In competition RC Aerobatics, some of the most widely discussed maneuvers are those that occur while the aircraft is in a stalled state. Two that come to mind are the spin and the snap roll, which is also commonly referred to as the flick roll. Not only are these maneuvers challenging to perform, but they are also difficult to judge.
Different airplanes will not have the same characteristics when it comes to stalls. Lighter airframes with thick airfoils may need to have a higher pitch angle relative to the horizon to stall, compared to a model that has a thin airfoil and a higher wing loading. Now it is evident why these two maneuvers are somewhat controversial for those sitting in the judging chairs!
I will examine the 11/2-turn positive spin found in the 2014 Basic International Miniature Aerobatic Club (IMAC) schedule, and the snap roll. Having a firm understanding of how to execute these maneuvers, some of the key elements a judge will look at, and a few setup techniques, will improve a pilot’s scores. Without further hesitation, let’s get down to business!
The 11/2-Turn Positive Spin
Before attempting this maneuver, you must understand the key control inputs needed to execute positive and negative spins. The term positive indicates that up-elevator, combined with the same direction of rudder and aileron input, must be used throughout the spin. If a negative spin is to be flown, you would need to apply opposite aileron and rudder, coupled with down-elevator input.
All spins will begin from horizontal flight and change in altitude should not exist on the entry line. To enter the spin, decrease the airspeed by reducing throttle and applying elevator input to maintain altitude. As the aircraft slows, the pilot must add more elevator.
The airplane’s pitch angle will change to get the aircraft to achieve a spin break. If you looked at the aircraft’s CG point, the flight path along the CG would remain the same throughout this entry line.
The airplane will stall when the pilot runs out of elevator authority and the nose will drop. If the airplane does not stall and begins to “mush,” add more elevator throw or move the CG slightly back.
The airplane has stalled and a wingtip has dropped, determining the direction of rotation. If the aircraft drops its left wingtip during the stall, the pilot should apply full left rudder, continue to hold full up-elevator, and apply a touch of left aileron input. Some airplanes need aileron input to rotate.
If the airplane breaks and then hesitates before the rotation, a downgrade will occur. After the desired amounts of rotations are complete, establish a vertical downline, and gently pull to an upright exit.
When the airplane is approaching or has stalled, it may crab into the wind, and the yaw attitude of the aircraft relative to the prescribed degree of rotation (in this case, 11/2 turns) may rotate more or less than the prescribed rotation amount. This is acceptable because it is a wind-corrected maneuver.
Throughout this maneuver, the aircraft should autorotate around the CG at a constant rotation speed. If a visible yaw component during the rotation is not present, the speed of the aircraft will likely increase and this is a variation of an aileron roll. If no autorotation is present, the pilot will receive a score of zero. Be cautious!
Snap Roll Fundamentals
Similar to the spin, a snap roll can either be positive or negative. A positive snap roll requires up-elevator and the same direction of aileron and rudder input, and a negative snap roll requires down-elevator combined with opposite aileron and rudder. When starting out in Aerobatics, many pilots confuse the inputs needed for a negative snap roll. Instead of applying opposite aileron and rudder input, they apply these commands in the same direction. These are the incorrect inputs, and if done during a competition, the judge will zero that maneuver.
Similar to the spin, this is a stalled maneuver. It is often difficult to judge because a visible pitch break must occur as well as the autorotation of the snap roll. Because it is difficult to see whether these two elements occurred at the same time or sequentially, there is no judging requirement that states whether these two elements must occur at the same time.
It is common to show a pitch break and then execute the rotation, but no line element should occur between the pitch break and the rotation. If a line is visible, meaning that a large delay exists, the maneuver receives a score of zero. It is also mandatory that visible yaw is shown during the rotation of the snap roll.
If you are required to perform a positive snap roll on center from upright level flight, in most horizontal snaps it is common to enter with a fairly high entry speed ranging from 60% to 75% of throttle input. Follow these steps to begin the snap roll:
1. Quickly apply a generous amount of elevator input to engage the pitch break.
2. Being careful not to show a line segment between the pitch break and the autorotation, begin releasing some of the elevator input while applying full aileron and rudder input in the same direction. If viewing the aircraft from the side, it should rotate around the CG.
3. Release all control inputs after the set number of rotations has been performed. If one under rotates or over rotates the desired number of rotations, a penalty of a half point for each 5° is given.
During the snap roll, if all commands are held yet the roll rate and/or the rotation in yaw changes, the servos on the ailerons and/or rudder may be experiencing blow-back. This means that although a given control surface may exhibit a certain amount of measured deflection on the ground, it may exhibit less deflection in the air when the airplane is at speed.
To prevent this, always ensure that you use high-quality servos with adequate torque. Servos such as the A6270 HV servo from Spektrum offer 582 ounce-inch at 6.0 volts. High-quality 4-40 pushrods/turnbuckles such as Pro-Links from Hangar 9 should be exclusively used to ensure that no pushrod flex exists. High-quality hardware is a must, and most aircraft manufacturers offer suggestions concerning equipment in their instruction manuals.
Final Thoughts
You should now be familiar with the proper control inputs, a few judging elements, and lastly, multiple setup techniques in pursuit of the perfect score. These two maneuvers are difficult to perform well, but with proper practice techniques, they are possible.
Visit the IMAC website, listed in “Sources,” for more information about the judging elements for both of these maneuvers, as well as all maneuvers found in the IMAC sequences. Knowledge is crucial to success!
Until next time, fly hard!
