use of flaps


The total weight of air displaced by an airplane must equal or exceed the weight of the aircraft for it to remain airborne. This air weight is obtained by the velocity of the relative wind over the lifting surfaces. An aircraft on approach and using flaps has most of the airflow over the top of the wing and horizontal tail. Flaps change the wing lift capability by changing camber and wing area.

Where the G-limits of a light aircraft are 3.8 positive and 1.5 negative the use of flaps make a significant change in these figures. The flap positive load falls to 2.0 and the negative approaches 1.0. What is means is that flaps should not be used during turbulence or high stress manoeuvres. Flaps should be removed in a spin since the recovery from the spin can result in a high G-load pull out.

Flaps are not used as brakes. Flaps are used to increase and maintain lift at slower speeds. During the flare the flaps change horizontal energy to vertical energy that is used to decrease the sink rate prior to touchdown. Fowler type flaps, as on Cessnas, deflect air downward and the gap created on flap deployment helps increase airflow over the wing's trailing edge. In the flare the flaps allow an energy conversion from horizontal to vertical. The air is reflected downward into the ground effect region. This allows the aircraft to be slow, nose high and controlled as it floats to touchdown.

Flaps contribute primarily to the landing approach angle by increasing the 'braking effect' of drag. The drag is used initially to increase the approach angle without a corresponding increase in speed. There is no appreciable (required or created) change in approach speed as distinguished from no-flap speeds. When the approach slope is changed into the roundout and flare, speed is quickly decreased. It is this decrease in speed, the horizontal slowness of possible ground contact that protects the aircraft structure. The more flaps available and used, the slower the speed, the slower the touchdown and shorter the rollout.

Flaps can change the camber or curve of the wing airfoil by adding 1/4 of the wing cord without changing the critical angle of attack. The efficiency of the wing at different speeds can be changes as required. When flaps are added the zero lift line changes, as does the angle of attack. Exceeding this angle with or without flaps will cause a stall. As flaps change the trailing edge of the wing the chord line (zero lift line parallel to the relative wind) between the leading edge and trailing edge of the wing is changed.

With each additional degree of flaps both lift and drag are added but in different proportion. Initially the gain in the coefficient of lift exceeds the increase in drag and reduces the stall speed. What is means is that the stall angle of attack changes with every change in flaps. Every addition of flaps increases the stalling angle of attack and reduces the manoeuvring speed (energy) margin. The first 15-degrees of flap will increase the lift and shift it toward the rear of the wing. This gives a corresponding pitch down movement of the nose. Over 15 degrees of flap the amount of drag begins to exceed the amount effective lift and there is a significant increase of wing induced downwash on the horizontal tail surfaces which causes a corresponding pitch up of the nose. The excess drag provides superior glide path control and approach aim. Flaps replaced the slip as a descent device for landing accuracy. Spoilers are a still better means of giving the drag effect. Gliders, with spoilers, habitually hit between 10' aiming markers.

Full flaps function best at a range between 1.3 and 1.6 of stall speed. Any slower, the stall angle of attack and narrow manoeuvring speed (energy) margin makes actual constant level flight close to the ground in this configuration very difficult. Flaps increase the lift factor but decrease the angle of attack prior to stall. The critical angle of attack to flying attitude range of the wing is much less with than without flaps. This is one of the reason stalls with flaps are a 'surprise'.

Depending on the manufacturer, the aerodynamic geometry of the flaps will affect trim. Except for the C-152 all Cessnas with 40-degrees of flap extension appear to have a one full turn of trim TO ten degrees of flap relationship if power is 1500 RPM. 

The manufacturer's recommended landing is using full flaps. Full flap landings provide the best aim to the runway. Side benefits are to reduce wear and tear on the aircraft by slower ground contact, less tire wear, and less required braking. Once you have acquired reasonable mastery of full flap landings you should include periodic landings with no flaps and partial flaps. The no-flap landing is the best one to use when practicing slips to a landing.

Less than full flap landings will flatten the glide angle and make touchdown point accuracy more difficult. The approach speeds are the same with or without flaps for the C-150, 60-kts. In the roundout with less than full flaps the pilot must be aware that the lower drag will mean greater lift during flare. Using the same yoke movement as with full flaps will cause a balloon. This is especially true if the flare is close to the ground. For every three full flap landings you would be well advised to make at least one partial flap landing to maintain the correct 'touch'.

The discussion about flaps is directly related to nose-wheel shimmy issues. The partial flap landings are far more apt to result in nose-wheel first landings at a much higher ground-contact speed. A few such landings will lead to nose-wheel shimmy.

There is another side to the partial-flap coin. The pilot who first learns to do such landings is more likely to use them as an option. Cessna, in a 'commercial' decision reduced the degree of flaps available in order to increase the useful load of its aircraft. This gave us the C-152 and later models of the larger Cessna with only 30 degrees of flap. Only those who learned on earlier Cessnas can appreciate the difference in landing precision that was lost.

The learning 'Law of Primacy' says that under sufficient stress you will resort to the way a process is first learned. A student who is taught the 'easy' way to fly without trim is entering a world of hurt in flying high performance aircraft. A student who has never learned how to use and anticipate power effect will find transition to a twin a carnival experience. Poor initial instruction from the very beginning is the most insidious because the student has no prior reference on which to base an opinion.

Flap Effects

The good/bad effects of flaps on an aircraft are multiple

Increase lift
Increase drag
More abrupt stall
Lower stall speed
Decrease climb rates
Change pitch attitude
Increase approach angle
Change trim requirements
Decrease distance to lift-off
Far narrower aerodynamic stall range.
The use/misuse of flaps is a judgment situation

Pitching Moments

Pitching moments makes the aircraft rotate around its centre of gravity either nose up or nose down. Any single change in configuration usually requires an offsetting adjustment of trim. The use of flaps is a change in aircraft configuration as is the landing gear. In a C-182RG the simultaneous retraction of landing gear and 10-degrees; of flaps have offsetting pitch changes so that no trim adjustment is required. A nice piece of engineering as is the 1:1 relationship that exists between notches of flaps and a full turn of trim in many Cessnas.

Just adding flaps will cause an aircraft to pitch up or down depending on how the change in lift and drag created is positioned around the centre of gravity. Flaps lowered on high-wing usually cause a nose-up pitching moment as the camber changes the lift around the centre of pressure. The resulting drag also causes a nose-up pitch. Low-wing aircraft flaps causes drag that pitches the nose down. Only the aggregate of pressures of lift and drag determines the direction of pitch change.

Flap extension also affects the airflow over the horizontal tail surfaces and thereby affects its lift. This lift is normally a downward force and the flow from the flaps can have a greater effect than either the camber or drag. The extension of the gear can, at various points of the extension cause either up or down pitch changes. This gear effect tends to be more marked in low-wing aircraft than in high-wing. The last configuration changes that can have effects are in power or propeller changes.

Putting on Flaps

The use of flaps is the most practical way to lower the liftoff speed and touchdown speed and thereby shorten the takeoff and landing distances. Flaps increase drag, shift the lift/angle of attack relationship, reduce lateral control and the manoeuvring load factor. The flap can increase the angle of climb or descent and reduce float. Flaps make it possible for the pilot to improve his landing approach judgment and aim. Along with this improvement comes a slower ground contact speed but not necessarily a slower approach speed. 

Vary your use of flaps to improve your mastery of the aircraft. Every extension of flaps through various settings will give a predictable change in flight and performance characteristics.  Rate of climb is always less with use of flaps during climb. This is a requirement by the FAR's. You should cycle the flaps through various settings of power and trim during your training until you can both predict and anticipate what will happen. In Cessnas the addition of flaps while maintaining the same speed by trim corrections will improve over-the-nose visibility. Flaps without trim adjustments will generally cause low-wing aircraft to pitch down. In level flight, adding power will cause pitch up and reducing power will lower the nose somewhat in proportion to the amount of flaps extended. Good operating practice calls for the maximum application of flaps as crosswind conditions allow.

The student should practice using a count to apply Cessna flaps where an indent is not installed. Too many things can go wrong with aircraft control if the attention (eye) is focused on the relatively slow movement of the flap indicator to a desired position. Different models (years) of Cessna have flap switches that operate differently and even opposite. Practice use of the flap switch to determine in which position it must be held, will neutralize, or stick. The most common switch must be held down to lower flaps, will centre when released from down, and will bring the flaps all the way up when set (but not held) in the up position. Thus, in order to milk up the flaps in small increments the switch must be held between the fingers and moved accordingly.

Taking off Flaps

Milking the flaps is a required skill in certain slow flight and go-around situations. At these times full power should first be applied, throttle and then C. H. and a minimum level attitude attained before removing any flaps at all. Any time the go-around airspeed is less than 60-kts the aircraft should be held in level flight and the flaps should be milked up. Milking requires that the flap handle is held throughout as brief spurts of movement with the handle raise the flaps a bit at a time. As more airspeed is acquired the flaps may be brought off more quickly. When climb speed is attained the flap switch may be placed in the up position and a climb attitude established.  The flaps motor cuts off when flaps are full up or down.

The danger of teaching the dumping of flaps to a student lies in the law of primacy causing a future retractable pilot using the gear lever instead of the flap lever on the post landing rollout. There is some argument as to the best operation of flaps after landing. Because of a proclivity for gear retraction accidents to occur to those pilots who practice bringing up flaps on landing. (They use the gear lever instead of the flap lever). Recommendations have been made that flaps be left down until clear of the runway and stopped. In line with the learning law of primacy such a practice has much to recommend it.  I strong wind conditions the flap rollers may jump from their tracks.  The flap motor and gears are capable of seriously bending and twisting the flaps when the rollers are off track.

However, there are wind conditions when the ground control of the aircraft necessitates getting the flaps up as soon as possible. Also there is a tendency for many pilots to apply brakes with the flaps down in such a manner as to lock the tires or to skid. The aerodynamic lifting of flaps, even under a light wind, is such that tire damage can result. The practicality of economics says to bring up the flaps on touchdown. If the landing shock causes the pilot to allow the yoke to move forward, the flaps can cause a condition known as 'wheel-barrowing'. This means that the lift from the flaps when added to the yoke position is sufficient to lift the main wheels off the pavement. This means the only ground contact is the nose wheel. Such a 'wheelbarrow' condition results in instant loss of control and a ground loop (very sharp turn). There are many landing situations where the yoke is held still or moved back and up. There are none where the yoke should be moved forward after touchdown.

Flaps and Descent Angle

The installation of flaps on aircraft makes possible a controlled steep approach. This has improved the ability of pilots to judge their arrival at the runway. There is an additional safety factor in using flaps. In the event of engine failure, the removal of flaps will make a significant increase in glide distance. A C-150 with full flaps has a glide angle of about 11 degrees in a no wind condition. VASI lights are usually at 3 degrees. Under most conditions a White over White VASI is acceptable in a Cessna until short final. Any head wind can increase the glide angle proportionate to the wind velocity. The untrained eye is able to detect angular differences when they exceed 5 degrees. A diagram of the runway showing a steep approach with a 5 degree angle of error will show how much more accurate the steep approach is. Compare this to the aiming error likely if the 5 degrees is drawn to the runway from a shallow approach.

Short Approach with Full Flaps

For pure simplicity and accuracy, the short approach wins. Downwind do the prelanding check. Abeam the numbers pull Carb Heat, reduce throttle to 1500 RPM. Hold altitude just long enough (5 seconds) to have airspeed reach the white arc. Apply full flaps. Fly 60 kts. No trim will be necessary. Turn base. Turn final. Roundout. Flare. Back on yoke and throttle. Rollout. Cleanup. If this were the only landing taught solo in five hours is possible. 10-degrees of flaps selected on downwind will not create a problem unless the crosswind is at 90-degrees and over 15-knots. Between 5 and 15-kt crosswinds limit flaps to 20-degrees. Above 15-kts no flaps should be used. The addition of airspeed and power can increase rudder effectiveness. It is rudder power that determines ability to maintain the nose parallel to the runway centerline. There is no crab wind correction on final. The wind is compensated for by a wing low, opposite rudder, half Dutchroll correction as required to keep the aircraft course aligned with the runway and the nose straight (parallel) to the runway. The wind velocity will change as you descend so will your aileron and rudder applications. Proficiency in the Dutch roll makes these changes reflexive.  

If you are unable to maintain the nose parallel to the runway heading with full rudder, increase the speed to gain more rudder authority. The increased speed and rudder power let you bring the nose into a parallel line with the runway. If such a lowering of the nose for speed causes a descent below the desired glide path, apply full power and then back off as required to maintain approach speed. The 'Dutch roll' skill is required to keep such changes and adjustments smooth.

Flaps in a Crosswind Landing

Flaps provide a surface area for a crosswind to act upon; the more flap the more surface. The upwind flap is more affected than the downwind flap. In the wing low, slip approach the lowered wing partially shields the flap and helps keeps the flight path aligned with the runway. The more flap used the less it is shielded and the more rudder required for lateral control. This lateral control difficulty increases as the flap extension reaches 40 degrees and the crosswind component reaches 90 degrees. Better rudder power can only be attained by an increase in airspeed. The crab-kick crosswind landing is another way of accomplishing a successful landing even at slow speeds.  However, timing of the touchdown  'kick' is very critical to prevent damaging side loads to the landing gear and gear box..

The stronger gustier and more nearly 90 degrees the wind is to the runway the fewer degrees of flaps should be used. Under certain gusty strong wind conditions it is possible for the flaps to blank out the elevator and horizontal stabilizer from its normal flow of air. When the elevator/ horizontal stabilizer stalls the nose goes straight down--NOW. Many Cessna manuals say that slips are not to be made with flaps. A slip can blank out the tail surfaces. However, this restriction does not apply to the wing low slip used to maintain runway alignment during cross wind landings.

The selection of flaps in crosswind conditions can be delayed until on final. No flaps should be added within 200' of the runway because of the possibility of airspeed control problems. Using less than full flaps in crosswinds should not change the approach speed but may increase the touchdown speed to the benefit of rudder control required for keeping the nose parallel. The approach attitude of a no flap landing is closer to the actual landing attitude than is the flap landing. No flap landings will take longer to decelerate so the flare to landing attitude will take longer with greater margins of error possible. Have plenty of runway for no flap landings because you are much more likely to make a judgment error as to where touchdown will occur. Any flaps used in a crosswind should be removed immediately on ground contact to prevent a weathervane turn from occurring

No Flap Landing

A no-flap landing uses more runway, requires more braking, and lacks obstacle clearance capability without requiring slips. You can determine a no flap approach speed by using the no flap calibrated stall speed and multiplying by 1.3. Refer to the POH to get the IAS. POH speeds are always based on gross weights unless otherwise stated. The requirement of more runway is due to a shift in the touchdown point caused by a shallow glide angle. An obstruction will require a slip to allow adequate runway in most situations.

The no flap landing is best practiced in all conditions but best used in gusty conditions. Such a landing may never be needed but should always be available. Flap landings occur close to aerodynamic stall and compromise control effectiveness. No flap landings retain a desirable crispness of control with some sacrifice of stall speed. This control may be required with the higher speed of touchdown and greater leverage of any swerve.

The no flap landing lacks the accuracy of flap landings due to the shallower glide angle. Downwind do the prelanding check. Abeam the numbers pull Carb Heat, reduce throttle to 1500 RPM. Hold altitude and heading. Trim down three full turns for 60 kts. Since no flaps will be used the downwind will need to be extended. This is a judgment call and affects accuracy. Turn base and fly 60 kts. Turn final and fly 60 kts. The reduction of power is now the only desirable accuracy adjustment. Full power may be added. (See "Decelerating approach") Once power is off a slip is an acceptable adjustment. (See slips) Roundout. Flare. Touchdown. Cleanup.

The power off no flap landing is not recommended as a continual practice because it can shock cool the engine. The procedure is as above except that there is an apparent rapid descent. This causes the pilot to attempt to slow the descent by raising the nose. Don't! You will lose airspeed and lose the flaring capacity that goes with the proper airspeed.

Flap Emergency

A simulated emergency-landing situation that deserves instructional attention is that of engine-failure on short final. Create the following situation on a 5000' or more runway. Arrive at short final with full flaps, at least 1500 RPM and the slowest approved approach speed. At 400' take off the power. The student should immediately remove all flaps and use the yoke to maintain the same approach speed. The initial reduction of power should make it obvious that the aircraft will be unable to reach the runway in its full flap configuration. The immediate removal of flaps will cause a sink of nearly 200'. These negatives are soon seen to be offset by the flatter glide and extended glide path made possible by the absence of the flaps. When done smoothly, touchdown should occur about 2000' down the runway.  Introduce this procedure shortly before solo.

One of the greatest procedure rules for an emergency is: "Undo what you just did. This applies directly to flaps. If you put in flaps and something untoward happens, take them off NOW. The effect in a split-flap application can be reduced by applying flaps incrementally.

Flaps Indicators

The most practical way to lower flaps without indent stops is to use the indicator only as a check. A 1-2-3-4- count on the flaps switch can be timed to give 10 degree flap application. Every individual will need to perfect their own count for a particular aircraft because of individual variations of speech.

The application of flaps will depend on the situation with variations from the normal 10 degrees before turning base, 20 degrees on base, and full flaps on final. If you plan to do slips it is best not to use any flaps. If there is a crosswind the stronger it is the fewer degrees of flaps the better. Flaps in cross winds will vary also according to pilot capability.

During a closed traffic practice session the amount of flaps may be varied. Going around and around repeating the same procedure with the same mistakes is not the way to improve. It is vital that the pilot keep track of the trim position as it relates to flap position. Any unanticipated yoke pressure is a warning about flap position or trim position.

Flaps are a source of drag that permits a steeper approach and greater landing accuracy for a given approach speed. Flaps reduce the aerodynamic stall speed. This reduction effectively reduces touchdown speed, shortens landing roll distance, improves forward visibility, and improves landing accuracy. Extending flaps increases the effective angle of attack of the horizontal tail.

Cessna in its original designs used 40 degrees of flap but this was reduced to 30 degrees where gross weights were increased. This was to meet both go-around requirements and potential accident liability. The flare control required for different flap settings will vary so landings should be practiced at each setting.

Slips with full flaps

If the landing approach is so high that even after full flaps, power off, and 55 kts IAS a slip is required this is indicative of poor planning and procedures. The use of slips in a flap-equipped aircraft is indicative of misjudgment. In addition, the POH (Pilot's Operating Handbook) for Cessna 150 and 172 expressly recommends against the use of flaps when slipping the aircraft. With flaps down, it is possible for the airflow that normally flows over the wing back to the horizontal stabilizer to be interrupted. The flaps "blank out" the stabilizer and elevator. It stalls.

This causes an abrupt, straight down nose attitude.

Use and Non-use of Flaps

Flaps are usually certified only to 2-Gs.
Aircraft can be slipped with and without flaps.
Normal landings use the maximum flap extension.
When on the ground in windy conditions remove your flaps asap
Severe misjudgement of a situation will require both flaps and a slip.
The flap motor is capable of bending the flaps if it is off its guide tracks.
You should practice no-flap landings to maintain that region of your skills.
The greater the crosswind component the less flap extension based upon your skills.
The use of flaps allows a pilot to maintain altitude in the pattern while close to the airport.
Abide by the white-arc flap use limitations of the airspeed indicator to avoid eventual flap failure.
Flaps are to allow a steeper angle of decent and better touch down aim without an increase in airspeed.
On engine failure in the pattern, consider removing any flaps to extend gliding range. at approach airspeed.
If you are into turbulence sufficient to slow below the yellow zone of the airspeed indicator, don't use flaps.

Irreversible Split Flap Emergency

Reduce power to get lowest controllable airspeed.
Make initial turns shallow into jammed control