PPL(A) 080 Principles of FlightRevision notes and practice questions

PPL 080 Principles of FlightFree sample, 10 questions

1. 1Air flows steadily through a venturi tube at low subsonic speed. Compared with the wide entry section, what happens in the narrow throat?

   1. Velocity decreases and static pressure increases.
   2. Velocity increases and static pressure increases.
   3. Velocity increases and static pressure decreases.
   4. Velocity is unchanged and static pressure decreases.
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   Answer C. The continuity equation says that the mass flow is the same through every section, so the air must speed up where the section narrows. Bernoulli's equation then says that the sum of static and dynamic pressure stays constant along a streamline: as dynamic pressure rises, static pressure falls. The same mechanism produces the low pressure over the curved upper surface of a wing. Thinking that a squeezed flow means a higher pressure is the classic trap.

2. 2At a constant angle of attack and air density, a wing produces 6,000 N of lift at 50 kt TAS. What lift does it produce at 100 kt TAS?

   1. 12,000 N
   2. 18,000 N
   3. 24,000 N
   4. 48,000 N
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   Answer C. Lift is L = ½ρV²SCL. With density, wing area and CL (fixed by the angle of attack) unchanged, lift varies with the square of the TAS. The speed ratio is 100 ÷ 50 = 2, so lift is multiplied by 2² = 4: 6,000 × 4 = 24,000 N. Assuming that lift doubles with the speed gives the tempting 12,000 N. In level flight lift must equal weight, which is why the pilot lowers the angle of attack as the speed increases.

3. 3Which statement about the induced drag of a light aeroplane in level flight is correct?

   1. It decreases as speed increases and is lower on a wing of high aspect ratio.
   2. It increases with the square of the speed, like skin friction and form drag.
   3. It is independent of the speed and depends only on the area of the wing.
   4. It increases as speed increases and is reduced by fitting wingtip winglets.
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   Answer A. Induced drag is the by-product of lift: the wingtip vortices create downwash, which tilts the lift vector rearwards. At low speed the wing needs a high angle of attack, so the vortices are strong and induced drag is high; it falls roughly as 1 ÷ V² as the speed increases. A high aspect ratio or winglets weaken the tip vortices and reduce it. The drag that rises with the square of the speed is parasite drag (form, skin friction and interference).

4. 4With a clean wing, an aeroplane stalls at an angle of attack of 16° in wings-level flight at maximum mass. At a lower mass in a steep turn, same configuration, at what angle of attack will it stall?

   1. At a smaller angle, because the load factor is higher.
   2. At a larger angle, because the aeroplane is lighter.
   3. At a larger angle, because the stalling IAS is higher in the turn.
   4. At 16°, but at a different indicated airspeed.
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   Answer D. For a given configuration at low Mach number, a wing always stalls at the same critical angle of attack, whatever the speed, mass, attitude or load factor. Mass and load factor change only the speed at which that angle is reached: a lower mass lowers the stalling speed, a higher load factor raises it. That is why an aeroplane can stall at any speed and in any attitude. Flaps, ice or contamination do change the critical angle, but they are excluded here because the wing is clean and the configuration unchanged.

5. 5Compared with the clean configuration, what is the effect of extending trailing-edge flaps on the wing?

   1. CL max increases and the stall occurs at a higher angle of attack.
   2. CL max is unchanged, but the stall occurs at a lower angle of attack.
   3. CL max decreases, but drag is reduced at a given airspeed.
   4. CL max increases and the stall occurs at a lower angle of attack.
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   Answer D. Trailing-edge flaps increase the camber of the wing (and its area for a Fowler flap), which shifts the lift curve upwards and to the left: CL max increases, but the critical angle of attack is reached earlier. They also add drag, which allows a steeper approach, and because the required CL is obtained at a smaller angle of attack the aeroplane flies a given speed at a lower nose attitude. A higher stalling angle is the effect of a leading-edge slat or slot, not of a flap.

6. 6After a gust, an aeroplane pitches back towards its trimmed attitude, overshoots it and then oscillates with increasing amplitude. How is its longitudinal stability described?

   1. Statically unstable and dynamically stable.
   2. Statically stable and dynamically unstable.
   3. Statically neutral and dynamically neutral.
   4. Statically stable and dynamically stable.
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   Answer B. Static stability is the initial tendency after a disturbance: here the aeroplane starts back towards its trimmed attitude, so it is statically stable. Dynamic stability describes the motion over time, and oscillations that grow are dynamically unstable. A statically unstable aeroplane can never be dynamically stable, because it diverges from the start.

7. 7A pilot rolls into a left turn using ailerons only, and the nose briefly yaws to the right. What causes this, and which design feature reduces it?

   1. The torque reaction of the engine; a fin offset to the left of the centreline.
   2. The extra induced drag of the down-going right aileron; differential or Frise ailerons.
   3. The extra induced drag of the up-going left aileron; ailerons with more down than up travel.
   4. The dihedral effect of the raised right wing; a larger fin and rudder area.
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   Answer B. This is adverse yaw: the down-going aileron on the rising right wing increases its lift and therefore its induced drag, which pulls that wing back and yaws the nose away from the turn. Differential ailerons move further up than down, and Frise ailerons push the nose of the up-going aileron into the airflow, both adding drag on the descending wing to balance it. The pilot corrects the rest with rudder in the direction of the turn, keeping the balance ball centred.

8. 8A light aeroplane is certified in the normal category under the traditional CS-23 / FAR 23 rules (the rules in force before the 2017 rewrite of CS-23). What are its limit manoeuvring load factors, flaps up?

   1. +4.4 and -1.76
   2. +6.0 and -3.0
   3. +2.5 and -1.0
   4. +3.8 and -1.52
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   Answer D. The traditional CS 23.337 and FAR 23.337 set a positive limit manoeuvring load factor of 3.8 for the normal category and a negative one of 0.4 times that value, that is -1.52. The utility category goes to +4.4 and -1.76, and the aerobatic category to +6.0 and -3.0. The values +2.5 and -1.0 belong to large transport aeroplanes under CS-25. The limit load factor is the largest load expected in service; the structure must carry 1.5 times that value, the ultimate load, without failing. Since 2017 CS-23 amendment 5 has moved these figures out of the rule into industry standards, but the limits of the light aeroplanes in service remain those of their flight manual, built on these traditional values.

9. 9Why does the blade angle of a propeller decrease from the root to the tip?

   1. To reduce the mass of the blade near the tip and the stress on the hub.
   2. To keep the tip speed below the speed of sound at full power.
   3. To make the tip stall first, which keeps the root effective at high power.
   4. So that every section, though faster towards the tip, meets a similar angle of attack.
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   Answer D. The blade angle is measured between the chord and the plane of rotation, and the local relative airflow combines the forward speed with the rotational speed, which grows with the distance from the hub. Twist reduces the blade angle towards the tip so that every section works at a similar, efficient angle of attack. A fixed-pitch propeller is efficient at one speed only, whereas a constant-speed propeller changes its blade angle to hold the selected rpm: fine pitch for take-off, coarser pitch in cruise.

10. 10Which statements about the forces on an aeroplane in steady, unaccelerated flight are correct?
    1. In straight and level flight, lift equals weight and thrust equals drag.
    2. In a steady straight climb, lift is slightly less than the weight.
    3. In a steady straight climb, thrust is greater than drag.
    4. In a steady straight climb, lift is greater than weight, which is what makes the aeroplane climb.

    1. 1 and 4.
    2. 1, 2 and 3.
    3. 1, 3 and 4.
    4. 2 and 3.
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    Answer B. In level flight the four forces balance in pairs: lift against weight and thrust against drag. In a steady climb, part of the weight acts backwards along the flight path, so thrust must equal drag plus that component, and lift only has to balance the rest of the weight, which makes it slightly smaller than the weight. Excess thrust, not excess lift, makes an aeroplane climb.