Every pilot wants more useful load, and Piper provides 242 more pounds of it in its new Piper Meridian single-engine turboprop, thanks to an increase in maximum takeoff weight. And the company also switched to the new Meggitt Magic 1500 autopilot that offers the performance and features a pilot expects in a turbine airplane.
When Piper introduced the Meridian in 2001 it had a 4,850 pound maximum takeoff weight, which was capped at that level by the FAA requirement that single-engine airplanes stall at 61 knots or less at maximum certified weight. With more than 1,000 shaft horsepower available from the Pratt & Whitney PT6A-42A engine, power to lift more weight was never a question. The Meridian wing and other primary structure also had the heft to carry extra weight. But the stall speed rule was limiting what the Meridian could carry.
Piper had a couple of choices available to increase the takeoff weight of the Meridian. One was to demonstrate improved crashworthiness to get a couple extra knots added to the maximum stalling speed. Socata had achieved this with its TBM 700, and Pilatus had done the same with its PC-12. The other option available was to reduce the stalling speed so the airplane could carry more weight up to the 61-knot maximum.
Piper engineers decided to focus on reduced stalling speed to achieve the increased weight goal. The primary driver in their decision was to control empty weight. Increasing the max takeoff weight adds to the useful load only if the empty weight of the airplane doesn't increase in the process.
One way to reduce stalling speed is to improve the efficiency of the wing flaps. The 61-knot maximum applies to the landing configuration, so any improvement in flap effectiveness would do the trick. But the Meridian flap is already very long in span, consuming most of the trailing edge of the long, slender wing. And the design of the flap itself, and its extension angle, is pretty well optimized, so very little, if any, improvement was available by modifying the wing flaps.
The other available option to reduce stalling speed is to micromanage airflow over the wings and tail. Airflow can be tailored by some sort of a small device on the airframe that generates a high-energy wake when the wing is flying at certain angles of attack. These devices go by many names, but the term vortex generator (VG) covers the category because it best describes what the devices do-generate a vortex.
The energy in the vortex that streams aft from a VG is used to direct air to flow the way the aerodynamicists want it to over the wings or tail. It would seem logical that air wants to flow straight aft over an airfoil, a straight line being the shortest distance, and all of that. But when a wing or tail is operating at a high angle of attack, many forces interact to cause the air to try to flow in all sorts of directions, including spanwise, or even forward. The energy in the vortex created by a VG acts as a fence to keep air flowing in the desired direction, which is aft. Years ago many jets had physical fences in the form of a chordwise blade that directed airflow near the stall, but the physical stall fences have virtually all been replaced by some form of VG.
Piper experimented with various VG configurations and came up with a series of 92 on the wing and 80 on the underside of the horizontal stabilizer that reduced stall speed. The tiny VGs are bonded to the surface and weigh only a few ounces. The VGs resemble little I-beams with beveled leading and trailing edges and are a fraction of an inch high. They add an immeasurably small amount of drag at cruise angle of attack, but as the airplane approaches a stall, they help keep the airflow attached to the surface and smoothly flowing aft to a lower airspeed. Thus, the Meridian can weigh 5,092 pounds and stall at 61 knots, compared to 4,850 without the VGs. In addition to being extremely light, the VGs require no maintenance, and you can dispatch with as many as five missing.
The only other change to accommodate the higher takeoff weight is some strengthening in the landing gear area. Though the VGs could easily be added to any existing Meridian, the changes to the landing gear make a retrofit to the higher takeoff weight impractical at this point.
