Technique: Learning to loop

Looking for pitch in all the wrong places…

At first glance, the loop looks like the simplest of all aerobatic maneuvers, but it’s really one of the more complex and involves much more than going fast and hauling back on the stick.  The key to flying a loop properly is knowing where to look, and—hint!—it’s not just straight ahead.

New aerobatic students tend to keep their eyes glued over the nose. That might work driving a car on the highway, but in an airplane performing a loop (or any other over-the-top maneuver) all that a student staring straight ahead sees is blue sky.

To maintain orientation, the pilot should look out the sides of the aircraft during the initial climb, and straight overhead to spot the horizon as the airplane comes over the top. Only as the airplane begins its descent on the back side of the loop should the pilot’s gaze shift forward over the nose.

The visual cues a pilot sees during a loop also require some interpretation. While looking out the sides of the aircraft during the climb, for example, the pilot can gauge much more than simply the airplane’s pitch. He or she must also pick up subtle signs of unwanted yaw and roll, and correct for them. It’s only by comparing the positions of the wing tips relative to the horizon that the pilot knows whether the aircraft is yawing (or “dragging” a wing). And at the top of the maneuver, when the pilot looks up (down) and sees the horizon, it’s immediately apparent whether the wings are properly level.

If you’re dragging a wing, there’s a yaw problem, and rudder corrects it. If you’re not level (in an inverted attitude) over the top, there’s a roll issue, and aileron fixes it.

External visual cues also help a great deal in staying oriented, so practicing over straight farm fields (or roads or power lines) can be beneficial.

Once an aerobatic student is looking in the right places, the next step is calibrating his or her arm for the pull, and the loop involves lots of pulling: hard at the bottom when the airplane is going fast, and light at the top when it’s slow. (There’s no pushing on the stick in a loop, only varying amounts of pulling.)

The sweet spot for the pull is typically about 3.5 Gs. Pull harder and you induce unwanted, energy-killing drag, as well as greater stress on the airframe. Pull too lightly and you may run out of airspeed before making it over the top. Most aerobatic airplanes have G meters, and a glance during the initial pull should be enough to tell you (like Goldilocks) whether you are pulling too hard, too soft, or just right.

As the airplane approaches the top of the maneuver, you should feel light in your seat. The target here is weightlessness—zero Gs. Airspeed at this point can be critically low, sometimes below stall speed. But at zero Gs, the wings aren’t making any lift, and wings that aren’t making lift can’t stall.

Gravity hasn’t forgotten you or your airplane, however, and as the heavy front end begins to point toward the ground and airspeed increases, airflow attaches to the wings in the normal fashion and you can start pulling out of the dive. The pull is gentle at first, and more firm as airspeed builds.

Pulling too hard at any point in a loop can create some surprising results. At the top of a loop, excessive pulling can lead to an unwanted departure that sometimes turns into a half snap roll—or a spin—and the pilot finds himself level (and very slow) at the point in which he expects to be inverted and accelerating. Pulling too hard on the back side of a loop can make it feel as though you’re driving down a rutted and bumpy country road. Relax the back pressure slightly and the ride will smooth out.

If the airplane you’re flying is equipped with an angle-of-attack indicator, it will illustrate an abstract concept that pilots learn to recite during ground school but rarely fully comprehend—which is that an airplane can stall in “any attitude” and at “any airspeed.” For me, it took stalling on the back side of a loop (while pointed straight down at a high power setting) for that idea to sink in.

During the initial pull into a loop, the airplane has a high airspeed, a high angle of attack, and is producing a large amount of lift (and high Gs). As the airplane floats over the top of the maneuver, it’s at low speed, a low angle of attack, and producing almost no lift (and no Gs). Gaining speed on the back side of the loop, airspeed becomes high again, angle of attack increases, and the amount of lift (and Gs) goes up along with them.

A constant-speed prop is a real luxury in a loop as you can go to high power and rpm and simply “set it and forget it.” Fixed-pitch props usually require full power in the initial climb and throttling back during the descent to avoid exceeding rpm redlines.

The loop introduces pilots to new sensations such as relatively high Gs and weightlessness, all in the same maneuver. It forces pilots to seek and interpret attitude information in new ways (pitch and yaw information comes from looking out the sides of the aircraft, and roll
by looking overhead). Angle of attack should approach, but never exceed, the critical angle.

Loops enable pilots to feel how the wing flies throughout its full range of speed and angle of attack, and experience—and even enjoy! —the physical demands imposed by both high Gs and weightlessness. It’s quite a ride.

Source:  AOPA



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