THEORY OF GLIDER FLIGHT
In 1988 the Soaring Society
rewrote the Joy Of Soaring originally written by Carle Conway. I was
asked to assist in the rewriting. The following revision is part of what I recommended.
It is a good Introduction to the Theory of Glider Flight. Bret Willat
WING, FUSELAGE AND TAIL
Most gliders consist of
three basic components: the wing, the fuselage, and the tail surfaces. The
wing supports the glider, and the ailerons and tail surfaces provide stability
and control. The fuselage performs the dual function of holding the wing and
tail in proper relationship to each other and of providing cockpit space for
the pilot. If one understands how the wing functions, the rest of the theory of
glider flight follows naturally and logically. Understanding helps in learning
the practical techniques of flight.
The way a wing works is not
obvious, compared, for example, to the functioning of a wheel on a car. A
wheel can be seen turning, bearing against the solid earth to apply power or to
change direction. The wing works invisibly in an atmosphere that yields before
the passage of the glider. But the air has substance and offers resistance,
supporting the wing even as it gives way to it.
What the wing does is to
drive a mass of air downward, producing an equal and opposite upward reaction
upon itself in accord with Newton's 3rd law of motion. (Oddly enough, under
most conditions the top of the wing contributes far more to this action than
the bottom, which explains why glider pilots are so particular about wiping
off the tops and less finicky about the lower surface, OK, the bottom is harder
to reach.) The upward reaction on the glider is made up of two force factors:
lift, which acts upward at right angles to the direction of motion; and drag,
which acts rearward, or parallel to the direction of motion. The values of
these two forces, expressed as a ratio, describe the glide performance of the
craft at a given time. If lift is twenty times as great as drag (expressed
L/D~2O:1), the glider will move forward twenty feet for each foot of altitude
lost. In soaring, lift is the hero and drag is the villain.
The ability of a given wing
to generate lift is in proportion to the density of the air, to speed, and to
the angle at which the wing strikes the air. Denser air produces more lift
because the wing has firmer substance to push against, so the reaction upon
itself is greater. More speed gives more lift because the wing can deflect more
air downward in a given time. Glider speed is dependent on the pull of gravity
or the tug of the towrope.
The third factor in the
effectiveness of a wing is the angle at which it strikes the air. This angle
of attack is important to the pilot because he controls it directly by
moving the stick fore and aft, which has the secondary effect of changing the
wing's lift and drag (which means angle of descent) and the glider's airspeed.
Decreasing the angle of
attack reduces lift, while increasing it generates more lift-but only up to a
point. At an angle of attack in the area of 12 to 18 degrees (depending on the
shape of the particular airfoil), the wing begins to lose its ability to
deflect air downward and instead begins to produce a turbulent wake behind its
upper surface. With only a slight additional increase in the angle of attack,
lift decreases rapidly and drag 'builds up vastly, so the rate of sink
increases; control is greatly reduced, and the wing is said to be stalled. (This
is usually the condition with the control stick all the way back.) To regain
lift, the angle of attack must be reduced (stick forward), returning the wing
to the range of angles of attack where the air flows smoothly over the top
surface and is deflected downward.
FUNCTIONS OF THE CONTROLS
Control surfaces are provided to allow the attitude of the glider to be
changed about its three axis. The pilot can blend these three movements to
produce any desired change in attitude. Moving the stick sideways depresses one
aileron and raises the other, making the lift of the two wings unequal and
rolling the glider right or left around the axis of the fuselage. Other results
also appear which will be discussed later in this chapter, when we take up
turning.
The vertical and horizontal
tail surfaces are small airfoils which work like a wing to deflect the airflow
and so produce equal and opposite reactions upon themselves. Moving the stick
fore and aft moves the elevator and lowers or raises the nose of the glider so
long as airspeed is adequate. A less superficial description of this action is
that the elevator controls the angle of attack of the wing.
The rudder pedals are
connected to the rudder in such a way that pushing the left pedal moves the
glider's nose to the left. This may not seem logical when compared to the way a
sled is steered; however, the use of the rudder is not to steer the
glider but to align its fuselage so as to minimize (and occasionally to
maximize) its drag. For this reason, the logic of steering is unimportant; the
rudder simply yaws the glider around its vertical axis.
STABILITY
Stability of direction and
bank are interrelated and complicated subjects of more interest to designers
than to pilots. Few gliders will return to a wings-level glide without help
from the pilot; in fact, few will long remain in that condition if left to
themselves. For good reason, designers rely on the pilot to handle directional
and banking stability.
Pitch stability, possessed
by gliders in widely varying degrees, is of great interest to the pilot. This
quality appears as a resistance on the part of the craft to flight at speeds
other than that for which it is trimmed. If trimmed for 40 mph, for example,
the pilot might have to exert a strong forward pressure on the stick to fly at
80 mph. Should he relax the pressure the glider would, after a series of pitch
oscillations, return to 40 mph.
Gliders
with high pitch stability require elevator trimming devices either on the
control surface or adjustable springs in the control system to permit the pilot
to fly the glider at any reasonable speed without having to hold a steady
pressure on the stick. A few glider models have been deliberately designed with
neutral stability and do not generate a stick load by changing speed. These
gliders do not require a trim.
GLIDE PATH AND SPEED CONTROLS
When making an approach for
a landing the pilot must be able to steepen his glide without gaining speed.
He also needs a means to check his speed when diving to lose altitude rapidly.
Several of the devices for providing this control are of interest to all glider
pilots.
Spoilers,
which are used on most training gliders, are devices of various styles, some
perforated, which emerge from the top or bottom of the wing, or both. They may
be hinged or may slide out vertically from slots in the general vicinity of
the main spar. The prime purpose of spoilers is to break up the smooth flow of
air over a portion of the wing, "spoiling" the lift. Secondarily,
drag is increased. (Recall that L/D determines the angle of descent.)
Dive
brakes are devices whose primary purpose is to increase drag. Most dive brakes
are a part of the wing, and so also decrease lift. The choice of names depends
on whether the accent is on decreasing lift (spoilers) or increasing drag
(dive brakes). Fuselage dive brakes and drogue parachutes have no direct effect
on lift, working entirely by increasing drag.
Since
there are a number of sailplanes available which have drogue parachutes as
supplements to spoilers and flaps, it should be noted that they are extremely
effective speed-limiting devices. However, their functioning in gliders has
not been 100% reliable, and a failure to open usually comes at the most
inconvenient moment. Further, once deployed, they cannot be controlled or
retracted and redeployed. Another handicap is that the backward pull of the
chute upon the tail of the glider substantially reduces the effectiveness of
both rudder and elevator. Perhaps these problems will one day be solved;
meanwhile, too much reliance should not be placed upon the drogue parachute.
Flaps, which are used on
some gliders, work differently from spoilers. Flaps are hinged portions of the
trailing edge of the wing that, when lowered, change the camber or cross sectional
curvature of the wing so as to increase both lift and drag. One type of flap
has moderate up and down travel for high-speed cruising and thermalling; the
function of speed control is handled by separate spoilers. Another type of
flap functions in the same way for cruising and thermalling, but dispenses with
the supplementary spoiler, powerful drag being provided by lowering the flap as
much as 90 degrees to the chord line.
Several
models of gliders have Fowler flaps, similar to those used on many airliners. Fowler
flaps lower and at the same time slide backward on tracks so as to increase
the area of the wing, to change its camber, and to provide a slot which smooths
the airflow on the top surface of the flap. The Fowler flap increases drag
moderately when partly extended for thermaling, and very greatly when fully
depressed for speed control.
All
the above devices tend to reduce speed, other factors remaining the same, and
when designed to keep maximum diving speed at a safe figure, are called
"terminal velocity limiting."
EFFECT OF AIR DENSITY ON PERFORMANCE
The density of the air
becomes less as altitude and temperature increase. In thinner air a wing must
be driven faster to produce a given amount of lift, all other things being
unchanged. Fortunately, a change in air density affects the airspeed indicator
in the same way as it does the wing, since they both function by the dynamic
reaction of the air. Therefore, a pilot can rely on the airspeed indicator
under most conditions without making a correction for density. Stalling speed,
red-line speed, and glide ratios throughout the performance range of the glider
are constant in terms of airspeed indicator readings. True airspeed, which is
the indicated airspeed corrected for the density factor (disregarding the
question of instrumental error), has a bearing on cross-country strategy and
the possibility of flutter; its use in navigation is nil because of the
irregular gait by which gliders wend their way across the land.
EFFECT OF LOADING ON PERFORMANCE
A heavily loaded glider goes
forward and down faster than when lightly loaded. The glide ratios are
the same for both loading conditions, but occur at different airspeeds. This
may seem strange until one realizes that an increase in weight does not affect
either lift or drag, the determining factors of glide ratio. Unfortunately, all
the airspeeds on the performance curve of the heavily-loaded glider are
higher, including stalling speed, and worst of all from the soaring standpoint,
the range of thermaling speeds.
On a normal cross-country
flight the heavily loaded glider makes better time between thermals but takes
longer to regain the altitude lost. Both its minimum thermaling circle and its
rate of sink (because of the steeper bank required) are greater than for a
lightly loaded glider. When the thermals are strong the "lead sled"
runs away from the "floater." When the thermals are weak, the
floaters stay up and make progress while the heavy jobs are fighting simply to
stay up, or perhaps are unable to do so.
Jettisonable water ballast
is carried in some gliders in an effort to combine the best qualities of both
types. This solution is incomplete, however, since water cannot be taken on in
flight when thermals strengthen in the early afternoon.
As things stand now, no
glider is ideal under all conditions. Each type is designed for certain
assumed conditions and represents the designer's idea of the best compromise
between speed and thermaling ability.
THE TURN
Newton's First Law of
motion (loosely stated) says that a moving body will keep moving in the
same direction until a force acts to turn it. The force that turns the
glider is the lift of the wing.
In level flight, wing lift
works upward, supporting the glider but not turning it. By banking the craft,
some of the lift is stolen for the purpose of overcoming inertia and
centrifugal force and thus pulling the glider around the turn. The proportional
part of the lift devoted to turning and the part still devoted to vertical
support vary with the angle of bank, as can be graphically described by the
familiar parallelogram of forces.
While turning, the pilot
must increase total lift so the proportional part supporting the
glider continues adequate
for the task. Speed, angle of attack, or both must
be increased. These changes also increase total drag and steepen the glide
path. Thus, a steeper bank results in a higher sink rate, an important fact to
remember in the art of thermalling.
The correct way to bank a
glider is with the ailerons and rudder, the rudder being used entirely to keep
the fuselage streamlined. The wrong way to bank is with the rudder alone.
Should a pilot push right rudder leaving the ailerons neutral, the fuselage
would pivot to the right around the yaw axis. This would be skidding. A
secondary effect of the skid would cause the wings to bank to the right. The
lift of the wing in the banked attitude would then produce a right turn. This
procedure is faulty for two reasons. The first concerns the high drag set up
by the skidding fuselage which causes loss of altitude. The second concerns
the effect if the wing is close to stalling, when the skidding turn may put the
glider into a spin.
TOTAL
DRAG
One of the major Concerns to
a glider pilot is to try to reduce his drag to a minimum. The total drag of a glider in flight is
composed of INDUCED drag
and PARASITE drag. Parasite drag
is all drag which is not directly
associated with the development
of lift. The wing surface, even at zero lift
produces "profile" drag due
to the skin friction and form. Other
components of the glider much as
rivets, fuselage, tail, tires etc., Contribute to drag because of their own form smoothness and
skin friction.
Parasite drag increases with speed
varying as the square of the velocity, ie. if
you were flying at 200 mph it would have four times as much parasite drag as it would have at
100 mph. consequently, parasite drag will be of greatest importance at high
speeds. At speeds just above stall, parasite drag is only 25% of the total
drag. It is obvious that at high
speeds the better the aerodynamic cleanness of the glider the better to obtain high speed performance.
Induced drag is the drag associated with
the creation of lift. Therefore, if you
increase the lift you also increase
the drag. By increasing the angle of attack, you
will: increase lift, increase
drag, and reduce your speed. (see diagram of drag Curves)
UNDESIRED SIDE EFFECTS OF THE TURN
In turning a glider, the pilot must counteract five undesired side effects, undesired in the sense that if they did not already exist, the designer would not design them into the aircraft. During student training, the development of habit patterns to overcome these side effects, is perhaps the largest single factor in learning to fly. Success will be much easier when the student understands the causes behind his vexations.
The first side effect is called adverse yaw,
and is encountered when banking into or out of a turn. . Adverse yaw is the drag on
the wing which is
raised, this is
due to the increase of angle
of attack by the
lowered aileron (Remember that when you increase the angle of attack;
you lower the airspeed, increase
the lift and increase the induced drag). To roll into a left turn,
for example, the pilot moves the stick to the left. This raises the left
aileron depresses the right aileron. Since the down aileron produces more drag
than the other, an undesirable yawing to the right (against the direction of
the turn) takes place, 
generating unwanted drag as
the fuselage moves sideways through the air. To balance out this adverse yaw
and so streamline the fuselage, the pilot applies left rudder with the left
aileron. This action is called "coordinating stick and rudder", and
the result is called a properly coordinated turn when the yaw string and the
slip-skid ball are centered.
Adverse yaw is mild at low
angles of attack but becomes severe when the wing is nearly stalled. For
example, in a tight turn which is close to an accelerated stall, the
application of full aileron to roll out can stall the low wingtip. The severe
drag on this wing yaws the glider into a dive. If the pilot now makes the
understandable error of trying to raise the glider's nose by pulling the stick
even farther back the result will be a sudden spin. To recover from such an
excessively tight turn, the angle of attack must first be reduced by moving
the stick forward, followed by use of rudder. Once you have a lowed angle of
attack use aileron/rudder to level the wings. A normal rollout results.
The diving tendency
is the second "I wish it wouldn't happen" in a turn. When the
glider is banked some of the wing's lift is transferred from the task of
support to that of pulling the glider around the turn, as previously explained.
In consequence, the rate of sink increases, stabilizing action causes the
glider's nose to drop, and the airspeed increases. If the pilot does not oppose
this reaction of the glider by moving the stick back, the airspeed will soon
stabilize at a higher level than formerly. To maintain the same speed in the
turn as in straight flight the pilot must use the elevator to keep the nose at
such a position on the horizon as to hold that speed. The steeper the bank, the
more back pressure on the control stick is needed.
The overbanking tendency is the
third annoyance in turning. Having established the desired angle of bank, the
pilot finds that he has to hold top aileron (against the angle of bank) to keep
the bank from steepening. In a glider this is true in all but the shallowest
banks because of the long span and normally small radius of turn. The outer
wingtip moves faster than the inner, so the outer wing has more lift, causing
the bank to steepen.
The fourth unwanted side effect, a yaw against
the direction of an established turn (not the adverse yaw caused
by aileron drag) appears because the faster-moving outer wing has more drag.
Rudder in the direction of the turn is needed to balance the wing's yawing
force, the amount being indicated by the yaw string or slip-skid ball. If these
are centered the turn is correctly coordinated, even though the controls are
crossed", which means rudder is applied on one side and aileron on the
other. "Crossed controls" is a heinous sin in powered flight.
." It should be noted that the same undesired effects of
a turn are in a powered aircraft, but are most noticed when in slow flight or
steep turns. Most of these effects are
more noticeable in the sailplane because of our long wings and slower speeds.
Power pilots please note that a glider is flown so the fuselage slips through
the air with minimum drag, ball/yaw string in the center, whatever position
of the controls is required to accomplish this end. The same should be true in
an airplane.
The
fifth and last (thank goodness) side effect of turning is the increase
in stalling speed. As was explained earlier, during a turn the
pilot applies back pressure on the stick to increase the angle of attack. Thus
the glider is closer to a stalled angle than in a level glide at the same
speed. Another way to look at the situation is that the load of centrifugal
force is added to the weight of the glider; this higher total load on the wing
raises the stalling speed. The thermalling pilot soon learns that he has to
increase his airspeed as he steepens his bank in order to keep from stalling.
It should be noted that the increase in stalling speed is caused by the
increased wing loading rather than by the angle of bank per se. In the
performance of aerobatic maneuvers such as a wingover the bank may be vertical
and the stick is not brought back; there is no turning and no centrifugal
force, and the glider's wing is not stalled even though the glider is momentarily
hanging almost motionless in a vertical bank.
The
following table gives an idea of the degree to which stalling speed increases
with the angle of bank in a properly executed turn. The percentage
increase is included so the reader can calculate actual values for any glider.
Figures are also given for a typical trainer and a high-performance sailplane.
|
Angle of Bank |
Load |
% Increase in Stall Speed |
Trainer Stalling Speed |
High-performance Stalling Speed |
|
00 |
1.0 |
0 |
31 |
46 |
|
300 |
1.18 |
8 |
33.5 |
50 |
|
450 |
1.4 |
18 |
36.5 |
54 |
|
600 |
2.0 |
40 |
43.5 |
64.5 |
|
750 |
4.0 |
100 |
62 |
92 |
|
900 A "properly executed
turn" is impossible |
||||
EFFECT OF WINDS ON TURNS
An observant beginner pilot (with
instructor) making a full-circle turn at an altitude of less than a thousand
feet while under strong and steady wind conditions, will be convinced that
glider turns are affected by the wind. In a downwind direction, even a steep
bank produces a turn of large radius over the earth and the glider goes very
fast. Turning in the upwind direction, the radius of the turn is very small
and the speed is visibly slowed. The student pilot knows that wind
affects the way the glider turns because he can see the effect just by
watching the ground. He feels that the airspeed indicator must be wrong, and
that the instructor, who is saying the wind doesn't affect the way a glider
turns, simply doesn't believe the evidence of his eyes.
Everything the beginner sees
is “absolutely so.” It is what he
cannot see, which is the motion of the parcel of air in which he is flying,
that lead’s to his incorrect conclusion.
He can’t even feel the strong wind that is causing his glider to behave
so eccentric all compared to the expected constant-rate turn.
An analogy may clarify the
situation. Consider a motorboat turning
in a river. The boat, in making a
constant rate turn, will soon run into its own wake, and the skipper had looked
over the side and watched his path over the river-bottom, he would have seen
the resultant of the circular motion of the boat and the straight motion of the
river. It probably would never occur to him that a boat's turning capability is
different in a river than on the placid surface of a lake because he can see
all the elements that are involved, unlike our beginner pilot.
When turning in a high wind the pilot makes no
change in his use of the controls. However, when he is concerned with his track
over the ground, as when in the traffic pattern or on the cross-wind leg of a
triangle flight, he must crab into the wind enough to compensate for the motion
of the air.
It should be noted that changes
in wind direction or velocity, as distinct from a steady wind, do affect the
way a glider handles. Such conditions, occurring in thunderstorms, wave rotors,
and in the 
disturbed
air close to the ground are future discussions.
