-by Pat Veillette
Flaps dont seem to get much respect, but they are a vital component on most aircraft. When used properly, flaps allow slower takeoff and landing speeds, thus decreasing runway distance requirements and lessening the severity of damage in the case of accidents.
On the other hand, flaps can degrade the aircrafts performance so much that the ability of the aircraft to remain airborne can be severely compromised. Flap management is so important in heavier aircraft that warning systems are incorporated to warn the pilot of incorrect flap positions for takeoff and landing.
Flaps increase the lift of a wing by increasing the camber of the airfoil. The results can be dramatic. For example, wind tunnel testing of a NACA 4415 airfoil shows it produces a lift coefficient of 0.4 at zero degrees angle of attack. When equipped with flaps deflected 60 degrees, the airfoils lift coefficient increases to 1.9 – nearly a fivefold increase in the lift for that angle of attack.
This characteristic explains why some after-market STCs for short takeoff and landing capabilities involve drooping the ailerons with the first notch of flaps, thus making the ailerons perform partly like a flap, to really improve the takeoff and landing performance of the aircraft.
Of course, the entire wing does not produce five times the amount of lift when the flaps are deployed because the flaps do not run the full span of the wing. However, the increase in total lift of the wing is still significantly improved by the extension of flaps.
While flaps are useful in many normal takeoff and landing operations, they can be a requirement if the field is short. The added lift from flaps allows the airplane to take off and land at a slower airspeed – and therefore shorter ground roll. For instance, the PA-28 Pilot Operating Handbook shows a takeoff distance of 2,100 feet with the flaps up, but only 1,750 when using 25 degrees of flaps.
Curiously enough, many GA aircraft manuals contain little information about no-flap landings. In most manuals, there are no speed references and no landing distances for partial flap or zero flap landings.
Some newer light airplanes do contain references, though they are far from complete. Cirrus airplanes, with POHs that rival any other GA airplane for completeness, make only a passing reference to no-flap or partial-flap landings, giving approach speeds for landings with flaps up, 16 degrees and 32 degrees, but no table of distances.
The POH for a new Cessna 206 gives this clue, found in a footnote to the short-field landing distance performance chart: If a landing with flaps up is necessary, increase the approach speed by 9 KIAS and allow for 45% longer distances.
The 9-knot increase in no-flap landing speed Cessna describes comes on top of a recommended approach airspeed of 67 knots, meaning the absence of flaps requires a 13 percent higher airspeed.
Interestingly, both manuals state at one point, When landing in a strong crosswind, use the minimum flap setting required for the field length. While thats standard procedure in flight training, the absence of manufacturer data describing what partial flap setting gives what field length at various weights and altitudes is troubling.
Flap use for takeoff is better documented, with most airplane manuals describing flaps-up and partial flaps takeoff distances and speeds. Generally the flaps-up performance is given for normal takeoff procedures and the partial flaps information is for short-field performance. This isnt always the case, however, as some airplanes just seem to handle better using partial flaps even for normal takeoff.
Setting the flaps to the takeoff position early in preflight usually wont hurt anything, except when the taxiway is contaminated with mud, ice or slush. In this case, it may make sense to leave the flaps retracted so you wont damage them with debris flung up from the tires.
Angle of Attack
In addition to changing takeoff and landing numbers, flaps also change the zero-lift angle of attack. For instance, the NACA 4415 airfoil produces zero lift at -4 degrees AOA. When flaps are deployed 60 degrees, the zero-lift angle of attack moves to -16 degrees.
The zero-lift angle of attack has important practical ramifications. When the flaps are deployed, the lower zero-lift AOA means the nose is lower for a given amount of lift, which gives you a better view of the runway. This is why most airplanes need some nose-down trim when the flaps are deployed. In many Mooneys, the electric flap motor and the electric trim work at complementary speeds. Deploy the flaps and trim down simultaneously and the two actions neatly cancel each others impact on pitch force as felt at the yoke.
In some airplanes, the zero-lift angle of attack is extreme enough to present some real challenges to transitioning pilots. During a normal approach in the DHC-6 Twin Otter, for example, the airplanes nose is at a very noticeable angle below the horizon. With go-around power and full flaps, the deck angle is below the flight path, leading to a feeling that the airplane is simply levitating.
Therefore, a pilot who pitches to a typical nose-up reference attitude with the flaps at 40 degrees would aerodynamically stall the airplane.
Conversely, if the aircrafts flap system is inoperative and you are left executing a no-flap approach, the pitch attitude will be much different than what you are used to.
The next time you practice a no-flap landing, notice that the pitch attitude is much higher, even though you are required to fly the no-flap landing at a higher airspeed. The higher nose pitch attitude makes the runway appear lower in the sight picture, tricking the pilot into believing that the aircraft is higher than normal.
The illusion creates a tendency to want to drag in a no-flap approach. Use whatever glide path guidance is available, such as a VASI, PAPI or ILS glideslope to help avoid this landing illusion.
The pitching moment induced by flap extension and retraction needs to be managed to make the flight smooth. A lot of pilots dont react in a proactive manner to counter the pitching moment. In a high-winged aircraft, the downwash from the flaps strikes the horizontal stabilizer and causes the nose to pitch up when the flaps are extended. This is particularly pronounced during the first 10 to 20 degrees of flap deployment.
This characteristic is most likely going to catch you by surprise when you extend the first notch of flaps on the downwind leg of a visual approach or when you pop in a notch upon reaching a fix during an instrument approach. This pitch-up is especially common to unprepared pilots abeam the numbers when they are concentrating on the runway or distracted by something else.
With that first notch, an overworked pilot will be distracted by monitoring the runway or concentrating on the approach. The nose will pitch up and the airspeed will begin to bleed off. Some flight instructors use the expression, flaps down, nose down to teach their students to trim nose down as soon as they extend the flaps.
In general, low winged aircraft do not suffer as significantly from the same phenomenon due to the wings downwash flowing below the horizontal stabilizer, thus creating no extra downloads on the tail as in the high-winged aircraft.
Flaps Add Drag
In addition to adding lift, flaps also increase drag, especially at full deflections. The first 10-20 degrees of flaps increase the wings lift rather markedly with only a relatively minor increase in drag. As the flaps extend beyond this point, the increase in lift is usually overshadowed by the increase in drag. The drag produced by full flap extension slows the aircraft or steepens its descent without inducing excessive airspeed.
Try flying at minimum controllable airspeed with the flaps fully extended and you will quickly realize you need to carry a lot of power to maintain altitude, especially in something like a Cessna 172. On a hot day at high density altitude or with a big load in the cabin, the aircraft may not be able to sustain level flight.
The upshot of this training is a full appreciation of the fact that full flap extension means youll need to use a lot of power to correct for drifting below glideslope or to otherwise arrest a premature descent. In many ways, full-flap operations feel like operating a multi-engine piston airplane on one engine.
As a matter of routine, I add power up to a pre-determined power level as soon as I extend the flaps to full. This helps prevent the airspeed from bleeding off too fast and allows the airplane to settle nicely to the desired final approach airspeed.
Drag can be a desirable characteristic to an aircraft on landing for a couple of reasons. Power response tends to be inconveniently slow when trying to increase the power from idle to full power. Usually a no-flap approach is flown with a relatively low power setting, and the momentary lapse in power response if you have to go around can seem like an eternity.
The added drag from flaps during an approach allows you to use a higher power setting without pumping up airspeed, which gives better engine response if you need to bolt. The higher power settings are especially noticeable in jets, which dont have the kind of throttle-to-engine-speed response of a piston. In addition, any turbine airplane that uses bleed air for de-icing and anti-icing of the engine inlets and wings gets some assistance from having more power producing more bleed air.
Full-flap drag can also assist in making steep descents, such as if ATC gives you a slam dunk or youre trying to make a short field with obstacles on final. Sure, you can dive for the runway, but that builds energy youll have to dissipate before flaring to land, and the drag from full flaps helps increase descent rate without building speed.
But that drag is a double-edged sword. If you have to go around, full flaps can make airplane control a handful. Nearly a third of the go-around accidents in a recent year involved an attempt to go around with the flaps still in the fully extended position.
You will find that many airplane manuals recommend retracting the flaps to a mid-way position right away after applying throttle in the go-around sequence. For example, the POH for a Cessna 172 recommends 1) Throttle-FULL OPEN, 2) Carburetor Heat-COLD, 3) Wing Flaps-20 Degrees (immediately), 4) Climb Speed-55 KIAS, 5) Wing Flaps-10 degrees (until obstacles are cleared) RETRACT (after reaching a safe altitude and 60 KIAS).
Be very careful about retracting the flaps too quickly, especially if the aircraft hasnt started accelerating yet. If the aircraft is slow and close to the ground, retract the flaps in increments to allow the airplane to accelerate properly as the flaps are being raised. A sudden and complete retraction of the flaps at a very low airspeed can cause a loss of lift resulting in the airplane settling into the ground.
The go-around, one of the most critical and least practiced of landing procedures, needs to be familiar and automatic. Some aspects of go-arounds are highly airplane specific. The big electric flaps of a Cessna 206 create huge amounts of drag and move very slowly, while a similar size, weight and speed Piper Cherokee Six has manual flaps you can pop up or down in an instant.
Pre-Flight Action
Inspecting the flaps during your preflight is an often-overlooked task. Most aircraft manuals recommend lowering the flaps to allow for a full visual inspection of the hinges, tracks and attach points. You want to make certain the flaps operate to the full down position, evenly and without binding.
Inspect the rollers and push rods to make certain they operate smoothly and dont show signs of excessive mechanical wear or corrosion that could induce a failure. Make certain the retaining bolts and cotter pins are in place. The condition of the flap itself should be inspected for any signs of popped rivets, bending, warping or cracking.
Most flap assemblies are not stressed to take considerable g loads. The aircraft I fly for a living has a 2.73 g limit in the clean configuration, and a 2.0 g limit when configured. Flaps were designed to provide high lift for takeoff and landing maneuvers. They werent designed for the added stresses of maneuvering flight. Flaps should be used for takeoff and landing, and not for radical maneuvers.
Treat your flaps well. While the top of the white arc is the limit for lowering the flaps, you are treating your airplane much better if you can manage your airspeed so as to lower the flaps at speeds less than the top of the white arc. Retract them well before reaching that airspeed.
This lessens the stress on the mechanical supports within the flap mechanism, which helps to reduce fatigue stresses and increases the operability of your flaps for a longer time period.
Flaps dont get the respect they deserve because its easy to take them for granted as optional equipment. But the next time youre making a short-field approach or takeoff, you may want to think about how much is riding on them working their magic with lift and drag.
Also With This Article
Click here to view “Tricks and Traps.”
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Pat Veillette is an aviation researcher and a pilot of transport aircraft.
