IMAC aerobatics: Cross-box maneuvers

Model Airplane News, Feb 2002 by Wolanski, Dan

MODEL AIRPLANE NEWS

FLIGHT TECHNIQUES

Cross-box maneuvers are designed to allow an aerobatic aircraft to travel perpendicular to the original flight path to correct for wind drift during competition. These maneuvers are essential in full-scale International Aerobatic Club (IAC) competition, since the pilots do not use wind correction as we do in International Miniature Aerobatic Club (IMAC) competition. Since IMAC rules permit wind correction, there is arguably no reason to use cross-box maneuvers. IMAC does, however, allow these maneuvers because we try to emulate full-scale competition, and it also gives the pilot a chance to use the entire aerobatic box to fly his sequence. Most competition maneuvers can be converted to cross-box maneuvers by executing a 1/4 or 3/4 roll during the upor down-line portion of the maneuver. For example, take the Hammerhead; a standard Hammerhead flown at the end of the box can be flown as shown in Figure 1. Now add a 1/4 roll in the down line, and the model will exit perpendicular to the flight path (see Figure 2).

The same can be said for a simple "Humpty Bump." Adding the 4 roll on the way up (up line) can allow you to adjust the diameter of the top radius while flying in the cross-box direction. The 1/4 roll on the way down allows you to exit parallel to the initial flight path but slightly farther in or out (see Figure 3).

END OF THE BOX

Flying perpendicular to the runway is not something we practice very much in IMAC. After all, many of our planes travel fast and would be out of sight quickly. Therefore, while attempting a cross-box maneuver, it may be very difficult to tell if you are, in fact, perpendicular to the runway. During crossbox maneuvers, most pilots look at too much of the wing during the rolling elements. What usually happens is that as the pilot attempts to fly perpendicular to the flightline, the plane actually heads toward the center of the box (slightly toward the pilot). The problem shows up after the cross-box maneuver, when the pilot attempts to pull into a vertical line and notices that the plane is not heading straight upward but is actually leaning in toward the center of the box.

To help correct this tendency, start by flying the cross-box maneuver directly in front of you. Try to exit straight and level while heading your plane directly away from you. After you've done this, slowly move the maneuver (a little bit at a time) away from your centerline and to the end of the box. After you've done the maneuver about 10 times, the model should be in its proper location and you will have a better picture in your mind's eye of how it should appear at the end of the box.

THE NEXT LEVEL

Once you have a better understanding of how perpendicular flight should look, it's time to take it to the next level. Instead of doing a simple 1/4 roll during the Hammerhead maneuver, change it to a 3/4 roll on the down line and exit cross-box inverted (depicted by dashed lines in the figures). Inverted cross-box maneuvers begin to appear in Advanced IMAC sequences.

Now perform another maneuver in the back corner of the box and exit it coming straight back in; this will give you a better idea of what your plane looks like going out (and coming back in) at 90 degrees to the flightline. Once you are comfortable with one side of the box, start practicing on the other side. Remember: if your plane constantly leans in toward the center of the box, your inboard wingtip is too low or, more than likely, you are not traveling perpendicular to the runway.

Once you become comfortable with both sides of the box, begin experimenting with different maneuvers. Try flying maneuvers such as a Figure-8, a Tail Slide and a Shark's Tooth. Maneuvers such as 1/4 and 3/4 rolling circles can be entered and exited from the cross-box position. Once you get the hang of it, the sky's the limit for possible flight combinations that will allow you to use the entire aerobatic box.