What happens to discussions about lift during your flight instruction? Does the focus shift to learning flight maneuvers instead of understanding lift? Do you understand what makes the airplane turn when performing a maneuver? Why do airplanes perform better in cooler air? Why is the air clear on top of a smog layer? Can it be warmer at a higher altitude? Does air temperature affect our climb capability? What causes lift or sink near mountains? What's the difference between turbulence and sink?
Let's start with angle of attack. In Anyone Can Fly, Jules Bergman describes angle of attack as "the angle at which the wing 'attacks' the air to create lift." That description is something pilots can relate to. The elevator controls angle of attack. Hold that thought.
An equally important concept is the lift vector. Unlike a car, an airplane operates in three dimensions, so we must begin thinking in those terms. For simplicity, most training manuals use a profile view to illustrate the forces acting on an aircraft: lift, gravity, thrust, and drag. Unfortunately, many students perceive that the lift vector is perpendicular to the ground when nothing could be further from the truth. Regardless of the aircraft's attitude, the lift vector runs through its vertical axis. Imagine the lift vector as an arrow running out the top of the aircraft. A 30-degree bank results in a 30-degree lift vector.
We use aileron to establish bank and rudder to control adverse yaw, but the elevator is the only flight control that moves us through the vertical plane. How? Because the elevator controls angle of attack. In other words, lift controlled by the elevator pulls the airplane through its lift vector.
Let's take the aircraft out of horizontal flight for a moment. When the nose drops after a stall, pulling back on the yoke makes recovery possible because the angle of attack increased when the nose fell - but don't pull back too soon, or you'll precipitate a secondary stall. The vertical plane was used to bring the nose on the horizon. Looking from a side view, if you started the same pull with enough airspeed and maintained the optimum angle of attack, the aircraft would complete a 360-degree turn through the vertical plane. While referred to as a loop, this maneuver is nothing more than a turn in a vertical dimension. If we started in a bank, we would have completed an oblique loop. When a jet fighter pursues a target, the pilot rolls the aircraft to put the target symbology on the target, then applies back pressure to bring weapons to bear. Once again, this is nothing more than an oblique turn.
Throughout these maneuvers, the primary control is the elevator, although coordinated rudder may be required to keep the turn-and-slip indicator (ball) centered. Remember, rudder and aileron have nothing to do with creating lift or turning an airplane. They are used to change the plane of motion.
When bank is increased, lift must also be increased or the aircraft will descend. To increase lift, pull back on the yoke to increase the angle of attack. We feel the effect of gravity through the additional lift. Typically, a 60-degree bank turn requires two Gs' worth of aft stick pressure, if altitude is to be maintained. More bank requires more lift, therefore you feel more G forces.
An airplane experiences lift because of relative low pressure over the wing and higher pressure below it, and a range of factors affect lift. Perhaps the most significant element is heat. When oil is heated, it thins. Air is no different. The hotter the air, the thinner it becomes. Because the air molecules in warm, thin air are farther apart than in colder, denser air, thin air can't produce the same amount of lift near critical angles of attack It's the pressure differential that allows an airplane to climb, and there's simply less differential to work with when the air is less dense. Therefore, climb performance decreases. To determine the effect of temperature on lift, you must compute the density altitude.
No aircraft escapes the effect of density altitude. To prove that point, I offer the following. I was flying a McDonnell Douglas A-4 Skyhawk - a military attack jet very different from the Cessna trainer of the same name - off a high desert airfield with 600 gallons of fuel in external tanks, a target, and 30,000 feet of steel cable. I was concerned about density altitude not only because of lift, but also its effect on engine thrust. The performance chart showed my takeoff was in the cautionary area - it was possible to take off, but performance would be minimal. Since my mission was towing a target for a weapons test, I elected to go. My acceleration check was fine, and I rotated on speed. The nose gear lifted, followed by the mains, but the airplane wouldn't climb out of ground effect. I remained in ground effect until I gained airspeed. Two miles past the runway, I was finally able to raise the gear.
How does this relate to light airplanes? First, statistics prove that pilots fail to check their weight and balance and density altitude before taking off. A trainer is significantly affected by density altitude because of its limited power. Not only does density altitude affect the fuel-air mixture, but you also might not have sufficient runway to take off. As I proved in the A-4, just because you break ground doesn't mean you can clear obstacles. Know your airplane; use the manual. For pilots flying retractable-gear aircraft, consider delaying retraction until the runway is no longer usable. Drag increases significantly with the gear in transit.
Another category of lift exists: that created by weather and terrain. How do birds soar? Simple; they work this kind of lift, otherwise known as orographic lift. Three types of lift fall under this category: thermal, ridge, and mountain wave. Thermal lift is caused by uneven heating of the Earth and can climb a sailplane more than a thousand feet per minute. This type of lift is most prevalent during early afternoon when the sun is at its peak. Any aircraft can work thermal lift; just remain within the confines of the convective current. Since that involves spiraling at slow speed, it is impractical for most powered airplanes. Cumulus clouds are the result of convective activity. As these clouds mature, they produce a pocket of sink more pronounced than the area of lift. The only means of escaping this sink is to increase airspeed and fly out of it.
Ridge lift is caused by an upslope condition. Wind riding up the hillside is forced vertically, thus creating lift. Hang gliding is popular at Torrey Pines, California, because there is constant lift where the ocean winds meet its cliffs. Ridge lift does not produce significant altitude gain.
Mountain waves create the most spectacular lift. Imagine a stone in a fast-moving stream. The water riding over the stone rises, but just behind it the water is driven downward and into a vortex. A mountain wave is no different, creating an area of lift, a downdraft, and rotor turbulence severe enough to cause structural damage to an aircraft. The wave most often occurs when a river of relatively stable air initially lifted by a mountain range perpendicular to the airflow forms a sine wave bound by more stable air and is amplified over distances as far as 200 miles downstream of the range. Glider pilots view lenticular clouds - an indicator of mountain wave - as a chance to set an altitude record, but fixed-wing pilots should avoid mountainous terrain under such conditions and stay clear of the cotton-like rotor clouds marking severe turbulence.
This article is intended to stimulate thought and discussion for you and your instructor. To fly safely is to think in three dimensions and understand the basic concepts of flight. Knowledge is power, and as a pilot you need the power that understanding and respecting lift provides.
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