I think there's a good chance your analysis is correct. The quickest way to stiffen the spar would be to glue another thin piece of wood to the bottom of the spar. If I remember correctly the bending strength goes up by either the 3rd or 4th power of the depth. Watch the TE also. A too weak trailing edge can flare upward causing the LE to tuck under. I like for both spars to be equal in deflection. You can measure this during construction and you can also observe it during flight.
----- Original Message -----
From: calgoddard
To: Indoor_Construction_at_yahoogroups.com
Sent: Tuesday, January 20, 2009 5:24 PM
Subject: [Indoor_Construction] plane won't recover after collision
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 descent 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.
The 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.
My guess is that failure of the 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 just square
balsa. It 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 0.30 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.
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