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Flight Control: No Yaw Control

  1. Aug 26, 2014 #1
    Regarding flight control for planes, there exist 3 axes: roll, pitch, and yaw. Something that I've been curious of recently is the ability to control a plane without direct yaw control.

    Let's go for an extreme example, imagine you only have control over a plane's pitch and roll. Although you would typically use the yaw to assist in turning a plane left and right, couldn't you accomplish turning left and right simply by rotating the plane by it's pitch and roll axis? Could you not roll the plane 90 degrees, and then use the pitch control, and then roll the plane back 90 degrees? This could indirectly steer the plane left or right, correct?

    In most scenarios, you would never roll the plane 90 degrees, but you could roll it less than 90 degrees and then alternate the pitch back and forth to the keep the plane level.

    I attempted to search for this concept but came up empty handed... For those of you experienced with planes, is this something that is ever done, or is it much easier to add a rudder for direct yaw control?
     
  2. jcsd
  3. Aug 26, 2014 #2

    cjl

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    Sure, you can absolutely fly a stable aircraft with no yaw control. You don't have to bank 90 degrees either - simply banking over a bit and increasing pitch to increase load factor as needed will result in a level steady-state turn.
     
  4. Aug 26, 2014 #3
    I'm having a tough time visualizing how banking and then adjusting the pitch would allow the plane to make a level turn. I assume it would turn the plane left or right but also cause the plane to move up or down as well.

    By adjust the pitch of the plane, you are essentially force the plane's nose to move about the roll-yaw plane. Say for example you were looking at a plane from behind, you then decide to the bank the plane to the left 45°. And because this plane has now been shifted by 45 degrees, you have also rotated the roll-yaw plane by 45°. The fact that the roll-yaw plane is no longer completely vertical, there exists both a vertical and horizontal component when compared to normal 3D space. Wouldn't pitching the plane upward for example move the plane to the left but also force the plane to move upward? How would this be a level turn?

    Hopefully my inquiry makes sense. Also, if you know of any good literature regarding this topic, feel free to pass it on to me.
     
    Last edited: Aug 26, 2014
  5. Aug 26, 2014 #4
    And also, apologies for posting this in the mechanical engineering forum. I did not realize there existed an aerospace engineering forum. Feel free to move this topic.
     
  6. Aug 26, 2014 #5

    rcgldr

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    When banked over, the total lift is the same, but the vertical component of lift is decreased, while gravity remains the same, so the aircraft would spiral downwards if additional up elevator was not input while the plane was banked in order to maintain a level turn.

    Also, there is some indirect yaw control, as the tail section will tend to act like a weather vane. At slow speeds, such as a glider in a thermal turn, the tail won't have sufficient weather vane effect, and the fuselage of the glider will droop at the tail. Rudder input is required to keep the fuselage level for a coordinated turn. At higher speeds, like a powered aircraft at cruise speed, the weather vane effect will reduce the amount of droop at the tail, so that the fuselage will be nearly (but not quite) level, and the turn will almost be coordinated without rudder input.
     
  7. Aug 26, 2014 #6
    Ahh I see, I was curious how banking would affect the total lift.

    Now regarding your description of indirect yaw control. You are saying that banking the plane whiles it's moving at a high speed will put forces on the plane's vertical stabilizers forcing it to align with the relative airflow? Thus rotating the plane about the yaw axis?

    Could you further expand on why the tail would droop when moving at slower speeds?
     
  8. Aug 27, 2014 #7
    Consider all of this as a departure from straight and level flight:

    In a bank the rising wing creates more induced drag (due to more lift). The extra drag will twist the plane on the vertical axis causing the tail to move toward the inside of the turn. This configuration is called a slip or slipping turn and the tail is lower than the nose (droopy tail haha -I'm borrowing that one). If the directional stability is great enough the air flow over the vertical stabilizer will counter this slip and cause opposite rotation around the vertical axis bringing the nose and tail back in line. At slow airspeeds the vertical stabilizer is less effective due to less airflow so the tail stays droopy without rudder input.

    In a turn (properly flown) the pilot will use the rudder to counter the twist as the bank is entered to maintain the coordination of the turn keeping the nose and tail in line.
     
  9. Aug 27, 2014 #8

    rcgldr

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    Both the vertical stabilzer and the fuselage itself will tend to align the the relative air flow, called weather vaning.

    When in a turn, the outer wing is moving faster than the inner wing. The drag on the wings produces a torque that tend to turn the nose outwards and the tail inwards. The greater the difference in speed between the inner and outer wing, the greater the torque. So assuming some fixed bank angle, the effect is greater at slower speeds due to a tighter turn. In addition for a tight thermal turn in a glider, the slower speed of the inner wing means that less lift is produced by the inner wing, so for a coordinated turn, the pilot needs to feed in some outer aileron (roll) input and positive rudder (yaw) input.

    For a glider, sometimes a streamer is setup to flow up the windshield, and for a coordianted turn the streamer will appear to be vertical relative to the pilot. Instruments inside the glider will also indicate a coordinated turn, but the pilot doesn't have to look down at the gauges if using the streamer as a guide.

    I fly radio control gliders, and since you view the model from the outside, when in a slow and tight thermal turn, you feed in opposite aileron to keep the bank angle from increasing, and you feed in positive rudder to keep the fuselage level (technically slightly nose down since it's always descending relative to the air), and the elevator input to control the gliders relative rate of descent and speed. You try to find updrafts that are moving upwards faster than the gliders descent rate while in thermal turn.
     
  10. Aug 27, 2014 #9
    I'm a complete rookie regarding flight dynamics and really need to sit down and open a book which outlines a lot of this stuff. I appreciate your help in explaining some of these concepts.

    If I wanted to create a plane design that did not feature a rudder to compensate for the adverse yaw effect, couldn't we adjust one of the aileron's so that it didn't have same angle magnitude of the opposite aileron (assuming the aileron's could be controlled independently of one another)?

    For example, if we were banking to the right. This would cause the left wing to move at a greater velocity than the right wing and would cause the left wing to experience greater drag. Would it be possible to adjust the right wings aileron angle so that the it was greater than the left aileron angle, thus balancing out the drag?

    Again, I need to open up a book that covers these concepts, but I appreciate all the help!
     
  11. Aug 28, 2014 #10

    AlephZero

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    The ailerons only provide enough force to roll the plane. They don't provide the extra lift. That comes from the fact that if the plane is turning, the wing on the outside of the turn is moving through the air faster than the inside wing, so the lift and drag forces are higher on the outside wing.

    That simple explanation isn't the whole truth, because planes are usually designed to be stable in flight, so that pilot doesn't need to correct every small movement caused by air turbulence etc.


    One basic thing about flight dynamics, which is obvious after somebody has pointed it out, is that the direction that the aerodynamic forces act on the plane are relative to the orientation of the plane in the air, not relative to the ground. The "lift" from the wings acts perpendicular to the wings, not necessarily vertically upwards. But of course the weight of the plane does always act vertically downwards. Thinking about the consequences of that is enough to explain a lot of the "non-intuitive" behavior of the plane, and the effect of the controls.
     
  12. Aug 28, 2014 #11
    Ailerons roll the airplane using a force generated around the longitudinal axis called lift. More lift on the outboard wing when entering a turn causes the wing to rise. Conversely less lift on the inboard wing causes it to drop. The lift to roll the airplane does cause drag. At low airspeeds, such as a glider, in a thermal, induced drag from lift (to cause the roll) is a much larger factor than parasitic drag from the faster moving wing. Most manuvering is at slower airspeeds.

    Airplanes ARE designed to be stable but thermaling a glider is done at very low airspeeds so contol inputs are often large to maintain coordinated flight.
     
  13. Aug 28, 2014 #12
    I see where I was unclear. My explanation was only regarding coordination entering the turn and "tail droop". The lift provides the force to turn...
     
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