The Cobra Calypso
Kit Review, March 1999
Those of you reading this probably are thinking, why would somebody want a plane like this? When I was bike racing, there was a saying, no bike shop should sell bicycles that cost more than $2000, if you need a bike of this caliber, get your sponsor to buy it for you. During the summers, I used to put more miles on my bike than my car, and I was self sponsored, so was the bike justified? But I digress.
No I don't fly F3B (anymore), Canada does not have a team; the plane is not ideal for F3J (I got the carbon version), and there's no F3F races within 9000 miles. I wanted this plane for fast flying on the bigger slopes and occasional thermal flying in windy conditions. Two reasons are the rationalization for acquiring this plane, I wanted to find out how much an improvement in performance is available by using the latest technology, and would this translate into more enjoyable flying. When you think about how much effort goes into the design and fabrication of this model, it's surprisingly cheap. In my line of work it costs more than $2000 to get a one page form letter out of your company's auditor.
The two grand is better spent on this plane, Get yours from Model Technology. I waited 3 months for one, in the middle of winter. It's white on top and a weird burnt orange color (which I requested) on the bottom. Very elegant looking.
The fuselage is narrow, and everything goes in a line. Pushrods are carbon shafts already installed. Servo placement must be exact, but is not a problem. My V tails had a fair amount of slop on the joiners, and the left stab was not aligned perfectly. These situations were solved by filling and drilling the alignment hole in the fuse, and then giving a coat of thick CA to the joiner pins until tight.
Wing servo installation is very tight in the thin wings. Internal pushrods and extremely short control horns means that the installation needs to be planned, tested, and revised until the right combination if servo placement and travel gets the job done without binding. This took more than 10 hours to do before it all worked, we'll see if it is worth it. The setup sheet gives all the control surface deflections for launch, speed/distance and thermal flying. The wipers on the ailerons were very tight, and full deflection up was not possible since the ailerons are hinged on the bottom. I ended up having to fix this by bending the top wiper back. The aileron was jammed into a full up position, about 20 degrees (allowing for springback), and then boiling water was applied to the wiper with a paintbrush. This was repeated until the aileron had some clearance, then the wiper was allowed to cool while still in the full up position. This gained about 5 degrees more up travel before binding. Flap travel is limited by the top control horn location, there's simply no way to get more than 50 degrees of flap motion, and I'm not willing to grind out the flap.
A two piece wing mounted on a pylon is beyond my ability to construct. The mounting setup consists of brass alignment pins, an ENORMOUS rectangular CF joiner rod, and inverted hold down screws. Very clean and ingenious. My plane takes 5 ounces of lead to balance at the rearmost recommended CG. Given that a molded plane is built to such exacting tolerances, surely the structure can be fabricated such that minimal dead weight in the nose is required! Final weight is 81 ounces, exactly the lowest listed weight on the spec sheets, no chance whatsoever to save weight, unless you are capable of a design and rebuild. Maybe WES Technik will develop a 3g servo with 50 oz-in of torque.
Ballast consists of melting lead into the provided brass tube, and cutting it into 1" long pieces. Maximum ballasted weight is 121 ounces. Should be a lethal weapon at that wing loading.
Testing was first done at the Cochrane slope, I got the ailerons hooked up backwards and it went straight in at about 50 mph. Just Kidding!!! First flight on these kits is a non-event compared to an original design. Build it straight, balance it and it's got to fly right. The wind was a smooth 15 mph and 4 degrees Celsius (40 F). I had given it a few hand tosses in the yard so I knew the elevator is OK. The wing has a fully movable trailing edge, there are no reference points for alignment of the ailerons and flaps. A handy dandy incidence gauge can be made from the printed template, I used sheet aluminum.
Climb out is dead straight, hey that's a relief. Take it up another few hundred feet and let 'er rip. Whoa, the elevator is too sensitive, better fix that. Check roll response, good at minimal deflections, even at low speed. Rolls are non-axial, kind of like a barrel roll, but no proverse yaw at low speed? Hmmm, I'm going to have to work on that one. Plane continues to tuck at high speed, so the CG is too far back for me. Inverted and the plane almost stalls, no wonder it's tucking in. Looking at the adjustable towhook range, and it doesn't look like you can get the hook all the way under the CG this far back. Camber is set to double the recommended amount (on the slider so I can fiddle around to find the best thermal setup) and the speed is still very high. Thermal turns are grooved at the recommended settings, but the speed range of the plane is way up there, even faster than the Diamond. The plane makes a high pitched whistle at speed, I don't know where it comes from since the entire plane is so clean, maybe the tail boom opening which is just blunt and open. Bank and crank is fun, will need to increase the elevator to camber mixing in speed mode to tighten up the turn.
Time to come in for a landing, set up high above the rotor and drop the flaps, plane tucks in and speeds up. No good. Adjust the down elevator compensation and try it again, much better. I know the landings would be a whole lot easier if the flaps could droop 80 degrees. Since this will not be a precision landing TD ship, I guess that I will live with the high landing speed. Roll response is SLOOOW with the flaps down, I'll have to work up a flap mix program to improve the roll control on final.
Off to the thermal field, after moving the CG forward 1/10". No problems on the winch, can't control the thermal turning radius at all. I look like a rank beginner doing semi wingovers while still going up. Adding another 1/4 ounce of nose weight smoothes the thermal turns out a lot, and the glide is less sinusoidal. The final CG is in the middle of the recommended range, but about 2mm behind the recommended thermal location, 3mm forward of the speed/distance location. Rudder coupling has been cut in half, now the turn entry is more axial. All I can say is fly this plane fast, fast, fast and it does fine. Even the F3B Diamond is easier to slow down and core a small thermal.
Control authority of the tail in normal flight seems adequate so perhaps the tail is just too small, particularly for the high moment of inertia of the wings (yaw axis), the wings weigh over 24 ounces each with servos and wiring.
Update Mar 18, '99. - I've been to the slope three days in a row to fly for a few hours, and the CG is pretty much OK at 2mm behind the recommended thermal position. In the dive, the plane will pull out and inverted requires noticeable amounts of down elevator. I can get the plane to do a tremendous flat spin using full up elevator and left aileron. Spin recovery requires at least 50 feet of elevation. The extensive yaw oscillations during recovery indicate to me that the heavy wings are overpowering the tail.
A few hours have been spent flying with 40 ounces of ballast and the tracking is smooth at full speed in racing style passes. It's very easy to fly at speed since I like a little bit of pullout after the full bank turns. Rolls are not axial unless the switch is in speed mode, less differential and no rudder coupling. Low speed handling in normal mode with the recommended settings is acceptable.
Update April 11, 1999. I've now spent about 6 hours more slope time and 4 hours more thermal time on the plane. The roll response at slow speed is not as fast as I like, and the aileron throw can't be increased beyond 15 degree of up. I've added exponential to the elevator so that the sensitivity is more to my liking, and you won't run out of travel when the flaps are down. This plane handles the same at high speed as at low speed, a tribute to the stiffness of the structure. This is really fun when the wind is over 30 mph and fully ballasted. For experimental purposes, a 4 square inch sub fin was added to see how the handling changed. High speed flight was unaffected, low speed handling was worse. So the fin area is the right size, rudder coupling will have to be fiddled to get the turn initiation profile I like. There is currently no rudder differential in the mix.
I've also moved the towhook forward until there is no chance of stalling it at full launch power. The launch settings are right on the recommended deflections, though there is still 90% differential on the ailerons. A more consistent launch rotation is achieved at less than max line tension to start. When the hook was back, I could throw it more vertically and still get a smooth transition. Zooms are snappy, vertical until the last possible moment and flick it over.
Thermalling camber is increased to 3.5 degrees of droop all the way across, but reduce the camber while circling to the minimum amount necessary to hold the desired turn radius. Snap flap can be switched in, but that doesn't make thermalling any easier for me. Examining the polar for the MH32 shows little increase in drag across a wide range of acceptable lift coefficients, and the plane seems to keep its sink rate reasonable at very high bank angles. The plane has frequently thermalled out at bank angles of 45 degrees and higher flying speeds, when other planes would probably have not. Remaining cored in a thermal is more difficult since the plane responses are more difficult to observe.
This is really a top design, as an engineer I am still amazed at the quality and design of the structure. Radio installation is the tightest and most finicky I've ever done, it took half an hour to feed the wiring leads from the nose to the wing pylon. You only have to do it once, so I guess it's not really that bad.
I would say that it's certainly too much plane for just hacking around, kind of like using a Formula 1 car to go to the mailbox. Not that you wouldn't enjoy the ride, just not the perfect tool. The performance difference between top designs is immeasurable until the last 5% of the envelope is explored, and normal flying just doesn't approach the edge of the envelope. I can't comment on F3B task performance since I don't fly F3B, and that's where the maximum performance is to be extracted.
Flight performance will be a matter of getting used to it and doing some direct thermalling comparisons against my other planes.
I've now flown the plane for a year, and a lot of the handling quirks have been taken out by adjusting the CG 2mm forward of the recommended Speed position, increasing the aileron differential by 2 degrees, and most importantly mixing in over 60% rudder differential. Now, all the turn entries and full rolls are completely axial. On tow, rudder input does not stall the plane, and tight thermal turns do not drop out. I originally went with the settings listed in the instructions, and boy they are so far off how I want a plane to fly it's a joke.
The handling improvement has changed my perception of the flight envelope so much, the servos appear to be the limiting factor in getting the plane to go where you want. To fix this, all of the Hitec servos have been replaced with Multiplex MC/V2 servos, FL on the ailerons, Micro on the flaps, and Micro Speed on the V-tail. I have also sanded off more of the aileron wipers to reduce the servo load at full deflection. Elevator response should be much crisper, and I'm running 50% exponential.
I've also gone to Lithium Metal batteries, which provide 6 volts on two cells, alleviating the extremely cramped radio tray. The 6 V supply should improve servo speed and torque as well.
With all of these changes, the plane is steady as a rock on tow and is docile enough to thermal out from hand launch height. I've also been practicing flying a long long way downwind to test the limits of upwind penetration and stability. Suffice to say this plane will return when everyone but the pilot has given up.
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