CRUISE

Question 1

Define endurance

Answer 1

Endurance = 1 / FF — 1 / TSFC x Thrust — 1 / TSFC x Drag (Thrust=Drag)

FF depends on Thrust and SFC
Thurst will be least when drag is least, which is Vmd
Drag curve can be called the fuel flow curve

TSFC = Amount of fuel needed to provide a certain thrust over a period of time
TSFC = FF per pound of thrust produced
TSFC = FF / Thrust

Question 2

When is endurance the greatest?

Answer 2

L/D ratio must be greatest which means drag must be minimum

Question 3

At what speed is endurance the greatest?

Answer 3

At minimum drag speed, Vmd

Question 4

Why isn’t Vmd an optimum speed to fly at?

Answer 4

Because of speed stability. Flying slightly faster gains speed stability and doesn’t decrese endurance that much because of the relative flatness of the drag curve

Question 5

Is Fuel Flow measured in weight or in volume and why?

Answer 5

FF is measured by weight rather than volume
Chemical energy in fuel is a function of mass
Volume expands or contracts with temperature

Question 6

How does the following factors affect Endurance?

1. Altitude
2. Weight
3. Configuration

Answer 6

1. Altitude:
   Climbing into less dense air decreases parasite drag but increases induced drag. Both curves moves so there’s no change in max endurance with altitude but the required speed will increase
   Endurance is lowest when TSFC is lowest. TSFC improves up to the tropopause (lowest temp)
2. Weight:
   Weight affects the total drag curve, but only the induced drag changes with weight
   Drag/FF decreases with weight but speed has to be reduced
   Left side of the drag curve gets “bent down”
3. Configuration:
   Will increase drag, but parasite drag specifically
   Total drag curve moves up left

Question 7

What is the relationship between best endurance and AOA?

Answer 7

Best endurance occurs at a specific AOA, regardless of weight

Question 8

What’s the relationship between best range speed and best endurance speed?

Answer 8

Best endurance speed is always slower than best range speed and always at a higher AOA

Question 9

Define range

Answer 9

Range is the distance that can be flown on the fuel available

Question 10

Define specific range

Answer 10

SR = NM / kg — TAS / FF — TAS / TSFC x T req —
TAS / TSFC x Drag

SR = Distance flown per unit of fuel 
FF = TSFC x Thrust req (Drag)

TSFC = FF / Drag
FF = TSFC x Drag

Question 11

When is range maximum?

Answer 11

Range is best when the ratio of TAS to Drag is the greatest. Drag = Fuel Flow
Drag curve is shallow at the bottom so an increase in TAS only gives a small increase in Drag/FF

Question 12

What speed is maximum range at?

Answer 12

Max range is at 1,32 Vmd

Question 13

Explain the concept “Cost Index”

Answer 13

Cost index can be 0 - 999
0 means that speed is 1,32 Vmd
Higher index means higher speed which can be beneficial since more flights can be done in a given time

CI = Time cost / Fuel cost

Question 14

How are the following factors affecting range?

1. Altitude
2. Weight
3. Wind

Answer 14

1. Altitude:
   SFC is best at high altitude, low temp and max rpm
   TAS increases with alt
   (SR = TAS / TSFC x Drag)
   Range increases with altitude
   TSFC starts to increase at very high altitude where efficient rpm is over 90 - 95%
   The higher the aircraft cruises the longer the range.
   This is because TAS is increased and only at high altitude will the engine thrust be low enough to
   operate at its design RPM.
2. Weight:
   Reduced weight influences the total drag curve via reduced induced drag
   Total drag curve gets bent down on the left side
   Max range speed increases due to a shallower tangent line which means that the ratio TAS to FF is higher
   AOA should remain the same
3. Wind:
   HW decreases range and speed increases
   TW increases range and speed should be decreased

In HW increase speed by half the HW component

Question 15

What is Manouvering speed (Va)?

Answer 15

Speed at which the a/c will stall before exceeding it’s design limit load factor in turbulent conditions or where flight controls suddenly are deflected in flight.

Question 16

How does Va change with weight?

Answer 16

Va decreases with weight
For every 2% reduction in weight, reduce Va by 1%
If weight decreases by 20%, decrease Va by 10%

Question 17

Describe how manouvering speed works and why Va has to decrease with weight

Answer 17

Let’s assume stalling angle is 18 degrees
Max load factor is 4

An a/c @ 2’500 lbs is flying at 110 kts and needs an AOA of 4,5 degrees
Decreasing weight to 1’800 lbs and the aircraft now only need 3 degrees AOA to fly at 110 kts
This means that the a/c can experience 6G:s before it stalls which means that the load factor limit will be broken before the stall…
If speed now is decreased to 95 kts with the 1’800 lbs weight, the AOA might have to go up to 4,5 degrees again.
At 4,5 degrees the a/c can will now stall exactly when the load factor limit is reached, which is good

Question 18

Difference between Long range Cruise (LRC) and Max Range Cruise speed (MRC)?

Answer 18

The advantage is that for LRC a 1 percent of range loss
is traded for 3 to 5 percent higher cruise velocity.
Since higher cruise speed has a great number of
advantages, the small sacrifice of range is a fair
bargain A relatively large increase in speed gives a relatively small decrease in range due to increased fuel flow

Question 19

What would a typical Cost Index value be for a B777?

What cost index would MRC and LRC be?

Answer 19

Typical value 90-150
MRC=0
LRC=180

Question 20

What’s the relationship between Angle of attack and best range and best endurance speeds?

Answer 20

Best range always occurs at a smaller angle of attack
than best endurance. It is critical to remember that best range and best endurance both occur at specific angles of attack, regardless of
weight.

Question 21

Why is it better for range to increase altitude?

How can it be seen on a graph?

Answer 21

Parasite drag decreases while induced drag increases
Total drag curve moves to the right but not up. This means that drag/fuel flow is the same but the speed has to increase.
Since total drag moves right the tangent line becomes shallower which means that the ratio of speed to drag/fuel flow increases which increases range

Question 22

Why are jet transports operated at high altitudes?

Answer 22

To make coincident the best operating conditions of the airframe with the best operating conditions for the engine [efficiency]. The higher the altitude, the higher the TAS at the airspeed coincident with the angle for the best L/D ratio [typically 1.32 Vimd]. Only at high
altitude will the thrust required be equal to the drag produced.

Question 23

What can we do to increase range as fuel is burnt?

Answer 23

The cruise speed could be reduced. This is because LRC speeds will occur at a particular pt. on the lift/drag curve. As fuel is burnt weight will decrease therefore the speed will have to be decreased to maintain the best LRC speed.

Question 24

How do we keep max range speed as weight is decreased?

Answer 24

At a lower weight the lift required is less so to be able to keep the same AOA the speed will have to slow down. Your plane has an angle of attack that minimizes induced drag, so as your plane gets lighter, you keep it fixed by dropping speed to adapt to your lower lift requirement.

Question 25

What does the bottom of the drag curve always correspond to?

Answer 25

The same AOA and L/D ratio

Question 26

How do we achive the same L/D ratio as weight reduces?

Answer 26

To achieve the max lift/drag ratio with a lower weight, the aircraft must be flown at a slightly slower speed when gliding and the resulting induced drag will be less.
Only induced drag changes with weight.

1. 2400/240=10
2. 2000/200=10

L/D=Weight/Drag