1. Control on the ground
ATC Chapter 2 & 3

2. Aim
To review principals of aerodynamic forces during taxi, take-off and landing

3. Objectives
State the differences between tail wheel and nosewheel aircraft
Describe slipstream and state how it affects aircraft performance
Explain torque effect and how it affects aircraft performance
Describe gyroscopic affects which are caused by the propeller on take-off
Explain asymmetric blade effect
State how ground effect affects the aircraft

4. 1. Nose wheel vs. tail wheel
Introduction
All propeller driven aircraft have directional problems due to an imbalance of forces caused by the propeller
We need to consider two types of aeroplanes when investigating aerodynamic effects on the ground, these are:
Nose wheel aeroplanes
Tail wheel aeroplanes

5. 1. Nose wheel vs. tail wheel
If an aircraft is to remain stable on the ground the centre of gravity must remain within an area bound by the wheels
The further the centre of gravity is from any of these boundaries the more stable it is
In both tail wheel and tricycle configurations the large surface area behind the centre of gravity will try to weathercock the aircraft
In a tricycle aircraft the CoG is ahead of the main wheels, this design is inherently stable, should a gust of wind try to weather cock the aircraft combination of the main and nose wheel will prevent it doing so

6. 1. Nose wheel vs. tail wheel
A tail wheel aircraft has the CoG behind the main wheels, this design is inherently unstable
If a gust of wind try's to weathercock the aircraft the centrifugal reaction force acting through the CoG acts in the same direction as the gust tightening the turn
If control is lost the aircraft will ground loop

7. 2. Slipstream Slipstream Effect
The air that is accelerated rearward by the propeller is called slipstream
A clockwise spinning propeller will create a clockwise rotation of the slipstream
This creates an asymmetric flow over the fin and the rudder
The slipstream strikes the left hand side of the fin creating an angle of attack
This created angle of attack creates a force on the fin, pushing it right
Creating a yawing moment to the left

8. 2. Slipstream Slipstream Effect
The magnitude of the slipstream is directly proportional to the magnitude of the thrust being produced
If thrust is  slipstream is
Remember thrust  velocity change, as velocity increases thrust decreases
Slipstream will be highest at low airspeed, high power settings such as during takeoff
For a pilot to counter the aerodynamic force of slipstream
Right rudder must be applied
If the aircraft is equipped, rudder trim can be used to assist

9. 2. Slipstream Design Features
There are a number of design features used to minimise the effect of slipstream, the first we will discuss is an offset fin
A fin that is not offset has the same amount of free stream air passing each side
When the fin is offset, an angle of attack is created between the fin and the relative airflow on the right side of the fin (clockwise spinning propeller)
The angle of attack creates an aerodynamic force which opposes the effect of slipstream

10. 2. Slipstream Design Features
The second design feature is an offset thrust line
To offset the thrust line produced by the propeller the engine must be mounted at an angle to the right
By offsetting the thrust line the nose of the aeroplane is pulled to the right, creating a yawing moment to the right
The yawing moment created opposes the yawing moment produced by slipstream
Offsetting the fin and thrust line is done by the manufacturers during the building and testing phases of an aeroplane, the pilot has no control over these forces

11. 3. Torque Effect Torque Reaction Remember Newtons third law of motion:
“For every action there is an equal and opposite reaction”
The rotation created by the torque reaction attempts to roll the aeroplane in the opposite direction
When the aeroplane is on the ground, the landing gear stops the aircraft from rolling
The rolling moment to the left creates an increased force on the left main landing gear
This force increases the amount of friction experienced on the left main landing gear compared to the right
This differential in friction force creates a yawing moment on the aircraft to the left
Yaw
Friction

12. 4. Gyroscopic Effect Gyroscopic Effect
The gyroscopic effect is most prominent on a tail wheel aircraft, when the tail is lifted off the ground
As the tail is lifted, the propeller plane of rotation (PPOR) is changed
The force to change the PPOR is applied to the top of the propeller, the resultant force due to procession will be experienced on the right side of the propeller (clockwise spinning propeller)
The force on the right side of the propeller creates a yawing moment to the left
The magnitude of the gyroscopic effect is directly proportional the PPOR change force, the greater the rate of PPOR change the greater the gyroscopic effect
Yaw

13. 5. Asymmetric Blade Effect
When a propeller moves through the air, with the propeller at right angles to the relative airflow:
The up and down going blades travel the same distance in the same time
The angle of attack on both the up and down going blades are the same
This creates an even amount of thrust being created by both the up and down going blades
If the propellers plane of rotation is inclined the down going blade must travel a greater distance than the up going blade through one half revolution
This causes more thrust to be produced by the down going blade
Additionally the incline angle creates a greater angle of attack on the down going blade in comparison to the up going blade further increasing the amount of thrust produced
Up going blade
Down going blade

14. 5. Asymmetric Blade Effect
The propeller is now generating more thrust on the down going blade side, compared with the up going blade side
When looking at a clockwise spinning propeller, more thrust is being produced on the right hand side of the propeller
This imbalance of thrust creates a yawing moment, with a clockwise spinning propeller the moment is to the left
Yaw
Down going blade
To counter the asymmetric blade effect’s yawing moment to the left the pilot must apply right rudder
Up going blade
Asymmetric blade effect is experienced during the take off roll on tail when aeroplanes as the propeller is not at right angles to the relative airflow, once the tail is lifted and the propeller becomes perpendicular to the relative airflow, asymmetric blade effect ceases

15. 5. Ground Effect Ground Effect
To explain ground effect we must first start by talking about induced drag
Induced drag is created from the pressure differential between the upper and lower surface of the wing and creates
wing tip vortices
In flight these vortices travel from the
bottom surface of the wing to the top

16. 5. Ground Effect Ground Effect
If the wing is close to the ground, approximately half of the wing span, the wing tip vortices are deflected
This deflection reduces the amount of induced drag
Total reaction is the vector addition of lift and induced drag
Total reaction  Lift + Induced Drag
During take off as the aircraft leaves the ground it experiences ground effect allowing the aircraft to be accelerated to the correct climb speed
However if the aircraft is heavy, rotated to a steep climb, flap retracted too early or is rotated at a low airspeed once out of ground effect the aircraft will descend and settle back on the ground

17. 5. Ground Effect Ground Effect
During landing ground effect is most significant and unavoidable
Ground effect is the main reason why the aeroplane floats for an extended distance during the flare, any excess speed during landing will aggravate the float
To avoid any negative effects of ground effect the pilot must:
For take off:
Rotate at the recommended speed
Do not retract flaps prematurely
On hot days at high elevation, lift off at a higher then normal speed and do not rotate to the normal lift off attitude until the correct speed is achieved
For landing:
The negative effect is float
Therefore is lightly loaded or too fast during the flare the float will be extended
This increase in float reduces the available stopping distance

18. Questions?