Mastering the crosswind
Author: Colin Aro
Article extracted from LightspeedAviation.com
The original article has been divided into four to facilitate the reader’s reading.
Turns and Slips
In the previous chapter, we looked at the effect of a crosswind on the aircraft’s flight path and the correction of that effect through the use of a crab angle. In both cases, the aircraft is pointed in a different direction than it is tracking. Therefore a crab angle is useful in correcting an aircraft’s ground track but useless for a crosswind landing due to the obvious potential for directional control problems.
Landing in a Crosswind
Obviously, the key to successfully negotiating a crosswind landing is to point and fly in the same direction as we approach the runway and transition to the ground. This helps immensely with directional control and avoids placing an excessive side load on the landing gear. Doing so requires the pilot to correct for crosswind drift WITHOUT a change in heading! The solution: the horizontal component of lift, the force that turns the aircraft…let’s digress a bit.
Turning Forces on an Aircraft
When the aircraft is banked, the lift vector is tilted and no longer directly opposes gravity. It has a vertical and horizontal component – the vertical component directly opposes gravity while the horizontal component is the force that turns the aircraft. Figure 1 illustrates the situation: the vertical and horizontal components add together to equal total lift. The vertical component of lift opposes gravity, so it has no direct bearing on the turning of the aircraft.
Its only effect is to keep the aircraft in level flight. Figure 2 depicts this – the force acting on the aircraft, the horizontal component of lift, deflects the flight path in the direction of bank. This produces a curved flight path in the direction of the horizontal component – a turn.
Rudder – What is it Good For?
As we know, the horizontal component of lift turns the aircraft – so what does the rudder do? The rudder coordinates flight. Too little or too much rudder results in a slip or a skid. Another way to describe a slip or a skid is that the aircraft’s heading change is too slow or too fast for the bank angle. Again, in both cases we’re not pointed where we’re headed, but this time we can take advantage.
We are more interested in the case of a slip, where the heading change is too slow for the bank angle. If the pilot goes as far as to use opposite rudder, there are moderate bank angles for which the heading change can be reduced to zero. This gives the pilot a horizontal force to oppose the crosswind component WITHOUT a change in heading – exactly what we were looking for. NOTE: the aircraft is slipping and therefore NOT in coordinated flight. That’s OK; we’re doing it on purpose! The aircraft is pointed in the same direction as it is travelling. See Figure 3.
With the aircraft on short final, the transition from crab to slip is made establishing the conditions in Figure 3. At this point, the aircraft’s longitudinal axis should be aligned with the aircraft’s flight path and the runway centerline in preparation for roundout and touchdown. Because the aircraft is slipping (and therefore producing extra drag) some power may be needed to compensate for the increased sink rate. Moving forward, this series will conclude with a discussion of roundout, touchdown, and rollout.