- #36
Dale
Mentor
- 35,337
- 13,562
Vectored thrust does indeed provide lift if it is vectored downwards. This is the principle of VSTOL aircraft
A.T. said:To make it more more intuitive think about a hole to the side, near the bottom. It clearly produces horizontal "thrust". Now attach a pipe to the hole that bends 90° downwards, and you have the same "thrust", but upwards, just like with the hole in the bucket bottom.
It does impart a vertical force.Danger said:It imparts torque to the bucket, along with the lateral force, but not lift.
None of that is need for the vertical force.Danger said:If you were to add two more bends to that pipe to have it eventually exit straight down under the centre of the bucket,
VSTOL's, as I specified in my post, utilize balanced thrust. The Raptor has a balancing lift fan far ahead of the main engine, the Harrier has 4 properly located ducts, etc.. In the case of the simple bent pipe, the upward component of the thrust would merely be upon the part of the pipe where it bends. That would impart a torque factor to the bucket, with its far-side bottom edge as the fulcrum.DaleSpam said:Vectored thrust does indeed provide lift if it is vectored downwards. This is the principle of VSTOL aircraft
That is completely irrelevant. The vertical force is there, regardless whether its torque is balanced by some other force.Danger said:VSTOL's, as I specified in my post, utilize balanced thrust.
It would impart a vertical force, which creates torques around some points.Danger said:That would impart a torque factor to the bucket
The original question was far more precise and specific than your, as usual, vague and ambiguous phrasing. It asked if a specific term still appears (is non-zero), if the variable mass system equation is applied to the bucket.Danger said:The original question asked whether or not a hole in the bottom of the bucket would act like a jet ...
That is unnecessarily rude, even for you. The original question, if you think back, was so incomprehensible that neither Dale nor I could understand it. The OP has since edited it. His first explanation, after we requested one, homed in on a hole in the bottom of the bucket and asked for the application of the movement equation due to jet effect, of which there is none. He also specified that the bucket is on the floor, which means that there would be no movement at all, even torque, unless it was a sealed and pressurized bucket.A.T. said:The original question was far more precise and specific than your, as usual, vague and ambiguous phrasing.
And that is the version I'm responding to.Danger said:The OP has since edited it.
The OP doesn’t ask about a "jet effect", whatever that might mean. It asks about a specific term:Danger said:jet effect, of which there is none.
And that term is not zero or "none".sergiokapone said:##u\frac{dm}{dt}## -- is jet reaction force
Which I didn't even see until I checked back to see what you were going on about.A.T. said:And that is the version I'm responding to.
Oh, really...? Have you read the title of the thread? "About jet reaction force"A.T. said:OP doesn’t ask about a "jet effect", whatever that might mean.
Yes, that how the OP calls the specific term it asks about. If your "jet effect" refers to the same term as "jet reaction force", then you are simply wrong, it's not zero.Danger said:Have you read the title of the thread? "About jet reaction force"
Sure, it is balanced, but it is still vectored. As long as you vector thrust downwards you will have a lift force upwards, even if it is not balanced (in which case you will have a torque in addition to the lift). Having the torque is a bad idea for a vehicle design, but it doesn't mean that there is no lift from a downward vectored thrust.Danger said:VSTOL's, as I specified in my post, utilize balanced thrust.
There has got to be some huge communications gap going on here. By your explanation, it would be self-contradictory to have a U-shaped tube mounted to the bottom. The jet thrust would be downward, but the bucket would be moving upward because of losing weight.DaleSpam said:Assuming that the jet of water is not hitting the scale, a bucket with a downward jet will be measured to weigh less than an identical one without a jet. It does in fact provide thrust, just not much.
Thank you for posting that. It leaves my answer intact (although I suspect that other's might still argue about it). To whatever extent there is any "thrust" at all, it is upon the remaining water, not upon the bucket. (Unless you want to include viscous drag of the water against the sides of the bucket, which would be simply ridiculous.)sergiokapone said:If in the bottom of the bucket with water we create an outlet, then water will flow down from it. Is the thrust of the water on the bucket? Explain the falsity of the positive answer to this question.
sergiokapone said:If in the bottom of the bucket with water we create an outlet, then water will flow down from it. Is the thrust of the water on the bucket?
What?Danger said:...but the bucket would be moving upward because of losing weight.
If you had a U-shaped tube mounted on the bottom such that the jet of water was squirting upwards then the bucket would be measured to weigh more than an otherwise identical bucket without the jet.Danger said:There has got to be some huge communications gap going on here. By your explanation, it would be self-contradictory to have a U-shaped tube mounted to the bottom. The jet thrust would be downward, but the bucket would be moving upward because of losing weight.
That should be pretty obvious to anyone who speaks English. (KFC doesn't count; in their case, a bucket is obviously meant as the bucket plus the chicken in it but that's a commercial term.)A.T. said:The answer to that depends what is considered to be "the bucket". Just the vessel, or the vessel + content?
You just contradicted yourself in the course of 2 sentences. Where does that equation mention just the vessel without content?A.T. said:in both cases there is thrust, as defined in the Variable-mass system equation, that acts either on the content or the vessel + content system.
That was in reference to Dale specifying that the bucket is on a scale, which obviously would be imparting an upward variable force. Without a scale, with the bucket simply sitting on the floor as the OP stated, the closest thing to an upward force would the the increase of the bucket's buoyancy in air and the decrease of both gravity attracting it and the floor pushing up without movement. I admit to both, but don't see how it can be considered a "jet reaction".A.T. said:What?
The positive answer is not false: http://en.wikipedia.org/wiki/Water_rocketsergiokapone said:If in the bottom of the bucket with water we create an outlet, then water will flow down from it. Is the thrust of the water on the bucket? Explain the falsity of the positive answer to this question.
What is the difference between a hole and a short straight vectoring tube?Danger said:In the case of a vectoring tube, that provides a structural connection between the tube and the bucket to transfer the force. In the case of a simple hole in the bottom, there isn't one.
I merely gave two answers for both possible interpretations of the question.Danger said:You just contradicted yourself in the course of 2 sentences
Simple... a short straight down tube is not a vectoring device. Its existence is totally irrelevant because it causes no different effect than a hole without it.DaleSpam said:What is the difference between a hole and a short straight vectoring tube?
No, you didn't. You made one sweeping statement that turned around and bit itself. I merely quoted it in separate sections.A.T. said:I merely gave two answers for both possible interpretations of the question.
No idea what you mean.Danger said:You made one sweeping statement that turned around and bit itself.
I think you are making a wrong conclusion. It causes no different effect than a hole without it, therefore there is no difference between a hole and a vectoring tube. Consider, if you put a hole on the side near the bottom then you clearly get thrust. Therefore a hole on the side is the same as a short straight vectoring tube on the side, both cause thrust to the side. Placing it on the bottom doesn't change any of that.Danger said:Simple, a short straight down tube is not a vectoring device. Its existence is totally irrelevant because it causes no different effect than a hole without it.
The water on the bottom of a bucket is also pressurized and the water reaction mass is also forceably ejected. The water rocket simply increases the pressure to make the effect obvious, but the water at the bottom of a bucket is already inherently pressurized by the weight of the water above it.Danger said:Do you not understand the irony of linking to the example of a water rocket? That is a pressurized system where the water reaction mass is forceable eject by an action against the interior or the bottle.
If it's just a dribble, then u is 0, and so is the thrust. But the water can be shooting out with high speed due to the pressure, and then u > 0.sophiecentaur said:If the dribble out of the bottom of the bucket...
I'm not surprised...A.T. said:No idea what you mean.
Good grief, this is maddening!DaleSpam said:there is no difference between a hole and a vectoring tube. Consider, if you put a hole on the side near the bottom then you clearly get thrust. Therefore a hole on the side is the same as a short straight vectoring tube on the side, both cause thrust to the side. Placing it on the bottom doesn't change any of that.
The water on the bottom of a bucket is also pressurized and the water reaction mass is also forceably ejected. The water rocket simply increases the pressure to make the effect obvious, but the water at the bottom of a bucket is already inherently pressurized by the weight of the water above it.
Again, there is no pressure that is associated with the bucket other than that supplied by the ambient air. You can't pressurize an open container.A.T. said:If it's just a dribble, then u is 0, and so is the thrust. But the water can be shooting out with high speed due to the pressure, and then u > 0.
For the bucket alone you don't need the variable-mass equation.sergiokapone said:Google translate says the bucket is
The water has a higher pressure than atmospheric at the bottom of the bucket:Danger said:The water in the bucket is not pressurized to above atmospheric,
No, bucket is with the water of course.A.T. said:For the bucket alone you don't need the variable-mass equation
It most certainly is. Ask any scuba diver if water has greater than atmospheric pressure.Danger said:The water in the bucket is not pressurized to above atmospheric
sergiokapone said:No, bucket is with the water of course.