Propellers & trimming 439


A propeller gives propulsive force from rotary motion of an engine  either directly or by gear reduction as shown in my jet engine article. Propellers are not only used in small aircraft they are also used in turboprops.

FORCES ACTING ON A PROPELLER

  1. Centrifugal force – it is the force that tends to pull the the blades away from the hub when turning .
  2. Torque bending force – caused by air resistance acting upon the blades in-conjunction with inertia causing blades to bend in opposite direction to direction of rotation.
  3. Aerodynamic twisting forces -caused because center of lift is ahead of center of rotation this tends to cause twist of the blade
  4. Thrust bending force – this force bends the blade forward at the edges due to high thrust and blade cross section.

TYPES OF PROPELLERS

  1. Fixed pitch – usually made of wood or metal and is found in one piece.
  2. Two position  –  this is a propeller which can have its pitch changed from one position to another angle by the pilot during flight.
  3. Ground adjustable pitch – in this case settings of pitch can only be adjusted with tools on ground before the engine runs.
  4. Constant speed – in this case a governor operated by the pilot at the cockpit is utilized to operate pitch changing mechanism that may either be electrical or hydraulic.
  5. Controllable pitch – pitch of the propeller can be changed in flight or while engine is running; a pitch changing mechanism is in this case operated hydraulically.
  6. Beta control – This propeller allows manual re-position of blade angle and often used by pilot during taxi.
  7. Reverse  – has the capability to have negative blade angle and produce a reversing thrust.
  8. Feathering – has the ability to turn the edge towards the wind and therefore eliminate drag & windmilling on event of engine failure. Feathering is when the blades are turned such that their middle to outer section is aligned with airflow to create minimal air resistance.

TRIMMING

Trimming is used in flight control. When an aircraft is trimmed it will maintain its altitude and speed without the pilot having to apply any load on cockpit controls i.e stick force.

METHODS OF TRIMMING

  1. Trimming tabs – usually found in the trailing edge flaps to adjust trim.
  2. Variable incidence tail plane
  3. Sprig bias system
  4. Center of gravity adjustment
  5. Adjustment of artificial feeling unit.

TYPES OF TRIMMING TABS

  1. Balance tab
  2. anti-balance tab
  3. Spring tab
  4. Servo tab

This trimming tabs will be described in a different article.

 

 


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