From rich@richstowell.com Fri Jan 11 02:18:40 2002 Path: cygnus.com!enews.sgi.com!harbinger.cc.monash.edu.au!news.mel.connect.com.au!news.xtra.co.nz!nntp-relay.ihug.net!ihug.co.nz!news-hog.berkeley.edu!ucberkeley!newsfeed.stanford.edu!postnews1.google.com!not-for-mail From: rich@richstowell.com (Rich Stowell) Newsgroups: rec.aviation.student Subject: Re: The Ball & Coordinated Flight Date: 10 Jan 2002 09:26:51 -0800 Organization: http://groups.google.com/ Lines: 123 Message-ID: <77e002cf.0201100926.702f0593@posting.google.com> References: NNTP-Posting-Host: 207.71.243.178 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Trace: posting.google.com 1010683611 21710 127.0.0.1 (10 Jan 2002 17:26:51 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: 10 Jan 2002 17:26:51 GMT Xref: cygnus.com rec.aviation.student:199717 With all the recent postings about the Slip/Skid Ball, I'm surprised no one has yet asked the following interesting question: ------------------------------------------------------------------ Is "coordinated flight" always the same as "ball centered" flight? ------------------------------------------------------------------ The answer is NO! The ball is an instrument, and as such, it suffers from the same three-letter word that all instruments suffer from -- LAG. As an instrument, the ball is least accurate while you are in the process of making dynamic changes/control inputs. The ball is most accurate "after the fact" -- when you are near a steady flight condition. The ball then allows you to fine tune your yaw control. A good demonstration of the inherent lag in the ball is a simple coordination exercise (called a Dutch Roll by aerobatic pilots--has nothing to do with the Dutch Roll instability of swept wing aircraft, though): Looking outside over the nose, and while keeping the nose on a point, smoothly apply aileron and rudder at the same time, in the same direction, and roll to say thirty degrees of bank. Then immediately switch your control inputs to the right--right aileron and right rudder--to an equal bank angle to the right. Then switch again. Continue rocking the wings left and right in this coordinated manner (just like you should be doing any time you're banking, be it into or out of turns). The biggest problems in this exercise--pilots usually don't apply enough aileron and often too much rudder. Also, the rudder input must exactly coincide with the aileron input. Not before, not after, but timed with the aileron. Most non-aerobatic instructors who try to teach this exercise often do so as a cross-controlled maneuver, to what benefit I don't know. This should be a coordinated maneuver (as evidenced by the feel in the seat of the pants and the nose of the airplane staying on a point on the horizon). If you try doing this exercise by looking at the ball and trying to keep it centered, the nose of the airplane will slew all over the sky and everyone in the airplane will get sick in a hurry. Another classic demonstration of limitations in the ball was done by William "Bill" Kershner. He placed two slip/skid balls on the instrument panel, one each in front of the two seats of a Cessna 150 Aerobat, and one if front of each seat in a Beech Aerobatic Sport. He then proceeded to do some spins. The two balls in each airplane were in opposite corners of the inclinometer--one to the left side, one to the right side. This is why during a spin, the ball cannot be used to determine which rudder to step on for recovery. As a variation of this, I routinely perform the classic skidded base-to-final turn into a spin (at altitude of course). As part of the demonstration, I often have the student look at the ball when I feel we are 1/2 second from spin entry. The ball actually straddles one of the two lines that we use to know where "centered" is. In other words, even though we are within 1/2 second of spin departure in the skidded turn, the ball is only 1/2 ball out of center. It clearly is not indicating how close we truly are to the impending spin departure. In fact, if we couple that left skidded turn into a spin with Kershner's experiment, we can imagine that prior to spin departure from a left skidding turn, both balls--the one in front of the pilot in the left seat, and the one in front of the pilot in the right seat--are off to the right of center somewhere. But as we get closer and closer to spin departure, the ball that will eventually be on the left side of the inclinometer in the left spin begins to move from the right to the left. Suppose it is just this ball that you have in your airplane. And even though you are skidding to the left, the ball eventually begins to move from right to left as you approach spin departure. At T-minus 1/4 second from spin entry you glance at this ball. Lo and behold, it's centered! Not really, it's just en route to the left as the airplane enters the spin. It just so happens that when you looked at it, things were apparently "lookin' good!" from a ball standpoint. An obvious demonstration of another one of the ball's inherent deficiencies is to roll the airplane to 180 degrees of bank--inverted flight--and see if the ball will stay on what is now the top of the hump of the inclinometer while you are in coordinated inverted flight. The ball cannot stay on the hump, and falls off to the side (though there is an air bubble that lives in the inclinometer--usually you can't see it in upright flight, but it makes itself known when inverted. One could, if one were so "inclined," look to see if the air bubble is centered to confirm coordinated flight!). The best advice I was ever given about the ball -- stop staring at it! (Unless you are in IMC, that is, where we must rely on our instruments. BTW, when we are in IMC, we do everything we can to keep the airplane as close to wings level, steady flight as possible so that our instruments will give us as close to real time information as possible--quite a bit different from VFR flying...) Fly the airplane and control yaw by sight (look at where the nose is going vs. where it should be), sound (propellers on Cessnas groan when you are uncoordinated), feel (in the seat of your pants), and aeronautical knowledge (climbing flight requires some right rudder, descending flight requires some left rudder, slow flight requires some right rudder, etc. -- we know this intellectually based on the aerodynamics of flight in light airplanes). Use the ball only to confirm and/or fine tune what you are already doing in terms of yaw control based on sight, sound, feel, and aeronautical knowledge. Oh, and to sense yaw accurately, you must not be leaning all over the place in your seat while you are maneuvering. You must learn to keep your body aligned with the airplane, NOT the horizon outside. If you are always leaning in your seat to stay "vertical" to the horizon, your body will not be aligned with the airplane's yaw axis and it will be much more difficult to sense yaw. Be safe, Rich Stowell http://www.richstowell.com