Why does fly fly?

Bernoulli's equation describes a relationship between static pressure and speed^2 (component of dynamic pressure). There are pressure distributions on a wing that do not correspond to the speed of the air as described by Bernoulli's law, because work is done. For example, in the vicinity of the the upper leading edge of a wing , you have a significant component of centripetal acceleration of air, that corresponds to a reduction in pressure with no change in speed.

Its not because work is done, it's because viscous forces are prevalent. Bernoullis law is just a special case of the Navier-Stokes Equations. The Navier-Stokes Equations always hold true, everywhere. Work being done would be an effect of a helicopter rotor or propeller blade when you are using momentum theory. (PS, I was giving warren a hard time about what he said for the forces on a wing. He's right, I was nitpicking).

I was being equally nitpicky about Bernoulli. Although the Navier-Stokes Equations hold in the real world (not sure how complicated turbulence makes this), classical Bernoulli doesn't because of the assumption that total energy is constant, or that total energy along a stream line is constant. The mass flow is constant, but the energy is clearly changed the case of a propeller, rotor, or turbine, and although the amount of energy changed involved is less with a wing, it's still there.

The "work being done" issue is mentioned in this Nasa article on propellers, and exit velocity. Not covered is what is happening at the outer edges of the decreasing diameter funnel of the main air stream.

" But at the exit, the velocity is greater than free stream because the propeller does work on the airflow. We can apply Bernoulli'sequation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed by the engine (by the propeller) [/i] violates an assumption used to derive the equation.[/i]"

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

I didn't read all the posts here, but I will give a very basic overview of what happens in low Reynolds number insect flight.

First of all, the statement that flies have 'flat plate' wings is flat out wrong. Micro Air Vehicles (MAVs) of a size somewhat close to that of flies (most slightly larger - drag fly size), do have chamber to them.

[1] http://www.terp.umd.edu/4.0/engineering/

The question I have is this. Can fly fly even if the wings are flat? If yes, then it proves that the camber of the wing is not the reason that fly can fly. It certainly helps to have camber

Airplanes do not fly (entirely) due to explanation given by Bernoulli's law, though it does contribute to the lift created by some wings. Bernoulli's law as a sole explanation of lift is one of the most enduring popular myths in physics.

Aerobatic airplanes have wings with symmetrical cross-sections. Most airplanes with asymmetrical wings are also capable of flying inverted. Airplane wings generate lift mainly due to their angle of attack -- they push air down, and the reaction force pushes them up. Simple as pie.

- Warren

I have to call BS on this one! No offense meant but the increased air rushing over the top of the wing creates a localize low pressure causing lift of the wing defeating the wind drag under the lower part of the wing at a slightly increased pressure.

I'm in Houston, where are all you astronauts and flight jocks to back me up with a better explanation?

increased air rushing over the top of the wing creates a localize low pressure causing lift of the wing defeating the wind drag under the lower part of the wing at a slightly increased pressure. I'm in Houston.

I already gave an explanation. There is a pressure difference between the top and bottom wing.


Forum Side note: why do you we have to have this discussion about how an airplane wing flies for the millionth time around here. Someone should just put a sticky that stays STOP ASKING about Bernoulli and wings. Good god, a thousand and one threads on this gets old fast. The OP didn't even ask about Bernoulli, so why are we even talking about it?

I get your idea, life is too short, huh. Some folks on this site can tell folks about the deflection of the wing of a fly on the upward motion as oppossed to the downward motion but you didn't mention that. Perhaps no one mentioned that the wing when going up is at a 75degree angle or that when it goes down it is at a 22 degree angle in stable flight. But then again maybe one of the experts you have access to has that high resolution slow motion video of the flight of a fly. Give them a call. I can give you a number if you need it.

Perhaps you could explain how your theory (air rushing over the top of the wing) applies to these pre-shuttle prototypes?

M2-F2 glider:



M2-F3 rocket powered version (max speed mach 1.6):

m2-f2, m2-f3

These designs were specifically meant for high altitude and/or vertical travel at supersonic speeds and still allow a controlled descent.

7:30 - 8:00 ***** Registration ($20), Coffee and Donuts ******
8:00 – 8:30 Mechanics Center Introduction: Chopra
8:30 – 10:00 Task-1 : Aeromechanics: Humbert
1.1 Fundamental bio-inspired principles of flapping flight physics - - Humbert/Dickinson
1.2 Dual-plane particle image flow diagnostics of flapping-wing unsteady aerodynamics - - - Leishman
1.3 DNS/LES/RANS analysis for rotary- and flapping-wing-based MAVs- - - Baeder/Yamleev
10:00 – 10:15 - - - Coffee Break
1.4 Flight dynamics simulation modeling of MAVs - - - Celi
1.5 Aeromechanics of revolutionary cyclocopter and flapping rotors - - - Chopra/Benedict
1.6 Bio-inspired flexture-based wings and airframes - - - Dickinson/Humbert
1.7 Avian-based wing morphing for agile flight - - - Hubbard

12:15 – 1:15 - - - - Lunch Break - - - - -

1:15 – 3:15 Task-2 : Ambulation: Full
2.1 Bio-inspired dynamic modeling and simulation with parameters for ground contact model - - Full /Goldman
2.2 Bio-inspired principles of appendage and actuator design - - - Full/Fearing/Wood
2.3 Ambulatory design of body and appendages - - - Full/Fearing
2.4 Bio-inspired crawling, running, climbing robots - - - Fearing/Full/Wood
3:15 – 3:30 - - - Coffee Break - - -
3:30 – 5:00 Task-3: Hybrid Aeromechanics/Ambulation: Fearing
3.1 Thrust augmented entomopter: a revolutionary hover-capable high-speed MAV - - - Chopra/Wereley
3.2 Bio-inspired hybrid aerial and terrestrial locomotion - - - Fearing/Full/Wood/Humbert
3.3 MBMAC: Multi-body Microsystem Analysis Code for rotary, flap and ground - - Masarati/Goldman/Chopra
5:00 Demonstrations & Reception (Kim Engineering Building Rotunda)

Location: Kim Engineering Building
8:30 – 9:00 - - - Coffee & Donuts - - -
9:00 – 11:30 Task-4: Multifunctional, Actuation and Propulsion: Wood
4.1 High performance microactuators - - - Smela/Fearing/Wood
4.2 Smart composite-based rapid fabrication of micromechanical and micromechatronic structures - - Wood/Fearing
4.3 Ultra-light multifunctional composite structures based on electrospun fabric - - - Shivakumar/Lingaiah
4.4 Chemical energy conversion system - - - Cadou/Jackson
4.5 Distributed propulsion system for power efficiency - - - Fearing/Full/Wood
11:30 – 12:00 ARL and Government Meeting
12:00 – 1:00 - - - Lunch - - -
1:00 – 1:30 TMG Meeting and Hot Wash

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.



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".

That is exactly correct. The NS equations are true - period. It is the NS equations that CFD solves.

That is called, 'momentum theory'.

Approximates. N-S equations have very few situations that can be solved exactly.

AKA "Actuator Disc Theory."