Originally Posted by ShoeDLG

I wouldn’t dismiss Peter Lissaman as a theorist. He was an accomplished designer as well.

I was commenting on the article, not the man.

Based on this and another story he wrote for "Air & Space" magazine, he was certainly an accomplished and entertaining writer.

Originally Posted by ShoeDLG

When you put a quadcopter over a scale it becomes an eggbeater.

So, if a vacuum cleaner is a one-prop quad (xlcrlee, #173)

and a quad over a scale is an eggbeater

Then I should be able to make omelets using four vacuum cleaners over a scale?

Is there any MOMENTUM for marketing such an idea ?

Or is this thread running out of momentum?

Originally Posted by jruley

.... is this thread running out of momentum?

Fear not, NO worries so long as the silly chickens keep laying their infertile eggs

I admit I don't have the stamina to read the whole thread. But I get the impression there are certain things which are not generally understood about the quad copter in the box.

When the quad copter is in the box, the net force on the inside of the box is equal to the weight of the air plus the weight of the quad copter. When the quad copter is hovering, it doesn't exert pressure directly on the box, but the air still does, and the air is the only thing exerting force on the inside of the box. So, since the mass supported is the same, the net force exerted by the air is now the weight of the air plus the weight of the copter. Forgive me if this has already been clarified, but it seemed like it hadn't.

Speaking of momentum transfer, if you approximate the rotor disks as infinitely thin discs that instantaneously transfer momentum to the air, that momentum transfer will match the weight of the quad copter closely, except for any aerodynamic effects on the rest of the quad copter. If you watch a particle of dust, you'll see that momentum transfer when it goes through the disc. Obviously a particle at any given spot in the box will have the same momentum. However, the next time you look it will be another particle. The particles in the box go through a cyclical change in momentum. Not sure if that's clarifying or muddying.
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As far as the original contention, I'm tired of people thinking there's some conflict between Newton and Bernoulli. How is the air supposed to move around without differences in pressure?

Originally Posted by lincoln

I admit I don't have the stamina to read the whole thread. But I get the impression there are certain things which are not generally understood about the quad copter in the box.

When the quad copter is in the box, the net force on the inside of the box is equal to the weight of the air plus the weight of the quad copter. When the quad copter is hovering, it doesn't exert pressure directly on the box, but the air still does, and the air is the only thing exerting force on the inside of the box. So, since the mass supported is the same, the net force exerted by the air is now the weight of the air plus the weight of the copter. Forgive me if this has already been clarified, but it seemed like it hadn't.

Speaking of momentum transfer, if you approximate the rotor disks as infinitely thin discs that instantaneously transfer momentum to the air, that momentum transfer will match the weight of the quad copter closely, except for any aerodynamic effects on the rest of the quad copter. If you watch a particle of dust, you'll see that momentum transfer when it goes through the disc. Obviously a particle at any given spot in the box will have the same momentum. However, the next time you look it will be another particle. The particles in the box go through a cyclical change in momentum. Not sure if that's clarifying or muddying.
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As far as the original contention, I'm tired of people thinking there's some conflict between Newton and Bernoulli. How is the air supposed to move around without differences in pressure?

Good post - except that there's no conflict between Newton and Bernoulli.

However ... only Newton defines the lifting force that keeps something up ... and that is defined as a reaction to the action of accelerating air downwards - by the device doing the work.

Examples ... A rocket. A turbofan. A jet. A wing. A rotor. An air hose - etc. etc. etc.

It's not rocket science. It's quite clear. Quite simple. NASA even has some pages on this.

No air moved down, no lift.

Does anyone understand the origin of the meaning of "Rocket Science"? At the end of WWII both the USSR and the USA did their best to grab the German rocket scientists for nationalistic security reasons. After the Russians got Sputnik up the USA tried and repeatedly failed using the Vanguard design. Final success came when the Redstone missile, etc., and other products, technologies AND "Rocket Scientists" of the "enticed" Germans at Huntsville saved the day.

The success came because those engineers & scientists were the result of European well-established and high-level education, vs the long-standing US social concept of eschewing "Higher Learning" (except for a few "Egg Heads"). Today even Asia far surpasses general US education: most US graduate schools are filled with non-American students, who turn out to be far better qualified than their Amertican counterparts. My experience indicates that a call to a random extension at Microsoft/Redmond or Boeing/Seattle has a ca. 85% likelihood of being answered by a VERY competent and VERY well-educated non-American. "Sad But True"

That noted: this thread DOES require several aspects of "Rocket Science", meaning deep and thorough EDUCATION; lacking in far too many (as in most) US-educated folk.

To start, this is incorrect:

Originally Posted by lincoln

When the quad copter is in the box, the net force on the inside of the box is equal to the weight of the air plus the weight of the quad copter.

and this attempt to reconcile that does not help:

Originally Posted by lincoln

When the quad copter is hovering, it doesn't exert pressure directly on the box, but the air still does, and the air is the only thing exerting force on the inside of the box. So, since the mass supported is the same, the net force exerted by the air is now the weight of the air plus the weight of the copter. Forgive me if this has already been clarified, but it seemed like it hadn't.

No point in further adding to the continuing series of foolish and childish "eggs" still being laid by the silly chickens who have not only NOT gotten a thorough education in aerodynamics and physics, but who rather prefer the comfort of their straw roost ..... all that is required is reading ALL of ShoeDLG's well-educated and factual posts until one actually understands them

Carry on, silly "cooped-up" [intellectually self-restricted] chickens ....

Originally Posted by xlcrlee

Does anyone understand the origin of the meaning of "Rocket Science"? At the end of WWII both the USSR and the USA did their best to grab the German rocket scientists for nationalistic security reasons. After the Russians got Sputnik up the USA tried and repeatedly failed using the Vanguard design. Final success came when the Redstone missile, etc., and other products, technologies AND "Rocket Scientists" of the "enticed" Germans at Huntsville saved the day.

Dear, oh dear, oh dear!

The Tsiolkovsky rocket equation was derived by Gospodin Konstantin in 1903:

https://en.wikipedia.org/wiki/Tsiolk...ocket_equation

This is the beginning of "Rocket Science". As opposed to random firecracker experiments leading to Received Wisdom passed down through Chinese families.

Robert Goddard developed the first liquid-fueled rocket in 1926. A mere provincial American, whose work was funded by the Smithsonian Institution and Guggenheim Foundation. Not NASA (which didn't exist yet).

https://www.nasa.gov/centers/goddard...r_goddard.html

Bright young German students like Werner von Braun wrote to Herr Doktor Goddard asking about his experiments. Their application of his work led to the V-2 weapons program, but the underlying SCIENCE was his.

The Russian rocket efforts were led by Sergei Korolev:

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

According to an article some years ago in "Air & Space", the Soviets never trusted "their" captured German scientists and put them to work designing consumer products.

So much for the alleged superiority of "European well-established and high-level education".

Originally Posted by xlcrlee

Does anyone understand the origin of the meaning of "Rocket Science"? At the end of WWII both the USSR and the USA did their best to grab the German rocket scientists for nationalistic security reasons. After the Russians got Sputnik up the USA tried and repeatedly failed using the Vanguard design. Final success came when the Redstone missile, etc., and other products, technologies AND "Rocket Scientists" of the "enticed" Germans at Huntsville saved the day.

The success came because those engineers & scientists were the result of European well-established and high-level education, vs the long-standing US social concept of eschewing "Higher Learning" (except for a few "Egg Heads"). Today even Asia far surpasses general US education: most US graduate schools are filled with non-American students, who turn out to be far better qualified than their Amertican counterparts. My experience indicates that a call to a random extension at Microsoft/Redmond or Boeing/Seattle has a ca. 85% likelihood of being answered by a VERY competent and VERY well-educated non-American. "Sad But True"

That noted: this thread DOES require several aspects of "Rocket Science", meaning deep and thorough EDUCATION; lacking in far too many (as in most) US-educated folk.

To start, this is incorrect:

and this attempt to reconcile that does not help:

No point in further adding to the continuing series of foolish and childish "eggs" still being laid by the silly chickens who have not only NOT gotten a thorough education in aerodynamics and physics, but who rather prefer the comfort of their straw roost ..... all that is required is reading ALL of ShoeDLG's well-educated and factual posts until one actually understands them

Carry on, silly "cooped-up" [intellectually self-restricted] chickens ....

Most of this mystery stuff can be explained by using one thing
logic
physics and aerodynamics are just subsets of logic.
Ya gotta be careful tho
what appears logical -may not be so

Originally Posted by xlcrlee

...all that is required is reading ALL of ShoeDLG's well-educated and factual posts until one actually understands them

Agreed. However, if you actually understood ShoeDLG's posts, you'd realize that he does not disagree with either of lincoln's points.

Originally Posted by richard hanson

Most of this mystery stuff can be explained by using one thing
logic
physics and aerodynamics are just subsets of logic.
Ya gotta be careful tho
what appears logical -may not be so

Please allow me to further elaborate, wise Richard:

1. BOTH Physics & Aerodynamics are intrinsically EMPIRICAL (means repeated and repeatable tests)

2. It is obviously illogical to assume that abstractions will always, ever or even sometimes fit measured and careful multi-person observation

And that is because ... despite our best wishes otherwise ... Reality is inherently "illogical" because it is far too complex and variable to fit our human-tailored "logical" simplifications. Even Plato's "Perfect Circle" which he supposed we can create (only) in our brains, is NOT even slightly "perfect"; instead we LIE to ourselves and tell ourselves, dismiss it, as being "perfect". We humans are really good at self-deception; true science (and some scientists) try to work around that.

Originally Posted by jruley

Agreed. However, if you actually understood ShoeDLG's posts, you'd realize that he does not disagree with either of lincoln's points.

1. I am not ShoeDLG and do not attempt to speak for him

2. I was referring to some specifics in lincoln's post


3. Perhaps you missed that?

... as you apparently did the fact that I was referring to the colloquial phrase "Rocket Science" and most pointedly NOT the actual study of rocket science

Originally Posted by jruley

Dear, oh dear, oh dear!

I said --be careful ! A friend who taught, wrote tests for advanced physics would remind me that "what you see may not be so"
The guy was a great conversationalist and teacher- we spent hours solving problems of he universe
My most memorable quote was what he said after we had discussed the various theories of how the universe formed and I asked which theory was likely correct.
He said "oh they could all be wrong".

Good Answer, Richard!

knowing one can be wrong is one of the best ways to maximize the eventual possibility of getting it right

Yeh
But I guess I could just look it up?
That seems to be an accepted approach
Then I could pass the blame on to whomever wrote the article.
That means I am never right or wrong.
Tada!

Originally Posted by lincoln

Speaking of momentum transfer, if you approximate the rotor disks as infinitely thin discs that instantaneously transfer momentum to the air, that momentum transfer will match the weight of the quad copter closely, except for any aerodynamic effects on the rest of the quad copter.

This is interesting. If you explore how a thin disk transfers momentum to the air, it's clear that the transfer is not instantaneous. It takes several disk diameters for the air's acceleration to take place. In the limit that your control volume "height" is very small compared to the disk diameter, the thrust on the disk is balanced entirely by pressure differential across the volume (there is negligible change in axial velocity). As you increase the height of your control volume, the change in axial velocity across the control volume will increase. You only need to make the height of the control volume a couple of diameters in order for the thrust on the disk to be balanced entirely by the rate at which the momentum of the air inside the volume is changing.

In the case of a rotor operating away from any solid boundaries, you can take a large collection of the air surrounding the rotor and show that the rate of change of downward momentum of that air is equal to the upward thrust of the rotor. As long as the volume of air your are considering is large enough (and it really doesn't need to be that large in the case of a rotor), you can increase or decrease the volume of air you are considering, and the rate of downward momentum change of that air will remain constant (and equal to the thrust). This is a solid indication that the thrust on a rotor operating away from boundaries is due to momentum exchange.

The same is not true for a steadily lifting wing. No matter how large you make the surrounding control volume, a change in its proportions will change the balance between lift and the rate of downward momentum transfer to the air. For cases of steadily lifting wings operating in bounded air (that's most of them), the presence of a large boundary far from the wing alters the balance between lift and momentum transfer in a way it does not for a steadily lifting rotor.

For obvious reasons, the same is also not true for a rotor operating near solid boundaries.