Some more construction photos from Dave Christensen, Celerity builder
We always get good feedback when we show pictures of our builders'
work. This time, feast your eyes on award-winner Dave Christensen's
Celerity wing and fuel tank construction!
|Leading edge tank-see related photos in the February 2005 newsletter|
In case you are wondering, Dave has won awards with his beautifully
constructed KR-2. It's easy to see why, when you look at the quality
of his work up close, even in a fully enclosed space such as the
inside of the wing.
|Christensen's aileron counterweight assembly|
Dave's aileron counterweight, shown above, is a modified copy of
the original-Dave's version is a bit more compact, which will save
a little weight.
Rather than going along with the plans that show the aileron
counterweight's actuator on one side of a rib and the weight on the
opposite side, Dave has brought his all together in one small area.
|Another view of Dave's counterwight and the aileron itself. The wing tip is to the left, off the picture.|
For us perfectionists, please pay close attention to the following
three items in Dave's pictures. First, note the plywood doubler on
the rear spar where the counterweight is mounted with four screws.
It's actually wedge-shaped to put the counterweight in line with
the adjoining rib! Second, please note the roller bearing that is
carrying the mass of the counterweight assembly's pivot tube. Finally,
note that even the hole Dave drilled through the rear spar for the
aileron actuator tube has been kept nice and compact. The result?
Another excellent installation, executed very nicely. And if you care
to look even closer, you can see little if any evidence of excess
glue at the corner blocking locations. (Of course, using clear
rather than white glue helps somewhat in this respect.) Note also
Dave's wing rib caps. As pointed out last month in our newsletter,
Dave is going with a slightly different wing skinning procedure, one
that he became familiar with while constructing his KR-2 and with
which he feels comfortable. Incidentally, that's the same method
that was used on the first Celerity prototype. Nice pictures, Dave!
Thanks for contributing.
Composite aircraft construction, from A to Space!
When you received your last newsletter, man had not successfully flown
around the world solo without stopping or refueling. Now, as of March
3, 2005, this "barrier" has also been broken. Aeronautical adventurer
Steve Fossett flew a small jet, the "GlobalFlyer", around the world
solo without refueling. This great achievement has put yet another
feather in the cap of Burt Rutan, GlobalFlyer's designer and builder,
who hardly needs any introduction to the rest of us. This came on the
heels of another of Rutan's creations making two successful flights
into space and safely back to earth last December in "SpaceShipOne"
and mother ship "White Knight", garnering the $10 million X-prize.
And of course by now it's almost ancient history that Dick Rutan
and Jeana Yeager flew Burt Rutan's "Voyager" around the world
without stopping or refueling over nine days back in December, 1986.
These remarkable aviation achievements once again put a relatively
quiet man back into the global spotlight, a gentleman whose notoriety
stems from his achievements covering the entire array of flight, from
homebuilts to spacecraft. Who would ever have thought, back in the
70's while watching Rutan's composite Longeze airplanes take flight,
that space flight would someday happen without the government getting
involved, using private funding, and that people would be purchasing
tickets for future space flights? A common denominator in Rutan's
achievements is the use of composite construction techniques. His
futuristic looking canard aircraft designs in the early 1970's (the
Variviggen, Varieze, and Longeze, which begat the Cozy, Velocity,
and E-Racer, among others), were all composite designs. So what
is this wonderful stuff we call "composite construction," and why
should you use this construction technique to build your airplane?
Is it really that easy to work with, and will it hold up over the
long term? The answers are; magic, it's the right thing to do, yes it
is, and so far so good. Here are some of my thoughts on composites,
along with technical information to help put you at ease about using
the stuff on your airplane.
Many times I have shown people my simple piece of one-quarter inch
thick Clark foam with fiberglass on both sides. They turn it over and
try to flex it--it's so lightweight and yet very rigid! The icing
on the cake is when I demonstrate the strength of the piece using my
stacked bricks demonstration. That's when they become believers.
At first, many onlookers give me the distinct impression that
they doubt anything so lightweight could be depended on to hold
an airplane with its fuel, occupants and baggage up in the air.
They just need to be shown how you can possibly make something so
strong out of such lightweight, dissimilar materials. (To be fair,
I must point out that most people also lack appreciation for the
fact that many airplanes have cloth skin, and that their wing ribs
are sometimes hardly thicker than the metal in a Campbells soup can!)
Not having a technical background, I imagine that many such observes
might think that "these things are made so light and flimsy, they're
probably just waiting to fall out of the sky!"
But that thin piece of foam and fiberglass is a prime example
of the proper application of materials to the intended purpose.
(i.e.--Engineering and technology say that you don't need a steel
plate to keep rain off your head--a light weight nylon umbrella
with wire ribs will do nicely.) And as Burt Rutan demonstrated, you
don't necessarily need lumber or aluminum or steel to make structural
Composite airplane skin
Both the Mirage Marathon and Celerity feature composite skin over a
wood structure. There are advantages with wood construction that we
believe are worthwhile, such as its documented strength, durability,
ease of construction, and sound deadening properties. Once we make our
wood frame structure, the next problem is deciding what to use for a
skin, fabric, more wood, or fiberglass? From a course in human anatomy
I learned that the skin is the largest single organ of the human body,
in both size and weight. Likewise, the skin of an airplane is large
and can make up a lot of the total empty weight. (For ultralights,
what's the first thing to go in order to save weight? Fuselage skin,
of course.) So the most obvious advantage of composite construction
should be realized when it is used to construct the skin. Also, the
use of composite materials for the skin allows the builder to craft
beautiful, smooth, complex shapes that are much more difficult to
achieve using the other types of aircraft covering. The big problem
covering an airplane with fiberglass is that fiberglass alone is
not a great material for airplane skin--when you make it thick
enough to achieve the desired rigidity, it becomes awfully heavy.
The solution is to combine it with a material that will make the
fiberglass skin stiffer without adding a significant weight penalty.
A composite skin is the perfect solution.
Definition of composites
Now what is composite construction exactly? My dictionary defines
"Made up of disparate or separate parts or elements;
compound...(Naut.) noting a vessel having frames of one material and
shells and decking of another, esp. one having iron or steel frames with
shells and decks planked; composite-built."
That sounds almost like a tube and fabric airplane, doesn't it? Yes,
the definition technically fits. However, in the world of experimental
aircraft construction the term "composite" has a particular meaning
that has been gained through popular usage. When people say
"composite" airplane they are usually thinking of something that is
very smooth and fast, made possible by materials such as fiberglass
and resin, a combination that yields much more strength than either
the cloth or the resin alone. Fiberglass has made that possible.
An excellent material for aircraft construction, it is relatively
inexpensive, easy to work with, and quite durable. Extensive use of
fiberglass to build airplanes has literally changed the face of the
homebuilt aircraft movement in recent years and helped foster a new
generation of builders that expects classy, hot designs. Now it's
also changing the face of commercial small airplane manufacturing
with Cirrus, Lancair's Columbia, and numerous other new entries in
the field of FAA certified composite airplanes made for the public.
Soft, weak foam makes fiberglass stronger!
Although fiberglass is a wonderful material, some more work had to be
done to make it lighter weight and more rigid, especially for large
skin panels. Eventually rigid foam came to the rescue. The use of
foam for cores was preceded by more expensive core material, which
was also difficult to manufacture. It had been common knowledge
for some time that lightweight, strong panels could be made by
bonding sheets of fiberglass or thin aluminum to the edges of a
lightweight honeycomb type core made from thin aluminum or even paper.
Unfortunately a significant problem arises in manufacturing with
honeycomb--it's difficult to bond the outer skin material to the edges
of the honeycomb. Therefore the initial application of honeycomb core
was limited to cases where the weight savings was important enough to
justify its high cost, such as expensive jets and spacecraft. As an
alternative, light weight wood was (and to a limited extent still is)
used as a core material. A modern day example is the end-grain balsa
wood core used in AeroCore composite strips. The strips are laid
over bulkheads and their edges bonded together. This way you can
make a fuselage shell rather quickly. In this application the balsa
wood grain runs perpendicular to both the length and the width of the
strips. A disadvantage with this or any other manufactured component
is the cost--you have to pay for someone to make up those specialized
core panels. And since it must be purchased in finished dimensions
and shapes, it also places limits on the builder's flexibility.
The obvious solution for those who want to hold their costs down is to
manufacture their own composite parts with foam, fiberglass and resin.
Another advantage with foam is that it can be easily shaped by the
builder to make any form desired, overcoming the problem of wet
fiberglass layups not having any shape or form. In kit airplane
factories, huge, expensive female molds are typically used over
and over again to make fuselage halves and wing panels. In making
these kits, the mold serves a tool. But with "moldless" construction
utilizing foam covered with fiberglass, the foam doubles as a form
or mold and also as a vital structural component of the piece.
Foam and fiberglass sandwich panels are inexpensive and easy to construct
Remember where I said that the skin is a very large part of an
airplane? It certainly is--figure on about 450 to 500 square feet
for either the Celerity or the Marathon! Therefore the unit cost
(price per square foot) becomes quite an important factor. The unit
weight is also significant. Adding one ounce per square foot to the
skin will increase total weight 30 pounds.
So even minor savings of weight and cost in the skin of an airplane
repays the builder with a multiplied benefit. Since there's quite
a bit of work involved, especially finishing, the labor costs add up
quickly in those kits which have a finished skin. These costs are of
course added to the sales price. Many of our builders are on a tighter
budget, and they know they can save a lot of money by "making their
own sandwiches" instead of buying a kit airplane. Even though three to
four layers of fiberglass are required, along with several gallons of
resin, the material needed to make the entire skin (including foam)
will run about $1,250. You would pay many times that amount for
premolded panels. But the real beauty of it is when you start working
with these materials, and you find out how easy it is for even a first
time builder. And don't worry, because even if you make a mistake
(you will--we all do) it's relatively simple to fix because of the
nature of the materials used, and the repairs won't cost you very much.
In a typical composite wing panel, glass cloth provides much of the
strength. The glass cloth can be any one of several materials, woven
in several different ways, and available in many weights. Glass cloth
is woven from fiberglass strands, carbon fibers, or KevlarŪ material.
Fiberglass is most commonly used in homebuilt aircraft projects. There
are two basic types of glass cloth, unidirectional and bidirectional
(or "BID"). All the fiberglass strands run in one direction with
unidirectional material, and the builder orients these strands in the
direction that tension will be applied. A few small cross-strands
hold it together during construction. Bidirectional cloth has
strands woven in both directions, and it offers many variations
such as more strands in one direction than another, satin weave
with finer strands, twill, crowfoot weave, etc. The weight (ounces
per square yard) largely determines the strength of the fiberglass.
We use 3.16 oz and 3.8 oz BID for the Celerity and Marathon. (6 oz is
used to make the fuel tanks) There are also two grades of fiberglass
cloth, "E" and "S". The E-glass cloth is somewhat less expensive,
has adequate strength, and is generally preferred over S-glass.
Carbon fiber (sometimes referred to as graphite) is used in special
applications where a very strong reinforcement is desired, such as a
sailboat mast or a wing spar. I have not worked with carbon fiber,
but I am told that it is more difficult to work with than fiberglass.
Since our airplanes are designed with the wood structure carrying the
loads, we wouldn't need carbon fiber. DuPont KevlarŪ (of bulletproof
vest fame) is very good for certain limited applications, especially
if abrasion resistance is important. It is reportedly difficult to
work with and doesn't have enough advantages to make it the material
of choice for aircraft construction.
Resin and hardener
The resin that is used to saturate glass cloth protects the fibers
and keeps them in proper orientation. Epoxy resin, the most popular
type for aircraft construction, is the only resin used for Marathon
and Celerity construction. It does not shrink over time like other
resins and is easy for the first time builder to use. The other two
resin types commonly used with fiberglass are called polyester and
vinyl ester. Polyester resin is used extensively for boat building
and some mold making. Although it's nice to work with, it tends to
shrink over time causing cracks in the paint. While vinyl ester resins
are used for some aircraft construction, but they are a chemical stew
that's complicated to mix up and use. Mixed and used properly, epoxy
resin is the superior material for homebuilt aircraft construction.
The mixing directions must be followed strictly, with the proper
amount of hardener mixed with the resin, never too much or too little
of either component. Unlike polyester resins, you cannot speed up or
slow down the hardening process by varying the amount of hardener used.
Although the resin and hardener react at room temperature, a process
of post curing at elevated temperatures increases its strength.
This can be accomplished by parking your aircraft in the sun.
(To be continued--see next issue! Ed.)
© 2010, Mirage Aircraft, Inc, Tuscon, AZ