Lift 5
Probability ofWakeTurbulence Encounter
Certain separation minima are applied by Air Traffic Control (ATC), but this does not guarantee avoidance. ATC applied separation merely reduces the probability of an encounter to a lower level, and may minimize the magnitude of the upset if an encounter does occur. Particular care should be exercised when following any substantially heavier aircraft, especially in conditions of light wind. The majority of serious incidents, close to the ground, occur when winds are light.
WakeTurbulence Avoidance
If the location of wake vortices behind a preceding or crossing aircraft are visualized, appropriate flight path control will minimize the probability of a wake turbulence encounter. Staying above and/or upwind of a preceding or crossing aircraft will usually keep your aircraft out of the generating aircraft’s wake vortex. Unfortunately, deviating from published approach and departure requirements in order to stay above/upwind of the flight path of a preceding aircraft may not be advisable. Maintaining proper separation remains the best advice for avoiding a wake turbulence encounter.
Ground Effect
When landing and taking off, the closeness of the wing to the ground prevents full development of the trailing vortices, Figure 5.21, making them much weaker. Upwash and downwash are reduced, causing the effective angle of attack of the wing to increase, (ref: Figure 5.15). Therefore, when an aircraft is “in ground effect” lift will generally be increased and induced drag (CDi) will be decreased. In addition, the reduced downwash will affect both longitudinal stability because of CP movement, and the pitching moment because of changes to the effective angle of attack of the tailplane, (Ref: Figure 5.23).
Figure 5.21
Lift 5
91
5 Lift
Lift 5
The Impact of Ground Effect
The influence of ground effect depends on the distance of the wing above the ground. A large reduction in CDi will take place only when the wing is very close to the ground, (within half the wingspan).
For a representative aircraft with a 40 m span, (Ref. Figure 5.22):
•At a height of 40 m, the reduction in CDi is only 1.4%.
•At a height of 10 m, the reduction in CDi is 23.5%, but
•At a height of 4 m, the reduction in CDi is 47.6%
|
60 |
|
|
|
|
|
|
|
|
|
|
|
Percent |
50 |
|
|
|
|
|
|
|
|
|
|
|
Reduction |
|
|
|
|
|
|
|
|
|
|
|
|
in |
40 |
|
|
|
|
CL |
Constant |
|
|
|
||
Induced |
|
|
|
|
|
|
|
|||||
Drag |
30 |
|
|
|
|
|
|
|
|
|
|
|
Coefficient |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
CDi |
20 |
|
|
|
|
|
|
|
|
|
|
|
10 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
0 |
0.1 |
0.2 |
0.3 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
0.9 |
1.0 |
1.1 |
|
0 |
|||||||||||
|
|
|
Ratio of wing height |
to span |
(h/b) |
|
|
|||||
Figure 5.22
The height of the wing above the ground when the aircraft is in the landing attitude is influenced by its mounting position on the fuselage. From the graph in Figure 5.22 it can be seen that the last few metres makes a big difference to the reduction of CDi. In general, it can be said that a low wing aircraft will experience a greater degree of ground effect than an aircraft with a high mounted wing.
92
Lift 5
High and LowTail Characteristics
While ground effect may possibly change the aerodynamic characteristics of the tailplane in its own right, a low mounted tailplane will have its effective angle of attack modified by the changing downwash angle behind the wing. A high mounted tailplane may be outside the influence of the changing downwash angle and not suffer the same disadvantages.
Normal |
5 |
Downwash |
|
|
Lift |
Down load |
|
on |
Tailplane |
Downwash Decreased |
|
by Ground Effect |
|
Decreased |
|
Down load |
|
on Tailplane |
|
Figure 5.23
93
5 Lift
Lift 5
" NORMAL" DOWNWASH
POSITIVE
CAMBER
Tailplane (Every illustration)
NEGATIVE
CAMBER
SYMMETRICAL
REDUCED
DOWNWASH
ANGLE
INCREASED
UPFORCE
DECREASED
DOWNFORCE
DECREASED
DOWNFORCE
Figure 5.24
Influence ofTailplane Camber on Pitching Moment
It can be seen from Figure 5.24 that the type of tailplane camber does not influence the pitching moment generated when downwash from the wing changes. Decreased downwash will always result in an aircraft nose-down pitching moment. The opposite will be true of increased downwash.
Downwash will change not only because of ground effect, but also when flaps are operated
and when a shock wave forms on the wing at speeds higher than MCRIT, so appreciation of this phenomena is a key element towards a full understanding of Principles of Flight.
94
Lift 5
Lift 5
Figure 5.25
Tailplane Angle of Attack
Angle of attack is the angle between the chord line and the relative airflow. The relative airflow has three characteristics:
•Magnitude - the speed of the aircraft through the air; the True Airspeed (TAS)
•Direction - parallel to and in the opposite direction to the aircraft flight path, and
•Condition - unaffected by the presence of the aircraft.
Air flowing off the wing trailing edge (downwash) cannot be defined as relative airflow because it does not conform to the definitions. Neither is it possible to think strictly of a tailplane angle of attack. Airflow which has been influenced by the presence of the aircraft (direction of flow and dynamic pressure) must be thought of as Effective Airflow. And the angle between the chord line and the effective airflow must be thought of as Effective Angle of Attack.
Consider Figure 5.25. Airflow from direction A gives the tailplane zero (effective) angle of attack. Airflow from direction E, F or G would be an increase in (effective) angle of attack. If airflow from direction G is now considered, flow from F, E, A, B, C or D would be a decrease in (effective) angle of attack. The term “negative angle of attack” is not used.
95