Re: Fuel consumption

Date:         18 Jun 97 02:36:56 
From: (Darren Rhodes)
Organization: Loughborough University
References:   1
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On 16 Jun 97 21:35:40 , Marc Schaeffer <> wrote:

>This question is related to fuel consumption for commercial aircraft. In
>several books I found that the fuel consumption for a B737-500 (which I
>take as example) is of 2700 l/h at cruising altitude and at normal
>cruising speed (908 km/h). I will call this value FCCA.
>I would like to know the typical values for the following parameters :
>- Fuel consuption from starting of the engines until reaching of the
>cruising altitude. This assumes a plane at MTOW, departure 10 minutes
>after having started the engines, a constant *typical* climbing rate.
>The value can be in liters or US gallons or l/h or gal/h. This value
>will be called FCTO.
>- What time would it take to reach cruising altitude with these
>- Fuel consumption from the moment where the aircraft leaves the
>cruising altitude until it touches ground. This assumes again a constant
>descend rate, MLW, no reversing of engines and shutdown of the engines 5
>minutes after touching ground. This value will be called FCGR.

In terms of the engine, it common to quote fuel consumption as lb/hr
per pound of thrust produced. Now thrust decreases with increasing
altitude and speed. Therefore SFC (in lb/hr/lb) will increase
throughout the climb. A typical takeoff value may be 0.30 lb/hr/lb
whilst the cruise value may be 0.55-0.60 lb/hr/lb.

For the climb to cruise, the climb rate will not be constant. Aircraft
normally climb at constant EAS, thus true airspeed increases during
the climb. Also thrust decreases during the climb so the climb rate
will decrease all the time. Top of climb for aircraft performance
calculation is normally assumed to at a climb rate of 300ft/min. Time
to climb for short-medium haul aircraft is 25 mins or less. The
747-400 on the other hand takes around 33 mins.

Thus, the problems with developing an analytical expression are that
SFC, thrust, drag and climb rate all vary during the climb.

The only way to get reasonable answers is to do a stepwise calculation
with say 5,000ft altitude steps for the climb and similar for descent.
You can then at least detemine thrust at the start and end of each
step and hence average climb rates. The time taken can then be used to
determine the fuel burnt and hene the reduction in aircraft weight.

The weight reduction is very small though, no more than 2-3 percent.

Hope that helps