Devnote Tuesdays: The Greener Pastures Edition

[lincourtl]

<offtopic>

Umm, actually, no. At least, not once you get to any appreciable speed.

The reason wings are cambered are so that the air has a longer path over the top of the wing than underneath. Ie, the same amount of air has a further distance to travel in the same amount of time. Hence the air on top of the wing is less dense than the air underneath the wing, effectively sucking the wing (and whatever it's attached to) up.</offtopic>

http://www.explainxkcd.com/wiki/index.php/Airfoil

[Aethon]

Rowsdower! Nooooooooooooo!

A good man, with a tough job, done well. Good luck mate. You can pop in and say hello once in a while can'tcha??

At least he sort of weaned us off of himself a bit, instead of tearing himself away like an old bandage.

Edited February 4, 2015 by Aethon

[Jarin]

Airfoils are friggin' weird. I have yet to find a solid explanation for how wings actually work aside from "yeah, it's bloody complicated"

[micha]

Ok, slightly more complicated: It's a combination of airfoil (wing) shape and angle-of-attack.

My main point was that planes (mainly) fly because they are sucked up, and not because they push air down. There is certainly some downwash at positive angles-of-attack, but it contributes comparatively little to overall lift.

But yes, it's complicated. And with enough thrust (MOAR BOOSTERS) pretty much anything will fly.

[KerbMav]

This any help?

Mods: I know, this looks like OT, but as aerodynamics are a big part of 1.0 and mean a lot of change for KSP, I think everyone should know what is going on "up there".

[pizzaoverhead]

Airfoils are friggin' weird. I have yet to find a solid explanation for how wings actually work aside from "yeah, it's bloody complicated"

Here's another mental model to use. Each wing spins a vortex of air, forcing it downwards as reaction mass.

http://amasci.com/wing/rotbal.html

[Xacktar]

We'll all miss you, Rowsdower!

I hope your next community appreciates you as much as we do!

[2001kraft]

Good luck to Rowsdower

As for the update details:

Can't wait to see the new spacesuit and animations and everything!

There goes my hype. Lol.

[CaLVin-K]

My main point was that planes (mainly) fly because they are sucked up, and not because they push air down. There is certainly some downwash at positive angles-of-attack, but it contributes comparatively little to overall lift.

This isn't correct, very little of the airplanes lift is generated via the Bernoulli principal (being sucked up) the vast vast majority (95%+) of an airplanes lift if caused by forcing air down. This is why at slow speeds and during take off you will see and hear pilots deploying the airplanes flaps, slats and the like, these devices change the shape of the wing to allow it to force more air down at slower speeds and are retracted during cruising flight because they increase drag at high speeds more than they increase lift. However even at cruising speeds airplanes have a positive angle of attack (2-3 deg) to generate lift. The shape of the wing is the way it is not because it causes lift, a flat board will provide almost the same amount of lift, but because it will decrease the drag caused by the airflow going over the top of the wing. If airplane wings were these flat boards then the airflow over the top of the wing (at positive angles of attack) would be chaotic with many edge vortices and turbulent airflow causing a lot of drag on the wing. The shape of airplane wings allows the air to flow over the top in a way called "laminar flow" which means that the airflow is relatively smooth and orderly, decreasing drag.

Edited February 4, 2015 by CaLVin-K

[Starwaster]

A wizard did it.

[Aethon]

I can't believe we still can't come to a consensus about how a wing creates lift. I was a pilot. I know very little about aerodynamics, but I'm beginning to think what I've been taught about how wings create lift my whole life is wrong, and what I thought was right as a boy building model airplanes was right.

[RW-1]

This isn't correct, very little of the airplanes lift is generated via the Bernoulli principal (being sucked up) the vast vast majority (95%+) of an airplanes lift if caused by forcing air down. This is why at slow speeds and during take off you will see and hear pilots deploying the airplanes flaps, slats and the like, these devices change the shape of the wing to allow it to force more air down at slower speeds and are retracted during cruising flight because they increase drag at high speeds more than they increase lift. However even at cruising speeds airplanes have a positive angle of attack (2-3 deg) to generate lift. The shape of the wing is the way it is not because it causes lift, a flat board will provide almost the same amount of lift, but because it will decrease the drag caused by the airflow going over the top of the wing. If airplane wings were these flat boards then the airflow over the top of the wing (at positive angles of attack) would be chaotic with many edge vortices and turbulent airflow causing a lot of drag on the wing. The shape of airplane wings allows the air to flow over the top in a way called "laminar flow" which means that the airflow is relatively smooth and orderly, decreasing drag.

Gads, as a CFII, it pains me to say your are half wrong. it is a combination of both principles, but in truth goes way further ... but I'm only going to give generalities below, a lot of what you posted above is misinformation, especially about laminar flow, and the use of slats and flaps.... let alone that if you tried to apply those to a helicopter rotor (a specifically designed airfoil that is designed to maintain CG while rotating angles of attack with collective, wouldn't work at all)

Which camp is correct? How is lift generated?

When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object.Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.

What is the argument?

Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift.

The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time. If we know the correct velocity distribution, we can use Bernoulli's equation to get the pressure, then use the pressure to determine the force. But the equal transit velocity is not the correct velocity. Another incorrect theory uses a Venturi flow to try to determine the velocity. But this also gives the wrong answer since a wing section isn't really half a Venturi nozzle. There is also an incorrect theory which uses Newton's third law applied to the bottom surface of a wing. This theory equates aerodynamic lift to a stone skipping across the water. It neglects the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas.

The real details of how an object generates lift are very complex and do not lend themselves to simplification.

For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel's father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.

Edited February 4, 2015 by RW-1

[Nemrav]

See Veritasiums take on it, then look at NASA's explanation, they both get the idea across, both Bernouille and Newtonian physics fully account for it.

See you Rowsdower, never have I ever seen a dev team that pays such close attention to their community.

DON"T FORGET TO CONTINUE POSTING !!!!!!!111!!!!!

[FleshJeb]

KERBAL

{

name = Rowsdower Kerman

type = Crew

brave = 1.0

dumb = 0.0

smartS = True

badS = True

state = UnAvailable

ToD = TooSoon

}

[cantab]

One of the errors that contributes to misunderstanding lift is to look at an airfoil like this: (ignore the slats and flaps)

and think it's at 0° angle of attack. The angle of attack is properly measured from the chord line - the line joining the leading and trailing edge - and not the bottom surface of the wing. In the picture above you can see that the leading edge is slightly higher than the trailing edge, and thus the "real" angle of attack is non-zero.

[DaPatman]

Clear skies, Rowsdower.

[Bill Phil]

This isn't correct, very little of the airplanes lift is generated via the Bernoulli principal (being sucked up) the vast vast majority (95%+) of an airplanes lift if caused by forcing air down. This is why at slow speeds and during take off you will see and hear pilots deploying the airplanes flaps, slats and the like, these devices change the shape of the wing to allow it to force more air down at slower speeds and are retracted during cruising flight because they increase drag at high speeds more than they increase lift. However even at cruising speeds airplanes have a positive angle of attack (2-3 deg) to generate lift. The shape of the wing is the way it is not because it causes lift, a flat board will provide almost the same amount of lift, but because it will decrease the drag caused by the airflow going over the top of the wing. If airplane wings were these flat boards then the airflow over the top of the wing (at positive angles of attack) would be chaotic with many edge vortices and turbulent airflow causing a lot of drag on the wing. The shape of airplane wings allows the air to flow over the top in a way called "laminar flow" which means that the airflow is relatively smooth and orderly, decreasing drag.

Explain why high angles of attack result in a stall, then.

A stall is a sudden loss of lift. If your angle of attack is high, the airstream loses connection with the surface of the wing, causing the sudden loss of lift.

[Technical Ben]

"This week i’ve finally gotten around to adding more of the planned design concern tests to the Engineer’s Report. Weeks ago i was getting real concerned that the tests were too heavy on the cpu, so i had to apply some re-factoring on the algorithms and now, with half the tests implemented, it looks like a non-concern."

Wait, "Check is engine attached" is CPU intensive? (I guess the hint is to either not ask the CPU to test every femto second, or your testing something more complicated?)

[Rowsdower]

Thank you for the kind words, everyone. I'm seriously gonna miss the heck out of being around the community so often. I'll try not to make myself a stranger

[CaLVin-K]

Explain why high angles of attack result in a stall, then.

A stall is a sudden loss of lift. If your angle of attack is high, the airstream loses connection with the surface of the wing, causing the sudden loss of lift.

the high angle of attack leads to an increase in drag, showing the plane, eventually if not corrected the plane stops producing enough lift to keep itself airborne, hence a stall.

[Dodgey]

the high angle of attack leads to an increase in drag, showing the plane, eventually if not corrected the plane stops producing enough lift to keep itself airborne, hence a stall.

No this isn't true, there is a distinct difference between air breaks and spoilers on an aircraft.

The function of an air break during landing is to increase the drag of an aircraft slowing it down.

The function of spoilers is to disrupt the air flowing over the top of the wing causing a stall, this is not used to slow the aircraft down but instead to reduce the lift allowing the wheels gets grip in order to break the aircraft. Stalling a wing does very little to slow it down.

While it is true that air being deflected off of the bottom of the wing does create some lift (thank you Newton) lift is also majorly created by air being accelerated down over the top of the wing, creating a down draft. Thanks to Newtons third law we can see as the air is accelerated downwards the wing is accelerated upwards.

If there is going to be any more debating on this I move we start a thread in the science labs, it seems like it would be more fitting there.

[Bill Phil]

the high angle of attack leads to an increase in drag, showing the plane, eventually if not corrected the plane stops producing enough lift to keep itself airborne, hence a stall.

Exactly. But if the wing is shoving more air down and getting more force from that, than wouldn't it increase lift?

[CaLVin-K]

The slower the airplane is moving the less lift and drag are created by the wing, eventually the force of shoving air down isn't enough to keep the plane up.

[Dodgey]

The slower the airplane is moving the less lift and drag are created by the wing, eventually the force of shoving air down isn't enough to keep the plane up.

You specifically said that stalling a wing increases drag significantly enough to slow down the aircraft such the increase in lift (due to the increased angle of attack) is not enough to compensate for the decrease in speed.

I just want to confirm this, are you saying that if I were to deploy the spoilers on an aircraft at cruising altitude and velocity, instead of stalling at its current speed (which would happen if lift was created by flowing over the top of the wing) the aircraft would instead decelerate abruptly to below the stall speed at which point it would stall. Is that correct?

[CaptainKipard]

If there is going to be any more debating on this I move we start a thread in the science labs, it seems like it would be more fitting there.

Start one and post pictures, because this all sounds like magic to me.