Cal, my guess is that the problem relates to the CG location. You said
that you moved it forward to offset the stall in the glide. I would
suggest that you move the CG back and trim the stall out with incidence
changes.
LeRoy Cordes
AMA 16974
Chicago, IL
In God We Trust
On Tue, 20 Jan 2009 22:24:19 -0000 "calgoddard" <calgoddard_at_yahoo.com>
writes:
> 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.
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>
>
LeRoy Cordes
AMA 16974
Chicago, IL
In God We Trust
____________________________________________________________
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Received on Wed Jan 21 2009 - 12:38:59 CET