Inclinometer error during slow flight at high angles of attack

5 Answers

1. 
   

   
   

   Nathan Parker on May 11, 2011

   The “ball” is a measure of lateral acceleration, not side slip per se.  Side slips generate a fuselage side force that tends to accelerate the aircraft laterally, so that measure of lateral acceleration is a good standin for side slip, but not always.  One exception that comes to mind is a multi-engine aircraft; with one engine out and the airplane in a zero-sideslip bank, the ball will be about half out, even though there is no sideslip.  What it’s measuring is the component of gravity that is acting along the lateral axis of the airplane, which is slightly banked.
    
   A reverse scenario is setting up for a power on stall; when you have enough rudder to counteract the left-turning tendencies, a ball-centered aircraft will be in a steady sideslip to the left for the same reason that a multi-engine aircraft will sideslip to the left with a left failed engine.  The airplane tends to break to the right.
    
   The only real measure of sideslip is a yaw string on the outside of the airplane, so I’m skeptical that your observation that the nose is moving is an indication of lack of coordination.  I’d put it down to “heading control.”
    
   Personally, I feel that it’s poor pedagogy to teach students to reference the ball during 99% of flight maneuvers, then teach them something different when setting up for a straight ahead stall.  The Airplane Flying Handbook says for turning stalls, reference the ball again.  In actuality, this works well for straight-ahead stalls, too, and that’s what I teach.  If students maintain heading with coordinated flight controls, they can maintain heading during the setup for the stall with no problem.  There is a slight tendency to break to the right, due to the reason I gave above, but this can usually be avoided by a smooth, gentle stall.  Regardless, the PTS only calls for maiintaining heading during the setup for the stall; breaking straight ahead is not a criterion for the maneuver.
    
    
    

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2. 
   

   
   

   Chris Parks on May 23, 2011

   I’m going to disagree with Nathan Parker.
   “The “ball” is a measure of lateral acceleration, not side slip per se.”
   I believe it is both.  When lateral acceleration is experienced (tromp on a rudder, suddenly add or subtract power) the ball swings wide, then settles somewhat.  The initial movement might be that lateral acceleration and may be what student pilots try to react to, resulting in some single leg pumps and improper emphasis on the ball.
   I feel that instruments are overly emphasized and referenced in initial training, though they should be referenced properly.
   Let’s say you notice your student consistently flying half a ball out.  I’d point it out to the student, then draw their attention outside to correct.  Hold the correction for a few seconds, then glance back inside to see what the ball is doing.  By drawing the attention outside proper sight picture is emphasized, and by delaying the peek inside the acceleration error is minimized and reduces chasing and emphasis.
   “A reverse scenario is setting up for a power on stall; when you have enough rudder to counteract the left-turning tendencies, a ball-centered aircraft will be in a steady sideslip to the left for the same reason that a multi-engine aircraft will sideslip to the left with a left failed engine.  The airplane tends to break to the right.”
   I’m not sure which airplane you’re experiencing this in but it is not the norm for a single engine airplane.  Besides, a sideslip to the left results in a break to the left (wing falls opposite the ball).  A proper power-on stall with a centered ball will break straight ahead, if it breaks at all.
   “The only real measure of sideslip is a yaw string on the outside of the airplane, so I’m skeptical that your observation that the nose is moving is an indication of lack of coordination.”
   Not for a single engine airplane in a tractor (prop in front) configuration.  Ever heard of spiraling slipstream?  But for a twin or a glider, yeah, what you say is true.  Get used to a level wing with a high nose attitude and watch left (pilot) or right (CFI) of the nose to stop adverse yaw, i.e. keep the nose from swinging left or right.
   “Personally, I feel that it’s poor pedagogy to teach students to reference the ball during 99% of flight maneuvers, then teach them something different when setting up for a straight ahead stall.”
   Referencing the ball is a great way of cross-checking your outside view, for all maneuvers.  Just don’t get sucked into staring at it.  Reference it, go outside for the change, make the change, pause, check inside again.  Wash, rinse, repeat.
   Nathan, in reading your post again I think your students might be holding some slight left wing down coupled with too much right rudder to compensate.
   Micah, the inclinometer is a pretty simple mechanical device.  It simply measures gravity / weight pulling from the center of the instrument’s location.  The fact that it is in front of the left seat shouldn’t matter in a properly performed stall.  If your students are consistently half off means they need to adjust their perception.  But reference earlier in my reply.  It is impossible in some stalls to look over the glareshield for a reference but instead direct their attention (and yours) to beside the glareshield to the visible horizon.

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3. 
   

   
   

   Nathan Parker on May 25, 2011

   “I believe it is both.”
   By what magic does the little ball in a closed tube of glass figure when the relative wind isn’t aligned with the longitudinal axis of the airplane? 😉 It can’t.  The ball isn’t much different from setting a ball on top of the instrument panel and watching it slide one way or the other.  The lateral acceleration is an indirect measure of sideslip.  The curvature of the tube is designed to calibrate the quantity of lateral acceleration; one ball width is equivalent to, as I recall, about .7g.  For aircraft that don’t generate much of a fuselage side force, the ball isn’t useful as a measurement of coordination.
    
   “Besides, a sideslip to the left results in a break to the left (wing falls opposite the ball). ”
   Only if you make the incorrect assumption that a ball out is always associated with a sideslip; it isn’t.  A ball-centered power on stall is in a sideslip to the left; a simple vector analysis can show that this is true, as can a yaw string, which works fairly well behind a propeller, because I have done it.  The flow towards the wingtip on the downwind wing acts much like a swept wing, which tends to have a wingtip stall due to the thickened boundary layer toward the tip.
    
   “in reading your post again I think your students might be holding some slight left wing down coupled with too much right rudder to compensate.”
   That would be a reasonable guess if you were not aware of the fact that this airplane is in a sideslip to the left during a ball-centered power on stall.  You can actually cause a straight ahead stall by letting the ball remain half out by using insufficient rudder.  Maybe that’s what your students are doing.  🙂
    
   You really have to understand that all of our instruments are only indirect measurements of the thing they purport to measure;  if you don’t understand what they *really* measure, you won’t know when you can’t trust them.

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4. 
   

   
   

   Nathan Parker on May 25, 2011

   Here is an excerpt from “Airplane Stability and Control”, by Malcom Abzug, p. 338:
    
   ——————-<snip>——————–
   In addition to a very small level of inherent static directional stability, the B-2’s all wing shape has next to no side-force derivative in sideslip, or C[greek letter omitted].  This creates a flight instrument problem, in that the normal ball-bank component of the turn and slip instrument cannot function as an indication of airplane sideslip.  The standard ball-bank component is a lateral accelerometer, calibrated to produce one ball width at a tilt angle of 4.5 degrees, or a lateral acceleration of 0.08 g in level flight.
   ——————-<snip>——————–
    
   So my memory of the lateral acceleration of one ball width was off by an order of magnitude; in retrospect, .7g is a huge amount, so that alone should have tipped me off that the number was wrong.

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5. 
   

   
   

   Andy Neumann on Jul 11, 2011

   Micah,
   I’ll admit, I have not noticed the phenomenom you are talking about.  I’ll summarize my answer with this: if the wings are level and the heading is constant, the ball should be in the center.  Whether the airplane is sideslipping or not is unimportant.  So, assuming your inclinometer is truly installed properly and working properly, I can only say that your wings must not be perfectly level or the airplane must be slowly changing heading. 
   I have flown many airplanes in which the inclinometer was slightly tilted, thus confounding the most devoted “keep the ball centered” pilots.  But I always know I am coordinated in level flight if the wings are level and my heading is constant.  You don’t need an inclinometer to figure that out–just watch the horizon and pick a landmark.  If the landmark isn’t drifting left or right, then I know I’m coordinated in level flight.  Conversely, if I have to hold the wings banked just to maintain a constant heading, then I know I’m NOT coordinated.  In normal flight, if the airplane is banked, it should be turning.  If it’s banked, but not turning, again, I know I’m NOT coordinated. 
   If I was you, I would watch closely.  Is your student maintaining wings level?  Is your student maintaining heading?  If so, ignore the ball!
   Also–keep in mind that in slow flight, you need more rudder to counteract adverse yaw because you need more aileron input to get the same roll rate.  A lot of students correctly increase right rudder in slow flight, but then they forget to keep coordinating rudder with the ailerons.  Bigger aileron inputs require bigger rudder inputs.  Your students might not be picking this up and it’s showing up in slow flight.  Just a guess. 
   Good question and good luck!

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