The Mystery of Fly Flight: Debunking Bernoulli's Law Explanation

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Discussion Overview

The discussion revolves around the mechanisms of insect flight, particularly focusing on the inadequacies of Bernoulli's law as an explanation for how flies achieve flight. Participants explore various aspects of wing structure, flapping motion, and the physics involved in lift generation, with a particular emphasis on the differences between insect and bird flight.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants argue that Bernoulli's law fails to explain fly flight due to the flat nature of fly wings, contrasting with the curved wings of birds.
  • Others suggest that the lift generated by wings is primarily due to the angle of attack and the downward push of air, rather than solely relying on Bernoulli's principle.
  • A participant mentions that insect wings may stretch and acquire camber during flight, which could contribute to lift in a manner similar to glider wings.
  • It is noted that the mechanics of insect flight are complex, involving different flapping patterns and interactions with air that are not fully understood.
  • Some contributions highlight that the flapping motion of wings creates a downforce on the air, which is essential for maintaining flight.
  • There are claims that the pressure distribution around wings does not always align with Bernoulli's law due to the work done at the air-wing interface.
  • Participants discuss the role of Reynolds number in insect flight, suggesting that smaller insects experience different aerodynamic conditions compared to larger ones.
  • One participant mentions the use of high-speed cameras in wind tunnel experiments to study fly flapping motion.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the mechanisms of insect flight and the applicability of Bernoulli's law. There is no consensus on the primary factors that contribute to lift in flies, and the discussion remains unresolved.

Contextual Notes

Some claims about wing structure and flight mechanics depend on specific definitions and assumptions that are not universally agreed upon. The discussion also touches on unresolved aspects of insect flight dynamics and the limitations of current understanding.

  • #31
Jeff Reid said:
And yet they still produce adequate lift at low sub-sonic speeds, and with the "hump" on the bottom of the wing instead of the top. Compare the angle off attack of the F104 (essentially a jet powered missle with tiny wings) to the M2-F2. The M2-F2 picture is a bit deceiving since the photo is angled a bit.

The M2-F2 and M2-F3 are good examples to "disprove" equal transit theory as the cause of lift, and I just find them interesting, as they are fairly unique.

I rotated the picture 2 degrees right to make the ground appear level, still the upper surface of the m2-f2 is nearly horizontal, but it could be in a "flare" since it's landing, but the wheels are still up, so even though it's fairly low, it's got some gliding time left to deploy the landing gear.

m2f2.jpg


Also the top surface isn't completely flat, it tapers at the tail, but the main point is shape of the lower surface, and the fact that the bottom surface is the "longer path".

ECN-1088.jpg

You have to be careful, just by looking at a picture gives you no indication of the AoA of those two aircraft. All you're seeing in this photo is the flight path angle - not the AoA.

Despite this, it is a very interesting aircraft design! It looks like a delta wing with the pilot all the way forward for stability in pitch.
 
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  • #32
Cyrus said:
That is exactly correct. The NS equations are true - period. It is the NS equations that CFD solves.
Approximates. N-S equations have very few situations that can be solved exactly.

Cyrus said:
That is called, 'momentum theory'.
AKA "Actuator Disc Theory."
 
  • #33
FredGarvin said:
Approximates. N-S equations have very few situations that can be solved exactly.

AKA "Actuator Disc Theory."

N-S doesn't have a closed form solution, implied in my post was the necessity to use something like CFD to solve them. That does not make the N-S equation an approximation. The solution is an approximation to the N-S equations, but the Equations are not an approximation.

*The only real "approximation" is that the gas particles follow a continuum.

As for Actuator Disc Theory, tom-a-to, to-ma-to.
 
  • #34
I left half way into the talks because I have work to do. But what I did see was pretty interesting. A guy from CalTech had optical sectioning images of a fly. Basically, they use special infrared laser beams to scan the fly and you can see all the internal structure of the fly just like a 3d MRI. The fly has two muscles that cause the complex flapping motion. One main muscle always powers the flies wings, while these two muscles adjust the tension, and thus the equivalent spring constant to change the flapping properties. Pretty non-intuitive. There were also a video of a honey bee inside a wind tunnel given a wind gust disturbance, and a high speed strobe video of a fly with a piece of string tethered onto its back to keep it stationary.
 
  • #35
Look this was shown on discovery...
actually the wings of the fly create whirl of wind[sorry for that..I can't find the exact word now...hope u understand what I want to convey]...on its lower back side...this is the main cause of lift
 
  • #36
lividz said:
Look this was shown on discovery...
actually the wings of the fly create whirl of wind[sorry for that..I can't find the exact word now...hope u understand what I want to convey]...on its lower back side...this is the main cause of lift

You do realize, I was sitting in a room with Dr. Michael H. Dickinson and other experts from the around the country that deal with this...right?

http://dickinson.caltech.edu/old/index.html
 
  • #37
Birds and insects fly in several modes. The flapping wing and flap-slap are the most well know.

It takes a few cord lengths to generate the captive vortex encircling the wind before it can develop good lift (Wagner effect). The figure eight motion (and who knows what else?) enables hovering flight.
 
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  • #38
^so...I didnt read it at that time..but thanks for info...
 
  • #39
lividz said:
^so...I didnt read it at that time..but thanks for info...

Oh, OK, Lividz. Now I understand your post.
 
  • #40
^no that was targeted @cyrus:

You do realize, I was sitting in a room with Dr. Michael H. Dickinson and other experts from the around the country that deal with this...right?

No, I just realized that!
I posted it just by reading the very 1st post...and as u may b knowing they don't show the hardcore details on shows abt such things...

neways...ok...coz its my mistake...that I posted a useless post .. just by knowing a small fraction of the whole scene...

but cyrus post conveys that there r sum guys who can explain it to me in much more details than I could ever get from other sources...thats good...

Lemme go thru the posts till now...
 
  • #41
<Moan> Please take the time to type in full English or don't bother posting.
 
  • #42
lividz said:
^no that was targeted @cyrus:



...

try to use the quote feature next time:wink:
 
  • #43
Cyrus said:
The solution is an approximation to the N-S equations, but the Equations are not an approximation.
That was my point. One rarely solves the N-S equations. They approximate them.
 
  • #44
Check out this particular study on dragonfly wings and cross sectional corregations that result from veins in the wings.

http://jeb.biologists.org/cgi/reprint/203/20/3125.pdf"
 
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  • #45
Phrak said:
Check out this particular study on dragonfly wings and cross sectional corregations that result from veins in the wings.

http://jeb.biologists.org/cgi/reprint/203/20/3125.pdf"

Thanks for this article, I'm going to forward it to my advisor! Very interesting results too!
 
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