Spins

Spin modes are steady (sometimes oscillatory) states of developed spins. Spin mode modifiers (e.g. erect/inverted, steep/flat) are used to characterize different spin modes (cf. Table 10.3 on Page 10.25 of USAF TPS Textbook, “Flying Qualities”, Vol. 2, Part 2 (1986)).

The same aircraft can exhibit several spin modes, which the airplane can either achieve through different entries, or by pilot control inputs during the spin. Some spin modes may be recoverable, while others may not be. In order for an airplane to become certificated for intentional spins, it must not possess any unrecoverable spin modes. However, sometimes certain spin modes can be hard to discover in an airplane, because they are difficult to enter and require a special entry or transition from another spin mode with a specific sequence of control inputs.

The beginning of the video “Stall/Spin Studies Relating to Light General Aviation Aircraft” (1973) from the NASA Langley Research Center illustrates different spin modes (a steep one, and a flat one). As one can see in the video, they have different recovery characteristics. Note that in the video, a tiny parachute pops up to mark visually, when recovery control surface deflections have been actuated. For more information and further spin research videos, see the Flight Research section of our literature list.

An in-spin control input refers to a control input with the spin direction of rotation, e.g. rudder in the direction of the spin or stick (ailerons) in the direction of the spin in the case of an upright spin. “Pro-spin” on the other hand refers to the control input helping the spin to continue (anti-spin would be one that helps recovery). These are two distinct concepts, which must be distinguished. For instance, in-spin aileron can be an anti-spin control input in fuselage-loaded aircraft, because it reduces the yaw rate of the airplane due to inertial coupling.

In a spin drag points straight up, while lift points horizontally towards the spin axis (it provides the centripetal acceleration which keeps the aircraft on its helical flight path around the spin axis).

This can be easily seen if one remembers that there are three fundamental forces in flight: weight, thrust, and resultant aerodynamic force (RAF). The RAF is typically artificially decomposed into a component parallel (and opposite) to the flight path – called drag -, and a component perpendicular to the flight path (and in the plane of symmetry of the aircraft) – called lift. (The remaining third RAF component is sideforce in the wind frame.) Since in a developed spin, the flight path is nearly vertical, the statement in the previous paragraph follows.

The sideslip angle equals approximately the bank angle minus the flight path inclination angle to the vertical (due to its helical shape) – a result we will derive in our course.

The sideslip direction is not indicative of the direction of rotation of the spin. Furthermore, the inclinometer ball does not measure sideslip, it measures sideforce at the location where it is mounted. One more reason not to rely on the inclinometer ball to determine the direction of the spin – simply ignore it.

Let us first state that some airplanes can enter completely unrecoverable spin modes, where no control input will recover them. So let us restrict our discussion to airplanes which will recover readily with the standard spin recovery technique.

Even then, whether such a rudder-only recovery will be successful, depends on the airplane make and model and the spin mode it entered. Some airplanes may recover, others do not. During a normal spin in a Pitts, the airplane will stop spinning with full back stick, when full opposite rudder is applied. Examples of airplanes which do not recover, however, include the Piper PA-38 Tomahawk and the American Champion Decathlon, as illustrated in the following two videos available on YouTube:

1. In this first video from the NASA Langley Research Center, test pilot Jim Patton conducts a series of spin tests in a Piper PA-38 Tomahawk. At 4:55 minutes into the video, Patton announces a “rudder-only recovery attempt”, and the airplane does not recover from the rotation until he moves the elevator control forward.
2. In this second video by Robert Dumovic (20:20 minutes into the video), Dumovic enters a spin to the left and then tries to reverse the direction of the rotation of the spin from left to right with full right rudder while maintaining full back elevator. As he correctly predicts before the maneuver, the American Champion 8KCAB Decathlon does not stop spinning to the left, and he has to move the elevator forward to get it to reverse direction. It is to be noted that the Decathlon is a very docile basic aerobatic trainer with excellent spin recovery characteristics, when the proper procedure is applied. Dumovic’s demonstration shows how deadly a misinterpretation of the standard PARE recovery technique can be, if the pilot erroneously waits with the E for Elevator until the rotation stops, because the airplane may never stop spinning.

This is the so called “elevator-only recovery technique”, and it is absolutely not recommended as a spin recovery technique, because it crucially omits the opposite rudder and may not work in many aircraft. However, it is interesting to study it nonetheless. Forward elevator alone can be sufficient in some cases to recover an airplane from a spin. A pilot may encounter this phenomenon unintentionally when trying to perform an accelerated spin, and going too far or too quickly with the forward elevator to accelerate the spin. On the other hand, in some aircraft one can go full forward on the elevator during an accelerated spin, and the airplane will not recover without opposite rudder.

Recovery will happen if the elevator is able to exert enough of a pitch down moment (against the pitch up moment from inertial coupling) to decrease the angle of attack below critical angle of attack. In many airplanes this requires the rotation of the airplane to be slowed down first with opposite rudder, but not in all.

NASA test pilot Jim Patton demonstrates a successful elevator-only recovery in this NASA Langley video, starting at 6:30 minutes. Notice how the spin accelerates as a result of the elevator forward motion, before the airplane recovers (this is a typical response even during PARE – the forward elevator accelerates the rotation briefly before recovery starts to happen, because a steeper attitude brings the mass of the airplane closer to the spin axis, therefore decreasing the moment of inertia around that axis – angular momentum conservation takes care of the rest).

This question relates to a couple of remarks in the previous question, and inquires about the apparent conundrum that – on the one hand – flat spins are observed to rotate faster than steep spins, but – on the other hand – that forward elevator (and a correspondingly steeper attitude) will accelerate a spin. This appears to be a contradiction.

The explanation and calculation, how these two observations can be reconciled, can be found on pages 10.70 through 10.78 of USAF Test Pilot School, “Flying Qualities Textbook,” Volume 2, Part 2, 1986.

Forward elevator accelerates a spin based on the discussion of the previous question. Forward elevator may also terminate a spin, if the pilot manages to get the angle of attack below the critical angle of attack: when the wing gets unstalled, the spin will stop (though it might in some cases transition into a steep spiral if rotation continues to persist).

The above is a wrestling match, as you go forward on the elevator for recovery. You get closer to unstalling the wing, but the rotation rate will also increase, leading to a larger pitch-up moment due to inertial coupling. If you win the wrestling match, you recover. If you lose, the airplane is unrecoverable.

One way to help you win the wrestling match is to try to decelerate the spin before you apply the elevator. That is why recovery procedures for most airplanes require opposite rudder first, and forward elevator later (with various degrees of delay, depending on make and model of airplane) – for this reason and because the down elevator shields the rudder from the airflow, making it less effective.

See this video on YouTube of a spin test conducted by the NASA Langley Research Center in the 1970s, featuring Joe Brownlee as test pilot. The unrecoverable spin starts 12:00 min into the video. Brownlee eventually bails out, and the airplane spins to the ground.

An out-of-control recovery procedure is applicable to many situations and must be executable without thinking. The idea is that the pilot can perform the procedure instantly, without analyzing the situation. Due to the speedy control input, the aircraft might be prevented from entering a worse situation, like a spin. An example of an out-of-control recovery procedure would be actively setting all controls neutral (not letting go of all controls). 

A spin recovery procedure, on the other hand, is explicitly designed to recover an aircraft from a spin with maximum effort. It is oftentimes more effective in terminating the spin than the out-of-control procedure, but it usually requires knowledge of spin direction and spin sense (erect or inverted). It may take several seconds for the pilot to determine those, especially if the pilot may be disoriented and a careful analysis is warranted (e.g. in the case of a steep inverted spin, there is a serious possibility that the pilot may misinterpret the spin direction, because yaw and roll are opposite).

The spin recovery procedure must be compatible with the out-of-control recovery procedure and differ as little as possible (i.e. it would be highly undesirable if the out-of-control recovery procedure facilitated spin entry, e.g. in the case that the pilot is already entering an incipient spin, when she/he applies it). 

Yes, it does. But in many rectangular wing general aviation airplanes it does not occur (at least not with reasonable control inputs and power off). And in others the roll is not nearly as violent as sometimes claimed. Fairly rarely does the plane go “over the top and lays itself on its back, surprising the pilot,” in a nearly instantaneous process. Typically there is time to react and to prevent the airplane bank angle from going past vertical in the opposite direction, at least if the pilot anticipates it (though there are exceptions).

This is contrary to skidding turns (with in-turn rudder), where even from a mild 20-degree bank angle turn, the airplane may – upon stall – break past vertical bank angle faster than pilot reaction time. We posted a brief video on our Stability and Control Course page illustrating departure from slipping and skidding turns for two different aircraft.

However, it is to be noted that in some aircraft rather quick over-the-top spin entries from slipping turns do really occur (though some may need a substantial amount of out-turn rudder to cause the over-the-top spin entry). It is therefore vital that you become intimately familiar with the stall/spin characteristics of your aircraft make and model in a variety of attitudes and power settings.

A couple of examples of over-the-top spin entries from videos available on YouTube:

• Bruce Williams in an Extra 300L: The slipping turn spin demonstration begins 2:40 min into the video, with spin entry occurring at 3:17 min. (It is to be noted that a Pitts S-2C, for instance, does not exhibit this kind of behavior, unless the controls are seriously mishandled.)
• In the USAF training video “Ejection Decision – a second too late!” (1981), an F-111 can be seen departing into an over-the-top spin during a 2g accelerated stall test (starting 13:59 minutes into the video, with the spin entry occurring around 14:40). After the spin departure, the airplane looks briefly like it may recover, but eventually becomes unrecoverable, to the point where the test pilot has to eject, after the spin chute deployment goes wrong. Swept wing airplanes can have stall/spin characteristics which are very different from rectangular wing aircraft.

One also has to pay attention to the details and interpret the situation properly. Rich Stowell mentions at the beginning of his book “The Light Airplane Pilot’s Guide to Stall/Spin Awareness” (2007) that slips tend to have a lower stall/spin potential than even coordinated flight (see “Myth 3” in his introduction). But then in his video “Live Spin Demonstrations over Cascade Airport”, available on YouTube, he demonstrates in a SuperDecathlon an over-the-top spin (at 1:33 min, “Spin 4: Left Climbing Turn, Over-the-Top”) from a slipping left turn with right rudder. Notice, however, that the stall horn does not become active until the airplane is actually noticeably banked to the right. What does this mean? Spin entries do not happen until stall is achieved, so this is not an over-the-top spin from a left slipping turn, this is a normal spin entry from a right skidding turn with right rudder. What is really happening in this video is that yaw-roll coupling from the right rudder deflection first slowly rolls the airplane over the top to the right, at which point the rudder input leads to a skidding right turn, and only then does the airplane stall and depart into a spin. Plenty of time for the pilot to react (the over-the-top motion is gradual). We assume Rich Stowell knows this and has just added the over-the-top spin for dramaturgical effects to his video.

Knowing that one’s airplane make and model does not go over the top power-off can be exploited for safety. For instance, during an engine failure after takeoff turnback maneuver, the pilot may choose to leave the rudder alone if she/he is concerned about inadvertently overdoing the rudder input in the heat of the stressful emergency situation, thereby preventing a possible spin entry from a skidding turn with too much rudder.

Equip it definitely with a turn-and-slip indicator, not with a turn coordinator. The needle of the turn-and-slip indicator measures yaw rate only and remains useful during inverted spins, indicating which rudder needs to be pressed (the opposite). The symbolic airplane of the turn coordinator, on the other hand, measures a combination of yaw and roll rate, because the gyroscope is mounted at an angle. While this may be handy for keeping the wings level during normal instrument flight and remains useful in upright spins, the turn coordinator becomes entirely useless during an inverted spin, because in inverted spins yaw and roll rates are opposite each other – one cannot use it to determine the (yaw) direction of the spin. (The same property of inverted spins may confuse the pilot as well, who might mistake the direction of roll for the direction of yaw, and press the wrong rudder during recovery.)

Practicing spin recovery by instruments only is useful not only for IFR emergencies, but also if you ever get confused about the direction and sense of a spin. A turn-and-slip indicator and an angle-of-attack indicator can be real life savers. But even with just the turn-and-slip indicator you will at least get the rudder correct. The military – suspicious of outside references – practices spin recovery primarily by instruments.

The standard entry into an inverted spin is the same as for an upright spin, except from an inverted flight attitude: elevator comes full forward, and full rudder in the direction of the spin is applied at the moment of the stall. However, many aircraft have much more up-elevator authority than down-elevator authority. Down-elevator authority may be insufficient in some aircraft to start a spin entry. In such cases, the pilot can use a roll-coupling entry into an inverted spin, as is described in the USAF TPS Textbook, “Flying Qualities”, Vol. 2, Part 2 (1986), Chapter 10, Page 10.118. Roll rate in one direction and yaw rate in the other direction can be used together to lift the nose higher than would be otherwise possible with the down-elevator alone, thus achieving a higher angle of attack and a clean inverted spin entry.