THOROUGHLY MODERN MOONEY
WE FLY OVATION ULTRA
By attaching tiny pieces of specially cut aluminum to an airfoil in a particular order, an aircraft owner can dramatically improve that aircraft’s low-speed handling qualities, including stall behavior, and at a relatively inexpensive price. If this sounds too good to be true, it’s not, as anyone who has installed a vortex-generator kit on their aircraft will readily attest.
Vortex generators alter the flow of relative wind across the surface of an airfoil. A kit consisting of dozens of bits of extruded aluminum, each approximately 2 to 3 inches in length and half an inch tall on a small single-engine aircraft, are attached at an angle to each other along the width of a wing, and sometimes to the vertical stabilizer and beneath the horizontal stabilizer. Each individual piece of aluminum creates a vortex-generator blade and is placed at roughly a 30-degree angle left and right to that same relative wind. The result is the vortex generators form prominent V shapes when viewed from the front of the wing.
When the air hits the sides of the vortex blades, it hops over to fill the low-pressure area being created on the backside, generating tiny tornado-like vortices in the process. The magic of the vortex generator is that spinning air stays attached to the top of the wing at lower speeds and higher angles of attack, postponing separation, or what we know as a stall. The spinning air at slow speeds now acts very much like it does when the wing is moving faster through the air, improving control response and reducing stall speeds. That translates into shorter takeoff and landing distances.
The result of adding vortex generators to a conventional twin-engine airplane can also be
impressive, especially as the airplane approaches its velocity minimum control, or Vmc, speed. The effect can be very pronounced when the aircraft is flying at a slow airspeed close to the ground. Vmc represents the lowest calibrated airspeed at which the pilot can expect to maintain directional control after the failure of one engine.
The aircraft’s Vmc speed is calculated in the worst possible scenario too, with one failed engine still windmilling, the landing gear down and full power being demanded from the good engine. If the aircraft should slow below Vmc, there is usually not sufficient rudder to maintain directional control and the aircraft will begin to roll. Inducing a rolling motion on a stalled wing could lead to a spin.
On a twin, the vortex generators are also added to the vertical stabilizer ahead of the rudder, in addition to the standard set on the wing. Vortex generators improve rudder effectiveness in line with the vertical stabilizer, the same way they improve wing controllability, normally allowing the aircraft to stall straight ahead before it ever begins to roll.
Vortex-generator installation involves gluing dozens of aluminum pieces, each weighing just fractions of an ounce, to various surfaces on the wing or vertical stabilizer, a process that can be completed in a single day. A pilot can help with the work too, as long as they’re under the watchful eye of a certified mechanic. A vortex-generator kit for a Cessna 172 costs about $1,450 and delivers an 8 percent reduction in stall speed.