A few years ago, while I was lecturing in the aviation department at a university in my home state, I was teaching an advanced aerodynamics and aircraft systems course when I made a joke about the FAA and equal transit theory being a cute fictional love story between two air molecule atoms wrenched apart by a wing’s leading edge but fortunately reunited after some turbulence. The class just stared awkwardly back at me. I then asked if anyone had any idea what I was talking about, because usually my jokes about the FAA (and the TSA for that matter) were typically met with some degree of laughter. One student raised his hand and said they had been taught this “theory” and were expected to understand it because the local FAA-issued DPEs (designated pilot examiners) at the flight academy required them to know this. Frankly, I was stunned. I understand that sometimes in order to get though a checkride (even in the airlines) you have to repeat some stupid thing or another that an examiner wants to hear in order to move on. Yet, as I reiterated this point and then asked if everyone actually understood how planes really achieve lift and fly, the blank stares resulted in suspending that day’s lecture and introducing them to the fundamentals of Newtonian physics.
But let’s start with equal transit, which perhaps many private pilots out there have read about in FAA texts and have possibly had to explain on their certifying checkrides. This physics and chemistry-defying scheme suggests that air molecules traveling over the longer upper surface of a wing (as a result of camber, the asymmetrical design between the top and bottom curves of an aircraft wing from leading edge to trailing edge) must move faster in order to meet back up with slower moving air flowing along the bottom of the wing. The result of this molecular affinity and the insistence that the atoms of air must reunite simultaneously on the backside of the wing is just false, because, as Newton would tell you, there is no physical law requiring air molecules that start together, finish together at the trailing edge of a wing before drifting off into space to live happily ever after.
In fact, the air over the top of the wing is indeed faster and actually arrives at the back of the wing way before the under-flowing air does for reasons of complexity we won’t go into here. The continuing thought of equal transit though, is that because of the air pressure differences generated above wing (low pressure) and below wing (high pressure), a lifting force is created. As a result, high-pressure air seeking a low-pressure area means the air molecules below the wing are desperately seeking their above wing partners enabling the plane to go up into the sky.
As a quick aside and just for fun, if this is how lift is generated – a curved (cambered) wing with this whole high pressure below seeks low pressure above thing, then how do fighter jets achieve lift when these wings have effectively zero camber (or curve) in them? Their wings are flat – so how do the molecules know where to go to provide lift? Moreover, if you've ever folded and flown a paper airplane, they fly just fine too and without any curved surfaces either. And I wouldn't even know where to begin with an aerobatic plane with cambered wings flying upside down at an airshow…
What equal transit is attempting to instill into the lift equation is Bernoulli’s 1738 fluid-dynamics principle where a constricted fluid (air is a fluid) flows faster resulting in lower pressure. This is what happens when you put your thumb over the end of a water hose to create a forceful spray. Although pressure differential is a component of lift, the much more complicated nature of its generation is more accurately explained by a combination of Bernoulli’s equation and Newton's Third Law of Motion (commonly referred to equal and opposite reaction), occurring in the downward deflection of air as a plane moves forward.
Force is produced when a mass is accelerated (Newton’s Second Law), and as a plane (which has mass) accelerates, lift is the force that delivers and holds it in the air in an equal and opposite manner. Since lift is a force, it’s also a vector and thus possesses magnitude and direction. The direction of lift being perpendicular to the relative wind, while drag is parallel to it – two of the four forces taught to all flight students, with the other two of course are thrust and weight.
I don’t know who at the FAA way back when decided that this was what they were going to tell pilots about lift and flight, but I do know that if you ask the folks at NASA’s Jet Propulsion Laboratory about equal transit they will laugh you out of the building. Maybe the FAA thought it was easier to explain, or it fit in with someone’s pet explanation of lift and it just stuck, or perhaps like many things at the FAA, it’s simply left over from the 1920s and hasn’t evolved yet.
Lift is not uncomplicated, but it’s certainly not made any easier by fiction and half-truths.
Speaking of, next month we’ll tackle that delightful “cushion of air” experienced down low on the runway…
