Week 4: Four Forces and Equilibrium, How to Get a Pilot's Licence, Turning Flight

The four forces

Every aeroplane in flight is pulled and pushed by four forces:

• Lift holds it up: made by air flowing over the wings
• Weight pulls it down: gravity acting on the aeroplane's mass, through the centre of gravity (CG)
• Thrust drives it forward: from the engine and propeller
• Drag holds it back: the air resisting the aeroplane's motion

Notice the vertical arrows are drawn much longer than the horizontal ones. That's deliberate (see "How big are they?"). Source: Pooleys Vol 4 (10th ed.) p.3–4.

Balanced pairs = equilibrium

In steady straight and level flight, lift balances weight and thrust balances drag. With both pairs balanced the total (resultant) force is zero, so the aeroplane neither speeds up nor slows down: it is in equilibrium, and keeps the same speed and direction until the pilot or a gust changes something. That is Newton's First Law of Motion working in front of you.

Source: Pooleys Vol 4 (10th ed.) p.4.

How big are they?

The pairs balance, but the pairs are not the same size as each other. In the cruise, a training aeroplane's lift (and weight) is roughly ten times its drag (and thrust), a lift/drag (L/D) ratio of about 10:1. The engine only has to match the drag, not carry the whole aeroplane.

STRETCHIf thrust only needs to match drag, and drag is a tenth of the weight, why can a glider stay up with no engine at all?

Source: Pooleys Vol 4 (10th ed.) p.4.

Equilibrium: equal forces, different places

In straight and level flight the opposing forces are equal: lift equals weight and thrust equals drag. With no resultant force the aeroplane carries on in steady, unaccelerated flight. Lift and weight are much the larger pair.

The four forces do not act through one point, though:

• Weight acts straight down through the centre of gravity (CG), which moves as fuel burns off or as passengers and cargo move
• Lift acts through the centre of pressure (CP), which moves with every change of angle of attack
• Thrust acts along the propeller shaft
• Drag acts parallel to the relative airflow, through a point that shifts with angle of attack, airspeed, flap and landing gear

The turning moment of any force on the aeroplane depends on the size of the force and its distance from the CG.

Source: Pooleys Vol 4 (10th ed.) p.65–66 and p.69.

Couples and pitching moments

A couple is a pair of equal, parallel forces acting in opposite directions along different lines of action. The pair produces no resultant force, but it does produce a turning moment, whose size depends on the magnitude of the two forces and the distance between their lines of action.

• Lift-weight couple: CP behind the CG gives a nose-down pitching moment. CP in front of the CG gives nose-up.
• Thrust-drag couple: drag line above the thrust line gives a nose-up pitching moment. Drag line below the thrust line gives nose-down.

For the two moments to match, the big forces (lift and weight) need their lines of action fairly close together, while the much smaller thrust and drag need rather more distance between theirs.

STRETCHLift and weight are far larger than thrust and drag. How can two couples built from such different forces produce the same turning moment?

Source: Pooleys Vol 4 (10th ed.) p.66–68.

The tailplane: the final balancing force

A perfect balance between the four forces and their two couples is rarely possible, so there is usually a residual pitching moment. The tailplane (horizontal stabiliser) counteracts it. It is simply another aerofoil, usually of symmetrical section, that can produce an upward or downward force depending on its angle of attack to the local airflow.

The residual moment is usually nose-down, so the tailplane normally produces a downward force, giving a nose-up moment. Because its moment arm from the CG is long, a small force is enough, which is why the tailplane is small compared with the mainplanes. The pilot varies the force with the elevators (held by hand or by trim) or, on some types, by moving the whole tailplane.

Aeroplanes are designed so the forces and couples are in approximate equilibrium in the cruise, leaving the tailplane only a small balancing job. How loading moves the CG is covered in POF-1.2.

Source: Pooleys Vol 4 (10th ed.) p.68–69.

Roll control: the ailerons

The ailerons are the primary control in roll. They sit on the outboard trailing edge of each wing and always move in opposite directions: one goes up and reduces the lift of its wing, the other goes down and increases the lift of its wing. The pilot moves them with sideways control column or by rotating the control wheel.

The unequal lift creates a rolling moment. Its size depends on the lift difference and on the moment arm to the CG, which is why the ailerons are placed outboard. The rule: the down-going aileron is on the up-going wing, and the up-going aileron is on the down-going wing.

Source: Pooleys Vol 4 (10th ed.) p.91–92.

Adverse aileron yaw

A down-going aileron effectively increases its wing's camber and angle of attack, so that wing makes more lift and more drag. The up-going aileron does the reverse: less lift and less drag. The lift difference rolls the aeroplane, but the drag difference (differential aileron drag) holds the rising wing back and yaws the nose away from the direction of roll. This is adverse aileron yaw, also called aileron drag. It is hard to spot unless you look for it, and it is mainly a low airspeed problem, most obvious in a slow turn.

Source: Pooleys Vol 4 (10th ed.) p.92.

Design fixes, and the pilot's fix

Good design has largely removed aileron drag from modern trainers, using one or more of three ideas:

• Differential ailerons: the up-going aileron (descending wing, inside the turn) deflects through a larger angle than the down-going one, adding parasite drag that tends to yaw the aeroplane into the bank. Adverse yaw is reduced, not eliminated.
• Frise-type ailerons: as an aileron rises its nose pokes out below the wing into the airstream, adding drag on the descending wing; the down-going aileron's nose stays tucked in. Frise ailerons can also work differentially.
• Coupled ailerons and rudder: the rudder moves automatically to yaw the aeroplane into the bank.

Whatever remains, the pilot removes with rudder. Rudder's primary effect is yaw, its secondary effect roll; the ailerons' primary effect is roll, their secondary effect yaw. Using rudder to cancel adverse yaw while the aeroplane is rolling is one of the most important elements of aeroplane control.

Source: Pooleys Vol 4 (10th ed.) p.92–93.

Roll is followed by yaw

Once banked, the lift is tilted and its sideways component has nothing to balance it, so the aeroplane slips towards the lower wing. That sideways airflow hits the fuselage and the big keel surfaces such as the fin, mostly behind the CG, and the nose yaws progressively towards the bank. With the ailerons back to neutral the yaw continues, and the aeroplane will drift into a spiral descent and lose height unless the pilot levels the wings or applies back pressure.

STRETCHTwo yaws point opposite ways during a turn entry: adverse yaw (away from the roll, while the ailerons are deflected) and the secondary-effect yaw (towards the bank, once banked). Which one is the rudder mainly fighting on the way in?

Source: Pooleys Vol 4 (10th ed.) p.94.

UK national licences: the UK PPL and the NPPL

Alongside the Part-FCL licences sit the UK's own national licences. A UK PPL has no expiry date, but the rating you want to use must be kept valid with a Certificate of Revalidation. Its privileges are set out in Part A of Schedule 8 to the ANO 2016. One difference to remember: the holder of a UK PPL(A) may not fly out of sight of the surface unless they hold an IMC Rating or an IR.

The National Private Pilot's Licence (NPPL) arrived in 2002. It is administered by the Light Aircraft Association under authority delegated by the CAA, and is available for aeroplanes, motor gliders, microlights and helicopters. In general it can be used only in UK airspace on UK-registered aircraft, unless another country's aviation authority has specifically agreed otherwise.

Source: Pooleys Vol 2 (17th ed.) p.170–171.

NPPL(A) with an SSEA or SLMG rating

A simpler licence than the Part-FCL PPL. Minimum requirements for issue:

• 22 hours dual, including 1 hour of instrument appreciation
• 10 hours solo, including a solo cross-country with two full-stop landings at two aerodromes other than the departure aerodrome
• The 9 Part-FCL PPL(A) exams
• A Navigation Skill Test (NST) and a General Skill Test (GST), each at least one hour; the NST comes before the qualifying cross-country
• Minimum age 16 for first solo, 17 for licence issue
• Medical: an NPPL medical declaration, or a Part-MED Class 1, 2 or LAPL medical

Limits: day VFR only, UK airspace, no more than 140 kt IAS maximum continuous cruise, no more than 2,000 kg maximum take-off weight. More than 4 persons on board, retractable undercarriage or a variable-pitch propeller each need differences training signed off in the logbook.

Source: Pooleys Vol 2 (17th ed.) p.171.

NPPL(A) with a Microlight rating

Microlights are classed as single-seat or two-seat, three-axis or weight-shift. They carry no more than two people, with a maximum authorised total weight of 300 kg (single-seat landplane), 450 kg (two-seat landplane), 330 kg (single-seat amphibian or floatplane) or 495 kg (two-seat amphibian or floatplane). Wing loading is limited to 25 kg per square metre and/or the stalling speed (VSO) to 35 kt calibrated airspeed.

• Full licence: at least 25 hours of instruction, including 10 hours solo
• Restricted licence: available after 15 hours, of which 7 solo; flight stays within 8 miles of the departure airfield
• Five exams: Aviation Law, Flight Rules and Procedures; Human Performance; Navigation; Meteorology; Aircraft (General)

Microlights operate on a Permit to Fly, and the only aerial work permitted is instruction. Syllabus and flight test details are published by the BMAA.

STRETCHThe aeroplane rating asks for 9 exams, 22 hours dual and 10 solo; the microlight rating for 5 exams and 25 hours including 10 solo. What does each extra requirement buy the pilot in terms of what they may fly?

Source: Pooleys Vol 2 (17th ed.) p.172–173.

Staying valid, and the medical

The NPPL itself is valid for life; its ratings last 24 months. To revalidate, either pass a General Skill Test, or log 12 hours of flight time including 8 hours as PIC, 12 take-offs and landings and 1 hour with an instructor, with 6 of the hours flown in the 12 months before expiry. A microlight rating is kept valid in the same way as the SSEA/SLMG rating.

Minimum medical for UK licences: UK Medical Declaration for the NPPL(A) (SSEA, SLMG or Microlight), the UK PPL(G) and the UK PPL(BA); Part-MED Class 2 for the UK PPL(A) and PPL(H); a Part-MED certificate for the NPPL(H). A Part-MED LAPL, Class 1 or Class 2 certificate is valid in place of the Pilot Medical Declaration (PMD).

Licence rules change. These are the requirements in the captured edition of Pooleys; check the current CAA and BMAA requirements before advising anyone.

Source: Pooleys Vol 2 (17th ed.) p.172–173.

What a medium level turn is

Exercise 9a. A medium level turn is flown at a constant height, with a medium angle of bank (30° or less), at constant power and in balance. Later in training come climbing turns, descending turns, rate 1 turns and steep turns (45° of bank or more).

The forces: banking tilts the lift, and its horizontal part is the turning force (the centripetal force). Nothing opposes that force, so the aeroplane is no longer in equilibrium and is pulled into the turn. More bank gives more turning force. Tilting the lift also shrinks its vertical part, so you add back pressure as you bank: a higher angle of attack, more total lift, and the vertical part balances the weight again.

Source: Pooleys Vol 1 (15th ed.) p.141–142.

What changes in the turn

• Nose attitude is higher than in straight and level flight, because of the back pressure. It is right when the aeroplane neither gains nor loses height.
• Airspeed falls a little, usually about 5 knots: more lift means more induced drag. Accept it.
• Stalling speed rises: about 7% higher at 30° of bank. At normal speeds there is still a safe margin.
• The aeroplane tends to overbank: the outer wing travels faster and makes extra lift. Simply hold the bank you want with the ailerons.

Judge the bank against the natural horizon and confirm it on the AI (attitude indicator). In a side-by-side cockpit the nose looks higher against the horizon in a left turn than in a right turn, seen from the left seat.

Source: Pooleys Vol 1 (15th ed.) p.142–143 and p.147.

Balance: keep the ball in the middle

While the aeroplane is rolling, aileron drag (adverse yaw) swings the nose away from the roll. Counter it with rudder pressure in the direction of the roll, both rolling in and rolling out; the extra pressure comes off once the turn is steady. After that the ball tells you the balance: ball out to the right, more right rudder, and the same on the left.

Source: Pooleys Vol 1 (15th ed.) p.144–145 and p.149.

Flying the manoeuvre

• Before entry: at the height and airspeed you want (altimeter steady, VSI at zero), in trim. Look out: into the turn, then opposite and behind, then scan back. Pick a reference point to roll out on.
• Entry: roll on bank with aileron, balance with same-side rudder, add back pressure to hold height.
• In the turn: bank with ailerons, balance with rudder, height with elevator. Gaining height: more bank or lower the nose. Losing height: less bank or raise the nose. Work the LAI cycle.
• Roll out: start about 10° before the reference point. Roll off bank, balance with rudder, gradually release the back pressure. Look out again.

Do not trim in the turn: it is a transient manoeuvre. On the panel the DI shows the heading changing while the altimeter stays put.

STRETCHWhy start the roll-out before the reference point, and what happens to the height if you forget to release the back pressure?

Source: Pooleys Vol 1 (15th ed.) p.147–151.