One thought on “Mysteries of Flight: The Downwind Turn

  1. by Ron Hughes
    If I remember correctly we are taught when an airplane becomes airborne, the airplane is relative to the air only. The airplane does not know or care which way the wind is blowing. All aerodynamic forces and accelerations are exactly the same in a mass of air that is moving relative to the earth (wind) as air that is not moving relative to the earth (calm). The curvature of the earth is not enough to make any difference in the performance of the airplane.
    The following is an old man’s opinion about the downwind turn.
    As far back as this old man can remember gravity has held this good earth together. Gravity is a constant force that is on 24/7/365——366 on leap year. Gravity cannot be canceled or turned off. The center of gravity of the earth should be the reference and pivot point for all calculations. Your position on this earth is expressed in degrees of latitude and longitude. (This would be a good place to mention we should set aside all linear motion math, formulas, physics, and logic because we are dealing with circular motion.)
    Let’s do some flying—-we are in an airplane at 5000 ft. AGL cruising at 120 knots using 70% power with no wind. (As we all know a knot is one nautical mile an hour—a nautical mile is one minute of one degree around the earth at sea level. There are 60 minutes in a degree. We are traveling around the earth at two degrees an hour.) Let’s do a 360 degree rate one turn. The airplane is banked and power is added to compensate for the increase in drag. Airspeed remains at 120 knots. After the turn is established, the aerodynamic forces and accelerations remain constant until the airplane flies into its own wake and /or level flight is established again. The ground track is a circle.
    While flying straight and level at 120 knots airspeed we notice the GPS and the inertial navigation system begin to show a slower ground speed. After a 30 second time span the ground speed is steady at 90 knots but the airspeed has remained at 120 knots. We have flown into a 30 knot head wind. Thrust and drag in the time allowed have done their job at governing the airspeed. Drag tried to increase but the trust could only propel the airplane at 120 knots through the air, so the airplane experienced a negative acceleration relative to the earth. Let’s do another 360 degree rate one turn. (Remembering the airplane is relative to the cg of the earth) After the bank and power are established, the wind is now blowing toward the bottom of the wing increasing the pressure on the bottom of the wing and pulling air away from the top side of the wing decreasing the pressure. This increases the lift the wing is producing. (This has the same effect as flying into an updraft while flying level). The airplane is experiencing a positive acceleration in the same direction the wind is blowing. At the same time the airplane is also experiencing a positive acceleration relative to the earth because trust and drag are governing the air speed as the drag is trying to decrease but the trust won’t let it. From 90 degrees to 180 degrees the trust has to convert the kinetic energy of the air (wind) to motion relative to the earth. With the air speed steady at 120 knots, the ground speed is now close to 150 knots. The airplane has experienced a positive acceleration relative to the earth because the kinetic energy of the air has been converted to motion relative to the earth.
    As the turn continues to the upwind, the air is now moving toward the top side of the wing and taking air away from the bottom of the wing causing a loss of lift so the airplane continues its movement in the same direction as the wind. When the turn is completed the ground speed is down to 90 knots and the airspeed is at 120 knots because trust and drag have done their job of governing. The airplane has experienced a negative acceleration relative to the earth. This airplane was traveling at one and a half degrees an hour around the earth before the turn. During the turn, the speed around the earth increased to two and a half degrees an hour, then back to 1 ½ degrees an hour. This is proven by the inertial navigation system that use accelerometers for sensors. If the airspeed remains constant, it appears this turn is impossible to do without experiencing positive and negative acceleration relative to the earth. The ground track is similar to lower case e cursive writing. All of this flying was done on the front side of the power curve.
    As all pilots know an airplane can fly level at two different airspeeds using the same amount of trust. One on the front side of the power curve and the other on the backside of the power curve. When flying level in the backside of the power curve into a headwind an airplane requires less trust to fly level than when flying with a tailwind. When flying with a tailwind, the trust vector is now meeting the wind vector at an angle and cannot covert all of the kinetic energy of the wind to circular motion relative to the earth. There is also a loss of lift flying with a tailwind because the air is moving toward the low pressure side of the wing and moving air away from the high pressure side of the wing. The airplane has to fly farther around the earth in the same amount of time to get the same amount of air going past the wing. This requires more thrust to make up for the loss of conversion of the kinetic energy of the air.
    If the airplane is capable of converting the kinetic energy of the wind to motion relative to the earth and the pilot flies the airplane correctly —the downwind turn should be as safe as flying in calm conditions. Airplanes that are under powered, overloaded, and have extra drag should use extreme caution in strong windy conditions because the acceleration rates relative to the earth are slower and the airspeed cannot be maintained.
    The downwind turn and wind shear are similar—–the difference being they have changed roles. The airplane is changing velocity and direction relative to the earth in the downwind turn. The air is changing velocity and/or direction relative to the earth in wind shear. The time span that it takes to change velocity and direction relative to the earth make the difference between safe and unsafe operation of the airplane.
    If you encounter wind shear while turning downwind the airplane may not be able to accelerate fast enough relative to the earth to maintain airspeed.

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