Bill;
This calculation gives you the minimum spar cap size and the maximum
deflection at the maximum load. The stress in the spar caps (sigma) will be
reduced in proportion to any increase in the cross section area of the spar
caps. Therefore, if you double the size of the spar caps the stress and the
tip deflection go to half the previous value. Then you can stand on the
winch without any problems.
Matt Gewain
-----Original Message-----
From: Bill Harris [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, August 08, 2001 8:57 AM
To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
Subject: [RCSE] spar-sizing calculations
I'm building a Windfree nostalgia sailplane with strengthened wings, and
utilizing a balsa-core carbon spar construction with the built-up wing, per
the Allegro-Lite.
Using Mark Drela'a "spar sizing" suggestions at:
http://www.charlesriverrc.org/articles/allegro2m/spar_sizing.txt
I get a spar that uses .042 and .028 single tapered (to .007) CF laminate
with a spar width of .25" and a spar height of .60" with a .25" steel
wingrod. This should withstand a "reasonable" winch load for this class of
sailplane, as well as be very light. In a way, this is a similar to the
"de-rated" spar used on the Allegro-Light-Electric.
All looks good until I get to the estimated tip deflection calculation.
Yikes!!! It comes out to be 28 inches! Not much I can do here... the only
variable I could work with is the spar height, and that can't change.
What does this "tip deflection" mean? Under what conditions does it occur?
>From the formula d= b^2 sigma / (4Eh), there is no input for wing/winch
load, so I assume that this is the maximum deflection that the _spar_ will
take before failure. I'm guessing that the D-tube sheeting and LE/TE might
also fail before this maximum spar deflection is reached.
It may not matter, but I'm picking at the details on this building
exercise...
--Bill
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