Cracking the code

You know the basic answer—lift and money—but you might not know the details behind it. We are constantly changing our airplane’s lift in flight, and while we likely understand what steps we have to take—add more power here, more back-pressure here—we might not understand the aerodynamic whys behind those steps. Understanding the lift equation can help clarify why you do what you do during each of your maneuvers.

If the lift equation seems like going above and beyond, it is. But historically speaking, the best pilots also understand the hows and the whys. Aerodynamics for Naval Aviators says, “…the safety and effectiveness of flying operations will depend greatly on the understanding and appreciation of how and why an airplane flies. The principles of aerodynamics will provide the foundations for developing exacting and precise flying techniques and operational procedures.” If it’s important to our friends Maverick and Iceman, it should be important to us.

First, what is lift, really? Here’s ANA’s technical definition: “The net force developed perpendicular to the relative wind. The aerodynamic force of lift on an airplane results from the generation of a pressure distribution on the wing.” If the lift pushing you up is greater than the weight and drag pulling down, you initiate a climb. When it is lower, you start a descent. Makes sense.

Philosophically, the lift equation is the Rosetta Stone of aviation, giving us mere mortals the means to decode the magic math of flight. Beautiful in the simple way it explains a complex component of flight, the lift equation is the answer to all things airplane. Understanding it will illuminate the logic behind each maneuver and you’ll understand why you’re doing what you’re doing (beyond just because your flight instructor told you to). When it clicks, you’ll feel like you’ve learned a new language overnight.

As pilots, there are a few things we can manipulate in flight to change our airplane’s lift while performing maneuvers. The character that looks like a lower-case p is rho, which represents air density. Density changes with altitude, but at typical general aviation levels, it won’t change much and is more fixed than the other variables in this equation. Since we can’t significantly modify rho, we won’t pay much attention to it right now. But we can change our velocity and our coefficient of lift, and sometimes, our wing area. 

L/D_max

Another important component of lift is its opposite, drag, which is calculated in nearly the same way as lift. For each airfoil, there is a point at which the coefficient of lift and angle of attack create the highest lift for the lowest drag. This is called the L/D_MAX. L/D_MAX is especially important when considering gliding distance in power out situations.

V: Change our velocity

Let’s start with V for velocity. The way we change our velocity is pretty straightforward. We can change our power setting, and we can change our altitude, switching potential energy into kinetic energy. Because velocity is squared, it generally does the most for us.

C_L: Change our coefficient of lift

The C_L is “a function of the shape of the wing and angle of attack.” Changing our angle of attack is straightforward—we modify pitch with elevator control. We have some control over the shape of the wing as well. “But wait!” you say. “Our wing is fixed! We cannot change it!” Not so. We can! And fun fact, when we do, it technically becomes a new airfoil. When we add in flaps, we change the shape of our wing and change how much lift it produces. We can also change the shape of the wing and increase lift with leading edge devices like slats. Now you know why all those STOL airplanes have such fancy wings.

A: Change our wing area

This one is a bit harder to do and less common in GA airplanes. But the primary way we do this is through wing devices like flaps. At some point, you may be asked what is the “best” type of flap, and maybe you already know that the answer is Fowler. But why are Fowler flaps more effective at maximizing lift and drag than other flaps? Because they, unlike others, don’t just change the shape of the wing and add lift—they also add wing area. By adding area, there is more wing to make more lift.

Think through it

Let’s apply what we’ve learned and think through the whys of each step in a steep turn. Our goal: a coordinated, constant-altitude 45-degree banked turn, a 1.4-G maneuver. In a bank, some of the lift produced by the wings acts horizontally. That leaves less lift supporting the aircraft’s weight vertically, so to stay level, we will need our total lift to increase, which means the “weight” we feel (load factor) will increase.

So, we roll into our turn, add a little power, add a little back-pressure. The power and back-pressure change our airspeed and angle of attack, increasing our velocity and coefficient of lift. With the increase, we can now create enough lift to compensate for our weight and stay level in the turn. As we come out of the turn, we take that little bit of power back and reduce our back-pressure because we’re now going back to our normal state of flight.

In cruise at steady state unaccelerated flight, our velocity is doing the most for us. We’re level, have a high velocity, and the wing is clean and flapless. The wing has a relatively low angle of attack, meaning we’re relying less on changing the wing’s coefficient of lift to maintain altitude. As we speed up after leveling off, we even have to lower the nose some to stay level. Because velocity is squared, this allows our other variables, in this case our coefficient of lift and our wing area, to remain as they are, or as we’ve seen, reduce in the case of coefficient of lift.

Now, let’s consider when we get slower but still want to stay level, like when we’re getting ready to enter the pattern, don’t quite want to add flaps yet, and as we take power back, don’t have that velocity squared helping us out. What do we do? We pitch up a little and change our angle of attack. We know that’s the right move as pilots, and the math tracks. When we change the angle of attack, we’re changing the coefficient of lift part of the equation—we make our C_L higher to compensate for our V decreasing. That way, we still get an L that matches the airplane’s weight.

Understanding lift really comes together in a graceful and perfect flare, when you get a perfectly buttery landing by balancing your lift with your tools and knowledge. If you understand the whys to this level before your checkride, you’ll be set up well to ace both the oral and the flight portions, and for a future of thoughtful flying.

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