Aircraft range equation

[earthgoodboy]

hi all,

Im currently a aerospace engineering and I have some questions that I have been wondered about the range equation.

I just studied about the breguet range equation (R=v/sfc L/D ln(wi/wf)). The equation is pretty simple within many assumptions such as constant speed constant L/D... How can the manufacturer such as Boeing or Airbus come up with the quoted range in the aircraft's specification? Since the breguet's equation is quite ideal and far from realistic, should there be a better way to determine the range?

is there a more precised formula has been developed? or the value just comes from the flight test alone?

I also think that there are many conditions of payload/fuel load, but how come the manufacturer come up with only 1 range in the specification? what condition do they usually use?

I know there are many questions and takes long to answer, if anyone could suggest me articles or books would be already much appreciated.

thanks

 

[GregLocock]

They model each part of the flight, under specific assumptions. Breguet is a nice piece of maths but ignores too much of the flight envelope of a current a/c to be helpful.

To an aspiring aeronautical engineer I can do no more than recommend the NASA/LARC archive of papers, which will give you a great historical perspective, and an appreciation of the level of analysis people did in the years BC.

http://ntrs.nasa.gov/?method=aboutntrs

Cheers

Greg Locock


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[SWComposites]

this is a good starting ref:

http://www.amazon.com/gp/search?index=books&linkCode=qs&keywords=9024727243

 

[rb1957]

as above they break a flight down into many stages/segments small enough that you can make simplifing assumptions (reasonably constant altitude, probably constant M, etc) and determine distance flown for each stage.

 

[earthgoodboy]

thanks for all the reply

rb1957, is that what the manufacturer really do to estimate the range. do know have any paper or ref i can look at?

 

[rb1957]

it's what i'd do. OEMs don't need to simplify things down to a single simple equation that (as you've pointed out) overly simplifies the variables at play.

a piece-wise discrete segmentation of an overall flight is an easy way to restrict the simplifying assumptions (constant this, constant that) that you'd get a reasonably accurate answer.

i'd expect that they run flight simulations that either are piece-wise discrete segments, or continuously varying varibles.

Raymer might go into this in more detail, or other Flight Performance texts.

 

[moon161]

Raymer, aircraft design a conceptual approach is a good book.
Also look at Jan Roskam, Aircraft design, vol 1, preliminary sizing. Worked examples in both, IIRC.

Both show the method of writing several design flight segments using simplifying assumptions and performance/weight estimates based on trade studies. The product of all your segment fuel fractions gives you your takeoff weight over structure & payload fraction.

 

[LefterisAeronautics]

Usually the range is a given specification I think. You take for granted for example that the range will be 3000 miles and you calculate the rest based on that.

Lefteris
Dep. of Mechanical Engineering and Aeronautics

 

[IRstuff]

For any realistic development, you start with a specification, and design to the specification. In the case of an aircraft, you start with a maximum range, payload, etc. Then, you design the plant to meet the requirements. Any single equation is unlikely to capture all the requirements

TTFN
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[rb1957]

range as part of a specification is a target ... once you've defined the design (drag, speed, fuel quantity) you might find that you can only achieve your desired range with a limited payload ... which is a typical range-payload chart.

Quando Omni Flunkus Moritati