Monday, November 25, 2013


There is something to be said about the sudden and violent break from a smooth flight after, and this is relative to the pilot experience, a brief flirtation with a shudder of imminent stall. This departure from the comforts is an aerodynamic oops. You know the kind when a surgeon cuts a vein accidentally, well not exactly that but pretty close. Both the pilot and the surgeon then sweat it out. They either are able to salvage from that experience and live to tell the tale or go down in flames. Lives can and are lost if experience is short and hubris is long.
So what of this departure?

Imagine a smooth flight back to your home, wherever home is. And the last few minutes as you get into the pattern altitude of your home-base airport on the downwind, you feel the drift from the crosswind. You adjust accordingly, but the drift forces you closer and closer to the runway as you continue on the downwind leg. The runway is now instead of being at the tip of your low wing aircraft, it is actually half way to the fuselage, close! You put another wind correction angle to your downwind and arrest the drift. Happily you feel comforted by the inputs. As you turn base, the aircraft now seems to jet across the approach end of the runway and your field of vision. Ah but you are prepared, you took an extra-long downwind leg just for this very reason as you fought the drift. So now you bank into the wind, but it needs more and more bank angle. You look at your airspeed and it is 1.2 VSo. You hold the bank and as you do, you feel the shudder. “It’s that damn wind shear!” something cries out and just as you realize this at 500 feet above the ground, the aircraft nose falls heavily. You’re instincts tell you, “Pull up! Pull up!” and if you do, all visuals are lost except the momentary rush of trees, or bushes or even flat well-manicured piece of land.

The silent sweat that is pouring down your back is a testimony to your understanding of the well-known aerodynamic limits of any airfoil. Exceeding that limit is a virtual calamity at low altitudes but can be salvaged at higher altitudes provided you have experienced and felt and trained for that knowledge.

The wing has a leading edge and a trailing edge, drawing a line between those two points gives us the chord line. It is this chord line that interacts with the relative wind. 

The term “relative” is relative based on how the thrust of the aircraft and its attitude is interacting in relation to the wind. For example, the fighter jet with its after-burners lit will have enough velocity to force a relative wind below its wing surface at near vertical and maintain a lift until it doesn’t. 

However if one were to have unlimited thrust as in the STS Space Shuttle with 5,6 million pound force then one could stay vertical and fly into space. 

You see it is all relative!

A classic example is as a child you might have put your hand out of a travelling car. If you faced the palm of your hand parallel to the ground and slowly changed the angle of the palm in reference to the oncoming air, your hand had a tendency to go up. “Eureka, I’ve found lift” you would think and yell. If you continued changing the angle, a point came where the hand simply was pushed back by the wind. That is as close to knowing the angle of attack function of an airfoil. Once the limits of the relative angle to the wind and the chord line of your hand exceeded, the drag exceeded the lift and push-back was the result. Try it someday, and feel the pressures if you haven’t before.

Angle of Attack is the most ingenious and simplistic measure of this knowledge. In mathematical terms the formula goes something like this: L = (1/2)*dv^2s(CL).
Where L = Lift
d = density
v = velocity
s = surface area of the airfoil
CL = Coefficient of Lift.

Lift, keeps the aircraft up in the air, is essentially helped by only two of these factors. The v in velocity and the CL as in Coefficient of Lift are the modifiers of any such departure from flight. Velocity however is limited in its endeavor to a certain extent since a relative wind change can occur at any speed, altitude and attitude. So that leaves us with the CL. Next question is what is this CL?

Coefficient of Lift expresses the ratio of the lift force to the force produced by the dynamic pressures times the area. It is the complex dependencies of the 3-dimensional airfoil (wing surface) and the air viscosity and compressibility. Below 200 miles per hour the latter has little reference, while the former still plays a part and of course the wing tip "downwash" that reduces the CL. So the measure of each airfoil is then mathematically derived at and gives us its aerodynamic limit. Knowing this helps the pilot in ascertaining where and when the failure might happen and how much margin should he or she give to prevent hat breakdown. 

How does one change the CL on an airfoil? Well my dear Watson, that is easy. Change its shape! How you say? Well you have the ability to deploy flaps that changes the chord line and the therefore its relationship to the relative wind and adding slats as airlines do, that further changes the geometry and increases the margin between stall and safe flight. Next time you fly with a competent instructor, allow him or her to demonstrate the stall characteristics of the airplane.

Stalls: Experience the departure stalls and approach to landing stalls. Consider recovery from a stall with the least loss of altitude. Consider stalling in clean configuration (without flaps and gear deployed)and then just as imminent stall occurs (that buffeting feeling) let the instructor put in the approach flaps and see as the chord line changes to the relative wind, the aircraft goes back to smooth flight without the burbles and shudders, albeit still at a higher angle of attack.

We fly with many different gauges in the aircraft and now with the glass cockpit, we fly with loads of information that keeps pummeling us for our attention. Only one instrument that has the capacity to keep us safe is missing from 95% of all the certified and non-certified general aviation aircraft. You guessed it, it is the Angle of Attack (AOA) instrument. Having this instrument is a simple safety measure that keeps the pilot in the know of when the airfoil capability is being exceeded. Try it out yourself. The AOA indicator will warn you well in advance when the violent break is about to happen. Based on our mathematical derivation, we also know that different airfoils e.g. An F16 wing has a different AOA then a Mooney or a Bonanza, have different critical angles of attack.

So there you have it. Keep the blue skies above you and the green earth below. Land when you want to and fly as often as you can.

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