Ground Lesson: Airplane Flight Control

Description: It’s important to understand the flight control systems a pilot uses to control the forces of flight and the aircraft’s direction and attitude. 

 

References:

 

Pilot Handbook of Aeronautical Knowledge-Chapter 5

 

Objective: Students will gain knowledge and understanding of the elements related to the operations of systems. 

 

Elements: 

 

• Primary flight controls 

• Secondary flight controls

• Trim controls 

 

Instructor Action

 

Primary flight controls - Intro 

• The rudder controls the yaw which rotates the airplane around the vertical axis

• The elevator controls the pitch along the plane’s lateral axis.

• Ailerons control roll about the longitudinal axis.

• Movement of any of the three primary flight control surfaces changes the airflow and pressure distribution over and around the airfoil. 

• Inputs by the pilot affect the lift and drag produced by the airfoil/control surface combination

• Allows the pilot to control the plane about its three axes of rotation 

• Low airspeeds make the controls respond to feel sluggish and soft since there is less aerodynamic pressure

• High airspeeds, the controls feel firm and has a rapid response

 

 

A. Ailerons 

1. The aileron controls the roll which causes a movement along the plane’s longitudinal axis

2. Attached to the outboard trailing edge of each wing. 

3. Connected via cables, pulleys and/or push-pull tubes to a control wheel

4. Moving the yoke to the right causes the right aileron to deflect upwards and the left aileron to the deflect downward. The opposite is true for a left turn.

   1. Adverse Yaw

      1. Downward deflected ailerons produce more lift but produce more drag.

      2. This added lift will create more drag and will slow the wing down.
      3. Drag slows down the wing and causes a yaw opposite of the bank so you must counter with rudder
      4. The adverse yaw is a result of differential drag and the slight difference in the velocity of one of the wings
   2. Countering Adverse Yaw
      1.  The rudder is used to counteracting adverse yaw.
      2.  The most amount of rudder used will be at low airspeeds, high AOA, and with large ailerons.
      3.  The rudder is less effective at lower speeds. 
         1. Differential Ailerons: One aileron is raised a greater than the other aileron and lowered for a given movement of the control wheel. This produces more drag on the descending wing (raised aileron) which cancels some adverse yaw. 
         2. Frise Type: The aileron that is being raised has its leading edge into the airflow and creates drag which helps equalize the drag created by the lowered aileron. This aileron also forms a slot so air flows smoothly over the lowered aileron making it more effective at high AOA. 
         3. Coupled Ailerons and Rudder: This is when the ailerons and rudder are link controls via interconnected springs which help correct the aileron drag by automatically deflecting the rudder.
         4.  Flaperons: Combines both aspects of flaps and ailerons. It can also be lowered together to function as a set of flaps. A mixer is used to combine the separate inputs into this single set of control surfaces callers flaperons.

B. Elevator

1. Controls pitch about the lateral axis from the control column in the flight deck by a series of mechanical linkages. 

2. Aft movements of the control column deflect the trailing edge of the elevator surface up which causes the tail of the plane to down and the nose to pitch up.
3. The up elevator position decreases the camber of the elevator and created a downward force. 

   1. T-Tail

   1. The elevator is above most of the downwash from the propeller and reduces noise and vibrations.

   2. Forces required to move the nose of the t-tail are greater as the downwash from the propeller does not help in pushing the tail.

   3. At high AOA with low airspeed and an aft CG, it is more susceptible to a deep stall which results in the wake of the wing impeding on the tail surface.

   2. Stabilator

   1. A one-piece horizontal stabilizer that pivots from a central hinge point. 

   2. It is much more responsive than a conventional elevator design. 

   3. Antiservo tabs are used on the trailing edge to decrease sensitivity.

movements of the control column deflect the trailing edge of the elevator surface up which causes the tail of the plane to down and the nose to pitch up

 

 

 

 

 

 

C. Rudder

1. Controls the movement of the aircraft about its vertical axis via a movable surface hinged to a fixed surface in this case to the vertical stabilizer. 

2. Controlled by the pedals. 

3. When the rudder is deflected into the airflow, a horizontal force is exerted in the opposite direction. 

4. When you push the left pedals, the tail moves to the right and yaws plane to the left. 

 

 

 

 

 

Secondary Flight Controls

FLAPS

1. 1. High lift device used on aircraft which are attached to the trailing edge of the wing to increase both the lift and induced drag for any given AOA. 
   2. Flaps allow a compromise between low landing speed and high cruising speed as it can be retracted back and forth
      1. Plain flap: Increases the airfoil camber resulting in a coefficient of lift at a given AOA but creates a lot of drag and a node down pitching moment as the CP moves aft. 
      2. Split flap: Deflected from the lower surface of the airfoil and produces more lift than the plain flap. Drag is created by the turbulent air patterns it produces. 
      3. Slotted flaps: Increases the lift coefficient more than the previous flaps. A duct forms between the flap well when flaps are lowered where high energy from the lower surface is ducted to the upper surface which accelerates the upper surface boundary layer and delays airflow separation. 
      4. Flower Flaps: Changes the Camber of the wing and increase. the wing area. It sides down backward on tracks instead of rotating down on a hinge Creases a lot of lift in the first sequence.
2. Leading Edge Device
   1. HIgh lift devices can also be applied to the leading edge of the airfoil.
      1. Fixed slots: Directs air to the upper wing surface to delay the airflow separation at higher angles of attacks as it increases the wing chamber. 
      2. Movable slats: Leading-edge segments that move on tracks which means at a low angle of attacks, each slat is held flush against the wings leading edge by high pressure that forms at the leading edge. As soon as the AOA increases, the high pressure moves aft below the lower surface of the wing, allowing the slats to move forward allowing the air below the wing to flow over the upper surface. 
      3. Leading-edge flaps: Performs similarly like trailing edge flaps to increase the coefficient of lift and the camber of the wings. 
      4. Leading-edge cuffs: It extends the leading edge down and forward. This causes the airflow to attach better to the upper surface of the wing at the higher angles of attack, lowering the plane's stall. 
3. Spoilers
   1. High drag devices deployed from the wings to spoil the smooth airflow reducing lift and increasing the drag
   2. can be used to counter adverse yaw so the raised wrong can have its spoilers deployed to increase and equalize drag.
   3. Reduces ground roll during landings.

Trim Controls 

Trim

1. A single trib tab attached to the trailing edge of the elevator and manually operated by a small, vertically mounted control wheel. 

2. Placing the trim control in the full nose-down position will move the trim tab to its full up position so that the airflow over the horizontal tail surface forces the trailing edge of the elevator down. 

3. If the trim tab is set to the full nose-up position, the tab moves to its full down position, so that the airflow under the horizontal tail surface hits the tab and forces the trailing edge of the elevator up. 

4. Establish the desired power, pitch, attitude, and configuration first then trim.

1. Balance tabs

1. Looks like trim tabs but they coupled to the control surface rod so that when the primary control surface is moved in any direction, the tab automatically moves in the opposite. 

2. Servo Tabs

1. A small portion of a flight control surface that deploys in such a way that it helps to move the entire flight control surface in the direction that the pilot wishes it to go. 

2. Decreases pilot workload and de-stabilize the aircraft. 

3. Antiservo Tabs: Works the same way as to balance tabs but they move the in the same direction as the trailing edge of the stabilator so that it can decrease the sensitivity of the stabilator. 

4. Ground Adjustable Tabs: A nonremovable metal trim on the rudder which is only movable on the ground. Adjustments are necessary until the aircraft no longer skids left or right.

5. Adjustable stabilizer: instead of having trim tabs, some planes have an adjustable stabilizer. Linkages provide the horizontal stabilizer about its rear spar via a jack crew mounted on the leading edge of the stabilizer. Jackcrews powered by wheel or crank