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 memorise 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 centre 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 aeroplane is in equilibrium: lift balances weight, thrust balances drag, and no net turning moment acts about the centre of gravity. That last clause is the one students skip — equilibrium means every force and every moment sums to zero.
- Weight — vertically downwards through the centre of gravity, in any attitude.
- Lift — perpendicular to the relative airflow, through the centre of pressure. Not "upwards": 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 aeroplane the centre of gravity sits deliberately ahead of the centre 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 aeroplane left to itself pitches nose-down, towards 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 downwards 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 centre of pressure, which on a cambered aerofoil 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 aeroplane. 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 towards 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
| CL | Coefficient of lift — how hard the aerofoil is working. Set by aerofoil shape, flap setting and, above all, angle of attack. |
| ρ (rho) | Air density. Falls with increasing altitude, temperature and humidity — the three ingredients of density altitude. |
| V | True airspeed, squared. Double the TAS at constant angle of attack and you quadruple the lift. |
| S | Wing area. Fixed, except where Fowler-type flaps extend rearwards and genuinely increase it. |
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 manoeuvring 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 aeroplane 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 centre of pressure moves rapidly rearward — the characteristic nose-down pitch. For light-aeroplane aerofoils that angle is typically in the region of 15° to 16°.
An aeroplane stalls at a critical angle of attack, not at a speed. For a given configuration that angle does not change with weight, load factor, bank, altitude, attitude or airspeed. Every "stall speed" is simply the speed at which, in one set of conditions, you happen to reach it. Change the conditions and the speed changes; the angle does not — which is why you can stall at any airspeed and in any attitude, and why the one recovery action that addresses the cause is to reduce the angle of attack.
What actually changes the stall speed
| Increased weight | Increases, with its square root. A 21% weight increase raises stall speed by 10%. |
| Increased load factor | Increases, with its square root — the same rule, since load factor is apparent weight. |
| Bank (level turn) | Increases. Load factor in a balanced level turn is 1/cos of the bank angle. |
| Flap lowered | Decreases. Trailing-edge flap raises camber and CLmax, while typically reducing the stalling angle. |
| CG moved forward | Increases. A larger tail download means the wing must carry more. |
| Power on | Decreases slightly: slipstream energises the inboard wing, and thrust partly supports the aeroplane. |
| Ice, frost, contamination | Increases, sometimes dramatically. Roughness destroys CLmax, with no reliable figure — hence frost is removed, not flown with. |
| Increased altitude | No change in IAS — TAS at the stall rises, indicated stall speed does not. |
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:
| Bank | Load factor | Stall speed |
|---|---|---|
| 15° | 1.04 | × 1.02 |
| 30° | 1.15 | × 1.07 |
| 45° | 1.41 | × 1.19 |
| 60° | 2.00 | × 1.41 |
So an aeroplane 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 aeroplane's own aerodynamic qualities — pre-stall buffet — or by a device such as a warning vane or a reed horn, so a light aeroplane 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.
Bank angle, load factor and stall speed is the calculation the paper keeps coming back to. Drill it until the numbers are automatic.
Explore the v1prep question banksFrom 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 aeroplane, 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.
| Stage | What it looks like |
|---|---|
| Incipient | The first turn or so, while roll, yaw and pitch rates are still settling. Recovery is prompt and costs comparatively little height. |
| Fully developed | The rates have stabilised: steady rotation, a low and steady airspeed, a high rate of descent, and a nose-down attitude that is usually less steep than it feels from inside. |
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 aeroplane departs towards the inside of the turn with no height in which to recover.
Fly the recovery your flight manual publishes — it is type-specific, and it is the one that was flight tested. The classic light-aeroplane sequence is: close the throttle, ailerons neutral, full rudder opposite to the direction of rotation (read the direction from the turn indicator, not from the world outside), control column progressively forward until the rotation stops, centralise the rudder, then ease out of the dive without a secondary stall and without over-stressing the airframe. Every step has a job: closing the throttle removes slipstream and gyroscopic effects, neutral ailerons avoid deepening the stall on the down-going wing, rudder kills the yaw that is driving the rotation, and moving the column forward is the only action that unstalls the wing.
Two things make a spin far worse, and both return later on this page: a centre 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 rearwards; 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: within about one wingspan of the surface 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 idealised 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 aeroplane, 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 aeroplane; a jet endures longest at VMD, because a jet burns fuel roughly in proportion to thrust rather than to power.
Best glide speed varies with weight; best glide angle does not. Lift/drag ratio is an airframe property, so a lighter aeroplane glides exactly as far from a given height — it simply achieves that angle at a lower speed. Best glide speed changes with the square root of the weight ratio, so at 80% of maximum weight the right speed is about 10% below the published figure. In wind, fly slightly faster into a headwind for best range over the ground, slightly slower with a tailwind.
Glide performance stops being theory the moment the engine stops, and the commonest reason a serviceable engine stops is carburettor 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 aeroplane reduces drag, so it accelerates back towards 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 aeroplane 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-aeroplane climb angles the lift shortfall is tiny, but the principle is the point: an aeroplane 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.
| Vx — best angle of climb | Most height per unit of horizontal distance. Set by maximum excess thrust, since climb angle is (thrust minus drag) divided by weight. Used to clear an obstacle. |
| Vy — best rate of climb | Most height per unit of time. Set by maximum excess power, since rate of climb is excess power divided by weight. Used to reach cruise altitude soonest. |
For a typical light piston aeroplane 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.
Nose attitude is not climb performance. Below Vx you are climbing at a worse angle, not a better one — and you are nearer the stall, at high power, at low level, where a wing drop has nowhere to go. If an obstacle looks close, fly the published Vx precisely: every degree above the best-angle attitude buys less height and less margin at once.
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 aeroplane'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 aeroplane'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 aeroplane wants right rudder as the power goes on. On types whose propellers turn the other way, every effect reverses with them.
| Effect | Mechanism | Worst when |
|---|---|---|
| Torque reaction | Newton's third law: the engine turns the propeller one way, so the airframe is rolled the other. Felt as a roll to the left, taken by the wheels on the ground and by aileron in the air. | High power, low airspeed |
| Slipstream effect | The slipstream leaves the propeller as a corkscrew and strikes one side of the fin, yawing the nose left. Designers offset the fin or the engine to cancel it at cruise power, not at full power. | High power, low airspeed |
| Gyroscopic effect | A force applied to a spinning disc takes effect 90° further round in the direction of rotation. Raise the tail on a tailwheel take-off and the nose-down input precesses into a yaw to the left. | Pitch or yaw changes at high RPM |
| Asymmetric blade effect | Also called P-factor. With the propeller axis inclined to the airflow, the down-going blade meets the air at a greater angle of attack than the up-going blade and produces more thrust, so the effective thrust line moves off centre and yaws the nose left. | High angle of attack: climb, go-around |
Notice that three of the four are worst in the same place — high power, low speed, nose high. That is the climb after take-off 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:
| Category | Positive limit | Negative limit |
|---|---|---|
| Normal | +3.8g | −1.52g |
| Utility | +4.4g | −1.76g |
| Aerobatic | +6.0g | −3.0g |
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 aeroplane gets lighter
VA, the design manoeuvring speed, is where the aerodynamic and structural boundaries meet: the speed at which full and abrupt deflection of a single control takes the aeroplane 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.
VA decreases as weight decreases. This is the one students answer backwards, because heavier usually means slower. The reasoning: 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 aeroplane reaches CLmax at a lower speed, so the whole balance point moves down. The same gust also accelerates a lighter aeroplane more, which is the identical physics arriving from outside. Use the figure scheduled for your actual mass in the flight manual.
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 manoeuvring or rough-air speed and accept the ride.
Static and dynamic stability in three axes
Stability is what the aeroplane 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 aeroplane returns towards 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 favourite question, so state it once and sharply: static stability is necessary but not sufficient. An aeroplane cannot be dynamically stable without being statically stable, but plenty of statically stable aeroplanes are dynamically neutral or even unstable.
| Motion | About which axis | Called | Provided mainly by |
|---|---|---|---|
| Pitch | Lateral | Longitudinal stability | Tailplane area and moment arm; CG position |
| Yaw | Normal (vertical) | Directional stability | Fin area and keel surface behind the CG |
| Roll | Longitudinal | Lateral stability | Dihedral, high wing, sweepback, keel surface above the CG |
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 aerofoil at zero angle of attack in balanced flight. Yaw the aeroplane 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 aeroplane starts to sideslip towards 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 aeroplane 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 aeroplane back towards 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 centre 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 aeroplanes.
Roll and yaw are coupled, and the balance produces two named misbehaviours. Spiral instability — strong directional with weak lateral — lets a small bank tighten slowly into a descending turn; it is common in light aeroplanes. 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 manoeuvrability, 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 centre 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.
| CG towards the forward limit | CG towards the aft limit | |
|---|---|---|
| Longitudinal stability | Greater — larger static margin | Reduced — smaller static margin |
| Stick forces | Heavier; sluggish in pitch | Lighter; responsive, even twitchy |
| Tail download | Greater | Smaller |
| Stall speed | Higher | Lower |
| Trim drag | More — slightly lower cruise speed and range | Less — marginally better cruise |
| Take-off and landing | Longer run, harder to rotate | Shorter run, easier rotation |
| Stall and spin recovery | Better — strong nose-down tendency | Worse — nose-down tendency weak or absent |
Carry the reasoning behind the stall-speed row into the exam intact: CG forward means a longer arm to the centre of pressure, so a larger nose-down moment, so a larger tailplane download, so the wing must produce more lift for the same aeroplane weight — and it reaches CLmax at a higher airspeed. That extra lift is also extra induced drag, hence a nose-heavy aeroplane's sluggish cruise and climb.
Forward CG or aft CG, stick forces, stall speed and recovery: the paper tests the reasoning, not the recall.
Explore the v1prep question banksThe aft CG limit is a recovery limit. Beyond it, longitudinal stability may be neutral or negative, and the natural nose-down pitch at the stall — the thing that makes a normal recovery straightforward — weakens or disappears. Recovery from a fully developed spin may become impossible — an aft CG takes away the very nose-down pitch that the recovery described above depends on — and the very light stick forces make it easy to run past the limit load factors without noticing. A tail-heavy aeroplane feels pleasantly light and quick right up to the point where it will not do what you need.
The forward limit, by contrast, is an authority limit: balancing a nose-heavy aeroplane needs increasing up-elevator, and beyond it there may not be enough left to rotate on take-off — 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 aeroplane. Check the landing condition as well as the take-off 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.
Practise the way you'll be examined
v1prep's PPL Principles of Flight bank follows the EASA learning objectives question by question, with a worked explanation for every answer — including the ones you got right for the wrong reason.
Start the PPL question bank →