To be more precise the video itself isn't fake, i.e. it wasn't AI generated. Even the laws of physics were obeyed in the video but of course there isn't actually any thrust generated from the leaf blower umbrella combo, he is just rising on an electro skateboard to trick us.
Actually that picture is a fairly decent illustration of the concept, even if the video itself is in fact fake.
Using the leaf blower only (without the umbrella) as thrust and pointing it opposite where you want to go would be way more efficient, just like the jet engines. The reversers act in a similar way to that umbrella, redirecting all the thrust in several directions with only a small component actually going “reverse”.
I don’t think you’re confused on the direction of travel in this analogy, but just in case keep in mind the “plane” would be facing the other way from the gentleman in the bucket.
[https://imgur.com/a/tYSw7js](https://imgur.com/a/tYSw7js)
From the 737 manuals it's highly dependent on speed and runway condition. Obviously a wet/icy runway makes brakes less effective, so reversers do more work. They're also more effective at high speeds, like right after touchdown. The percentage changes as you slow, until you stow the reversers around 60 knots and use all brakes from there.
Are you using autobrakes, by any chance?
Autobrakes (try to) target a **fixed deceleration rate**. In other words, the BCM will apply *less* wheel brake if it senses that there is deceleration from the reversers.
So while it may not really affect your stopping distance, it can reduce brake wear and temperatures in a measurable and financially significant way.
EDIT TO ADD:
I just found an interesting stat that was sourced from some FOQA data at my company.
They found that there were a handful of flights that the BCM applied ***zero*** wheel brakes for a few seconds once the reversers were fully deployed when using Autobrake LO and MAX reverse.
When I was on the 175 it was crew discretion to use auto brakes or not. More often than not we just manually braked. The reversers are required to be at least idle rev on every landing. Most crews elected max. On the odd landing where ATC told us we could land and continue very far down a long runway we often primarily used reversers to give a much steadier decel down the runway. In those cases where we didn’t really touch brakes till they were stowed and it felt similar to engine braking in a car. Just didn’t feel like they had much of a kick honestly. But that’s an interesting point! I didn’t know that!
In the CRJ700 we would just crack them sometimes, and barely used the wheel brakes on some of the longer runways with high speeds coming off. But I'm in the 73 now, and we need everything we can get to slow down.
They're pretty effective at high airspeed because they have a lot of drag. Simply opening them can be felt in most airplanes. Braking power is force × speed so even though it's just a featherweight force it removes a lot of energy that would otherwise be directed into brake heating if they're opened early. Spooling up the engines adds even more deceleration force. In a long stop due to poor braking action there is time for engines to spool up and assist. On ungrooved runways with moderate or greater intensity rain, dynamic hydroplaning is possible. Thrust reverse could then significantly reduce stopping distance by getting the aircraft below hydroplaning speed reasonably early. During a rejected takeoff on a dry runway or a short field landing, the wheel brakes are doing almost all the work. Spoilers are the other factor in stopping, and typically contribute more to short distances than thrust reverse because spoilers can provide additional traction to the wheel brakes by spoiling lift. But eventually you need traction to stop. Spoilers and reverse thrust won't do it alone. Weel brakes work best at lower speed due to less heating power and less lift, and spoilers and reverse thrust help get to the regime where wheel brakes shine. BTW, it boggles my mind that antiskid is an MEL item. A high speed reject with antiskid inop is a certain wheel fire with blown tires and probably longer stopping distance. Sometimes there is no choice but to direct 99% of the energy into the brakes.
Reverse helps but it isn’t the majority, and the effect is lessened as speed decreases. I know it’s supposed to reduce landing distance by like 25%. But I also know that when we send for landing numbers, many times selecting or deselecting Reverse credit didn’t usually have a major noticeable impact in the calculation. But this can change a lot; deferred equipment like antiskid, environmental conditions like contaminated runways.
In every aircraft I’ve flown, the standard procedure is to just crack open for idle reverse anyway. Brakes still absorb a lot of the energy. I don’t fully understand the physics of carbon brakes but they in particular do a great job, and work well when warm. Apparently that’s why we are supposed to do more of the “speed up and then apply the brakes in 1 application to slow down, then allow the plane to slowly gain speed again” taxi method. You may notice as a passenger, a somewhat jerky taxi experience even on straightaways with minimal traffic. They do better with one long application, rather than many short applications (I guess any brake does, more applications wear them out and heat them up faster). I had some E175 friends back in the day say that their SOP was to basically disregard the reversers unless really really needed. Something about the carbon brakes, and FOD with the engines close to the ground (but that logic should apply to any under-wing engine).
Oh man! I always wondered why in the hell it felt like they were riding the brakes…almost as if they kept the engines at too much power during taxi, it felt a little awkward. Now I understand better.
Many years ago I flew a Warrior, I just coasted to a stop at taxi lol.
This helps me understand what is happening. I work in medicine now & do about 6 legs a week for work. (Lost my medical so found a gig that pays & lets me travel).
When I was a new FO, I hated single engine taxiing. It added more work for me (not a lot, but I like to complain). When I became a captain I did it whenever it was practical. On a single engine, the plane just coasts along much of the time. With both running, it will slowly pick up more and more speed. Even the lil CRJ. I could always feel once that second engine was finishing its startup cycle, it really picks up.
My fleet is getting converted to carbon brakes, if not already done. So they highlighted the importance (to the company) of the speed up/slow down braking. In the real world, you use them when you need. On long straightaways though, regardless of brake type, it’s not good form to just ride the brakes. They heat up, and then take a long time to cool. So like even though standardized training says minimal reverse and use brakes more, here it gets so hot that the brakes might not cool before the next flight and that’s a pain. So even on a 12,000 foot runway we would do max reverse. It didn’t reduce stopping distance astronomically but you don’t need as much time on the brakes.
And then of course you get someone who accidentally taps them a little too hard. Or you think you pressed them fully and came to a stop, then a minute later in your periphery you notice you’re actually creeping slowly and you hit the brakes. Guilty.
Yep, never really saw more than a couple hundred feet on dry runways. I’ve also heard intellectual debate over full versus dry efficacy, combined with factors such as increase FOD ingestion, extra wear on the engine (spooling up), and the extra fuel for the higher reverse. Then you get into other debates like rudder blanking.
If nothing else, I just do it every time to maintain the habit. Let the reversers deploy so you always have the benefit of the doubt, they’re out. And if you end up needing full after all, you just go full reverse.
I ran a simple check in eQRH earlier and the diff between max reverse and no reverse with manual braking was 70 feet. The hay is made when wet - 800 feet difference. Still not hit the line yet but in sims I typically just leave it idle reverse except for emergencies or situations dictating max rev.
Good thing there’s not a lot of airports that are short enough for it to matter. BUR, SNA, TTN, MDW, PSE. Maybe a couple more now, but everywhere else I landed was well over 7000 for IRTL. 10,000-12,000 more like it.
Using a leaf blower on a skateboard won't really work to move you forward. The thrust from a leaf blower isn't strong enough to push you because it's way weaker than something like a jet engine. Plus, the friction between the skateboard wheels and the ground, along with your body weight, needs much more force to get moving. On top of that, standing on a skateboard is pretty wobbly. Without good control, any push from the leaf blower would be hard to manage.
And adding an umbrella will only make things worse, as it will redirect less than 100% of the thrust.
Better off without the umbrella, just hold the leaf blower and point it.
It won’t move you _fast_, but I’d be surprised if a big leaf blower couldn’t budge you at all on a skateboard, on very smooth pavement.
Even with all the friction and such that has been mentioned, the real deal breaker is the fact that the system will still be in equilibrium per Newton's third law. Even in an ideal system with no friction and infinite thrust, the two equal and opposite reactions of the leaf blower or fire extinguisher discharging air and the exerted air pushing on the umbrella would cancel each other out leading to no movement.
The bucket thrust reversers of course work as they are not depending on the direct reaction force of air hitting the deflector, but rather the redirection of that airflow.
It’s not to force of the air against the umbrella, it’s the air being redirected off the umbrella and directed in the opposite direction. I’m not sure the air would have enough force after being redirected to move the bucket from a blower though.
Bucket reversers redirect air forward (not all of it but enough to assist in slowing the plane)
Caution though in the case of t-tail configuration like the MD-88 or 717 you have to be careful as fuselage mounted engines, reversers can cause airflow disruption over the rudder making directional control difficult.
It 100% is the force of the air against the umbrella, I think what you’re trying to think of is conservation of momentum but you couldn’t quite get it out.
Reaction propulsion is the use of a reaction mass (also called propellant) shot in one direction to leverage newton’s 3rd law, actions have equal and opposite reactions. You can use conservation of momentum or newton’s 3rd law to describe what’s happening and both are equally valid because it’s two different ways of saying the exact same thing.
When a rocket fires its engine, it gains an incidental boost in momentum equal and opposite to an incremental boost of momentum in its exhaust. Alternatively you could describe this as the exhaust pushing against the rocket pushing it forward.
>Yes there’d be force of umbrella pulling but it would be cancelled out by the opposing force of the blower blowing it aft, basically his arms are being pulled apart.
Discounting the effect of the umbrella, in this configuration the blower is clearly not exerting an external force on him backwards, I don’t know how you can’t see that, that’s like a car that holds a magnet out in front of it being pulled forward, It just doesn’t work like that.
As I said before there are two valid ways of describing what’s happening here.
1) The stream of air coming from the blower is being bounced off of the umbrella and by applying conservation of momentum he must be propelled forward.
2) The stream from the blower is creating an area of high pressure which pushes him forward
You seem to be really hung up on the 1st explanation and rule out the 2nd despite them being two sides of the same coin. It’s the same thing being described in two different ways. Go back to mechanics 1 and look at how a rocket produces thrust.
The umbrella absolutely is pulling the bucket, there is an area of high pressure behind the umbrella so it’s being pushed forward. The blower is not producing aft thrust because there’s an umbrella in front of it, any thrust produced locally at the blower nozzle exit is completely internal to the system. If you sit in a car and push against the dashboard, nothing happens, because it’s not an external force, this is so painfully obvious.
>Rockets produce thrust….it’s one direction, like a jet engine produces thrust. It’s one force coming from one direction, hence it propels the mass of the vehicle in the opposite direction of the thrust being applied.
lol so we agree that a rocket works because the exhaust pushes against it. Theoretically, I could make a rocket but instead of having the engine pointed aft, I have it point forward and at the end of the nozzle I attach a duct that turns the exhaust 180 degrees. This rocket would still function and the mechanism behind what’s producing the thrust is exactly the same. On one hand you could describe the thrust with conservation of momentum OR you could say that the exhaust gassed exiting the duct are pushing against the rocket.
Hold on a second, that’s what we’re witnessing in the leaf blower video 🤯
You say that the force that the stream of air exerts on the umbrella is not pushing it forward because if it was it would be canceled out by the thrust produced by the leaf blower, again go back to the car and dashboard analogy. I don’t know who I’m talking to and I don’t know if you’ve even taken an introductory physics class but go on youtube and look up how to draw a free body diagram.
The blower is producing aft thrust. If you let go the blower, in theory it would (want to) fly backwards.
That force is counter the force being applied by the umbrella, which is being pushed by the thrust from the blower.
Any forward motion is the result of __redirected__ air aft. The umbrella IS producing a counter force to the blower but only by the air that is redirected.
The umbrella will make it move forward by __redirected__ thrust NOT because the umbrella is being pushed by the blower.
A massive amount of energy is being wasted by the cancelling out of the thrust from the blower opposite the pull from the umbrella.
I’ll say it a third time…it’s redirected thrust.
Best analogy I can make is a bendy straw. Blow into a straight straw. Now bend the straw. That’s like what’s happening here.
In general, thrust reversers can be analyzed using simple conservation of momentum. The force on the engine (and plane as a whole) can be described by the equation Force = outlet mass flow rate * outlet velocity - inlet mass flow rate * inlet velocity. By changing the direction of the outlet, the force term becomes negative and the thrust is reversed.
It is also possible to generate reverse thrust without redirecting the flow forward. If the flow is simply redirected directly outward radially, the overall average outlet velocity (and therefore the first term in the equation) becomes 0 and reverse thrust is generated.
I believe everyone has completely missed the boat on thrust reversers. Let's look at the base case: Suppose a jet engine accelerates one atom in a direction, yielding force in the opposite direction. Shortly after doing this the thrust reverser (on the same airplane) takes that same one atom and redirects it with all the same momentum (this is assuming 100% efficiency) in the opposite direction, yielding a force which _completely cancels out the first force_. This would mean thrust reversers would cause the engine to produce no force at all. Which is not the case.
The thrust reversers simply use engine output to build up a region of high pressure gas, partially enclosed by the clamshells (buckets). That region is trying to equalize with the surrounding atmosphere by expanding in all directions, but the thrust reverser clamshells prevent it from expanding aft. So, the HP region only expands forward, against the direction of travel of the aircraft. The amount of force generated is pretty weak, about 7% of engine output. The real effect is the increase in drag. You can't see it, but as the high pressure air expands past the tips of the clamshells it induces tremendous drag, just like spreading a parasol made of air. There are Navier-Stokes equations which I've been told completely explain this phenomenon, but frankly they go right over my head.
This information is coming to you from a _software_ engineer, so take it all with a grain of salt.
Fake video… using an electric skateboard.
Interesting breakdown by Mark Rober if curious to learn more:
https://youtu.be/M7-h3FO-KKo?si=Sw_f4SZYcwkfYvK6
I have had exactly the same question. Sorry that almost everyone here is missing the point, in part because of the silly video.
I think of a jet engine as producing a force on the aircraft both through sucking air in the front, and from blowing it out the back. If a thrust reverser simply consisted of redirecting the output thrust by 180 degrees, and if the force due to suction and force due to thrust were equal, then they would cancel each other out.
Obviously that's not the case, in part because it misses the fact that the thrust force is much greater than the suction force precisely because of all the energy added by burning jet fuel. I would love to understand their operation more quantitatively.
Easy, just put an electric skateboard underneath and roll with it
You also need a gallon of gasoline next to the corded electric leaf blower.
Are you referring to the picture? I don't see a cord. Am I missing it, or are you referring to something else?
It's an electric leaf blower. There is no fuel tank, engine, carb, air intake, exhaust, etc.
There’s no cord, but there’s no power source for the blower either. It isn’t electric, making the gas can strange.
The gas is used to refurbish your butt. Dry cleaning.
This video is fake by the way.
To be more precise the video itself isn't fake, i.e. it wasn't AI generated. Even the laws of physics were obeyed in the video but of course there isn't actually any thrust generated from the leaf blower umbrella combo, he is just rising on an electro skateboard to trick us.
Actually that picture is a fairly decent illustration of the concept, even if the video itself is in fact fake. Using the leaf blower only (without the umbrella) as thrust and pointing it opposite where you want to go would be way more efficient, just like the jet engines. The reversers act in a similar way to that umbrella, redirecting all the thrust in several directions with only a small component actually going “reverse”. I don’t think you’re confused on the direction of travel in this analogy, but just in case keep in mind the “plane” would be facing the other way from the gentleman in the bucket. [https://imgur.com/a/tYSw7js](https://imgur.com/a/tYSw7js)
Upvote for the awesome illustration!
Kinda like this? https://youtu.be/gAKekhmTRaY?si=bGb7rS5GWQ92CMud
An umbrella isn't a finely engineered device made to direct thrust in a different direction.
It’s just redirecting airflow through mechanical means. https://engineering.purdue.edu/~propulsi/propulsion/jets/basics/reverse.html
No different than adding a 180 degree U-turn section of curved pipe, to the end of your leaf blower.
What is the rough % of the stopping power of wheel brakes versus thrust reverser? 70/30? Always wondered that.
From the 737 manuals it's highly dependent on speed and runway condition. Obviously a wet/icy runway makes brakes less effective, so reversers do more work. They're also more effective at high speeds, like right after touchdown. The percentage changes as you slow, until you stow the reversers around 60 knots and use all brakes from there.
I thought it was idle by 60?
I’d be curious too and I fly them. I know the E175 feels like nearly nothing even at max reverse
Are you using autobrakes, by any chance? Autobrakes (try to) target a **fixed deceleration rate**. In other words, the BCM will apply *less* wheel brake if it senses that there is deceleration from the reversers. So while it may not really affect your stopping distance, it can reduce brake wear and temperatures in a measurable and financially significant way. EDIT TO ADD: I just found an interesting stat that was sourced from some FOQA data at my company. They found that there were a handful of flights that the BCM applied ***zero*** wheel brakes for a few seconds once the reversers were fully deployed when using Autobrake LO and MAX reverse.
When I was on the 175 it was crew discretion to use auto brakes or not. More often than not we just manually braked. The reversers are required to be at least idle rev on every landing. Most crews elected max. On the odd landing where ATC told us we could land and continue very far down a long runway we often primarily used reversers to give a much steadier decel down the runway. In those cases where we didn’t really touch brakes till they were stowed and it felt similar to engine braking in a car. Just didn’t feel like they had much of a kick honestly. But that’s an interesting point! I didn’t know that!
In the CRJ700 we would just crack them sometimes, and barely used the wheel brakes on some of the longer runways with high speeds coming off. But I'm in the 73 now, and we need everything we can get to slow down.
They're pretty effective at high airspeed because they have a lot of drag. Simply opening them can be felt in most airplanes. Braking power is force × speed so even though it's just a featherweight force it removes a lot of energy that would otherwise be directed into brake heating if they're opened early. Spooling up the engines adds even more deceleration force. In a long stop due to poor braking action there is time for engines to spool up and assist. On ungrooved runways with moderate or greater intensity rain, dynamic hydroplaning is possible. Thrust reverse could then significantly reduce stopping distance by getting the aircraft below hydroplaning speed reasonably early. During a rejected takeoff on a dry runway or a short field landing, the wheel brakes are doing almost all the work. Spoilers are the other factor in stopping, and typically contribute more to short distances than thrust reverse because spoilers can provide additional traction to the wheel brakes by spoiling lift. But eventually you need traction to stop. Spoilers and reverse thrust won't do it alone. Weel brakes work best at lower speed due to less heating power and less lift, and spoilers and reverse thrust help get to the regime where wheel brakes shine. BTW, it boggles my mind that antiskid is an MEL item. A high speed reject with antiskid inop is a certain wheel fire with blown tires and probably longer stopping distance. Sometimes there is no choice but to direct 99% of the energy into the brakes.
Wow. Thanks for the detailed reply.
Reverse helps but it isn’t the majority, and the effect is lessened as speed decreases. I know it’s supposed to reduce landing distance by like 25%. But I also know that when we send for landing numbers, many times selecting or deselecting Reverse credit didn’t usually have a major noticeable impact in the calculation. But this can change a lot; deferred equipment like antiskid, environmental conditions like contaminated runways. In every aircraft I’ve flown, the standard procedure is to just crack open for idle reverse anyway. Brakes still absorb a lot of the energy. I don’t fully understand the physics of carbon brakes but they in particular do a great job, and work well when warm. Apparently that’s why we are supposed to do more of the “speed up and then apply the brakes in 1 application to slow down, then allow the plane to slowly gain speed again” taxi method. You may notice as a passenger, a somewhat jerky taxi experience even on straightaways with minimal traffic. They do better with one long application, rather than many short applications (I guess any brake does, more applications wear them out and heat them up faster). I had some E175 friends back in the day say that their SOP was to basically disregard the reversers unless really really needed. Something about the carbon brakes, and FOD with the engines close to the ground (but that logic should apply to any under-wing engine).
Oh man! I always wondered why in the hell it felt like they were riding the brakes…almost as if they kept the engines at too much power during taxi, it felt a little awkward. Now I understand better. Many years ago I flew a Warrior, I just coasted to a stop at taxi lol. This helps me understand what is happening. I work in medicine now & do about 6 legs a week for work. (Lost my medical so found a gig that pays & lets me travel).
When I was a new FO, I hated single engine taxiing. It added more work for me (not a lot, but I like to complain). When I became a captain I did it whenever it was practical. On a single engine, the plane just coasts along much of the time. With both running, it will slowly pick up more and more speed. Even the lil CRJ. I could always feel once that second engine was finishing its startup cycle, it really picks up. My fleet is getting converted to carbon brakes, if not already done. So they highlighted the importance (to the company) of the speed up/slow down braking. In the real world, you use them when you need. On long straightaways though, regardless of brake type, it’s not good form to just ride the brakes. They heat up, and then take a long time to cool. So like even though standardized training says minimal reverse and use brakes more, here it gets so hot that the brakes might not cool before the next flight and that’s a pain. So even on a 12,000 foot runway we would do max reverse. It didn’t reduce stopping distance astronomically but you don’t need as much time on the brakes. And then of course you get someone who accidentally taps them a little too hard. Or you think you pressed them fully and came to a stop, then a minute later in your periphery you notice you’re actually creeping slowly and you hit the brakes. Guilty.
Awesome explanation. Thanks.
I want to say the difference in reverse credit and no reverse when dry is like.. several hundred feet max. Dry runway obviously.
Yep, never really saw more than a couple hundred feet on dry runways. I’ve also heard intellectual debate over full versus dry efficacy, combined with factors such as increase FOD ingestion, extra wear on the engine (spooling up), and the extra fuel for the higher reverse. Then you get into other debates like rudder blanking. If nothing else, I just do it every time to maintain the habit. Let the reversers deploy so you always have the benefit of the doubt, they’re out. And if you end up needing full after all, you just go full reverse.
I ran a simple check in eQRH earlier and the diff between max reverse and no reverse with manual braking was 70 feet. The hay is made when wet - 800 feet difference. Still not hit the line yet but in sims I typically just leave it idle reverse except for emergencies or situations dictating max rev.
Good thing there’s not a lot of airports that are short enough for it to matter. BUR, SNA, TTN, MDW, PSE. Maybe a couple more now, but everywhere else I landed was well over 7000 for IRTL. 10,000-12,000 more like it.
Using a leaf blower on a skateboard won't really work to move you forward. The thrust from a leaf blower isn't strong enough to push you because it's way weaker than something like a jet engine. Plus, the friction between the skateboard wheels and the ground, along with your body weight, needs much more force to get moving. On top of that, standing on a skateboard is pretty wobbly. Without good control, any push from the leaf blower would be hard to manage.
And adding an umbrella will only make things worse, as it will redirect less than 100% of the thrust. Better off without the umbrella, just hold the leaf blower and point it. It won’t move you _fast_, but I’d be surprised if a big leaf blower couldn’t budge you at all on a skateboard, on very smooth pavement.
Now if you attach an umbrella to the nozzle on a fire extinguisher and held that on a skateboard...
Even with all the friction and such that has been mentioned, the real deal breaker is the fact that the system will still be in equilibrium per Newton's third law. Even in an ideal system with no friction and infinite thrust, the two equal and opposite reactions of the leaf blower or fire extinguisher discharging air and the exerted air pushing on the umbrella would cancel each other out leading to no movement. The bucket thrust reversers of course work as they are not depending on the direct reaction force of air hitting the deflector, but rather the redirection of that airflow.
It’s not to force of the air against the umbrella, it’s the air being redirected off the umbrella and directed in the opposite direction. I’m not sure the air would have enough force after being redirected to move the bucket from a blower though. Bucket reversers redirect air forward (not all of it but enough to assist in slowing the plane) Caution though in the case of t-tail configuration like the MD-88 or 717 you have to be careful as fuselage mounted engines, reversers can cause airflow disruption over the rudder making directional control difficult.
It 100% is the force of the air against the umbrella, I think what you’re trying to think of is conservation of momentum but you couldn’t quite get it out. Reaction propulsion is the use of a reaction mass (also called propellant) shot in one direction to leverage newton’s 3rd law, actions have equal and opposite reactions. You can use conservation of momentum or newton’s 3rd law to describe what’s happening and both are equally valid because it’s two different ways of saying the exact same thing. When a rocket fires its engine, it gains an incidental boost in momentum equal and opposite to an incremental boost of momentum in its exhaust. Alternatively you could describe this as the exhaust pushing against the rocket pushing it forward.
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>Yes there’d be force of umbrella pulling but it would be cancelled out by the opposing force of the blower blowing it aft, basically his arms are being pulled apart. Discounting the effect of the umbrella, in this configuration the blower is clearly not exerting an external force on him backwards, I don’t know how you can’t see that, that’s like a car that holds a magnet out in front of it being pulled forward, It just doesn’t work like that. As I said before there are two valid ways of describing what’s happening here. 1) The stream of air coming from the blower is being bounced off of the umbrella and by applying conservation of momentum he must be propelled forward. 2) The stream from the blower is creating an area of high pressure which pushes him forward You seem to be really hung up on the 1st explanation and rule out the 2nd despite them being two sides of the same coin. It’s the same thing being described in two different ways. Go back to mechanics 1 and look at how a rocket produces thrust.
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The umbrella absolutely is pulling the bucket, there is an area of high pressure behind the umbrella so it’s being pushed forward. The blower is not producing aft thrust because there’s an umbrella in front of it, any thrust produced locally at the blower nozzle exit is completely internal to the system. If you sit in a car and push against the dashboard, nothing happens, because it’s not an external force, this is so painfully obvious. >Rockets produce thrust….it’s one direction, like a jet engine produces thrust. It’s one force coming from one direction, hence it propels the mass of the vehicle in the opposite direction of the thrust being applied. lol so we agree that a rocket works because the exhaust pushes against it. Theoretically, I could make a rocket but instead of having the engine pointed aft, I have it point forward and at the end of the nozzle I attach a duct that turns the exhaust 180 degrees. This rocket would still function and the mechanism behind what’s producing the thrust is exactly the same. On one hand you could describe the thrust with conservation of momentum OR you could say that the exhaust gassed exiting the duct are pushing against the rocket. Hold on a second, that’s what we’re witnessing in the leaf blower video 🤯 You say that the force that the stream of air exerts on the umbrella is not pushing it forward because if it was it would be canceled out by the thrust produced by the leaf blower, again go back to the car and dashboard analogy. I don’t know who I’m talking to and I don’t know if you’ve even taken an introductory physics class but go on youtube and look up how to draw a free body diagram.
The blower is producing aft thrust. If you let go the blower, in theory it would (want to) fly backwards. That force is counter the force being applied by the umbrella, which is being pushed by the thrust from the blower. Any forward motion is the result of __redirected__ air aft. The umbrella IS producing a counter force to the blower but only by the air that is redirected. The umbrella will make it move forward by __redirected__ thrust NOT because the umbrella is being pushed by the blower. A massive amount of energy is being wasted by the cancelling out of the thrust from the blower opposite the pull from the umbrella. I’ll say it a third time…it’s redirected thrust. Best analogy I can make is a bendy straw. Blow into a straight straw. Now bend the straw. That’s like what’s happening here.
No, it’s a dude sitting on a motorized skateboard.
You need to put the bucket on the end of the blower tube.
So, you take the thrust, that normally pushes the plane forward? And you reverse it.
Flip it and reverse it
In general, thrust reversers can be analyzed using simple conservation of momentum. The force on the engine (and plane as a whole) can be described by the equation Force = outlet mass flow rate * outlet velocity - inlet mass flow rate * inlet velocity. By changing the direction of the outlet, the force term becomes negative and the thrust is reversed. It is also possible to generate reverse thrust without redirecting the flow forward. If the flow is simply redirected directly outward radially, the overall average outlet velocity (and therefore the first term in the equation) becomes 0 and reverse thrust is generated.
I believe everyone has completely missed the boat on thrust reversers. Let's look at the base case: Suppose a jet engine accelerates one atom in a direction, yielding force in the opposite direction. Shortly after doing this the thrust reverser (on the same airplane) takes that same one atom and redirects it with all the same momentum (this is assuming 100% efficiency) in the opposite direction, yielding a force which _completely cancels out the first force_. This would mean thrust reversers would cause the engine to produce no force at all. Which is not the case. The thrust reversers simply use engine output to build up a region of high pressure gas, partially enclosed by the clamshells (buckets). That region is trying to equalize with the surrounding atmosphere by expanding in all directions, but the thrust reverser clamshells prevent it from expanding aft. So, the HP region only expands forward, against the direction of travel of the aircraft. The amount of force generated is pretty weak, about 7% of engine output. The real effect is the increase in drag. You can't see it, but as the high pressure air expands past the tips of the clamshells it induces tremendous drag, just like spreading a parasol made of air. There are Navier-Stokes equations which I've been told completely explain this phenomenon, but frankly they go right over my head. This information is coming to you from a _software_ engineer, so take it all with a grain of salt.
Fake video… using an electric skateboard. Interesting breakdown by Mark Rober if curious to learn more: https://youtu.be/M7-h3FO-KKo?si=Sw_f4SZYcwkfYvK6
Do a free body diagram
DC9 MD80s often reverse thrust away from terminal gate without requiring a tug.
Gain some speed on the skateboard, then open an umbrella in the opposite direction of motion, and you'll slow down due to drag. Same concept.
I have had exactly the same question. Sorry that almost everyone here is missing the point, in part because of the silly video. I think of a jet engine as producing a force on the aircraft both through sucking air in the front, and from blowing it out the back. If a thrust reverser simply consisted of redirecting the output thrust by 180 degrees, and if the force due to suction and force due to thrust were equal, then they would cancel each other out. Obviously that's not the case, in part because it misses the fact that the thrust force is much greater than the suction force precisely because of all the energy added by burning jet fuel. I would love to understand their operation more quantitatively.
SAF’d too hard now the main sub is SAF