Flex in Blanik Vertical Stab

Our club Blanik L-13 has on several occasions had the lower rudder hinge
lift out of the bearing. This results in the rudder cables (under tension)
pulling the rudder forward so that it rubs on the Vert Stab spar. Today
when we took it to the shop, the AP looked it over and determined that the
vertical stab could rock forward slightly. Since the rudder is held in
place vertically by the top hinge, when the stab rocks forward, this lifts
the rudder and allows it to come out of the lower bearing.

Once solution is to reinforce the rear of the fuselage to keep the vert stab
from rocking forward. The price quoted is almost more then the Blanik is
worth. We're waiting for replies from other Blanik repair shops to see if
there are alternatives.

Has anyone else experienced this with an L-13? If so, how was it resolved?

Thanks,
John Scott

On Aug 17, 4:48*pm, "John Scott" wrote:
Our club Blanik L-13 has on several occasions had the lower rudder hinge
lift out of the bearing. *This results in the rudder cables (under tension)
pulling the rudder forward so that it rubs on the Vert Stab spar. *Today
when we took it to the shop, the AP looked it over and determined that the
vertical stab could rock forward slightly. *Since the rudder is held in
place vertically by the top hinge, when the stab rocks forward, this lifts
the rudder and allows it to come out of the lower bearing.

Once solution is to reinforce the rear of the fuselage to keep the vert stab
from rocking forward. *The price quoted is almost more then the Blanik is
worth. *We're waiting for replies from other Blanik repair shops to see if
there are alternatives.

Has anyone else experienced this with an L-13? If so, how was it resolved?

Thanks,
John Scott

I doubt you have found the real problem yet.
If the fin moves as you describe, something is pretty wrong. Look for
missing or loose rivets or a cracked component.
Good Luck
UH

On Aug 17, 2:03*pm, wrote:

I doubt you have found the real problem yet...

I agree with Hank: The fore-aft play of the stabilizer may be a
contributory factor in the lower hinge disengagement, but I bet you'll
find that the primary cause is a misalignment elsewhere in the rudder
pivot system.

That said, I have always been some combination of amazed and unsettled
by the relative casualness of the drag/thrust attachment of the L13
(not L23) vertical stabilizer.

The stabilizer is basically attached to the dorsal "chine" along the
top of the fuselage where the right and left skins join. If you look
closely at the forward end of the vertical stabilizer root rib, you
see three or four 3/8" or so holes for the screws that attach the
stabilizer to the chine. Near as I can tell (and feel free to
demonstrate otherwise), there are no special considerations to
reinforce the chine in that area or to reinforce its connection to the
nearby bulkheads.

As a result, even on a healthy Blanik there is a somewhat generous
amount of fore-aft play to the stabilizer. Pressing the stabilizer tip
forward and aft just affects the ellipticality of the fuselage cross-
section in the area of the stabilizer forward attach.

Don't get me wrong, I think the stabilizer attachment is perfectly
airworthy as-designed and as-built. The longitudinal flight loadings
on the vertical fin are relatively modest under normal circumstances,
and the attach is demonstrably adequate for those loads. But I do
somewhat question the long-term serviceability of the assembly,
especially when the aircraft is moved and handled by untrained hands.

Thanks, Bob K.

Second that -

From a couple of years back when someone made a ground loop with ours
my recollection is that the whole plot is largely held in placed by
"Bulkhead 14" which is the complex pressing made out of some unusual
alloy. (Probably common in USSR in 1970 - unobtanium in South Africa in
200x)The principle is that it is a cone shape ending at this bulkhead.
Any distortion of the cone (e.g. dents in the sides of the fuselage) ,
or damage to the end plate will compromise the strength.

Bulkhead 14 is the one that the rudder hinges attach to (rudder post
bulkhead), as does all sorts of hardware in the end of the fuselage cone.

What we had was a tailwheel mod that caused the bottom of this assembly
to collapse under the torsion/compression force. When it was reverse
engineered by a specialist AMO - the alloy used proved too brittle and
it failed on the second or third landing. The resulting horisontal crack
had similar effect to your reported problem. In our case it allowed the
back of the rudder to lift.

Might be worth a look to see if this is the problem.

The second repair was not too expensive - given that it is an
"experimental" here, and we had the apprentices at the local airways
repair shop do the work as a project. (It helps to have a D-check
mechanic in the club...)

The real party trick apparently, was finding an alloy that could give
the spring/stiffness/strength of the original part.

wrote:
On Aug 17, 4:48 pm, "John Scott" wrote:
Our club Blanik L-13 has on several occasions had the lower rudder hinge
lift out of the bearing. This results in the rudder cables (under tension)
pulling the rudder forward so that it rubs on the Vert Stab spar. Today
when we took it to the shop, the AP looked it over and determined that the
vertical stab could rock forward slightly. Since the rudder is held in
place vertically by the top hinge, when the stab rocks forward, this lifts
the rudder and allows it to come out of the lower bearing.

Once solution is to reinforce the rear of the fuselage to keep the vert stab
from rocking forward. The price quoted is almost more then the Blanik is
worth. We're waiting for replies from other Blanik repair shops to see if
there are alternatives.

Has anyone else experienced this with an L-13? If so, how was it resolved?

Thanks,
John Scott

I doubt you have found the real problem yet.
If the fin moves as you describe, something is pretty wrong. Look for
missing or loose rivets or a cracked component.
Good Luck
UH