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8.2.16 My glider never came down and flew away. What is wrong?




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

8.2.16 My glider never came down and flew away. What is wrong?

If it went in a straight line, you need to re-trim the glider
to circle as it glides. Perhaps your field was too small. Find
a larger place to fly.

If neither of these is the case, you probably just found a
thermal. Air is not static. It moves around due to uneven
heating and cooling. A hawk circling overhead, without
flapping its wings is in a thermal. When air is heated, it
rises. When air is cooled, it sinks. Whatever is in that air
goes up or down with it, be it bird, balloon, rocket, or
airplane. If the air is rising faster than the sink rate of
your model, the model will rise in the air. In general, this
is good, as it allows your model to fly much longer. It stops
being good when you lose the model!

This is a "good" problem. It means you've solved most of the
problems you've encountered, and have (had?) a pretty good
glider. Picking thermals is an art that is beyond this FAQ
(but we'll try anyway, shortly). Now we have to find a way to
get the glider back. These devices are called dethermalizers
(DT) because they are designed to get your model out of a
thermal.

This is done by transforming a good glider into a bad
glider. There are two parts to this transformation. The first
is some sort of timer, to cause the action to occur when you
choose. The second is an actuating device that de-stabilizes
the glide.

Timers come in several forms. Most common is dethermalizer
fuse. This looks more like cotton rope, and burns very slowly,
typically 1/4" per minute. By having this fuse burn a string
or rubber band, we can actuate a device in flight. Be sure to
use a snuffer tube (short piece of brass or aluminum tube the
extinguish the burning material) with the fuse, to prevent the
fuse from falling free and starting a grass file. Other more
sophisticated timers are built from small spring wound motors,
or a viscous fluid like STP or silly putty with a piston
slowly moving through the fluid

http://www.mindspring.com/~thayer5/ffpages/tips/timer/putty1.gif
http://groups.msn.com/FreeFlightOnline/interestingstuff.msnw?action=ShowPhoto&PhotoID=62
http://homepage.interaccess.com/~ndzied1/Detherm/Open.jpg
http://homepage.interaccess.com/~ndzied1/Detherm/Closed.jpg

Some are even electronic
http://www.geocities.com/rdh82000/Picoalt/index.htm.

There are many actuating devices used. The simplest is a drop
weight. Since we often need to add weight to the nose of a
glider when trimming, this weight can be dropped, with a
string going either to the tail or INSIDE wing (if you go to
the outside wing, all you will do is change the glider from a
left turn to a right turn, or vice versa). By shifting the
weight, the glider will now severely stall (tail), or spiral
(inside wing) into the ground.

The "beer can" DT was popular at MIT because of its first
step, empty a can of beer! A piece of the aluminum can is
deployed as a flap from the INSIDE (turn side) of the
fuselage. This acts as a drag break, and causes the glider to
slowly spiral down.

Often a DT consists of a flap, either on the wing or stab,
that pops up and alters the trim of a glider, causing it to
spiral dive or stall. One problem with these is that if not
set properly, they can mess up the trim of your glider,
eliminating the need for a DT in the first place.

Another problem with many DTs, especially those that produce a
stall or gentle spiral, is that in a strong thermal, they may
be insufficient to recover the model. Finally, the DT action
may bring the glider down so hard that it is damaged on
landing.

I like the pop up wing DT used on the Gold Rush (see reference
below). The entire wing is hinged, and pops up about 60
degrees. This effectively turns the entire wing into a drag
break, sending the fuselage straight down. The model lands
nose first, protecting the delicate tail from damage. A
variation of this totally cuts the wing loose, except for a
string that ties the wing to the tail. The fuselage falls like
an arrow, nose first, with the wing fluttering behind. Another
nice feature for the serious competitor is that the hinge pin
can be removed, making the model very easy to pack for
shipping.

 

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