6. cruise i

6. CRUISE (I)6. CRUISE (I)

Performance JAR 25Performance JAR 25

CRUISE (I)CRUISE (I)

Introduction

Specific range

Maximum range cruise

Long range Cruise

INTRODUCTIONINTRODUCTION

While aircraft performance during takeoff and initial climb is mainly related to airworthiness requirements, the objective of studying the cruise performances is decreasing Direct Operative Costs (DOC).

These operative costs include:

• Fixed costs (taxes, insurance…)

• Flight time related costs (crew, maintenance…)

• Fuel-consumption related costs

These DOCs can be minimised by making the right speed and flight level choices.

INTRODUCTIONINTRODUCTION

Due to compressibility effects of the air, the polar curve of the aircraft changes at high Mach numbers. This will have a notorious influence on cruise performance.

SPECIFIC RANGESPECIFIC RANGE

The specific range is the distance covered per fuel unit. Therefore, the SR is:

In addition, the TSFC (Thrust Specific Fuel Consumption) is the relationship between the Fuel Flow and the thrust provided with that FF:

FF

Cs · M

FF

TAS SR

T

FF TSFC


Remember that, in straight and level flight, thrust equals drag:

Joining together the three previous formulas, we obtain the final one for the specific range:

W· C

C D T

L

D

W· TSFC

C · CC

M SR

s

D

L


Now we are ready to make some conclusions. SR depends on:

W· TSFC

C · CC

M SR

s

D

L

Aerodynamic characteristics of the aircraft

Engine performance Weight


If TSFC or weight are increased, the specific range will decrease. On the contrary, high M (CL / CD) values will determine great specific ranges.

In the graph below it can be noted how the value of M (CL / CD) changes at different Mach numbers.

M (CL/CD)

M

MAXIMUM RANGE CRUISEMAXIMUM RANGE CRUISE

For a given altitude there is a speed that will give the largest range. That speed is known as maximum range cruise speed (MMRC). The advantage of the MMRC is that the fuel consumption for a given distance is at its minimum. It also corresponds to the maximum distance an aircraft can fly with a given fuel quantity.

SR

MMMRC


MMRC decreases as the weight decreases due to fuel burn-off, at a given altitude.

PA = constant: Weight ↓ MMRC ↓


If we consider a given weight (instead of a given altitude), MMRC increases as the pressure altitude increases.

W = constant: PA ↑ MMRC ↑


On the other hand, for a given speed there is an optimum altitude that will give the maximum range. This altitude increases as weight decreases.


We have seen different maximum range speeds and altitudes when the other parameters remain constant. These provide relative maximum ranges; that is, the maximum range for a specific altitude or Mach number.

The absolute maximum range can only be achieved at a unique altitude and Mach number (for a given weight). This range would be a “maximum of maximums”.

The maximum range is given by the maximum value of M (CL/CD).


TEMPERATURE INFLUENCE

Temperature has a negligible effect over range. If the aircraft flies through a higher temperature area at a constant Mach number, the thrust levers must be moved forward to compensate the loss of thrust, thus increasing fuel flow.

But at the same time, that temperature rise increases the TAS, balancing the equation to keep the SR* constant.

* Remember that SR = TAS / FF.


WIND INFLUENCE

If we consider ground SR (GS/FF instead of TAS/FF), wind has influence over it: tailwind increases SR and headwind decreases it.

In addition, MMRC will also change depending on the wind: increases with headwind and decreases with tailwind.


CENTER OF GRAVITY INFLUENCE

If center of gravity is moved rearwards, drag will be reduced and hence fuel consumption. Instead of the tail having high negative angles of attack (fwd CG), it will have low positive ones. This will increase the specific range.

However, cruise performance charts discard this effect as far as Maximum Range and Long Range speeds are concerned. The CG position does not change MMRC.

Upwards force to compensate positive pitching moment.

Picture has been exaggerated for better understanding

Low angle of attack, low drag.

LONG RANGE CRUISELONG RANGE CRUISE

MMRC is usually very close to the point of minimum drag. Remember that below that point the speed becomes unstable. Thus, in practice, continuous thrust adjustments would have to be done to maintain MMRC. These adjustments would result in increased fuel consumptions, so that the maximum range could not be achieved.

Since MMRC is impractical a higher speed was established, known as long range cruise speed (MLRC). Flying at MLRC sacrifices range in 1% only, and reduces flight time considerably.

This speed is commonly used if one engine fails during the cruise phase.


The 1% loss compared to the maximum specific range is largely compensated by the cruise speed increase due to the flatness of the curve.


All factors affecting MMRC also affect MLRC in the same way. To summarise, we can say that:

FOR A GIVEN WEIGHT:

MLRC increases if pressure altitude increases.

FOR A GIVEN ALTITUDE:

MLRC decreases if weight decreases.

FOR A GIVEN MACH NUMBER:

Optimum altitude increases when weight decreases.


WIND INFLUENCE:

MLRC increases with headwind, and decreases with tailwind.

TEMPERATURE INFLUENCE:

None.

CENTER OF GRAVITY INFLUENCE:

A position backwards will increase specific range, but change of MLRC is not considered.



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6. cruise i