Cruise

Question 1

What is specific range?

What factors will increase and decrease SGR?

Answer 1

Distance per unit fuel.
SGR= GS/FF in Nm/kg

Increase SGR with:

• Increase in Mach number
• Increase in L/D ratio

Decrease SGR with:

• Increase in SFC
• Increase in weight

Question 2

What is achieved during cruise at ‘max range Mach number’ (Mmr)?
Draw the graph for it

Answer 2

Mmr corresponds to the minimum fuel consumption and max distance per unit fuel.

Question 3

How does weight affect Mmr?

Draw the graph

Answer 3

As a/c weight decreases during cruise (PA constant), specific range increases, as the Mmr decreases.
So if you keep a constant Mach number at a constant altitude, the efficiency will decrease.

Question 4

How does pressure altitude affect Mmr?

Draw the graph

Answer 4

If weight is kept constant and PA increases, Mmr increases.

Question 5

What is an advantage of a long range cruise Mach number (Mlrc)?
Draw the graph

Answer 5

Mlrc has a 1% higher fuel burn for a relatively larger increase in speed.
SR(LR)=0.99SR(MR)

Question 6

When is Mlrc advantageous over Mmr?

Answer 6

Coat index (CI)= cost of time/cost of fuel

If CI is high (cost of time is large), you will want to arrive faster to travel at a speed close to VMO.
If CI is low (cost of fuel is large), you want to burn fuel the most economically at the expense of time in the air, so travel closer to Vmr

A CI of 30kg/min means the cost of running per min is equivalent to 30kg of fuel

Question 7

How does PA affect Mlrc?

How does weight affect Mlrc? Draw graph for this one

Answer 7

Increase in PA whilst a constant weight will increase the Mlrc.
Increase in weight whilst a constant PA will increase the Mlrc.

Question 8

What is the equation for direct operating costs?
Draw a graph for this
Where does Mmr and Mecon lie on this graph?

Answer 8

DOC= Cc + CfF + CtT

Where Cc is fixed costs
Cf is fuel costs per unit fuel and F is fuel units
Ct is time related costs p/h and T is trip time

Mmr lies at the minimum fuel cost.
Mecon lies at the DOC minimum

Question 9

How does PA and weight affect Mecon?

Answer 9

PA increases with constant weight: increases Mecon

Weight increases with constant PA: increases Mecon

Question 10

How does cruising at a constant Mach number affect fuel consumption?
Draw a graph

Answer 10

As weight decreases in cruise the gap between the Mach number and Mmr (which decreases) increases, so this means fuel consumption increases as it is not operating at the optimum.

Question 11

What must happen to altitude if you want to maintain optimum efficiency whilst maintaining Mach number?
Draw a graph

Answer 11

As weight will decrease, to maintain M and efficiency you must climb

Question 12

For a given Mach number if the a/c is flying at the optimum altitude, what L/D occurs?

Answer 12

Is at the best L/D ratio.

Any increase in altitude will decrease the max L/D (Cl/Cd).

Question 13

How does the optimum altitude change with weight and Mach number? What equation can be made?
Draw a graph

Answer 13

For a given PA if weight increases, optimum altitude decreases and SGR decreases.
Decrease in Mach number decreases the optimum altitude.
W/Ps=k

Question 14

How does HWC/TWC affect Mmr and SR?

Answer 14

HWC: decrease SR and increase Mmr
TWC: increase SR and decrease Mmr

Question 15

Why might flight at a lower altitude than the optimum be beneficial?

Answer 15

If the there are more favourable winds at a lower altitude, SGR may increase although the SR may decrease due to increased fuel consumption. However the increase in SGR at some point is more beneficial than the SGR at the optimum altitude.

Question 16

How does temperature and weight affect the max cruise altitude (in terms of limit Mach number cruise rating)?

Answer 16

Each engine has a limited max cruise rating indicating the max temperature the engines can sustain.
Weight and altitude constant, temperature increase… decrease limit M#.
Temperature and altitude constant, weight increase… decrease limit M#.

Question 17

What is the max cruise altitude (in terms of TA vs TR)?
How does weight, temperature and M# affect this altitude?
Draw a graph

Answer 17

If an a/c flys at a constant Mach number, thrust must be increased if this Mach number is to be maintained at a higher altitude. The max cruise altitude here is the altitude at which for a given weight the Mach number can be maintained with max cruise thrust.
An increase in temperature, weight and M# will all decrease max cruise altitude.

Question 18

Draw a graph explaining lift range.
What is the equation for lift limit?
What defines these limits?

Answer 18

LL=mg=0.7SPsCl(max)M^2
Note) each Cl(m)M^2 corresponds for a Ps (PA) for a given weight.

Low M# limit due to stall of airflow separation due high AoA, low IAS.
High M# limit due to stall due effects of compressibility (shock)

Question 19

What is load factor equal too? (S/L, turn and lift limit)
What is the buffet limit?
When on this graph is M#(min)=M#(max)?
Draw a graph (refer pg144)

Answer 19

LF(S/L) = L/W
LF(turn) = 1/cos0
LF(LL)= [0.7SPsCl(m)M^2] / mg

Buffet limit is essentially where a decrease/increase in speed at a given weight/altitude will result in a stall.
Coffin corner is when M(min)=M(max) and is also the max LF

Question 20

How does altitude affect LF(max) and lift range? Draw graph.

Why is there a load factor limit and what is it typically set to?

Answer 20

Increase in altitude will decrease LF(max) and decrease lift range.
Operationally (not regulatory) load factor limit is set to 1.3 as this way it is certain buffeting will never occur. If the a/c is lighter, you can fly higher before this is reached (1.3 buffet limited ceiling).

Question 21

What is the highest recommend altitude set as?

Answer 21

Lowest if:

• Buffet ceiling
• Climb ceiling (300ft/min)
• Max certified altitude (structural)
• Max cruise altitude (thrust limited)

Question 22

Describe a step climb.

Draw a diagram.

Answer 22

As weight decreases, optimum altitude will typically increase.
A climb of 2000ft will occur in RVSM airspace and below FL290.
A climb of 4000ft will occur above FL290.
+/-2000ft of optimum altitude will give 99%SR(max).