Spinning on purpose - or sometimes not

This post is encouraged by the previous item about parachute dropping. There, we had three examples of surprise spinning. All were aggravated by too much aft load shift, but one followed up with behaviour that might have put it in a higher aerobatic category. Did one of the videos show us a glimpse of crossover spin technique, divided into two tiny parts? Perhaps, but the curious notion of spinning in a transport aircraft while dropping parachutists suggests the first subject of this website - unexpected connections, if that’s where your imagination leads.

I expect most pilots are still taught something about the spin, its recognition and the correct recovery technique required for the aircraft type. I would hope so where possible. The spin is a ‘departed’ manoeuvre, in other words not part of the normal manoeuvring intended for the design, and most pilots will not experience an accidental one. But it does happen, particularly when preoccupation with the approach to an unfamiliar and small landing field diverts attention from the basics. Low speed combined with some element of turning is all it takes, and the result sometimes ticks the serious accident box.

History

By the end of the Edwardian era powered aviation had come quite a long way, and the search for better performance, more stability and more logical control systems had also introduced higher energy accidents. But there seems to be a common problem running back to the earliest days - an unintended loss of directional control leading to a rotating descent which defied normal control instincts. Lt Wilfred Parke’s public recovery success in August 1912 is remembered as a defining point of spin recovery knowledge, although it was a kind of sideslipping spiral dive he had experienced - and to this day the related but different spin and spiral dive behaviours still get confused.

Naturally, experience and understanding was rapidly gained by brave experimenters, but because aircraft were not designed specifically around their spin behaviour and use, spin training was not introduced until after the start of WW1. At the beginning of this conflict some in high military office believed that aircraft would be of little use anyway - except as aerial means of reconnaissance and artillery spotting. But German single seat fighters and the rapid loss of defenceless spotters quickly demonstrated the flaws in this assumption.

By the end of WW1, performance and controlled manoeuvrability had improved considerably, and aerobatic manoeuvring as a way of life for some had come to stay. Between the wars, aerial entertainment flourished, and the variety of daredevil acts are too many to number. John Becker of the Pompano Air Centre was a 17 year old ‘How low can you throw’ stunt parachutist in the 1930s. This airshow item was presented as a competition, to ramp up the uncertainty factor. He went on to be an ‘over the hump’ USAF transport pilot in WW2, and I met him, alive and well in the 1990s, so he must have never got it completely wrong. Of course an airshow commentator and his powers of dramatic improvisation are always an essential part of this kind of public entertainment, and the actual risks braved by the aerobatic display pilot may (and should) differ significantly from the public’s extreme assessment. Today, such dichotomy could just be an alternative truth of the aerial stage, called marketing in everyday life.

A search for Ernst Udet will reveal much about a colourful aerobatic character who became very popular in the US. A member of Richthofen’s flying circus he became the highest scoring German pilot to survive the war, and continued the flying showman’s lifestyle throughout the 1920s and 30s. His skill and experience was called upon to help German aviation in 1938, but this management job took the glamour and fun out of flying, and he shot himself in 1941.

Aerobatic contests pre-1960

Books have been written about this subject, but until 1960 or thereabouts, aerobatic competitions suffered from a basic problem for the competitors - ‘How do we know what the judges are looking for? This is another way of asking ‘Is it fair?

There was no easy answer to this, but by 1960 a structured syllabus of manoeuvre analysis had appeared, based on various shorthand systems of manoeuvre depiction, and by the 1964 world championships the aerocryptographic system devised by Spanish Jungmeister pilot, José Luis Aresti Aguirre, was officially adopted.

A similar problem of the previous quality judgment was identified to me by a New York theatrical acquaintance, involved with the teaching of theatre. I found that my assessment of a tryout performance was often at odds with professional judgment. She explained that there are basic rules. ‘If we went to an airshow I might say I liked a particular display, and you might point out that it had been reckless, badly flown and potentially dangerous.’ I got the point.

Aresti symbols: an introduction

What did the 3 parachute dropping aircraft do?

The KingAir90 performed a 1¾ positive spin, after which some other stuff took place before an approximation to level flight occurred - the end of the figure. The red triangles indicate the satisfactory take off positions for an acceptable centre of gravity.

The C47 Dakota did one of these - a 1½ spin. The long and steep dive out of camera shot did not show the official finish of this figure, but we can assume that the required normal flight was finally achieved.

The Cessna’s unconventional figure combination is not in the official contest agenda, but will be explained later.

Aresti basics

Each figure begins and finishes in level flight, either erect or inverted. The start of a figure is indicated by a small circle (sometimes dot), and the finish by a short vertical line. Negative lift coefficient flight (upside down flying) is depicted by a dashed line.

These symbols indicate a roll, stall turn and loop, three fundamental basics. There follow a half loop and half roll (roll off the top), and we finish with a level flick or snap roll. This last makes use of spinning aerodynamic behaviour, with additional dynamic content (more kinetic energy).

The detailed and analytical Aresti dictionary, understandable by readers from Australia to Accrington or Zanzibar to Zagreb, has become the competition bible. Some might argue that this formality takes personal creativity away from the daredevil stunting popular with an airshow crowd with unspecified expectations, but it does provide a platform for far-flung worldwide competitors to meet every two years on a reasonably level and safe playing field.

The spin as deliberate aerobatics

One would not usually consider the spin an aerobatic precision manoeuvre, but its inclusion in the Aresti dictionary suggests that it had became popular and commonplace among between-the-wars training and recreational circles, once the terror of its departed mysteries had been mastered.

Assuming straight and level decelerating flight to the stall, correct spin behaviour should follow - similar to beginner training. Following the specified number of rotations the competition pilot is required to recover on the correct heading and assume a vertical dive, after which level flight is regained. As well as correct spin appearance, the entry and recovery are also evaluated. Gratuitous multiple rotating wastes height energy and represents little extra skill value - even though an airshow crowd may privately thrill at the prospect of disaster.

But entertainment and masterly competition are not the same. A single turn spin is most common in formal competition, with additional 90⁰ increments up to two rotations as convenient in practice. It is not a high scoring figure, and a competitor will not voluntarily invest more than minimum points in the spin in a freely-chosen programme. The Aresti difficulty coefficients reflect the amount of rotation to a small extent, but the basic natures of the spin, positive or negative, and both entry and exit lift coefficients (upside down, right way up entries, pushing or pulling out) are more significant factors for the allocation of difficulty coefficient, and therefore score value.

Varieties of the spin . . .

This post title figure depicts a one turn positive spin. It is the lowest valued spinning item in the catalogue.

The inverted equivalent looks like this, and is worth a good few more difficulty points:

Aresti correctly ranked it as more difficult because right-way-up people (most of us) find it more challenging, but aerodynamically it is the same thing upside down: inverted straight and level, throttle back, reduce speed in level flight (stick forward) until the machine stalls, apply rudder, and so on.

( . . . rudder?

Here’s a timely and relevant digression. Some years ago I had some interesting conversation in Barbados with a windsurfing Swiss military pilot. He was one of the full time nucleus of the Swiss people’s air force and currently a member of the 6-man Patrouille Suisse Hunter-mounted aerobatic team, otherwise training others on the F5 mini-fighter. Gregor told me he thought he would try out the F5’s inverted stalling behaviour one day, flew upside down at a moderate speed, pushed the stick forward and applied right rudder. ‘It rolled to the left, I was amazed,’ he said.)

He had discovered a fundamental feature of inverted autorotation. An inverted aircraft obeys its controls in the normal way, and the pilot continues to see the correct picture, but from a new upside down seat mounted under the fuselage, so to speak. A ground-based observer interprets aircraft behaviour as seen from outside, so an inverted spin observed to be to the left requires right rudder from the upside-down pilot. I will leave this subject here, but invite thought as homework, (and suggest a look at Lyn Williams’ excellent drawing of this concept in brother Neil’s book, ‘Aerobatics’).

The recovery patter is aerodynamically similar to the upright version: rudder opposite to the yaw element (as the pilot sees it), and stick back until the rotation stops. Then we have the vertical dive with appropriate recovery to the required kind of level flight. All competition figures start and finish with level flight, but this can be positive or negative (right way up or upside down). This technical point means that a spin may have to begin with the wrong kind of starting attitude. This is where ‘crossover’ comes in - crossing over to the opposite lift coefficient without picking up a ton of speed.

Tips and tricks

At the world aerobatic championships in 1972 - my first - I flew the standard Zlin 526, the only pilot to do so at this event. This Zlin did good inverted spins, especially as it was designed as a basic trainer for right-way-up flying, and I had no problem with this manoeuvre. But the unknown (unseen) sequence started with a crossover entry to an inverted spin. I’d never done this before. Neil Williams gave me some very rare advice about a minimum speed start - below the normal stall speed:

“Slow up in normal level flight as for a positive spin, then, just before the stall point bring the stick all the way back to wash off some additional speed. The judges won’t notice it.” He was right. “Then stick straight into a front corner and opposite rudder. As the nose drops and rotates give a brief vroom of engine for increased elevator and rudder effect.”

It worked a treat, and many years later at an Advanced World Competition in Kansas an Irish competitor flying for Britain in a borrowed Yak 55 asked for the same advice. I explained the technique, adding that once the onset of spinning was evident he should slide his ailerons into the middle, keeping the stick fully forward. In-spin aileron tends to resist a spin and encourage a spiral dive; in this special case it’s use is a necessary cheat to encourage the start, but does not help the developed manoeuvre. The result was excellent, and the centring of aileron calmed the roll in favour of true spinning, allowing the full wing to assume its natural character; and the nose swept up in a convincing inverted spin.

Only one turn was required, and his recovery was correct, but a bit late - understandable when one considered the excellent flat spin attitude achieved half way round. I was the UK judge at this international, FAI sanctioned meet: clinical, analytical, impartial, internationally respected. But at the video conference to decide whether he had exceeded the null points 45⁰ overshoot error I voted with the confusing-attitude-perspective excuse camp. I wasn’t confused, but my UK vote outnumbered the null point merchants so the Irish pilot got a score for the figure. Does this make me a bent judge? Only slightly, but to see my advice carried out so convincingly by a relative beginner, and the Yak become a negative frisby in a mere 270⁰ of rotation from normal straight and level, was a coach’s dream. He deserved plenty of marks for his courage, and especially trust in his unfamiliar teacher. I don’t think the name Riley had anything to do with tactical judging.

What did the parachuting Cessna Caravan do?

This is not an easy question, but there is a wealth of informative snippets delivered by the numerous jumpers’ GoPro clips. First we see the aircraft roll 90⁰ to the left as the left wing stalls, shedding those jumpers clinging to the outside rail, and perhaps some others at the doorway. Unlike the Beechcraft and DC3/C47 footage, the Cessna does not enter a steep nose down spin. It appears to briefly hold its knife edge attitude, as if the pilot has applied opposing top rudder (right), and also cancelled positive spin tendency with down elevator.

The next freefaller’s image shows the Cessna flying upside down, in a creditable straight and level attitude. How did it get there so quickly? Perhaps there are some clues in more on-board camera footage. First we hear the stall warning horn sound. Then we see a flurry of jumpers leave the exit, as if sucked out by explosive decompression.

In the following cabin confusion we can see webbing strap items on the side rail hanging upwards. The Cessna is flying upside down.

The next jumble of clips show a variety of personal attempts to get to the doorway - and get the hell out of there, no doubt. But whether by walking in the air, crawling up the wall, dragging oneself across the ceiling . . ? Who can say?

The Caravan only has one engine, so asymmetric power is not a major directional factor. Whether the excess weight at the back, and perhaps reducing speed, required full down elevator I don’t know. This seems to be the situation before both stalls displayed by both the Beech and Douglas aircraft featured in the previous ‘Dropping parachutes’ blog.

Does the Cessna flight manual say ‘If the stall warning sounds apply full down elevator’? Possibly; probably not in so few words, but if the Polish pilot knows more about aerobatics than dropping parachutes from a Cessna Caravan this event might makes some sense. However, the information from a few YouTube clips is usually entertaining but seldom as comprehensive as a critical observer would like.

Conclusion

Following the previous blog post this essay presents some thoughts but no conclusions about these events. Every second of life is accompanied by risk: in fact life itself is always fatal. Bearing this in mind, ‘extreme’ is a much overused word when describing other people’s hobbies, and human fragile opinions will always vary.

The recreational sport of skydiving has mushroomed - big time - since my 1960s introduction to it and the Dragon Rapide. It’s relatively available today, but the challenge can fulfil a human instinct for the occasional reminder of our mortality, and the tactics for postponing its result. Eve discovered this when she took the first bite of the apple.

The three loss of control incidents already described were not supposed to happen. They were all caused when an enthusiasm for the excitements of life overwhelms the discretion borne of the experience and understandings of those who wish to do the same thing more than once. Skydiving lift accidents do occur, but not often. These three unusual events did not result in injury, although the man who had to deflect himself over the Cessna’s tailplane has been pointed out to me. I have not heard his comments

The Cessna Caravan appeared to first stall and auto-rotate to the left by 90⁰, corrective right rudder then appeared to maintain an instant of knife-edge. Did it then perform a further negative 90⁰ flick roll to the left (push and right rudder required), ending up in inverted flight in a level attitude?

The Aresti catalogue does not subdivide flick (US ‘snap’) rolls below a half rotation, but we can easily write ¼ beside the symbol. Here’s the shorthand for the combination flown (perhaps):

Cessna Caravan hesitation crossover quarter snaps, positive then negative

I’ve not seen one of these described in official circles.