THE PHYSICS OF FLIGHT; a thread for CIT

[Hokulele]

Until that time, there is little for me to add.

<snipped the mathy goodness>

For those who like pictures to go with their equations, this webpage helped me visualize this a bit better.

http://www.chebucto.ns.ca/Recreation/BSC/crclmotn.html

 

[jaydeehess]

Wow. Listen to the experts. Airliners are now custom built so that they can't pull-up over the Pentagon but they can do just about everything else. Maybe we can fly one to Mars. As long as we get an amateur pilot.

In all of the spectacular examples of severly damaged aircraft listed here, none of them are large , heavy passenger aircraft.

That you cannot fathom what difference that makes indicates your utter lack of any technical accumen.

If you have something constructive to add to this thread,either to the OP or the derail, let's have it by all means. Your personal incredulity is worthless in a technical discussion.

 

[TjW]

But how does an F-4 fly?

It's very simple. They're so ugly, the earth repels them.

 

[X]

Just to clarify before anyone asks, there is no scientific mystery about lift, just that most of the simplified explanations one usually encounters in educational materials are not quite correct. Some of the details, in particular how the boundary layer forms from rest and whether or not it remains attached, are rather complicated, but the derivation of lift (the force) from a snapshot of steady flow is pretty simple; whether you approach it from the Bernoulli pressure perspective (as in the writeup here) or in a Newtonian sense (equal and opposite, etc.) you will get the right answer.

Yes, I could have worded that better.
Below are three examples, for those who want to know more. As we only touched briefly on these during the Aircraft Performance and Dynamics course I took, they made sense, but only just.

Circulation Theory of Lift
Lifting Line Theory of Lift
Momentum Theory of Lift


Oh, and the bit about the Gee Bee replica was awesome. I love those things. They look like plump little bumblebees. One of the few airplanes that actually qualifies as "cute".

I thought they stopped flying them due to the stability issues. Or did the R2 address that problem?

Wow. Listen to the experts. Airliners are now custom built so that they can't pull-up over the Pentagon but they can do just about everything else. Maybe we can fly one to Mars. As long as we get an amateur pilot.

On the off chance you aren't trolling (apologies to R. Mackey for replying):

No one in this thread (or in other threads) said an airplane could not pull-up over the Pentagon.
At least, not as a final point like you have it.
The people who have said such things have said them in regards to the specifics of the situation. Speed, aircraft capabilities, distance from Pentagon, etc.

What this thread is addressing is the physical limitations of aircraft. All aircraft can "pull-up". No one denies this. But pulling up height "y" over distance "x" at speed "v" is a specific scenario. While the aircraft may be able to pull-up in a general sense, the described maneuver may be beyond it's physical capabilities. It can still pull-up, but at either slower speed or greater distance.

To draw an analogy: CIT's infamous Hockey Stick flight path involved a descent, then a short, sharp pull to horizontal. This is not possible. The aircraft is not physically capable of generating the required lift to do that maneuver.
However, a slower, more gradual pull to horizontal (as was demonstead by R. Maceky, I beleive) is well within the aircraft's physical limitations.

So the people saying the pull-up is impossible are responding to a specific scenario in which it is indeed beyond the abilities of the aircraft.

Now, please. If you have something relevant to ask (clarification, more information, sample problems), then do so.

Otherwise, kindly stop trying to derail my thread into a mess of personal attacks and useless vitriol. That goes for all of you. I don't want to see this thread get AAH just because you can't hold your tongue.

It's very simple. They're so ugly, the earth repels them.

Nominated. I laughed.

 

[R.Mackey]

Oh, and the bit about the Gee Bee replica was awesome. I love those things. They look like plump little bumblebees. One of the few airplanes that actually qualifies as "cute".

I thought they stopped flying them due to the stability issues. Or did the R2 address that problem?

You and me both. When I was a wee lad and I went to see The Rocketeer, hardly 30 seconds into the movie I was filled with dread, because I knew they were going to crash one of those poor things.

Ha. No. The R2 didn't address it at all, was merely a slightly downrated plant for cross-country racing.

The real issues with the Gee Bees were, in order, (a) flaky engine technology of the day, (b) extremely high wing loading, requiring a very hot landing and getting you in trouble fast if the engine conked out, and (c) relatively small and ineffective control surfaces. The designers made extremely few sacrifices -- they were openly courting death in pursuit of the fastest plane humans could build. Nearly all of the originals crashed, many fatalities, and that plus the Great Depression sank the company.

However, from Scott Crosby and others, the plane is actually not as deadly as once thought. The main problem seems to be that most pilots simply weren't ready for anything with such extreme performance. With practice and training, the replica fleet has had a decent service record.

It'd be a swell R/C kit, I've seen those for sale but never flying. The real thing flying is spectacular.

 

[ElMondoHummus]

Okay, combining the OP and the "single-wing" videos, I just want to clarify: Do we all agree that the reason the F-15 was able to continue flight was because the body itself provided lift that would offset whatever asymmetric force was induced by the loss of one wing? And the flight was aided by fly-by-wire computing, as well as a crazy-high thrust-to-weight ratio? Whereas with an acrobatic "show plane" (I don't know what the actual term is) probably derives all its lift from the wings alone, and could not continue without fatally spiraling out of control? That's what I take from all the commentary.

I'd argue that this is an on-topic question, since it's one about applying the principles illustrated by the OP to a pair of specific cases. At any rate, that's sort of what I'm taking the consensus to be. Anyone agree? BJE's post ended up not being a derail, but rather a terrific opportunity to apply the principles in the OP.

 

[Reheat]

And the flight was aided by fly-by-wire computing, .......

I'd argue that this is an on-topic question, since it's one about applying the principles illustrated by the OP to a pair of specific cases.

Except for the "fly-by-wire" part it's a pretty good synopsis of why the F-15 was able to fly and the model/full scale aerobatic aircraft was not. (Check Mackey's explanation again.) Add the presence of a rather large rudder surface area and you've nailed it.

This is an educational thread and I agree that this subject is on topic. It helps to emphasize basic principles visually and conceptually when various parts are missing. In fact, everything except the attempted derail by the twoofer have been on topic.

Again, cudos to X for spending the time to construct and post this. Education is the key to unraveling the ignorance surrounding many of the events of 9/11.

 

[rwguinn]

Okay, combining the OP and the "single-wing" videos, I just want to clarify: Do we all agree that the reason the F-15 was able to continue flight was because the body itself provided lift that would offset whatever asymmetric force was induced by the loss of one wing? And the flight was aided by fly-by-wire computing, as well as a crazy-high thrust-to-weight ratio? Whereas with an acrobatic "show plane" (I don't know what the actual term is) probably derives all its lift from the wings alone, and could not continue without fatally spiraling out of control? That's what I take from all the commentary.

I'd argue that this is an on-topic question, since it's one about applying the principles illustrated by the OP to a pair of specific cases. At any rate, that's sort of what I'm taking the consensus to be. Anyone agree? BJE's post ended up not being a derail, but rather a terrific opportunity to apply the principles in the OP.

The reason that the "Show plane" thing looks plausible is that Knife-edge flight does derive the lift from the extreme Angle of Attack of the fuselage (Very large nose-up angle) and the available thrust. Given sufficuent aauthority from the remaining aileron, the scenario is at least possible, albeit very unlikely.
Some full size (rider scale)aerobatic aircraft are approaching model airplane capabilities--vertical hovering, very high A-o-A rolling manuevers, and sustained knife-edge flight--and even knife-edge loops.
And models are approching full-scale size--it is not uncommon to see 50% Extra 3XX models.
With enough power and control authority, you can fly a brick.

 

[ElMondoHummus]

Except for the "fly-by-wire" part it's a pretty good synopsis of why the F-15 was able to fly and the model/full scale aerobatic aircraft was not. (Check Mackey's explanation again.) Add the presence of a rather large rudder surface area and you've nailed it.

How'd I miss that? I normally keep an eye out for Ryan's posts. Thanks for pointing it out.

Yeah, I never remembered the Eagle having FBW; such a big deal was made of the F16 having it, and it debuted later than the F-15.

-----

Ok. So from all these explanations, it seems to me that not any plane could get away with this. You'd need a large rudder, and some serious thrust; like rwguinn says, enough power, enough control surface, fly a brick. So something like certain acrobatic planes, the F-15, and other planes meeting that criteria could pull off a knife-edge while missing a wing, whereas for a plane - just randomly selecting from this thread - like a KC-135 would almost certainly not.

Or a 757. Or a Gulfstream G-550. Or a Sopwith Camel (yeah, lookit that teeny rudder... poor thing must have rudder envy ).

-----

And yes, let me add my thanks to X. Threads like this provide excellent background to one of the basic concepts abused in the 9/11 conspiracy fantasies.

 

[R.Mackey]

Okay, combining the OP and the "single-wing" videos, I just want to clarify: Do we all agree that the reason the F-15 was able to continue flight was because the body itself provided lift that would offset whatever asymmetric force was induced by the loss of one wing? And the flight was aided by fly-by-wire computing, as well as a crazy-high thrust-to-weight ratio? Whereas with an acrobatic "show plane" (I don't know what the actual term is) probably derives all its lift from the wings alone, and could not continue without fatally spiraling out of control? That's what I take from all the commentary.

Well, remember, lift opposes weight. With the starboard wing gone on the F-15, it lost all the lift on that side, but also all the weight. In ordinary flight you have both wings carrying the body between them, so weight and lift don't have to derive from the same places, although the average does -- you want the aggregate center of mass to more or less line up with the aggregate center of pressure.

After losing a wing, the center of mass and pressure both shift, and this is what causes rotation -- you get a force couple that causes a roll. But how much they shift is dependent on velocity, angle of attack, and so on. At very low speeds, the heavier wing will droop, but at such low speeds the aircraft is basically falling anyway. At high speeds, the wing generates a huge surplus of lift that rolls the wing upward. The pilot needs to find the sweet spot in between.

That sweet spot will be at pretty high speed, but also high angle of attack. The latter results in relatively inefficient wing performance -- moderate lift, but also pretty high drag -- not to the point of stalling, but getting there. At high AoA no wing shape is going to be particularly efficient, and the L/D of the wing will approach that of the aircraft body itself; both will be effectively bluff bodies rather than proper airfoils. Kind of like the Space Shuttle, which on final approach is almost insensitive to attitude, basically a giant ceramic brick.

If you watch the video of that F-15 landing, he comes down very fast and very steep. Quite a harrowing landing. This is partly because with one wing, you certainly want to get down as soon as possible, but also partly because of the effect above.

The F-15 is one of the best aircraft to attempt such an absurd stunt because, as Reheat correctly notes, it also has enormous control surfaces, and these can overcome strong aerodynamic problems. The huge engines don't hurt either -- high AoA also lets you supplement lift with thrust.

Most "show planes" i.e. aerobatic planes would not be able to do this because they're built for low speed maneuvers, and thus their wings are quite large compared to their bodies, unlike the F-15 where the body is substantial. But it is entirely possible for a stunt plane to fly knife-edge. This isn't conducive to safe landing, however.

The reason that the "Show plane" thing looks plausible is that Knife-edge flight does derive the lift from the extreme Angle of Attack of the fuselage (Very large nose-up angle) and the available thrust. Given sufficuent aauthority from the remaining aileron, the scenario is at least possible, albeit very unlikely.
Some full size (rider scale)aerobatic aircraft are approaching model airplane capabilities--vertical hovering, very high A-o-A rolling manuevers, and sustained knife-edge flight--and even knife-edge loops.
And models are approching full-scale size--it is not uncommon to see 50% Extra 3XX models.
With enough power and control authority, you can fly a brick.

Absolutely right. It also bears pointing out that R/C models are aircraft, not some totally different species. If a model can do it, a full-size plane can do it provided performance can be suitably scaled (e.g. thrust to weight) and we don't smack into a new flow regime after we scale up (viz. Mach limits). This is often just a question of money and practicality.

ETA: In case it hasn't been mentioned yet, some designs don't need wings at all.

 

[Homeland Insurgency]

What this thread is addressing is the physical limitations of aircraft. All aircraft can "pull-up". No one denies this. But pulling up height "y" over distance "x" at speed "v" is a specific scenario. While the aircraft may be able to pull-up in a general sense, the described maneuver may be beyond it's physical capabilities. It can still pull-up, but at either slower speed or greater distance.

Yes that was my point. It's a contradiction to all the past debunker claims of just how much past recommended limitations an airliner can go. I mean it would have to if it were to pull off what was claimed. From that aspect you are not helping.

Thank you.

 

[dtugg]

Yes that was my point. It's a contradiction to all the past debunker claims of just how much past recommended limitations an airliner can go. I mean it would have to if it were to pull off what was claimed. From that aspect you are not helping.

Thank you.

You know nothing; nobody cares what you say. You have zero expertise in this area. Neither do I. But I do not choose to ignore comments from experts so that I can continue to believe some ridiculous fantasy.

 

[X]

Yes that was my point. It's a contradiction to all the past debunker claims of just how much past recommended limitations an airliner can go. I mean it would have to if it were to pull off what was claimed. From that aspect you are not helping.

Thank you.

I'm sorry, I have no idea what you are getting at.

Could you please state your question clearly, so that I can give you an answer?

 

[bje]

The reason that the "Show plane" thing looks plausible is that Knife-edge flight does derive the lift from the extreme Angle of Attack of the fuselage (Very large nose-up angle) and the available thrust. Given sufficuent aauthority from the remaining aileron, the scenario is at least possible, albeit very unlikely.

Snopes just picked it up on Nov. 6:

http://www.snopes.com/photos/airplane/onewing.asp

 

[Mangoose]

Snopes just picked it up on Nov. 6:

Cool, but they missed the most obvious thing that shows that the video is a fake. When you see the plane flying in the beginning of the video, it has a white trim between the orange and grey painted portions. When it lands, there is no trim at all -- the paint is just orange and grey. Unless the plane is repainted while in flight, we must conclude: two paint schemes = two planes, cleverly edited together.

 

[Dog Town]

Yes that was my point. It's a contradiction to all the past debunker claims of just how much past recommended limitations an airliner can go. I mean it would have to if it were to pull off what was claimed. From that aspect you are not helping.

Thank you.

No...thank you! For once again proving, your pathetic understanding, of the definitions provided by cultural literacy. Pfffttt......

 

[Big Les]

Yes, I could have worded that better.
Below are three examples, for those who want to know more. As we only touched briefly on these during the Aircraft Performance and Dynamics course I took, they made sense, but only just.

Circulation Theory of Lift
Lifting Line Theory of Lift
Momentum Theory of Lift

Ah, OK. I was (re)taught (as a non-scientist bumpkin) that it was primarily the deflection of air by the wing that produced lift. But even your initial purely pressure-based explanation is better than the model often shown by schoolteachers of air going faster over one surface than the other = lift.

 

[X]

But even your initial purely pressure-based explanation is better than the model often shown by schoolteachers of air going faster over one surface than the other = lift.

Actually, air moving faster over one surface than the other is exactly what I said. That speed difference is what causes the pressure change.
I just went into a little more detail on why the air flowing over the wing changes speed.


Air pressure can be separated into two sources: static pressure and dynamic pressure.

Static pressure is the pressure the air would have if it were not moving at all. This is the one driving lift.
Dynamic pressure is linked to the kinetic energy of the fluid. The main difference is that instead of trying to find the kinetic energy using mass (hard to do with the atmosphere) is uses density. This does not matter as much in lift (except, as you will see shortly, in reducing the static pressure), since the air is flowing over the wing rather than

The rest of this results from Bernoulli's principle (the following formula is for incompressible flow only, but it can be adapted to compressible flow):

[latex]P_{0} + \rho g h = P_{dynamic} + P_{static} + \rho g h = \frac{1}{2} \rho V^2 + P_{static} + \rho g h = constant[/latex]

where

P₀ = total pressure
P_dynamic = dynamic pressure
P_static = static pressure
g = gravity
h = height (altitude)
V = velocity
and rho = density​

Now this of course seems backwards at first glance. Greater velocity means more pressure, since it's got that V² term sitting right in there.
This is why the Bugatti Veyron only needs 280 horsepower to reach 150 mph, but needs over 1,000 horsepower to reach 250 mph. 60% more speed requires three and a half times as much power. All to overcome the drag caused by pushing through the dynamic pressure.

The trick here is that the above equation is constant.

At a constant altitude, we can ignore the height term, and we get:

[latex]constant = P_{static} + \frac{1}{2} \rho V^2[/latex]


As I said, it is principally the static pressure generating the lift, so let's re-arrange this equation to show the static pressure. I'm going to assign "C" to represent the constant.

[latex]P_{static} = C - \frac{1}{2} \rho V^2[/latex]


So, increased speed means reduced static pressure.
The difference in static pressure between the upper and lower surfaces of the wing is what generates lift.
This pressure difference is caused by the air moving faster over the top of the wing than over the bottom (think about it: if the air went the same speed over both surfaces, the static pressure would be the same on each surface, and there would be no lift).

So, what your teachers told you was perfectly acceptable. They just omitted the details behind it. To be fair, most schoolchildren don't have the background (in physics or math) yet to be able to follow the explanation I gave.


~ X (currently thinking of incorporating this post into the discussion)


Also be aware that total pressure is sometimes referred to as stagnation pressure (the pressure a moving stream of fluid would have if it were brought to a stop with no loss of energy). This is how Pitot tubes (those long thin rods you see projecting from the front of aircraft, usually with "remove before flight" tags hanging off them) determine airspeed, altitude, and more.

 

[X]

PDF version 1.0!

Alrighty!

I know a few of you have been waiting for this.

I have finished the first draft of the PDF file.

I added some information (1), removed hyperlinks (2), and fixed a rather glaring error I'm surprised wasn't picked up on (3).


(1) Based off suggestions and posts in this thread
(2) Replaced with a short glossary
(3) In Section 2.3, I originally said that lift is due to reduced dynamic pressure above the wing a. This is incorrect. As I showed in my last post, lift is due to reduced static pressure above the wing.


I look forward to feedback, especially from scientists, real engineers, pilots, and laypeople.

Comments on accuracy and readability, requests for clarification, and suggestions of additional topics are welcome, and indeed requested.

Sadly, the pdf file is too large to attach (a little more than twice the forum's attachment limit). So I have loaded onto afree online file-hosting site.

If this link does not work, let me know:

Edit: Link deleted. See post 101.

 

[Hokulele]

Downloaded. Thanks!

In a quick skim, I noticed a broken Latex link in section 3.3.