Pull-Pull Elevators
04-01-2003 | 06:14 PM
#1
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From: Grand Forks, ND
Pull-Pull Elevators
A question for the more experienced-
When you set up a pull-pull for a split surface do you simply hook up two cables to the servo arm? or do you do a bellcrank to keep them seperate? neither? I am really at a loss and have never seen one of these in person before.
Thanks for the time,
When you set up a pull-pull for a split surface do you simply hook up two cables to the servo arm? or do you do a bellcrank to keep them seperate? neither? I am really at a loss and have never seen one of these in person before.
Thanks for the time,
04-01-2003 | 06:41 PM
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From: Bedford, UK
Pull-Pull Elevators
I don't know if this helps but here's what I'm thinking...
On electrics you thread the pull pull cable through the outermost hole in the servo arm, across the top of the arm and back down through the opposite hole. You adjust your elevators (the kevlar can slide across the top of the arm) then C/A the cable in place on the arm.
For giant scale you could do the same thing on a large bellcrank, utilizing 2 "EZ keepers" to lock the cabes in position once adjusted. Since the cables run through the same holes, there's no assymetry (assuming that the belcrank is centrally mounted).
Another way would be to use 4 wheels, one on each elevator half and two bolted together on the servo. In both systems you've got 2 loops of cable that terminate in ball links at the surface.
Another way is 4 cables, 8 ball links bolted to the bellcrank and surface. Whether bolted top and bottom of the bell crank or one on top of the other, you will have some differntial in throw due to different geometry's.
I use a bellcrank on large planes because I've had servo's get "rachety" on pull pull and find that the case tops have worn throwing the gears out of alignment. 2 servo's on a big Cap had the center gear pin walk out of the top.
Central Hobbies sells a nifty belcrank that you mount in the tail, just in case you get tired of visuallizing cats cradles with fishing line.
Chris
On electrics you thread the pull pull cable through the outermost hole in the servo arm, across the top of the arm and back down through the opposite hole. You adjust your elevators (the kevlar can slide across the top of the arm) then C/A the cable in place on the arm.
For giant scale you could do the same thing on a large bellcrank, utilizing 2 "EZ keepers" to lock the cabes in position once adjusted. Since the cables run through the same holes, there's no assymetry (assuming that the belcrank is centrally mounted).
Another way would be to use 4 wheels, one on each elevator half and two bolted together on the servo. In both systems you've got 2 loops of cable that terminate in ball links at the surface.
Another way is 4 cables, 8 ball links bolted to the bellcrank and surface. Whether bolted top and bottom of the bell crank or one on top of the other, you will have some differntial in throw due to different geometry's.
I use a bellcrank on large planes because I've had servo's get "rachety" on pull pull and find that the case tops have worn throwing the gears out of alignment. 2 servo's on a big Cap had the center gear pin walk out of the top.
Central Hobbies sells a nifty belcrank that you mount in the tail, just in case you get tired of visuallizing cats cradles with fishing line.
Chris
04-02-2003 | 01:52 AM
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From: Perth, AUSTRALIA
Pull-Pull Elevators
I've had pull pull on a few airplanes and I'm starting to get jack of it to be honest.
It's hard to get the geometries exactly right so you don't get differential on the elevator halves...
But at any rate the easiest way to connect the cables is to run one cable to both halves, i.e. one v shaped cable to both top halves and one v shaped cable to both bottom halves.
Put a crimp connector on the line, pull the cable through a hole in the arm/bellcrank and then thread the cable back through the connector, and down to the elevators.
I use pull pull connectors and sullivan clevises and it all works well.
it's still time consuming and IMHO not that accurate.
It's hard to get the geometries exactly right so you don't get differential on the elevator halves...
But at any rate the easiest way to connect the cables is to run one cable to both halves, i.e. one v shaped cable to both top halves and one v shaped cable to both bottom halves.
Put a crimp connector on the line, pull the cable through a hole in the arm/bellcrank and then thread the cable back through the connector, and down to the elevators.
I use pull pull connectors and sullivan clevises and it all works well.
it's still time consuming and IMHO not that accurate.
