How do aircraft fly upside down?

  • #1
Can anybody tell me how aircraft fly up side down? I'm an under grad aero student and cant see how aircraft do. Can any aircraft theoretically fly up side down?:smile:
 

Answers and Replies

  • #2
J77
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paddy-boy66 said:
Can anybody tell me how aircraft fly up side down? I'm an under grad aero student and cant see how aircraft do. Can any aircraft theoretically fly up side down?:smile:
You just need to adjust the angle of attack (http://en.wikipedia.org/wiki/Angle_of_attack)
 
  • #3
LURCH
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Aerodynamically, yes, any aircraft can fly upside down, but for most ut is very innefficient. Also, some have structural limitations that won't take the strain, and many have engine limitations that will stop the flow of fuel.
 
  • #4
but doesn't the resultant lift always act in one direction relative to the "upper" surface so if its upside down wont the "lift" be downwards? Am i being stupid in saying this?:confused:
 
  • #5
J77
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Angle of attack's like sticking your hand out of the car window - if you tilt your hand, you feel the force upwards or downwards - same for a plane.
 
  • #6
Gokul43201
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paddy-boy66 said:
but doesn't the resultant lift always act in one direction relative to the "upper" surface so if its upside down wont the "lift" be downwards? Am i being stupid in saying this?:confused:
No, you've fallen prey to the most pervasive fallacy in aerodynamics - the Equal Transit Time fairy tale. Google that phrase, and you'll find many places that show how it's wrong.
 
  • #7
Danger
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That having been said, aeroplanes that are designed for aerobatic manoeuvres have specialized wings, control surfaces, and engines. Stagger-wing biplanes are particularly well-suited to it.
 
  • #8
Astronuc
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paddy-boy66 said:
Can any aircraft theoretically fly up side down?:smile:
Well considering that some aircraft fly upside down, it is actual rather than theoretical. Practially, it is limited to small aircaft, acrobatic and jet fighters, and the like.

Many aircraft, particularly large aircraft are not designed nor build to fly upside down, and most likely would disassemble (wings falling off or engines failing) if they tried to fly upside down.

Generally, the angle of attack is greater when the craft is inverted.
 
  • #9
Averagesupernova
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Hmmmmmmmmm. I'll have to try that with a 747 on my flight sim.
 
  • #10
Danger
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Averagesupernova said:
Hmmmmmmmmm. I'll have to try that with a 747 on my flight sim.
Do you also have a crash-team sim? :biggrin:
 
  • #11
thats awesome thanks guys! i'd like to see a 747 flying upside down :biggrin:
 
  • #12
russ_watters
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Here's another way to think about it: normal wing flying upside down is just a poorly shaped wing flying right-side-up.
Astronuc said:
Well considering that some aircraft fly upside down, it is actual rather than theoretical.
I think, perhaps, by "any", he meant every. Though pretty much any *airfoil* is capable of producing negative lift (or positive lift while inverted), many *planes* cannot fly inverted for various other reasons (structural, fuel flow, lift/drag ratio, etc).
 
  • #13
Hootenanny
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I believe that a 747 would theoretically would be able to fly upside down, however, it's wings and tail fin would be ripped off by the forces involved in executing the role.
 
  • #15
so then how do aircraft fly perpendicular to the ground, or is there a component of the thrust equal to g?
 
  • #16
Danger
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They can't do it for long. In the case of aerobatic machines, the fusilage provides lift, although not as much as the wings. With military jets, it's more a matter of pure thrust vs. gravity. Don't try it with a Cessna 150.
 
  • #17
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paddy-boy66 said:
so then how do aircraft fly perpendicular to the ground, or is there a component of the thrust equal to g?
It's called a knife edge.

For one thing, you get body lift as Danger said. But that is very minor. You also get some lift from the tail fin which is now acting as a wing, but again thats only minor. And if you look carefully, you will always notice that the nose of the airplane is pointed upwards. This is for good reason, becasue the tail is pointed down. This means the thrust has a component in the upwards direction. Thrust is the name of the game here. You need to have a very high thrust/weight ratio in order to do this, or your airplane will fall out of the sky.

Here is a picture for clarity:

http://www.rcgroups.com/articles/ezonemag/2005/may/7/knifeedge.jpg

http://www.wtp.net/DBEST/webimages/lonniecaprice.jpg

http://moleski.net/rc/pix/dc3.jpg
 
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  • #18
russ_watters
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cyrusabdollahi said:
You want to fly an airliner upside down, sure here it is:

If done properly, the forces are not a problem, nothing would be ripped off.

It's called a chandelle and it's a 1g manuver. If your airplane is going to fall appart at 1g, you have bigger problems. :wink:
I don't want to get too much into semantecs, but since it was a 1g, transient maneuver, I don't know that I'd call that "flying upside-down".

I suspect an airliner could structurally handle flying inverted, but that its wings would not be efficient enough to keep it in the air (high angle of attack required = too much drag).
 
  • #19
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It's because Santa Clause is your Parents:

airfoil.png


Ah, xkcd -- has an answer for everything! Thank you Randall.

But, for the record, I agree with thrust and momentum being the critical components; though it doesn't mean the Bernoulli principle isn't real.
 
  • #20
I ended up over here by way of the XKCD forums. This seems like a neat place, I look forward to looking around and maybe learning some new physics.

Regarding how airplanes can fly upside down:
http://forums.xkcd.com/viewtopic.php?f=7&t=64908&start=92

Regarding how airplanes can fly knife-edge: It is NOT the vertical component of the thrust. Most high-performance airplanes have a thrust-to-weight ratio of just over 1, so they'd have to point almost straight up before the vertical component of the thrust can offset the weight. Airshow airplanes do knife-edge flight by getting lift off the fuselage, i.e. by flying like a lifting body and using the fuselage as a wing. Anything will produce SOME lift if you get it going fast enough and orient it at the right angle ;]
 
  • #21
I don't want to get too much into semantecs, but since it was a 1g, transient maneuver, I don't know that I'd call that "flying upside-down".
One thing is inverted, positive-g flight, like a barrel roll. Yes, almost any airplane can do that, as Tex Johnston demonstrated on the Dash 80. The aerodynamics are the same as regular flight.

Another thing is sustained negative-g flight (which is what is usually meant by "flying upside down"). The aerodynamics are different. The wings need to fly upside down, the engines need to work harder (because the wings are draggier, and because the higher angle of attack means that the fuselage is making more drag and that the engines are pointed upwards so not all the thrust is in the horizontal direction), the structure is loaded differently (but most airplanes are certified for the minus-one-g condition, so nothing would break apart), the systems might malfunction (if they rely on gravity to drive fuel, oil, hydraulic fluid, and other things through the pipes in the right direction), and - most importantly - the tail needs to stay down so as to maintain a nose-up attitude so that the wings still work. That's the hardest part. Airliners and general-aviation airplanes are simply too stable: The pilot can push the yoke all the way forward, and the elevators are simply not powerful enough to over-power the stabilizer and keep the nose from sinking. Aerobatic airplanes and fighter jets have smaller stabilizers set at smaller angles, and bigger and more powerful elevators, so they can keep the nose up and hold the necessary angle of attack. But a Cessna or a 747, probably not.
 

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