A Few Good Measuring Tips
Sport Aviation - 7/86
By Tony Bingelis
Those
of us who were born and raised in the good of U. S. of A. grew up using
the common ruler as our standard unit of measurement. We learned to
judge sizes in relation to the 12 inch ruler (one foot) and the
yardstick (3 feet). It is, therefore, easy for most of us to
immediately visualize the approximate size of a 2 inch square piece but
not so easy for us to get a mental image of how large a 50 millimeter
square would be (both are approximately the same size . . . two
inches).
It is not
surprising then, that we prefer our aircraft plans to be depicted in
the standard inches and feet units that we grew up with. On the other
hand, our European friends and others living in almost any other part
of the world, would much rather build their airplanes using the metric
system. Frankly, I, too, prefer plans dimensioned in the metric system
because I detest working with fractions. Whole millimeter numbers are
much easier to work with.
Really,
it doesn't matter what unit of measurement is used in the plans you
have, provided you have the correctly dimensioned measuring tools to
work with. What does matter is the ability to accurately measure your
work with the tools (scales and rulers) you are using. Measuring
accuracy can, in many instances, be improved by learning how to
properly use your basic measuring tools and by improvising whenever
necessary to obtain the measurement you need with the minimum of
effort. Let me pass on a few good measuring tips to make my point.
How To Use The Basic Measuring Tools
The
basic measuring tool is the ruler, or as it is known in engineering
circles, the rule. Most builders use a flexible steel rule for making
their small measurements. The handiest of these are the steel 6 inch
rule and the 12 inch rule. Steel rules can be purchased calibrated in a
variety of measurement units . . . in inches and fractions, or in
decimals instead of halves, quarters, eights or 16ths of an inch, and
also in millimeters and centimeters. The accuracy of measurement lies
not in the calibration standard of the scale as much as it does in how
you transfer the measurement from the plans to your work. Let's take
using a steel rule for example. Many builders don't realize that the
greatest measuring accuracy is obtained when the steel rule is stood on
edge against the material being measured and marked. This technique can
help eliminate even the slight errors that result when your eye is not
directly over the scale when marking the point of measurement.
A steel tape is used a bit differently.
It
is used to measure larger dimensions and to establish aircraft
alignment of wings and landing gear, by triangulation. A 50 foot steel
tape is used for this purpose because a steel tape will not stretch
measurably, and an accurate reading is possible provided the tape is
pulled tight to remove the slack. Accuracy will generally be better if
you can get someone to hold the tape at one end against a specific mark
or point while you read the dimension at the other end. An example of
this is measuring the distance from the rudder post to a wing tip and
comparing the readings for each side.
A
smaller steel tape, 8 feet to 14 feet long, may be handier for most
other large measurements for the same reason . . . it does not stretch.
Here's another tip.
Fitting Fuselage Crossmembers
Anyone building a wood aircraft will be confronted with many opportunities to accurately cut and fit crossmembers.
These
wood strips must be cut at the correct angle and to the correct length
for fitting into a specific location. Sometimes you will get the angle
and length right the first time, but often, when you have to correct
the angle to obtain a better fit, you learn to your chagrin that the
piece is too short. A clever little marking aid can solve this vexing
situation and speed up your progress as well.
I
first learned about this gadget many years ago from a long time EAA
friend, Gene Darst of Beaumont, TX. Gene called it a Preacher's Helper,
if I remember correctly. The significance of that label escapes me
unless its use helps deter one from having to voice his frustration
with profane expletives.
Anyway,
the simple little device really works. It permits you to mark a
cross-member on all four sides. This alone insures that you will be
able to make the cut to the exact angle needed. Another benefit is
gained because the line marked for the cut will be made slightly
outboard, thereby minimizing the risk of cutting the piece too short.
That brings up an important point.
Don't Cut On The Line
A
common mistake a new builder may make is to cut right on the line he
has marked. This may be all right for the first cut if he remeasures
the length and remarks the opposite end of the piece before cutting it
off. However, it is absolutely wrong to allow yourself to get in the
habit of cutting through the marked line. All saw blades have width . .
. most of them are much wider than a marked line. It is easy to see,
therefore, that when a cut is made down the center of the line you will
be losing at least 1/32" of the measured material length. Multiply this
error by two cuts and your piece will be too short.
Here's
a good rule to follow. Always make your saw cut so that at least some
of the marked line remains on the part you intend to use.
Dividing Any Dimension Into Equal Spaces
Often
there is a need to divide a given length of material into equal spaces
for the installation of screws or rivets, for example. This is easily
accomplished without help from your computer or finger counting
arithmetic. Place one end of a ruler on one edge and swing the free end
of the ruler down until the number you want touches the opposite edge
of the space you want divided. Using a ruler and slanting it so that
the number 7 touches the far edge you will automatically have 7 equal
spaces after you make a mark at each inch interval. The example shown
in Figure 3 has the number 5 contacting the right hand margin.
This will give you 5 equal spaces as shown. You don't have to use a
ruler for this system. You could use a strip of wood or a piece of
cardboard marked every two inches, for example, and numbered in
sequence from one through whatever length is necessary. Look at the
drawing again and think it through. It should be easier to understand
than all these words.
Measuring Round Stock and Tubing
This
one should be a snap to understand. If you have a good eye
(figuratively speaking, of course) you can generally do a good job of
estimating the diameter of common tubing sizes. Sometimes, however, you
can be fooled. Obviously, when the tube diameter is quite large, or has
some uncommon diameter, you had better measure it to be sure. There are
a number of ways to measure the outside diameter of round stock. Figure 4 shows
three of the simplest ways to do this. Of course, if the tubing
diameter must be measured accurately to the nearest thousandth of an
inch, a micrometer or vernier caliper should be used instead.
Determining Bolt Lengths Required
There
are areas in an airplane where it is very difficult to determine the
correct bolt length to use. Nevertheless, there is a way to do this
without having to resort to that time wasting "trial and error"
technique we are all so familiar with. One troublesome location that
comes to mind is the firewall area. You often forget the thickness of
the section through which the bolt must go . . . especially in a
composite or wood aircraft. Then, too, trying to figure out how much
extra length will be taken up by some bracket or fitting adds to the
problem. Some folks solve this problem the easy way. That way is
illustrated in Figure 5. Simple, isn't it?
Making Large Inside Measurements
It
is hard to obtain an accurate dimension for an inside measurement with
a steel tape. You really can't use a yard stick as it may be too long
to fit in the space to be measured. At other times access is a problem.
I'll bet if you thought about it long enough you would come up with the
same solution as that shown in Figure 6.
All
you need are a couple of sticks and a spring clamp. Slide the sticks
outward to contact the sides of the space you want measured and then
clamp the sticks together with a spring clamp. Remove your super
measuring tool and measure the length from one end to the other with a
yardstick or steel tape. Voila! Instant success with a high degree of
accuracy, too.
Mental Mutterings
Most
aircraft designers go by the book when they dimension their plans for
the builders. In many instances this highly professional rendition
really creates unnecessary confusion, anxiety and work for the amateur
builder. I will cite a few examples.
Let's
say the design of the spar shown in the plans provides for a 5 degree
dihedral angle. How many builders can accurately measure a 5 degree
angle twice and get the same figure? Wouldn't it be more kindly for the
designer to give the dihedral dimension in both inches and degrees? A
builder would then have two ways to check his work.
The
same complaint can be raised for control surface travel limits. When
rudder travel is typically given as 25 degrees left and 25 degrees
right, how do you measure that (don't scale the drawings, they say)?
Measuring flap travel is equally awkward (read that as difficult) . . .
especially when Fowler flaps are installed. But then, since most
builders didn't flunk readin' 'n 'rithmetic in school maybe this
problem is no problem for them . . . time consuming, perhaps??
