THOR­OUGHLY MOD­ERN MOONEY

WE FLY OVA­TION UL­TRA

Flying - - FRONT PAGE - By Rob Mark

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.

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