COCKPIT By Harold Holmes (EAA 220238), CFI 1038 Inverrary Lane Deerfield, IL 60015

EXPLORING SPINS

ISPORT N THE NOVEMBER issue of AVIATION I discussed the Aerodynamics of Stalls. An appropriate follow-up to the stall article is one on spins. I have taken the liberty to use a few illustrations from the

Visualized Flight Maneuvers Handbook in order to help clarify some of the spin procedures and concepts. First of all, the spin can be defined as an aggravated stall which results in what is termed "autorotation". As the Handbook illustration shows, the airplane follows a corkscrew path in a downward direction (see Diagram 1). As the plane is spinning the wings are producing some lift and the airplane is forced downward by gravity (FAA, p. 154, Ref. 2). Further in the article, I will present a more detailed analysis of the spin. As a pilot examiner, I have personally witnessed an aversion to stalls on the part of a fairly large number of applicants. I can also recall a total of six private applicants who actually ended up in an inadvertent spin during the flight test. In all six cases, the applicants were not able to recover from the spin. In view of this, I feel it is important for all pilots to learn the

cause of a spin and the proper methods of prevention and/or recovery from the spin. This will also relieve some of the mental anxiety and will help prevent the inadvertent spin. Recently, I had the unique opportunity of flying with Woody Woods who

and I thoroughly discussed the four spin modes we planned to test. These included the normal, accelerated and flat spins - both power off and power on. The plan was to do the series of four upright spins and then to repeat them inverted. As an observer, I was impressed with the thoroughness of the walk-around inspection and preflight briefing which included emergency egress and parachute use. The IAC training program appears to be excellent in looking at it from my viewpoint as a flight examiner. Ever since reading Gene Beggs' article on emergency spin recovery in SPORT AVIATION I could not wait to see it for myself. The day for the flight was ideal in that the sky was clear and the air smooth. After reaching an altitude of 5,000 AGL, and following the completion of a series of clearing turns in the Pitts S2A biplane, I warmed up by doing a normal upright one-turn spin and recovery to the left. Except for the controls being extremely sensitive, the spin seemed to be normal and I used the normal spin recovery sequence of applying opposite rudder to stop rotation, application of forward-elevator movement, neutraliz-

ing of the rudder as the spin stopped and finally the return to level flight. This first spin enabled me to take note of the normal rotation speed for this particular aircraft. We climbed

is an acrobatic instructor, an enthusiastic EAAer and President of

back to altitude, recleared and proceeded to go into the accelerated spin. Gene Beggs has coined the term 'accelerated spin' to describe the type of

lAC's Chapter 70 in Crystal Lake, IL. Woody is also a Captain with United

spin that develops when the stick is moved toward the neutral position

Airlines. Prior to the flight, Woody

after the spin has developed, either

30 DECEMBER 1984

Diagram 1.

The spin is an aggravated sta which results in autorotation

ENTRY PHASE

Clear area (minimum altitude tor spins — 3,000 teet above ground level) don't f o r g e t c l e a r i n g turns. (Altitude loss about 500 teet per 1 turn—lasting 3 seconds. Incipient s p i n o c c u r s from the time airplane

stalls and rotation starts and spin axis becomes vertical or near vertical. To enter spin: • Apply carb heat. • Reduce power to idle slowly.

• Increase angle of attack to begin normal power

off stall. (Note: Airplane must be stalled and yaw introduced before it will spin.) THE STALL

At the moment airplane stalls, ap-

BREAK

pty full rudder in direction of desired spin. Reduce power to idle. (Check mfr.s' recs.—

placards, flaps, etc.)

NOTE: Inadvertent spin — reduce power to idle and use normal recovery techniques.

inverted or upright. I read his articles with great interest, but actually doing these spins and experiencing what Gene accurately describes as the awesome rotation rate attainable in the accelerated spin really gets your attention! We moved the stick all the way to the forward stop and then all the way back while continuing to hold in spin rudder. We could change the rotation rate substantially but not stop the spin. This phenomena will be discussed further plus other valuable lessons to be learned in future articles. Our next area of investigation was the upright flat spin, first power-off and then power-on. A normal right spin was established and after about a half turn left aileron was added. The wings leveled with the nose approximately 20 degrees below the horizon and the rotation rate increased dramatically. The next spin was to the left, and while applying opposite aileron to flatten the spin, full power was added. This changed the pitch attitude to approximately 10 to 15 degrees below the horizon. Again the rotation rate was substantially increased. This completed the upright series. Now the fun really begins. With a crisp half roll to inverted, throttle closed, stick eased forward to the inverted stall, full rudder puts us into the inverted spin. We found that acceleration in the inverted spin was even more dramatic and one could see if it was encountered for the first time - unexpectedly - how disorienting and

frightening this could become. We completed the series with a normal entry to an inverted spin, stick full forward and full right rudder. After a half turn, full right aileron was added along with full power. While rotating at about 360 degrees per second, Woody's calm voice over the intercom pointed out that the nose was right on the horizon. This was easily confirmed by the late afternoon sun low on the horizon. We counted the turns as we passed the sun - 1 , 2 , 3, 4, 5 - throttle closed, stick released, full left rudder and in -'/i of a turn the ground stopped its blurring rotation abruptly. Rudder neutral and we pulled out. What a thrill! Seeing this emergency recovery technique makes one a real believer. The stall/spin has been cited as a casual factor in approximately 30% of all fatal general aviation accidents. We need to realize that the typical aircraft designer, including the designer of a homebuilt aircraft, does not have a reliable means of determining stall/spin characteristics prior to prototype flight tests. This is why NASA initiated an extensive general aviation stall/spin research program at Langley Research Center. The NASA research findings have been made available to EAA members at past EAA Conventions. These findings included results of spin model tests, static and rotary balance wind tunnel tests, airplane flight tests and analytic studies of typical general aviation configurations. In NASA's analysis of fatal acci-

dents, 24% occurred at take-off, 40% in flight and 36% while landing. The median pilot experience was 400 hours of flight time with '/-i having over 1,000 hours of flight time. NASA studies show that most accidents were caused while the pilot was distracted. NASA research also recommends that in order to keep pilots from falling victim to distractions three steps be taken: 1) Provide stall warning device; 2) Improve pilot training; and 3) Design airplane less susceptible to inadvertent stalls. Since stalling and spinning are major causal factors in fatal general aviation accidents, we as pilots operating homebuilt or general aviation aircraft need to know the causes of the stall/spin accident. Examination of the circumstances suggests that the majority of these stall/spin fatalities occur at low altitude and involve inadvertent loss of longitudinal or lateral directional control (spin entry) and ground impact before the spin becomes fully developed. During my recent spin venture with Woody Woods, I noted that we lost between 500 to 700 feet per turn, lasting about 3 seconds in a normal steep spin (see Diagram 1). At no time did we recover at less than 3,000 feet AGL. I didn't have time to check altitude loss in the flat and accelerated spins. In previous articles I have stressed the fact that if the ball is out of center - uncoordinated - a wing will always drop at the beginning of a stall. Also, I have tried to point out that when this happens the nose will yaw in the direction of the low wing. This is where that most important control, the rudder, comes into play in stall recovery before the spin ever devel-

Diagram 2. RECOVERY PHASE

Apply positive opposite rudder until rotation stops. Then neutralize rudder-ailerons.

Diagram 3. THE DEVELOPED SPIN

Apply brisk and positive forward (or neutral) pressure on yoke or stick to unstall wing.

Airplane attitude, angles

and motions are stabilized from turn to turn —.,.

flight path close to verti->£>( cal.

After recovery from rotation pull up smoothly from straight

dive to level flight with steady control pressure. (Do not exceed Vne redline airspeed.) Gently raise nose to horizon. Carbureton heat "OFF." Learn to distinguish objects on ground.

Spin is maintained by a balance between aerodynamic and inertia forces and moments. Rotation about the center of gravity — a rotary motion.

Reference for o r i e n t a t i o n should be on horizon.

Spinning motion involves rolling, yawing and pitching while airplane is at high angles of attack and

Return to cruising power. Practice spins in both directions.

sideslip.

SPORT AVIATION 31

ops. The correct amount of opposite rudder must be applied to prevent the nose from yawing toward the low wing. In this case, if the pilot maintains directional control and does not allow the nose to yaw toward the low wing, the wing will not drop any farther and a stall recovery is initiated. A spin can be prevented at any altitude and the foregoing method is especially vital at low altitudes. By allowing the airplane to yaw during the stall, the airplane will also slip in the direction of the lowered wing. Now, the relative air meeting the side of the plane and the vertical fin will cause a weathervaning tendency into the relative wind. If the resulting yaw is allowed to continue, a spin condition is imminent. In Diagram 2 we can visualize what happens during a spin. You will note that the rolling motion is around the longitudinal axis. This is caused by the lowered wing having a greater angle of attack caused by the upward motion of the wind in relation to the surfaces of the airplane. We can say that the lowered wing is stalled out. Now, the upper or rising wing has a lesser angle of attack because the relative wind is striking it at a smaller angle. The upper or rising wing has more lift than the lower wing, which causes the airplane to roll around the longitudinal axis. These differential amounts of drag on the wings cause the spinning motion. From our earlier statements, the spin not only involves rolling, but also yawing and pitching while the airplane is at high angles of attack. Now for the prescription. First, a lot of practice of slow flight and stalls using distractions by the CFI is important. The approach to landing (power off) stall with a properly executed go-around following the stall recovery is the most important maneuver in flight training. Immediate corrective action is necessary to prevent the stall, thus preventing the spin from happening. In the case of an inadvertent spin with power on, cut power first because power aggravates the spin and causes an abnormal altitude loss during spin recovery. While spinning, it is important to

determine which direction - left or

right - the airplane is rotating. An advantage of Gene Beggs' method is that the pilot does not need to know what kind of spin he or she is in, or the direction of rotation - the recovery procedures are the same whether spinning upright or inverted, flat or

normal, power on or off (see Ref. 1). This system makes it simple in that all that is necessary is to go against rudder pressure, the one that is the hardest to push. Apply full rudder in the direction of heaviest pressure and once the rotation has stopped, neu32 DECEMBER 1984

Diagram 4.

EVALUATION (Recovery)

• Reduce power to idle. • Neutralize ailerons. • Hold full rudder in opposite direction of rotation. (Note: refer to turn needle of turn coordinator to est. direction of rotation — not to the ball indication). • After rudder reaches stop, move yoke briskly forward to break stall. (See Mfrs. Recs.) • Maintain control inputs until rotation stops. • When rotation stops, neutralize rudder and smoothly recover from dive. • Retract flaps before exceeding flap extension speed. Note: Observe aircraft certificates, data sheets and placards for determining if the spin is approved. ;

tralize the rudder and recover from the dive. In modern acrobatic aircraft such as the Pitts or Eagle, it is essential to look straight over the engine cowling under the top wing. This will prevent confusion as to the direction of yaw. Looking over the top wing, especially when inverted, can actually result in reverse sensing of yaw. Doing this will prevent disorientation and confusion and will aid in determining the direction of the spin. Along this line, NASA test pilots found the visual and kinesthetic (seat of the pants) spin environment sufficiently different from other flight maneuvers to warrant special preparation. NASA test pilots discovered that after a certain number of spin turns in a given time, additional mental channels appeared to open and pilots were able to perceive and process more information from the instruments and from visual and force cues (see Ref. 4). The amount of spin exposure needed to attain this state probably varies among individual pilots, according to NASA. I can well recall my Navy flight training during WWII when we were trained on how to recover from spins using only instrument references, namely needle, ball and airspeed, which is all we had

then. This method of training, no

doubt, saved many lives, including mine on one occasion. In view of my recent spin experience flying with Woody, I think all of us need to re-examine spin recovery techniques. Perhaps this is something

that EAA, FAA and NASA could research together. This suggestion is based on my empirical observations. Our future plans include additional articles on spins as well as video tapes depicting various types of spins and recovery techniques.

I

References 1. Beggs, Gene - Aerobatics With Beggs, EAA, SPORT AVIATION, August 1984, p. 15. 2. Federal Aviation Administration, Flight Training Handbook (AC 61-21A), U. S. Dept. of Transportation. Flight Standards Service, Washington, DC, 1980, pp. 154157.

3. Holmes, H. J., Flight Maneuvers Manual, Palatine, IL. Haldon

Books, 1984. 4. NASA, The Effects of Configuration Changes on Spin and Recovery Characteristics of a Low-Wing General Aviation Research Airplane, Langley Research Center, 1979, p. 1-4. 5. NASA, Wing Design for Spin Resistance, Langley Research Center (AIAA Paper No. 84-2223), 1984, P-1.