Mythbusters: Blow your own sail

[Trev]

Hi, I know this has been done but a quick read through on here does not seem to address the household discussion we had on this topic. The mythbudgets claimed to have defeated Newtons laws by putting a sail on one end of a fan boat and turning the fan around. I predicted it would not work, but by the end of the show the boat appears to be pulled along by the sail, albeit slowly.
Reading the existing thread I could not see the explanation I landed upon as follows, perhaps you can tell me why I am wrong.
The boat is being propelled by atmospheric pressure. The moving body of air between the fan and the sail has a lower pressure relative to the still air around the boat at right angles to the flow of air to the sail. As a consequence the air around the boat rushes into the space and results in increased mass pushing on the sail.

 

[A.T.]

The mythbudgets claimed to have defeated Newtons laws

Did they really claim that? The behavior is completely consistent with Newtons laws, and similar to thrust reverses on airliners.

The moving body of air between the fan and the sail has a lower pressure relative to the still air around the boat at right angles to the flow of air to the sail. As a consequence the air around the boat rushes into the space and results in increased mass pushing on the sail.

Nobody had any doubts how the force on the sail is created. The issue was whether it can be greater than the recoil of the fan in the opposite direction.

 

[Trev]

From memory they did say "appears to have violated Newtons Laws".

Re your second point my apologies, re reading the post over a little more time, that the boat moves forwards because air is accelerated from an already moving speed by the propeller, and then comes to a dead stop, or indeed a negative speed at the sail, creating a greater force against the sail than the reaction force against the propeller.

The conversation goes on to discuss at some length 'suck' which is of course another way of say atmospheric pressure, I think my point would be to make that the same pressure is pushing in from the sides of the column of air between the propeller and the sail, and this is contributing, as the system otherwise would be lossy to the extent that the force on the sail could not exceed the force on the propeller, but I am late to the conversation.

Still, nice forum, look forward to poking around in here, I'll read with greater care before posting next time :-).

 

[DrStupid]

From memory they did say "appears to have violated Newtons Laws".

Maybe in the sense of "it looks like it violated Newtons Laws". If I recall correctly they explained the effect as thrust reversal.

 

[DaveC426913]

I suppose the original conclusion that the fan+sail can't work is based on the assumption that air has no inertia. If it starts at the fan and hits the sail and simply stops, then the forces cancel out.

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.

So, yes, like thrust reversal in a jet engine, but ... reversed. So, thrust reversal reversal.

 

[A.T.]

I suppose the original conclusion that the fan+sail can't work is based on the assumption that air has no inertia.

If that had been the assumption, then the conclusion would have been that no wind-powered vehicle can work. The wrong assumption is more likely that the collision of the air mass with the sail just stops the air, like in a inelastic collision. While in fact it can reverse it's momentum.

 

[Andrew Mason]

From memory they did say "appears to have violated Newtons Laws".

Re your second point my apologies, re reading the post over a little more time, that the boat moves forwards because air is accelerated from an already moving speed by the propeller, and then comes to a dead stop, or indeed a negative speed at the sail, creating a greater force against the sail than the reaction force against the propeller.

If the air is being sucked in from the back and pushed toward the sail then it is not possible for the boat to move forward due to the air pushing on the sail. No matter how you configure it, the force on the air caused by the propeller is going to be equal and opposite to the force on the propeller(/boat) caused by the accelerating air.

If the air is being sucked in from the front and pushed toward the sail and allowed to flow out the back then the forward motion is due to the force of the air on the fan, not the sail. This, of course, is how an airboat works - like a propeller airplane.

If the air is being sucked in from the side and the sail is used to deflect the air toward the rear, then the boat could move forward. This is just a less efficient method of moving air toward the rear. The air that is deflected backward pushes forward on the sail and, therefore, the mast pushing the boat forward.

There may also be another more complicated reason that has to do with the shape of the sail as an airfoil if there is external movement of air. A sailboat can sail into the wind, for example. It is not pushed by the wind. Rather the sail shape creates a low pressure region on the curved side of the sail and the boat is drawn forward by the difference in pressure.

AM

 

[rcgldr]

As long as the sail is large enough and shaped so that the reversed flow doesn't significantly interfere with the flow being sucked in by the fan it will work. Assuming still air and the ground as a frame of reference, the change in momentum of the air sucked in by the fan is m v, where v is the speed of the air sucked in as it goes from zero to the speed it exits the fan at. The change of the momentum of the air off the sail just has to be reversed from forward to backwards with some non-zero backwards velocity so that the change in momentum of the air off the sail is greater than m v, perhaps 1.25 m v would be enough.

As the blow your own sail boat moves forward, then it's sail also moves a horizontal column of air forwards with it, part of which eventually ends up being sucked in by the fan, so that the air being sucked in by the fan has an initial non-zero forwards velocity.

As an analogy, imagine there are a bunch of balls floating in the water, and that the boat has a trampoline like surface for a sail. You grab a ball from the water and throw it at the trampoline where the ball ends up moving backwards relative to the water after it bounces off the trampoline sail. As the boat moves forwards you grab another ball from the water and repeat the process.

A bit off topic, a sail boat can sail into the wind because it interacts with both the air and the water. A sail boat on a frictionless surface could not sail upwind. Ignoring the drag component, the water and sail have a Newton third law pair of force component perpendicular to the direction of travel of the boat (wrt the water), which allows the sail to divert the apparent wind to produce thrust (wrt to direction of travel of the boat). The coefficient of lift versus the total drag on the sail boat determines how small the angle to directly upwind the sail boat can sail upwind. In the case of an ice boat, the drag is much less and an ice boat can tach with an upwind component of speed greater than the speed of the wind. So can an DUWFTTW (directly up wind faster than the wind) cart that uses a prop driven by it's wheels.

Full scale demo of blow your own sail by MythBusters:

 

[A.T.]

If the air is being sucked in from the back and pushed toward the sail then it is not possible for the boat to move forward due to the air pushing on the sail.

Of course it's possible. See MB video above and a more controlled experiment below.

No matter how you configure it, the force on the air caused by the propeller is going to be equal and opposite to the force on the propeller(/boat) caused by the accelerating air.

Wrong, see post #6.

 

[Andrew Mason]

Andrew Mason said: ↑
No matter how you configure it, the force on the air caused by the propeller is going to be equal and opposite to the force on the propeller(/boat) caused by the accelerating air.

Wrong, see post #6.
Of course it's possible. See MB video above and a more controlled experiment below.

You are saying that Newton's Third Law does not apply? Read my post carefully.

AM

 

[A.T.]

You are saying that Newton's Third Law does not apply?

No I'm saying this part is wrong:

If the air is being sucked in from the back and pushed toward the sail then it is not possible for the boat to move forward due to the air pushing on the sail.

 

[Andrew Mason]

That is a slightly different issue. I was responding to your #9 post.

AM

 

[thankz]

wouldn't this be considered a standing wave?

 

[Andrew Mason]

No I'm saying this part is wrong:

Andrew Mason said: 浡
If the air is being sucked in from the back and pushed toward the sail then it is not possible for the boat to move forward due to the air pushing on the sail.

For the boat to move forward there has to be a net movement of air rearward. But that can't happen if you are moving air into the fan from the rear at the same rate that you are pushing it backward (and that assumes a perfect elastic rearward reflection from the sail, which is never going to happen). A reverse thruster on a jet engine works because the fuel gases combined with the air are pushed toward the direction of travel faster than the incoming air. And that only works for a brief time because the air supply rapidly decreases.

AM

 

[A.T.]

For the boat to move forward there has to be a net movement of air rearward.

Yes

But that can't happen if you are moving air into the fan from the rear at the same rate that you are pushing it backward

Yes, so you have to push it backwards more, which the sail does.

(and that assumes a perfect elastic rearward reflection from the sail, which is never going to happen).

No, it happens as soon as the reflection from the sail is not perfectly inelastic, but still far from perfectly elastic.

A reverse thruster on a jet engine works because the fuel gases combined with the air are pushed toward the direction of travel faster than the incoming air. And that only works for a brief time because the air supply rapidly decreases.

A jet engine accelerates air, just like a fan does. There is no reason why a thrust reverser should work on one, but not on the other.

And that only works for a brief time because the air supply rapidly decreases.

A thrust reverser can work continuously, not just for a brief time.

 

[Andrew Mason]

Yes, so you have to push it backwards more, which the sail does.

How? If I am over-simplifying something, please correct me. But you will have to explain it.

No, it happens as soon as the reflection from the sail is not perfectly inelastic, but still far from perfectly elastic.

A jet engine accelerates air, just like a fan does. There is no reason why a thrust reverser should work on one, but not on the other.

Here's my reasoning: A jet engine crams air and fuel into a chamber and ignites it. It expels more gas rearward than was brought in and the stream is much narrower and faster than the incoming stream. So if you were to direct that stream forward, the reverse thrust would be greater than the forward propulsion caused by the fan blades sucking air in from the front. A fan or propeller just pushes air. It does not concentrate it. The rate at which mass flows into the propeller has to equal the rate at which mass is expelled behind the propeller. Since the area behind the propeller is the same as the area in front, there is no concentration of the air so it can't expel the air at a greater speed than it takes it in. So putting a thrust reverser behind a propeller and turning up the forward propeller speed would not work. To create reverse propulsion with a propeller you have to reverse the pitch so that it is pushing air forward.

A thrust reverser can work continuously, not just for a brief time.

Where does the intake air come from if the plane is not moving forward or moving forward quickly enough to supply the intake fans?

AM

 

[A.T.]

How?

The fan accelerates air from 0 to v. The sail reverses the direction from v to something < 0. Which of the two constitutes more momentum transfer, and will thus exert more force on the boat?

 

[rcgldr]

The air flow through a fan does get concentrated, except that due to viscosity, the flow interacts with the surrounding air. The air accelerates as it approaches the plane of the fan (or propeller), then it's speed remains about the same as it crosses the plane of the fan (or propeller), where there's a pressure jump (the pressure jump is how the fan (or propeller) perform work on the air). The now higher than ambient air continues to accelerate as it's pressure returns back to ambient. Ignoring viscosity affecting the surrounding air, the mass flow rate remains constant, so the cross sectional area of the flow decreases as the flow speed increases. There is a NASA article about this:

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

A.T.'s post pretty much explains why the sail does not have to be 100% elastic.

About the reverse thrusters on a jet, at some airports, the terminals are blast tolerant, and commercial jets can use reverse thrusters to back away from the terminal.

 

[CWatters]

Force = mass * acceleration so let's look at the acceleration of the air...

The air going through the fan is accelerated from 0 to X m/s.
At the sail the air is reflected or "accelerated" from X to -X m/s.

So the rearward force on the fan is proportional to X and the forward force on the sail is proportional to 2X.

So there is a net forward force proportional to X.

Edit: Clearly the sail may not reflect all the air but it doesn't have to. If any is reflected there will be a net forward force.

 

[Andrew Mason]

backward

A.T.'s post pretty much explains why the sail does not have to be 100% elastic.

I never said that it had to be 100% elastic. I said that there has to be net airflow rearward. You can do that any way you want. You could do it with a jet engine with the sail or something like a metal plate directing the expelled gases rearward. I just have difficulty understanding how you could get a net rearward stream of air if you are sucking it in from the rear at the same rate as you are pushing it toward the sail. If the flow of air into the fan is vdm/dt and the rearward flow deflecting from the sail is -vdm/dt (which is the best you can get), how do you get a net rearward flow of air? In actual practice, the rearward flow will be much less than v due to dispersion of the air.

AM

 

[A.T.]

the rearward flow deflecting from the sail is -vdm/dt (which is the best you can get)

Why should that be the best you can get?

 

[A.T.]

If you have -vdm/dt of force on the sail

You don't. If the sail reverses the flow from v to < 0, then the force on the sail is more than vdm/dt.

 

[DaveC426913]

A jet engine crams air and fuel into a chamber and ignites it.

A fan or propeller just pushes air.

Both of them add energy to the air. One from fuel the other from electricity.

They do work on the air and that is translated to work on the craft.

Your assertion that a "fan just pushes air" seems to assume it doesn't add usable energy to the system. A jet engine simply adds somewhat more.

 

[rcgldr]

how do you get a net rearward flow of air? In actual practice, the rearward flow will be much less than v due to dispersion of the air.

The videos are proof that the blow your on sail models can move forwards, although the speed is slow since the sail also acts as a air dam pushing the air in front of it forwards. Momentum is conserved, and if the sail boat moves forward, then there has to be a net opposing change in momentum of the air. Since the back flow velocity off the sail is less than the forwards flow out of the fan, the back flow must involve a larger volume (mass) of air, so the flow that originated from the fan and deflected by the sail apparently interacts with the surrounding air.

The fan is also operating in a near static situation, so I'm not sure how much of the intake flow starts off with a component perpendicular to the flow at the fan. I'd be curious to see a smoke test done on the intake side of a fan.

 

[Andrew Mason]

The videos are proof that the blow your on sail models can move forwards, although the speed is slow since the sail also acts as a air dam pushing the air in front of it forwards. Momentum is conserved, and if the sail boat moves forward, then there has to be a net opposing change in momentum of the air. Since the back flow velocity off the sail is less than the forwards flow out of the fan, the back flow must involve a larger volume (mass) of air, so the flow that originated from the fan and deflected by the sail apparently interacts with the surrounding air.

I think that is the key. It has to impart backward momentum to a larger mass of air than is flowing out of the fan.

You will notice in the film that it only provided forward net force if they kept changing the fan's direction. If the fan is kept pointing directly toward the sail the pressure builds up behind the sail and can escape only sideways (and unless the sail is perfectly positioned, more escapes from one side than the other and the boat starts going in circles). However, if the fan is then directed off the sail, the air under pressure between the fan and sail can escape backward and this rearward momentum of the air produces forward momentum of the boat. Unless that pressure is allowed to build up and then released by changing the fan direction, there would be no forward momentum to the boat.

You can get a watermelon to move toward the shooter by putting a hard metal sheet in front of it and hitting it with soft hunting bullets. The bullets flatten on the initial impact and in passing through the watermelon built up pressure in front of the bullet inside the watermelon. When the bullet reaches the other end of the watermelon, the contents explode out of the melon in a forward direction sending the melon backward with more momentum than the bullet carried into it. Unless that pressure is built up and then released to a much larger mass of watermelon than the bullet mass, there is no way the watermelon will move backward.

AM

 

[A.T.]

It has to impart backward momentum to a larger mass of air than is flowing out of the fan.

No, it does not have to be a larger mass, if the change in velocity is greater. See post #17.

You will notice in the film that it only provided forward net force if they kept changing the fan's direction.

Watch the other film:

 

[rcgldr]

... it does not have to be a larger mass, if the change in velocity is greater.

The change in velocity explains the difference in forces at the fan and at the sail resulting in a net forwards force. The other issue is that momentum is conserved, the air, boat, (and the water) can be considered a closed system. If the sail boat is moving forwards, then the net air flow has to be backwards.

 

[A.T.]

The change in velocity explains the difference in forces at the fan and at the sail resulting in a net forwards force. The other issue is that momentum is conserved,

That's the same issue, not another issue. The net force is there because momentum has to be conserved.

If the sail boat is moving forwards, then the net air flow has to be backwards.

Yes, so?

 

[rcgldr]

If the sail boat is moving forwards, then the net air flow has to be backwards.

Yes, so?

So AM is wondering how that happens, when the backwards velocity of the air off the sail is less than the forwards velocity of flow feeding into the fan (this is an assumption, but seems reasonable). My guess is that the backwards flow involves more air due to the flow originating from the fan affecting the surrounding air, increasing the amount of air reflected off the sail.

 

[A.T.]

So AM is wondering how that happens, when the backwards velocity of the air off the sail is less than the forwards velocity of flow feeding into the fan (this is an assumption, but seems reasonable).

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.

 

[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.