Aerodynamics: Swept Wings, Speed & Stalls Flashcards
What is anhedral?
The downward inclination of a wing from the root to the tip.
What is lift?
The phenomenon generated by an aerofoil due to pressure differences above and below the aerofoil.
Describe how you would design a high-speed aircraft wing
Thin, minimal-chamber, swept wings.
In designing a high-speed wing, you need to consider first the requirement for economical highspeed performance in the cruise configuration. However, you also have to consider the restraints on
the design of the need to keep airfield performance within acceptable limits and the need to give the
structural people a reasonable task.
There are several interactive design areas of a wing. Some are purely for lift, some are a
compromise between lift and speed, and some are purely for speed. For the high-speed requirements
of a wing, the design would focus on sweep, thickness, and chamber.
The degree of sweep, thickness, and chamber used for the final high-speed wing design depends
on their many interactive compromises that culminate in directly fulfilling the wing’s high-speed
requirement and inversely the wing’s lift and structural requirements.
How does a swept wing aid the increase in its critical Mach number (Mcrit
) speed?
The swept-wing design increases its Mcrit speed because it is sensitive to the (airflow) airspeed
vector normal to the leading edge for a given aircraft Mach number. A swept wing makes the velocity
vector normal (perpendicular) (AC) to the leading edge a shorter distance than the chordwise
resultant (AB). Since the wing is responsive only to the velocity vector normal to the leading edge, the
effective chordwise velocity is reduced (in effect, the wing is persuaded to believe that it is flying
slower than it actually is). This means that the airspeed can be increased before the effective
chordwise component becomes sonic, and thus the critical number is raised.
Describe how you would optimize the lift design on a swept wing.
To optimize the lift design on a swept wing, you would need to (1) examine and develop the lift
design areas of the clean wing and (2) add high lift devices to the clean wing to a degree that satisfies
our main lift concern, that of adequate airfield performance.
What advantages does a jet aircraft gain from a swept wing?
The advantages a jet aircraft gains from the swept wing are (1) high Mach cruise speeds and (2)
stability in turbulence.
1. High Mach cruise speeds. The swept wing is designed to enable the aircraft to maximize the high
Mach speeds its jet engines can produce. The straight-winged aircraft experiences sonic disturbed
airflow, resulting in a loss of lift at relatively low speeds. Therefore, a different wing design was
required for the aircraft to be able achieve higher cruise speeds. The swept-wing design delays the
airflow over the wing from going supersonic and, as such, allows the aircraft to maximize the jet
engine’s potential for higher Mach cruise speeds. Additionally, the swept wing is also designed
with a minimal chamber and thickness, thereby reducing profile drag, which further increases the
wing’s ability for higher speeds. (See Q: Describe how you would design a high-speed aircraft
wing, page 19.)
2. Stability in turbulence. Ironically, a disadvantage of the swept wing is its poor lift qualities,
which lends itself to an advantage in that it is more stable in turbulence compared with a straightwinged aircraft. This is so because the swept wing produces less lift and therefore is less
responsive to updraughts, which allows for a smoother, more stable ride in gusty conditions.
What disadvantages does a jet aircraft suffer from a swept wing?
Because the swept wing is designed for high cruise speeds, it suffers from the following
disadvantages as a consequence:
1. Poor lift qualities are experienced because the sweep-back design has the effect of reducing the lift
capabilities of the wing.
2. Higher stall speeds are a consequence of the poor lift qualities of a swept wing.
3. Speed instability is the second consequence of poor lift at lower speeds for the swept-wing
aircraft. Speed is unstable below minimum drag speed (VIMD) because the aircraft is now sliding
up the back end of the jet drag curve, where power required increases with reducing speed. This
means that despite the higher coefficient of lift (CL
) associated with lower speeds, the drag penalty
increases faster than the lift; therefore, the lift-drag ratio degrades, and the net result is a tendency
to progressively lose speed. Thus speed is unstable because of the drag penalties particular to the
swept wing. (See Q: Explain speed stability, page 22.)
4. A wing-tip stalling tendency is particular to a swept-wing aircraft mainly because of the high local
CL
loading it experiences. Uncorrected (in the design), this effect would make the aircraft
longitudinally unstable, which is a major disadvantage.
Where does a swept wing stall first, and what effect does this have on the aircraft’s attitude?
A simple swept and/or tapered wing will stall at the wing tip first if not induced/controlled to stall at
another wing section first by the designer.
This is so because the outer wing section experiences a higher aerodynamic loading due to the
wing taper, which causes a greater angle of incidence to be experienced to a degree where the
airflow stalls at the wing tips. The boundary layer spanwise airflow, also a result of sweep, further
contributes to the airflow stalling at the wing tips.
A stall at the wing tip causes a loss of lift outboard and therefore aft (due to the wing sweep),
which moves the center of pressure inboard and therefore forward; this produces a pitch-up tendency
that continues as the wing stalls progressively further inboard.
A wing-tip stall is resolved in the wing design with the following better aerodynamic stalling
characteristics:
1. Greater chamber at the tip; this increases airflow speed over the surface, which delays the stall.
2. Washout or twist, which creates a lower angle of incidence at the wing tips and delays the effect of
the outboard wing loading that causes the stall.
Explain speed stability.
Speed stability is the behavior of the speed after a disturbance at a fixed power setting.
The behavior of an aircraft’s speed after it has been disturbed is a consequence of the drag values
experienced by the aircraft frame.
Speed is said to be stable if after it has been disturbed from its trimmed state it returns naturally to
its original speed. For example:
1. An increase in speeds leads to an increase in drag, thus causing a return to the original speed.
2. A decrease in speed leads to a decrease in drag, thus causing a return to the original speed.
Speed is said to be unstable if after it has been disturbed from its trimmed state the speed
divergence continues, resulting in a negative speed stability. For example:
1. A decrease in speed leads to an increase in drag, which causes a further decrease in speed, thus
causing a negative speed divergence.
2. An increase in speed leads to a decrease in drag, which causes a further increase in speed, thus
causing a positive speed divergence.
What is Mach number?
Mach number (MN) is a true airspeed indication, given as a percentage relative to the local speed of
sound; e.g., half the speed of sound = 0.5 Mach.
What is the critical Mach number (Mcrit
)?
Mcrit
is the aircraft’s Mach speed at which the airflow over a wing becomes sonic—critical Mach
number.
The aircraft’s Mach speed is lower than the airflow speed over a wing. A typical Mcrit speed of
0.72 M experiences sonic Mach 1 airflow speed over the upper surface of the wing. Subsonic aircraft
experience a rapid rise in drag above the critical Mach number, and because the aircraft’s engines do
not have the available power to maintain its speed and lift values under these conditions, the aircraft
suffers a loss of lift.
Describe the characteristics of critical Mach number (Mcrit
)?
- Initial Mach buffet, caused by the shock waves on the upper surface of the wing as the aircraft
approaches Mcrit
, is usually experienced. - An increase in drag because of the breakdown of airflow causes the stick force to change from a
required forward push to a neutral force and then a required pull force as the aircraft approaches
and passes Mcrit
. - A nose-down change in attitude (Mach tuck) occurs at or after Mcrit
. - A possible loss of control.
Describe the changes in the center of pressure as an aircraft speed increases past the critical
Mach number (Mcrit
).
The center of pressure moves rearward on a swept wing as the aircraft passes its Mcrit
for two
reasons:
1. The shock waves on the wing’s upper surface occur toward the leading edge because of the greater
chamber, which creates the greatest airflow velocity to be experienced at this point. This upsets
the lift distribution chordwise and causes a rearward shift in the center of the lift (center of
pressure).
2. The swept wing tends to experience the shock-wave effect at the thick root part of the wing first,
causing a loss of lift inboard, and therefore, the lift force now predominantly comes from the
outboard part of the swept wing, which is further aft because of the wing sweep.
What is Mach tuck?
Mach tuck is the nose-down pitching moment an aircraft experiences as it passes its critical Mach
number (Mcrit
).
Mach tuck is a form of longitudinal instability that occurs because of the center of pressure’s
rearward movement behind the center of gravity (see preceding question), which induces the aircraft
to pitch down (or the aircraft’s nose to tuck).
What is the purpose of a Mach trimmer?
The purpose of a Mach trimmer is to automatically compensate for Mach tuck (longitudinal
instability) above Mcrit
.
What is a Mach trimmer, and what is it used for?
A Mach trimmer is a system that artificially corrects for Mach tuck above the aircraft’s Mcrit by sensing the aircraft’s speed and signaling a proportional upward movement of the elevator or variable-incidence stabilizer to maintain the aircraft’s pitch attitude throughout its speed range up to its maximum Mach demonstrated flight diving speed (MDF).
Note: Mach trimmers allow for an aircraft’s normal operating speed range to be above its Mcrit. In the event of a Mach trimmer failure, there is usually an imposed reduced Mach maximum operating speed (MMO) value so that a margin is retained below the Mach speed at which the onset
of instability occurs.
What are the effects of compressibility?
Compressibility is the effect of air being compressed onto a surface (at a right angle to the relative
airflow), resulting in an increase in density, and thus dynamic pressure rises above its expected
value. It is directly associated with high speeds. (See Chapter 5, “Atmosphere and Speed,” page
115.)
There are two main effects of compressibility:
1. Compressibility error on dynamic pressure reading flight instruments; e.g., air speed indicator
shows an overread error that is greater the higher the aircraft’s speed. (See Q: Describe
equivalent air speed [EAS], page 121.)
2. Compressed air is experienced on the leading edge of the wing, which disturbs the pressure pattern
on the wing and causes the disturbed air shock-wave/drag effect at the critical Mach number.
Explain speed margins.
A speed margin is the difference between the aircraft’s normal maximum permitted operating speed
and its higher certified testing speed.
For a piston-engined propeller aircraft:
VNO is the normal operating maximum permitted speed.
VNE is the higher, never exceeded operating speed.
VDF is the maximum demonstrated flight diving speed, established during design certification
flight trials.
The piston-engined propeller aircraft enjoys a relatively large margin between VNO and VDF and
has very little overspeed tendencies. Therefore, the speed margin for a piston-engined propeller
aircraft is not very significant.
For the jet aircraft:
VMO/MMO is the maximum indicated operating speed in knots or Mach number. This is the normal maximum operating speed, which ensures an aircraft’s structural integrity and adequate handling qualities.
VDF/MDF is the maximum demonstrated flight diving speed in knots or Mach number
established during the design certification flight trials. This flight diving speed incurs reduced
aircraft structural integrity and often a lower level of handling qualities.
The jet aircraft’s margin between VMO/MMO and VDF/MDF is relatively small, and because of its low cruise drag and the enormous power available from its jet engines, especially at low altitudes,
the jet aircraft has a distinct overspeed tendency. Therefore, the speed margin on a jet aircraft is very significant.
Explain maneuverability margins/envelope.
Maneuverability margin/envelope is contained by its upper and lower speed limits, which are either
(1) between the aircraft’s stall speed (VS
) at the bottom end of its speed range and its VDF/MDF speed at the top end of its speed range or (2) between 1.2/3 VS
(representing a safe operating limit
above the stall) at the bottom end of its speed range and VMO/MMO at the top end of its speed range.
What is coffin corner?
Coffin corner occurs at an aircraft’s absolute ceiling, where the speeds at which Mach number buffet and prestall buffet occur are coincident, and although trained for, in practice, they are difficult to distinguish between. Therefore, a margin is imposed between an aircraft’s operating and absolute ceiling. (See Q: What is the absolute/maximum service ceiling? Page 209.) Mach number and the slow speed stall buffet are coincident at coffin corner because a stall is a function of indicated air speed (IAS) and Mach number is a function of the local speed of sound
(LSS), which itself is a function of temperature. For a constant Mach number (which is the normal mode of speed management), the IAS decreases
with altitude due to the decreasing LSS. To prevent the IAS decreasing to its stall speed, the Mach number must be increased, which results in an increasing IAS.
For a constant IAS, the Mach number increases with altitude due to a decreasing LSS and temperature to a point where the IAS exceeds Mcrit. To prevent the Mach number exceeding Mcrit
, the IAS must be reduced, which results in a decreasing Mach number.
Therefore, there comes a point at the aircraft’s absolute ceiling where the aircraft can go no higher. This is so because it is bounded on one side by the low-speed buffet and on the other by the
high-speed buffet because the stall IAS and the Mcrit values are equal. This is coffin corner, and this effect restricts the attainable altitude by the aircraft.
Explain why an aircraft stalls.
An aircraft stalls when the streamlined/laminar airflow (or boundary layer) over the wing’s upper
surface, which produces lift, breaks away from the surface when the critical angle of attack is
exceeded, irrespective of airspeed, and becomes turbulent, causing a loss in lift (i.e., the turbulent air
on the upper surface creates a higher air pressure than on the lower surface). The only way to recover
is to decrease the angle of attack (i.e., relax the back pressure and/or move the control column
forward).
What properties affect an aircraft’s stall speed?
An aircraft will stall at a constant angle of attack (known as the critical angle of attack). Because
most aircraft do not have angle of attack indicators (except “eyebrows” on some electronic flight
instrument system displays), the pilot has to rely on airspeed indications. However, the speed at
which the aircraft will stall is variable depending on the effects of the following properties.
1. Weight
a. Actual weight
b. Load factor, g in a turn
c. Effective weight/center of gravity position
2. Altitude
3. Wing design/lift
4. Configuration
5. Propeller engine power
How does the stall speed vary with weight?
The heavier the aircraft, the higher is the indicated speed at which the aircraft will stall.
If an aircraft’s actual weight is increased, the wing must produce more lift (remember that the lift
force must equal the weight force), but because the stall occurs at a constant angle of attack, we can
only increase lift by increasing speed. Therefore, the stall speed will increase with an increase in the
aircraft’s actual or effective weight.
The stall speed is proportional to the square root of the aircraft’s weight.
What wing design areas delay the breakup of airflow (stall)?
- Wing slots are the main design feature that delays/suppresses stall speed. A slot is a form of
boundary layer control that reenergizes the airflow to delay it over the wing from separating at the
normal stall speed. The wing therefore produces a higher coefficient of lift (CL
) and can achieve a
lower speed at the stall angle of attack. - Lower angle of incidence and a greater chamber for a particular wing section, e.g., wing tips.
