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Old 12-17-2012, 09:25 AM   #76
pilotofdoom
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Originally Posted by Pulsar View Post
Air doesn't magically go faster over the top than the bottom. That's the age-old easy and pretty much incorrect answer.
Windtunnel tests show otherwise. Not the best photo, but it gets the point across. Photo Link. Also Youtube Link.

As far as wings curved, to prevent turbulence is one of the reasons. Keeping the flow laminar vice turbulent is key to achieving a high L/D ratio, essentially giving good fuel economy. This was key for the P-51 airfoil and to the success of that aircraft, amongst other design choices. Furthermore, sometimes turbulence will help delay the onset of the stall by delaying airstream seperation, see vortex generators on top of wings, mostly in small aircraft looking for extremely low takeoff and landing speeds.

Wings also have camber to generate more lift at a lower AoA, preventing the wing from needed excessive AoA at lower airspeeds to generate the same amount of lift. I believe it also improves L/D ratio, but I'd have to look up a few airfoils in my trusty Abbott and Von Doenhoff 'Theory of Wing Sections' to be sure. Symmetrical airfoils work just fine; mostly used by aerobatic aircraft since they spend quite a bit of time in inverted flight compared to most aircraft, something that positively cambered airfoils can do, just not do as well as symmetrical airfoils.

While yes, Newton’s Third Law basically tells us that air getting shoved downwards creates a force, and thus the reaction force is the wing getting shoved upwards is valid, especially for hovering helicopter theory, there's more to that to an airfoil. There's also Bernoulli's principle, where fluids that experience an increase in velocity results in a decrease in pressure. If you put static ports along the length, top and bottom, of an airfoil, and sum the static pressures (using the vector perpendicular to the port on the airfoil), you can accurate measure the total lift of an airfoil.
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Old 12-17-2012, 08:00 PM   #77
KillerCharlie
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Wow this thread is old.

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Originally Posted by Pulsar View Post
It's like the blind leading the blind in this thread. Get educated. This is highly technical, not 'guess and hope you get it right'.
I'm an aerodynamicist at a little airplane company that starts with a B. I've designed wings and wingtips that have been wind tunnel tested and flight tested, as well as some stuff that's about to go into production. I know a little about lift and drag - not just what they teach you in school, but real life do-or-die transonic aerodynamics.

Here's one of the places I visit from time to time:
http://www.youtube.com/watch?v=vD828...=results_video

Last edited by KillerCharlie; 12-17-2012 at 08:35 PM.
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Old 12-17-2012, 08:27 PM   #78
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As far as wings curved, to prevent turbulence is one of the reasons. Keeping the flow laminar vice turbulent is key to achieving a high L/D ratio, essentially giving good fuel economy.
To take this another step, at higher speeds (where you have to worry about compressibility and shocks), the camber (upper surface curvature) is done in a way to get the shock's strength and position just right - this keeps drag down and keeps the flow attached.

I'm not aware of any aircraft that use camber to increase alpha - usually you just change the wing twist (leave the airfoil alone). An airplane's angle-of-attack mostly matters during takeoff/landing to reduce tail strikes and increase pilot visibility.
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Old 12-17-2012, 10:09 PM   #79
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[QUOTE=pilotofdoom;34383994]Windtunnel tests show otherwise. Not the best photo, but it gets the point across. Photo Link. Also Youtube Link.
QUOTE]

I don't think you understood my point.

The air over top of a wing doesn't travel faster because it has farther to go. That explanation is fundamentally flawed and has confused new aeronautics students for decades.

The air travels faster because the angle of attack and curvature form a low pressure zone. This gives the air particles a downward velocity vector. In combination with their horizontal vector, they end up moving faster.

But NOT because they 'have farther to travel' as has been explained so many thousands of times.
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Old 12-18-2012, 04:37 PM   #80
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[QUOTE=Pulsar;34387714]
Quote:
Originally Posted by pilotofdoom View Post
Windtunnel tests show otherwise. Not the best photo, but it gets the point across. Photo Link. Also Youtube Link.
QUOTE]

I don't think you understood my point.

The air over top of a wing doesn't travel faster because it has farther to go. That explanation is fundamentally flawed and has confused new aeronautics students for decades.

The air travels faster because the angle of attack and curvature form a low pressure zone. This gives the air particles a downward velocity vector. In combination with their horizontal vector, they end up moving faster.

But NOT because they 'have farther to travel' as has been explained so many thousands of times.
So what happens when a plane does a loop-de-loop or flies upside down? Airplane wings don't look symmetrical across a horizontal axis, so how does it work? Ultimately, the goal is to get the air particles above the wing to travel faster, right?
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Old 12-19-2012, 08:26 AM   #81
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[QUOTE=Sleepingforest;34391134]
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So what happens when a plane does a loop-de-loop or flies upside down? Airplane wings don't look symmetrical across a horizontal axis, so how does it work? Ultimately, the goal is to get the air particles above the wing to travel faster, right?
An aerobatic aircraft or a fighter aircraft will have symmetrical aerofoils (in most cases) as they require less dynamic stability than aircraft such as general aviation/training aircraft. Less dynamic stability also leads to improved maneuverability.

Basically, these aircraft will fly at an angle of attack that is always positive to generate lift via the coanda effect (think putting your hand outside the car window and rotating it to make it lift up your arm), rather than a pressure differential (as seen on heavy transport aircraft).

So there are two ways to generate lift. One is through the pressure differential (Bernoulli's principle), and the other is through the coanda effect (basically Newton's third law). In the case of symmetrical aerofoils, Newton's third law is king.
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Old 12-19-2012, 09:55 AM   #82
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An aerobatic aircraft or a fighter aircraft will have symmetrical aerofoils (in most cases) as they require less dynamic stability than aircraft such as general aviation/training aircraft. Less dynamic stability also leads to improved maneuverability.

Basically, these aircraft will fly at an angle of attack that is always positive to generate lift via the coanda effect (think putting your hand outside the car window and rotating it to make it lift up your arm), rather than a pressure differential (as seen on heavy transport aircraft).

So there are two ways to generate lift. One is through the pressure differential (Bernoulli's principle), and the other is through the coanda effect (basically Newton's third law). In the case of symmetrical aerofoils, Newton's third law is king.
Coanda effect? Really?? Now these explanations are going off the deep end. All airfoils generate lift the same way, the mechanism does not differ from one to the next.

The coanda effect involves a jet of fluid attaching to an object. Airfoils don't involve jet flows at all.

What do I know though?

Last edited by KillerCharlie; 12-20-2012 at 12:10 AM.
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Old 12-25-2012, 03:25 PM   #83
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I would expect the shuttle to be aerodynamically handicapped by express design. Maybe they had to structurally make it aerodynamically inefficient so as to exploit the reentry for deceleration? A more aerodynamic efficient design might make the reentry much harder or more fuel expensive with the atmospheric resistance having less impact on its speed of reentry?

It is possible the space shuttle could have had a more aerodynamic friendly design, but they deliberately choose to make it less so as an aspect of fundamental design choice.

I like to think of it as a paper rocket thrown by our hand. Our hand are the SRBs and the external fuel tanks. The rocket by itself just goes up and comes down, hopefully at the trajectory we wanted it to do so. After the rocket leaves our hand, it will have to depend significantly upon the external environmental dynamics to control flight and the return trip. Of course you will have to imagine tiny midget astronauts inside this paper rocket in here
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Old 12-25-2012, 06:53 PM   #84
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I think things have gotten off track. The question isn't really if the shuttle can theoretically fly given enough airspeed, because the fact that it glides shows it is airworthy. The real question is if the shuttle "as designed" can do it. I'm sure if Boeing took the airframe and did some modifications for atmospheric flight and stripped out the main engines and installed some jet conventional jet engines it could be done. The CG would need to shifted and control surfaces would need to be modified. Bottom line is that the space shuttle is not purpose built for atmospheric flight other than a controlled decent and none of it's currently install propulsion systems can change that and it wouldn't be capable of fullfilling any space mission at that point.

Bottom line, theoretically the shuttle can be modified to take off from a runway under its own power, but the space shuttle as designed cannot do it.

Last edited by Anteaus; 12-25-2012 at 07:23 PM.
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Old 12-26-2012, 10:42 AM   #85
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Wow this thread is old.



I'm an aerodynamicist at a little airplane company that starts with a B.
beechcraft?
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Old 12-27-2012, 01:46 AM   #86
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beechcraft?
No, I was joking when I said "little" company... check the link I posted.
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