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4.1
LET IT SNOW -- BUT NOT ON YOUR LAZAIR
This
winter we have received three reports of Lazairs, tied down each case, the weight of the snow caused a strut to
buckl~,
In only damage, but In the other two, the wingtips and D-cells
were outside, being damaged by snow loads.
In one incident, the buckled strut was the damaged when the wing
struck the ground.
We
did a few measurements and calculations to determine how~ much load
might be caused by snow on the wings. By weighing a plastic bucket full
of snow, we arrived at a density of 13.3 pounds per cubic foot.
This was done on a relatively warm day when the snow was quite
dense, but there will no doubt be days when the snow density will be
even higher. Using this
measured value of 13.3, and a wing area of 142 square feet, a six inch
snowfall would produce a snow weight of g44 pounds.
Based on a gross weight of 340 pounds, this snow load would be
the equivalent of -2.8 g's.
Hudson's
"Engineer's Manual'3 gives the density of snow as 5 to
12 pounds per cubic foot, fresh fallen, and 15 to 50 pounds per cubic
foot when wet or compacted. Using
the upper limit, a layer of snow jua~ Ov~ dfl Łn~k ~ could
exceed the design limit of -2 g's: All of these calculations have
assumed an equal load distribution over the length of the wingspan. For a snowload, this is a reasonable assumption, but normal
flying loads tend to be more concentrated at the wing root and diminish
toward the tip, so a snow load will tend to stress the strut more than a
flying load of the same av~ge value.
What
does all this mean?
4.2
PINNED ROD ENDS
Several
months ago we received a letter from a builder claiming that a BEP
pinned rod end broke as he was tightening the nut.
We pulled a random sample of twenty units from stock,~ did a
torque test on them, and they all passed with no problems.
For two months, there were no more reported BEP problems, and the
first incident was chalked up to a "ham fisted mechanic".
Then suddenly we received several reports of BEP's being broken
during installation. These
failures occurred shortly after we phased In a new batch of BEP's from
the manufacturer, so we pulled another sample and discovered that about
90 percent of them could be broken if the nut were tightened to a torque
of about 60 inch-pounds. We returned the whole batch to the manufacturer and
they replaced It
with another batch. We
tested the new batch and obtained about the same yield as the batch we
returned. We have now been
told that the specified torque for these units is 50 Inch-pounds and the
breaking torque is about 54 inch-pounds.
Based on experience to date, it is obvious that this is not a
sufficient margin, so we have discontinued the use of this particular
part. We are now manufacturing our own BEP's using a standard BE
rod end with a specially machined 10-32 capscrew and a shoulder
standoff. These units will
withstand a torque in excess of that which would normally be applied
during installation, and have been designed so that if the nut is
overtightened, the standoff~will yield before the capscrew, resulting In
an inherently safe failure mode (with the rod end captivated on the
capscrew).
We
do not advocate arbitrary replacement of the previous style BEP's since
those which do not break during installation will not break in service
(there Is a stress reduction in th~ BEP caused by cold flow of the
F18/19 mixer plate). However,
anyone ordering a BEP as a replacement part will receive the new design.
4.3
MYLAR WING COVERING
Since
the last update, we have continued our efforts to extend the life
expectancy of the covering material. The obvious solution is to use a
mylar
which has been treated with an ultraviolet inhibitor.
We have tried several samples of UV inhibited mylar, but
unfortunately, one of the steps in the UV treatment involves a flame
heating process. This
stress relieves the mylar and virtually eliminates its shrinkability.
We have tried samples of many other materials (including Lexan
which has recently become available in thin film form) but have not yet
found a material which combines all of the required qualities (including
strength, transparency, shrinkability and UV resistance).
We are presently testing and evaluating many types of films
including vinyl, acrylic, oriented polystyrene, Tyvek and Tedlar.
In
addition to these, we will be looking at off-the-shelf and custom
manufactured laminates which combine the properties of two or more
materials.
Based
on tests conducted to date, mylar still appears to be the best material
for the application, and therefore we will continue to use it until we
can prove that something else is better.
In the meantime, we suggest that you follow the guidelines
provided in Item 11 of Update Number 3, and check future Updates for new
developments.
4.4
PUSHROD WEAR
Although
no serious problems have been encountered, we have seen a couple of
Lazairs with measurable wear on the 1/4 inch ruddervator pushrods (T26)
where they pass through the F32 guides.
The amount of wear does not seem to be a direct function of the
time on the airframe, as the most wear seen to date was on an aircraft
with only sixty hours on it, while our company demonstrators with
several hundred hours on them show virtually no wear.
One possible explanation is that dust or other airborne
contaminants are trapped by the grease and act as an abrasive and/or
corrosion accelerator. We
are now considering the use of graphite as a lubricant rather than
grease, but we have not had sufficient test time yet to make any firm
recommendations on lubricants. However, we do recommend that the
pushrods be checked for wear at least once every twenty flight hours.
Since the pushrods are made from thick-wall tubing, they can
tolerate a noticeable amount of wear without posing a serious problem.
However, any pushrod which looks like it is worn should be
checked with a micrometer or vernier caliper
by measuring the diameter of the worn section and comparing it to
the measured diameter in a section where there is no wear.
If the difference exceeds .030 inches, the pushrod should be
replaced.
4.5
KEEP YOUR TIPS UP
In
the last update, we advised against making design changes in the Lazair
unless you are qualified to predict the consequences of the changes.
Since then, we have received a report of a situation which
illustrates the point rather dramatically.
Most
Lazair owners are probably used to having people ask why the wingtips
are turned up rather than down (since the trend on many light airplanes
is toward down turned wingtips). Down
turned tips tend to increase lift, but since the Lazair was designed
around a very high lift airfoil, the additional lift provided by
downturned tips is not necessary. What
~ necessary, is a smooth airflow over the ailerons (to increase their
effectiveness), and a force which will lift the leading wingtip if the
aircraft tends to slip sideways.
One
Lazair owner decided to redesign his aircraft by installing the wingtips
upside down. Fortunately,
he had the foresight to have the aircraft test flown by his local
Ultraflight distributor who is a commercial pilot with a wealth of
flying experience. During
the initial phase of the test flight, while executing gentle maneuvers,
everything appeared normal. However,
after entering a turn with a very high angle of bank, the aircraft began
to slip into the turn and refused to come out of it.
Only by using every bit of his skill and knowledge was the pilot
able to regain control, and in doing so, he lost nearly 600 feet of
altitude. This Is exactly the same characteristic which was related to
us at Oshkosh this summer by the owner of a Mirage (which
incidentally has down turned wingtips).
There
is nothing in this world which is so perfect it cannot be improved - not
even the Lazair - but the message should be clear.
If you do not know what you're doing) don't change it!
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