Flight Mechanics

In demonstrating VMCL:

•the rudder force may not exceed 150 lb.

•the aeroplane may not exhibit hazardous flight characteristics or require exceptional piloting skill, alertness or strength.

•lateral control must be sufficient to roll the aeroplane, from an initial condition of steady flight, through an angle of 20° in the direction necessary to initiate a turn away from the inoperative engine(s), in not more than 5 seconds.

Factors Affecting VMCL

Aileron Effectiveness

At low airspeed, dynamic pressure is low which reduces the effectiveness of all the flying controls for a given angle of displacement. This effect on the rudder has already been discussed, but the ailerons will be affected in a similar way. At reduced airspeed, greater aileron displacement must be used to obtain the required roll response. The ‘down’ aileron on the left side will also add to the yawing moment because of its increased induced drag and may stall the wing at low IAS (high CL). Adequate aileron effectiveness is clearly very important when considering VMCL because this minimum control speed contains a roll requirement, not just directional control.

Summary of Minimum Control Speeds

CS 25.149 sets out the criteria to be used when establishing the minimum control speeds for certification of a new aircraft. The speeds so established will be included in the aircraft’s Flight Manual.

From careful study of the above extracts, several things can be noted:

•nose wheel steering may not be used when establishing VMCG. Its use would artificially decrease VMCG. In service, when operating from a slippery runway, nose wheel steering would be ineffective, so it might be impossible to directionally control the aircraft when at or above the stated VMCG.

•VMCL includes a roll requirement, not merely directional control, as with the other speeds.

•The thrust developed by an engine depends on the air density, and so thrust will decrease with increasing altitude and temperature. The yawing moment due to asymmetric thrust will therefore decrease with altitude and temperature, and so control can be maintained

at a lower IAS. VMC therefore decreases with increasing altitude and temperature (higher density altitude).

12

Flight Mechanics 12

395

12

Mechanics Flight 12

Flight Mechanics

Performance with One Engine Inoperative

It was shown on page 369 that an aircraft’s ability to climb depends upon the excess thrust available, after aerodynamic drag is balanced. If a twin-engine aircraft loses an engine, total thrust is reduced by 50%, but the excess thrust (the thrust, minus aerodynamic drag) is reduced by more than 50%, Figure 12.28. The ability to climb may be reduced as much as 80%.

Single-engine Angle of Climb

Angle of climb is determined by excess thrust available. Climb angle will be a maximum when the aircraft is flown at the IAS where excess thrust is a maximum (maximum thrust to drag ratio). Since thrust decreases with forward speed and total drag increases below and above the minimum drag speed (VIMD), the best angle of climb is achieved at a speed below VIMD but a safe margin above the stall speed. The airspeed for maximum angle of climb is VX for all engines operating and VXSE for best single-engine angle of climb.

Single-engine Rate of Climb

Rate of climb is determined by excess power available. Power is the rate of doing work and work is force times distance moved, so power is force times distance moved in a given time, i.e. thrust or drag times TAS (thrust or drag because they are both forces and TAS because it is the only speed there is!). Although thrust reduces with forward speed, total power available increases because of the speed factor. Similarly, power required is a measure of drag times TAS, so excess power available determines the available rate of climb. The airspeed for best rate of climb is VY for all engines operating and VYSE for best single-engine rate of climb.

VY and VYSE are higher than VX and VXSE and provide a safer margin above both stall and VMCA. Under most circumstances VY and VYSE are the best speeds to use. On small twin-engine aircraft

VYSE is marked on the Airspeed Indicator by a blue radial line and is called ‘blue line speed’.

Conclusions

At a given altitude, airspeed and throttle position, excess thrust depends on the amount of drag being generated, and this will depend on configuration, weight and whether turns are required to be made. The control surface deflections required to balance asymmetric thrust will also cause an increase in drag. It is essential, therefore, that after losing an engine, particularly during take-off or during a go-around, drag is reduced and no turns are made until well away from the ground.

Drag can be reduced by feathering the propeller of the inoperative engine, raising the undercarriage, carefully raising the flaps, closing the cowl flap on the inoperative engine and banking the aircraft no more than 5° towards the operating engine. Flying at VYSE (blue line speed) with maximum continuous thrust on the operating engine will provide maximum climb performance and optimum control over the aeroplane.

396

Flight Mechanics 12

THRUST
				THRUST
	EXCESS			REQUIRED
				(DRAG)
			EXCESS
MINIMUM
DRAG				THRUST
VS				AVAILABLE
	VX	VXSE	V I MD
65	70	75	85	I AS
VMCA		SAFETY MARGI N

BOTH ENGINES

ONE ENGINE

			POW ER
			AVAI LABLE
POWER
VS			POW ER
			REQUI RED
		EXCESS
			POW ER
			AVAI LABLE
	EXCESS
MI NI MUM		V Y
POW ER
	VI MD		VYSE
65	70	85	IAS
VMCA
	SAFETY MARGI N

Figure 12.28 Excess thrust & excess power.

Flight Mechanics 12

397

12 Questions

Questions

	1.	In straight and level powered flight the following principal forces act on an aircraft:
		a.	thrust, lift, weight.
		b.	thrust, lift, drag, weight.
		c.	thrust, lift, drag.
		d.	lift, drag, weight.
	2.	For an aircraft in level flight, if the wing CP is aft of the CG and there is no thrust/
		drag couple, the tailplane load must be:
		a.	upward.
		b.	downward.
		c.	zero.
		d.	forward.
	3.	When considering the forces acting upon an aeroplane in straight and level flight
		at constant airspeed, which statement is correct?
12		a.	Weight acts vertically toward the centre of the Earth.
		b.	Lift acts perpendicular to the chord line and must be greater than weight.
Questions		c.	Thrust acts forward parallel to the relative wind and is greater than drag.
		d.	Lift acts in the same direction to the aircraft weight.
	4.	The horizontal stabilizer usually provides a down load in level flight because:
		a.	the main plane lift is always positive.
		b.	the lift/weight and thrust/drag couples combine to give a nose-down pitch.
		c.	the lift produced is greater than required at high speed.
		d.	this configuration gives less interference.
	5.	The reason a light general aviation aircraft tends to nose-down during power
		reduction is that the:
		a.	thrust line acts horizontally and above the force of drag.
		b.	centre of gravity is located forward of the centre of pressure.
		c.	centre of pressure is located forward of the centre of gravity.
		d.	force of drag acts horizontally and above the thrust line.
	6.	To give the best obstacle clearance on take-off, take-off should be made with:
		a.	flaps partially extended and at best rate of climb speed (VY).
		b.	flaps partially extended and at best angle of climb speed (VX).
		c.	flaps retracted and at best rate of climb speed (VY).
		d.	flaps retracted and at best angle of climb speed (VX).
	7.	The angle of climb is proportional to:
		a.	the amount by which the lift exceeds the weight.
		b.	the amount by which the thrust exceeds the drag.
		c.	the amount by which the thrust exceeds the weight.
		d.	the angle of attack of the wing.

398

Questions 12

8.In a climb at a steady speed, the thrust is:

a.equal to the aerodynamic drag.

b.greater than the aerodynamic drag.

c.less than the aerodynamic drag.

d.equal to the weight component along the flight path.

9.A constant rate of climb in an aeroplane is determined by:

a.wind speed.

b.the aircraft weight.

c.excess engine power.

d.excess airspeed.

10.Assume that after take-off a turn is made to a downwind heading. In regard to the ground, the aeroplane will climb at:

a.a greater rate into the wind than downwind.

b.a steeper angle downwind than into the wind.

c.the same angle upwind or downwind.

d.a steeper angle into the wind than downwind.

11.What effect does high density altitude have on aircraft performance?

a.It increases take-off performance.

b.It increases engine performance.

c.It reduces climb performance.

12.During a steady climb the lift force is:

a.less than the weight.

b.exactly equal to the weight.

c.equal to the weight plus the drag.

d.greater than the weight.

13.In a steady climb the wing lift is:

a.equal to the weight.

b.greater than the weight.

c.equal to the weight component perpendicular to the flight path.

d.equal to the vertical component of weight.

14.During a glide the following forces act on an aircraft:

a.lift, weight, thrust.

b.lift, drag, weight.

c.drag, thrust, weight.

d.lift and weight only.

15.For a glider having a maximum L/D ratio of 20:1, the flattest glide angle that could be achieved in still air would be:

a.1 ft in 10 ft.

b.1 ft in 20 ft.

c.1 ft in 40 ft.

d.1 ft in 200 ft.

Questions 12

399

12 Questions

Questions 12

16.To cover the greatest distance when gliding the gliding speed must be:

a.near to the stalling speed.

b.as high as possible within VNE limits.

c.about 30% faster than VMD.

d.the one that gives the highest L/D ratio.

17.If the weight of an aircraft is increased, the maximum gliding range:

a.decreases.

b.increases.

c.remains the same, and rate of descent is unchanged.

d.remains the same, but rate of descent increases.

18.When gliding into a headwind, the ground distance covered will be:

a.less than in still air.

b.the same as in still air but the glide angle will be steeper.

c.the same as in still air but the glide angle will be flatter.

d.greater than in still air.

19.During a ‘power on’ descent the forces acting on an aircraft are:

a.lift, drag and weight.

b.lift, thrust and weight.

c.lift, drag, thrust and weight.

d.lift and weight only.

20.If air brakes are extended during a glide, and speed maintained, the rate of descent will:

a.increase and glide angle will be steeper.

b.increase, but glide angle will remain the same.

c.decrease.

d.remain the same.

21.An aircraft has a L/D ratio of 16:1 at 50 kt in calm air. What would the approximate GLIDE RATIO be with a direct headwind of 25 kt?

a.32:1

b.16:1

c.8:1

d.4:1

22.During a turn the lift force may be resolved into two forces; these are:

a.a force opposite to thrust and a force equal and opposite to weight.

b.centripetal force and a force equal and opposite to drag.

c.centripetal force and a force equal and opposite to weight.

d.centrifugal force and a force equal and opposite to thrust.

400

Questions 12

23.In a turn at a constant IAS, compared to straight and level flight at the same IAS:

a.the same power is required because the IAS is the same.

b.more power is required because the drag is greater.

c.more power is required because some thrust is required to give the centripetal force.

d.less power is required because the lift required is less.

24.In a turn at a given TAS and bank angle:

a.only one radius of turn is possible.

b.the radius can be varied by varying the pitch.

c.the radius can be varied by varying the yaw.

d.two different radii are possible, one to the right and one to the left.

25.As bank angle is increased in a turn at a constant IAS, the load factor will:

a.increase in direct proportion to bank angle.

b.increase at an increasing rate.

c.decrease.

d.remain the same.

26.Skidding outward in a turn is caused by:

a.insufficient rate of yaw.

b.too much bank.

c.too much nose-up pitch.

d.insufficient bank.

27.For a turn at a constant IAS if the radius of turn is decreased, the load factor will:

a.increase.

b.decrease but bank angle will increase.

c.decrease but bank angle will decrease.

d.remain the same.

28.An aircraft has a stalling speed in level flight of 70 kt IAS. In a 60° balanced turn the stalling speed would be:

a.76 kt.

b.84 kt.

c.99 kt.

d.140 kt.

29.An increase in airspeed while maintaining a constant load factor during a level, coordinated turn would result in:

a.an increase in centrifugal force.

b.the same radius of turn.

c.a decrease in the radius of turn.

d.an increase in the radius of turn.

Questions 12

401

12 Questions

Questions 12

30.How can the pilot increase the rate of turn and decrease the radius at the same time?

a.Shallow the bank and increase airspeed.

b.Steepen the bank and increase airspeed.

c.Steepen the bank and decrease airspeed.

31.If an aircraft with a gross weight of 2000 kg were subjected to a total load of 6000 kg in flight, the load factor would be:

a.9 g

b.2 g

c.6 g

d.3 g

32.Why must the angle of attack be increased during a turn to maintain altitude?

a.Compensate for increase in induced drag.

b.Increase the horizontal component of lift equal to the vertical component.

c.Compensate for loss of vertical component of lift.

d.To stop the nose from dropping below the horizon and the airspeed

increasing.

33.Two aircraft of different weight are in a steady turn at the same bank angle:

a.the heavier aircraft would have a higher “g” load.

b.the lighter aircraft would have a higher “g” load.

c.they would both have the same “g” load.

34.For a multi-engine aircraft, VMCG is defined as the minimum control speed on the ground with one engine inoperative. The aircraft must be able to:

a.abandon the take-off.

b.continue the take-off or abandon it.

c.continue the take-off using primary controls only.

d.continue the take-off using primary controls and nose wheel steering.

35.What criteria determines which engine is the “critical” engine of a twin engine aeroplane?

a.the one with the centre of thrust farthest from the centreline of the fuselage.

b.the one with the centre of thrust closest to the centreline of the fuselage.

c.the one designated by the manufacturer which develops most usable thrust.

d.the failure of which causes the least yawing moment.

36.Following failure of the critical engine, what performance should the pilot of a light, twin engine aeroplane be able to maintain at VMCA?

a.Heading, altitude, and ability to climb 50 ft/min.

b.Heading only.

c.Heading and altitude.

402

Questions 12

Questions 12

403