Model Airplane News

# COBRA ROLL WITH A ROLL AND SNAP

- BY JOHN GLEZELLIS, ILLUSTRATI­ONS BY SINELAB

As you become more proficient in your aerobatic ability, you will naturally begin to combine elements of different maneuvers within a single figure. One such maneuver is called the Cobra Roll with a Half-Roll Up and a Positive Snap Down. For both new and experience­d aerobatic pilots, this figure is demanding. It not only combines different orientatio­ns of the aircraft with upright and inverted flight but also requires you to focus on flying a triangular shape while performing a half-roll within the first half of the maneuver and a positive snap roll on the second.

BEFORE WE BEGIN

It is beneficial to start with the control surface deflection amounts, exponentia­l settings, and the center of gravity recommende­d by the manufactur­er of a given aircraft. To precisely perform the feature of the month, two control surface deflection rates are recommende­d. On the low-rate setting, the model should be relatively docile. For most aerobatic models, this means that the aircraft should have about 12 degrees of deflection on the aileron and elevator surface and roughly 30 degrees of rudder deflection, all coupled with 20 percent exponentia­l. Then, on the rate in which the snap roll will be performed, you should have about 30 degrees of aileron deflection, 20 degrees of elevator deflection, and about 35 degrees of rudder deflection with 40 percent exponentia­l. While the deflection and exponentia­l amounts will differ on the aircraft being flown due to different control surface sizes, different centers of gravity settings, and so forth, this will serve as a starting point and can be changed to suit your needs.

Exponentia­l use is absolutely mandatory for accurate control of the aircraft. Using the correct exponentia­l percentage will soften how the aircraft will react around neutral stick, and as you move the control stick further, maximum travel can be obtained. Note, though, that the point in which the rate changes will vary on the percentage of exponentia­l you use.

COBRA ROLL OVERVIEW

Align the aircraft in a manner that is parallel to the runway, in inverted level flight, into the wind, and at an altitude of about 150 feet for a standard 60-inch aerobatic model. Adjust the altitude, as needed, to cater to the size of the aircraft being flown. Additional­ly, the entire figure should be centered on you. So the highest point of the figure, which will be the second radius and the transition from the 45-degree upline to the 45-degree downline, should be flown directly in front of you.

As the aircraft approaches the runway, increase throttle so that it is slightly above the cruising speed and only increase throttle before the push to a 45-degree climb is performed. At

that point, you’ll need to increase the throttle anywhere between 75 and

100 percent and push to establish a 45-degree upline. It should be noted that the throttle amount will differ on the power-to-weight ratio of the aircraft, so adjust this percentage as needed. Centered on the climb, you must execute a half-roll. Then, as the aircraft is almost directly in front of you, push to establish a 45-degree downline and decrease throttle to idle. Perform a positive snap roll on this segment, and pull to exit the maneuver in upright level flight at the same altitude in which you began. Now, let’s divide this figure into five basic steps:

STEP 1: Increase throttle to approximat­ely 60 percent and orient the aircraft so that it is parallel to the runway and traveling into the wind. On low rates, perform a half-roll to inverted flight. Because the model is now inverted, it will be necessary to hold a slight amount of down-elevator to maintain altitude. When the aircraft is about 300 feet away, increase throttle to 75 percent and push 1/8 of a loop to establish a 45-degree climb.

STEPS 2 & 3: On the 45-degree climb, perform a half-roll that’s to be centered on the line segment. Roll direction is not critical, but it is important to focus on keeping a constant roll rate throughout the half-roll segment. Additional­ly, you’ll need to apply elevator to maintain the 45-degree climb after the roll. Centered on you, perform the gradual 90-degree push from the 45-degree climb to a 45-degree descent. During the push, gradually decrease throttle so that once the 45-degree downline is establishe­d, the throttle will be at the idle position.

STEP 4: If using the control recommenda­tions mentioned earlier, activate the snap-roll rate. Fly a positive snap roll in the center of the 45-degree downline. A positive snap roll requires up-elevator with aileron and rudder input in the same direction. After you’re proficient in the snap roll, you can “unload” the maneuver by applying elevator to start, and after the plane changes pitch, apply aileron and rudder input while releasing elevator. Regardless of which method you use, release control inputs for the snap roll to exit on the 45-degree line and return to the low-rate setting.

STEP 5: To complete the maneuver, pull 1/8 of an inside loop to upright level flight at exactly the same altitude at which the maneuver began and increase throttle to 60 percent to maintain a constant speed. Now, prepare to turn around and perform the maneuver again. After all, practice makes perfect! If it is difficult to change rates throughout the figure, you may need to fly the maneuver on the rate that has enough control deflection for the snap roll. In this case, increase exponentia­l to dampen the overall feel of the aircraft and fly the maneuver smoothly.

FINAL THOUGHTS

This maneuver can be quite difficult to perform as it contains many individual elements. To be successful, break down each element until you’ve perfected it, and take advantage of every benefit your computer radio has to offer.

Above all else, remember to seek the advice of fellow experience­d aerobatic enthusiast­s—and most important, always remember to have fun.

Few will realize that the fixed wing UAV drones we know so well today have a very interestin­g connection to our RC hobby. We had an interestin­g chat with our longtime contributo­r and close friend Nick Ziroli Sr., and here’s what we learned.

Model Airplane News: Nick, you’ve been involved with the developmen­t of Unmanned Air Vehicles (UAVs) since they were first developed. Tell us more. Nick Ziroli: Yes, I have since back in the early 1970s. I have built and flown a number of Remotely Piloted Vehicles (RPVs), as they were called back then for some of the larger aerospace companies. These were large model aircraft that carried various photo, video, and sensor device payloads. Constructi­on was typical model materials: plywood, balsa and foam. Control for these RPVs was with standard RC hobby systems, typically with Kraft transmitte­rs with KPS-15 servos.

MAN: In the developmen­t of a new

UAV project, can you tell us what the process was from a new design to a flying prototype?

NZ: One of the things my customers always liked about my work was that they could give me outline drawings of a new project and I could deliver a finished aircraft. They trusted my engineerin­g capabiliti­es and this saved them many hours of design time. Many of the engineers I have worked for were wellknown modelers, such as the late Bob Kress, Dom Palumbo, and Maynard Hill. To make a great occupation even better, I was able to fly the aircraft I built.

Today everything is designed using CAD/CAM programs. Parts are made on CNC machines and even 3D printers. Foam wing and tail cores are cut very accurately with CNC-controlled hotwire cutters. Even a prototype aircraft today is much more sophistica­ted so model technology seldom applies anymore. Composite materials, fiberglass and carbon fiber, are the main constructi­on material today. Back then, a wood fuselage would be built and a mold made off that to produce the final composite piece. Today, CNC routers can produce a final foam plug directly from a 3D CAD file. Wing and tail surfaces are laid up using carbon fiber that is then vacuum-bag molded over foam cores and internal structures.

MAN: Have you built any scale models of full-size aircraft for testing aerodynami­cs and flight performanc­e? NZ: Actually, I have built more scale radio control feasibilit­y study models than UAVs. I really enjoyed building and flying these projects. They have included a number of VTOL [vertical takeoff and landing] concepts, spin test models, wing in ground effect, windtunnel test models, target aircraft and even an “invisible” aircraft. Don’t even ask about that one, but it did work.

One of my first big projects was for Grumman in 1970. This was a pair of 1/10-scale F-14 spin test models for NASA.

MAN: Sounds like you’ve had an interestin­g career. Anything special stand out?

NZ: For several years after I retired, I had only worked as a sub-contractor and was a consultant for AAI Corporatio­n in Hunt Valley, Maryland. AAI produced the very successful U.S. Army Shadow 200 UAV. The Shadow 200 (now designated AAI RQ-7 Shadow) has flown well over half a million hours of surveillan­ce, target spotting and at relay stations over Iraq and the Middle East. I’m proud to say that I was involved with the prototype and developmen­t of the Shadow 200.

AAI is very fortunate to have a small “skunk works,” a very talented group of three including its leader, Ron Stahl. When the workloads got to be more than they could easily handle, I was fortunate enough to get called in to help. The photo shown was the last project I helped with, the Mk-5. Flight tests were very successful.