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2017-11-20

  Notes  

 
 

FUEL  MANAGEMENT
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RANGES,  FUEL LOADS  &  FUEL  CONSUMPTION

 
The physics which never happened ... outside Atlantic Air Combat™
 
 

INTRODUCTION

To put it simply : the amount of time an aircraft can spend in the air is limited by the quantity of fuel carried and how fast the aircraft engine(s) consume the available fuel. The distance that aircraft may travel is then depended on its airspeed during this allocated time.

Other factors influence the above basics elements.  To only mention a few : the aerodynamics of the aircraft, its engine(s) efficiency at various altitude, actual atmospheric pressure and temperature, not forgetting the wind and the transported payload or the aircraft weight in general.

Atlantic Air Combat™ is not a flight simulation game : it's an arcade game with elements of simulation.  As such AAC will do away with atmospherics and thermodynamics, and with weights only taken in consideration to compute acceleration, only the follwing elements will be involved as far as fuel management is concerned :
aircraft fuel load : known or modified
aircraft range : known or modified
aircraft speed (max, cruise, etc) : known or modified
engine power settings : the greater the power output, the greater the fuel consumption

Engine Specific Fuel Consumption (SFC)  &  AAC Aircraft Specific Fuel Consumption (ASFC)

Engines are characterized by a specific fuel consumption (SFC) which can only be seen as a background data as an actual engine functioning at full power at sea level or at cruise settings at altitude are VERY different things.  Example :
 Engine Take-Off Output @ Sea Level SFC Cruise Output @ 35,000 ft SFC
 J57P29W 10500 lbf 0.770 lb/lbf/hr 3350 lbf 0.909 lb/lbf/hr

Atlantic Air Combat™ is not going to use such intricated notions.  From the elements taken into consideration an aircraft fuel consumption (ASFC) will be determined for each aircraft.  Example :
 Aircraft Engines Thrust Cruise Speed
at 80% power
Known Range Endurance Fuel Load ASFC in lb/lbf/hr
 Aircraft 8×10500 lbf 84000 lbf 455 knots 6370 nmi 14 hours 259052 lb 259052÷14÷84000÷0.8=0.275

The ASFC of 0.275 is thus an aircraft related linear value which has little relationship with the known SFC of 0.770 for the chosen engine.  But applying this fictional value the other way around gives the desired range : 8400 lbf × 8 × 14hrs × 0.275 lb/lbf/hr = 258720 lbs (fuel allocation is exhausted, with a little 332 lbs left).  If a different power setting is used than the suggested 80% cruise setting, it will have a direct impact on the effective range of that aircraft in AAC.  Therefore, ranges indicated in AAC are indicative calculated  pieces of information only.

Engines enjoying an extra power level (afterburner or water injection on a jet engine; emergency war power or methanol injection on a piston engine) will see their fuel consumption increase in the same linear manner unless we adopt a different value for power settings beyond 100% (this particular subject is still under scrutiny)

Fuel Load

Atlantic Air Combat™ refers to two weight values for fuel : 7.6 lbs/gallon or 0.76kg/litre or 6.320 lbs/US gallon for jet/turboprop fuel (JPX in AAC parlance) and 4.9 lbs/gallon or 0.49 kg/litres or 4.0799 lbs/US gal for piston engines AvGas (AGS in AAC parlance).  It differs slightly from the internationally accepted value of 7.866 lb/gallon for JP4 fuel and 4.8879 for AvGas(fuel weight ratios at 15°C/59°F and 760mm of mercury barometric pressure).

Payload and Range

Contrary to real life, range in AAC is only dependent on the amount of fuel available and fuel consumption - the weight of the aircraft and the modifications thereof (fuel consumption, released ordnance) are not incorporated in the concept, nor drag nor anything else.

Aicraft Specifications in Atlantic Air Combat™

AAC aircrafts are inspired by aircrafts from early sixties or earlier, and their key specifications (speed, ..., range) are similar.   Still, there are differences.   Because AAC details are approximated or because AAC has increased or lowered performances compared to the original.   As far as fuel management is concerned, these differences could originate from :

aircraft fuel load

: standard or supplementary, internal and/or external, fuel capacities may be increased or decreased

engine change

: several vintage aircrafts  are fictionally "rebuilt" and "re-engined" for AAC purposes

These modifications are designed to meet a specific purpose or the result of AAC authors fantasy.   These revised performances rest on the following principles to keep them plausible and easy to guesstimate and compute.

EFFECT OF MODIFIED FUEL LOAD ON RANGE

The basic formula for range calculation could be expressed as follows :

 Range : speed
SFC
× lift
drag
× Natural
Logarithm of
        (aircraft full weight)        
(aicraft full weight - fuel weight)
Source

However, a simplified formula is used in AAC to guesstimate the range of a modified aircraft.  Speed, SFC and lift-to-weight ratio are disregarded as they would be the same as the reference aircraft to which it would compare. This leaves a simple logarithm computing related to a known range value.

 Range : will be related to : Natural
Logarithm of
        (aircraft full weight)        
(aicraft full weight - fuel weight)

Then a simple proportional rule is applied to determine the indicative range of the modified aircraft.  Example : the AAC BB52J10, inspired by the B-52B, carries more (external) fuel than the original :

  Weight
Empty
Fuel
US gal
Weight
×6.328
TOW           Range
Ferry
B-52B 165280 39550 250272 415552 Log of 415552 ÷ 165280=2.5142 0,921967   6380 nmi
BB52J 165280 43550 275584 440864 Log of 440864 ÷ 165280=2.6674 0,981095 0,981095 ÷ 0,921967 × 6380 = 6789 nmi

EFFECT OF MODIFIED SFC ON RANGE

The only practical use of known engine SFC is to figure out the effect on indicative range by analogy : if an aircraft, carrying so much fuel would see its engine fictionally replaced by another one with a different SFC the modified aircraft would have a proportionally modified range:

  Engine SFC Range
Ferry
            Range
Ferry
B-52B J57P29W 0.770 6380 nmi              
BB52J J57P31W 0.700     6380×0.770÷0.700     7018 nmi

EFFECT OF MODIFIED FUEL LOAD  and  SFC ON RANGE

A fantasy aircraft variant may end up with both a fictional fuel load and engine, supposedly with a different SFC.  In that instance, the new indicative range will be guesstimated in two steps :

 1 
Engine SFC Range
Ferry
            Range
Ferry
B-52B J57P29W 0.770 6380 nmi              
BB52J J57P31W 0.700     6380×0.770÷0.700     7018 nmi
 2 
Weight
Empty
Fuel
US gal
Weight
×6.328
TOW            
B-52B 165280 39550 250272 415552 Log of 415552 ÷ 165280=2.5142 0,921967   7018 nmi
BB52J 165280 43550 275584 440864 Log of 440864 ÷ 165280=2.6674 0,981095 0,981095 ÷ 0,921967 × 7018 = 7468 nmi
 
 

___________

 

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Summary

Year

L/D

-

Concorde

1963

8.0

-

Flying Wing

-

17.0

-

-

-

-

Boeing

B52A

1954

20.0

N.A.

P51D Mustang

1941

91.0

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Subsonic Jet Aircrafts

Year

L/D

Airbus

A320

-

17.0

Boeing

B707.320

-

19.4

Boeing

727.200

-

16.4

Boeing

747.100

-

17.7

Boeing

767.200

-

19.0

Douglas

DC3

1935

14.7

Douglas

DC8

-

17.9

Douglas

DC9

1966

16.5

Douglas

DC10

-

17.7

Ford

Trimotor

1927

12.0

Fokker

50

1966

16.0

Lockheed

L1011 Tristar

-

17.0

Wright

Flyer 1

1903

8.3

-

-

-

-

Birds

L/D

House Sparrow (passer domesticus)

4

Herring Gull (jarus argentatus)

10

Common Tern (sterna hirundo)

12

Albatros (diomeda exulans)

20

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-