ppl_05_e2

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 8: PROPELLER THRUST

An

anticlockwise rotating

propeller, as

viewed from the pilot’s seat, will cause a powerful pistonengine driven, tail-wheel propeller aircraft to swing to the right when the tail is lifted on the take-off run.

Figure 8.26 Gyroscopic precession in a propeller.

Asymmetric Blade Effect.

Asymmetric blade effect is felt by the aircraft when the axis of rotation of the propeller is inclined upwards to the horizontal path being followed by the aircraft, or, which amounts to the same thing, when the plane of rotation of the propeller is not vertical. This situation arises during the initial take-off run of a tail wheel aircraft, before that part of the take-off run where the pilot raises the tail.

Figure 8.27 The asymmetric blade effect.

As the aircraft accelerates forwards with its tail wheel still in contact with the ground (and considering any half-rotation period of the propeller), the down-going blade will move through a greater linear distance than the up-going blade (see Figure 8.27).

Consequently, the relative airfow over the down-going blade is of a higher velocity than the up-going blade. From Equation (12) you will see that an increase in relative velocity v will cause an increase in thrust.

Furthermore, an increase in the velocity of the relative airfow, will cause an increase in blade angle of attack. (Look back at Figure 8.20 for a graphical illustration of this fact. You will see that for a given propeller rotational speed and a given airspeed, any increase in the relative airfow vector will lead to an increase in blade angle of attack.)

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Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 8: PROPELLER THRUST

Therefore, the down-going propeller blade will also have a greater angle of attack (CL) than the up-going blade. This, too, leads to an increase in thrust on the down going blade, as can be deduced from Equation (12).

Take-off Roll and Type of Undercarriage.

For a tail-wheel aircraft with a propeller rotating anti-clockwise (as seen by the pilot), then, the Gyroscopic Effect and the Asymmetric Blade Effect will combine to generate more thrust on the left hand side of the propeller than the right hand side of the propeller, causing the aircraft to tend to swing to the right during the initial takeoff roll.

Figure 8.28 A “tail-dragger” - the De Havilland Chipmunk, fitted with a tailwheel.

A tail-dragger (see Figure 8.28) with a clockwise rotating propeller will, of course, tend to swing to the left.

The asymmetric effect will continue until the tail is lifted, thus putting the axis of rotation of the propeller disc at right angles to the path the aircraft is following.

However, asymmetric blade effect is dependent on thrust, and is proportional to forward speed, so it is not a signifcant factor in the initial part of the ground roll for a low-powered, tail-wheel aircraft. But the effect may become increasingly apparent as the ground roll continues, if the aircraft maintains its tail-down attitude.

Asymmetric effect on a tail wheel aircraft is not of any signifcance during the landing roll when the throttle is closed, as very little thrust is produced by the propeller; in fact it produces mainly drag. However, if the throttle is opened again, smartly, to initiate a go-around, while the tail wheel is still on the ground, asymmetric effect may make itself felt (Note that the asymmetric blade effect is sometimes referred to as ‘P-factor effect.’)

Note, then that during the take-off roll, the gyroscopic effect and asymmetric blade effect, are experienced by tail-wheel aircraft only.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 8: PROPELLER THRUST

Direction of Swing.

In the case of a propeller rotating anti-clockwise as seen by the pilot, Slipstream Effect, Torque Effect, Gyroscopic Effect and Asymmetric Blade Effect all induce a swing to the right, and all effects reinforce one another.

For an aircraft with the propeller rotating clockwise when viewed from the pilot’s seat, these effects would combine to induce a swing to the left.

So, as a fnal illustration, imagine you are commencing a take-off in a tailwheel aircraft with an anticlockwise rotating propeller (as seen from your pilot’s seat). You should, then, be prepared for a possible swing to the right when you start to roll. If your aircraft has a powerful engine, yaw to the right during the take-off roll, as the tail is lifted into the fying position, will almost certainly be signifcant.

Consider, then, in this situation, to what extent this swing to starboard might be exacerbated by a cross-wind from the right which acts on the tail, pushing it even more to the left. The increase in right swing could be considerable. Of course, a cross wind from the left will tend to negate the right swing on take-off. If your aircraft had a clockwise rotating propeller, viewed from your seat, it would, of course, tend to swing to the left on take-off. A left swing will be amplifed by a cross-wind from the left and diminished by a cross-wind from the right.

CONCLUSION.

In this chapter, we have looked at the basic principles of how a propeller produces thrust from the points of view of both simplifed momentum theory and the rotating aerofoil theory. Both theories appear in specialist text books on propellers, and both give a credible explanation of thrust generation. Text books for pilots under training tend to concentrate on the rotating wing-theory, or Bernoulli explanation, of thrust, whereas momentum theory is usually found only in more scientifcally oriented literature. We have thought it best to mention both theories, so that the reader is aware of how complex, and sometimes controversial, theories of propeller thrust can be.

Several aspects of propeller theory, though, are common to both the rotating wingtheory interpretation and the momentum-theory interpretation of propeller thrust; these are: propeller twist, the variation of thrust with aircraft forward speed, and angle-of-attack considerations. It is those aspects of thrust which are common to both theories of propeller operation which are the ones you need to learn for your pilot examinations. In order to prepare your examinations, be guided by the questions at the end of this chapter and by the key-points in the text margins.

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Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 8: PROPELLER THRUST QUESTIONS

Representative PPL - type questions to test your theoretical knowledge of Propeller Thrust.

1.A propeller blade is twisted along its length in order to:

a.give a progressively increasing blade angle from root to tip

b.give a progressively increasing angle of attack from root to tip when the propeller is rotating

c.compensate for the decreasing linear speed of the blade from root to tip

d.maintain the most effcient angle of attack along the whole length of the propeller blade when the propeller is rotating

2.Blade angle _________ from the hub to the tip of a propeller blade in order to maintain an optimal ____________ from hub to tip during propeller rotation.

a.	increases	angle of attack
b.	decreases	geometric pitch
c.	increases	effective pitch
d.	decreases	angle of attack

3.As an aircraft with a variable-pitch, constant-speed propeller accelerates along the runway:

a.the angle of attack will decrease and the engine RPM remain constant

b.the blade pitch angle increases, maintaining a constant angle of attack and constant RPM

c.the angle of attack will remain constant and the engine RPM will increase

d.the linear velocity of the propeller tip will gradually decrease

4.In a dive, with the throttle setting constant, the engine RPM of an aircraft ftted with a fxed-pitch propeller will:

a.decrease as the airspeed increases

b.remain constant whatever the airspeed

c.increase if the airspeed is allowed to increase

d.decrease as long as the throttle setting is not changed

5.In a single-engine, propeller-driven aircraft, the torque reaction of a clockwise rotating propeller (as seen from the pilot’s seat) will tend to cause:

a.left roll and right yaw during take off

b.left roll and left yaw during take off

c.right roll and right yaw during take off

d.left yaw and right roll during take off

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 8: PROPELLER THRUST QUESTIONS

6.Which of the following combinations will decrease the angle of attack of a fxed pitch propeller blade?

a.Increased TAS and increased RPM

b.Increased TAS and decreased RPM

c.Decreased TAS and increased RPM

d.Decreased TAS and decreased RPM

7.Which of the following combinations will increase the angle of attack of a fxed pitch propeller blade?

a.Increased TAS and increased RPM

b.Increased TAS and decreased RPM

c.Decreased TAS and increased RPM

d.Decreased TAS and decreased RPM

8.The advantage of a constant speed propeller over a fxed pitch propeller is that:

a.a greater maximum thrust is available

b.a higher maximum effciency is attained

c.more blade surface area is made available

d.optimal effciency is achieved over a wide speed range

9.The angle of attack of a fxed, coarse-pitch propeller on a touring light aircraft:

a.will be lower during the take-off run than in fight

b.will be optimal in all fight conditions

c.will be most effcient at the cruising speeds published in the pilot’s operating manual

d.will decrease with decreasing airspeed at constant engine RPM

10.A propeller blade is twisted from root to tip:

a.to provide maximum thrust at the root

b.to provide maximum thrust at the tip

c.so that propeller effciency remains high, at any engine RPM

d.so that thrust remains approximately constant along the whole length of the propeller blade, at any engine RPM

11.The angle of attack of a fxed-pitch propeller designed for cruising fight is:

a.optimal for steady cruising fight only

b.increases with an increase in TAS

c.decreases with an increase in RPM

d.will always be positive in a power off glide

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Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 8: PROPELLER THRUST QUESTIONS

12.Propeller-blade angle of attack is the angle between the blade chord-line and the:

a.plane of rotation of the propeller

b.aeroplane’s gradient of climb

c.the airfow relative to the propeller

d.helix angle

13.What is the purpose of increasing the number of propeller blades?

a.To reduce noise

b.To improve power absorption

c.To increase the effciency of the variable pitch mechanism

d.To enable a longer undercarriage to be ftted

14.What would be the gyroscopic effect of a clockwise rotating propeller (viewed from the pilot’s seat) on a single-engine, tail-wheel aircraft as it raises its tail during the take off run?

a.The aircraft would yaw to the right

b.The aircraft would yaw to the left

c.The aircraft would roll to the right

d.The aircraft would roll to the left

15.During the take-off roll, what effect does torque have on an aircraft with an anti-clockwise rotating propeller, as seen from the pilot’s seat?

a.Weight on left wheel decreased, weight on right wheel increased

b.Weight on left wheel increased, weight on right wheel remains constant

c.Weight on left wheel increased, weight on right wheel decreased

d.Weight on right wheel increased, weight on left wheel remains constant

16.Which of the following defnitions of propeller parameters is correct?

a.Blade Angle is the angle between chord line and the relative airfow

b.Critical tip speed is the propeller speed at which there is a risk of the fow separating at some part of the propeller

c.Blade angle of attack is the angle between the blade chord line and propeller’s plane of rotation

d.Geometric pitch is the theoretical distance that the propeller travels forward in one rotation

17.Which of the following gives the most correct explanation of why a propeller’s blade angle decreases from root to tip?

a.To compensate for the change in blade cross section from root to tip

b.To provide increased thrust at the root

c.To provide increased thrust at the tip

d.To compensate for the increase in rotational velocity from root to tip.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 8: PROPELLER THRUST QUESTIONS

18.On take off, why is it that a tail-wheel aircraft displays a greater tendency to swing than a nose-wheel aircraft?

a.Because the propeller of a tail-wheel aircraft always rotates anticlockwise

b.Because torque effect in a tail-wheel aircraft counters slipstream effect

c.Because the tail-wheel aircraft is subject to asymmetric blade effect and gyroscopic effect, in addition to slipstream and torque effect

d.Because the nose-wheel aircraft is not subject to torque effect and slipstream effect

19.When power is applied, why do aircraft in straight and level fight show a tendency to yaw?

a.Because of the gyroscopic effect of the propeller

b.Because of the slipstream effect and torque effect of the propeller

c.Because most aircraft are ftted with an off-set fn

d.Because the pilot always applies rudder when increasing power

20.On an aircraft ftted with a fxed pitch propeller, why does a change in airspeed always cause a corresponding change in engine RPM, even though the pilot may not move the throttle lever?

a.Because a change in airspeed causes a change in inlet manifold pressure which affects engine power output

b.Because an increase in airspeed causes a decrease in propeller angle of attack , thus reducing propeller torque, while a decrease in airspeed has the opposite effect

c.Because of the asymmetric blade effect

d.Because of the slipstream effect

21.Choose the most correct answer from the options below. As a coarse blade pitch is effcient at cruising speeds, why should a pilot ever choose to select fne pitch?

a.In order to minimise fuel consumption

b.Because of noise limitations on climbing away from the airfeld over built up areas

c.In order to increase engine RPM to the most fuel effcient level

d.In order to optimise the aircraft’s performance on take-off

22.Which of the answers below is the most correct? What performance advantages does an aircraft possess if it is ftted with a fne pitch propeller?

a.It will produce maximum thrust at higher cruising speeds

b.Its engine will be less likely to overheat in a climb, at high RPM and relatively low forward speed

c.The propeller will give optimal thrust for the take-off and initial climb

d.It will be able to fy at high speeds without exceeding the maximum permissible engine RPM

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 8: PROPELLER THRUST QUESTIONS

23.What are the advantages of a constant-speed propeller?

a.Cruise performance will be improved

b.Take off performance will be improved

c.The engine can never over-speed whatever the circumstances

d.It provides an effcient blade angle of attack over a wide range of airspeeds

24.What type of aircraft would most-likely be ftted with a fne, fxed-pitch propeller?

a.A touring aircraft

b.A glider tug

c.A high-performance, military, turbo-prop training aircraft

d.A turbo-prop passenger aircraft

Question	1	2	3	4	5	6	7	8	9	10	11	12
Answer
Question	13	14	15	16	17	18	19	20	21	22	23	24
Answer

The answers to these questions can be found at the end of this book.

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CHAPTER 9

THE FOUR FORCES AND

TURNING FLIGHT

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Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 9: THE FOUR FORCES AND TURNING FLIGHT

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