Updated October 2026
Four forces act on every airplane in flight: lift, weight, thrust and drag. In steady, unaccelerated flight they balance, so the upward forces equal the downward ones and the forward forces equal the rearward ones. A wing makes lift by turning the oncoming air downward, and Bernoulli's pressure difference and Newton's third law are two descriptions of that same event.
That paragraph is most of what the FAA knowledge test asks about aerodynamics, and almost none of what you need in the airplane. The sections below fill in the rest: how a wing works, where drag comes from, why the airplane is stable, and what changes in a turn. Each section points to a page that goes deeper, and the full list of spoke pages is further down.
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The Four Forces Of Flight
The FAA's Pilot's Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25C, chapter 5) defines them plainly. Thrust is the forward force from the engine and propeller. Drag is the rearward, retarding force from disturbed airflow. Lift acts perpendicular to the flight path. Weight is the combined load of the airplane, people, fuel and bags, pulling straight down through the center of gravity.
| Force | Direction | Opposed By | What Changes It In The Cockpit |
|---|---|---|---|
| Lift | Perpendicular to the flight path | Weight (in level flight) | Angle of attack, airspeed, flaps |
| Weight | Straight down, through the CG | Lift | Loading and fuel, decided on the ground |
| Thrust | Forward, along the propeller axis | Drag | Throttle |
| Drag | Rearward, along the flight path | Thrust | Airspeed, flaps, gear, angle of attack |
"Lift equals weight and thrust equals drag" is the version everyone memorizes, and the handbook calls it an incomplete simplification. What it actually says is that there can be no unbalanced forces in steady, straight flight. In a steady climb, part of the weight acts backward along the flight path, so thrust has to cover drag plus that share of weight. Your climb performance on a hot day lives in that margin, which is why the density altitude page belongs in this group.
How A Wing Makes Lift (Bernoulli, Newton And Why "Equal Transit" Is Wrong)
A wing at a positive angle of attack bends the airflow. Air speeds up over the curved upper surface and its pressure drops, which is Bernoulli's principle: as the velocity of a moving fluid increases, its pressure decreases. The PHAK adds the Newton side: the wing pushes air down behind it as downwash, and the equal and opposite reaction pushes the wing up. NASA's Glenn Research Center states it directly. Both explanations are correct, because they describe the same force, one by adding up pressures and the other by adding up the turning of the flow.
The explanation to unlearn is "equal transit." It claims air going over the longer top surface has to hurry to meet the air underneath at the trailing edge. NASA points out that nothing requires the two to meet. In fact the air over the top arrives at the trailing edge first, moving much faster than equal transit predicts. The theory also cannot explain a flat paper airplane, a symmetric airfoil or an airplane flying upside down, and all three fly.
The takeaway for a student pilot is short. Lift comes from how much the wing turns the air, and the main thing you control is the angle at which it meets the air. Our page on angle of incidence vs angle of attack takes that angle apart.
The Lift Equation In Plain English
Lift = CL × ½ × ρ × V² × S
CL is the coefficient of lift. It rises with angle of attack up to the critical angle, and again when you lower flaps. ρ (rho) is air density, which falls on hot days and at high fields. V is true airspeed. S is wing area, fixed unless the flaps extend backward as well as down.
Read it like a pilot. Velocity is squared, so doubling the airspeed gives four times the lift at the same angle of attack. That is why a slow airplane needs a high angle of attack, and why the only way to fly slower in level flight is to raise the nose. Density sits on the same line as speed, so thin air has to be made up with true airspeed. The airplane lifts off at the same indicated airspeed in August, but at a higher speed over the ground. And because weight has to be matched by lift, a heavier airplane needs a higher angle of attack at every speed, which is why stall speed rises with weight.
Induced Drag, Parasite Drag And The Drag Curve
Parasite drag is the drag that has nothing to do with making lift. The PHAK splits it three ways: form drag from the airplane's shape, interference drag where airflows meet at junctions such as the wing root, and skin friction from the air rubbing the surface. It grows with the square of airspeed. Go twice as fast and you have four times the parasite drag.
Induced drag is the cost of making lift. High-pressure air under the wing spills around the tips into the low pressure above, forming wingtip vortices and downwash that tilt the lift vector back. It is largest at high angles of attack and falls with the square of airspeed, so it dominates when you are slow.
Add the two and you get the total drag curve, shaped like a U. At one speed total drag is lowest and the wing gives the most lift for the least drag, L/DMAX. Best glide speed sits close to that point; in our Piper Warriors it is 73 knots. Slower than the bottom of the curve, induced drag climbs so fast that holding altitude takes more power the slower you go. That region is where slow flight lives, and the slow flight and stalls page shows how it feels from the seat.
Center Of Gravity, Center Of Pressure And Stability
Weight acts at the center of gravity. Lift acts at the center of pressure, which moves forward as angle of attack increases and aft as it decreases. Designers place the CG ahead of the center of lift, which makes the airplane nose-heavy, and balance it with a downward force on the horizontal tail. Much of that tail-down force comes from the wing's own downwash, and it grows with airspeed.
That arrangement is what makes a trainer stable in pitch. Slow down and the tail-down force weakens, so the nose drops and speed builds back. Speed up and the nose rises. Static stability is that first tendency to return toward trimmed flight. Dynamic stability is whether the oscillations that follow shrink, stay the same or grow. A well-loaded trainer damps them out in a few cycles.
Move the CG aft and every margin shrinks. The PHAK names an aft CG as the most critical loading for stall recovery, because the nose has less tendency to drop on its own. Weight and balance calculations exist for exactly that reason, and our guide to weight and balance for student pilots walks through them step by step.
Ground Effect, Wingtip Vortices And Load Factor
Ground effect starts to matter when the wing is within about one wingspan of the surface. The ground interferes with the wingtip vortices and the downwash, and induced drag falls. The PHAK's figures are about 1.4 percent less induced drag at a height of one span, 23.5 percent at a quarter span and 47.6 percent at a tenth. A low-wing trainer like the Warrior or the DA20 has its wing close to the runway in the flare, which is why both float down the runway when you arrive fast. The same effect is your friend on a soft-field takeoff and your enemy if you lift off below a speed the airplane can hold once it climbs out of it.
Those wingtip vortices are also wake turbulence. The handbook's rule is that they are strongest behind an airplane that is heavy, clean and slow, which describes a business jet on final to runway 16R. Our page on wake turbulence at Van Nuys covers how to stay above and behind them.
Load factor is the ratio of the load the wing carries to the airplane's weight. In a level turn it depends only on bank angle: 1.41 G at 45 degrees and 2.0 G at 60, per the Airplane Flying Handbook (FAA-H-8083-3C, chapter 10). Stall speed rises with the square root of load factor, so a 50-knot stall becomes about 60 knots at 45 degrees and about 70 at 60. The same chapter notes that a standard-category light airplane is built for up to 3.8 G, which a level 75-degree turn exceeds.
Mean Aerodynamic Chord, Aspect Ratio And Wing Loading
These three numbers describe the wing's shape and the load on it, and they explain a lot of what an airplane feels like.
Mean aerodynamic chord (MAC) is, in the PHAK glossary's words, the average distance from the leading edge to the trailing edge of the wing. On a tapered wing the formal calculation weights the wider root more heavily, but the idea is an average chord. It matters because larger airplanes often express the CG as a percentage of MAC, so "CG at 25 percent MAC" means a quarter of the way back along that average chord.
Aspect ratio is wingspan divided by average chord. Long, narrow wings have high aspect ratios, smaller tip vortices for their size and less induced drag. Wing loading is weight divided by wing area: the pounds each square foot has to hold up.
The Diamond DA20-C1 is a good worked example. Its published span is 35 ft 8 in and its wing area about 125 square feet. Divide area by span and the average chord is about 3.5 feet. Square the span and divide by the area and the aspect ratio comes out a little over 10, high for a trainer and a direct result of its motorglider ancestry. At its 1,764-pound maximum weight the wing loading is about 14 pounds per square foot. High aspect ratio is why it glides so far; light wing loading is part of why a light airplane feels every bump on a summer afternoon.
Where The Aerodynamics Shows Up In Our Fleet At Van Nuys
All 20 of our airplanes are owned by the school and maintained in our own shop. Each has an aerodynamic personality you learn by feel. Most of the lessons happen over the Simi Valley practice area, northwest of the field beyond the Santa Susana Mountains, where there is room to stall, slow down and turn without anyone sequencing you.
| Airplane | Tail Numbers | Wing | What It Teaches |
|---|---|---|---|
| Cessna 162 Skycatcher | N60225, N7030B, N6039M, N179LC, N5208V, N52049 | High wing, light airframe | Wing loading: delightful in smooth air, busy in afternoon turbulence; useful load decides every flight |
| Piper Warrior PA-28-161 | N3034Q, N6312C | Low wing, semi-tapered, visible dihedral | A well-warned, straight-ahead stall; ground effect float when fast |
| Diamond DA20-C1 | N306EF, N308EF | Low wing, high aspect ratio | Long glide, speed control on final, spins (utility category) |
| Cirrus SR22 Turbo | N422SN | Low wing with a leading-edge cuff | A stall that nods straight ahead because the inboard wing stalls first |
| Piper Seminole | N311EF | Low wing, two counter-rotating engines | Asymmetric thrust and engine-out control |
Two local conditions sharpen all of it. Summer heat raises density altitude at our 802-foot field, so the lift equation's ρ is working against you on an August afternoon. And mechanical turbulence over the Santa Susanas and the San Gabriels on a windy day is when maneuvering speed and load factor stop being written-test topics. For the procedure side, our Redbird MCX is the only full-motion flight simulator at Van Nuys Airport. Rehearse stall recoveries and the instrument scan there at $120 an hour inside a package before you fly them.
The Spoke Pages: Angle Of Attack, Adverse Yaw, Left-Turning Tendencies, Stalls, Maneuvering Speed
Each page below takes one idea from this hub and works it through with numbers from our airplanes.
| Page | What It Covers |
|---|---|
| Angle Of Incidence Vs Angle Of Attack | Chord line, relative wind, critical angle, dihedral and washout |
| Adverse Yaw Explained | Why the nose swings the wrong way, slips, skids, Dutch roll and load factor |
| Left-Turning Tendencies Explained | Torque, spiraling slipstream, P-factor, gyroscopic precession and right rudder |
| Pressure Altitude And Density Altitude | Formulas, a calculator and hot-day performance at Van Nuys |
| Maneuvering Speed Explained | VA, the square-root formula and why it falls with weight |
| Slow Flight And Stalls | Power-on and power-off stalls and the recovery order |
| Steep Turns And Ground Reference Maneuvers | 45-degree turns, overbanking and wind drift |
| Commercial Pilot Maneuvers | Chandelles, lazy eights and the 50-degree steep turn |
| Crosswind Landings And Forward Slips | Crosswind components and flying a deliberate slip |
| Short-Field And Soft-Field Landings | Ground effect on purpose, and performance on a tight runway |
| Airplane Controls | Ailerons, elevator, rudder, trim and flaps |
| Weight And Balance For Student Pilots | CG, moment and the loading envelope |
| Wake Turbulence At Van Nuys | Wingtip vortices behind business jets |
| Unusual Attitude Recovery | Nose-high and nose-low recoveries |
| Upset Recovery Training | Upset and spin training from Van Nuys |
| CFI Spin Endorsement | Spins in the DA20 under 14 CFR 61.183(i) |
| How Fast Do Planes Fly | Trainer and airliner speeds, knots and Mach |
| How High Do Planes Fly | Service ceilings and why airliners cruise high |
Related Reading
- Private pilot written exam, where 14 CFR 61.105(b)(10) puts aerodynamics on the test.
- Private pilot oral exam guide, how examiners turn these topics into scenarios.
- Private pilot maneuvers, every checkride maneuver in one place.
- The airplanes you'll fly, the whole fleet with specifications.
- Aviation glossary, 150 terms including the ones on this page.
Aerodynamics FAQ
What Are The Four Forces Of Flight?
Lift, weight, thrust and drag. Lift acts perpendicular to the flight path, weight acts straight down through the center of gravity, thrust pulls forward and drag holds the airplane back. In steady, unaccelerated flight there are no unbalanced forces.
Does Bernoulli's Principle Explain Lift?
It explains half of the description. Faster air over the top of the wing has lower pressure, and that pressure difference is real. Newton's third law describes the same lift as the reaction to the wing pushing air downward. NASA's Glenn Research Center calls both explanations correct.
What Is Induced Drag?
Induced drag is the drag created by making lift, mostly through wingtip vortices and downwash. It is greatest at high angles of attack and low airspeeds and falls with the square of airspeed, which is why it dominates in slow flight.
What Is Mean Aerodynamic Chord?
It is the average distance from the leading edge to the trailing edge of the wing. Weight and balance data for larger airplanes often states the center of gravity as a percentage of mean aerodynamic chord.
What Is Ground Effect?
Within about one wingspan of the ground, the surface interferes with the wingtip vortices and induced drag falls. The FAA handbook gives a 23.5 percent reduction at a quarter span and 47.6 percent at a tenth, which is why a fast low-wing trainer floats in the flare.
How Does Bank Angle Change Load Factor?
In a level turn, load factor depends only on bank: 1.41 G at 45 degrees and 2.0 G at 60 degrees. Stall speed rises with the square root of load factor, so a 50-knot stall speed becomes about 70 knots at 60 degrees of bank.
Learn Aerodynamics In The Airplane, Not Just The Book
The fastest way to make lift and drag feel real is to slow an airplane down and hear it change. A discovery flight is $229 for 60 minutes in a Piper Warrior out of Van Nuys, you fly it, and it is credited toward training if you enroll within 24 hours. Planning the whole private pilot certificate? The Standard package is $10,400 with $2,500 down, and a free consultation with an instructor will show you where every topic on this page fits.
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