F1B stab whiplashing

Over this weekend at a contest, I had two horrible consecutive flights on one of my F1Bs, caused by the hammer piercing through the Mylar covering on the right side of center rib pair, causing the model to serially stall down. The question, of course, is that caused it.

It turns out my monofilament lines to the rudder were too tight (it was very hot and dry).  As a result, the tail boom (a standard Ukrainian Aluminum/CF tail boom, 12 mm at its base) was slightly bowed under tension.  Consequently, at the instant the rudder line was released, the tail boom had a whiplash reaction.  The stab, preached over a stab mount and held down by the hammer’s screw, attached to the tail boom with a rubber band loop, can pivot against the front edges of the stab’s mount. These pivoting movements are elastically constrained by the stab’s rubber band loop. Because the stab is an independent object, its inertia causes it to counter rotate the initial whiplash of the tail boom projected into stab’s plain.  With the right combination of these factors the stab’s sideway pivot could be large enough to allow the hammer’s screw to pierce into the adjacent Mylar covering.

The simplest solution is to stick a piece of tape or thick Mylar over the back of the stab extending the range under the hammer’s screw, left and right. One can also add small balsa triangles at each side of the center rib pair for better support. The top surface has to be very smooth.

The other issue is the line tension that can trigger the sideway whiplashing.  I’m using a raw 8 Lb monofilament line. A thinner monofilament line (6 Lb) is stretchier and it can’t build such high tension.  Some fliers use non-stretching woven Dacron lines which require appropriate tube-guides at the points the lines enter and exit the tail boom to reduce abrasion plus an element giving them their elasticity on the timer’s side. The line release allows the elastic element to contract and the tube-guides add line friction, slowing the line’s sudden release thus damping the tail boom’s whiplash reaction.  Maybe one should continue using monofilament lines to the rudder together with small tube-guides at the entry and exit points to dampen the line’s release speed?

Aram