A question for the guys who know their aerodynamics.

[Finkeren]

One thing I have never been able to understand about flying, is how inverted flight is even posible in a standard fixed wing aircraft. 

 

From what little I understand about aerodynamics, the airfoil is designed to generate a lift force in one specific direction - directly upwards when the plane is flying straight and level.

 

Flipping the aircraft on its back to fly inverted should (in my understanding) mean, that the lift force is no longer cancelling out the gravitational pull on the aircraft to keep the aircraft at a given altitude but rather is working in the same direction as gravity to pull the plane towards the ground even faster than in a free fall.

 

Why then doesn't a plane fall like a rock out of the sky, when flying inverted? In most flight sims I've flown it's certainly a lot harder to keep the plane level, the aircraft clearly "wants" to get back into a more "normal" position, and you have to actively fly it at all times (let go of the stick, and it immediately drops its nose towards the ground). In BoS it feels mighty unstable and unsafe - as I imagine it should. Yet it's still posible with some practice to keep it flying almost straight and level for a significant amount of time, and the altitude lost is nowhere near what you'd experience in a free fall.

 

Clearly inverted flight is posible, I've seen it done on several occasions in the real world, even at very low altitudes, but I don't understand why.

 

Can someone with knowledge of aerodynamics explain to me (or provide a link) that explains, what happens with the forces working on a plane in inverted flight?

Edited December 5, 2013 by Finkeren

[-MG-Cacti4-6]

You bring up some very interesting points. One thing you do need to understand however is that the wing spar is designed in such a way to provide stable air flow both over the top of the wings and horizontal stab and underneath it. So in essence you have to two different vectors of force acting on the wings, one from the top, one from the bottom, in the form of air pressure. Now with all of that out of the way, air doesn't flow over the wing at the same speed, it flows over the top of the wing faster than it flows under the bottom.So, why is it that there is a tendency to nose down during inverted flight?

 

It goes back to airflow. because air flows over the bottom surface of the wing slower, it generates lift this way because the air pressure is higher. thats why in un-trimmed flight, most a/c have a tendency to nose up until the bird either stalls, or there is a balance achieved between the pressures acting on both sides of the wing. So naturally, when an a/c is inverted, this process is also flipped upside down. 

 

Also keep in mind, manufacturing and engineering have come quiet a long way since the mid 30's when most ww2 a/c were developed, so thanks to inefficiency in the wing itself, this effect can be even more pronounced than say, in an a/c built to today's standards

Edited December 5, 2013 by -MG-Cacti4-6

[vonrickenbecker]

While most airfoils are designed to give more lift in one orientation than another, it is mostly the angle of attack that dictates how much lift is generated (and in what direction). Flying inverted is certainly possible, it's just not quite as efficient as flying right-side up. Actually, I remember reading once that the P-51 had a symmetrical airfoil wing (top and bottom surfaces identical), which is part of what gave it it's high speed capability.

[TheCheese]

I think the main thing to understand is along the lines of what baronvonvlemson said, namely the angle of attack. In normal flight, if you increase the angle of attack of your airfoil, it will continue to function until a certain point, meaning that though it is tilted upwards, some air is still flowing over the top, and that air is still flowing faster than the air slamming into the underside of the wing.

 

If your airfoil is inverted, and perfectly horizontal, then likely yes, you will plummet downwards, however if you push the stick forwards and change the angle of attack of the wing, the same effect as flying upright is invoked, where lift can increase as air flows over the top of the wing and over a longer distance than the air flowing beneath the wing

 

[Finkeren]

Ok, so if I understand this correctly: The airfoil on the wing doesn't just provide lift in one direction, it depends on angle of attack? So by keeping the nose well above the horizon while inverted, I keep the AoA of the wings positive so they still generate lift in the opposite direction of the gravitational pull (meaning that now the lower preasure is on the bottom of the rather than on the top?

 

Am I understanding this correctly.

[Sparrer]

S!

 

because of :

 

 

 

 

Thats why you need to push your stick a little bit to fly inverted

[OBT-Psycho]

yes, as the diagram shows it well. And the example of the Mustang is the perfect one. P-51 wings were designed to have a laminar airfoil, both air up and under the wing go the same speed if the wing is leveled with the airflow. But as they have a slight AoA, they provide lift. no matter where is the sky and were is the ground

[Finkeren]

Thanks TheCheese.

 

That diagram cleared things up quite a bit for me. I've seen illustrations of the airflow over an airfoil before ofc, but I had never seen it illustrated in an inverted position as well.

 

I get, why it works now, and why it's clearly less effective than flying normal, with the airfoil being optimised for "upright" flight.

 

Thanks a lot.

[Sparrer]

S!

 

Just put your hands like a wing through window when driving fast a car.....then change the angle a little bit...you will feel the reaction of air as a force even .......if your hand is not designed as a wing

[JtD]

Am I understanding this correctly.

Yes, you are. I attached another chart showing the lift over angle of attack relation for a NACA 23015 airfoil, pretty common one on WW2 fighters. You can see that at negative angles of attack, there's a negative lift coefficient down to about -1.4@-15°. This is what keeps your aircraft in flight when inverted.

[DD_bongodriver]

Also there are symmetrical airfoils, I believe the Pitts uses them, they generate lift purely on angle of attack and will fly the same inverted as they do upright.

[Finkeren]

One more question: If the wing generates lift the same way in inverted flight, just less effectively, why does it feel so unstable in some flight sims? In BoS especially the LaGG seems quite unwilling to stay inverted, and RoF it's even worse. Does that have something to do with wing dihedral, or is it because the horizontal and vertical stabilisers work differently, or maybe something else?

[ImPeRaToR]

I don't feel like going outside to test my aerodynamics so I can't really tell you how good they are

[DD_bongodriver]

Yes, dihedral is a stabilising feature, when inverted it becomes anhedral and this is unstable.

[NonWonderDog]

S!

 

because of :

 

 

 

 

Thats why you need to push your stick a little bit to fly inverted

Ehh, that picture's not right at all. (Edit: the flow lines are correct; the annotations are wrong.) It can all be described by Bernoulli's equations unless you're on the edge of space (that picture is only valid for the space shuttle re-entry).

 

The explanation that "the air has to go further on top" is so simplified to be meaningless, but it is true that lift in an airplane is entirely due to pressure differentials.

 

The way aero engineers think about it, there's a fact of life called the "Kutta condition." To wit: "a body with a sharp trailing edge which is moving through a fluid will create about itself a circulation of sufficient strength to hold the rear stagnation point at the trailing edge." In other words, since an infinite pressure would have to exist at a perfectly sharp trailing edge in order to turn the flow, the flow does not turn around a trailing edge. In order for the flow to not turn, a wing has to create circulation proportional to angle of attack. At a positive alpha, the circulation means the air is moving faster above the wing. Faster air is at lower pressure, therefore lift. (And in case you were wondering: Yes, egg beaters DO work as wings, since they create circulation. They're not very controllable, of course.)

 

It's hard to put into words, which is why the grade-school explanation is so bad and misleading.

 

Its also much more complicated when you allow for compressible flow and stalls.

Edited December 5, 2013 by NonWonderDog

[Finkeren]

Yes, dihedral is a stabilising feature, when inverted it becomes anhedral and this is unstable.

Ok, so a plane with a pronounced dihedral, such as the P-40, will generally be more unstable, when flying inverted?

[DD_bongodriver]

Ok, so a plane with a pronounced dihedral, such as the P-40, will generally be more unstable, when flying inverted?

 

in principle yes.

[HansHansen]

First, as already mentioned, the P51 had a laminar airfoil, not a symmetrical one. Thats a huge difference. Laminar airfoils have their highest thickness further back (compared to the normal high lift airfoil) to keep the air flowing laminar over the wing for a longer distance, reducing drag.

 

Second, symmetrical airfoils are only used for "hardcore" aerobatics aircraft to give them similar characterstics for both normal and inverted flight, but produce less lift in general.

 

 

I am guessing another factor that may contribute to instability is that, only to keep flying straight while inverted, you already have huge AoA, so you are closer to a stall all the time. (except symmetrical airfoils)

[Furio]

One more question: If the wing generates lift the same way in inverted flight, just less effectively, why does it feel so unstable in some flight sims? In BoS especially the LaGG seems quite unwilling to stay inverted, and RoF it's even worse. Does that have something to do with wing dihedral, or is it because the horizontal and vertical stabilisers work differently, or maybe something else?

 

I did some rough sketches, hoping they can be useful.

About ROF, it’s easy to understand. WW I airfoils were very thin, highly cambered (curved) and had little efficiency even upright. Inverted they produced very little lift and enormous drag.

 

 

 

WWII era types had much better airfoils. They were almost invariably fixed with some incidence relative to the fuselage, so they produced lift while the fuselage stood horizontal. Say that this incidence was 2 or 3 degrees.

 

 

Now, flip your plane inverted. You need to push your nose the same 2 or 3 degrees to reach an incidence of zero degrees. Then you need to push more to add incidence enough to produce lift. If the airfoil is unsymmetrical (in the LaGG it was an NACA 23012), these 2 or 3 degrees will not be enough. Say that you’ll need 4 or 5. If this number is correct, in total your nose will change attitude as much as 8 degrees.

 

 

It this attitude, your plane produces a lot of drag. You are pushing a lot on the stick, using most of its travel just to maintain level flight. Looking at the horizon, all your visual reference is much different while inverted.  Sum these facts, and you’ll understand the feeling that the plane is “unwilling to stay inverted”.

[Furio]

Talking of wing incidence, the Withley bomber came to my mind. It was probably the plane with the higher incidence ever built. Look its fuselage nose-down attitude in level flight and think the attitude it would have inverted!

 

 

 

[Uriah]

 Faster air is at lower pressure, therefore lift. 

Once I understood a bit more I did not like term 'lift'.  It should be 'push'.  The Wright brothers discovered the proper angle of attach by mounting (I believe a small bicycle wheel) horizontally on their bicycle a playing card.  As one of the brothers road the cycle the passing air forced the playing card to stay at a particular angle.  It surprised them what the angle was.  They also discovered the 'right' shape of the air foil by using a small (no bigger than a six feet).  But what they did not know is that the best shape of the air foil depends on how fast you can move the wing through the air.  As the Wright brothers' plane could only fly very slowly the air foil had to be rather slim and wide.  Everyone for a long while thought that is the way it had to be.  But some German scientist discovered they could have a much fatter air foil and much more lift.  That resulted in an awesome WWI fighter plane.  Then during the jet age...

  The point is that the optimal shape of the air foil depends on what you want to do and how fast you can go through the air.

 

And that inverted flying by the acrobatic planes?  I've heard the wings are not fixed, they can actually alter the angle of attack from the body of the plane.  I have no idea if that is true.   

[DD_bongodriver]

And that inverted flying by the acrobatic planes? I've heard the wings are not fixed, they can actually alter the angle of attack from the body of the plane. I have no idea if that is true.

 

Not a feature I have ever heard of.

[Furio]

The Mignet "Flying Fleas" can do it. They change angle of attack of the forward wing to control pitch. You can easily see the push-pull rods going from the stick to the forward wing's trailing edge.

 

 

[HansHansen]

with those ailerons and big powerful engines, this thing surely will pull off some mad crazy aerobatic maneuvers

[-MG-Cacti4-6]

with those ailerons and big powerful engines, this thing surely will pull off some mad crazy aerobatic maneuvers

it has a ufo fm...needs rebalancing

[Gort]

Perhaps someone can explain, given the exemplary knowledge here, why there seems to be less lift behind an aircraft carrier, especially at night!

[Finkeren]

Talking of wing incidence, the Withley bomber came to my mind. It was probably the plane with the higher incidence ever built. Look its fuselage nose-down attitude in level flight and think the attitude it would have inverted!

 

 

 

Wow, that's pretty astounding. I've noticed the nose-down attitude of the Withley before, but didn't know the reason.

 

I guess it was done to help generate lift on take off and/or landing?

[JtD]

Yes, with the tricycle landing gear the incidence of the aircraft was limited on the ground, and in order to bring the plane into the air with a reasonably short take off run, you'd need extra incidence for the wing. Also, wing incidence helps to achieve a good horizontal attitude in flight, with minimized drag at cruise settings. If it had to fly nose up all the time, it would present a larger cross section, meaning more drag.

[Gort]

If you have an iOS device, try out this app. You can draw airfoil sections, add high lift devices, model all sorts of shapes and sizes, vary velocity and viscosity, etc, even add propellor flow. Relative lift, drag and CL can be calculated and compared.

 

https://itunes.apple.com/us/app/wind-tunnel-pro-hd/id450980034?mt=8

 

The basic version's icon shows exactly what you were inquiring in your post.

 

https://itunes.apple.com/us/app/wind-tunnel/id381971296?mt=8

[Sternjaeger]

that's a little amazing app!