Plane & Pilot
Tuesday, April 9, 2013

The Logic Of Flaps

Flaps make pilots’ lives easier—for those aviators who know how to use them

Contrary to what you might think, some types of wings can actually benefit from a reflex or negative flap setting. Maule used to equip its airplanes with a negative or reflex flap position.

The company claimed an extra three to five knots with the flaps in the -5 degree position. That's five degrees above a streamlined wing attitude. If that seems counter-intuitive, I've seen it work intermittently on Maules and fairly consistently on Dick VanGrunsven's RV series of homebuilts, adding two-three knots to cruise speed. No, it won't work on a Bonanza/Mooney/Malibu/Cessna 210/etc. The effect is totally dependent on the type of airfoil employed.

Care and feeding of flaps begins even before you start the engine. Assuming you can afford the battery power, make it a point to fully deflect the flaps and ascertain that the preset deflections correspond with the position indicator. Also, make certain flaps are fully stowed before hitting the starter to avoid having the prop throw rocks or other debris back into the bottom skin of the flaps.

In winter, it's also good practice to avoid taxiing through puddles of water or slush on your way to the run-up area, but especially so if you need to use flaps for takeoff. Water splashed onto the flap tracks or hinges can freeze and make it difficult or impossible to deploy flaps when you need them.

If your aircraft uses flaps for takeoff, wait until just before taking the runway to deploy them to the takeoff position, again to protect them from collecting dings during run-up. (You obviously may want to modify that advice if you're departing from a rocky or gravel runway.)

One myth that some bush pilots perpetuate is that you can decrease your takeoff distance by waiting until the aircraft is moving at 30-40 knots before selecting takeoff flaps. Not. The amount of additional drag generated by prepositioned flaps from brake release to liftoff is negligible. Like any aerodynamic surface, flaps have little or no effect on drag until the airplane is practically up to rotation speed.

Even if you do have the benefit of manual flaps that can be deployed with the flick of a wrist, better to set flaps to the takeoff position before power up than to add an additional procedure at the critical moment of rotation.

Once flaps are stowed, there's not much to worry about until it's time to use them again. Pay attention to the max flap deployment speed. Flaps are often huge aerodynamic surfaces, and though their true max operating speeds have been determined with a fudge factor included, better to be slightly slow during deployment than fast.


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