Flaps
“It’s gonna be a long hard drag, but we’ll make it.”
-Janis Joplin-
Wing flaps are one of the many, useful, performance-enhancing improvements to aircraft taken for granted by most pilots. They are the product of considerable thought, engineering, and development. Correct use of flaps serves at least four important functions: it reduces takeoff roll distance, decreases stall speed, reduces landing roll distance, and increases pilot visibility on landing and for manoeuvring. Wing flaps have proven to be a very useful development.
Wing flaps are almost ubiquitous on modern aeroplanes, but this has not always been so. It was 11 years after the first powered flight before flaps appeared at all and almost 20 years before they became a truly viable and practical addition to aeroplanes. Light training and personal aeroplanes did not begin to come with flaps as a standard accessory until the 1950s.
The Wright Flyer launched for the first powered flight on December 17, 1903. The first powered flight in Canada did not take place for another 6 years until February 23, 1909 when the Silver Dart, piloted by J.A.D “Douglas” McCurdy,[i] took to the air above the frozen waters of Baddeck Bay, Cape Breton, Nova Scotia. Neither the Wright Flyer nor the Silver Dart sported flaps.
The Dart was the product of an illustrious team led by Alexander Graham Bell and included Fredrick W. Baldwin, Lieutenant Thomas Selfridge[ii], and Glenn Curtiss. Much to the displeasure of Wilber and Orville Wright, Curtiss later went on to patent the aileron which resulted in a long and very interesting court battle between Curtis and the Wright brothers.[iii]
Flaps first appeared operationally in 1914 on the British built S.E. 4 biplane, a WWI fighter, although the pilots of the time, not believing they contributed to improved performance, rarely used them.
In 1920, Orville Wright and J.M.H Jacobs invented the split flap, a hinged flap on the underside of the wing that served to increase drag, allowing a steeper approach angle on approach. In the mid-1920s, Harland Fowler, a U.S. engineer, using his own time and money for the necessary research, developed the Fowler flap which was installed and tested on a number of aeroplanes during the years 1927 to 1929.[iv]
The Fowler flap, unlike previous efforts, does not just hinge down from the underside of the wing to increase drag, it also slides back from the wing when deployed, increasing both wing area and lift. The slot created between the trailing edge of the wing and the flap accelerates airflow over the top surface of the extended flap greatly increasing lift.
Fowler flaps and their variations can be seen on most training aircraft and larger flying machines sitting on the apron today.
In 1937, Piaggio Aero Industries in Italy, one of the oldest manufacturers of aircraft in the world, went on to develop double slotted flaps; Boeing introduced the triple slotted flap in 1964 for its 727 airliner.
As a pilot, our major interest, of course, is in how a device works and how and when we should use it to maximize the benefits of its performance-enhancing capabilities.
As we know, the primary purpose of flaps is to increase the lift-generating potential of a wing.
Lift is the product of four factors: the coefficient of lift (angle of attack for us pilots), air density, airspeed, and the surface area of the wing (L=1/2CpV2S). In unaccelerated flight, the product of those four lift-producing forces is also equal to the weight of the machine (L=1/2CpV2S=W).
Deployment of flaps increases the coefficient of lift and, in the case of Fowler flaps and their variants, also increases the surface area of the wing, allowing it to generate increased lift at the same airspeed or the required lift to support the aeroplane at a reduced airspeed.
Diagram 1 displays the force vectors involved during normal, non-accelerated flight with no flap deployed. The total lifting force produced by the wing acts at right angles to the chord line, the line extending from the centre of the trailing edge of the wing and passing through the centre of the leading edge. The vertical component of that lift—the portion of the lifting force that opposes gravity—acts at right angles to the relative wind, the direction of travel, and in a reciprocal direction to the weight vector.
The angular difference between these two forces, the horizontal component of lift, represents the force vector of induced drag, a product of the angle of attack. The greater the angle of attack, the greater the portion of the total lift becomes dedicated to producing induced drag.
Diagram 2 demonstrates what happens when flaps are deployed. The chord line, the line drawn between the trailing edge of the wing and the centre of the leading edge, is now “outside” the wing itself.
The lift produced by an aeroplane’s wing is a direct function of the angle of attack; deployment of flap sharply increases that angle of attack so, unless we reduce airspeed to compensate for the increased lift resulting from the increased angle of attack, our wing will generate more lift than required: vertical acceleration will result.
The increased angle of attack also sharply increases the amount of induced drag. Increasing drag results in reduced airspeed unless additional power, thrust, is also added. To avoid acceleration, either positive or negative, thrust must equal drag.
Deployment of flaps changes the camber, the curve of the wing, creating a wing cross section that results in increased ability to produce lift and increased drag. Once flaps are deployed, pitching the nose forward allows the pilot to have a better view of the terrain and airspace ahead while still allowing the aeroplane to achieve a sufficient angle of attack to produce the required lift for flight at a lower airspeed.
Correct use of wing flaps serves at least four important functions: it reduces takeoff roll distance due to increased lift at low airspeed, decreases stall speed, reduces landing roll distance, and, as just mentioned, increases pilot visibility at any given airspeed as the deployment of flaps, essentially, changes the angle of incidence.
Improper use of flap may result in increased takeoff distances, decreased climb performance, reduced speed potential due to excess drag and, in extreme cases, damage to the aircraft. In most aircraft, the total load factor the aeroplane can sustain is reduced when flaps are deployed. A quick look at your aircraft’s POH or AFM will quantify the load bearing penalty for use of flaps. For the C-172P, for example, in normal category the load factor limit drops from +3.8g to 3.0g; in the utility category the load factor limit drops from +4.4g to +3.0g.
Sometimes optimizing aeroplane performance is critical either for success of the current mission or success of the pilot’s or the company which owns and operates an aeroplane’s long term goals. Increased performance can mean greater capability to manage safe flight in difficult situations or it can mean saving money and time enabling a company or an aircraft owner to remain viable.
These days, every penny seems to count for more and more and margins of success are perhaps narrower than they once were.
The POH or AFM for the aeroplane you fly is always a good place to start for specific information on operation of your flying machine. The designers, engineers, and flight test personnel have put in long hours both on the ground and in the air to determine and quantify the best procedures for operating that particular aircraft. It is not excellent practice to second-guess those who have put so much time and energy into understanding and quantifying the performance specifications of the aircraft you are going to fly.
Correct use of flaps can shorten takeoff roll and increase initial climb performance. Taking a look at a typical training aircraft, the C-172P for example, 10 degrees of flap is specified when executing a short field takeoff either with or without an obstacle. In addition, in Section 4, Normal Procedures, the Cessna POH for the P model states that “Normal takeoffs are accomplished with wing flaps 0 degrees – 10 degrees. Using 10 degrees of wing flaps reduces ground roll and total distance over an obstacle by approximately 10 percent. Flap deflections greater than 10 degrees are not approved for takeoff.”[v]
Increasing lift produced at low speed on the takeoff roll by using flap decreases takeoff roll by 10% for this machine and allows the aircraft to achieve flight at a lower airspeed; rolling resistance from contact with the runway is relieved sooner; wear on tires and wheel components is reduced. We save money, time, and wear on the machine and get airborne sooner and in less distance. Over time, the cost and time savings to the aircraft operator can become significant.
We also note that Cessna does not recommend or authorize the use of more than 10 degrees wing flaps at any time for takeoff with this model. Greater flap deflection increases drag to the point where the advantages achieved with the minimal flap deployment are lost.
Proper use of flaps reduces stall speed and allows an increased safety margin when manoeuvring at low airspeeds. The C-172P manual, for example, provides some excellent sample numbers. At gross weight with no flaps, the machine will stall at 44 KIAS power off in level flight. With 30 degrees flaps under the same conditions, stall speed is reduced to 33 KIAS, a 25% reduction in indicated stall speed. When manoeuvring at low speeds, particularly in proximity to the ground, reduced stall speed is a good friend.
When manoeuvring at low altitudes for execution of a precautionary approach, for example, the POH specifies 20 degrees flaps and an airspeed of 60 KIAS. This allows the pilot to safely manoeuvre the aircraft for a careful look at a potential landing area or aerodrome with questionable surface conditions while maintaining a positive safety margin, improved visibility, and positive control of the machine.
On landing, proper use of flaps allows a safe approach at lower speed and provides enhanced visibility for the pilot. Referencing the C-727P POH, we find Cessna is not long on specifics. The POH says, “Normal landing approaches can be made with power-on or power-off with any flap setting desired. Surface winds and air turbulence are usually the primary factors in determining the most comfortable approach speeds.”
For short field landings, Cessna Aircraft Company is a bit more helpful, or at least more specific: “For a short field landing in smooth air conditions, make an approach at 61 KIAS with 30 degrees flaps using enough power to control the glide path.”
Outside of the training environment, normal landings are what most pilots carry out most of the time. Some useful guidance on speed and flap settings for maximal performance could be helpful information. When operating in anything outside normal conditions, pilot judgement becomes increasingly important.
Both approach speed and flap settings are determined by the ambient conditions: wind, length and condition of the intended landing surface, and turbulence. The generic approach speed formula is Vref = 1.3Vso where Vso is the stall speed for the aeroplane in landing configuration. Vso accounts for the effects of flaps, landing gear, landing weight, and centre of gravity location - factors that directly affect stall speed.
A slightly more sophisticated approach speed can be obtained by factoring in landing weight.[vi]
Landing distance, however, is not the determining factor for use of a landing surface for most aeroplanes. Most light aeroplanes will land in about half the distance required for takeoff. As long as the landing surface has sufficient length for a safe takeoff, including, for safety, a length available to accommodate accelerate stop distance, except in very unusual situations landing in the shortest possible distance is not normally a major consideration.
Modifying Vref to account for ambient wind conditions by adding the head wind component and half the gust factor will increase the safety margin and still allow the machine to achieve landing roll figures within a very acceptable range. Thus, approach speed becomes Vref = 1.3 Vso + headwind component + ? the gust factor. This will result in a ground speed on approach equivalent to the no wind ground speed and provide a safety factor for gusty conditions.
If the aeroplane would normally approach at, say, 70 KIAS with winds 20G30, an approach speed of 95 KIAS (70 + 20 + 5) would result in the same ground speed on approach as one would achieve in no wind conditions plus a factor for gusts.
To maximize the benefits of flaps on landing, use as much flap as is consistent with safety. This can help significantly on saving tires, brakes, wear on all the moving parts of the landing gear, and an increased stall safety margin on approach.
More flaps allow a slower touchdown speed, increased stall protection, and a better visual environment for the pilot, but reduces positive aerodynamic control on approach in difficult conditions, in crosswinds or gusty conditions, for example.
An old friend of mine used to say, “Set your flaps by the wind sock. If the wind sock is hanging down, use maximum flaps; if the wind sock is straight out, use none or minimal flaps; if the sock is half way in between, use half flaps.” It’s a simple guideline, but it hits the mark for operational simplicity.
Wing flaps are a wonderful addition to the performance-enhancing equipment on aeroplanes. Learning to use them properly to maximize their benefits is an excellent plan.
End Notes
[i] https://www.gov.ns.ca/nsarm/virtual/mccurdy
[ii] Lieutenant Selfridge became the first aviation fatality when on September 17, 1908, he died of head injuries following a crash of the Wright Flyer piloted by Orville Wright during a demonstration flight for the US Army at Fort Myers, Virginia.
[iii] https://www.centennialofflight.gov/essay/Wright_Bros/Patent_Battles/WR12.htm
[iv] https://www.centennialofflight.gov/essay/Evolution_of_Technology/High_Lift_Devices/Tech6.htm
[v] Information Manual, Cessna 1981 Model 172P, Cessna Aircraft Company, Wichita, Kansas, 1980
[vi] Vref = 1.3 Vso√landing weight/gross weight. When making this calculation, always use CAS and convert the final answer to IAS for operations. Note: most light aircraft do not provide information regarding approach speeds below gross weight. Do not second guess the manufacturer of your machine. If the number they provide as a minimum approach speed is, for example 61 KIAS, you become a test pilot executing an approach at a lower speed.
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