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?

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.

That sounds fun, what are you making it for?

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.

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?

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