Learn › Technical subjects › Principles of Flight

Principles of Flight

The airplane is not obliged to survive whatever you ask of it. This is the material that tells you what you are asking for — angle of attack, load factor, and the forces behind both.

Technical Subject Areas — FI.II.D

Why this subject exists

A Beech Bonanza was maneuvering low over a group of onlookers. Pulling out of a turn, the pilot loaded it past what the structure could carry, and the tail and a wing came off in flight. Both people on board were killed; that nobody on the ground was killed too was luck. The account here stands for a class of accident rather than one NTSB file, and the class is well populated. Nothing had been broken beforehand — the pilot asked the wing for more lift than the airframe could absorb, and got it.

The four forces

• Lift — from airflow over the wing, perpendicular to the relative wind
• Weight — airplane, crew, fuel and baggage, acting down through the center of gravity
• Thrust — from the powerplant, moving the airplane forward
• Drag — the aerodynamic resistance to that motion

In steady, unaccelerated flight the opposing pairs cancel: lift equals weight, thrust equals drag. Accelerate, climb or turn and a pair goes out of balance until the airplane settles into a new steady state.

The airfoil, and the one angle that matters

The chord line runs from leading edge to trailing edge. The mean camber line runs the same way but stays equidistant between the upper and lower surfaces, so it traces the wing's curvature; where the two lines separate, the airfoil is cambered. The relative wind is the direction the air moves relative to the wing, and the angle between it and the chord line is the angle of attack — what you are really controlling with the elevator.

Raise it and the wing makes more lift, up to a point. Past that point — the critical angle of attack — the airflow separates, lift drops sharply, drag climbs, and the wing is stalled.

That angle belongs to the airfoil in the shape it is currently in, and for the airfoils on trainers it sits somewhere in the region of 15 to 20 degrees. It takes no notice of airspeed, weight or attitude. It does take notice of the wing itself: lower the flaps and you have changed the airfoil, and ice or frost on the leading edge lowers the critical angle, sometimes by a great deal, so a wing that looks only lightly rimed can quit well before the number you were carrying in your head.

A stall is an angle, not a number on the airspeed indicator, which is why you can stall in a steep turn at a speed that felt comfortable a moment earlier. That is an accelerated stall, and it is a maneuver you first meet as an instructor demonstration, at altitude in a suitable airplane — not something to go and explore on your own. Most trainers stall at the wing root first, so buffet warns you while the ailerons at the tips are still in clean air.

Where the lift comes from

Two explanations describe the same event. Bernoulli: where a fluid speeds up its pressure falls, so air accelerating over the curved upper surface is at lower pressure than the air beneath, and the wing is pushed toward it. Newton's third law: the wing deflects air downward, and the reaction pushes the wing up.

Lift depends on the coefficient of lift, air density, true airspeed squared, and wing area. You can raise angle of attack, add airspeed, or lower flaps, which adds camber and on some designs area. Airspeed is the powerful one because lift varies with the square of velocity: double the airspeed at the same angle of attack and the wing makes four times the lift. Air density you cannot touch — hot day, high field, less to work with, the whole density altitude problem in a sentence.

Drag, and the back side of the curve

Parasite drag is what the airplane costs you for being a solid object in moving air: form drag from the shape of cowling and antennas, skin friction from air scrubbing the surface, interference drag where airstreams meet at the wing root. It grows fast with speed, like a hand out of a car window at 25 mph and then 75. Induced drag is the price of lift, inseparable from producing it, and greatest when you are slow at a high angle of attack.

Add them and total drag makes a U. Slowing down reduces drag on the front side, the region of normal command. Past the bottom you are in the region of reverse command, where flying slower needs more power — slow flight and short final, and why pitching up alone will not fix low and slow on final.

Wingtip vortices

The pressure difference that makes lift also spills high pressure air around the tip toward the low pressure above, trailing a spiraling vortex behind each wingtip. Behind a large airplane those can roll a light one past its aileron authority. Vortices are strongest when the generating airplane is heavy, clean and slow — an airliner departing or approaching.

They are invisible, and they move in ways you can plan around. They sink at several hundred feet per minute behind the airplane that made them, tending to level off some 500 to 900 feet below its flight path, and they drift with the wind. In still air they can persist for three minutes or more, so the airplane that laid them may be out of sight while its wake is still sitting where it left it. The avoidance guidance in AIM Section 7-4, Wake Turbulence, follows from that:

• Departing behind a larger airplane, rotate before its rotation point and climb above its climb path until you turn clear of its wake
• Landing behind one, stay above its approach path and touch down beyond its touchdown point
• Avoid following on a similar flight path within 1,000 feet of its altitude
• Watch the wind. The case the AIM singles out as calling for maximum caution is a light quartering tailwind, which can hold the upwind vortex on the runway and carry the downwind vortex over onto a parallel runway
• A helicopter in a slow hover taxi or stationary hover is its own hazard, its downwash throwing high-velocity outwash vortices out around it; AIM 7-4-7, Helicopters, tells pilots of small aircraft to stay at least three rotor diameters clear

Stability, controllability, maneuverability

Stability is the airplane's built-in tendency to return to its original flight path after a disturbance. Static stability is the initial tendency: positive starts back, neutral stays where the bump put it, negative keeps going. Dynamic stability is the response over the following seconds, and positive means the oscillations damp out.

The axis names cross over, which catches people on orals. Stability about the lateral axis is longitudinal stability, in pitch, set by the wing and horizontal tail positions relative to the CG and the size of that tail — the practical reason weight and balance matters, since loading aft erodes it. About the longitudinal axis it is lateral stability, in roll, from dihedral, sweepback, keel effect and weight distribution. About the vertical axis it is directional stability, in yaw, from the fin and fuselage side area behind the CG.

Controllability is how readily the airplane answers your inputs. Maneuverability is how easily it maneuvers and how well it takes the stresses of maneuvering. Stability trades against both: a stable airplane makes a poor aerobatic one, and a highly maneuverable one demands constant attention. A trainer is built stable on purpose.

Load factor and maneuvering speed

Load factor is the ratio of lift produced to the airplane's weight, in G. In straight-and-level unaccelerated flight the wings carry exactly the weight of the airplane, so you are at 1 G; in a level 60-degree banked turn they must produce twice that weight, so you are at 2 G. Two things follow: load factor is stress on the structure, and exceeding the limit breaks the airplane, as it did in that Bonanza; and it raises stall speed, so a hard pull can stall you at an airspeed that was safe wings level.

Airplanes are certificated normal, utility or acrobatic, with progressively higher limit load factors, and a normal-category airplane is built to a positive limit of 3.8 G. The airworthiness standards for these airplanes are in 14 CFR part 23, though part 23 became performance-based at Amendment 23-64 and the 3.8 G figure now lives in the accepted means of compliance behind the rule rather than in the rule text. The CFR is the authority and it changes. The limits you actually operate to are the ones in your POH and on the placards, and 14 CFR 91.9(a) is the section that requires you to comply with them.

Maneuvering speed, Va, is what stands between a maneuver and a bent airframe, and it is narrower protection than it sounds. At or below Va, one abrupt full deflection of a single control will stall the wing before the structure reaches its limit load — the stall relieves the load. That is the whole of the guarantee. It does not cover several controls used in combination, and it does not cover a control reversal; the FAA has said so plainly since the American Airlines 587 accident, and current POHs carry the caveat. Either of those can overstress the airframe below Va. Above Va, an abrupt input can break something with the wing still flying. Va also falls as weight falls, backward from most pilots' first guess, so take today's number from your POH rather than from memory.

Va is a maneuver-load speed and nothing more. Gusts are a separate account: the structural cruising speed Vno is the top of the green arc, the yellow arc above it is for smooth air only, and the speed to fly in turbulence is the turbulent-air or rough-air penetration speed your POH names for your airplane at its weight. Look it up rather than assuming it equals Va.

The left-turning tendencies

Four effects push a single-engine propeller airplane left, all worst at high power and low airspeed — takeoff and climb. All four assume a propeller turning clockwise as seen from the pilot seat, which covers most US-built engines but is not universal: counter-rotating installations and some non-US powerplants turn the other way and reverse every one of these. Your POH and engine data tell you which you are flying.

• Torque — with the propeller turning clockwise from your seat, the airplane tries to roll counterclockwise. On the takeoff roll that means extra weight on the left main, more friction there, and a pull left.
• P-factor — at a high angle of attack the descending blade on the right takes a bigger bite than the ascending blade on the left, moving the center of thrust right and yawing the nose left.
• Spiraling slipstream — the corkscrew of air off the propeller wraps the fuselage and strikes the left side of the fin, pushing the tail right and the nose left.
• Gyroscopic precession — the propeller is a gyroscope, and a force applied to it is felt 90 degrees later in the direction of rotation. Any displacement of the propeller's plane of rotation produces it, in yaw as readily as in pitch; the classic case is a tailwheel airplane raising its tail on the takeoff roll.

You need not sort out which is acting. You need right rudder, and more of it as angle of attack rises and airspeed falls.

Where you meet it next

On the checkride this starts easy and does not stay easy. You will be asked to name the four forces, then for something harder: what happens to stall speed in a 60-degree bank, why Va falls as fuel burns off, how you would depart behind a heavy jet. In the airplane it is the right rudder on every climb, the power slow flight needs, and the back pressure a steep turn wants — along with the limit on it.

It reappears across the rest of the ground school: performance and limitations, where load factor and CG stop being ideas and become chart entries; flight controls and systems, where the three axes came from; weather information, where the density and the gusts in this lesson are forecast; and risk management, where an airplane asked for more than it had is the recurring accident.

• The rest of the ground school — performance and limitations, flight controls and systems, weather information, risk management and accident prevention
• Density altitude — the day the air gives you less to work with
• Flight Kneeboard — for the POH numbers you should be flying to rather than remembering