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Homework Help: Calculating drag

  1. Apr 11, 2006 #1
    i am having trouble calculating various drag co-efficents for a dc -10 in cruise.

    i have found this data:
    Thrust: 153000 lb
    density: 0.00085 slug/ft
    V= 852ft/s
    AR= 7.5
    e= .8
    S= 3958ft^2
    Cl= 0.5

    I have also found the formulas
    Cd= D/ (density x S x V^2 /2)
    Cd= Cdo + ((cl^2) / (pi x AR x e))
    Cdi= (cl^2)/ (pi x AR x e)

    i have tried these but get odd answers. i need help to know whether my mistake is my data, my formulas or my calculation.
    help with graphing this data vs airspeed would also be appriciated
  2. jcsd
  3. Apr 11, 2006 #2


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    It appears that you have all the information you need to do the calcs. Perhaps it is a units issue? If you show us what you have we might be able to pinpoint something for you.
  4. Apr 11, 2006 #3


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    Gold Member

    Is that value for S right?

    It looks rather large indeed, but then I'm too snobby to work in imperial units to find out for sure.
  5. Apr 12, 2006 #4
    the answers that i got using these formulas were:
    for Cd = 0.125
    for Cdi= 0.0137
    for CDo = 0.11
    as cdi and cdo were so different i thought that there must be a problem.
    i was also unsure if the thrust value given was for one engine or for all three. is it an appropriate value for three engines?

    -in response to brewnog S= 317 m ^2
  6. Apr 12, 2006 #5


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    The thrust value you give is consistent with three engines. 50,000 Lbf sounds about right for a single engine.

    Did you do a dimensional analysis to make sure your units were all cancelled out?
  7. Apr 17, 2006 #6
    i did a dimensional analysis. but i'm pretty sure that i'm using all the right units. still having problems though. are the formulas correct and is the oswald efficency factor an approptriate value for a DC - 10?
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