Brodhead Pietenpol Association

I hope someone can help me with this problem. I finished the stick controls, and when the cables are tensioned for the elevator at neutral stick position the cables seem fine. When the stick is moved off center, (forward or back) the cable gets very loose. Now I know I've got the geometry correct according to the plans, so why does the cable go slack? After looking at the cable and pulley geometry, How do you not get slack?

Good Evening George:

Could you please send me a picture of that anomaly? In the meantime, I'll look through my building logs for the travel you asked for. By memory, I'd surely be sending you down the wrong path.

HD Rose
74N Bendigo Field

Hello George,
I was concerned about this "slack cable" issue during my build, and took care to make the bellcrank arms the exact length of the elevator horns, thereby insuring that a true parallelogram was formed when the cables were strung. I don't remember if the parts shown on the plans were of the same length - maybe if they're not, slack cables could result when the bellcrank and horn move into non-parallel planes. I don't have the "slack cable" issue, but maybe that's not the reason why - I just remember that thought process as I was making metal parts.
"planes&bikes"
Don Youngblood, N41YB @ LQK

Good morning George.
The slack elevator cables are definately caused by the walking beam arm and the elevator control arms not forming a perfect parallelogram. If made to plans they form a trapezoid, which is why they go slack. I made the arm of the walking beam the same as the arms of the elevator control horns which is 7-3/4 inches and my cables do not go slack.

In addition a characteristic of most Piets is the upper elevator cables rub on the horizontal stabilizer when the stick is forward and the elevators are down. This is caused by the position of the walking beam. If you move the rotation tube of the walking beam upwards a little, the control cables will not rub on the horiz stab. I don't remember exactly how far up this is, maybe an inch or so. You have to be careful that you don't move it too far else the cables may exit through the top of the turtle deck rather than the sides of the fuselage. What I did is to mount the horiz stab and the elevators including the elevator control horns. Then I took the walking beam assembly and held it in place with clamps. I then attached strings to the elevator control horns. Now check for clearance, proper exit on the fuselage and if the cables rub on the horizontal stab. Next move the location of the walking beam up or down as neccessary to achieve the desired result. I have attached a photo of what I did.

Hope this helps, Rick Schreiber
NX478RS
Valparaiso IN

Good Morning George!

While I'm not sure how comfortable you are about deviating from the original plans, I had spent considerable time working on this problem in my own Pietenpol, NX709HR. Attached are some photos for your use, should you desire. As I have designed and built this the cables do not go slack in either up or down positions, nor do they ever touch the leading edge of the horizontal stabilize; which both reasons were the cause for consideration of the design change. There are some neat things here to point out. I have built into my elevator walking beam stops for both up and down as you can see in the pictures. Additionally, I have also incorporated a neat tension device to pre-load the elevator stick, thus taking off a considerable amount of stick weight while in flight. This proved to be a very neat and important idea, and I enjoyed the flight yesterday as I began the process of seeing if my changes/modifications proved to be worth the time/weight.

As I have noticed here many have told you the limits are more about the space you have after you get your legs in the darn thing, but mine seems to work perfectly as I designed and built it. Hope the information helps.

Thanks,

Herb Rose
Bendigo Field 74N