topwinguk

I have been using pull pull controls on rudder for years but never tried them on elevator. Anybody got views on using them for elevator control?

Tony

khodges

My Feedback: (1)

None myself, but am also interested in knowing . Getting ready to build WWI bipe and the elevator cables exit between the front and rear cockpit on both sides, and I would like to include that in scale detail. Let the ideas commense!

Rodney

I've used pull-pull on both elevators and rudder on all my 1/4 scale (7 different models now) with great success. I wrote an article for MAN which was published back in May 1983 issue describing the technique I use in detail. I still think it is one of best ways although I admit to being prejudiced. Note that technique described makes it ultra easy to change out servos or lines and used common items easily obtained.

JoeAirPort

My Feedback: (41)

What I've learned from pull pull systems:

1) Unless you keep the cables perfectly parallel, it's gets complicated. That means the servo horn and the servo arm have to have the cables connect at the exact same distance from the rotation point (servo shaft and control surface hinge). If the cables cross, there will be slack in one of them at full defections unless you make some changes to the setup (too much detail to go into here).

2) They should not touch the fuse where they exit. This can make for some huge slots.

I would think that the simplist way to do a split elevator is to mount the servos in the tail on the side of the fuse so the cables are on the outside but it might be kind of ugly.

topwinguk

Thanks guys for replies. Joe, your comment on slack cables is a fair point but I use the Akerman steering geometry approach (like the steering mechanism in your auto) and ensure both cables are taut, only when the control surface is neutral and sweep the horns on the surfaces about 15* rearwards. This gives differential movement and prevents undue tightening of one cable due to unequal geometry. The airflow keeps tension, where you want it, on the surface. This I knew worked OK on the rudder but I was a bit unsure about elevator. Don't want an unneccessary problem on my new scratch built big bipe!!!

Spaceclam

actually, the points on the surface and the points on the control horn do not have to be the same distance apart, and you will not get slack it they are. see, if you move the servo horn 1" at the tip, it will pull the wire 1" in that direction. therefore, the line on the other side will move 1 inch the other way. so, you could have the point on the conttrol surface a mile out where the cable attaches, but it will still only pull one inch, however many degrees that will be. you will not get slack because the other side will move 1" where it connects as well. it's a matter of leverage. a longer moment at the control surfaces and a smaller moment on the servo side will result in less throw, more torque. it's a comon mistake to think that the wires have to be paralell. in many cases, crossing the wires will eliminate slack due to imperfect geometry.

Rodney

No the distances do not have to be the same on either end. You do have to watch where your attachement points are relative to the hinge line though. A common mistake with pull-pull is to have the cables to tight or the attachment points to the horns in the wrong placed relative to the hinge line. You also do not have to have the cables parallel, I run them around corners, around obstructions etc. with no problems as long as you have a decent guide system.

kregan

Can you give more info on what you use for guides?
What do you use to help guide the cable around small
curves etc.

Thanks

jaka

Hello!
Do not worry about having different control horn sizes...it doesn't really matter as long as you have the hinge line at 90 degrees to the attachment point of the wire.
I have used wires on many different airplanes from sailplanes and scale to F3D Pylonracers and do recommend them as they are lighter than any other system and doesn't transfer vibrations to the servo as carbon rods or balsa rods do.
Do not worry about having the wires parallel...it doesn't matter ....you can have them crossed if you like..and that makes the exit holes smaller.
I always try to mount my elevator horns inside the fuselage ...for looks only.
I often silver solder the elevator horn from soft 1,5mm thick iron and 3mm piano wire.
But now and then I use aluminum EATON arrows as elevator linkage to which I drill,CA glue and a solder 3mm piano wire (6cm-10cm long), to which I secure the control wire.
The wires i use are bought at a sport fishing store, thinnest possible fishing wire.
See pictures of my GP GEE BEE R-2

Regards!
Jan K
Sweden