Tuesday, August 6, 2013
The Big Picture
Fighter’s-eye view, born trainer and a sport-flying funship
A final note: Several hundred Sky Arrows continue to serve in a variety of roles including law enforcement, environmental research, surveillance patrol and as news eye-in-the-sky. Lots of gear can be cobbed onto the belly and nose as compared to what choppers can—at a fraction of the cost. It has been certified in several categories: S-LSA (#21), European J-VLA, FAA part 23. There's even an Experimental Amateur Built (EAB) kit, and a UAV version is under study.
Sky Arrow is a seasoned, proven S-LSA that joins the chorus of quality-made, high-personality sport aircraft available today. I was pleasantly surprised—I bet you will be, too.
Pusher Or Tractor: Which Is Better?
|In propulsion, each has clear advantages, but it comes down to a matter of how the designer applies it. For example, many amphibs are pushers, since keeping the prop as far away from water and the cockpit as possible minimizes water spray erosion.
The Sky Arrow as a pusher gains superb forward visibility and no prop strike risk: big pluses in flight training.
Generally, a tractor prop in clean air can deliver around 92% of engine power into usable thrust. A pusher interacts with airframe drag, cutting that efficiency to 87%. Both camps cite cherished viewpoints:
• More efficient prop blast: no prop-blast drag from the fuselage and wing root
• Enhanced wing efficiency: The wing sees only smooth relative wind.
• Superior visibility
• Improved slow-speed control: The prop-blast is closer to tail surfaces.
• Fuel is behind the crew, so leaks and engine fires will flow aft...although tail feathers are in more immediate fire danger.
• Improved L/D from prop-blast (faster than relative wind) over central wing.
• In a bad crash, the tractor engine can absorb much of the impact. A pusher's engine mass can smash forward into the cockpit. (Note: There's no case history where this has ever happened.)
• More passenger protective structure in front.
• Less prop damage potential from debris thrown up by wheels.
• Air cooling advantage: Pushers don't have cooling prop-blast—it's less of a factor with Rotax and other liquid-cooled engines.
• Lower prop noise, prop drag and vibration: Tractor props rotate through clean air. Pusher props turn in and out of the wing/ fuselages' laminar flow. Sky Arrow's prop also sees disturbed airflow from the engine cowling, pylon and fuselage's top half.
• Lower potential yaw and pitch changes from varying power settings. Pusher prop blast is closer to the tail feathers and more impactful.
These arguments are just the tip of the iceberg. You'll find a lot of debate on tractor vs. pusher online. Bottom line: At LSA speeds (120 knots and less) the relative advantages are likely negligible. Mission specifics (i.e. amphib vs. long range cruise) ultimately determine the designer's choice.
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