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Old 28th Nov 2013, 09:23
  #9 (permalink)  
3 Point
 
Join Date: Oct 2000
Location: South West UK
Posts: 370
Drift or sideslip???
Yaw or sideslip???
Roll or bank????

Such confusing use of terminology! How do we ever expect students to understand what we are talking about??

If we use precise terminology then things are much clearer.

Yaw is an angular motion, rotating around the vertical (or normal) axis of the aeroplane; it is controlled by rudder.

Sideslip is the angle between the aircraft's fore and aft (or longitudinal axis) and the relative airflow (not the wind!).

Roll is an angular motion, rotating around the fore and aft (or longitudinal) axis, it is controlled by ailerons.

Bank is the angular displacement of the aircraft's lateral axis from horizontal (ie are the wings level with the horizon).

Relative airflow is the direction of the air as it flows past the aeroplane.

wind is the direction of airflow across the earth.

Drift is the angle between the aircraft's heading (its longitudinal axis) and its track (its direction over the ground), drift is caused by wind.


So; to turn an aeroplane we need some sort of force acting towards the centre of the desired turn (a centripetal force). Normally we create this by rolling to an angle of bank which tilts the lift vector. The resulting horizontal component of the lift vector provides the centripetal force and the aeroplane turns.

If we use rudder to yaw the aeroplane but use ailerons to hold the wings level we will have an element of force acting horizontally across the aircraft (from one wing tip to the other), this is caused by the air loads on the side surfaces of the fuselage. There will also be an element of the engine thrust acting to the side because we are no longer pointing the aeroplane and hence the trust straight at the relative airflow; these two forces, in combination, will provide centripetal force to turn the aeroplane. That's how the Wrights did it but it's awfully uncomfortable and not very effective so we don't do it that way any more.

For a steady heading sideslip you can roll one way to a steady bank angle (side force is created by the horizontal component of lift) and then use rudder to sideslip in the opposite direction so the side forces from sideslip and bank are equal but in opposite directions and therefore cancel each other out. Presto, sideslip with no turning!

Maulkin, get your instructors to discuss this with you using a model to demonstrate; a picture paints a thousand words! And be careful of people explaining things using sloppy or incorrect terminology; many of the answers on here confused the h*ll out of me!

Happy landings and good luck in your training

3 Point

Last edited by 3 Point; 28th Nov 2013 at 14:39.
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