Type: RC Park Flyer
Skill level: Intermediate builder; intermediate pilot
Wingspan: 44 inches
Length: 34.5 inches
Weight: 26 ounces, minus battery
Motor: E-flite Power 10 brushless outrunner
ESC: Castle Creations Phoenix-45
Propeller: APC 11 x 5.5E
Battery: Three-cell 2200 mAh LiPo
Radio: Four channels, four Hitec HS-65HB servos
Construction: Balsa, plywood
Finish: Poly-span sealed with Aero-Glass clear
The Cobra II was released as a kit in 1968 by Harold DeBolt. I built my first one in the barracks while stationed at Little Rock AFB in 1969. This is my fifth if that tells you anything. The Cobra was designed as a Formula II (slow pylon) racer and aerobatic performer with “stand-way-off” P-39 looks. It appears on the kit box in the colors of Cobra II which won the National Air Races in 1946.
It was love at first flight. The airplane, as designed, has a wonderful package of flight characteristics. For my style of flying it had one moderate and one mild shortcoming. It was designed during the transition from reeds to proportional and thus had more dihedral than necessary for use with proportional. This gave rolls the appearance of having a “wallow.”
In all versions since, I have built the wing close to flat across the top spar thus imparting a minimal dihedral. This in effect moves more of the wing area further below the thrustline so the thrustline must also be lowered to clean up the rolls. Loss of dihedral reduces rudder effectiveness, so the rudder must be enlarged also. This triad of related changes gives the Cobra the ability to perform beautiful rolls.
It is well known that double taper wings are prone to tip stall. I used to love pulling out of spins right at the ground. With the first Cobra, I once pulled out very abruptly while hitting throttle at the same time. I say once because there was no airplane left for twice. Torque and tip stall ate the airplane. My final modification is 1/8-inch wingtip washout on the 40-size aircraft.
These changes make the Cobra II a great pattern type performer with no bad habits. For the electric version, I made one other change. To counteract the instant torque and large propeller almost 3° of side thrust was added. Incidentally, I don’t think Formula II ever really took off.
Why an electric Cobra? I’ve had seven or eight scale or aerobatic electric ARFs. Most of them were structurally flimsy and designed for 3-D flying which makes them less suitable for non 3-D flyers.
I am a dedicated Vintage RC flyer. We have this great secret that we have been trying to share with the modern flying fraternity. As a rule, the old designs fly better or maybe it would be more accurate to say they fly easier. They had to—the radios of that day left a lot to be desired.
The airplanes had to be somewhat self-flying and they were since they evolved from Free Flight. Today’s jet fighters cannot fly without complex computer flight inputs called “fly by wire.” In a way our models have gone in a related direction.
The great secret in vintage models is the force arrangement. This includes wing and stab proportions and nose and tail length proportions, but in particular it is the relationship of thrustline to wing incidence and stab incidence.
The author reproduced the plans with a copy machine, then laid the copies upside down on the wood and pressed the paper copies to the wood using a hot iron. This method imprints the wood, to make cutting accurate parts easier.
The fuselage sides are matched 1/16-inch, hard-grain balsa. The square blocks pinned to the work surface helps with alignment to ensure the tail post is perpendicular.
The fuselage has no lightening holes, but it has airflow passages through the cowl blocks, bulkheads, and wing sheeting.
Small triangle-shaped balsa protects the aileron servos and EZ Link connectors during landing. Photo by Jay Smith.
The landing gear was left off to save weight and eliminate drag. Finger holes were added to the bottom of the wing to assist in launching. Smith photo.
The servo tray can be sized to fit your servos of choice. The author found the Hitec HS-65HB to be a good match. Smith photo.
The author poses with the e-Cobra. Polyspan covering provides a glimpse of the beautiful structure underneath, while the painted stripes provide contrast in the air. Smith photo.
The e-Cobra is pleasant to fly, agile in maneuvers, and fast at high throttle while exhibiting no bad habits on landing. Smith photo.
Approximately 3° of right thrust is built in to counteract the torque of the E-flite Power 10 motor. A removable hatch provides easy access to the motor and battery. Smith photo.
The first flights were still with a bit nose heavy condition. The control throws were conservative with exponential dialed in. This airplane was not touchy at all. As of this writing, I need more tail weight and maybe a little more rudder because it doesn’t want to spin. The battery saddle could be moved back into F2 a little on the next one.
It is a very pleasant airplane to fly, agile in maneuvers and fast at high throttle. It slows down nice for landing with no bad habits. All in all, the vintage Cobra II made an even better scaled down modified for electric than expected. Also it doesn’t have a bad camera angle.
Thanks to Harold DeBolt for a timeless classic design. Thanks to Jay Smith for the great photo work.
Read the entire article in the May 2013 issue of Model Aviation!
Daniel Lee Grotzinger
12460 Van Spronsen Way
Indianapolis, IN 46236
Vintage RC Society
Landing Products (APC Propellers)
FAI Model Supply (Polyspan)