# Fan Descender Design Project

al_garnett
I have been tasked to design a controlled descender to be used on a building site to lower tools/rubble to the ground.

I'm trying to design a product like a fan descender like that seen at powerfan.co.uk

Could anyone help me with the theory side of its operation. Obviously it uses the fans air resisstance, but i need to complete calcs to prove my design would work. If anyone could point me in the right direction or to a suitable source of info it's be greatly appreciated.

Thanks
Alex

Mentor
I have been tasked to design a controlled descender to be used on a building site to lower tools/rubble to the ground.

I'm trying to design a product like a fan descender like that seen at powerfan.co.uk

Could anyone help me with the theory side of its operation. Obviously it uses the fans air resisstance, but i need to complete calcs to prove my design would work. If anyone could point me in the right direction or to a suitable source of info it's be greatly appreciated.

Thanks
Alex

Why use a fan? Can't you just use a rope and pulley?

Why use a fan? Can't you just use a rope and pulley?

A rope and pulley alone won't dump the potential energy of the descending mass.

OP: What is your education level? Can you equate rate of descent and weight of load to a power output? (via P = E/t and E = mgh)
From there you'll have a power output. The Powerfan appears to use a cone pulley to give a variable ratio presumably to slow the rate of descent as the descender nears the ground. This is important - you need the mass to accelerate only a portion of the descent otherwise the descent will either take too long (low constant a) or the impact speed will be to great (high constant a).

you need to transfer the power to the air via the fan- research fan power & shaft power - this may involve a gearbox.

al_garnett
A rope and pulley alone won't dump the potential energy of the descending mass.

OP: What is your education level? Can you equate rate of descent and weight of load to a power output? (via P = E/t and E = mgh)
From there you'll have a power output. The Powerfan appears to use a cone pulley to give a variable ratio presumably to slow the rate of descent as the descender nears the ground. This is important - you need the mass to accelerate only a portion of the descent otherwise the descent will either take too long (low constant a) or the impact speed will be to great (high constant a).

you need to transfer the power to the air via the fan- research fan power & shaft power - this may involve a gearbox.
From the two equations i have calculated the gravitational potential energy that the mass to be descended has. And using P=E/t, have set a time for descent, t. I now have the Power output in J/s. Am i right in saying this is the amount of energy i must dissipate each second for a constant velocity of the mass whilst descending?

Mentor
A rope and pulley alone won't dump the potential energy of the descending mass.
No, but whatever is holding the other end of the rope could.

Using what is basically an autorotating helicopter seems impractical to me. It has to be lifted back up to reuse and there isn't a good reason I can see for the device to be dropped off a building, to possibly be damaged or hurt someone. There is no reason why the device couldn't be attached to the axle of a winch; it would work exactly the same.

No, but whatever is holding the other end of the rope could.

Using what is basically an autorotating helicopter seems impractical to me. It has to be lifted back up to reuse and there isn't a good reason I can see for the device to be dropped off a building, to possibly be damaged or hurt someone. There is no reason why the device couldn't be attached to the axle of a winch; it would work exactly the same.
The device stays on the building, as the load/person is lowered its energy is dumped to the air by the fan via the pulley. Most rowing machines I've seen use the same concept. I'd guess traditional lowering devices use consumables (ie brake pads and discs) so there is an advantage there - less wear, potentially more reliable.

From the two equations i have calculated the gravitational potential energy that the mass to be descended has. And using P=E/t, have set a time for descent, t. I now have the Power output in J/s. Am i right in saying this is the amount of energy i must dissipate each second for a constant velocity of the mass whilst descending?
Yes that's right. It's gets complicated when you consider fans as they aren't linear devices and also the issue of acceleration I mentioned.

al_garnett
Yes that's right. It's gets complicated when you consider fans as they aren't linear devices and also the issue of acceleration I mentioned.

I read a post on lowering tethered payloads on mjgradziel.com which mentions that a fan of 1m diameter will be sufficient to lower the weight of an average person. DCould you suggest any equations to consider when trying to calculate the required size of the fan?

Mentor

That blog doesn't seem terribly reliable. Why the fascination with the fan concept? As Russ says, you can easily hurt someone on the ground with something like this. I doubt OSHA would approve the use at any job sites in the US, for example.

How could someone on the ground get hurt? Assuming the unit works as intended it performs similar to a block n tackle. I think there is some misunderstanding of how this thing works...

Mentor
How could someone on the ground get hurt? Assuming the unit works as intended it performs similar to a block n tackle. I think there is some misunderstanding of how this thing works...

I guess not. How is this lowering fan assembly guided? Is it self-navigating like a drone? Or is it cable guided down a channel? If cable-guided, why not just use the cable for lowering?

And BTW, the fan assembly needs to be a pair of counter-rotating fans, right?

I sure wouldn't want to be working on the ground with drones lowering heavy weights coming down from above. Even if they are 99% reliable, that means several close calls or accidents per week...

http://powerfan.co.uk/Default.aspx

Mentor
http://powerfan.co.uk/Default.aspx

Ah, thanks! That makes much more sense now. Sorry if I missed that you were talking about that kind of arrangement.

And there is an advantage in that the rope is lighter when it is pulled back up, and uses the roof-mounted fan to dampen the heavier load's trip down. Thanks!

You're welcome. It seems like an elegant solution to an age old problem to me :-D

al_garnett
Could anyone provide me with any equations i would need to use to calculate the size of the fan i will need to lower 150kg and also the diameter of the spool that the rope will be wrapped around?

You have a power input to the fan. You now need to find a fan that will dissipate that power at a reasonable RPM.
estimate a spool diameter based on common sense ( does it need to fit in the boot of a car or back of a semi truck? etc)
You should know the max impact speed of the lowered mass: are you lowering ming vases or trash or old ladies?
That'll give you a spool shaft RPM - Can you find a fan that'll dissipate the power at that RPM or do you need a gearbox?

These should get you started:
http://en.wikipedia.org/wiki/Specific_fan_power
http://en.wikipedia.org/wiki/Affinity_laws

I also suggest you look at how air resistance rowing machines are adjusted...

al_garnett
al_garnett
You have a power input to the fan. You now need to find a fan that will dissipate that power at a reasonable RPM.
estimate a spool diameter based on common sense ( does it need to fit in the boot of a car or back of a semi truck? etc)
You should know the max impact speed of the lowered mass: are you lowering ming vases or trash or old ladies?
That'll give you a spool shaft RPM - Can you find a fan that'll dissipate the power at that RPM or do you need a gearbox?

These should get you started:
http://en.wikipedia.org/wiki/Specific_fan_power
http://en.wikipedia.org/wiki/Affinity_laws

I also suggest you look at how air resistance rowing machines are adjusted...

I'm now struggling after checking out both of those wikipedia articles, struggling to find information also because as soon as Wattage is mentioned i get nothing but results for electric fans on google.

Would the calculations be easier if i used a centrifugal brake? to lower the 150kg, 20metres at a descent speed of 1m/s?