The February article Moment of Truth on takeoff techniques brought a flood of mail. The comments generally pertained to two parts of the article: the dynamic effect of crosswinds and the effect of headwinds on takeoff runs.
Regarding the effect of crosswinds on controllability, the most common misperception is that the sole purpose of applying aileron control into the wind is to increase ground friction on the upwind wheel and stop the airplane from drifting downwind. It is true aileron into the crosswind does indeed increase friction on the upwind tire and aids in offsetting wind drift, but the primary purpose is to prevent the windward wing from rising.
The greater the surface area presented to the crosswind component, the greater the resultant push. Since the vertical surface area aft of the CG is greater than that forward of the CG, the airplane tends to rotate about the CG into the wind -weather cocking. On tailwheel airplanes both the CG and the larger surface area are behind the main gear, which acts as a restraining point, and explains why tailwheel airplanes can be a bear in crosswinds.
To offset the weather cocking effect and maintain directional control, apply upwind aileron and downwind rudder. Aileron into the wind (and the increase in friction at the upwind wheel) increases the weather cocking effect. The upwind wheel acts as a restraining point and the downwind wheel wants to overtake and rotate about it, but rudder application and nose wheel steering offset the condition.
In a single-engine airplane, the rudder starts being effective when full power is applied and gets more effective as the speed builds. On multiengine airplanes, rudder is not very effective until attaining a designated minimum airspeed (Vmcg), which is usually slightly less than lift-off airspeed. Therefore, in a multiengine engine there is greater dependence upon nose wheel steering for directional control throughout the majority of the ground roll. Thus, it is apparent that the amount of elevator nose-down pressure required to maintain nose wheel effectiveness is dependent upon the particular make/model and class airplane being flown.
Without keeping aileron into the wind, the wing may rise and the crosswind acts on its undersurface. That adds to the pushing effect, causes greater friction on the downwind tire – which then acts as a restraining point – and allows the airplane to rotate downwind. The combination of the downwind push, the upwind wing rising and its resultant bank adding to the rotational force could result in a wheel-barreling effect in a tricycle airplane or a ground loop in a tailwheel airplane.
Thus, in pronounced crosswind takeoffs apply aileron into the wind, apply power before brake release and apply enough forward pressure on the stick to ensure positive nosewheel steering until liftoff airspeed is attained.
One aerodynamic fact that causes confusion is that, for a given crosswind component, the faster the airplane the less the crosswind effect. This leads pilots to believe that a faster airplane is easier to control in any given crosswind. However, this only applies with the airplane out of ground effect and without the wheels acting as restraining points. Because of these additional factors you experience somewhat of a different effect on the ground.
As the forward speed increases, the crosswind shifts and becomes more of a relative headwind and the aileron control is more effective. Therefore you only need to maintain enough aileron pressure to assure the crosswind component does not raise the upwind wing. With a strong crosswind component this procedure may result in the downwind wing lifting the downwind wheel entirely. The one-wheel takeoff may be acceptable in a strong crosswind (80 percent or greater of the maximum demonstrated crosswind component), but could be considered over-controlling in any less crosswind.
One of the oddities of crosswind takeoffs is that, based on airspeed versus wind correction angle, you may think the faster airplane experiences a smaller relative wind angle because it encounters less of a crosswind effect.
Nothing could be further from the truth. When I experienced my first jet checkout I thought crosswind takeoffs would be a snap. I was wrong. Although I experienced similar acceleration times as in a piston-powered transport airplane, the distance traveled was a bit longer and the push resulting from the crosswind acted a bit longer; meaning I had to work harder to offset the crosswind effect.
One of the reasons for this is that wind effect on the ground is a cumulative thing and is dependent upon speed and distance traveled. As the distance traveled is increased, so is the total crosswind effect or push.
Consider that a Cessna 172P takes about 22 seconds at max gross weight to accelerate to its takeoff speed of 51 knots and uses 925 feet of runway. A Cessna T210N at max gross under similar conditions will require a ground roll of 1,360 feet to accelerate to 72 knots, but will also take about 22 seconds.
The fact that the faster airplane requires less of a wind correction angle means that cumulative crosswind effect is roughly the same as with the slower airplane, not less. Its correct that a given crosswind component requires less of a wind correction angle from a faster airplane, but do not expect less of an effort to maintain directional control on the ground in a faster airplane. In fact, as the disparity between speeds and distances of the two airplanes increase, the pilot will have to work harder to maintain directional control.
Headwinds
Change the crosswind to a headwind, and some interesting things happen. The takeoff distance is shorter because headwind essentially gives the wing a head start toward its flying speed. Second, it takes less time to accelerate to the liftoff airspeed.
In the original article, I stated, The airplane accelerates to its liftoff speed in the same amount of time regardless of wind. This statement is incorrect. Somewhere along the line I tripped over my own tongue. I meant to say that the rate of acceleration was the same. The point being made is that for identical conditions the rate of acceleration was the same, but a headwind produced benefits from the time and distances to takeoff and a tailwind stretches the time and distance because of a lower average acceleration.
To examine the effect of wind on takeoff performance, specifically ground roll distance, consider the Cessna 172P. The Takeoff Distance Chart presumes the airplane is at a sea level airport at 20 degrees C. Liftoff airspeed is 51 KIAS and the zero wind ground roll distance is 925 ft.
In the Notes section, item three reads: Decrease distances 10% for each 9-knot headwind. For operation with tailwinds up to 10 knots, increase distance by 10 percent for each 2 knots.
Based on the above, the ground roll a 20-knot headwind would reduce the takeoff roll by 22.2 percent, reducing it from 925 feet to 720 feet.
We now have two values for comparison, a zero-wind condition with a final ground speed of 51 knots and a distance 925 feet, and a 20-knot headwind resulting in a final ground speed of 31 knots and a distance 720 feet. However, we need to determine the time to accelerate to liftoff. Having a feeling for time is important because it is one more parameter you can use to gauge the progress of the takeoff run. It helps you sense when something is not in order, even though the engine may be running smoothly.
First, convert the applicable airspeed from knots to feet per second. The 20-knot headwind becomes 33.78 fps and the liftoff speed of 51 knots becomes 86.139 fps. The average velocity is merely the initial velocity plus the difference between the final and initial velocities divided by two. With a zero headwind, the average velocity is 43.069 fps and with the headwind its 59.959 fps.
Knowing the average velocity and the distance allows you to calculate the time to liftoff. In zero-wind conditions, the ground roll (925 feet) divided by the average velocity (43.069 fps) yields a takeoff run of 21.5 seconds. With a 20-knot headwind, the shorter ground roll (720 feet) and faster average velocity (59,959 fps) means the takeoff run will be 12 seconds.
For more detail on aircraft performance, there are two worthwhile publications available from the Government Printing Office. The Airplane Flying Handbook, FAA-H-8083-3 (1999) and Aerodynamics for Naval Aviators, NAVWEPS 00-80T-80 (Rev. Jan 1965) both provide valuable insight into whats happening to the air around the airframe.
Spend some time studying the subject matter and applying it to your flying. Anything that increases your level of applied knowledge makes you a safer pilot. More importantly, it aids you in recognizing and dispelling the many misconceptions that have crept into aviation over the years and avoiding their pitfalls.
-by Tom Oneto
Tom Oneto is a 13,000-hour ATP/CFII, Part 121 and corporate pilot, and a former Designated Pilot Examiner.
