Aircraft entering a Spin
Stalling Effects, Spinning
Most aircraft accidents occur during the take-off or landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. In this section of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight.
If during the incipient or full stall the aircraft is not in balance, either by uncoordinated use of rudder or some other cause, the aircraft is likely to enter an autorotation called the spin.
When the aircraft enters a spin it is in a stalled flight condition where the aircraft follows a vertical spiralling descent path. The aircraft is stalled but at the same time rolling and yawing and one wing generating more lift than the other. The aircraft is autorotating as long as it is kept in the spin, some aircraft are able to fly themselves out of a spin by just letting go of the flight controls.
Entering a spin deliberately is easy: bring the aircraft in the stall and introduce a yawing motion by applying rudder, left or right. The wing will drop and the aircraft enters a spin and moves about all three axes: rolling, yawing, pitching and uncoordinated thereby losing altitude at low airspeed.
When fully set in a spin, the aircraft spirals downwards at a high rate of descend about the vertical axes and with the wings at a large angle of attack. Some upward force is generated opposite to the weight of the aircraft, slowing its descent. The outer rapidly moving wing has lower angle of attack (AOA) and more lift, adding to the rolling motion, contrary to the inner wing with a higher AOA and more drag and this condition helps the aircraft yawing.
This is when the spin starts. The aircraft must be at or beyond the point of stall, and with a difference in AOA between the wings the autorotation starts.
Autorotation is fed by roll and yaw. Roll is caused by the outer wing with lower AOA and more lift and the inner wing has less lift with a higher AOA. Yaw is caused by the inner wing generating more drag.
When a wing drops in normal flight the AOA increases and the wing generates more lift so that there is a natural tendency to roll back and the aircraft usually stabilizes by itself.
When a wing drops in stalled flight the increase in AOA causes the wing to be more deeply stalled resulting in less lift and the drop will continue. At this point the drag (CD) will increase leading into a yaw in the direction of the lower wing. This yaw leads to more roll and autorotation has started.
For autorotation to occur we need only one stalled wing.
Every aircraft has its own spin characteristics, but most will go through an early incipient spin where rate of roll and yaw are fluctuating and the pilot may feel some airframe buffeting. If action is taken at this point recovery will be almost instantly.
After these initial one or two turns into the spin it will become more developed, wings will be deeply stalled and the attitude will be flatter. Recovery will take more time and probably a turn or two.
Different CG positions will have their effect on stall/spin behaviour. A rearward CG will result in a flatter spin and difficulty with recovery. A forward CG makes spin entry a bit more difficult and the nose will be lower during spin. Recovery is much easier. Which emphasizes the fact that no aircraft should be flown with the CG outside of the limits.
Normally demonstrated by a qualified flight instructor during flight training (during aerobatics or aircraft familiarization). The exact procedure for a full recovery depends on the type of aircraft. But the standard, generalised procedure is as follows:
- Close throttle and flaps up, on some aircraft types flaps may not even be used on spin training
- Apply full opposite rudder to stop the spin rotation
- Hold until rotation stops, this can take a second or a turn or two
- Reduce the AOA by easing the yoke or stick forward
- The moment the spin stops, centralize rudder, level the wings and pull gently out of the dive
Again, the spin is a stalled condition of the aircraft. Unintentional spins can be avoided by not allowing the aircraft to stall (take immediate action to recover) and by avoiding uncoordinated flight which leads to autorotation when flying close to the stalling AOA.
Sometimes called graveyard spiral and often confused for a spin but the aircraft in a spiral dive is not stalled, its descending with a low nose attitude and high airspeed. Much like a steep turn that ended up descending very fast.
To recover from a spiral dive: close throttle, smoothly roll the wings level using ailerons and coordinated rudder, then ease out of the dive with the elevator. You will feel some G-forces and do make sure not to exceed VNE.