Design Considerations for the Carbon Dragon


Wondering why the airfoil was designed the way it was?  Thinking about changing a part of the design?  I wouldn't dream of changing the airfoil unless my name was spelled, "Roncz".  On the other hand, parts of the builder's manual leave a lot of decisions to the builder.  This webpage provides the limited insights I have discovered regarding the various design parameters of the Carbon Dragon.  Hopefully, knowledgeable readers who see this page will Email more insights to me (This email address is being protected from spambots. You need JavaScript enabled to view it.).  It should be noted that most of the individual parts for the Carbon Dragon are easy to build while the whole may appear daunting at first.  Mostly, the Carbon Dragon is an over-sized model airplane project with carbon and Kevlar added in keys.  (Model airplane builders are now using carbon also.)


Horizontal Stabilizer:
At least two builders are redesigning the horizontal stabilizer, H Stab.  They are building a rectangular section with symmetrical rib patterns.  This decision eliminates several design features built into the Carbon Dragon H Stab.

Wash-In and Boundary Layer:   In a letter to me, Jonathan Pitt points out, "The elevator [D-tube?] is oversized in order to reattach the boundary-layer at the hinge line. It is just another bit of aerodynamic tweaking that Irv Culver added to make a great flying sailplane.  Another point to look at is the small wash-in built into the horizontal stabilizer.  This wash-in helps to self-trim the stick forces as the airspeed increases ... but it can be a trouble spot when you attach the D-tube skin."  If you carefully draw a nose-to-trailing-edge line on each airfoil and carefully analyze the amount of camber above and below this line ... you will clearly see a built-in twist to the H Stab.  The over size D-tube is an old designers trick even used on the leading edge of aircraft doors.  In the latter case, the front edge of the door should stick out slightly, be perfectly flush ... but never receding.

Down Forces and Angle of Incidence:  Builder, Steve Arndt, points out that the sophisticated inverted airfoil shape provides a positive pitch function even though the horizontal stabilizer is installed with zero incidence.  Steve says, "By using an inverted airfoil, the CD is able to attain the pitch positive function of the stab/elevator with a minimum of parasitic drag.  A symmetrical airfoil would have to be pitched downward to accomplish the same effect and would generate much more drag."  He further warns builders  to install the H Stab parallel to a line through the center of the Tail Boom, not the top edge.  [Build it to the plans folks! ssa]

Double Taper:  The long, double-taper of the H Stab will create less induced drag.  Induced drag is the greatest at slow speed (whereas, parasitic drag is greatest at high speed).  Since the Carbon Dragon is designed to exploit micro-lift while flying at slow speeds, reducing induced drag is important. 



Wash-out:  The double-tapered wing design has a separate design for each of the 13 airfoil sections for the wing.  One of the prime features of this design is a built-in wash-out (as contrast to wash-in on the H Stab).  Wash-out is required to ensure the roots section of the wing stalls before the wing tips ensuring aileron control at the point of stall.  Also, I have noted that the wing-tip airfoils have a thicker cross-section than the roots; perhaps, to prevent the slower flying wing tip from stalling when thermalling at slow speeds with a very long wing.

Carbon Spar Caps Compression Strength and Root Fitting Shear-Out:  While one can find Tensile strength numbers for the carbon tows (yarn) in the 470,000 psi range (Aircraft Spruce Catalog), the compression strength is considerable less.  The values achievable depend on your ability to lay the carbon yarn in straight lines.  This can be achieved by pulling on both ends of the yarn while the epoxy sets ... but not in the spar cap since most of the carbon tows end inside the spar cap box.  So to be conservative, according to Gene Sandberg, Culver used 90,000 psi  in compression (page 38, Carbon Dragon builder's manual contains a Hercules table of typical tow properties ... don't use the numbers).  Builders are now using Carbon Rods in spar caps.  Drilling holes for attachment of the Spar root fittings may totally sever some of the carbon rods (depending on placement of the rods relative to hole locations).  This raises the question of sufficient strength to avoid Shear-Out or pull-out of the root fittings when under high G forces.  This concern is shared with the long wing Windrose, the Genesis and other sailplanes.  Carbon Tows or Yarn may interweave enough to provide addition support against shear-out.  If you use carbon rods ... test.  When proven methods for use of Carbon Rods are known, I will publish them herein.


More to come ......

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