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Help using a manometer

  1. Jan 18, 2015 #1
    First of all I'd like to apologise for my question, as it will probably be embarrassingly easy to answer but I don't have any idea.
    I've recently been given the role of running a manufacturing centre for F1 in Schools and everything is going well apart from one area.
    We have a wind tunnel to check the aerodynamics of small F1 cars made from balsa wood. The wind tunnel comes with a manometer which apparently gives an accurate reading for wind speed and also a display showing the amount of drag in grams.
    Could someone please explain what this means and how I can use the figures?

    Regards Dean
  2. jcsd
  3. Jan 18, 2015 #2
    Basically the instrument will allow you to measure the aerodynamic drag in grams on the F1 car for the selected wind speed. Different cars will have differing amounts of drag at the same speed.

    The car is mounted on or against the test stand and the wind speed adjusted to the desired speed. The manometer allows you to match speeds from test to test or at least measure and compensate for the difference. The drag in grams indicates how much force is needed to overcome the drag.

    I clipped this out of wikipedia:
    FD is the drag force, (This is indicated in grams on your meter)
    f7f177957cf064a93e9811df8fe65ed1.png is the density of the fluid, (Lookup on a table for your altitude and temperature)
    v is the speed of the object relative to the fluid, (Your manometer will indicate this)
    A is the https://www.physicsforums.com/wiki/Cross_section_(geometry) [Broken], and (You will have to measure the car for this)
    CD is the https://www.physicsforums.com/wiki/Drag_coefficient [Broken] – a https://www.physicsforums.com/wiki/Dimensionless_number [Broken] number.
    Last edited by a moderator: May 7, 2017
  4. Jan 18, 2015 #3
    Sorry the links back to Wiki got broken in the copy/paste.
  5. Jan 18, 2015 #4

    jim hardy

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    Does your manometer look something like this?
    Your manometer problem then is to convert from whatever are the units on your manometer to miles-per-hour or feet-per-second or something ?
  6. Jan 18, 2015 #5
    Doh! I translated manometer to anemometer.

    Edit: Although the manometer may be scaled for windspeed at STP.
  7. Jan 18, 2015 #6

    jim hardy

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    Dean are you a non-science major who's been thrust into this program unprepared?
    That's okay it has happened to me .

    Following is so very elementary it is either just what you need, or a farce.
    If the latter please do not take offense - it's just a shot in the dark.

    You mentioned "figures"
    can you post one, and give us a little more about your background?
    I looked up 'F1 in schools' , looks like a great science program
    but school administrations do not always match assignment to background.

    Dont take following as "talking down"


    Okay, maybe this is the snag

    engineers love to see everything on a graph, not a spreadsheet

    Basics of a graph:
    It shows the interrelationship between two things that you measure , those two things are called your "variables"
    To make a graph one establishes two scales one horizontal and one vertical
    like this


    Horizontal scale is named the "abscissa"
    vertical scale is named "ordinate"
    but hardly anyone calls them that
    we say horizontal and vertical because that's more mnemonic, or often X for horizontal and Y for vertical.
    and where they intersect is called "origin" or sometimes "zero, zero"

    To gather data for his graph
    one controls one of his variables , observes and writes down how the other behaves.
    He gets a series of paired values for his two variables.
    For each pair of values he puts a dot on his graph.

    it is traditional to use horizontal for the controlled variable
    and vertical for the observed variable
    so one chooses scales for horizontal and vertical that encompass all his numbers

    On the numberless graph here

    horizontal is controlled variable, speed
    vertical is observed variable drag
    probably for a seaplane taking off from water -
    you can see drag first increase with speed than decrease as the hull transitions from displacement to planing .
    (Ever water ski? If so you've felt that transition...)
    and as hull leaves the water there's no more water drag .

    So you'll measure drag at increasing speed and graph the results
    kid with lowest drag curve wins.

    Does your F1 apparatus also measure vertical force?
    Good multi-axis wind tunnels go back to Wright Brothers
    if you're near Dayton Ohio go see theirs at Air Force Museum.
    Last edited: Jan 18, 2015
  8. Jan 18, 2015 #7

    jim hardy

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    strictly speaking a gram is a mass
    and drag is a force so should be in Newtons,

    but most people dont use Newtons

    you'll be okay - just dont be surprised if your students try to trip you up.

    A gram force miht more correctly be called a 'pond' or 9.8 milliNewtons
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