The Burly Badger

Badgers are feisty little creatures that make enormous holes in my yard.They deserve respect as varmint killers.

This is the project description of the latest Thermal Duration plane that I am building. There are two members of the club who are flying Psycho 3's, and I am impressed with them as far as overall versatility. I have both the Bird of Prey and the Emerald for the light air conditions, and would like something slower than the Diamond or Cobra for the all-in-one contest plane. I still think that the Diamond is still the ultimate TD plane, especially now that mine weighs in at 80 oz, so this plane is going to have to be lighter by a significan margin to be able to float. So the target weight is a paltry 68 oz, and it MUST be indestructible on the Rahm winch. I would rather be able to go all out on the winch for the big launch during a contest, rather than worrying about a wing fold. The wings were splatter painted in the bag, and for future reference the paint weighs 0.2 ozs. per wing! Krylon Appliance Epoxy is the white, and regular Krylon Epoxy for the red. The angled paint job will make the plane crab through the air. Transparent red Monokote is used on the tail

Here is the nitty gritty of the design and construction. Anyone is obviously free to copy this design since is is a hybrid of several other planes, with nothing secret in the materials, construction and design. The discussion will present the choices that I faced when selecting the final option, not wanting to pour too many hours into the project. Overall it took about 70 hours to build this thing, and once it was started, little time was wasted. The process of bagging and fitting the wings is not as bad as you might think relative to a composite kit, you save some time in the rigging by pre-installing wires, blocks and servo cutouts that are pre-sized. All the plug in wings use the same alignment and fitting procedure, which is now routine compared to the first time try.

The wing is the thing, and a wing is defined by the airfoil and planform. Planform choices were: copy the Psycho, copy the Emerald, or design a new one. Out of the design tools available to me, the weakest tools by far are the planform tools. Mixing airfoils and the effect of stall angle estimates are pretty gross assumptions in the any of the tools I've seen for free. The Moonarc has a constant airfoil and uses a blunter leading edge to delay stall at the tip. I will use this method since a transitional airfoil is too much guess and hope. So I decide to mimic the Emerald with 2" added to the span, same chord at the taper breaks with slightly larger tip chord. Part of the span increase is lost because the fuselage I am using is 3/4" narrower than the Emerald fuselage. Flaps are 2.25", ailerons taper to 2" at the tip. The flap starts 1" from the wing root to allow for taping, and the ailerons extend another 2" further at the tip compared to the Emerald.

Which airfoil? All the latest planes seem to be narrowing down to the SA 7035 and the MH 32. Probably could go with either, but let's use the SA7035 in deference to the SD7037 on the Emerald. No modification to the airfoil for the entire span and no washout, so I will have to pay attention while thermalling.

I have a new NSP Edge fuselage lying around so might as well use that. I will adjust the tail volume for the reduced length. Come to think of it, the Emerald has too much tail area anyway (based on my flying feel), so leave the tail area the same on the shorter fuse. I find the 10.0 ounce weight of the fuselage objectionable, but cannot be bothered to build my own, or buy a new one. I like the shape, rather elegant..

Item	Estimate	Actual
Wing (each)	17.5	18.3
Skin	8.1	9.1
Spar	2.9	2.6
Core	5	5.2
Wires	.8	.8
Root rib	.4	.5
pushrods	.3	.8
Fuselage	10	10
Stab w/joiners	1.4	1.5
Rudder	.3	.4
Wing rod	3.8	3.8
Radio gear	11	11
Pushrods	.5	.5
Towhook, skeg, other	1.5	1.6
Tray	1	1
Wiring and tubing	2	1.8
Nose weight	3	2.7
Total	69.5	70.9

Where can the weight be trimmed? Well, the wings are really kinda strong, so probably can lose an ounce per side, particularly the 2.75 oz glass layer. Geez, the wings are so torsionally strong it's overkill. Using the rolling paper towel method got the fabrics very dry on layup, you can see small separation of the FG/CF layer in spots. I don't think I can skimp on any of the resin. The fuselage can get down to 8 ounces, losing 2 oz. A 1/2" diameter joiner would save1.4 oz., but this would be the weak link in a launch. A smaller plywood tray would save another 0.5 oz. The design goal was a strong plane at minimum weight so this is not very far off for a bagged wing. Turn me loose on a CNC machine and I'll get a plane in the 63 ounce range.

The spar is constructed using the vac bag method described on my page. This one is 24" long each side and made for a 5/8" joiner rod with 5 degree dihedral. CF 0.060 on top and 0.030 on the bottom, it is good for at least 64 ft lbs of moment. I'm going to try Dow Hi-D 60 for the inner panels to try to increase the strength, and regular blue Dow for the outers.

The layup looks too weak at the center joiner but the wingrod and joiner tube are not in the equation, so the overall wing strength is enough. The other point that is suspect is the end of the spar, so I have laid a patch of uni-cf over the end of the spar for transition. The CF covers the entire upper surface so that the ailerons and flaps are stiff. Bias cut FG is glued into the opening cutouts and a knuckle hinge fairing will be used on the flap. After closeout of the cutout, the ailerons and flaps are very rigid torsionally. To get a more accurate leading edge (LE), I bagged a 1" wide strip of FG over the pre-shaped LE before the main skins. As the main skins were closed up, a bead of splooey (stiff microballoon mix) was spread on the LE to make sanding easier. Next time I will dye the splooey to see when the LE is approaching the right shape during final sanding. Same method was used to get a razor sharp wing tip.

The underside of the wing shows the tapered CF fabric centered on the spar/thickest point of the wing. The spar ends somewhere between the two servos.

The full flying stab design is the same as the Emerald (which dates all the way back to the Aquila), with CF reinforcement for the ribs and trailing edge. I think that I now have seven of these, and they are getting lighter and lighter. I scrimped on the glue by getting tight joints and that has saved about another 3 grams. Stab flex though the front joiner seems to affect the planes with a 1/8" CF rod, so I have now standardized on 3/16" CF rod for the front, and a short steel pin for the rear. Aluminum tubes hold the joiner rods. The fin area of the Edge fuselage is less than the Emerald, so I've increased the rudder area to compensate.

No changes from a normal installation. Shoe Goo is the best stuff I've found for glueing in the servo trays, and the plywood tray extends from the front opening to the wing LE. A crush brace for the wing LE completes the structural mods. Pushrods are the 0.070 dia. CF rods in teflon tubes, available from CST. Three EPP bulkheads hold the rods in place in the tailboom (see picture above), and they are spot Shoo Gooed in the front half. No ballast tubes for this plane, lead slugs will be put into the wing joiner tubes. Servos are all Hitec, HS205MG for the flaps, HS85MG for the ailerons, HS81 for the rudder and HS205 for the elevator. I have over 40 Hitec servos now, with no failures in normal service. A Futaba slim receiver is velcro'd onto the tray.

A few hand tosses in the yard show the plane straight and settings are OK for the first heave off the slope. I almost toss the plane into the trees when the mighty heave and a flat glide is more than the width of the yard.

A few days later and the wind is perfect for flying at the radio tower hill. Fifty degrees and a 10 mph wind, this is mild for Calgary in mid November. A good throw and its flying nicely, just a few clicks of down trim to level. Check the CG and 3/8 oz is removed for proper trim. The turns are smooth and crisp, no tip stall or quirks. Although it's hard to tell what the speed range is going to be, there is good energy retention in the high speed turns and half pipes. The max speed runs show no sign of flex, it's really solid at speeds never encountered in thermal flying. For some reason, really large amounts of elevator comp are needed on landing, and the ratio is almost linear. Now let's get a towhook an it and see how it launches.

Xmas day in Calgary, and we are having record temperatures, it's 60 F and no wind. Off to the flat field with the winch and Matt is there with his Stork. I've set the towhook right on the CG, resulting is a series of popoffs, too slow to reach the down stick. The Stylus is programmed for 20% down elevator and the plane stands on it's tail, but rotation is a little slower. Still need a little more down elevator during the launch until the zoom. Maybe the towhook is behind the CG? Some programming is still needed to get a better transition from the dive to the zoom, not enough Elevator to Camber mix in Reflex mode. It's sunny enough to get some gentle but extensive lift cycles, and the Badger floats along for as long as you want in 100 yard circles. In tighter turns the tip stall is significant when the speed falls off to nothing, but you get plenty of warning as the nose comes up. Recovery is more sideways than down, maybe the fin is too large. Anyway, looks like this is the plane I will campaign in the contests for 2000.

The rest of the day is spent setting up the Stork, which, in the past, has augured in twice on final approach. The Futaba 7UGS is not flexible enough to do everything to control this plane to the limit. I'm impressed with the styling of the plane but hate the sloppy flap linkages. Sure it's not an F3B plane, but it still bothers me. Flying-wise, the performance is incredible, my weak winch limits the launch performance, but it is certainly beautiful to watch it fly.

I've been flying this plane most of the time and like it's overall versatility. With full camber on, the float is quite good and the turning radius is tight. Penetration is respectable, there's no ballast capacity so the limit seems to be around 15 mph wind. The wing is more than strong enough for full rip launches on mono line. In the local contests it has not been a hindrance, last year I would have flown either the Diamond or Emerald in all contests. The launch problem has been corrected as the CG was moved backwards.

At the NWSS contest it went up against 2 Psychos, 2 Stratos (Strati?), a Predator and some other older designs, beating them all quite handily on the first day in marginal conditions. Didn't fly on day 2. I still prefer the Emerald for TD in light winds, but flying your own plane is a statement in itself.

Update Sept. 2002

I have had this idea for a while and it was time to put into action. There has been a lot of talk about the importance of coordinated turns in improving the efficiency of low speed thermal turns. I have never seen anyone who could consistently fly the rudder as an improvement over the computer mix of aileron differential and rudder coupling. Realize that your rudder inputs do cause drag, and unless you save at least as much drag due to sideslip of the fuselage, it's not an improvement. Therefore it is time to seek technology for the answer. The solution is to install a gyro on the yaw axis and decouple the rudder. The plane is set up with aileron differential to have good coordinated turns at low speed with minimal rudder correction required. The rudder is connected to the gyro and no aileron/rudder coupling is used. To fly the thermal, the pilot holds a constant offset on the left (rudder) stick and flies the bank angle only.

Let's see what this does. A constant input into the gyro gives the command for the plane to hold a constant yaw rate. With a low dihedral plane, the bank angle should be constant with a constant yaw rate, the plane should not try to fall out of the turn. The gyro moves the rudder as necessary to have a constant yaw, the pilot adjusts the bank angle to fly the smooth circle. Set up properly, the gyro makes only small corrections to the rudder that would be too subtle for the pilot to handle to keep a smooth yaw rate. With my GWS Piezo Gyro, the gain is set to high.

Thermalling procedure is to set up the plane in a circle holding a constant position on the left stick. A suitable bank angle is found for the circle size and the gyro moves the rudder as necessary to maintain the constant yaw rate. If you can hold the bank angle at the appropriate amount for the size of the circle, the turn will be coordinated by the gyro, who knows whether it is applying opposing or favoring rudder!

The gyro also helps on launch to keep the plane on track. I installed a gyro switch to engage the gyro correction only when camber is on (thermal mode) and on launch. The rest of the time, the rudder is coupled and the plane trimmed to fly coordinated turns at cruising speed.

I can tell you that the method works. It is easier to hold a coordinated turn at slow speeds and high (>30 degree) bank angles. Pilot workload in putting the plane into the right part of the thermal is reduced and the plane can be flown more efficiently. The gyro will not correct bad thermalling behavior of an out of trim plane. The Badger has always been a sweet plane to fly, this enhances the slow speed efficiency beyond the capability of the pilot.