Peter Garrison - My mother, who would fortify herself for any aerial voyage with either Miltown or Chivas Regal, would later revisit with perverse relish each “air pocket” the plane had encountered.
I’M NOT SURE WHAT she believed an air pocket consisted of, but I suppose it was something like the nonsensical “region of low pressure causing an aircraft to lose altitude suddenly” that you still find if you Google the phrase today.
You would not suppose that awareness of air pockets could much precede the Wrights, but in fact the earliest reference I have encountered is in a biography of the Roman general and statesman Titus Quinctius Flamininus, who in 197 BCE liberated a number of Greek citystates from Macedonian control. Writing some 250 years after the event, the Greek author, Plutarch, describes how a large gathering of Greeks gave out such a shout of joy at the announcement of their emancipation that passing crows fell from the sky.
He pauses in his narrative to ponder this remarkable event. “The disruption of the air must be the cause of it. For the voices, being numerous, and the acclamation violent, the air breaks with it, and can no longer give support to the birds; but lets them tumble, like one that should attempt to walk upon a vacuum ... It is possible, too, that there may be a circular agitation of the air, which, like marine whirlpools, may have a violent direction of this sort given to it ...”
‘Air is not a fruitcake’
That mixture of credulity and careful analysis is characteristic of ancient authors; we moderns, or at least the irreligious among us, simply dismiss implausible stories of long past events as mere fables. But it is interesting to see that a firstcentury Greek, who was a new-born babe as far as the mechanisms of flight and the behaviour of air are concerned, nevertheless came pretty close to describing what today we would all “loss of lift” and “turbulence.”
Despite the passage of 2,000 years, Google’s “region of low pressure” is actually worse than Plutarch’s “disruption of the air.” Air is not a fruitcake, with thicker lumps scattered around here and there for aeroplanes to bump into. There are, indeed, regions of low pressure in the atmosphere, and regions of high pressure, and of all sorts of pressures in between, but they are hundreds or thousands of miles wide and in no sense “pockets.” Besides, pressure has nothing to do with it; if it were actually a deficit of something that caused an aeroplane or a crow to drop, it would be density, not pressure.
In fact, what causes an aeroplane to drop or to surge upward is not a change in the quality of the air. It is a change in its motion – indeed, a sort of “agitation.” You see how air swirls and eddies when a beam of sunlight illuminates airborne dust particles or shower steam. What you perceive in an aeroplane as bumps are differences in the direction and velocity of air movement as you pass rapidly from one region of the air mass to another. The aeroplane cannot instantly re-trim itself for each swirl and eddy through which it passes, and so the wing experiences transitory changes in angle of attack and airspeed. The lift force therefore changes, and the aeroplane rises or sinks accordingly. Bumps feel more sharp edged and percussive in a fast aeroplane than in a slow one, because the transition from one region to another is quicker.
There is a connection between weight, speed and turbulence that pilots are taught, but do not necessarily understand. We are taught that the safe speed for turbulence penetration gets lower as an aeroplane gets lighter. This seems counterintuitive; why
would a lightly loaded aeroplane not be better rather than worse equipped to sustain a given gust loading?
‘don’t fly over any rock concerts’
The reason has to do with angle of attack and lift coefficient – and this may be why it is not more easily understood. When an aeroplane encounters a gust, the change in angle of attack is the result of combining the speed of the plane with the relative direction and speed of the gust. For example, if a plane traveling at 100 metres per second (about 195 Ktas) encounters a vertical gust of 20 feet per second, its angle of attack changes by nearly four degrees (that is, a 15-to-one slope). That is a significant change, considering that the entire range of usable angles of attack, between zero lift and the stall, is only around 15 degrees.
How the aeroplane responds to that change has to do with both its speed and its wing loading. The higher an aeroplane’s wing loading, the higher its lift coefficient at a given speed. The lower its speed, the higher its lift coefficient at a given weight. Now we come to the bumpy part of the trip. Lift coefficient is a measure of how much of a wing’s lifting capacity is being used, relative to the air pressure generated by the plane’s forward speed. Its range in normal flight is from 0.1 to about 1.5 with flaps up.
A value of 1.5 means that the average pressure over the entire wing is one and a half times the pressure registered by the pitot tube and presented to the pilot as indicated airspeed. The lift coefficient of a wing at an angle of attack of zero depends on the camber of its aerofoils; but each change of one degree of angle of attack results in a change of about one-tenth of a unit of lift coefficient. (Actually, it’s slightly
less than a tenth, but let’s use one tenth for convenience.) This linear relationship has the following result. If the lift coefficient is 0.2 at an angle of attack of one degree, it will be 0.4 at an angle of attack of three degrees – two tenths increase for two degrees increase. But, by the same token, if it is 0.4 at one degree – that is, the wing loading is higher or the aeroplane is moving more slowly – it will be 0.6 at three degrees.
Notice the difference. The change in lift coefficient is 0.2 in both cases, but the relation to the starting value is different. In the first case the lift force increases by 100 percent, and the aeroplane experiences an acceleration of 2G; in the second, although the change in angle of attack is the same, it increases by only 50 percent, and the acceleration is 1.5G.
If you ponder this relationship, you will see that it has two important consequences. One is that aeroplanes with higher cruising lift coefficients – generally, ones with higher wing loadings or flying at higher altitudes, or heavy aeroplanes flying at lower indicated airspeeds – experience lower accelerations in a given level of turbulence.
The other, which is the more consequential, is that by slowing down, and therefore increasing your angle of attack and lift coefficient, you reduce the effects of turbulence. This is true in spite of the fact that as you slow down, the change in angle of attack for a given gust gets larger.
Pilots sometimes ask why, if an aeroplane is lightly loaded, its allowable G loading doesn’t go up. Surely an empty cabin puts less strain on the wing than a full one. The reason is that the entire structure of the aeroplane – not just the wing spar, but he engine mounts and battery box and gear uplocks and fuel tanks and what have you – are designed for the required limit load – 3.8 G for most “normal category”
aeroplanes – and so parts other than the wing are still vulnerable to overstress.
'Turbulence is not caused by low pressure but by vertical air movements'
That’s why it is always wise to slow down for turbulence – even if it is merely anticipated turbulence, and even if your plane is lightly loaded.
Slowing down has a second advantage as well.
As the lift coefficient gets closer to the stall, a sufficiently powerful upward gust will push it beyond the stalling angle of attack and the lift of the wing will diminish rather than increase. This is the basic idea of manoeuvring speed and turbulence penetration speed: to use the wing’s inherent inability to continue lifting beyond a certain angle of attack to protect the wing’s structure against excessive lift.
But just to be super-safe – heed old Plutarch and don’t fly over any rock concerts or football games.
Those people can really yell.
Peter Garrison – My mother, who would fortify herself for any aerial voyage with either Miltown or Chivas Regal, would later revisit with perverse relish each “air pocket” the plane had encountered. I’M NOT SURE WHAT she believed an air pocket consisted of, but I suppose it was something likeRead More →
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