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Thread: Question - thrust and lift basics

  1. #1
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    Default Question - thrust and lift basics

    I'm new to hovercrafts but have an intense interest in them. I have a couple of basic questions. I've read through several of the documents on this site concerning thrust and lift.



    Thrust

    1. From the equation in the thrustcalc.pdf, if I know a fans volume(cfm) and velocity(fpm)outputs @1"SP and the density of air, can I simply calculate Thrust as



    thrust=volume*velocity*density of air?



    2. Is 1" SP reasonable for a thrust fan?



    3. How does thrust translate to speed and acceleration in ideal (static) conditions?



    Lift

    1. What SP is reasonable for a lift fan?



    Thank you,

    Gilbert

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    Default Re: Question - thrust and lift basics

    This can all be done in units (OK my physics is a wee bit rusty but I'd hope I'm near the mark).

    1 - If you accelerate a mass of air m backwards to velocity V, then the hover of mass M will accelerate to velocity v (note capital letters )



    So for the thrust you need the speed of the airflow, and the quantity (which is m^3 * kg/m^3 - multiply to get kg )



    So yep that's essentially correct.



    2. - not sure I'll let somone else do that..



    3. - the key is in the equation N = kgm/s^2 (Thrust = mass*acceleration). So it's simply the Thrust in newtons divided by the mass of the hover = the acceleration (in an ideal world)

    In reality you'd have to subtract the resistive forces first, then work out F=ur /etc to get the top speed.







    Ha that's the most complicated thing I've done at work today

    Time to Jakes, 6 hours! (as long as the M6's clear )

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    Default Re: Question - thrust and lift basics

    Quote Originally Posted by Toby
    Ha that's the most complicated thing I've done at work today

    Time to Jakes, 6 hours! (as long as the M6's clear )


    I hope you are going to Charlies not Jakes as there will be racing going on there!!!!



    As for the original question I am sure that someone will answer it tuesdayish when they get back from the race meeting

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    Default Re: Question - thrust and lift basics

    Hi



    Thrust

    1. From the equation in the thrustcalc.pdf, if I know a fans volume(cfm) and velocity(fpm)outputs @1"SP and the density of air, can I simply calculate Thrust as



    thrust=volume*velocity*density of air?




    Yes



    2. Is 1" SP reasonable for a thrust fan?



    I normally use 50Pa which is equivelent to 0.2" water-gauge. 1.0" water-gauge would predict less flow and hence less thrsut, and so be more conservative.



    3. How does thrust translate to speed and acceleration in ideal (static) conditions?



    Acceleration = force / mass. Watch the units if in Imperial, as you get Accn = lb / lb which is nonsense! You finish up having to multiply by 'g' in Imperial but not in SI.









    Lift

    1. What SP is reasonable for a lift fan?





    This depends heavily on the craft size, weight and internal ducting details. For a typical small craft it might be in the region of 500-1000Pa.



    Cheers

    Ian




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    Default Re: Question - thrust and lift basics

    The others have pretty well answered your questions, so I will just add these comments.



    If working from manufacturers data (eg. The fan calculator programs) it should be bourne in mind that the static pressure figures are usually calculated from the fan total pressure. In most hovercraft thrust fans the greatest discharge velocity is around the periphery of the duct. If you measure the Pst at the periphery it will be lower than expected.



    For the purpose of calculation, you can simply consider the Pst to be Ptot-Pvel



    Velocity Pressure (Pvel) = 0.5 x Rho X Velocity^2 where Rho = 1.22Kg/mtr^3 (conventional)



    Like Ian, I would normally use figures of 50-100 Pa max (0.2 - 0.4 Inwg)

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