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LONG-EZ AILERON BELHORN FAILUREFrom CP58, Page 7 (January, 1989)
RAF has recently received
two separate reports of failures of one of the CS132L weldments, the belhorn, which drives
the aileron out in the wing roots. One of these belhorns has had lightening holes bored
through the .050 steel belhom and it cracked through one of these holes. However, the
second one was as received from Brock and it cracked across at the edge of the weld around
the tube. Prior to the failure of the belhorn this builder pilot had had to replace the
rod end that bolts to this belhorn, at least twice over the past 350 flight hours, due to
the rod end being "pounded out" until it was dangerously loose.
The belhorn failure occurred in flight and caused a few moments
of concern, but in both cases the Long-EZ was easily controlled. A disconnected aileron
will float trailing edge up. To keep the wings level, the pilot has to raise the trailing
edge of the operable aileron which, of course, will give a nose up pitching moment
requiring forward stick to fly level. The one operable aileron will provide reasonable
roll control and, of course, the rudders will roll the airplane by themselves. The
greatest hazard would be if the disconnected aileron pushrod, being loose in the
cowling/root of the wing area, ever managed to get itself jammed. Depending how much
lateral input there was at the time, you may or may not be able to correct the roll with
primary control system failure is cause for strong concern so we at RAF have designed,
built and flight tested a new aileron belhorn. Drawings for this new part have been sent
to Ken Brock Mfg. and Ken will have these parts available as soon as possible. We will
provide a drawing of the new belhorn in this CP for those people who would like to make
these parts themselves. (see sketch)
Why would this belhorn fail on two relatively low time Long-EZs
when we have literally dozens of Long-EZs with- 1000-plus hours and some with 1500-plus
hours with no failures and zero wear on the rod ends? Bill Freeman, Long-EZ builder/flyer
and a man whose specialty is working with vibration problems and who has a Master's degree
in Mechanical Engineering, has a theory with which we concur. The original control system
with aluminum push rod tubes apparently was OK. The natural frequency of this collection
of parts was not the same as the normal cruise excitation frequency of the engine/prop.
Changing the aluminum tubes to steel as called out in the CP may have moved the control
system into the excitation frequency of the engine/prop combination. Bill says that this
strongly suggests to him a spanwise vibration of the CS132L belhorn and CS129L pushrod at,
or near, its natural frequency, inducing a high-cycle fatigue failure in the CS132L
belhornn. The fact that the rod end bearings were beaten out is strongly suggestive of a
resonant vibration of the CS132L and CS129L pushrod. This vibration would have the bottom
end of CS132L and the aft end of CS129L moving spanwise, bending CS132L in the weak
direction with -high enough stress levels to initiate a fatigue failure in CS132L.
The new part, part number CS132L-R, has two arms instead of one
which will more than quadruple the stiffness of the system and will also provide redundant
links in the aileron system as well as providing positive retention of the rod end in the
event of a ball slipping out.
If you absolutely insist on flying before the new belhorn is
replaced, a careful examination of your CS132L belhorns are mandatory. Use a bright light
and a magnifying glass. Examine the area shown in the sketch while gently flexing the
CS132L left and right requires immediate grounding of the aircraft. Any sign of a crack
until the new CS132L-R is installed. Examine the rod ends bolted to the CS132L. Look for a
loose ball, or play in the rod end in the fore/aft plane. A worn rod end must be replaced
before flight and you should realize from the above discussion that a worn rod end almost
certainly indicates that a belhorn failure is imminent. If you have steel tube push rods
(CS132L), your belhorns are definitely more suspect. If you have worn rod ends, do not fly
until you replace the belhoms and rod ends. Even if everything looks OK, replace belhoms
within the next 25 hours of flight. The CAD plated CS132L belhorns should not be painted
since the paint may hide a crack.
This is a serious matter and should not be ignored. A primary
control system failure could result in a serious accident. Please report any cracked or
broken belhorns to RAF along with the number of hours on the airplane, whether you have
aluminum or steel push rod tubes and if you have experienced rod end wear or failure.
See the additional article below on similar "Aileron
Below is an excerpt of a letter received at RAF
"Thanks for all the good newsletters. Just to clarify, I
have had aileron flutter (see Ed. note). At 10 hours, I noted a lot of aluminum dust
behind the aileron hinges. In flight, I visually could see the tip of both ailerons as a
1/4' blur. I added leading edge weight and installed the Teflon hinge pin setup. At this
point, I had no visible vibration at 2000 ft at 120 mph, but still had vibration at 8000
ft., 160 mph. It remained this way for many hours of "hauling rides' but. less than 5
cross country hours. Note: I never was able to detect any vibration on the stick
I recently put more weight on the right aileron which was still
vibrating slightly at altitude. This extra weight was along the outboard end where I had
previously not had any. This finally cured the problem. Now the ailerons hang with the top
surface level. Note: The problem occurred when the ailerons balanced bottom surface level
as per plans. Note: Both ailerons had this problem. The left aileron is very accurate
dimensionally, the right's trailing edge rises 1/4" in the outboard 8" from a
straight line. Also, I have a good surface finish, laminar flow, as evidenced by wing drop
before the vortilons.
It is very hard to see the trailing edge of the aileron and
difficult to decide if it is indeed vibrating 1/4" or if your eye is just not that
sharp, but having fixed it, I can verify that it was not an optical illusion.
I feel that many Long-EZ's probably have this problem and their
pilots are not aware of it.
Again, there is no indication of stick vibration.
We have published Larry's letter as he wrote it because we
believe he experienced the same phenomena described above: Engine/prop excited
"forced vibration" driving his aileron at the same frequency as the engine/prop.
"Flutter" is an aerodynamic condition and is normally divergent, i.e., expands
to destruction. "Forced vibration" can continue as long as the source
(engine/prop) is maintained near the same frequency as the natural frequency of the
aileron. By overbalancing his ailerons to the top limit as called out in the plans, he has
(1) changed the mass of his ailerons thereby lowering the natural frequency of the
ailerons and, (2) repositioned the CG of the aileron relative to the hinge, thus reducing
the "forced vibration" input.
If your ailerons are vibrating at the trailing edge as Larry's
were, you must add more leading edge weight. Note: We checked several Long-EZs here at
Mojave and none of them exhibited any visible vibration at the trailing edge, however, all
of them show some signs of aileron hinge wear (black aluminum dust on the aileron,
particularly after flying through moisture).
Keep in mind that it may be difficult to spot. Have a passenger
in the rear seat look at the aileron trailing edges very, very carefully. Spend at least
30 seconds staring at the ailerons in level flight, in a climb, in a descent, and in left
and right turns. If any vibration is seen, re-balance the ailerons.
The easiest way is to get some lead ribbon from a golf pro shop
and stick it to the top of the aileron leading edges, full span- until it balances
top skin level. Lay up one ply of BID to permanently secure the lead to the aileron
leading edge. (see sketch, page 15)
We would like to thank Larry Bush for the excellent feedback on
this situation. This is the kind of information we all need to know about in order to keep
the large fleet of EZs flying safely and consistently.