Wright Stuff Plane Won't Recover

From: calgoddard <calgoddard_at_yahoo.com>
Date: Tue, 20 Jan 2009 22:36:25 -0000

Hi all:

I have coached Wright Stuff in Science Olympiad for several years.
Many of you have helped me to become a novice expert, at least at the
Wright Stuff level. Thank you.

I need some input.

I have a team of sixth graders that have built a decent Wright Stuff
plane. It is near minimum weight. At first it had motor stick
bending issues, but those have been completely solved with the
addition of a truss, similar to that sometimes used on F1D planes
only made of balsa tension members instead of wire.

Then the plane would "mush" or literally descend in a flat horizontal
attidue five feet during the descent phase of the flight, but only
after a nice climb and cruise. This was solved by moving the CG
forward.

The final problem happens when the plane has a head on collision. For
example, if it hits a beam or a curtain it momentarily stops and
dives like all indoor free flight rubber powered planes, but instead
of recovering and resuming flight in a nice level attitude, it
continues to rapidly descent at an angle of about thirty degrees,
until it crashes. Two bent prop shafts have resulted.

The main wing is 14 cm in chord by 40 cm in span, which comes out to
about a 35% aspect ratio. The inside wing panel is slightly longer
and the wing has about 1/8 inch wash in on the inboard trailing
edge. I estimate that the CG is about 2 3/4 inches forward of the
trailing edge of the main wing. Camber is about 6% on the ribs. The
wing has flat tip plates, which are pretty large. So the main section
of the wing has straight LE and TE spars and the tip plates are
vertical. My seventh graders (second Wright Stuff team I am
coaching) are getting very good times with their planes of similar
design, with none of the problems described in this post. They were
smart enough to build two planes.

My guess is that failure of the sixth grader's plane to recover is
due to the leading edge spar not being stiff enough, and that once
the plane is nose down, the main wing changes shape so that all lift
is lost. These are just sixth graders so the leading edge is made of
square balsa and not some fancy cross-section. The LE spar is 40 cm
long and is made of 1/16" square balsa. I seem to recall that this
spar weighed about 0.30 grams. It did not seem too floppy when
selected. Total weight of the wing, including tooth pick (bass wood)
wing posts (hard for the sixth graders to break them), is
approximately 1.70 grams.

My proposed fix it to glue a 1/8" by 1/16" by 40 cm piece with a
triangular cross-section to the leading edge to reinforce the same
and provide a better airfoil cross section at the same time. This
piece weighs about 0.22 grams and seems relatively stiff, or at least
as stiff as the existing leading edge spar to which it will be glued.
They can switch to a lighter plastic prop which has worked well in
the past, and still be around 7.0 grams, which is the minimum weight
allowed under the 2009 Wright Stuff rules.

Do you have any thoughts about what may be causing the dive and
failure to recover, and any proposed fixes? There probably isn't
enough time before the Wright Stuff competition for the sixth graders
to build a new wing. Like almost all Wright Stuff teams, we have
extremely limited access to the gym for flying. It seems like there
is basketball 24/7.

Thanks in advance for your suggestions.
Received on Tue Jan 20 2009 - 14:36:27 CET