Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: paul.fr on 28/10/2007 19:35:48

Title: how do hot air balloons come down?
Post by: paul.fr on 28/10/2007 19:35:48

maybe a simple answer, but i don't know it.

Title: how do hot air balloons come down?
Post by: another_someone on 28/10/2007 20:31:34

Switch off the heat, and let the balloon cool down (there may be a rip cord to allow an emergency release of hot air, but if this is there, I would imagine it is only used when you are already almost in contact with the ground).

Title: how do hot air balloons come down?
Post by: syhprum on 29/10/2007 05:39:57

I see one difficulty with bring a hot air balloon down, as one gets lower the air temperature gets higher hence the amount of lift avaiable decreases.
I see quite a few come over where I live and as they get lower there often seems to be frantic blasting with the burners to keep the rate of descent modest!

Title: how do hot air balloons come down?
Post by: eric l on 29/10/2007 09:02:19

Quote from: syhprum on 29/10/2007 05:39:57

I see quite a few come over where I live and as they get lower there often seems to be frantic blasting with the burners to keep the rate of descent modest!

I can imagine that !  When driving a car, one gets accustomed to immediate response to trottle, brake or steering wheel.  In a hot air balloon, the response to switching on the gas may take as long as a minute - which means a lot of descent.
With really experieced balloonists, you will observe this frantic blasting of burners only when they notice that the projected landing site they are aiming for is not really suited for some reason.

Title: how do hot air balloons come down?
Post by: DoctorBeaver on 29/10/2007 09:19:20

Nearly had 1 come down in 1 of the fields at the stables a couple of weeks back. He got down to, I reckon, 20' and realised it was the wrong field. He wanted to be the other side of the road.

It didn't half spook the horses!

Title: how do hot air balloons come down?
Post by: lyner on 29/10/2007 22:47:20

Quote

as one gets lower the air temperature gets higher hence the amount of lift available decreases.

It isn't as simple as that, is it? The air in the balloon gets compressed and its temperature will go up  and the density of the air, lower down, is greater so you have more buoyancy. If it worked the way you imply, the reverse process would take a balloon infinitely high and, yet, they all have a 'ceiling' which is, basically, imposed by the  heat energy put in with the flame. What I'm saying is that, unless you make a drastic change with the burner - or dumping hot air - the balloon is fairly stable, in principle.

Title: how do hot air balloons come down?
Post by: syhprum on 30/10/2007 08:11:05

Quote from: sophiecentaur on 29/10/2007 22:47:20

Quote

as one gets lower the air temperature gets higher hence the amount of lift available decreases.

It isn't as simple as that, is it? The air in the balloon gets compressed

I must disagree with the suggestion that the air in a hot balloon gets compressed, all the balloons that I have seen have a hole in the bottom about 2 meters diameter so there can be little difference in pressure inside or out

Title: how do hot air balloons come down?
Post by: chris on 30/10/2007 08:58:09

The air cannot be compressed - in fact the reverse is true - because if it were "compressed" then it would be denser than the air around the balloon, so it wouldn't rise!

Title: how do hot air balloons come down?
Post by: lyner on 30/10/2007 11:27:06

You mis-understand me. The air in the balloon is compressed because the surrounding air pressure increases as you go lower ( the air around it is compressed, too of course, by the atmosphere above it). The hole in the bottom and the flexible envelope allows air in an out and equalise the pressure.  It is the relative densities of the warm air inside and the cool air outside that gives you the lift.
A hot air balloon is, theoretically, quite a stable object, as far as its height is concerned.
As your height decreases, the gravitational PE of the ballon + its air decreases and the temperature of the air inside  MUST rise - to obey the energy conservation law, the warmer air will give you more lift and slow you down. If you let just a little hot air out, the balloon will drop a short distance until it reaches a new equilibrium height. It would be possible to let just enough hot air out  and to land at zero speed - but difficult to judge.
The reason for all the frantic burning of gas is that you need to do things quickly - to get down where you want on a steep path (avoiding trees and power lines). You have to build up speed and then apply the 'brakes' to avoid ploughing into the ground.

You don't need an envelope to see this effect; a (cumulus) cloud is formed when a body of air warms up near the ground. It expands and rises until its temperature drops (due to the gas laws) to bring its density equal to the surrounding air - it reaches a stable ceiling where the thermal energy it got at the ground is all transferred into gravitational potential energy.  The hotter the ground, the higher it reaches.