wind-force calculation

[tourist]

It is a skinny, mylar blimp.
Basically, it is a 84" by 9" cylinder. Its total mass will be 80g. I am at a loss as to how to find the effective force exacted upon this thing from the wind. :confused:
Any ideas? I'm thinking 10mph is a good figure.

[JetPlaneFlyer]

To calculate the drag force due to the wind you would use this formula:

Fd = 1/2.p.v^2.Cd.A

Where Fd = drag force
p = Air density
V = Wind Velocity (or forward speed)
Cd = Coefficient of drag
A = Reference Area

The coefficient of drag for a cylindrical shape would be something in the order on 0.3 'end-on' and 1.25 'side on'

It's explained more fully here: http://en.wikipedia.org/wiki/Drag_equation

Hope this helps.

Steve

[JetPlaneFlyer]

To put some rough figures on it...

If the wind comes ‘end on’ the force due to drag of the gas cylinder alone (no allowance for the gondola) at 10mph would be around 0.24N (24g)
‘Side on’ the force increases greatly to something like 7.31N (745g)

Gut feeling says this is underestimating the 'end on' drag due to my assumption on the Cd being a bit off.. Do a web search and you may find some better Cd figures for long cylinders. Also any gondola will increase the drag significantly.

[Tom Harper]

We've been here before - at length!

The blimp floats in the the air mass. It cannot be acted upon by 'wind'.
Unless it is tethered.

Tom

[JetPlaneFlyer]

Unless it is tethered.


That's exactly what i thought the question was about... Or maybe the 'relative wind' caused by forward flight if it's fitted with some sort of propulsion.

As Tom indicates... if the blimp is simply blowing along freely in a steady state wind then there is no wind 'force' felt by the craft because the relative wind velocity between the blimp and the air it's travelling in would be zero.

[tourist]

yes, 'relative wind'. Thanks, guys.

[tourist]

We've been here before - at length!

The blimp floats in the the air mass. It cannot be acted upon by 'wind'.
Unless it is tethered.

Tom

I tried searching, but I have bad luck with search engines.
search summary:
wind+blimp=no results
wind+airship=no results
drag+blimp=no results

Also, how can moving air molecules avoid transferring their energy to a surface? How can the blimp move in the same direction as the air molecules if they cannot act upon it? The blimp has mass. It cannot act just like a feather. The resultant vector of the billions of air molecules striking the mylar has to have some effect, right?

[Tom Harper]

More than you ever wanted to hear about the subject is located at:


http://www.rcgroups.com/forums/showthread.php?t=522513&highlight=air+speed+ground+speed

[Brandano]

Hmm, I tend to be a bit more praqmatical on this sort of things... tether it to a pole with a pulley on the end, tie the tether to a weight and plonk the weight on a set of scales. Any weight taken off the "no wind" weight is the amount of force exerted on the blimp.