How is it possible to fly inverted???

[profile]

Newton's 2nd law talks about what happens to an object when a force is applied to it, not the 3rd.

Of course air is deflected downwards as a result of the forces acting on the wing, but that is a RESULT of lift, not the CAUSE of lift!

 

[profile]

Symetrical wing works by having a positive AoA. A flat board will also produce lift, as will a brick, with a positive AoA. The issue is not producing lift, but being efficient at it. Obviously, you trade the ability for fairly efficient inverted flight for less efficient normal flight, but a non-symetrical wing will fly inverted also, just needs more AoA and is less efficient at it.

 

[paulsalem]

Symetrical wing works by having a positive AoA. A flat board will also produce lift, as will a brick, with a positive AoA. The issue is not producing lift, but being efficient at it. Obviously, you trade the ability for fairly efficient inverted flight for less efficient normal flight, but a non-symetrical wing will fly inverted also, just needs more AoA and is less efficient at it.

Profile,

I agree with you to a point.

Lift is produced by a postive AOA. Ok, well why? The wing/board/brick/hand out the window etc. pushes air down, air pushes back on wing/board/brick/hand out the window etc. (3rd law)

The reason a plane stays alot is b/c the wing pushes the air down and the air pushes back on the wing (3rd)

Newtons 2nd law Fnet=m*a , seems to only affect on the amount of force. Depedent on the acceleration of the air/wing and mass of the air/wing.

Agree?

 

[paulsalem]

Profile,


Also, along the same lines. Since air is a fluid, must not an aircraft dispace its wieght in air to stay aloft? Does it not do this by pushing down air?

 

[Thedude]

TheDude,
It sounds like you are suggesting that an airplane with a normal cambered wing cannot maintain inverted flight, that is simply wrong. It's just a matter of angle of attack. Clearly, it would be less efficient than a symmetrical airfoil. If that's not what you were implying, I apologize.

Thats is exaclty what I am saying. Take a C-152 Aerobat. It cannot maintain sustained level flight (been a few years but I beleive this correct) even if it did have the inverted fuel & oil system. The normal wing (light trainer) is just too inefficent for this to happen, ie. cannot generate enough lift when inverted. But you are correct in saying that it is just a matter of AoA.

Something I did forget to metion is having excess thrust. Having extra thrust can off set the reduction in lift due to an inefficent wing. (Remember what makes an a/c climb.) Kinda like flying in the region of reverse command.

I wish I could quote some references from my aerodynamics classes but they are all in storage . Ones to check out are Aerodynmics for Naval Aviators, Naval Avaitors Test Pilot Guide or this web site
http://quest.arc.nasa.gov/aero/question/aerotheory/


Does anyone remember when the FAA was teaching centripital force to make airplanes turn.

 

[mzaharis]

Best website I've seen for explaining lift phenomena:

http://www.av8n.com/how/htm/airfoils.html

While Bernoulli is in effect, it is not the old "the top of the airfoil is longer, therefore the air on top must go faster to meet up with the air on the bottom" BS. In fact (and this has been demonstrated in wind tunnels, CFD, etc.), the air on the top gets to the TE FASTER than the air on the bottom. The argument that the air goes faster on top to match the air that takes the shorter path on the bottom is grade school fiction.

Welcome to the circulation theory of lift, which is what airfoil designers use to do their jobs.

EDIT

See section 3.6 for a refutation of the "deflection lift" "theory". Section 3.8 discusses inverted flight and symmetric airfoils.

 

[VNugget]

Thats is exaclty what I am saying. Take a C-152 Aerobat. It cannot maintain sustained level flight (been a few years but I beleive this correct) even if it did have the inverted fuel & oil system. The normal wing (light trainer) is just too inefficent for this to happen, ie. cannot generate enough lift when inverted. But you are correct in saying that it is just a matter of AoA.

Something I did forget to metion is having excess thrust. Having extra thrust can off set the reduction in lift due to an inefficent wing. (Remember what makes an a/c climb.) Kinda like flying in the region of reverse command.

If it is really is the case that a 150 can't maintain inverted level flight, it would simply be because the CLmax is so low that you can't have enough speed* to create enough lift... not because you would enter some other mysterious regime of flight where the rules of aerodynamics change. Categorically saying that a cambered wing "can not" maintain inverted flight is wrong. Region of reverse command? Huh?

* This sounds like a nitpick, but it's actually important. You need airspeed, not thrust. (Ref. basic lift equation) If you don't have enough thrust to overcome drag and fly level, you can always do it in a dive (and still at -1G).

 

[coolyokeluke]

A new (old) theory that's gaining prevelance is the coanda effect. Check it out.

http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html

 

[Thedude]

If it is really is the case that a 150 can't maintain inverted level flight, it would simply be because the CLmax is so low that you can't have enough speed* to create enough lift... not because you would enter some other mysterious regime of flight where the rules of aerodynamics change. Categorically saying that a cambered wing "can not" maintain inverted flight is wrong. Region of reverse command? Huh?

* This sounds like a nitpick, but it's actually important. You need airspeed, not thrust. (Ref. basic lift equation) If you don't have enough thrust to overcome drag and fly level, you can always do it in a dive (and still at -1G).

Speed is important, but I assume that for disscusion purposes we are talking about steady state flight, straight and level. So descending while inverted would negate your argument. That is not flying inverted that is descending inverted.

The short version is CL=1/2 Rho V*2. So according to this, I can shove a rocket engine in my car and fly it around because I have now acheived V=velocity. I am talking real world , make it happen and show me kinda physics when I speak of the 152. Not enough thrust = not enough velocity.

Region of Reverse command is not mysterious regime. Every time I land my B-727 the approach is done in the area of reverse command as you will find with most transport cat a/c.

 

[mattpilot]

There is no "bernoulli vs newton". A look at the Bernoulli equations will quickly show you that they are all Newtonian equations modified to apply to aer/hydro dynamics.

uhmm ................. no. Bernoulli's equation is a derivative of the Universal Gas Laws. It has nothing to do with newtons laws. HOWEVER, lift can not be explained by bernoulli's equation or by newtons laws itself.

Bernoulli's principle is important in understanding WHY there is a pressure differential taking place across the wing. This pressure differential creates a FORCE below the wing that is pushing UP against the wing giving it LIFT. And this is where newtons laws come in. But to suggest that the major source of lift is created by induced lift (by that i mean air striking the underside of the wing and thus creating an equal and opposite upward force), and that bernoulli's principle is just a students simple answer is just redicules.

 

[VNugget]

Speed is important, but I assume that for disscusion purposes we are talking about steady state flight, straight and level. So descending while inverted would negate your argument. That is not flying inverted that is descending inverted.

The short version is CL=1/2 Rho V*2. So according to this, I can shove a rocket engine in my car and fly it around because I have now acheived V=velocity. I am talking real world , make it happen and show me kinda physics when I speak of the 152. Not enough thrust = not enough velocity.

Region of Reverse command is not mysterious regime. Every time I land my B-727 the approach is done in the area of reverse command as you will find with most transport cat a/c.

Descending while inverted does ont negate my argument, since it wasn't based on that. I said IF you don't happen to have enough thrust to overcome the drag, (do you know if that would be the case? I don't) then you can add to your thrust vector by descending.

Cl does not equal 1/2*rho*V^2 (I assume you meant to use use * as an exponent).

Cl = L / ((1/2)*rho*V^2*S)

or,

L = Cl*(1/2)*rho*V^2*S

Like I said, if you don't have enough Cl, you can make up for it with V. (Even in a descent. Getting some of your weight to add to your thrust does not mean that a wing is "not flying" anymore.) Which brings me to my next point. You implied that with an inverted airfoil, the only way this can work if you add some ridiculous amount of V to make up for a miniscule Cl, as per your car/rocket example.

Thanks but no thanks... let's do some actual investigation. Let's look at the airfoil data obtained expirementally by NACA in the 50's* for a sipmle cambered airfoil. Say, 2412 for example. In the positive AOA range, the Cl tops out at about 1.6, and in the negative range, at about -0.9, so the negative Clmax is about .56 of the positive. To maintain the same lift we'd have to multiply (V^2) by the reciprocal, or 1.77. The square root of that is about 1.33. Blah blah blah, get to the point? For an infinite NACA 2412 airfoil, stall speed increases by .33** if you invert it. Do you need to strap a rocket engine on to do be able to do that? Don't think so.

* Reference: Introduction to Flight, John D. Anderson, Jr. I tried to look for this stuff online, but couldn't find it. If you want, I'll scan the graphs for you.

** Incidentally, this means you big jet jocks can still fly almost the same approach speed inverted as upright, you just can't flare.

 

[huncowboy]

A new (old) theory that's gaining prevelance is the coanda effect. Check it out.

http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html

Thanks for the links in this thread. (The above and the rest as well.) I have just "explained" lift to a student pilot the other day... now I feel like an idiot.

 

[Vector4fun]

After y'all get through beating each other up over conflicting formulas for lift generated by airfoils, you can branch out to lifting bodies, and then sustained knife-edge flight.....

 

[UnAnswerd]

The argument that the air goes faster on top to match the air that takes the shorter path on the bottom is grade school fiction.

I'm surprised to hear that. That is exactly what is presented within my textbook. Can you really say that this theory is false???

 

[UnstableAviator]

PFM, it has to be PFM....

http://http://www.lerc.nasa.gov/WWW/K-12/airplane/foil2.html\


Visualize and understand.....or at least help.

I love this thing, at least it helps me visualize whats going on. By changing the camber and the AoA, as well as the thickness, it makes it easy to see how the pressure differential changes.

Remember, the yellow line on the graph is the lower surface, which is generally greater than the free air stream. Magenta is the upper surface, the critical surface for producing lift.

 

[VNugget]

I'm surprised to hear that. That is exactly what is presented within my textbook. Can you really say that this theory is false???

Yes, based on direct obervation. The top and bottom layers simply don't rejoin when lift is created.

http://www.amasci.com/wing/airgif.html

 

[profile]

I would recommend reading "Flightwise" by Chris Carpenter, available thru
www.amazon.co.uk.

 

[mattpilot]

Here's a question for everyone who disputes bernoulli's principle as the prime generator of lift.

How can an aircraft fly at a negative angle of attack and still develop enough lift to sustain flight?

Most WW2 fighters (probably modern day fighters too, but them are the only ones i know for sure), including the spitfire, flew at around -0.5 to -1 degree AoA in steady flight, because the Coefficient of Lift was still positive at that point thus still producing lift. Only chambered airfoils can do that. Symmetrical airfoils have their "angle of zero lift" at the 0 degree AoA point.

Therefore i conclude (and you conclude however you want), that the 'induced lift' theory (or newtons third) can not be the prime generator of lift. If you have a negative AoA, how would the relative wind strike the bottom surface of the wing and push the airplane up?

On the other hand, with a chambered airfoil at a negative AoA but still with a positive Coefficent of Lift, bernoulli's principle makes only sense. The airflow above the surface speeds up, thus decreasing pressure. The high pressure below the wing tries to 'unite' with the low pressure above the wing and therefore produces an upward force.


I did a quick search and came up with the following website: http://www.centennialofflight.gov/essay/Theories_of_Flight/Two_dimensional_coef/TH14.htm

Didn't read through everything, but it should prove helpful.

 

[mzaharis]

I'm surprised to hear that. That is exactly what is presented within my textbook. Can you really say that this theory is false???

I'm not saying it. The guys who designed the wing on they airplanes you fly say it. Most non-aerospace engineering textbooks apply a hackneyed version of Bernoulli, based on the "equal transit time" concept. It has been conclusively proven wrong since the earliest wind tunnel tests. It gets trotted out as a layperson's explanation for lift because it (incorrectly) uses some valid fluid mechanical concepts to provide an easy-to-understand explanation. Therefore, you see it in many non-engineering texts. While Bernoulli's equations do apply to the flow field around a wing, it is not that the air is uninterrupted prior to arriving at the wing, the air below the wing goes straight, and the air above the wing curves. There's a much more complex flow field around an airfoil than that. Especially for subsonic flight, the air in front of the wing is actually travelling in an upwards direction, and the air behind the wing is travelling in a downwards direction. This is called circulation, and is the fundamental property of airflow around a wing that defines how lift is generated.

Read the link, as it explains circulation theory of lift far better than I can. It's a pretty long slog, but it describes the basic principles behind what airfoil designers such as John Roncz and Michael Selig use in their analytical tools to design airfoils. The "air has a longer trip on the top of the wing than the bottom" stuff is what gets presented in non-aviation and general aviation textbooks, not academic aeronautical engineering textbooks. It is just not a full explanation that explains the full flow field around the wing.

I'd try to explain it here, but in my opinion, the author of the website used just about the minimum number of words and pictures necessary to explain the concepts, and they are far too much to post here.

http://www.av8n.com/how/htm/airfoils.html

BTW, the Coanda effect is also incorrect, as it only applies to an energized stream of air flowing over a curved surface. The airflow over a wing is not energized, unless you're referring to an unusual STOL design such as the Ukranian AN-72 or the US YC-14 prototype STOL transport, which uses engine exhaust to energize the overwing airflow.

http://www.av8n.com/how/htm/spins.html#sec-coanda

BTW, the parent website for these pages is a fantastic resource for the aerodynamics and physics of flight. It takes the principles used by aerospace engineers to design aircraft, and presents them in a manner that makes them useful to pilots, without "watering them down" too much.

http://www.av8n.com/how/

 

[mzaharis]

http://http://www.lerc.nasa.gov/WWW/K-12/airplane/foil2.html\


Visualize and understand.....or at least help.

I love this thing, at least it helps me visualize whats going on. By changing the camber and the AoA, as well as the thickness, it makes it easy to see how the pressure differential changes.

Remember, the yellow line on the graph is the lower surface, which is generally greater than the free air stream. Magenta is the upper surface, the critical surface for producing lift.

When I was back in high school in 1983, I was able to do a summer research experience program at University of Iowa's Fluid Mechanics Lab (I was SUCH a geek - oh, wait, I still am). I had a fun time working with a FORTRAN program that did pretty much the same thing as FOILSIM, but without all the pretty real-time animations. It had been written by a grad student as his thesis, and contained considerably more advanced math than I understood at the time. It took airfoil coordinates, and determined the pressure and velocity distributions based on a circulation theory of lift model (and the circulation vortex to satisfy the Kutta condition! - google it ;-) ). That's when I started to learn that what's taught in high school textbooks and popular aviation books was either very incorrect or very incomplete.