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Why we don’t understand heavier-than-air flight

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Re: Why we don’t understand heavier-than-air flight

#101

Post by jjk166 »

defaultname wrote: post #74
Someone1234 wrote: post #34

Earlier quoted context omitted.

If an aircraft flies level upside down it will lose altitude towards the ground (as opposed to right side up wherein given adequate thrust it should keep its current altitude). In order to stay at a fixed altitude upside down you have to bring the nose of the aircraft up several degrees (increasing based on air speed).

Every aircraft has the wing set at an incident angle relative to the axis of the fuselage. Usually to generate enough deflection force for level (relative to the fuselage) flight at cruising speed. Upside down flight requires you to basically inverse this deflection, but it isn't because of Bernoulli lift.

Wings can and generally do have zero degree angle of attack lift.

Re: Why we don’t understand heavier-than-air flight

#102

Post by tdy721 »

dreamcompiler wrote: post #73
tdy721 wrote: post #67

Rocketry is heavier than air flight. I submit we discovered it before lighter than air flight. I will hazard that I do have a nice understanding of how it works. I learned it from media like this: https://youtu.be/X4iMeKif488 I do think you’re dead wrong!

The common connotation of "heavier-than-air flight" and "lighter-than-air flight" is that air is the medium in which the flight takes place, and that air is essential for the flight to happen. The science of aerodynamics is necessary for describing how such flight works. That's not true for rockets. Rockets can fly in air, and a rocket's fins only work in air, but rockets don't have to use fins and rockets work fine…

Technically correct. Hey it’s only mostly flight until max Q, then it’s... spaceflight?

Star Wars ships come out of space backwards and I don’t get it...

Re: Why we don’t understand heavier-than-air flight

#103

Post by lowkey »

slavak wrote: post #99
lowkey wrote: post #56

Can confirm. I worked at Pratt & Whitney testing jet engines early in my career. At the time I read a similar article and spread it amongst my colleagues - the cognitive dissonance was palpable. As engineers we had been taught that lift was due to air above the wing traveling faster than air below the wing and thus creating lift by way of a pressure differential. The more accurate answer as seen in the article is tha…

The experts weren't _wrong_ in their understanding. Bernoulli (creating lift by way of pressure differential) and Newton (reaction to redirection of the flow downwards) are different ways of describing the same thing; integrating either the pressure or velocity vector of the airflow around the wing will give you the correct results for lift.[1] Whenever people argue about which interpretation of lift is correct I thi…

I commented elsewhere that there is no physical reason why the Bernoulli effect would cause the upper streamline and the lower streamline to reach the back of the wing at the same time - and to my knowledge there is no experimental evidence that it does. I may be wrong about that but I have never seen an adequate rebuttal.

Re: Why we don’t understand heavier-than-air flight

#104

Post by jjk166 »

lowkey wrote: post #98
afterburner wrote: post #82

Earlier quoted context omitted.

Causing a pressure difference is the same as causing a force vector. There's no gotcha here, it's just two ways of looking at the same thing.

My point about the Bernoulli effect is that there is no physical reason why the upper flow should move faster than the lower flow and in fact testing shows that they do not.

The upper flow does move faster, and must for the sake of vortex production.

Re: Why we don’t understand heavier-than-air flight

#105

Post by jameshart »

AstralStorm wrote: post #30
tomsto wrote: post #29

Earlier quoted context omitted.

This is true of a symmetrical aerofoil (e.g. most helicopters) but not for an asymmetrical aerofoil (most fixed wing aircraft). It is true that a slightly positive angle of attack generates more lift than none (because the pressure/lower side starts making a contribution)

Correct. Still incomplete. Angle of attack causes a vortex at the trailing edge which has nothing to do with raw air speed and everything to do with fluid dynamics (which involves speed but is much more complex) Short version is that you created a hole (lower pressure area) in air which it now tries to fill. Air and gasses have finite limited velocity known as speed of sound, which is why you get these pressure diffe…

Isn't the intent of a smooth aerofoil design to prevent the formation of vortices on the trailing edge? They're inevitable at the wingtip, but in controlled flight most wings are trying to produce laminar flow, right?

In my understanding, if you increase angle of attack sufficiently to generate vortices on the upper surface, then you aren't efficiently transferring downward momentum to the air your wing is shedding, and you lose lift, which causes aerodynamic stall. Am I missing something?

Re: Why we don’t understand heavier-than-air flight

#106

Post by lowkey »

afterburner wrote: post #82
lowkey wrote: post #56

Can confirm. I worked at Pratt & Whitney testing jet engines early in my career. At the time I read a similar article and spread it amongst my colleagues - the cognitive dissonance was palpable. As engineers we had been taught that lift was due to air above the wing traveling faster than air below the wing and thus creating lift by way of a pressure differential. The more accurate answer as seen in the article is tha…

Causing a pressure difference is the same as causing a force vector. There's no gotcha here, it's just two ways of looking at the same thing.

See my sister comment. The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline.

The problem with this theory is that there is no physical reason why both streamlines must arrive at the back of the wing at the same time - and per experimental verification, in fact they don’t.

Re: Why we don’t understand heavier-than-air flight

#107

Post by lowkey »

jjk166 wrote: post #104
lowkey wrote: post #98

Earlier quoted context omitted.

My point about the Bernoulli effect is that there is no physical reason why the upper flow should move faster than the lower flow and in fact testing shows that they do not.

The upper flow does move faster, and must for the sake of vortex production.

Please explain what you mean here in simple language. I don’t understand how you conclude that the upper flow must move faster and I don’t understand how it relates to vortex production.

Re: Why we don’t understand heavier-than-air flight

#108

Post by afterburner »

zazen wrote: post #97
afterburner wrote: post #95

Earlier quoted context omitted.

If you are asserting that the angle of attack does not affect the lift, you are wrong. A plane flying upside down is most certainly not using the same angle of attack as it does right side up. The real difference in performance is efficiency, the upside down plane is burning more fuel due to the increased drag from sub-optimal operation (a high angle of attack to overcome the optimization for right-side-up flying). N…

> If you are asserting that the angle of attack does not affect the lift, you are wrong. I am certainly not asserting that, and I'm baffled how you could have formed the impression that I was. You appeared to be attempting to rebut an argument in favour of the significance of angle of attack. We have another pointless internet misunderstanding on our hands.

I am replying to this:

"I usually just answer Socratically: "So how can (some) planes fly upside-down?" whenever I encounter the Bernoulli-adherents."

Which is a lazy and garbled gotcha attempt.

Re: Why we don’t understand heavier-than-air flight

#109

Post by DangitBobby »

afterburner wrote: post #86
DangitBobby wrote: post #55

Earlier quoted context omitted.

No, I have to conclude that the previous commenter didn't understand your point, which was the observation that heavy aircraft with very low sustained forward thrust (thrust that would not be sufficient for a helicopter to hover) results in sufficient upward lift to suspend the aircraft indefinitely, which is very surprising.

A helicopter doesn't rely on the forward thrust of the chassis, it relies on the forward thrust of the helicopter blades in rotation. Those blades are wings.

The way the thrust is generated is mostly irrelevant to this observation. The observation is about the mechanics of lift, which is some function of thrust combined with the wings, rotors, balloons, etc. You'd observe the same bizarre mechanics if the thrust were generated by releasing highly compressed air from a tank.

Re: Why we don’t understand heavier-than-air flight

#110

Post by zamfi »

beckingz wrote: post #70
HPsquared wrote: post #62

Earlier quoted context omitted.

For everyone other than the aforementioned physics grad student, it's a lie. For the physics grad student, it's a mystery.

Turns out, the world is really really complicated. So it's better to say that our models are simplified, not lies.

All models are wrong, but some are useful. (George Box)
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