The Logic Of Flaps

In that respect, Cessna pilots have a leg up on the rest of the flight training world. I'm not sure what percentage of aviators learn to fly in the little high-wing airplanes from Wichita, but it has to be a big number. Whatever it is, those lucky prospective aviators benefit from some of the most effective flaps in general aviation---huge barn-door surfaces that provide dramatic reductions in stall speed and impressive short-field performance. By definition, pilots trained in the marquee's products receive an education on the usefulness and effectiveness of flaps.

Sadly, not every airplane manifests the same advantages. I learned to fly in a variety of trainers, everything from an Aeronca Champ to a Piper Colt, and none of my trainers had flaps. If you didn't learn to control airspeed and master the slip, you might have a tough time putting an airplane down on a short runway.

Part of that learning process in the last century was in a Globe Swift with among the industry's quickest ailerons, most effective elevator and least effective flaps. I dearly loved that little Swift, but the Globe GC-1B had a pair of short-span/narrow-chord flaps that improved the pilot's view of the runway but didn't do much else.

Perhaps for that very reason, there were only two positions, full up or full down, and the latter setting required only two seconds to set. A final joke: The flap-position selector was shaped like a wheel and the landing-gear position knob was shaped like a flap. Go figure.

No matter how effective they are (or in the case of the Swift, aren't), flaps make a pilot's job exponentially easier in virtually all respects. They deploy from the trailing edge of the wings and increase the camber or curvature of the airfoil, improving the lift coefficient in the process. This allows the wing to develop the same lift at a lower speed, effectively increasing drag, reducing the stall speed and allowing the aircraft to approach at a slower velocity. This can significantly reduce landing roll, a valuable benefit for short field operation.

Some aircraft designs specify flaps for takeoff and landing, others for landing only. Using flaps during the takeoff run allows the aircraft to lift off sooner and use less runway. Many general aviation designs and practically all commuter and large airliners require flaps for both departure and arrival.

Flaps come in a variety of shapes and sizes, so many that it would take most of this magazine to analyze them all. Here are the four basic types.

1 From a manufacturing point of view, the easiest flap to produce is the plain, hinged flap. This typically mounts at three of four points along the aft wing and often extends to half span. It's usually mounted with hinges that allow it to deploy 20 to 40 degrees below the trailing edge. Plain flaps are popular with homebuilt aircraft because they're easy to construct and the operating mechanism can be as simple as a manual cable device. Piper Cherokees and Beech Bonanzas employ plain flaps.

2 A variation on the plain flap is the split flap that leaves an upper surface extending to the original trailing edge. The flaps deploy beneath the wing and don't allow air to escape through a slot. In this way, airflow across the top of the wing is unaffected. Cessna 300 and 400 series twins and DC3s use the split-flap concept. Split flaps were popular on WWII-era fighters. They're hinged several inches forward on the wing's trailing edge and allow the top wing surface to maintain its original shape during deployment.

3 Conversely, a slotted flap is specifically designed to create a slot at the flap leading edge thru which air can flow up and across the top-aft surface, allowing airflow to remain attached and thereby improve lift. This process effectively "recharges" the air flowing across the top of the flap, often helping to preserve some minor laminar flow and prevent a stall. Most high-wing Cessnas utilize slotted flap.

4 The Fowler flap is a more complex surface that slides aft on tracks as it translates down, actually increasing the wing chord as it deflects. Fowlers are more expensive to build and are therefore most common on high-performance aircraft of all types, from modern airliners to advanced technology general aviation aircraft. STOL machines, such as the Robertson conversions of the '70s and '80s, also used Fowler flaps to provide maximum short-field performance.

Some engineers use combinations of different types of flaps to achieve specific effects. The double-slotted Fowler flap creates two gaps as the flaps deploy, allowing more air to leak through two slots to the top surface and increase lift. The flaperon is a long, narrow flap that spans the entire wing trailing edge. In normal straight and level flight, flaperons act exactly like large ailerons, deflecting up or down to provide roll control. In takeoff or landing mode, flaperons droop symmetrically to help reduce landing speed and improve visibility while also providing effective roll control. The homebuilt Kitfox used flaperons.

Slats aren't flaps in the strictest sense, but they help accomplish the same mission. They're built into the leading edge of the wing and droop down and forward, again to increase the camber of the wing and reduce the stall speed.

Don't confuse spoilers with flaps, however. Spoilers provide nothing but drag, whereas flaps contribute some lift while adding drag. Unlike flaps, spoilers typically have no operating limits and may be deployed all the way to redline, whereas that's not the case with flaps.

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.

You'll typically extend flaps upon entering the pattern. Make it a point to do that with wings level rather than in a turn. Asymmetric flap deployment is rare, but if you're turning hard left and have only the left flap deploy, you could be in for an unpleasant surprise.

Similarly, don't make it a habit to extend gear and flaps at the same time. If both systems are operated electrically, you could overtax the alternator and pop a circuit breaker. That's not exactly an emergency, but it's one less thing to worry about if you've just entered a busy pattern.

If there's any major crosswind blowing, be cautious about extending full flaps in one swell foop. Try using half on downwind and the last half on base or 1/3 at a time on downwind, base and final, respectively. The FAA used to recommend no flaps for strong crosswind landings, but now, even they agree that full flaps should be used on practically every landing. Best to introduce them a little at a time, however, rather than all at once.

If you're a little high with flaps full down and power full off, think twice about using a full flap slip. Though slips may not be a problem on some aircraft, others don't react kindly to fully cross-controlled maneuvering at low altitude. The problem is often blanking the airflow across the tail and essentially nullifying elevator control. Some Cessna flight manuals warn against such antics.

If you do decide to try it in an aircraft that allows the maneuver, keep careful watch over airspeed. Full-flap slips can bleed off airspeed in a hurry, so you may need considerable forward yoke or stick to preserve your approach speed.

If the runway is short and there's a need to plant it on and stop it short, consider raising the flaps immediately upon touchdown to place more weight on the main gear and enhance braking action. If you have considerable time in the airplane and fully understand its landing characteristics, you might even try what some Mooney pilots call "autoland". Once you're established in the flare a foot or so above the ground, trip the flaps full up and be ready to cushion the touchdown with plenty of back elevator. If you do it right, you can generate greasers practically every time. Do it wrong, and you may not be happy with the result. As mentioned above, this trick is only for highly experienced pilots with plenty of hours in type.

Flaps are just one more tool in a pilot's bag of tricks. In conjunction with wing design, they have varying effects in different aircraft. Most of the time, they allow pilots to fly approaches slower. Fast may be better at cruise, but when it comes to landing, slower is safer.