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8.2.13 My glider shredded. What is wrong?




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

8.2.13 My glider shredded. What is wrong?

It was either not strong enough, or the motor was too
powerful. If the motor was too powerful, then the fix is
obvious: use a less powerful motor next time. Beware of cored
motors, they love to shred gliders. This includes the ignition
spike of the A10-3, B8, C5-3, and almost all composites. A few
composites, like the AeroTech/Apogee C4, D3, E6 and F10 are
designed for gliders.

Also beware of violent ejection charges on fixed pod
models. They can rip the pod off the fuselage. Some
reinforcing across the fuselage, pylon, and pod will fix this
problem. Also refer back to the discussion of grain direction
on pod pylons.

Sometimes the solution is as simple as ballasting the
pod. Gliders are often under optimum weight, so adding weight
to the pod slows the boost, increases the coast, can increase
the final altitude, and is dropped off before glide. I often
crimp a few fishing split shot to the shock cord to add weight
to the pod.

There are several things that can be done to strengthen
gliders. Spruce is often used for the fuselage to increase its
strength, but at a significant weight penalty. Wings can be
made of thicker wood, although this increases the weight of
the glider. When trying to maximize performance, it becomes
important to select the density of the balsa used in your
glider. Lighter wood (6#/ft^3) will save weight, while denser
balsa (10#/ft^3) is stronger. Use the lighter wood for wings
and stabs, the denser for fuselages, which is still lighter
than spruce.

You also need to consider the grain of the balsa. "A" grain
wood has the grain running perpendicular to the surface. It is
very flexible. It is not a good choice for wings, but is
excellent for sheeting built up surfaces, or rolling balsa
tubes. "C" grain wood has the grain running parallel to the
surface. It has a mottled appearance, and is very stiff. It is
ideal for wings and stabs. "B" grain is between A and C, and
should be used where stiffness is not an issue, such as
fuselages.

The SIG catalog is an excellent reference on the subject of
balsa density and grain. The balsa information from an old SIG
catalog can be found on the web at
http://www.mentornet.org/balsa3.htm.

Higher aspect ratio wings are weaker than low aspect ratio
wings. Try redesigning your wing or tail to lower the aspect
ratio.

An excellent way to strengthen balsa without adding much
weight is to tissue the glider wings. See below.

Two other ways to make lighter wings particularly on large
gliders are built up construction, and foam cores. A wing can
be built of balsa strips, and covered with tissue or
Monokote. This can yield a very strong but lightweight
wing. Foam is commonly used in RC models, and can be used in
some of the larger gliders (C-D and up) covered with
fiberglass or tissue. Uncovered foam from meat trays can be
used for some mini-motor designs. These techniques are beyond
the scope of this FAQ.

The leading edge of a wing is prone to nicks and dings from
running into things. This can be reinforced with a thin strip
of spruce, or a thin piece of nylon or Kevlar line glued along
the edge.

For the ultimate in strength and low weight, all parts of a
glider can be reinforced with composites like fiberglass,
carbon fiber or Kevlar. This is applied either with Amberoid
or an Epoxy resin.

I learned about composites from a book by Lambie, another by
Rutan (yes, THAT Rutan), and from the MIT folks including Mark
Drela. Mark set several indoor HLG records, worked on the
Deadaelus and other human powered vehicles, and lots of other
stuff that used balsa.

Mark's Upstart-4 can be found in the January 1981 issue of
Model Aviation on page 52. It was probably one of the first
IHLGs to feature carbon fibre.

 

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