WILL THE AIRPLANE SKID OR SLIP IF I PUSH ON THE RUDDER PEDAL IN LEVEL FLIGHT

9 Answers

1. 
   

   
   

   Micah on May 09, 2012

   Yes

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2. 
   

   
   

   Nathan Parker on May 09, 2012

   The field of aerodynamics doesn’t really draw a distinction between a slip and a skid; those are more pilot terms.  In an aerodynamic sense, an airplane is slipping any time the fuselage isn’t aligned with the relative wind, so when a pilot says he’s slipping or skidding, an aeronautical engineer would just say he’s slipping in both circumstances.
    
   The term “skid” is used by pilots for a particular slipping scenario in which the airplane is yawed towards a lowered wing.  If there is no lowered wing, then there is no skid, although the airplane might be slipping in either direction.

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3. 
   

   
   

   Dom on May 09, 2012

   My understanding is that would cause a skid. Initiates a uncoordinated turn with insufficient bank, ball would roll opposite the direction you’re turning.

   Slip is the opposite – wings banked with insufficient same direction rudder (or opposite rudder). Ball would roll same direction wings are banked.

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4. 
   

   
   

   Lucas on May 09, 2012

   You would skid in both cases because you would be drifting away from your radius of turn.
   (True) It isn’t well defined in aerodynamics, but physics help just as much. If the ball is in the opposite direction as the turn, the centrifugal force is grater than the centripital one and that is a skid on the other hand if the forces are pulling you towards the center of the turn you are slipping.
   Lucas
   Passfaaexams.com
    
    

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5. 
   

   
   

   Nathan Parker on May 09, 2012

   Gah, the stuff about centripetal force being less than or greater than centrifugal force is nonsense physics-wise.  Yes, I know that comes from the FAA, but the FAA has often been the source of garbage physics.
    
   Firstly, centrifugal force is a “fictitious” force and shouldn’t be included in a diagram depicting the forces in a turn.  An aircraft in turning flight has an unopposed centripetal force acting towards the center of the turn.  If there were truly an equal and opposite force (centrifugal force), then the airplane wouldn’t turn at all.  What we call centrifugal force is merely the resistance to a change in velocity that a mass exhibits; for instance, when you turn a corner in your auto, you feel slung towards the outside of the turn, but that’s only because your body wants to keep moving forward, but your vehicle is turning out from under you.  Your body continues to follow Newton’s first law in maintaining its original direction.
    
   Secondly, even if you were to calculate a value for centrifugal force, it would always be equal, but opposite, to centripetal force.  In other words, they can’t have different magnitudes, since they’re two sides of the same coin.
    

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6. 
   

   
   

   Dom on May 10, 2012

   I was going by what I remembered from the FAA hndbk of aero knowledge. Looks like its pg4-20 and 4-21. I thought I saw a better pic somewhere – jeeppesen maybe. Dont know how physics-accurate this is.

   “In a slipping turn, the aircraft is not turning at the rate appropriate to the bank being used, since the aircraft is yawed toward the outside of the turning flightpath. The aircraft is banked too much for the ROT, so the horizontal lift component is greater than the centrifugal force. [Figure 4-29] Equilibrium between the horizontal lift component and centrifugal force is reestablished by either decreasing the bank, increasing the ROT, or a combination of the two changes.

   A skidding turn results from an excess of centrifugal force over the horizontal lift component, pulling the aircraft toward the outside of the turn. The ROT is too great for the angle of bank. Correction of a skidding turn thus involves a reduction in the ROT, an increase in bank, or a combination of the two changes.”

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7. 
   

   
   

   Nathan Parker on May 10, 2012

   “In a slipping turn, the aircraft is not turning at the rate appropriate to the bank being used, since the aircraft is yawed toward the outside of the turning flightpath.”
    
   This is the closest to a coherent physics explanation.  I would put the answer this way:  “In a slipping turn, the centripetal force is less than the horizontal component of lift.”  The reason is that the horizontal component of lift is being opposed by two other forces: 1)  The yawed airplane produces a fuselage side force that works opposite the centripetal force, and 2) There is a component of thrust that also acts against the centripetal force.
    
   In a coordinated turn, the lift vector of the airplane is perpendicular to the wing span of the airplane.  In other words, it acts from your head to your butt; when the turn is uncoordinated, the lift vector is canted to your left or right, so that there is a component of force that pushes your butt towards the high or low wing.  That’s why the ball moves in the same direction.

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8. 
   

   
   

   Donnie on May 12, 2012

   I love the discussion about centripetal force, because that is “good physics”. (Finally calling things by their proper name and understanding Newton’s 1st law.) And I’m concerned that an examiner would not like to hear such an answer. They’d rather hear their published description of the “physics” and use of the HCL-centrifugal force diagram. (from a CFI applicant)

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9. 
   

   
   

   Nathan Parker on May 12, 2012

   “And I’m concerned that an examiner would not like to hear such an answer.”
    
   You’re quite right to raise that question. Realize first, though, that all examiners are different and you can’t really generalize about what examiners want to hear or what they don’t want to hear.  My view is that if you’re going to send in a candidate with non-FAA derived knowledge, they need to be a master of that area of knowledge, so that they can present the information in such a way as to make it irrefutable.  This means understanding where common explanations go wrong.  This is an awful lot harder on the CFI candidate and his instructor, and I can easily see why they might choose just to regurgitate FAA knowledge.  Even that isn’t risk-free, though, because an astute examiner could easily lead the candidate into indefensible positions, because he’s spouting nonsense:  “So, if you have a centripetal force that is equal and opposite to centrifugal force, that means the net force acting on the airplane is zero, so why does the airplane change direction in violation to Newton’s first law?”
    
   Better, I think, is for the instructor to teach with integrity and try to pass on the most accurate knowledge that he’s capable of.  If that causes problems with the views of others, then that can be addressed as a separate problem.  I’ve managed to persuade more than one examiner that they were wrong about something, although this is better done outside the context of a checkride.
    

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