Tuesday, November 6, 2012
The Alpha Bet
Is this the best-yet trainer for learning the ABCs of flight?
Good glide control is still required to avoid undue drama in the landing phase: Alpha is a slippery bird even with those flaps-down, draggy tips. The optimal landing routine includes slowing below 70 knots abeam midfield, pulling on one notch of flaps (15 degrees), then slowing to 50 knots for the second, final flap setting (25 degrees).
I made three landings, including two where we came in deliberately high to confirm that, in the absence of speed brakes, a strong slip helps sufficiently to degrade glide angle without building unwanted speed. No, it's not like throwing out a barn door as in a Cessna 172. That low-profile tail boom pulls substantially less drag than boxier airframes. But slipping is effective. I saw sink rates near 800 with no flaps and near 1,200 fpm with full flaps.
Touchdown was a breeze: I kicked her straight a few feet above that minuscule Brennand strip, countered the slight crosswind with lowered wing and a bit of rudder, then at under 50 knots, tugged an easy flare and set the bird comfortably down.
Roll response is a bit "dopier"—more sluggish—at 50 knots as expected, but effective rudder management helps maintain alignment right through the flare. Alpha really is easy to land...but keep that speed below 50 or expect a long float and possible go around.
I consider the Pipistrel Alpha Trainer to be a real breakthrough. Implemented at flight schools in sufficient numbers, it could become the Piper Cub for an entire generation of new pilots who will learn the importance of deft three-axis control and energy management...and have lots of fun in the process. The Alpha is a major step forward, and lest we forget, at a price that few LSA with conventional construction have been able to achieve. That alone should rightfully deliver it to training ramps across the country...and private hangars, too. Well done again, Pipistrel!
The Importance of Aspect Ratio
|Ever notice the difference between the wing of an albatross and a hawk? Although both are soaring birds, the albatross's wing is more efficient, because the ratio of its long span to the narrow chord bestows a higher-lift/lower-drag efficiency that the hawk will never achieve.
In nature, birds like the albatross that fly long distances have high-aspect wings, which minimize fatigue (lower fuel burn). Hawks, by contrast, benefit from low-aspect-ratio wings that give them better maneuverability on the hunt.
Aircraft designers invoke "high-aspect" wings such as in the Pipistrel line of aircraft, for applications where high efficiency is considered primary in importance, such as optimizing weight and fuel economy without sacrificing performance.
Consider two airplanes: the Piper Cub and the Pipistrel Alpha Trainer. The Cub has an aspect ratio of 6.9, while the Alpha sports 11.8. The Cub wing looks noticeably less "narrow" than the Alpha's. Predictably, the Cub wing produces less lift and more drag per square foot than the Alpha wing.
Not all airplanes need or want a high-aspect wing. Also, they can be more challenging to design: longer, narrower spans benefit from composite technologies that makes high-strength/low-weight, cantilevered (no struts) designs possible.
Remember the early biplanes, with two "stubby," strut and wire-braced, low-aspect wings? That was necessary in part because aerodynamicists, although they recognized the benefits of clean, streamlined structures and high-aspect-ratio wings, didn't have the materials and construction skills back then to build them.
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