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Flapping

Back in the early days of the last century, when most men wore hats and a great many sported moustaches, air traffic control wasn’t yet an entirely justifiable concept, and engines failed with regularity, one thing airplanes didn’t have was flaps. Certain airplanes flown today, such as the Piper Cub, still don’t have them. So who cares?

Back in the early days of the last century, when most men wore hats and a great many sported moustaches, air traffic control wasn’t yet an entirely justifiable concept, and engines failed with regularity, one thing airplanes didn’t have was flaps. Certain airplanes flown today, such as the Piper Cub, still don’t have them. So who cares?

What do full flaps buy you?  Most of the important reasons can be recited by almost anyone; you’ll land slower, and shorter. The primary purpose of flaps is to provide an increased range of control, with increased lift at the lower levels of flap deployment, along with a decrease in stall speed, and increased drag as the flap angle is increased (or as we say, “more flaps” are added, but all we’re doing is either increasing their angle or extending the distance that they extend backward, or both). Flaps allow steeper descents without the penalty of increased airspeed. Unlike ailerons, which are designed to move in opposite directions, they always move in the same direction. Although the relative amount of lift and drag provided by various flap systems varies, they effectively increase the camber of the wings and alter the chord line, and generally speaking, improve the wing’s performance throughout the “first” half of their extent of travel. During the remainder of their extension though, they create a relatively much greater increase in drag and much less additional lift. Most airplanes don’t take off very well after the 20 degree point of flap extension. The optimum for many is in fact, more like 10 degrees. (Most flaps change only camber and chord, but other more advanced Fowler flaps with their track and roller system, actually increase the surface area of the wings.)

If you were taught your lessons well, your flight instructor has probably already told you that you won’t always want to use full flaps. There are a few situations in which you will eventually find yourself (or perhaps already have) when full flaps can actually be a liability. Here are a few such cases.

If you’re landing at a fairly large airport, and from having looked at your AOPA Airport Directory (or the Airport/Facility Directory, or some other online or printed guide), you know that the FBO is much closer to the other end of the runway. There’s nothing wrong with practicing your short field approach skills, but unless there’s someone tailgating right behind you on final, or someone in the control tower asks you to clear the runway quickly, why spend several extra minutes taxiing when you can get most of the way there on (or just above) the runway itself? If the runway is 7000 feet long, I might not even bother to use flaps at all. When you can effectively “taxi” at least halfway there at 40+ knots, instead of the low single digits, doesn’t that make more sense?

Incidentally, even if there isn’t a mile and a half of runway in front of you, did you know that flaps can actually make it harder to land? For most tricycle gear airplanes, flap extension creates a noticeably lower nose-down attitude, and this definitely means greater pulling forces will be necessary to keep the nose wheel from hitting the runway before the mains. It is actually easier to make better landings using only partial flaps, especially in aircraft requiring greater elevator control forces. An airplane such as a Piper Cherokee, for which it is a bit harder to pull back, becomes a bit more stubborn with flaps fully deployed, and this could be a set-up for wheel barrowing and even loss of directional control.

The potential argument that not using full flaps means losing some of the benefits of reduced stall speeds is in theory correct. However, considering that the difference amounts to only one or two knots after the 20-degree point, it’s quite a small penalty. As an example, the stall speed of a Turbo Skylane RG at maximum gross weight and forward CG, using 20 degrees of flaps, is 43 knots. Using 40 degrees of flaps, it’s 41 knots. That’s a really small difference, wouldn’t you say?

The best case for excusing this small sacrifice in stall speed protection is probably during the go-around. If you think you might be fighting a losing battle with fierce crosswinds and you decide to retreat, or that windsock whizzing by is now indicating that you have a tailwind, or if a deer bounds onto the runway at the last minute (or someone just taxied out, despite your best efforts at telling the world you planned on landing there), then your workload becomes that much less of a hazard. Most airplanes will climb just fine, at least somewhat, with 20 degrees of flaps. (A few, such as a Cessna 182RG I can recall having flown, climb best with that flap setting.) And some won’t climb at all with full flaps. The juggling act with the flap handle, changing elevator forces, pitch changes, as well as power is not to be taken lightly in a high workload situation such as during a go-around or missed approach performed under any duress.

The next advantage to the use of partial flaps comes when the winds change suddenly with changes in altitude. In a wind shear situation, during which airspeed and altitude losses can be alarmingly great, partial flaps translate into excess power.

Another justification for the use of only partial flaps could come—as scary as it is-if you ever unwittingly stall the airplane during an approach to landing. At partial flap settings, you are much less likely to be behind the power curve. In a full-flaps scenario, typically the transition from being in control to being out of it, because of the rapidly escalating increase in rate of descent and changes in attitude, are the worst. With full flaps, it requires much more reserve altitude to recover from any upsets, and this is something you might not have.

There is one other reason why partial flaps can be better than full flaps: glide ratio. One of the primary features of flaps is, after all, to increase the descent profile. At such a time of course, flap retraction would be an imperative rather than an option, but the potential for confusion is arguably greater. That is because flap retraction from the full-out position would be accompanied by a proportionally greater short-term increase in sink rate (as well as the need to re-trim). I absolutely never use full flaps, until I know that I’d have to slip not to have the runway made, for reasons much related to this.

You might have been wondering if I was going to mention the effects of flaps during crosswind landings. It’s actually a bit of a double-edged sword. During the approach phase, using full flaps in fact makes it easier to control roll during gusty conditions, as the air spilling around from in front of fully deployed flaps somewhat increases the authority of the ailerons, making them more effective. Once on the ground though, full flaps just provides more surface area upon which the wind can have its way with you. (While retracting flaps on the rollout can be a hazard for those whose cockpits afford the potential to confuse it with retracting the gear instead, when it comes down to the advantages of the moment, its hard to argue principles if you’re really having a rollicking time out there.)

I’ll toss out one discretionary note of caution, which is that suddenly changing the way you fly (say from using full flaps to using only partial flaps on your very next landing, because you were so impressed with what you just read) comes with the potential for surprise. Treat any changes in operational philosophy and practices of control configuration gingerly, even the most ostensibly modest ones, because they could surprise you. Always seek dual instruction for anything you’ve not done before on your own, or with which you are not comfortable. In closing, if you can attain something of a “who cares?” perspective of ambivalence with regard to flaps, and be equally facile with them or without them, adapting as the need arises, that’s the best of all places to be.

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