Mark Drela's Supra 3.4m
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Home > Articles & Tips Index > Mark Drela's Supra 3.4m TD/F3J  Sailplane

Courtesy of Mark Drela, [ drela "at" mit.edu ], July 2004 - web presentation by Tomer Jackman,[Jackman_Tomer "at" EMC.com] Photos courtesy of Jeff Newcum


Supra 3.4m TD/F3J Sailplane

The Supra 3.4m TD/F3J Sailplane

Construction notes are included in the drawings, which you can download as Acrobat v6 PDF documents in the links below. If you need a copy of the Adobe Acrobat Reader, you can download it free from http://www.adobe.com/products/acrobat/readstep.html.


Introduction

The Supra wing is a slight modification of the Aegea wing. The sweep has been eliminated, mainly to reduce the flaps-down launch torsional loads by a factor of 3. This greatly reduces the amount of washout twist during launch, giving a better spanwise load distribution. The wing will still bend mightily, but this has relatively little effect, since the twist is almost nil. Besides the unswept planform, I've tried a number of additional new ideas on this ship:

  • Following the SuperGee philosophy, the fuselage pod is minimal, and the wing is on a pylon mount. I think it works, because the glider is amazingly quiet in a fast flyby. Surely a good sign. I may have overdone it on the pod size -- the radio installation was rather difficult. A 5% larger pod would be much more practical.
  •  The Supra's EDA of 6 degrees is typical on DLGs, but is quite large for this type of glider. The idea was to give it approximately neutral spiral stability at moderate glide speeds. This makes it extremely easy to fly precise thermal circles with little pilot workload. Once a moderate-bank circle was established, and I fed in the usual up-elevator trim, I could take my hands off the TX for almost a minute with the glider holding the circle nicely. Like with a DLG, using mainly the rudder for thermal circle adjustments works very well. It remains to be seen whether the large dihedral impairs landing precision in gusts.
  •  I also tried hard to get the extremities light. I tapered the tip spars in width and used 1.0 oz Kevlar on the tails and wingtips. Seems plenty sturdy on the Hi-load core.
  • All four wing servos have RDS drive on them . I used large-diameter hypo tubing for the shafts rather than the commercial solid wire shafts. The linkage is amazingly tight -- tighter than is possible with horns and pushrods I think. It also eliminates the need for electrical connectors across the outer wing joint. The drawback is that it's considerably more work than a horn setup, but not too bad I think.
  • Integral-bagged hinges. This is an adaptation of a hinge technique that Ib Jensen posted on Ezone a while back. The cores have the hingelines cut, faced, and wrapped with light glass before bagging. The main advantage is that the hinge is incredibly strong, since it no longer relies on the poor peel strength of the skin on the foam. Instead, the skin at the hinge ends up bonded to the light glass wrap after bagging. See the hinges.pdf sheet for details. If you want to try this technique, I suggest practicing on the vertical tail first.
  • The joiners are carbon rods in Kevlar tubes, buried in endgrain basswood and hard endgrain balsa between the spacaps. There's also light endgrain balsa for a few inches on each side of the centerline, to withstand potentially large pressure from the wing saddle. Foam everywhere else. I should point out that Hi-load 60 or Spyder are barely adequate for a spar core. Softer foams are not. If using lighter foam, it's also sufficient to use balsa only on the spar sides, since that's where the glass wrap is applying its load. For example, a 3/4" wide spar could have a light foam core 1/2" wide, with 1/8" endgrain balsa sides. The bond between the foam and balsa is unstressed, and could be made with 3M-77. Seems easier than a solid balsa spar core.
  • I also made a new V-mount mold for this larger boom, which is 5/8" o.d. at the stab location.
     

Plans and Info Sheets

Supra plans (PDF / DXF)
Wing plans (PDF / DXF)
Wrapped spar construction (PDF / DXF)
Integrated hinge construction (PDF / DXF)
Bends (PDF)
Horizontal stab plans (PDF / DXF)
Fin plans (PDF / DXF )
Fuselage pod plans (PDF / DXF)
Flap RDS (PDF / DXF)
Aileron RDS (PDF / DXF)
Alternative control horns (PDF / DXF)
Making kevlar tubes for joiners. (PDF)
Making endgrain-basswood joiner blocks. (PDF)
Kevlar piece layout for minimum waste. (PDF)
Alternative flap RDS drive with larger deflection. (PDF)
CG and control-throw setup. (TXT)
Program for the Royal Evo. (TXT)

Wing

Curved wingtip outlines : I sand the cores at the tip. On the Supra I also sanded away a little bit of the corner in the LE at the mid/tip section joint, so the whole outer panel looks like it has a moldie-like curved planform. I removed less than 1/16" of the corner, which is enough to give the curved-LE illusion. Restoring the airfoil is easy. You just match the shape of the core on both sides of the modification. Viewing the surface with a compact shallow light clearly shows the shape.

Achieving an accurate leading edge : Basically the same method as Phil Barnes. The main difference is that I feathered the edge of the Mylar so that it follows the airfoil
closer to the LE. I also iron the LE strip after application with a 170F iron. This reactivates and very firmly bonds the dried 3M-77 so there's no risk of detachment.

RDS pocket : A place for the pocket is cut out before the core is bagged, but the foam is left in it's place until the bagging is complete. Then the foam is removed, and the pocket is potted in place..

 

Spar

I used the foam piece cut from the wing core as the spar core. The numbers indicate that Hi-load 60 is rather marginal in compressive strength in this application, but only if we assume that the spar wrap has no compressive load capability. But the wrap surely will carry some compressive load, as will the surrounding wing foam, so the foam spar core is probably OK. Hasn't failed yet in Tom's wings. It's certainly easier and lighter than endgrain balsa. I wouldn't try this with any softer foam, though. It's prudent to use endgrain balsa in the spar center over the wing saddle.

From an Allegro-lite post (Warren M.) : "...Carbon sleeves (sic) also works well. I have been using them for years and results in a stiffer spar. At joiner boxes, I still use additional kevlar tow wrap as insurance. The sleeve is easier to apply, note that it necks down well but does not expand when choosing a size. The 3K sleeve is the equivalent of 8 oz fabric so it is heavier than two wraps of 3 oz fabric..."

The CF sleeve is indeed a good alternative to the Fiber Glass wrap. It is somewhat simpler to use than wrapping with glass. Also, the large 3K CF tows have considerable compression load capability, and probably offload the hard-pushed foam spar core better than the glass, but that's just a guess. Tom Kiesling used CF sleeve on his Supra spar.

The CF sleeve is best suited for the center panel, since it's relatively heavy and would be massive overkill for the tip panels. So if you wish to use CF sleeve, I recommend using it only for the center panel spar. For the tip spars, I would stick with the 1.5oz glass, and add the
CF sleeve only over the short joiner portion.

 

Horizontal tail (stab)

Vertical tail (fim)

The vertical tail has no sub-fin. This is an F3J requirement that the only thing protruding from the bottom of the fuselage is the tow hook. The boom has been designed to take the twisting moments generated by such a tail.

Fuselage pod and wing pylon

 Because of the slender pylon, ballast is installed via the front radio bay. The wing bolt passes through a hole in the ballast slug.

Core Specs

Use "Hi-load 60" for all cores.

Wing cores

The 2.4 oz CF carbon in the wing layup, could be replaced with 1.7 oz Kevlar for TD flying, since this is somewhat less violent than F3J.

Pannel Root airfoil Root chord Twist Tip airfoil Tip chord Twist span sweep (dxLE) skin thickness
1 AG40d(-2) 9.75" 
247.7 mm
0 AG41d(-2) 8.75" 
222.3 mm
0 31.5"
800.1 mm
0.25" 
6.4 mm
0.003" 
0.08 mm
2 AG41d(-2) 8.75" 
222.3 mm
0 AG42d(-2) 6.25" 
158.8 mm
-0.5 deg 23.5" 
596.9 mm
1.00" 
25.4 mm
0.003" 
0.08 mm
3 AG42d(-2) 6.25" 
158.8 mm
-0.5 deg AG43d(-2) 3.75"  
95.3 mm
-0.5 deg 12.0" 
304.8 mm
1.438"
36.5 mm
0.003" 
0.08 mm

Horizontal tail cores

Root airfoil Root chord Tip airfoil Tip chord span sweep (dxLE) skin thickness
HT14t* 4.5" 
114.3 mm
HT12 2.0"  
50.8 mm
13.0" 
330.2 mm
1.00" 
25.4 mm
0.002" 
0.05 mm

* HT14t is HT14 thickened to 8.0%

Vertical tail cores

Root airfoil Root chord Tip airfoil Tip chord span sweep (dxLE) skin thickness
HT13* 8.0" 
203.2 mm
HT12

3.5"  
88.9 mm

13.0" 
330.2 mm
1.75" 
44.5 mm
0.002" 
0.05 mm

*HT13 is HT14 thinned to 6.5%

Boom

A very useful addition is 5/16-16 threaded hole in the big end. This
lets me pop the boom off the mandrel with bolt and collar. Aluminum mandrel. Diameters are:
 

x D
0" 0.875"
2" 0.875"
48" 0.475"

The part from 2" to 48" is a straight taper. The Supra boom runs from 1" to 42". It's made from 150 g/m^2 prepreg I got from CST. 5 layers at big end, dropping to 4 layers at 22", and then 3 layers at 32". The two innermost full-length layers are +/-20 at the big end, and steepen to +/-35 at the small end. These give hoop and torsional stiffness. Prepreg is flat CF about 0.005" thick which already has heat-curing epoxy in it. Cure is either at 250F or 350F, depending on type of epoxy. The 250F is much more convenient for hobbyists, since a 250F oven box can be made from almost anything. It must be stored in a deep freezer, since the epoxy slowly hardens at room temperature. It comes with a nonstick paper backing. At room temperature the epoxy is the consistency of taffy. It makes the prepreg tacky at room temperature, sort of like Post-It adhesive, which makes it very easy to work with. There no wet-layup mess. You just apply it and cook it. But it absolutely must be compacted somehow during cure. I roll it onto the mandrel one layer at a time. Then I do a spiral wrap of peel-ply, then absorbent paper, then I wrap tightly with heatshrink tape. Then I place it in a 250F oven. First the outside heatshrink tape tightens, then the epoxy liquifies and the excess soaks into the paper as the carbon fibers get compacted, then the
epoxy gels and finally solidifies. It's important not to shrink the tape with a heat gun, because the epoxy will gel too early, so the excess will not bleed out.
 

At the field

Video: Mark hand launching his Supra - Mark  winch launching his Supra - Mark landing his Supra



If you are interested...

Q: If one wanted to make a 120-inch variant of the Supra wing, could you offer any suggestions regarding where and how you'd decrease the panel span lengths?
A: I'd multiply all span wise positions by 120/134, and round off to nearest inch or 1/2 inch for convenience.

Q: Could you please send me the Supra graphic that you used on your wing?
A: It's not a graphic in the usual sense. It's simply the word "Supra" in one of the curvy Illustrator fonts. I printed it out on plain paper and cut out the letters with a #11 knife to make a stencil mask. I then airbrushed over the mask onto the mylar with just the red and blue tip colors, blending them in the middle to make the purple.


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