# Oops, My Error: Inverted Flight, Look in the Mirror?

edit: I made a mistake I think with a fundamental aspect of lift (that sustaining lift fundamentally consumes power, even ignoring form and friction drag…resolving this)

Have you ever tried inverted flight with some of the airliners? A380, B747, E175, etc. They all fly inverted. Would IRL physics actually allow this? After all, the maximum curve in the airfoil is at the top of the wing.

Why look in the mirror? You are a solid object, which takes up space such that air is vacant from the volume you occupy. And (I’ve seen it estimated) the human body has a glide ratio of about 1:1 (skydivers moving forward).

Take a step forward. The air must rapidly fill the void where your body was.

A wing moves forward. Air must rapidly fill the void of its prior location. The faster the wing moves, the bigger the volume void revealed that needs to be filled per unit time. This results in an ongoing pressure deficit, redirecting the motion of passing air. A pressure deficit that increases with speed.

Just make sure this low-pressure area is on top (to lift the wing up), which pulls passing air down as a response to filling the void (Newton’s 3rd law, below).

Where does the low-pressure area originate? All along the trailing boundary of the wing, where the vacated void is continuously being exposed.

Look at the trailing boundary of any wing. The trailing boundary is always on top of the wing (the curve’s shape is always contoured this way). The AoA adds to the total trailing boundary.

In inverted flight AoA creates practically all of your trailing boundary at the top of the wing. You can test it and see that AoA is pretty essential when you are flying upside down.

The contour of the airfoil has the primary purpose of optimizing lift to drag ratios, and stall performance. A flat plank, for example, flies well when it’s outside of its stall range. But its non-stall range is quite narrow.

(note: the airfoil is usually contoured so that it can produce some lift at zero AoA, but this contour is not essential to producing lift; AoA is your main tool for producing and controlling lift)

The trailing boundary void connects the notion of low pressure holding up the wing from above to Newton’s 3rd law: air forced down is equal to the force of lift holding you up: The wing is pulled up, because air is forced down.

Air forced down, cuts into the clouds

Flat wings fly:

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It bugs me I didn’t mention:

Any asymmetrical solid moving through the air turns the air which is the essence of lift. But I would be wrong not to add turbulent vs laminar flow as the air is separated by the solid, limits efficiency of the force obtained.

Stall behavior is because the flow of air or a fluid doesn’t like bending around abrupt changes in path. But all of the aircraft airfoil shapes you would expect to have a good laminar flow range while inverted.

Though the speed range for lift and stall behavior you would expect to change (the airfoil bump is now on the bottom)

Non-laminar flow (abrupt path change):

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My big mistake? I thought lift “ideally” doesn’t consume energy. That’s how you can be held in the sky with little energy from the engines (fuel burn is only to overcome inefficiencies like skin drag and loss from wake vortices). I thought lift ideally comes for free from bending the passing air.

Wrong(?). Downward movement of air as seen by outside observers, requires energy to be spent to mobilize the air from being stationary: ADB Antonov An-225 Mriya UR-82060 Amazing landing in the fog / 9.01.2022 RWY27 EPRZ Rzeszów Jasionka - YouTube

So a volume of air with mass weight equal to the aircraft, needs to be accelerated down, at the same rate an object falls to the earth, to create lift equal to the aircrafts weight.

This energy siphoned off from the passing air for this downward acceleration, has to be paid for by engine fuel burn.

It’s just that there is so much kinetic energy in the aircrafts forward motion that the % siphoned off is small.

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What’s the point of this thread other than explaining what you posted?

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IF has seriously infected my interest in why aircraft fly. I thought I knew. But the more one seeks to grasp an honestly understandable explanation, the more barriers seem to be thrown up.

A comment about non-acrobatic IF aircraft being able to fly upside down has once more rekindled that spark in that curiosity.

The big curiosity: In surveying source after source, why does there seem to be no common consensus on why aircraft are held in the sky. This includes expert sources. It’s piecemeal and often contradictory.

What am I missing in this mystery?

edit: Maybe part of it is I missed induced drag’s direct role in the downwash angle(?):

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I appreciate the thought you’ve put into this… Although there’s no harm in analyzing it, you might be looking too deep. As far as I’m concerned, the common consensus is that Newton’s 3rd law and Bernoulli’s principle explains lift. Full stop. Any upside down airfoil would still be subject to Newtons 3rd law explaining lift just like your hand would be as you stick it out the window of your car while driving. I wouldn’t say IF is perfect physics-wise, but by my logic, an upside down airliner would fly better than a rock or a skydiver. I don’t want to be the one to test it out IRL tho. Appreciate the curiosity once again…

Maybe have a look at the lift equation? Keeping in mind the factor speed has on lift. No mention of induced drag, thats irrelevant in a way. If you go too slow and reach a critical AoA, you’re no better than a rock.

Good luck

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Hi, thank you for your reply. My current obstacle is: “How do I explain to a novice (or myself even) why engine thrust doesn’t have to equal weight to keep an aircraft from falling?” (how this occurs in keeping with conservation of energy)

In searching for an answer, I’d maybe go to the drag curve:

The reason you get the advantage over hovering flight (I assume) is that you have the lift-induced drag curve you can move down as you go faster.

If, on the other hand, you go up the induced drag curve, assuming you’re still in flight, you get to a point where induced drag exceeds available engine power.

Any thoughts on how to explain how forward flight allows engine power far below vertical hover power? It just seems that the lift drag curve is fundamental to explaining conservation of energy in the face of reduced engine power requirements with speed.

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What in the world is this thread?

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IF inspired magic of flight.

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Also:

There is a lot stuffed inside the lift coefficient Cl. This includes AoA which of course is a key factor of lift generation and, I assume, AoA is closely connected to induced drag.

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Lets look at the exchange of energy between an aircraft in low-speed SLUF and a standard environment (the flow field it is in). On a very small level, we can see that the aircraft disrupts the airflow through viscous forces (parasitic drag) and through differential pressure in the flow creating vortices (induced drag). When these forces are multiplied by a distance, it becomes the work done by the system (the aircraft), W.out. Now, if we want to keep our aircraft in SLUF, we need to have a W.in which is equal to W.out, in order to maintain the energy of the aircraft. Our W.in created simply by having an engine. As you can infer, our engine need only generate the force which is equal to drag in order to keep it at a specific altitude and speed (energy level).

Another way you might look at it is by examining an engineless glider. In this example we will assume that the aircraft maintains only a constant speed, with our altitude being variable. Therefore, KE = const. We can see that work is done on the aircraft through drag, therefore either potential energy or kinetic energy must decrease in order to maintain a balance of energy. Now we just said that KE must remain constant, so PE must decrease. In this manner, I can actually calculate the drag on my aircraft solely with a variometer.

If I know my glider weighs 1000 lbs (~32 slugs) and is losing 200 ft/min at 50 knots, I can determine the drag force on my aircraft to be roughly 40 lbs. If I had an engine that could output 40 lbs, I would have a fully powered aircraft.

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Thank you. I certainly appreciate the detail you put in to sorting out what is what.

I agree the rate of work (power = energy per unit time) consumed from the aircraft (for lift) has to equal the rate of work supplied.

So the general case to cover both powered and unpowered flight, per unit time:

Drag acting over a distance (work consumed) = Thrust acting over a distance (work supplied) +
Gravity acting over a distance (work supplied)

The distance of course being in the direction of the respective force. And gravity only works if you travel in its direction (descent, as in a glide to replace thrust being zero).

The equality above is obviously needed for constant speed (unaccelerated) flight.

So the issue I had/have, is: “How does Lift fit in with that power equality?”

To get enough force up to oppose W (aircraft weight) you need to change some of the passing air’s momentum to down (Newton’s 3rd law).

You need this induced downward momentum of air to be
W = Maird X Vdown. So that’s just force as a change in momentum. It’s a change because the air’s speed in the down direction went from 0 to Vdown as it passed the wing. Maird is mass of air moved in the down direction.

So to get enough of this change in downward momentum of air, you get to trade off between Maird vs Vdown.

So is the problem?: this downward work on the air (we see as downwash as an aircraft passes through a patch of cloud) which causes the momentum change of W,

it consumes some of the aircraft’s kinetic energy as: (1/2)Maird X Vdown^2

So momenum needed for lift is proportional to Vdown; but energy consumed to produce that momentum is proportional to Vdown squared.

…there’s a potential conclusion in here somewhere(?), my brain is shutting down; need to come back to it in a bit…:)

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This is quite the thread, very interesting to read and have not seen someone mention newtons laws in quite some time. Excellent job on the mathematical analysis, and in theory all this is correct.

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Huh? 😂😂😂😂😂

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IF engages my aviation bug in a big way. This topic is about how the mind deals with understanding the magic of flight. How can something like an A350 carry enough fuel to fly to the opposite side of the planet, or a 747 can glide for miles at engine idle(?)

A key part (and current obstacle for me) of that whole picture is how to simply understand the low power consumed by lift due to forward motion (lowers fuel consumption; stretches the glide).

It maybe makes sense the faster you go, the more mass of air gets turned by the wind. Still thinking about how to see it simple and clear. If, any ideas I’m all ears.

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yea i know your point, wing irl dont allow fly inverted cause of physics, aerodynamic and more, but, its a game, doesnt even get close to a realistic fs, the physics arent the best, so every aircraft can fly inverted, but hey, you cant expect too much, first at all, its a mobile flight simulaotr with a small group, so, you cant expect msfs, or xplane physics. two: it works for a correct procedure, the game isnt made for fly upside down, thhe game expect that you fly with al the correct procedures, so if you dont follow these, its easier to found bugs.

i mean, its hard to follow all the irl physics. if you want more realistic flight experience, you an go to another place, because, IF, never, i think, NEVER, will improve the physics when it works, its how the world works, if it work, why change? look at the capitalism, and look at the socialism, one work, aonther no, so the one that works, is most common to use, another example, why use bernoulli’s theory who is not accepted at all, because also exist the 3rd newton law that explain that too, but, why we use bernoulli? because it work, it work for make the planes and explain in an easy way why planes fly.

Hi, thank you for your reply. But actually, my point is the opposite. Sorry for the confusing title.

I believe wings can fly upside down given the right conditions: airflow has to turn smoothly around edges (roughly laminar). and there has to be sufficient additional AoA (Angle of Attack) and IAS. (Also, flaps don’t work in reverse, so don’t use them)

As Cody stated:

Because airliners aren’t meant to fly upside down, there are a lot of unknows in whether the engineering will allow us to hold the inverted wing at the appropriate angle and speed for sustained flight … But in principle wings can fly upside down.

I like that IF allows us to test it out. It does indeed seem to require more AoA and higher speeds. So the physics feel like the right direction the best I can tell.

Say you have a 50/50 chance it works. Irl, no way one would try it. In IF? Who wouldn’t try it. I just appreciate they allow the aircraft to fly upside down, and with a pretty decent feel.

Also, the IF physics in general, I’m very happy with it. No simulator has perfect physics. Not even the most sophisticated simulators used by the airlines. But how much attention is paid to the physics is pretty amazing.

My criticism was not with IF physics, but in a gap in my understanding in a key aspect of the physics of flight. I think I have an answer for that now. I want to say it more concisely though…

Does this make logical sense?: directly from the FAA’s “Chapter 5,
Aerodynamics of Flight”
PHAK Chapter 5 (faa.gov)

"rotation of these vortices, it can be seen that they induce
an upward flow of air beyond the tip and a downwash flow
behind the wing’s trailing edge. This induced downwash has
nothing in common with the downwash that is necessary to
produce lift. It is, in fact, the source of induced drag.

Downwash points the relative wind downward, so the more
downwash you have, the more your relative wind points
downward. That’s important for one very good reason: lift is
always perpendicular to the relative wind. In Figure 5-11, you
can see that when you have less downwash, your lift vector
is more vertical, opposing gravity. And when you have more
downwash, your lift vector points back more, causing induced
drag. On top of that, it takes energy for your wings to create
downwash and vortices, and that energy creates drag."

Induced drag is from vortices downwash which has “nothing in common” with downwash for lift? But lift downwash creates a lift component pointed aft, causing induced drag?