PPL Principles of Flight: The Complete Guide

Principles of Flight is the subject that explains all the others. Get it right and mass and balance, performance and half of your handling training stop being lists to memorize and become consequences of four forces and one equation. This guide works through the core of the EASA PPL(A) Principles of Flight syllabus — the parts everything else depends on — in the order that makes sense: the forces and their couples, lift, the stall and the spin, drag, the climb, propeller effects, the flight envelope, stability, and what the center of gravity does to all of it. Where a figure matters it is here; where a relationship matters more, we have said why.

The four forces, and why they don't act through one point

In steady, straight, level flight at constant speed the airplane is in equilibrium: lift balances weight, thrust balances drag, and no net turning moment acts about the center of gravity. That last clause is the one students skip — equilibrium means every force and every moment sums to zero.

• Weight — vertically downward through the center of gravity, in any attitude.
• Lift — perpendicular to the relative airflow, through the center of pressure. Not "upward": lift is only vertical with the wings level and the flight path horizontal.
• Thrust — approximately along the propeller shaft axis.
• Drag — parallel to the relative airflow, opposing the flight path.

The lift-weight couple and the tailplane download

On the conventional light airplane the center of gravity sits deliberately ahead of the center of pressure, creating a nose-down pitching moment — the lift-weight couple. It is designed in as a trim arrangement and a safety feature: an airplane left to itself pitches nose-down, toward a lower attitude and a flying speed, and it does the same if the tail download is lost. Balancing it is the tailplane download, a small force acting downward at a long moment arm behind the CG.

Do not carry that into a stability question, though. Stability is the production of a restoring moment when the angle of attack changes, and it depends on the CG lying ahead of the neutral point — not ahead of the center of pressure, which on a cambered airfoil migrates with angle of attack and is exactly why a wing on its own is unstable. The stability section below does that job properly.

Hence the sentence examined constantly: the wing must produce more lift than the weight of the airplane. Lift equals weight plus tail download — a surplus that costs trim drag and raises the stall speed, both penalties growing as the CG moves forward.

The thrust-drag couple

Thrust and drag do not act along one line either. In the common arrangement the thrust line lies below the drag line, so power gives a nose-up couple opposing the nose-down one; lose the engine and that contribution disappears, so the nose pitches down toward a safer speed. Geometry varies between designs, the principle does not — a power change is also a pitch change.

Principles of Flight interlocks with several of the other papers — see our guide to all nine EASA PPL theory subjects.

Lift and the lift formula

Almost every lift question at PPL level collapses into one equation:

Lift = CL × ½ρV² × S

The pilot changes CL in an instant by moving the control column and with it the angle of attack, V over seconds with attitude and power, S in steps with the flap lever, and ρ only by choosing a different altitude — never at the height they are already at. Slow flight is that hierarchy and nothing more: lift must stay equal to weight plus tail download, so if V falls, CL must rise — the angle of attack must increase. That one trade-off explains the circuit, the approach and the stall.

Note also that ½ρV² is dynamic pressure, which is what a pitot-static airspeed indicator actually measures: not how fast you are going, but how hard the air is hitting the pitot tube. A given IAS therefore means a given angle of attack at a given weight and configuration, at any altitude — strictly that is true of CAS, since IAS also carries instrument and position error, small in the cruise but growing at high angles of attack. It is why stall speeds, flap limiting speeds and maneuvering speed are published as indicated airspeeds and stay valid as you climb. Climb speeds are the exception, and for a reason worth understanding: Vx and Vy are set by excess thrust and excess power rather than by a fixed angle of attack, so they change with altitude even in indicated terms — see the climb section below. The same logic scales up to the airliner: see A320 V-speeds explained.

Angle of attack and the stall

Angle of attack is the angle between the chord line and the relative airflow — not pitch attitude, not angle of incidence, and with no fixed relationship to either. An airplane 45° nose-down can be at a very high angle of attack.

As it increases, CL rises roughly linearly and boundary-layer separation creeps forward from the trailing edge. At the critical angle of attack separation becomes general across the upper surface: CL falls away sharply, drag rises steeply, and the center of pressure moves rapidly rearward — the characteristic nose-down pitch. For light-airplane airfoils that angle is typically in the region of 15° to 16°.

What actually changes the stall speed

Two of those rows are Meteorology questions in disguise: density altitude and airframe contamination both come back at you in the met paper — see our top 25 PPL Meteorology questions.

Work the bank case through — it is the one that kills people. In a balanced level turn:

So an airplane stalling at 50 kt wings-level stalls at about 71 kt in a level 60° turn. Note how gently the penalty builds to 30° and how sharply beyond 45° — the geometry of an over-banked final turn with the speed already low.

Warning comes as buffet, sloppy controls and, at 1g, a high nose attitude, plus the clear and distinctive stall warning that certification requires. Note what certification actually demands: CS-23 allows that warning to be furnished either by the airplane's own aerodynamic qualities — pre-stall buffet — or by a device such as a warning vane or a reed horn, so a light airplane with no artificial warning system can be perfectly legally certificated. Washout, the wing twisted so the root sits at a greater angle of incidence than the tip, makes the root stall first, so the ailerons stay in attached airflow and keep working into the stall.

That margin at the tip does not make aileron a safe recovery input. A down-going aileron increases the local angle of attack, so picking up a dropped wing with aileron can take the part of the wing already closest to the stall straight past it. Prevent further yaw with rudder, unload to reduce the angle of attack, and level the wings once the wing is flying again. Preventing the yaw is not a detail: yaw at the stall is how a stall becomes a spin.

From the stall to the spin: autorotation and recovery

A spin is not a separate phenomenon to be learned separately. It is a stall with yaw, sustained by the wings themselves — which is why every instructor spends the stall exercise preventing yaw.

Autorotation is the mechanism. Yaw at the point of stall rolls the airplane, and once it is rolling the two wings see different angles of attack. The down-going wing meets the airflow at a greater angle: already beyond the critical angle, it loses more lift again and gains more drag. The up-going wing meets it at a smaller angle, so it is less deeply stalled, keeps more lift and has less drag. The lift difference sustains the roll and the drag difference sustains the yaw, so the motion feeds itself. Three ingredients are needed together: the wing stalled, one wing more stalled than the other, and yaw.

The classic entry is not a training exercise. It is the skidding turn from base to final: bottom rudder to tighten the turn, back pressure to stop the nose dropping, speed already low. Stall, yaw, and the airplane departs toward the inside of the turn with no height in which to recover.

Two things make a spin far worse, and both return later on this page: a center of gravity at or behind the aft limit, which weakens or removes the nose-down pitch the recovery depends on, and any loading outside the certificated envelope, where the published recovery was never tested at all.

Drag: induced drag, parasite drag and the total drag curve

Drag comes in two families that behave in opposite ways with speed. Parasite drag (also zero-lift drag) is the cost of pushing a shape through the air — profile drag, meaning form drag plus skin friction, together with interference drag and anything hanging in the airflow. It has nothing to do with producing lift and is proportional to the square of the speed.

Induced drag is the price of making lift. The pressure difference across the wing drives a spanwise flow that rolls up into trailing wingtip vortices, and the resulting downwash tilts the effective lift vector rearward; that rearward component is induced drag. Depending as it does on CL squared, at a given weight in level flight it is inversely proportional to the square of the speed — fly slowly and it dominates. It is worsened by weight, load factor and low speed, and reduced by high aspect ratio, wingtip devices, and ground effect: it becomes noticeable within about one wingspan of the surface and pronounced below about half a span, where the ground interrupts the downwash and induced drag falls away sharply.

The total drag curve and VMD

Add the two and total drag is a bucket: high at low speed from induced drag, high at high speed from parasite drag, with a minimum between. That minimum is VMD, the minimum drag speed, sitting on the idealized polar where induced drag equals parasite drag. Since level-flight lift is fixed by weight, minimum drag also means maximum lift/drag ratio — so VMD is at once the speed for the best glide angle, roughly the speed for maximum still-air range in a propeller airplane, and the dividing line between normal and reversed command. VMP, the minimum power speed, lies below it and gives maximum endurance — again in a propeller airplane; a jet endures longest at VMD, because a jet burns fuel roughly in proportion to thrust rather than to power.

Glide performance stops being theory the moment the engine stops, and the commonest reason a serviceable engine stops is carburetor icing.

The region of reversed command

Above VMD everything behaves as intuition expects: fly faster and the thrust needed to hold height increases. Below VMD it inverts — flying slower requires more thrust, because induced drag rises faster than parasite drag falls. That back of the drag curve is the region of reversed command. Keep the two currencies apart, because the exam does: thrust required is least at VMD, but power required is drag multiplied by speed, so it bottoms out lower still at VMP. Between VMP and VMD, slowing down still reduces the power needed even though drag is already rising; only below VMP does flying more slowly demand more power as well.

The problem down there is speed instability. Above VMD a disturbance that slows the airplane reduces drag, so it accelerates back toward trim and the speed looks after itself. Below VMD a disturbance that slows it increases drag, which slows it further; nothing arrests that but the pilot. It is the aerodynamics of the dragged-in, low, flat approach, and why recovery from low-and-slow is power first, attitude second.

Climb performance: Vx, Vy and why they converge

Start from what holds the airplane up in a climb, because the exam does. In a steady climb at constant speed on a climb angle gamma:

• Lift = Weight × cos(gamma) — lift is less than weight in a climb, not more.
• Thrust = Drag + (Weight × sin(gamma)) — thrust must beat drag and haul the weight component up the slope.

At light-airplane climb angles the lift shortfall is tiny, but the principle is the point: an airplane climbs because of excess thrust, not excess lift. Pulling back trades speed for height — a zoom, not a climb. It buys a few hundred feet once and then stops; sustained climb performance is bought with power.

For a typical light piston airplane Vx is lower than Vy. Take both from the flight manual rather than a rule of thumb — they are type-specific, and several manuals schedule them with weight.

Why Vx and Vy converge with altitude

Climb, and a normally aspirated engine loses power as the air thins while what the airframe demands at a given IAS stays much the same. Both excess thrust and excess power shrink, and as they do, Vx increases and Vy decreases in indicated airspeed terms. They meet at the absolute ceiling, where the rate of climb is zero and exactly one speed will still maintain height. Because the last few hundred feet take so long, the more useful figure is the service ceiling, conventionally the altitude at which a light piston airplane's best rate of climb has decayed to 100 ft/min — other categories use different conventions, so quote the one your manual uses.

Weight, flap and density altitude all degrade both angle and rate. Wind is the exception that gets examined: it does not change your rate of climb or your climb angle through the air, only the flight path over the ground — so a headwind genuinely helps you clear an obstacle while the airplane's own performance is unchanged. Density altitude, meanwhile, is where Principles of Flight meets the met and altimetry papers: cold, dense air gives you a better climb even as it makes your altimeter over-read — see cold-weather altimetry.

Propeller effects: torque, slipstream, gyroscopic and asymmetric blade

Four separate effects come from bolting a large rotating disc to the front, and the paper expects you to name and distinguish them. Take the common case: a propeller turning clockwise as seen from the cockpit, which covers most Lycoming- and Continental-engined singles. Every one of the four then produces a yaw or a roll to the left — which is why the airplane wants right rudder as the power goes on. On types whose propellers turn the other way, every effect reverses with them.

Notice that three of the four are worst in the same place — high power, low speed, nose high. That is the climb after takeoff and the go-around, which is where the rudder work is real, and where an unbalanced, yawing climb is the first link in more than one stall-spin accident.

The flight envelope: EASA load factor limits, VA and VNE

Load factor is lift divided by weight, quoted in g. It is not only something turbulence does to you: every time you pull, turn or recover from a dive, you choose one. The flight envelope, drawn as the V-n diagram, plots load factor against airspeed and fences in the combinations the airframe is certificated for. Its three boundaries have three different reasons.

• The left-hand boundary is aerodynamic. It is the stall: at a given speed the wing cannot generate more than CLmax, so it cannot generate more than a certain load factor. Since stall speed rises with the square root of load factor, that boundary is a curve rather than a line.
• The top and bottom boundaries are structural. They are the positive and negative limit load factors, the most the structure must carry without permanent deformation. Beyond them lies the ultimate load factor, 1.5 times the limit, which the structure must survive briefly without failing — a certification figure, never a working one.
• The right-hand boundary is VNE, never exceed, the red line on the ASI. Below it sits VNO, the top of the green arc and the maximum structural cruising speed; the yellow arc between the two is for smooth air only.

CS-23 sets the limit load factors by category, and your flight manual states which category you are in — some light types are certificated in more than one, with different loading for each:

With flap extended the permitted load factor is very much lower, which is one of the reasons VFE exists and why flap comes up before the speed does, not after.

VA, and why it falls as the airplane gets lighter

VA, the design maneuvering speed, is where the aerodynamic and structural boundaries meet: the speed at which full and abrupt deflection of a single control takes the airplane to its limit load factor and no further. Below VA the wing stalls before the structure is overloaded, and the stall is the safety valve. Above VA it is not: full deflection can break something before the wing gives up. Even below VA, rapid full-deflection reversals are outside what the figure covers.

Gusts belong to the same picture: a vertical gust changes the angle of attack abruptly, and the faster you are flying, the more load factor that change produces. Hence the standard airmanship, which is now a structural argument rather than a comfort one — in turbulence, slow to the maneuvering or rough-air speed and accept the ride.

Static and dynamic stability in three axes

Stability is what the airplane does about a disturbance without the pilot, and two different questions are being asked. Static stability is the initial tendency after a disturbance, before any time passes: positive, it starts to return; neutral, it stays where the disturbance left it; negative, it diverges further.

Dynamic stability is what the motion does with time. A statically stable airplane returns toward trim, overshoots, comes back, overshoots less. If the oscillations decay it is dynamically stable; if they hold their size, neutral; if they grow, unstable. The relationship is a favorite question, so state it once and sharply: static stability is necessary but not sufficient. An airplane cannot be dynamically stable without being statically stable, but plenty of statically stable airplanes are dynamically neutral or even unstable.

The naming catches everyone out once: stability in roll is called lateral but happens about the longitudinal axis, and stability in pitch is called longitudinal but happens about the lateral axis.

Longitudinal. A pitch disturbance changes the angle of attack the tailplane sees, and the tailplane generates a restoring force at a long arm behind the CG — increased by a larger tailplane, a longer arm and a more forward CG. The CG position at which that moment vanishes is the neutral point; the distance from the actual CG to it is the static margin, a direct measure of pitch stability.

Directional. The fin is a symmetrical airfoil at zero angle of attack in balanced flight. Yaw the airplane and it meets the airflow at an angle, generating a side force behind the CG that weathercocks the nose back into the airflow. Any keel surface behind the CG helps; anything ahead of it — a long nose, a deep forward fuselage — works against it.

Lateral. Roll is different, and the difference is examinable: a pure roll disturbance produces no restoring moment by itself, and one appears only once the airplane starts to sideslip toward the lower wing. Dihedral is the primary means — the sideslip meets the lower wing at a greater effective angle of attack and the upper wing at a smaller one, so the lower wing lifts more and rolls the airplane level. A high wing adds to it, and for a reason worth getting right: in a sideslip the flow around the fuselage produces upwash over the lower wing and downwash over the upper, rolling the airplane back toward level — an effect often quoted as worth several degrees of effective dihedral. It is not a pendulum effect, and that is the trap the question bank sets: the center of gravity hanging below the wing does not by itself produce a restoring rolling moment. Keel surface above the CG does help, but it is a separate contribution. Sweepback does the same job on faster airplanes.

Roll and yaw are coupled, and the balance produces two named misbehaviors. Spiral instability — strong directional with weak lateral — lets a small bank tighten slowly into a descending turn; it is common in light airplanes. Dutch roll — strong lateral with weak directional — is a poorly damped oscillation of roll and yaw together. Designers deliberately accept some spiral instability instead, because a spiral develops slowly enough to correct. Stability always trades against maneuverability, and fly-by-wire transports take that to its conclusion, flying with relaxed natural stability and letting the computers supply it: see A320 flight control laws explained.

What center of gravity position does to stability, stick forces and the stall

The CG envelope is not an administrative box. Both limits exist for aerodynamic reasons, and knowing which reason belongs to which answers most of the CG questions in the paper.

Carry the reasoning behind the stall-speed row into the exam intact: CG forward means a longer arm to the center of pressure, so a larger nose-down moment, so a larger tailplane download, so the wing must produce more lift for the same airplane weight — and it reaches CLmax at a higher airspeed. That extra lift is also extra induced drag, hence a nose-heavy airplane's sluggish cruise and climb.

The forward limit, by contrast, is an authority limit: balancing a nose-heavy airplane needs increasing up-elevator, and beyond it there may not be enough left to rotate on takeoff — or, the more common trap, to hold the nose up in the round-out with full flap.

Finally, the CG travels in flight. Burning fuel shifts it, in a direction that depends on where the tanks sit relative to the CG, and baggage is often the longest lever arm on a light airplane. Check the landing condition as well as the takeoff condition — a loading inside the envelope at brakes-off can finish outside it. The numerical work belongs to Flight Performance and Planning; the reasoning is Principles of Flight, and it returns with more depth at CPL theory and Instrument Rating theory level.

Common questions

What are the four forces of flight?

Lift, weight, thrust and drag, in equilibrium in steady level flight — and the moments about the center of gravity sum to zero as well. Weight acts vertically downward through the CG, lift perpendicular to the relative airflow, thrust roughly along the propeller axis and drag parallel to the airflow. Because the tailplane carries a download, the wing lifts more than the airplane weighs.

What is the lift formula?

Lift = CL × ½ρV² × S. CL is the coefficient of lift, set mainly by angle of attack; ρ is air density; V is true airspeed, squared; S is wing area. The practical consequence is the whole of slow flight: if speed falls, the angle of attack must increase to keep lift equal to weight.

At what angle of attack does an airplane stall?

At the critical angle of attack, typically in the region of 15 to 16 degrees for light-airplane airfoils. The angle matters far more than any speed: for a given configuration it is fixed, and does not change with weight, load factor, bank, altitude or airspeed. That is why an airplane can be stalled at any airspeed and in any attitude.

Why does stall speed increase with bank angle?

Because bank increases load factor, and stall speed rises with the square root of load factor. In a balanced level turn the load factor is 1 divided by the cosine of the bank angle: 1.41g at 45 degrees, 2g at 60. So an airplane that stalls at 65 kt wings-level stalls at about 77 kt in a level 45-degree turn.

Does stall speed change with altitude?

The indicated stall speed does not change appreciably; the true airspeed at which you stall increases. The airspeed indicator measures dynamic pressure, and a given dynamic pressure at a given weight corresponds to a given angle of attack whatever the air density. That is why stall speeds and most other V-speeds are published as indicated airspeeds.

What is the difference between Vx and Vy?

Vx is the best angle of climb speed — most height per unit of horizontal distance — set by maximum excess thrust. Vy is the best rate of climb speed — most height per unit of time — set by maximum excess power. On a typical light piston airplane Vx is the lower of the two, and with altitude Vx increases and Vy decreases until they meet at the absolute ceiling.

What is induced drag and how do you reduce it?

Induced drag is the by-product of producing lift: the pressure difference across the wing drives a spanwise flow that forms trailing wingtip vortices, and the resulting downwash tilts the effective lift vector rearward. In level flight it is inversely proportional to the square of the airspeed, so it dominates at low speed. Reduce it by flying faster, flying lighter, and with high aspect ratio, wingtip devices and ground effect.

What is the region of reversed command?

You meet it in the airplane before you meet it on a graph: nose high, speed low, still sinking, and pulling back only deepens the sink. It is the back of the drag curve — below VMD, where flying more slowly needs more thrust rather than less, and below VMP more power as well. The hazard is speed instability: slowing increases drag, which slows you further. Recovery is power first, attitude second.

What is the difference between static and dynamic stability?

Static stability is the initial tendency after a disturbance: positive means it begins to return, neutral means it stays where it was put, negative means it diverges. Dynamic stability is what the resulting motion does over time — oscillations that decay, hold their size, or grow. Static stability is necessary but not sufficient: an airplane cannot be dynamically stable without it.

Why does maneuvering speed VA decrease as weight decreases?

Because VA is the speed at which the wing reaches CLmax exactly at the limit load factor, so it scales with stall speed — with the square root of the weight ratio. A lighter airplane reaches CLmax at a lower speed, so its VA is lower. Below VA the wing stalls before the structure is overloaded; above it, full control deflection can overload the structure first.

How does center of gravity position affect stability?

A forward CG increases longitudinal stability: heavier stick forces, a higher stall speed, more trim drag, better stall recovery. An aft CG reduces it: lighter stick forces, a lower stall speed and marginally better cruise, but degraded stall and spin recovery. The aft limit is a recovery limit; the forward limit is an elevator authority limit — enough to rotate and to flare.