Airplanes need yaw

Turning flight

What seems banal with today's aircraft was considered risky in the early days of aviation: turning. Slopes of more than a few degrees were hardly manageable. This was partly due to the ineffective steering around the longitudinal axis - before ailerons came up, there was wing twisting or nothing at all, so turns were flown exclusively with the rudder. But partly also due to a poor understanding of what happens aerodynamically in this flight condition.

The more bank angle, the more important it is to use the rudder

Separate rudders for all three axes made it easier to master turns. In addition, many aircraft types make it easy for the pilot to turn only with aileron and elevator - at least flat. However, the more bank is involved, the more important it is to use all oars in a coordinated manner. Why? Curve exercises with different inclines give the answer. Let's start with a little bank. It should remain just as constant as the altitude, and the aircraft should neither slide nor push, i.e. the longitudinal axis of the aircraft should form a right angle to the radius of the circle. This is the case when the ball stays in the center in the level or in the turn and bank indicator.

Conveniently, you choose an altitude of a full thousand feet so that control is easy. And don't forget: the air space behind the aircraft must also be clear before the cornering exercises begin. A slight aileron deflection brings the machine into an inclined position. Shortly before the desired “bank” is reached, the rudder is neutralized. Because the projected area of ​​the wings has decreased due to the inclination, the (vertical acting) lift is also lower. The pilot compensates for this deficit by pulling the elevator and thus increasing the angle of attack.

The rudder prevents slipping in turns

In aircraft with differentiated deflection ailerons, he can ignore the rudder in flat turns; the ball stays in the middle of its own accord: because the inside aileron deflects more upwards than the outside deflection downwards, there is more resistance inside than outside - the aircraft is turned into the bend without the pilot having to help around the vertical axis. If, on the other hand, the ailerons deflect the same amount up and down, the machine slips: the nose points outwards. The negative turning torque is responsible for this. "Negative" because the machine yaws against the direction of the curve when it rolls into the curve. Reason: On the wing on the outside of the curve, the downward deflecting aileron not only produces increased lift, but also induces more drag, while both of these decrease on the wing on the inside of the curve.

Differentiated deflection ailerons require little pedal support

As a result, the outer wing lags behind the inner one - the plane slips. The pilot can prevent this with the rudder; he “coordinates” the curve. If he fails to adjust the aircraft attitude around the vertical axis, the fuselage plowing diagonally through the air generates increased resistance. But braking is exactly the opposite of what is desirable when turning.

This becomes clear when the angle is greater. While the elevator hardly needs to be pulled in flat turns to compensate for the slight loss of lift, the wing generates significantly less lift in steep turns. It now needs a correspondingly more angle of attack. Even more bank angle, even more angle of attack - until at some point the critical angle of attack is reached at which the flow breaks off. If the angle of attack is approaching the limit, but the aircraft is not supposed to lose altitude, it needs more speed. So engine power. The limit of bank angle without loss of altitude is reached when the aircraft is just fast enough at full throttle that it generates the necessary lift just below the critical angle of attack. An exercise helps to find the right rudder control and power setting when turning. To do this, fix a point on the windshield that is on the horizon before turning it in. A fly, a smudge of dirt or the fingertip of the hand resting on the panel are suitable. In addition, one notices a feature in the landscape that lies behind it. Once the desired incline has been reached, try using the elevator to keep the reference point on the horizon. The following observation scheme has proven itself during the circling: bank position - height - horizon - sphere - travel.

Asymmetry when cornering

Right and left turns may require different rudders: Airplanes with a right turning propeller need less rudder deflection in left turns than in right turns due to the propulsion effects. The oars may even have to be operated slightly in opposite directions in order to keep the ball in the middle. However, only an assumed asymmetry arises from the side position of the pilot in cockpits with seats arranged next to one another. Curving to the right, the nose of the aircraft appears to dip below the horizon, while turning left, on the other hand, to come up. In fact, the pilot seated on the left is not looking over the middle of the cowling, but over the muzzle to the right of it. This leads him to incorrect elevator corrections. Shortly before the full circle is completed and the selected landscape feature appears behind the reference point, the bank is reduced and the elevator is released. It takes a few circles before an inexperienced pilot can hold his ground and turn around at the right moment. Above all, steeply flown circles rarely succeed straight away.

Large inclines are also more demanding because of the increased stress on man and machine. Twice the acceleration due to gravity acts at a 60 degree bank angle. The stall speed is 1.41 times as high as in normal flight. Why actually? Because the "smaller" wing due to the inclination provides less lift than is leveled. As a result, the machine has to fly faster if the angle of attack can no longer be increased. In addition, the stall speed in turns can be increased by the machine yawing.

Sitting on the left, the nose of the aircraft seems to plunge into right turns

If the pilot deflects the rudder too little, the air flows around the wing not only in the longitudinal direction, but also transversely. In the case of low-wing aircraft, the upper side of the wing on the outside of the curve is then partially in the lee of the fuselage - which is why the disturbed flow here may lead to a premature demolition. Low-wingers then roll outwards into the stable against the direction of the curve. Neglecting the rudder when cruising and even more when climbing is less dangerous, but reduces performance. The speed and rate of climb increase significantly as soon as the pilot kicks the ball in the middle.

Reversal curve after loss of sight

Extreme inclinations rarely occur in everyday flying. Sometimes during evasive maneuvers or when there is a strong crosswind when turning into the final approach and the pilot closes the curve so as not to overshoot the runway axis. Little travel and a large bank angle: a dangerous combination close to the ground!

Curves with moderate inclines are more common. To be able to fly cleanly - this is exactly what is important when turning around after losing sight. Markings on the turn indicator are helpful. If you bring the wing tip of the aircraft symbol with the marking to the left or right below the neutral position into cover and then turn off again after one minute, you are flying exactly on the opposite course: The instrument is calibrated to a rotation rate of three degrees per second; this corresponds to the standard curve for instrument flight.

Do not rely on instruments for reversal curves

The turn indicator does not show the incline, although the hanging surface of the airplane symbol suggests this. (Older instruments have a vertical pointer and markings on the side of it.) That is why the airplane symbol on the ground twitches when you roll around a curve where no inclination is involved. If in a reversal curve it is important not to come out laterally offset (by the curve diameter) on the opposite course, but on the track flown in front of the curve, the rule applies: memorize the starting course, fly the standard curve, and if there is a 80 degree deviation from the starting course, change to the standard curve in the opposite direction and maintain this until the opposite course to the course before the maneuver is present. This variant of the turnaround is helpful, for example, if you want to be above a previously used catching line after turning around, for example a motorway. So it is not just a question of style, efficiency or safety to master turns properly. It can just be useful.

Text & drawings: Helmut Mauch, illustrations: Eric Kutschke aviator magazine 05/2012

  • Bank
  • Turning flight
  • Rudder steering
  • Rudder
  • Ailerons
  • Aileron linkage
  • negative turning torque
  • Elevator
  • Reversal curve