# Drag equation for a blimp needed

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I'm designing a blimp - a non-rigid, helium filled airship of about 15 metres (45 feet). Diameter about 3 metres (9 feet).

I'd like to know what drag the blimp will produce so I can select the motor sizes and consequently battery sizes - all of which reflects into the payload ability.

How can I calculate the drag? I know that it is largely done experimentally using a wind tunnel, but what is the draq equation that I am to begin with?

What is a good way to measure drag in a wind tunnel?

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well you could get fill the blimp get a spring balance and connect it up to your blimp and then drag it along and see how much force it requires for a certain speed. it should be somewhat accurate. if you really need to you could make a data plot and work out a rough equation for its drag with respect to velocity.

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That sounds fun, what are you making it for?

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I'm designing a blimp - a non-rigid' date=' helium filled airship of about 15 metres (45 feet). Diameter about 3 metres (9 feet).

I'd like to know what drag the blimp will produce so I can select the motor sizes and consequently battery sizes - all of which reflects into the payload ability.

How can I calculate the drag? I know that it is largely done experimentally using a wind tunnel, but what is the draq equation that I am to begin with?

What is a good way to measure drag in a wind tunnel?[/quote']

Cross sectional area in feet X air speed in feet/s squared X .0012 slugs/cubic feet ( half the density of air at sea level)

X Drag Coefficient (maybe 0.1 to 0.2)

should work out to drag in lbs.

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Thanks lads for your assistance. The blimp is for aerial photography (AP). It's just a modest sized blimp for sailing around during the day to take pics of real estate. It doesn't need to be 15 metres for AP but I intend to put it to other uses.

Thanks JCM for that equation... got the equation somewhere in metric too?

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Thanks lads for your assistance. The blimp is for aerial photography (AP). It's just a modest sized blimp for sailing around during the day to take pics of real estate. It doesn't need to be 15 metres for AP but I intend to put it to other uses.

Thanks JCM for that equation... got the equation somewhere in metric too?

Cross sectional area X velocity squared X density of the fluid (air) divided by 2 X Cd (drag coefficient again probably 0.1 to 0.2)

I think if you plug in the numbers in any units you want you get a force. I generally use f-p-s for aerodynamics and always have that equation in my head but I think it works out in metric with the same coefficient (though the coefficient can change with velocity, fluid and scale due to viscous effects on the flow

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