don't expect the best looking control geometries to be the most symmetrical. I think the angles generated by the bellcrank arm and flap rod are the main culprits, but are compensated by a bit of asymmetry elsewhere.

These spreadsheets seem to me to be the only way to make control output uniform. That having been said though, we fly our planes without a lot of trouble in that regard anyway. I think we compensate for asymmetrical controls almost automatically.

We may not want uniformity. I flew a plane that came in second at a world champs that seemed a lot more sensitive in up
than down. That would give more leverage at the top of the hourglass. Still, I make the control response of my planes a straight line. I have a notion to make a machine that measures leadout and control surface travel and take it to the Nats and measure a bunch of planes.

I just get these thoughts so bear with me please. Anyone tried solid wire leadouts?

Anyone tried solid wire leadouts?

They were more popular in bygone days than they are now.  It is simple to assume that the leadout wire makes a sharp corner at the wingtip.  A solid wire would be another spring, which would at least make the analysis more difficult. 

I think people who still use them like to hear the rattle on the way to the flying field.  It reminds them of the old days.

Do you mind sharing what type of filament you are you using to print your bellcranks? 

ABS, Hatchbox brand, I think.  I haven't optimized them yet, but a preliminary 5-incher took a surprising 150 lb. pull, so I think they may be viable.