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
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.
The good/bad effects of flaps
on an aircraft are multiple
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 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
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
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
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
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
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
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.
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.
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.
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
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
You should practice no-flap landings to maintain that region of your
The greater the crosswind component the less flap extension based upon
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
Make initial turns shallow into jammed control