The dumb questions thread

[George S]

Draw a circle representing the copter. Draw an up arrow labeled +f, and a down arrow from the circle labeled -f. Force vectors acting on the copter This boys and girls is called a free body diagram in mechanics and is a very very useful tool when used by experienced adults.Now listen closely...when +f = -f the copter is hovering. Are you with me so fat? Good. Okay so far jokodo? Good boy.

Now, the down force is -mg, the mass of the helicopter times gravitational acceleration from Newton's Laws. The up force is thrust from the helicopter blades. Those things that go round and round and round.

Now boys and girls a question for your homework in basic mechanics. A copter has a weight which can be measured on a scale on the ground, When the copter is hovering on what part of the copter bears the total weight?

After we answer that question boys and girls we will move on to the next step in the analysis.

No part of the copter bears the total weight. The low pressure air just above the spinning blades provides the total lift. The atmosphere provides the lift.

It seems to me that as the copter is leaving the scale it will show less weight as the copter rises.

What is with the dimensional analysis? The two forces (m*g and lift), no matter the units, cancel. +f (due to gravity) = -lift (due to differential air pressure on the surfaces of the spinning wing.)

____
Notes:
Injecting real-world considerations: Some of the lift is ground-effect. The above is a simple hover well above the ground. The propeller effect (the blades are not only the wings but also the propeller) blows some wind down which contributes to a small portion of the lift.
Another: The copter is getting lighter every moment as fuel is burned. Hovering is not really possible except in thought experiments.

 

[Jokodo]

I went through this to no end on the other thread derving pressure in pascals in a gas tank.

Read the link on statistical mechanics used to derive pressure in N/m^2 in a tank of gas. It is straightforward. The downward motion of the air created by the rotor blades exerts a pressure in Newtons/m^2 on the scale. Press on a scale with your hand and you exert pressure in pascals.The scale displays equivalent mass proportion to force your hand imparts to the scale.

Conservation of energy says the kinetic energy imparted to the air by the blades must equal the the weight of the copter, m*g. For steady state hover thrust must equal weight of the copter.

So what you're saying is, Newtons per square meter and Newtons per square meter per second are the same thing and can be used interchangeably. In other words, time doesn't exist.

For all your posts you have not learned a thing.

I'm learning new things everyday. I'm in a new line of work where I only started a couple months ago, and some aspects are still challenging.

When it comes to understanding that you can't equate quantities of differing dimensions, you're the one who has to learn.

 

[Kharakov]

@ steve.

Total mass of air accelerated downwards per second = gravitational mass of copter * 9.8 mps + frictional acceleration of air passing copter downwards.

What mass of air does a 1 kg point mass (zero volume) copter with super teflon non friction skin (who cares... it's a point) displace in a downwards direction per second as it hovers?

 

[bilby]

Draw a circle representing the copter. Draw an up arrow labeled +f, and a down arrow from the circle labeled -f. Force vectors acting on the copter This boys and girls is called a free body diagram in mechanics and is a very very useful tool when used by experienced adults.Now listen closely...when +f = -f the copter is hovering. Are you with me so fat? Good. Okay so far jokodo? Good boy.

Now, the down force is -mg, the mass of the helicopter times gravitational acceleration from Newton's Laws. The up force is thrust from the helicopter blades. Those things that go round and round and round.

Now boys and girls a question for your homework in basic mechanics. A copter has a weight which can be measured on a scale on the ground, When the copter is hovering on what part of the copter bears the total weight?

After we answer that question boys and girls we will move on to the next step in the analysis.

No part of the copter bears the total weight.

Actually, the rotor hub does. Without lateral movement relative to the air, a hovering helicopter is effectively suspended from the rotor hub, as only the rotor provides any lift; If you stopped the engine, hooked a crane to the rotor hub, and lifted the copter off the ground, the forces on every part of the machine other than the rotor blades themselves, and the torque effects (which act normal to lift, and are therefore negligible) would be the same.

Hovering is possible, despite the continuous change in mass due to burning fuel - the rate of fuel consumption is fairly constant, and can be compensated for by a constant small adjustment to the collective. A bigger problem is variable wind; unless it is a perfectly still day, there is a larger and less predictable adjustment required to the cyclic in order to stay in place. And ground effects will have a potential impact on lift too - but above a (presumably flat) scale of effectively infinite radius, those effects too should be constant. In principle hovering in place should be possible to an arbitrary degree of precision, assuming an arbitrary level of pilot skill. In practice, there will likely be some noticeable drift in all three dimensions about the intended hover location.

It is hilarious that Steve has presented a high school level physics problem, and still managed to get so much wrong, while making condescending remarks to everyone who points out his rather basic errors. Dimensional analysis isn't particularly hard, but this is the third thread I have seen where he completely fails to see that he has it wrong. It's become so tiresome that I put him on ignore - but I still see his stuff if people quote it, and I have to laugh.

 

[Wiploc]

Actually, the rotor hub does.

I think it would bear a little more than the total weight. Some of the air it thrusts downward will hit the body of the copter, wasted thrust.

A bigger problem is variable wind; unless it is a perfectly still day...

I'm assuming the scale is a flat surface extending to infinity, and that there is no wind. For obvious reasons, I won't assume a vacuum.

 

[bilby]

Actually, the rotor hub does.

I think it would bear a little more than the total weight. Some of the air it thrusts downward will hit the body of the copter, wasted thrust.

A bigger problem is variable wind; unless it is a perfectly still day...

I'm assuming the scale is a flat surface extending to infinity, and that there is no wind. For obvious reasons, I won't assume a vacuum.

Yeah, I tried assuming a spherical helicopter in a vacuum, but the idea never got off the ground

 

[steve_bank]

I did some net reading on copter aerodynamics. Yes it is mare complicated than simple downdraft which is why I said hovering just above a large scale.

At hover ignoring secondary effects the rotor shaft sees a lateral centrifugal force of the rotating blades. The rotor shaft sees the weight of the helicopter as a tension load along the shaft. When you sit in a seat your weight is transfer through the rigid structure to the shaft and the blades.

The rooters see the weight load distributed on the blades in gs or pounds per squatter For the helicopter to hover there must be a vertical vector mg on the copter. That force is felt on the blades. Air is pushed down creating a plosive up vector on the rotors. It is not entirely true, imagine the entire coptor including the blades as rigid.

The coptor can never be weightless. At hover if you stand on a scale you will get your 1g body weight.

 

[steve_bank]

https://www.youtube.com/watch?v=lVeP6oqH-Qo
https://www.bbc.com/news/science-environment-30797983

The original question was if bids taking flight in a closed contained made it lighter. Mythbusters first used birds. The result was nosy but the average Wight did not change. They repeated with a helicopter and the weight remain steady.


For weight to stay constant in the observed helicopter experiment the force of air must equal the mg force acting on the helicopter. From Newton's law of equal and opposite reaction there must be a force on the blades equal to mg of the of the helicopter.
Somebody at Stanford repeated the bird experiment with more senate equipotent and could see weight change as wings flapped.

 

[Bomb#20]

You can work through the dimensional analysis at your leisure.

Okay, let's discuss your dimensional analysis error. The gravitational energy of the helicopter at hover is steady state energy. The kinetic energy in the air forced down is continuously delivered to air as it arrives above the rotors, so the helicopter transmits a certain amount of kinetic energy to the air per second that it's hovering. When you claim "The kinetic energy in the air forced down equals the gravitational energy of the helicopter at hover.", how many seconds does it take for the helicopter to give the air an amount of energy equal to the helicopter's gravitational energy?

I went through this to no end on the other thread derving pressure in pascals in a gas tank.

Read the link on statistical mechanics used to derive pressure in N/m^2 in a tank of gas. It is straightforward. The downward motion of the air created by the rotor blades exerts a pressure in Newtons/m^2 on the scale. Press on a scale with your hand and you exert pressure in pascals.The scale displays equivalent mass proportion to force your hand imparts to the scale.

Conservation of energy says the kinetic energy imparted to the air by the blades must equal the the weight of the copter, m*g. For steady state hover thrust must equal weight of the copter.

You keep getting the forces right and getting the energy wrong. Then when people correct you on the energy you keep lecturing them about the forces, as though it proved what you said about energy. Stop doing that. Pay attention to what people tell you instead of assuming they're ignorant students and you're a professor. You aren't a professor. Stop being so bloody arrogant.

Conservation of energy does not say the kinetic energy imparted to the air by the blades must equal the the weight of the copter. That's ridiculous. Kinetic energy and weight are in different units. They can't be equal.

 

[Jokodo]

Dumb question: Is there a deep physical reason why blue pigments tend to be more persistent than those of other colours?

You know how a formerly pink poster that's been hanging outside (even behind glass) for a couple of months, or a magazine that's been sitting on a table next to the window turns bluish as the red pigment fades more rapidly under the light?

I was thinking about maybe the reason being that red pigments (i. e. the ones that absorb blue light) degrade faster because they are exposed to more energetic radiation, which blue pigments reflect. But I don't know enough to judge whether that's plausible.

Or is it just a matter of historical contingency, just an accidental fact of life: We just so happened to stumble across a blue-reflecting molecule that's cheap to produce and easy and safe to handle and which, by mere coincidence, is more durable than any otherwise comparable red pigment?

On a probably unrelated note, red pigments also tend to wash off more easily. When you squish a fresh bilberry/whortleberry/blueberry, the juice is red with just a hint of purple. But the stains on your hands after you've nibbled a handful and tried and failed to remove the evidence are pure blue. Similar with blue red wine stains. I was going to ask: Does that have anything to do with red pigment molecules having different shapes that make them less adhesive? -- but it appears the answer lies in the  Anthocyanin contained in both, and which is turns blue when it's removed from the acidic environment of the fruit or wine.

 

[Angry Floof]

Different colors are made from different materials that give the ink or dye that color. I would ask what the blue ink and red ink are made of and what's different in the chemical makeup.

 

[steve_bank]

I went through this to no end on the other thread derving pressure in pascals in a gas tank.

Read the link on statistical mechanics used to derive pressure in N/m^2 in a tank of gas. It is straightforward. The downward motion of the air created by the rotor blades exerts a pressure in Newtons/m^2 on the scale. Press on a scale with your hand and you exert pressure in pascals.The scale displays equivalent mass proportion to force your hand imparts to the scale.

Conservation of energy says the kinetic energy imparted to the air by the blades must equal the the weight of the copter, m*g. For steady state hover thrust must equal weight of the copter.

You keep getting the forces right and getting the energy wrong. Then when people correct you on the energy you keep lecturing them about the forces, as though it proved what you said about energy. Stop doing that. Pay attention to what people tell you instead of assuming they're ignorant students and you're a professor. You aren't a professor. Stop being so bloody arrogant.

Conservation of energy does not say the kinetic energy imparted to the air by the blades must equal the the weight of the copter. That's ridiculous. Kinetic energy and weight are in different units. They can't be equal.

Show ,e a free body diagram that makes your point. Are you arguing the weight of the helicopter does not apear on the rotor blades or places the rotor shaft in tension, Tension Means stretching as opposed to compression or torque.

You stand in a hovering helicopter. The floof keeps you from falling and through your feet your body creates a pressure, force per unit area, on the floor of the helicopter. F = ng where m is your body mass.

Where vectorially is that force per unit area counterbalanced on the helicopter? Are you arguing there is no vertical force exerted on the blades or is it magic that keeps the helicopter up?

In fixed wing level flight vectorially what counteracts mg of the aircraft to keep it up? Don't just say lift dies it. What are the forces acting on the wings?What is the force per unit area on the wing? It is the same question.

 

[Jokodo]

Different colors are made from different materials that give the ink or dye that color. I would ask what the blue ink and red ink are made of and what's different in the chemical makeup.

That seems to be an answer in favour of "accident of history".

Is there only *one* commonly used blue/red ink each?

Because I've *never* seen the opposite happen: a purple print turning red under prolonged exposure to sunlight. Have you?

 

[Jokodo]

I went through this to no end on the other thread derving pressure in pascals in a gas tank.

Read the link on statistical mechanics used to derive pressure in N/m^2 in a tank of gas. It is straightforward. The downward motion of the air created by the rotor blades exerts a pressure in Newtons/m^2 on the scale. Press on a scale with your hand and you exert pressure in pascals.The scale displays equivalent mass proportion to force your hand imparts to the scale.

Conservation of energy says the kinetic energy imparted to the air by the blades must equal the the weight of the copter, m*g. For steady state hover thrust must equal weight of the copter.

You keep getting the forces right and getting the energy wrong. Then when people correct you on the energy you keep lecturing them about the forces, as though it proved what you said about energy. Stop doing that. Pay attention to what people tell you instead of assuming they're ignorant students and you're a professor. You aren't a professor. Stop being so bloody arrogant.

Conservation of energy does not say the kinetic energy imparted to the air by the blades must equal the the weight of the copter. That's ridiculous. Kinetic energy and weight are in different units. They can't be equal.

Show ,e a free body diagram that makes your point. Are you arguing the weight of the helicopter does not apear on the rotor blades or places the rotor shaft in tension, Tension Means stretching as opposed to compression or torque.

You stand in a hovering helicopter. The floof keeps you from falling and through your feet your body creates a pressure, force per unit area, on the floor of the helicopter. F = ng where m is your body mass.

Where vectorially is that force per unit area counterbalanced on the helicopter? Are you arguing there is no vertical force exerted on the blades or is it magic that keeps the helicopter up?

In fixed wing level flight vectorially what counteracts mg of the aircraft to keep it up? Don't just say lift dies it. What are the forces acting on the wings?What is the force per unit area on the wing? It is the same question.

The upward thrust produced by the rotor blades may well be equal to the gravitational acceleration the helicopter is experiencing. As long as the copter is hovering in place, that's exactly the situation.

But that doesn't make it equal to its kinetic or potential or any other kind of energy. Power != energy. Incompatible units. The gravitational acceleration of the copter is equal to the acceleration of the air molecules produced by the rotor (-ish: assuming no friction and a perfectly vertical beam of air no part of which hits the body of the helicopter), but neither can be expressed in Joules.

 

[steve_bank]

Show ,e a free body diagram that makes your point. Are you arguing the weight of the helicopter does not apear on the rotor blades or places the rotor shaft in tension, Tension Means stretching as opposed to compression or torque.

You stand in a hovering helicopter. The floof keeps you from falling and through your feet your body creates a pressure, force per unit area, on the floor of the helicopter. F = ng where m is your body mass.

Where vectorially is that force per unit area counterbalanced on the helicopter? Are you arguing there is no vertical force exerted on the blades or is it magic that keeps the helicopter up?

In fixed wing level flight vectorially what counteracts mg of the aircraft to keep it up? Don't just say lift dies it. What are the forces acting on the wings?What is the force per unit area on the wing? It is the same question.

The upward thrust produced by the rotor blades may well be equal to the gravitational acceleration the helicopter is experiencing. As long as the copter is hovering in place, that's exactly the situation.

But that doesn't make it equal to its kinetic or potential or any other kind of energy. Power != energy. Incompatible units. The gravitational acceleration of the copter is equal to the acceleration of the air molecules produced by the rotor (-ish: assuming no friction and a perfectly vertical beam of air no part of which hits the body of the helicopter), but neither can be expressed in Joules.

Correct. The fuselage of the helicopter does not provide lift. All weight of the helicopter including cargo and passengers is connected mechanicaly to the rotors. Imagine a hang glider at constant altitude in a thermal. At constant altitude the weight of the person plus harness and rigging reacts in a a pounds per square inch on the wing surface,. Total weight is distributed across the wing.

Energy is a scalar. Power is rate of change of energy,
Energy E = Joules
Power P = dE/dt Watts

From the link on deriving pressure in a tank using statistical mechanics.

The blades striking the air particles changes average momentum of the gas and the change in momentum crates a vertical force in newton's on the blade., derived from Newton's Laws. When the downward column of air strikes the scale surface the change in direction of particle by Newton's Laws results in a downward force on the scale.

There fore at any point in time during hover the average total kinetic energy in the moving air partcles must equal mg of the helicopter.

You can look at from Bernoulli's Equations which itself is a continuity equation. A continuity equation is an expression of conservation and defines energy and mass input and output of system. The system in this case is air and the rotating blade. Conservation says energy and mass of the roaring blade, energy added to the air, and the energy to counter gravity for hover must all add up. That is the beauty and the power of LOT.

Whoevers I got stuck on a problem I resorted to LOT.

When you start looking at things through LOT seemingly complex problems become simpler. Electric circuits, fluid mechanics, aerodynamics it is all the same. Specific equations fill in the blanks for matter, energy, and entropy or losses.

 

[barbos]

This is becoming way more complicated than it needs to be.
Imagine whole earth surface being a scale, then everything (including air) above it will be registered. The fact that something does not have direct contact with the scale is irrelevant. The only effect is due to the altitude because weight of the object depends on the distance to the earth but it's rather small because radius of the earth is so much larger than helicopter altitude (1/r^2 factor)

 

[bilby]

This is becoming way more complicated than it needs to be.

I agree wholeheartedly.

Imagine whole earth surface being a scale,

OK... Now I am trying to work out how that could possibly be engineered. Are you sure that this isn't getting even more over complicated?

then everything (including air) above it will be registered. The fact that something does not have direct contact with the scale is irrelevant. The only effect is due to the altitude because weight of the object depends on the distance to the earth but it's rather small because radius of the earth is so much larger than helicopter altitude (1/r^2 factor)

Yeah, I am pretty sure that this isn't making things less complicated.

Let's try this - Imagine a helicopter hovering. As it isn't moving vertically, we can confidently assert that the lift is exactly equal to the weight.

Keep your scales for weighing vegetables at the supermarket.

 

[fromderinside]

Great. Now all we need to know is the planet, air pressure, and altitude where equivalence is achieved to determine gravity force. Or am I missing something. After all I'm not a physicist nor even an aeronautical engineer who ever had the budget to conduct such an experiment.

Why not reduce this to a lab problem with known materials and reliable instruments to measure effects of specific masses against the unknown force in question. I'm pretty sure Newton would agree.

How about something where one has to calculate the average effect of sound at standard temp pressure on inner hair cells located at resonance in the Basilar membrane after a 1000 hz. 60 DBA tone arrives there in sones.

 

[steve_bank]

Gravity was initially measured using spheres and as delicate a balance mechanism as could be made at the time. It is an attractive force that can not be shielded. The Cavendish Experiment.

https://en.wikipedia.org/wiki/Cavendish_experiment

Drop a cannon ball from 10,000 feet and technically the Earth and ball are falling towards each other. Due to relative mass the effect of the ball on the Earth is imperceptible.Gravity is a force. A spring-mass bathroom scale is a force meter. The spring deflection is proportional to f = ma = mg.\

The Earth is not a scale.

 

[steve_bank]

As a junior engineer interviewing at Intel the manager asked me a question.

In 0g a sealed sphere is filled with oxygen. A very small candle is somehow lit at the center of ball. Will the candle stay lit as long as the O2 is not used up?