The Fiat
The Fiat is what I've decided to call this thing, since it will be at my command. The design objective was to develop a larger competition glider which has a speed range greater than the Emerald, but less than a F3B plane. What I was willing to give up are minimum sink rate and a brutally tight thermal turning radius. I have owned and flown the Thermal Eagle and the Diamond, which I found too difficult to thermal in light air, low wind conditions. The Bird of Prey is just too slow for windy conditions and the Emerald is not quite quick enough.
The ugly ugly paint job on the center panel is a mottled grey/white, as a result of not letting the epoxy paint cure on the mylar before the glass was laid. Only sporadic blotches of paint stuck to the wing, the rest remaining on the mylar.
Construction notes:
The wing is carbon uniweb over pink (26 oz/cu ft) foam with a big ol' spar in the center panel. Bias cut 3 ounce fiberglass over the CF makes the wing torsionally rigid. There's a 12" long by 5/8" diameter CF rod to take the center panel load. The tip panels have a sub spar to hold the joiner tube. The weak link in the wing is a 3/8" CF rod used as the tip panel plug ins, though it is still good for 200 lbs. of line tension.
The fuselage is a butchered from a Open Class RG15 Super V, though I don't know why it is called a Super V when it has a fin. Mark Levoe is still supplying them with a little notice. I hacked the wing fairing off the top like a convertible and laid in a wing saddle with a center bolt (3/16" steel). Yeah Yeah, the stress concentration factor on the notch is 3, so I laid in a CF ring and balsa brace to take the hoop stress. The main bolt screws down into the same block as the towhook so the fuse is along for the ride. A #8 nylon bolt is used at the trailing edge for wing alignment. Below is the before and after the conversion.
The two CF X's across the wing opening add stiffness. Torsionally, the fuselage is just a stiff as before once the wing is bolted into the saddle. You can also see the CF pan (CF/plywood/CF) added to the belly which holds the skeg, towhook and wing holdown bolt.
Specs
3.15 meter span, center panel is 254 mm chord X 635mm span, tapers to 187 X 736, 120 X 204. Tip panel TE sweeps forward 25 mm, though I don't know what effect this has.
Wing area - 70.9 dm2, 1100 sq in. Aspect ratio - 14
Weight - 2.25 kg (78.5 oz), for a wing loading of 10.3 oz/sq ft.
Rudder, elevator, flaps, and ailerons. Flaps and ailerons are 22% of chord.
RG15 airfoil thinned to 8.7%, transition to 1% camber over the tip panel. Used the three panel LIFTDIST.XLS to eyeball a shape.
CG at 45% of root chord.
I'm going to have a lot of fun flying this thing over the summer. On to the Next Project!
Update: Feb 8, 99: Yahoo!, it flies..I call it the Fiat. The first flight of the plane is uneventful, other than launching it into a 20 knot wind on the slope. Guessed at the control settings and it flew fine. Hard to tell what was going on since the airspeed was 25 to 60 mph all the time. No issues with penetration or wing flex in speed run turns though. It finally came out at 78 oz. with nose weight, skeg, taping and fairings. Now it's off to the winch, with the trimming procedure described in the trimming article. Final tally is about $300 in material and 110 hours of labor, germination to first flight. You design and build planes for a lot of reasons, saving money is not one of them.
Update February 11 thru 15, we are having unusually warm weather for this time of winter, the thermal field is clear of snow and the temperature is above freezing. So it's off the the thermal field with the winch to test it out. The climb is adequate for a low camber section, not as good as the SD7037 winged planes. Float is acceptable with the camber (about 4 degrees) cranked in. The thermals are strong enough to speck out, once the low speed turns were dialed in with a little less differential than what I'm used to on a plane this size. Now the extra weight is becoming apparent, at 8 ounces lighter the handing would improve. The next try will get the weight into the target range, and the RG15 camber can be reduced proportionately. Flaps need to be slightly bigger to slow down more for landing, next time they will be increased to 25%, ailerons response in fine. The plane has flown in calm to 20 mph winds without ballast, so the speed range is acceptable.
Update April 12, 1999. I'm putting more time into thermal flying this plane than any other, getting used to the responses to lift. The extra wing area seems to help in the wiggles and bumps when flying into lift. What is more interesting is the clear responses to sink, the tail just drops and the plane crawls, telling you to get out of there now!
Other than that, not much to report, the design parameters are no great departures from the accepted norms of TD design, so there isn't going to be any sudden leap forward in performance. I'm happy that it flies acceptably, has no quirks, and is unique.
November 28, 1999. Now that building season is here, I'm taking a shot at a more engineered center panel, to lighten the plane up. Lots of tail volume so I will stretch the span of the center panel out to 60", for a total span of 3.404 m.
October 1, 2000. The new center panel is finished and flew for the first time. A small adjustment for CG and the plane flies a little better, more float and still smooth in the thermal turns. The new center panel came in at 34.7 ounces, with servos. This is a little more than I wanted, but the spar I designed is serious overkill. The spar has 0.060" by 0.5" caps with a braided carbon sleeve for the wrap. The center 20" of the spar has a 1/2" diameter solid carbon rod in place of the shear web, the breaking strength of the spar at the midpoint is over 200 ft lbs with a safety factor of 2. The final flying weight is now 86 ounces, but it still floats well and turns crisply. And this is what it looks like now that we're done:
The aspect ratio is increased to 15 on a wing area of 1200 sq.in. Even though the weight has increased, the extra wing area lets the plane really zip up the line on the new winch. Flaps have been increased to 25% of chord, using knuckle hinges. I've also gone to internal pushrods as can be seen from the photo below, nothing hanging out and really solid linkages with no springy center.
From the underside, only partial CF layup is needed for the required strength. To the right is the fitted blue styrofoam box for the plane.
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