The use of rudder to maintain the runway centreline on take-off
A common misconception regarding this type of question is to assume that the wind is blowing the whole aircraft off the centreline, thereby needing a turn to correct this. However, in practice it doesn't quite work like this.
First of all, you need to understand how winds are described...
Winds come from the direction stated. So a northerly wind comes from the north. An easterly wind is from the east, and would be written 090. A wind of 130/15 is FROM the bearing 130 at strength 15 kts. (One way of remembering this is to think about the North wind. Its cold because it comes FROM the cold north.)
Let's assume you are making a take-off from Runway 18, with a westerly wind at 15 knots.
- You are on Runway 18, so when ready for take-off you are pointed due
SOUTH.
- A westerly wind comes from the west, so it is coming from your RIGHT hand
side.
- Now this wind will hit the aircraft, and the tail has a nice vertical
surface area that catches the wind more than anything else. Now imagine the
wind is just a big hand gently pushing the tail on its vertical surface. This
pushes the tail a little on its right hand (starboard) side. As the wind is
from the WEST, it will push the TAIL towards the EAST (to port), which rotates
the aircraft in a clockwise direction.
(An easier way of remembering this is that aircraft tend to 'weathercock' INTO wind. So, if you did nothing, ultimately the aircraft would end up facing the wind. This 'weathercocking' effect is important. For example, you need to understand this in order to grasp the subtleties of NDB 'tracking' versus 'homing'.)
- In this example, a weathercock into the wind means you are turning RIGHT.
You are turning INTO the wind.
- So to control this turning motion, you need a little bit of LEFT rudder to
counteract this wind induced turn to the RIGHT so that you track the
centreline. As you gain speed, the amount of rudder you need will reduce.
Caveat: You would also need to use a little aileron too for a
crosswind take-off, but that wasn't the subject of these questions.
Remember that other factors such as propellor torque and also propellor
slipstream on the rudder also cause yaw.
Comment: Of course, in reality, pilots just make these (small)
adjustments with rudder in all sorts of crosswind conditions (usually) without
needing to think too much about it ;-)
References:
See this forum discussion. This is a great posting (with some good commmentary) that was originally published as part of John Brandon's FlySafe Tutorials, which are very well written.
