erikv

Question of the movement about the lateral axis in spins (light a/c):

Some of my books suggest that during a spin, most a/c will try to recover from the stall, stall again, recover, etc. This results in, but is not described as, pitch oscillations during the spin.
Another book describes the movement during a spin as a corkscrew path while falling nose down.

From what I've seen spinning gliders and some light a/c, I have difficulty accepting the first theory. I do agree that the aircraft pitches, but tend to believe that it pitches up continuously as a spin is a combined movement about all three axes, similar to a barrell roll.

Your thoughts?

Erik.

Mad (Flt) Scientist

Been a while since I had to worry about spinning but...

Spin characteristics vary widely, as do the mechanisms driving the spins. But one of the common components is that the wings are stalled (angle of attack on each wing greater than stall AoA). When a roll rate is imparted to the aircraft, this causes a moment which drives the roll rate, rather than opposing it as in 'normal' flight. This is a major contributor to the spinning motion.

If the aircraft (or even one wing) were to 'unstall' then the effect would be to cause a strong damping to the roll motion; this would either bring the aircraft out of the spin (most likely) or cause it to transition into another spin mode, likely in a violent fashion. While stalling and unstalling might be associated with an aircraft with several spin modes transitioning between them, I don't see it as a component to a stable spin mode.

Weight and Balance

A few years back I rode along on several spins flights in a Katana, and clearly noted some very slow pitch oscillations during an otherwise stable upright spin. I didn't think much about it at the time, but I'm sure that the gentle amplitude and long period could not have resulted from something as major as unstalling - restalling - etc.

My personal theory would be some small change in the flow field around the wing or stabilizer, changing the pitching moment of the airfoil, but leaving the wing well and truly stalled.

wyvern

erikv,

You are correct in believing that the aircraft in a spin pitches up continuously, although there may be variations in the amplitude of the pitch-up.

To visualise this, imagine a light aircraft in a spin, and isolate pitch by "suspending" the aircraft from the tip of its out-spin wing.
The aircraft is now in the "spin" with its wings being vertical, with no yaw (other than it is descending) or roll. In order to follow the flight path of the "spin", (which is now like riding the "wall of death" in a fairground) the nose has to pitch up constantly.

Also consider the recovery action, whose main component is "stick forward until the spin stops". It is felt that pilots who have had difficulty coming out of a spin have been too ready to try something different to the standard recovery action, when they should have held the stick forward "until the spin stops".

I follow the rule of thumb - if it is difficult to get the aircraft to spin, it is likely to be difficult to get it to recover.

Mad (Flt) Scientist

Be careful what you mean by "pitches up". Especially in exaggerated and abnormal attitudes the "normal" correlation between body angular rates and attitudes is lost.

So while the spinning aircraft may have a nose-up pitch rate (q) it may also have a stable pitch attitude (theta) and hence zero rate-of-change-of-pitch-attitude (theta-dot).

And some people use "pitch-up" when they more properly mean "rate of change of angle of attack" - yet another variation on the theme.

northwing

A spin is an equilibrium state in which the aerodynamic forces tending to oppose roll, pitch and yaw rates are balanced by gyroscopic forces tending to increase them. The gyroscopic forces arise because the aircraft is rotating about all 3 axes at once. Thus roll rate, combined with precession about the yaw axis, gives rise to a torque about the pitch axis. Yaw rate, combined with a pitch rate, gives rise to a torque about the roll axis and so on. Hence the motion is self sustaining. All 3 rates can become oscillatory, particularly when the roll inertia is low compared to the yaw and pitch inertia. In the Hunter, for example, roll reversals are common in the spin (to the tune of 3 sick bags in my case). In the Jaguar spin all 3 axes are oscillatory and while the spin looks like a totally random falling leaf the rates recorded by on board gyros are all nice regular oscillations - all at different frequencies.

Mud Clubber

Central to the spin behaviour is the relationship between roll and pitch moments of inertia. Most light aircraft will have a stable balance. If you then check out the control matrix you can see the relationship between the rates of roll and pitch, which should explain why roll and yaw can cause the pitch oscillations.
Is it mostly the case that when light aircraft enter a fully developed spin it is "buried", ie both wings are actually stalled but the inner one more than the outer. The pitching moment effects of this with the stick fully back can cause the old "nodding dog" effect as the nose tracks around the horizon, the same as it would in a straight and level stall.

chris-the-duck

Just a small thought from an uneducated amateur..... surely if the aircraft recovered from its stall, the spin would stop, as the stalled characteristic is central to the spin. Presumably, the pitch variation is just an oscillation on the pitch axis.

I am willing to stand corrected on this, but I remember learning "no spin without a stall"!

Genghis the Engineer

CtD, you're correct; what commonly happens is that the aircraft recovers into a spiral dive. Recovery from this is fairly painless, but (a) the pilot must identify that this has happened, the "picture" can be very similar, and (b) Vne can be exceeded quite easily.

G

grob103

Genghis,

You say the picture can be very similar. I only fly gliders, but is the engine in a powered craft so loud that the rapidly increasing wind noise wouldn't be obvious?

If not, I'm going to have to learn to rely a lot less on my ears for speed control when I do my PPL!

Genghis the Engineer

We tend to wear headsets or helmets when flying powered aircraft.

G

LOMCEVAK

Re spiral dives. The anti-spin moments present in some aircraft at certain cgs and AUWs mean that a spin will devolve into a spiral dive after one or two turns. The motion may look identical to the spin, but the big clue is the increasing control forces needed to hold the control deflections as IAS increases.

grob103

Genghis, LOMCEVAK,

I'll keep an eye out for the stiffening controls next time I'm
looking to lose a few thousand in a hurry

On a related note, I presume there's nothing inherently dangerous from "holding" a spin using back-pressure and rudder... If so, I need to have a word with one of my (former) instructors!

Thanks again.

[Edit... I'm thinking specifically of the SZD Junior here]

Genghis the Engineer

If you want to hold a spin, then full back stick and pro-spin rudder are the conventional way to do this. The only reason not to is if the specific aircraft type isn't cleared past a given number of spins - usually because it winds itself up into something hard to recover from.

So, almost certainly no reason to feel aggrieved at your instructor.

G

mstram

Wyvern wrote:
"I follow the rule of thumb - if it is difficult to get the aircraft to spin, it is likely to be difficult to get it to recover."


Hmm, where did you get that "rule of thumb" ? I would guess it be *easier* to recover if it was difficult to start.

E.g. C172, needs ..or as "assisted" into a spin by slight addition of power, otherwise just tends to spiral dive instead of spinning.

Power off, release controls, auto-recovers (at least after 2-3 spins anyway !!!)

Mike

Genghis the Engineer

Wyvern's rule of thumb is actually described in considerable detail in Darrol Stinton's book "Flying Qualities and Flight Testing of the Aeroplane".

There are two reasons why an aeroplane may be spin-resistant. One is that, primarily through a combination of intertia ratios and high directional stability, the aircraft will not sustain autorotation - these are very safe and what the FAA's "spin resistance" criteria are really written around.

What Wyvern (and Darrol) are mainly referring to is the other reason, which is control authority. Some aeroplanes lack the control authority to get themselves into a spin (the C150 for example often needs a touch of power to help it). These can (note can, not must) be the killers because having got into the spin, they may then lack sufficient control authority to get out. this is likely to be true for an aircraft with low rudder power and moderate directional stability since at, say, Vs+5 where spin-entry is generally initiated, if the controls don't have the authority to get you into a spin, at Vs, where dynamic pressure is still lower, the control may not have enough authority to kill the yawing action that defines the spin.

The same can of-course be theoretically true of the elevator or all-flying tailplane. Although I've little experience of the type, I seem to recall somebody telling me that this applied to the Harrier and some strange control inputs are needed for spin recovery - is there a Harrier expert in the house?

G

grob103

Genghis,

Thanks. It's very reassuring to know that my preferred method of losing excess height (various kinds of stall entry and resultant spin practice) is safe, even if I hold for a few revolutions more than "normal" to observe more closely what's happening.

Still, it scared the hell of out me the first time I did a spin in the Junior - I didn't realise you could go that far past vertical!

grob103