Mythbusters: Blow your own sail

[Andrew Mason]

If you want to get the net airflow created by the boat as a whole, then you have to compare the velocity of the air off the sail, with the initial velocity of the air before it was affected by the boat at all. With the boat at rest, this initial air velocity is zero, so any backflow from the sail means a net backflow, and thus a net forward force.

What about the rearward force on the fan, which is connected to the boat? If the same amount of air is moving forward into the fan as is moving backward from the sail, the backward moving air has to be moving faster. I am saying that can't happen. However, if there is much more air moving backward - even though it may be moving backward more slowly than the air that is moving forward into the fan - it can carry more momentum.

If the fan simply builds up a higher pressure behind the sail and the releases it suddenly (by changing the direction of the fan output or just stopping the fan) the fan transfers energy to a large mass of air (pressure x volume = energy) and then sends that mass rearward. That, it seems to me, is what is happening here.

AM

 

[A.T.]

the backward moving air has to be moving faster.

Faster than zero, which was the velocity of the air before it was affected by the boat in any way. This net difference in the air's momentum determines the net force on the boat.

 

[Andrew Mason]

Faster than zero, which was the velocity of the air before it was affected by the boat in any way. This net difference in the air's momentum determines the net force on the boat.

At any given moment you have air moving forward into the fan. Unless you have more air moving backward at a slower speed than the air that is being sucked into the fan, you will have a net movement of air forward and the boat moving forward. That is what I am trying to avoid. The only way I can see that it can be done is if you have a much larger mass of air moving backward at a speed that is slower than the forward moving air but carrying more rearward momentum (due to the larger mass) using much less energy than the output of the fan.

AM

 

[A.T.]

At any given moment you have air moving ...

You have to compare the final momentum of the air, to the initial momentum zero momentum of the air. Not to some intermediate state where the air has already been accelerated. Whatever the air does in between is irrelevant. If at the end the air leaves the sail with backwards momentum, then it has gained backwards momentum from the boat and the boat has gained forwards momentum.

 

[Andrew Mason]

You have to compare the final momentum of the air, to the initial momentum zero momentum of the air. Not to some intermediate state where the air has already been accelerated. Whatever the air does in between is irrelevant. If at the end the air leaves the sail with backwards momentum, then it has gained backwards momentum from the boat and the boat has gained forwards momentum.

Ok. I agree. But the final momentum of the air includes the forward moving air moving through the fan.

AM

 

[rcgldr]

You have to compare the final momentum of the air, to the initial momentum zero momentum of the air. Not to some intermediate state where the air has already been accelerated. Whatever the air does in between is irrelevant. If at the end the air leaves the sail with backwards momentum, then it has gained backwards momentum from the boat and the boat has gained forwards momentum.

Consider the air and the boat as a closed system. Even though the air gains backwards momentum, it could appear that the mass of the air is being shifted forwards by the interaction of fan and sail, but the center of mass of the system doesn't move since there are no external forces, so some form of circulation occurs where the mass of the affected air is shifted backwards while the mass of the boat is shifted forwards.

 

[A.T.]

But the final momentum of the air includes the forward moving air moving through the fan.

No. The final state of the air is leaving the sail backwards. That's where it's interaction with the boat ends, so that's the momentum that has to be compared to the the initial zero momentum.

 

[A.T.]

it could appear that the mass of the air is being shifted forwards by the interaction of fan and sail

What matters for the force on the boat is the change of the air's momentum, not of the air's position. The air starts with zero momentum and ends up with backwards momentum, so the boat receives forward momentum.

the affected air is shifted backwards

Yes obviously, since the air receives backwards momentum.

 

[Andrew Mason]

What matters for the force on the boat is the change of the air's momentum, not of the air's position. The air starts with zero momentum and ends up with backwards momentum, so the boat receives forward momentum.

So where does the air flow through the fan get its forward momentum from, if not from the fan? You seem to be deliberately omitting the rearward force on the fan from this air flow.

AM

 

[A.T.]

You seem to be deliberately omitting the rearward force on the fan from this air flow.

You seem to be deliberately ignoring my post #17 where I compare the forces on the fan and sail.

Also note that you don't have to care about the individual interactions, if you know that the net effect of them all is giving the air backwards momentum. It then follows from momentum conservation that the net force on the boat is forwards.

 

[Andrew Mason]

You seem to be deliberately ignoring my post #17 where I compare the forces on the fan and sail.

No, I agree with that. I am just pointing out that for a steady-state system that is providing continuous forward net force on the boat, a net mass of air has to be moving backward relative to the boat. I am suggesting that the only way this can occur is if the rate of rearward mass flow is greater than the rate of forward mass flow passing through the fan. Since the rearward speed of the air coming off the sail is going to be less than the speed of the air entering the fan, I am suggesting that a greater mass of air moves backward in a given time period $\Delta t$ than is moving forward (through the fan) in the same period.

AM

 

[A.T.]

I am just pointing out that for a steady-state system that is providing continuous forward net force on the boat, a net mass of air has to be moving backward relative to the boat.

If everything started from rest, then yes.

I am suggesting that the only way this can occur is if the rate of rearward mass flow is greater than the rate of forward mass flow passing through the fan.

If by "rearward mass flow" you mean the flow rate at the sail, then no, it doesn't have to be greater than the flow rate at the fan.

Since the rearward speed of the air coming off the sail is going to be less than the speed of the air entering the fan, I am suggesting that a greater mass of air moves backward in a given time period Δt\Delta t than is moving forward (through the fan) in the same period.

Yes, there is a greater mass of air moving back, than forward. But this doesn't require a faster flow rate at the sail. Note that the amount if air moving forward between fan and sail is approximately constant, while the amount of air moving back accumulates over time, as air is released with backwards momentum from the sail.

 

[paddywwoof]

the simplest way of thinking about it, as you say, is as a black box where you don't worry about what's happening inside. All that matters is the net flow around the box. For the box to experience a forward force the air must be experiencing a backward force and have net flow in that direction.

Traditionally (i.e. approximately) 'free' fans suck fluid from all around them (i.e. you can't really suck directionally) but blow directionally. However putting a sail in the way of the fan's exhaust effectively guarantees that the net outflow is in any direction but forward. i.e. the air is sucked in from pretty well all directions but it leaves mainly sideways but with a slight rearward component.

 

[Carno Raar]

I don't understand why this is a "thing".

If I blow air or water out a pipe for propulsion, and I bend the pipe into a U, it's obvious and easy to demonstrate that I will reverse direction. So the stuff hits the sail and the sail acts like a pipe bend. Either I am missing something vital or the sail is just confusing some people because sails do that (people don't believe we can sail upwind or sail at higher than the wind speed).

I am guessing that Mythbusters having to angle the fan to move is an issue of directional control because their boat has no keel (this type of boat is designed for swamps (very calm, shallow water) and is flat bottomed with no projections under the hull.

A mechanically propelled ship can rely on small fins sticking out the side for stability control (like a missile) but a sailboat experiences some quite extreme forces from the wind on the sail and needs a keel both for stability and to "grip" the water for propulsion when the wind is offset to the desired direction of travel. You may also have to lean out over the side to maintain its balance, like in Andrew Mason's avatar picture. Anyway if you'd only asked a sailor, he would have told you why a keel-less sailboat is moving in a small circle instead of forwards :-)

I don't know why aircraft jet can't reverse thrust indefinitely. I guess it's to do with breaking the deflector or not being designed to move backwards. Boats with marine waterjets can certainly reverse until they run out of fuel or break something, by deploying a "reversing bucket" over the jet nozzle.

 

[A.T.]

If I blow air or water out a pipe for propulsion, and I bend the pipe into a U, it's obvious and easy to demonstrate that I will reverse direction.

Exactly. And the mass flow rate at the bend is the same as at the propeller in the pipe. With an open system (no pipe) there is obviously diffusion of the flow, but that is in no way required for the concept to work.

I don't know why aircraft jet can't reverse thrust indefinitely.

Who says they can't?

 

[Carno Raar]

Who says they can't?

It's some technical limitation on the plane or its engines, and not anything to do with new physics. No need to panic! :-)

 

[A.T.]

It's some technical limitation on the plane or its engines

Any references on that?

 

[Carno Raar]

Any references on that?

I'm not a pilot (unless you count the time I flipped a catamaran yacht) so I just Googled it. Google found a post on a pilot's forum explaining why a particular airliner can't reverse thrust below 60 knots. http://www.pprune.org/tech-log/438954-question-reverse-thrust.html#post6168002

 

[A.T.]

Google found a post on a pilot's forum explaining why a particular airliner can't reverse thrust below 60 knots. http://www.pprune.org/tech-log/438954-question-reverse-thrust.html#post6168002

Here the quote:

The primary reason for stowing reversers below eighty knots or so is that as the aircraft slows, the potential for exhaust gas re-ingestion increases, as well as the potential for reverse flow gasses to cause or permit foreign object ingestion. Direction to stow reversers is there to protect the engine. Ingestion of exhuast by products can cause a flameout, and some engines aren't very stable in deep reverse at slow speeds; they may compressor stall and flame-out on their own. Reverse thrust isn't very effective at low speeds, and offers little advantage.

So it's not like they can't do it, but more that it is being avoided, because it creates some risks.

 

[rcgldr]

I don't know why aircraft jet can't reverse thrust indefinitely. I guess it's to do with breaking the deflector or not being designed to move backwards.

Earlier in this thread I mentioned that at some airports the terminals are blast tolerant, and commercial jets can optionally use reverse thrust to back away from the terminal.

 

[Intrastellar]

But imagine replacing the fan with a ballbearing gun, and the sail with a plate of steel. The ballbearings will rebound off the plate and escape to the rear of the vehicle, producing thrust. We don't normally consider air to have inertia, but if you get enough of it moving, it certainly will.

The plate of steal is connected to the ballbearing gun in the sail and fan example. When the ballbearing gun fires a ballbearing, the gun will get pushed back, and thus will the plate. When the forward moving ballbearing hits the backwards moving plate, they will both stop by momentum conservation.

Step one: ballbearing gets fired
gun+plate and ballbearing will have an equal and opposite momentum by momentum conservation

Step two: ballbearing hits the plate
since gun+plate and ballbearing have equal and opposite momentum, they will come to a halt when they collide, by momentum conservation

@A.T. : Is Dave's quoted post above the premise that you are defending ?

 

[jbriggs444]

The plate of steal is connected to the ballbearing gun in the sail and fan example. When the ballbearing gun fires a ballbearing, the gun will get pushed back, and thus will the plate. When the forward moving ballbearing hits the backwards moving plate, they will both stop by momentum conservation.

Momentum conservation permits a range of outcomes. This includes an outcome where the ball bearing stops dead at the plate (like a wad of putty) and an outcome where the ball bearing rebounds and gains a rearward velocity. If the collision is elastic then the putty-like outcome is forbidden and the rebounding outcome is assured.

 

[rcgldr]

This includes an outcome where the ball bearing stops dead at the plate (like a wad of putty) and an outcome where the ball bearing rebounds and gains a rearward velocity. If the collision is elastic then the putty-like outcome is forbidden and the rebounding outcome is assured.

To continue this analogy from AM's issue, the ball bearings are not on the boat, but sucked up from distance behind the boat, so ball bearings that float on the water.

 

[A.T.]

When the forward moving ballbearing hits the backwards moving plate, they will both stop by momentum conservation.

As jbriggs444 said, momentum conservation allows that the ball ends up moving backwards, while the boat ends up moving forward.

 

[A.T.]

To continue this analogy from AM's issue, the ball bearings are not on the boat, but sucked up from distance behind the boat, so ball bearings that float on the water.

What is "the issue" translated to this analogy? Here it is obvious that no "surrounding balls" need to be affected by the thrown balls for the boat to move forward.

 

[rcgldr]

To continue this analogy from AM's issue, the ball bearings are not on the boat, but sucked up from distance behind the boat, so ball bearings that float on the water.

What is "the issue" translated to this analogy? Here it is obvious that no "surrounding balls" need to be affected by the thrown balls for the boat to move forward.

AM's point is that the non-moving (wrt water) source of the thrown balls is at some fixed distance behind the boat (versus already on the moving boat). The balls are picked up from behind the boat (like the air beign sucked into the fan), and then thrown forwards at the sail (like the flow from the fan when it hits the sail). Since the videos prove this works, and since the center of mass of the system doesn't move, then "circulation" results in the balls (or air) being displaced backwards as the boat moves forwards. AM's issue is an explanation for this "circulation".

 

[A.T.]

then "circulation" results in the balls (or air) being displaced backwards as the boat moves forwards. AM's issue is an explanation for this "circulation".

If by "circulation" you mean that the used balls eventually end up behind their pick-up position, then I agree that it happens. And I think the analogy makes obvious why, without involving a greater mass by affecting surrounding balls.

 

[Andrew Mason]

The circulation is problem. This is due to the continuous operation of the fan. There is no question that a fan sucking in air from behind and compressing a volume of air behind the sail and then stopping and letting that compressed "blob" of air bounce backward will cause a net forward push on the boat. That results in a net rearward flow of air. But if the fan is continually drawing the same amount of air forward as is flowing backward, I am having difficulty seeing a net rearward flow of air. So I have to conclude that more air is moving backward than is moving forward. If the difference is large enough, the lower velocity but larger mass of air can carry more momentum (with considerably less flow energy).

I would suggest that the boat would get more forward momentum if they kept stopping and starting the fan. This is consistent with the fact that the forward impulse to the boat is realized only when the fan output is continually changing, being directed away to one side and then moved back directly behind the sail.

AM

 

[A.T.]

But if the fan is continually drawing the same amount of air forward as is flowing backward,

The amount moving forward is constant, while the amount moving backwards accumulates since the fan was switched on.

Maybe it will help you to think about the ball analogy. If you throw the next ball when the previous ball bounces back, then there is only one ball moving forward. But there are many balls that bounced before and are still moving back.

 

[Andrew Mason]

The amount moving forward is constant, while the amount moving backwards accumulates since the fan was switched on.

Maybe it will help you to think about the ball analogy. If you throw the next ball when the previous ball bounces back, then there is only one ball moving forward. But there are many balls that bounced before and are still moving back.

How does the motion of the air after it leaves the sail affect the boat? The mass flow that causes a force on the boat (ie. at the fan and the sail) should be the only mass flow that is material to the motion of the boat, no? If the rearward mass flow at the sail is exactly equal and opposite to the forward mass flow on the fan, I don't see how you get forward momentum to the boat. That is why I am suggesting that the rearward mass flow is greater.

AM