by Spike Spencer
Spinning is a complex subject for analysis and it is difficult to make generalisations that are true of all aircraft. One type may behave in a certain manner while another may behave completely differently under the same conditions. I have to admit that the maths surrounding this is about as interesting as stray Bosons, Gluons and Charmed Quarks. So, if you are a professional Aerodynamicist of the Newtonian, Inertial or Wake Theorist schools, what follows will probably offend you greatly. If so - tough. You can write your own article can’t you ? (but you must phrase it such that mere mortals can comprehend)
“If having applied full opposite controls, the model continues to spin ... pray.”
To say that this is potentially misleading is a masterful understatement. After an exchange of emails, I was persuaded to ‘stick my neck out’ and write an article with the aim of generating feedback and further debate on this subject, or perhaps even a few ‘I learned about flying from that’ articles.
Having revised the subject in various scientific papers and in my copy of AP3456 (the CFS standard reference text) I was somewhat daunted by the scope of the subject and the reams of mathematical proofs and vector diagrams that already took several books to even start to explain. However, with the strict aim of making the topic accessible to non-professionals, I eventually drafted some suitable text in a way that managed not to upset my fixed-wing subject expert colleagues at work.
What is a Spin ?
Firstly, it is NOT the same as a spiral dive. In a spiral dive the aircraft will be simultaneously rotating about all three axes but the relative airflow over the wings remains at an angle of attack below the stall and all flying controls have their normal effect on the relevant axis of rotation. The helical path taken by the aircraft may look like that in a true spin but this is misleading; the radius of rotation about the Earth-vertical (or descent-path) axis will be significantly greater than a true spin where this axis tends to be closely coincident with the ‘inner’ wingtip.
A spin (usually) starts from a ‘simple’ stall. Speed is reduced while progressively applying more and more elevator to try to maintain height (we won’t go into a debate over the difference between height/ altitude/level). This results in increasing Angle of Attack to try to maintain lift until the wing’s Stall Angle is reached. At this point lift generated by the wing starts to collapse (but does not go away completely) and the stall is associated with a significant increase in drag. The effect on the aircraft is that weight wins its battle against lift and the aircraft starts to descend rapidly at a high angle of attack. The nose will almost certainly drop to some angle below the horizon but, because of the increasingly high rate of descent and its effect on the Angle of attack, the wing remains fully stalled.
It is at this incipient stage of the spin that the wing will demonstrate that no man-made object is perfect. Due to tiny differences in wing rigging, or even a fly on one leading edge or the pilot inserting an inadvertent yaw (rudder) input, one wing will stall slightly earlier than the other; that wing will start to descend first and the airframe will roll about the longitudinal axis. The downgoing wing will become even more deeply stalled with associated drag rise, the ‘rising’ wing will be ‘less stalled’ and with (relatively) lower drag. The resulting drag difference between the left and right wings will cause the downgoing wing to slow down relative to its opposite number and the aircraft will start to autorotate about the yaw axis. This drag effect will slow the inner wing even more, making its stall condition even deeper and so on. Note that the ensuing spin is a direct result of the stalled wing and the autorotation effect, we have made no mention of Rudder or Aileron inputs.
This is the point at which the mathematical ‘flute music’ really kicks in and everything affects everything else ! You will be mightily relieved to know that you will not be subjected to a single equation in this article - so read on.
Note: Autorotation in a spin by a fixed-wing aircraft is not the same as the helicopter equivalent of 'gliding', also termed "autorotation". I do not intend to discuss that here.
To enter a spin deliberately, approach the stall in level flight by progressively reducing forward speed while applying up elevator to maintain height. At the point at which the stall commences, apply all remaining up elevator to ensure the wing is fully stalled while simultaneously inducing the initial autorotation by applying full rudder in the desired direction of spin. Ailerons should be kept central throughout. All things being equal the aircraft will enter a fully developed spin which will probably stabilise within a few turns, and will stay that way while these control inputs are held.
Factors affecting the spin characteristics
Most model aircraft and many fullsize (civil) light aircraft will recover from a spin merely by centring the controls. However, there are many which will continue in a fully-developed spin and require positive action to regain controlled flight. There are some for which no effective spin recovery has been established but Darwinian selection ensures that these types are in the minority.
For those who are aware of gyroscopic precession, each of these moments can be equated to a gyroscopic rotor of equivalent mass rotating about each of the primary axes. When all are rotating simultaneously but are physically connected together (the airframe) the secondary effects get somewhat involved. So as well as primary and secondary aerodynamic effects, we also have primary and secondary inertial effects. [It is at this stage that my brain starts to hurt] I commend those boffins who need to cope with this stuff but it is no good telling me what the aircraft is supposed to do, I need to know what it actually does do - we are back to hands-on experimentation.
Spin recovery at the Incipient stage
Within the first turn or so, the spin is not fully established and few, if any of the forces involved will be in equilibrium. If the pilot did not intend to spin, the actions taken in the first half turn will be critical to recovery with minimum loss of height. The spin started fundamentally with a stalled wing and this must be unstalled and working again as soon as possible. Any yaw present must also be ‘nipped in the bud’ or autorotation will establish rapidly. It is at this stage that misuse of the ailerons can be critical. Remember that the ailerons only affect yaw as a secondary effect, the primary yaw control is RUDDER and it is that control which should be used to overcome the undesired yaw present in the onset of autorotation. Use of aileron to try to lift the ‘falling’ wing before the whole wing is unstalled will tend to reinforce the incipient spin. This is because a downgoing aileron increases the angle of attack thereby deepening the stall of the inner wing. Also, adverse yaw will increase the retardation of the inner wing adding to the autorotation effect. Recovery from an incipient spin will be to counter the ‘wing drop’ caused by the undemanded yaw by applying top rudder, and to unstall the wing by removing any up-elevator. If you have one, judicious application of engine thrust might also be a good idea which will give more slipstream over the rudder and increase airflow over the wing by assisting forward acceleration. Once the spin is fully established things may well be different.
I remember a few years ago seeing photographs of a beautifully built large-scale model Comet racer which came to grief on its first public flight. The model entered (was allowed to enter) an incipient spin soon after takeoff (trying to climb too steeply/ too slowly ?) and after about 3⁄4 turn impacted the ground. The pilot later stated that "... it would not recover". The photo taken immediately before impact clearly showed the rudder almost central but the aileron on the lower wing inside the turn deflected fully downwards.
Recovery from a fully developed spin
As stated above, the primary features of a spin are a stalled wing and autorotation. Recovery is initiated by the pilot by actions aimed first at opposing the autorotation and then reducing the angle of attack to unstall the wings. The aircraft may then be recovered from the ensuing steep dive. Experience shows that the most successful recovery procedure is first to reduce the yaw and stop the autorotation by applying full-opposite-to-spin rudder. Holding this opposite rudder may take some time to act and the spin rotation may even speed up initially - stick with it and in most case the spin will stop. As soon as the rotation stops, centralise the rudder or else you may re-enter another spin in the opposite direction. If the spin continues with full opposite rudder applied, the elevator should be progressively moved downwards aiming to unstall the wings. Once the yaw has stopped and the wing is unstalled, the aircraft should be gently eased out of the dive and returned to normal flight. Pulling out too sharply from the dive could cause entry to an accelerated stall/spin with a possible ‘flick’ entry where things can happen rather rapidly - not ideal when the ground is getting close.
Many people ask, quite reasonably, why the elevator is not used first to unstall the wing - this is because in a fully developed spin the relative airflow is coming from almost directly below, down elevator usually increases the shielded area of the fin and rudder. By blanketing the rudder it is less effective and recovery adversely affected. Those types with V tails or high-mounted T tails may behave differently.
Attempted use of Aileron during recovery
Rudder is normally the primary control for spin recovery but where inertial effects are large, aileron deflection may be needed when rudder effectiveness is poor in a spin. There may be times when aileron is essential to recovery (no I can’t say in which sense) but in the final analysis it is its effect on the yawing moment which makes it work. Once in a fully- developed spin with some state of equilibrium and depending on the inertial and aerodynamic couplings of each individual aircraft (B/A ratio), ‘out-of-spin’ aileron may either be pro-spin or anti-spin, only testing will establish this. Therefore, until these aircraft characteristics are defined by experimental flying, the ailerons should remain in their neutral position.
Standard spin recovery
Always subject to specific-to-type advice:
• Throttle closed [gliders - n/a]
Spin fails to stop (fullsize)
When you run out of height and ideas, pull the handle and trust Martin-Baker. If you aren’t sitting on one of those, first check that you are not in the simulator, then make the appropriate cabin announcement as determined by Company instructions ["Captain Speaking speaking - we have a minor technical malfunction and we will be landing slightly early"] and then talk privately to whichever is your own God.
Spin fails to stop (model)
There is a lot you can do before resorting to prayer. If ‘standard spin recovery’ fails to work on a model, this is where those spin trials come into their own; if you know that certain aileron application is definitely anti-spin, now is the time to use it. If this parameter is not known beforehand, deflection of this control surface could render the spin ‘unsurvivable’. Bear in mind also that, in a fully developed spin, any control input may take several rotations to have a significant effect on the forces acting on the airframe, so don’t expect immediate results. With models we are not usually too concerned about airframe stresses so consider also what might happen with application of engine power if that is available. As well as thrust, we are now introducing yet another gyroscopic moment with the spinning prop which could vary significantly depending on the direction of spin rotation. It may help and it may not - you may wish to try this at a safe height. One of my ‘vintage’ models (Magnatilla) would spin beautifully and when full pro-spin controls were held on (including the appropriate aileron) and the throttle opened fully, the rate of spin would increase dramatically with both the wings and fuselage in a very flat attitude but the rate of descent reducing to a very low value. The inner wing was most definitely deeply stalled but here was proof that the whole wing was still producing significant lift ! To get it out again I had to close the throttle and apply full opposite rudder and half-down elevator to recover which took a few more turns. Had I let it spin all the way to the ground in this condition, the rate of descent was such that the arrival would not have done much damage. It sure was [predictably repeatable] fun and at school birthday ‘toffee-bombing’ sorties I was always asked to “Make it fly like a helicopter again, Mister.” In an inverted spin it was a right pig so I avoided that part of the flight envelope - but that is another story.
The factors which make any one model a ‘bad spinner’ are too numerous to fully debate here, but here are just a few:
• Masses distributed away from the Centre of Gravity - increases the inertial moments which require more powerful aerodynamic controls to oppose them. So keep wingtips light and beware if you put lead or servos in the tail to offset a heavy motor or long nose.
The argument that high aspect ratio (A/R) types are either more or less spin-prone than those with low aspect-ratio is so dependent on all the other dynamic factors that generalisation is fruitless. The Gloster Javelin was a low A/R delta and many will say that deltas don’t spin but that became somewhat irrelevant when the high tailplane was completely blanked by the wing in a so-called ‘super-stall’ rendering the elevator useless. I have known gliders which will spin beautifully both erect and inverted, recovering merely by easing off the controls used for spin entry. However, there are others of similar A/R in which spinning is prohibited because of their reluctance to recover [and I am grateful to the brave soul who established that fact beforehand].
With a model you are your own test pilot so, as well as ‘bog-standard’ spins, you can carefully explore the effect of aileron deflection on your particular model during spin recovery. Enter a spin in a repeatable manner and allow it to stabilise for a set number of turns before recovery. Does in-spin aileron deflection change the rate of spin rotation? Does this deflection make any change (helpful or detrimental) to the recovery? Is there any change to the number of turns taken before recovery is effective? Don’t make assumptions - observe your model behaviour carefully. Remember also that, unless you have exactly the same equipment layout and masses distributed throughout the airframe and wings, your model may well behave differently from an apparently identical one constructed by someone else. If you try different control inputs as well (power?) repeat the spin and recovery several times and change only one thing at a time or you will have no idea what is affecting what.
Spinning is not a ‘black art’ but the inertial and aerodynamic interactions can be extremely complex and will vary considerably between even apparently similar aircraft. It is a ‘departure from controlled flight’. As far as modellers are concerned, we don’t need to know ‘the science bit’ and the standard spin recovery nearly always works first time with the spin usually stopping after merely centring all controls.
Deeper understanding of the characteristics of each individual model aircraft is something that should be explored progressively by every competent pilot. This process should continue until the entry and exit parameters of his model are well known (here we have the advantage over our fullsize brethren who should abide by the Operating Manual derived from formal flight testing). With this practical knowledge and adequate height for recovery, the manoeuvre is safe and repeatable. If you have a model which for some reason is wholly unsuitable for spin recovery (it may get in - but will it get out ?) - keep well away from this regime. If you have attempted the spin trials suggested with such a model, it is likely that the Darwin effect will already have taken its toll with a single, terminal, spin !
I hope this article has gone some way to better understanding of spinning but I recognise that it may well generate more questions than answers. That’s fine, just start a new Forum thread - someone will be able to find the answer. In the meantime, happy stick-twiddling and have fun exploring the full flight envelope of your models; but remember, airspace above you is as useless as runway behind you.
Author's note: Acknowledgement is given for cartoons from "The White Sheet" and by Tugg. They may be subject to copyright and should not be used commercially without permission of the originator.