|
How to Keep Your Wings Attached and Trousers Dry by David Lian |
||||
|
You're on your first solo stall. Power is back to idle. Nose and wings are level while speed bleeds away. You feel elevator buffetting through the stick as the stall warning sounds. The nose will drop very soon and you'll recover from the stall. Piece of cake. Except there's one small problem. While busy holding the nose and wings level, you somehow missed a developing yaw. Unconsciously, you rested a little more weight on the left pedal and now - gosh, the nose drops and the left wing flicks down viciously. Taken by surprise, you respond in slow motion: forward stick, right rudder, full throttle and unconsciously, right aileron. No joy, the rotation seems to continue. You hear the sounds of rising airspeed and a racing engine as the rotation slows. You catch the right aileron and correct it - whew! The ground is getting closer - fast! So you pull back on the stick smoothly and firmly. Bang! Bang! Your wing spars fracture and you're now a wingless lawn dart! As I wrote this, my pulse quickened, as it probably did for those of you who fly. Parts of the situation above probably parallel our own fears or personal experiences when first learning to fly. But why should we worry about spins/spiral dives when flying sims? Being a lawn dart isn't a big deal - after all, it's not fatal! Besides, all the control laws of new fangled jet sims like EF2000, F22 ADF etc prevent spins don't they? True for the jet jockeys. But with some high fidelity prop sims like Fighter Squadron etc, if they are any good, you could experience inadvertent spins more often than you expect. This may be especially the case when you're in an intense dogfight against tough opponents. So let's delve a little more into spins and how to recover from them. Autorotation Let's look at an example in which the aircraft enters a normal spin to the left. Yawing (to the left in our example) at the point of stall causes a lift imbalance between each wing because the right wing is moving faster than the left. The lift imbalance causes a left roll and stalling of the left wing. Differential angles of attack for both wings also cause differential drag. Greater drag from the left wing gives rise to a left yaw, which then feeds back into the lift imbalance between the wings. Hence, a self-sustaining feedback loop is established between yaw and roll.
Incipient Spin The incipient phase of a spin occurs when the aircraft transitions from a horizontal to a vertical flight path. Airspeed increases during this time. What happens next depends on whether the aircraft enters a stable or unstable spin. To understand this, think of the aircraft as a simple stick (representing the longitudinal axis) with two unequal weights attached to each end. In turn, the stick is attached to a pole and can pivot about the attachment point. As the stick is rotated as shown, the weights drive outwards, tending to make the stick (aircraft) move towards the horizontal. Spin rotation speeds depend on an aircraft's individual characteristics but typical rotation speeds are around 360 degrees per second. Vertical descent speeds are typically in excess of 6000 feet per minute. For piston aircraft, the engine usually cuts out leaving the propeller to stop while turbine aircraft may experience engine flameouts. So spin recovery usually involves engine airstarts - always a fun exercise!
<>Stable Spin If the rotation is sufficiently fast, the aircraft's nose pitches up to a shallower nose-down angle. As this happens, both wings become fully stalled and airspeed stabilises. At this point, the aircraft is established in a stable spin with the controls remaining in the pro-spin position (full back stick and left rudder - in our example) on their own.
In a stable spin, equilibrium is reached between the natural pitch down tendency due to the aircraft's aerodynamic stability, and the pitch up tendency due to the inertia forces. Given the influence of mass distribution and the centre of gravity's (CG) location, it is easy to visualise the effects of an aft CG (decreased pitch stability) and greater mass dispersion (increased inertia forces). The spin becomes flatter (remember the dreaded flat spin in Top Gun?) and more difficult from which to recover.
|
Unstable Spin What happens when the inertia forces cannot generate sufficient pitch up motion to offset the pitch down motion due to the aircraft's aerodynamic stability? The nose simply pitches down, thereby unstalling the wings. No problem, after all, this is what we want - right? It turns out that life isn't as simple. Remember that entry into the spin involved significant yaw rate and angle? Under such conditions, the blanketing effect of the vertical tail means the elevators are ineffective in holding the nose up. The aircraft enters a spiral dive in which speed and G forces (outward from the spiral) build up very rapidly. If the pilot doesn't recover sufficiently fast, he will become an expensive lawn dart! You may have watched with perhaps morbid fascination, World War 2 movies of mortally wounded B-17 Flying Fortresses in their spiral dives. Many broke up as they went down, no doubt due to massive structural failure of their airframes. It was not a pretty sight. One can only imagine the terror of the crewmen pinned helplessly by G forces in the spiralling aircraft; knowing that only death would release them. In developing spiral dives, structural failure can occur as the pilot attempts to pull out of the dive. Wing structures appear to fail at loads below stated limits of the aircraft. For example, an aircraft rated to 6Gs may experience wing buckling or even wing loss at 4-5Gs, as the pilot tries to pull out of a spiral dive. Why does this happen? The reason more often than not, is due to the failed wing exceeding its rolling G limit. This and other interesting topics will be discussed in a later article. Alright, enough of theory and onto flying technique. Entering into a Spin The actions for entering into a stable spin are straightforward:
Recognising a Stable Spin
Note that if the airspeed does not stabilise but continues to increase, then the aircraft is in a spiral dive and the pilot must commence recovery immediately. Recovering from a Spin Most aircraft will recover through use of the following technique:
Closing Comments There are some types of aircraft in which the above "hands-off" recovery technique does not work. Such aircraft typically require positive forward stick to pitch the nose down to unstall the wings. This characteristic may be due to aircraft design, an aft CG location, greater mass dispersion or a combination of all three factors. My former chief flight instructor (one-time airforce test pilot) told me of a nasty experience with a De Havilland Chipmunk which refused to recover from a spin. When he entered the spin, he noticed it appeared to have a very shallow nose angle. Anyway, with the altimeter unwinding merrily towards 5000 feet, he began recovery. He became a little concerned when the aircraft simply refused to respond to the full opposite rudder. Then it dawned upon him that the very shallow nose angle indicated a flat spin! He then tried positive forward stick - no response. With precious little altitude left, he resorted to pitching the stick back and forth - timing each reversal to help increase the pitch oscillation. Grudgingly, the aircraft pitched out of the shallow rotation plane, after which a normal recovery followed. Unknown to him, the aircraft had been fitted with a larger engine and as a result, required a counter-weight in the rear fuselage. The increased mass placed at each end of the fuselage greatly increased the inertial forces generated during the spin, thereby causing the rotational plane to flatten. Following a hasty landing and shutdown, a young mechanic sauntered up to stick a "Spins Prohibited" placard in the cockpit. He had forgotten to do so a few days earlier! Next: G Limits and Structural Failure
|
|||
|
This material is copyrighted and may not be reprinted in any form without
permission of the publisher.
|
||||
