Questions you want answered.

That last one holds a special place in my heart.

If that's an F-15, Coldstream, it's got a 1-1 thrust ratio at fully loaded takeoff weight, so I suspect it could fly in some fashion *without* the wings...

NSMike wrote:

Yeah, I very rarely use ATMs, but in my experience, most don't carry anything larger than $20 bills, nor give you the option of which denomination bills you get. You're probably going to have to go in and ask for one from a teller.

Also, not sure why you need one, but if you're doing it, for example, to get a large denomination bill as a gift for a kid (niece/nephew, young cousin, etc.) because it would be a neat thing, it might be best to check in with the parents first, because you might just be adding an errand to their schedule - some stores are kinda crotchety about taking large denomination bills because of the risk of forgeries and because they don't like to keep large denomination bills in cash registers.

I don't need one, I was just listening to the 99% Invisible episode about US bank notes and I got curious.

Also in the part about (awesome) Australian polymer notes they didn't mention that we got caught bribing foreign officials to licence it from us. Embarrassing.

There's been talk of phasing out the $100 note here but it hasn't gained steam.

If I was going to get one, and for some reason the bank wasn't an option, I'd go to ALDI and see if the cashier there had one, because that's where the older people that take all their money out at a bank counter go I would think.

Robear wrote:

If that's an F-15, Coldstream, it's got a 1-1 thrust ratio at fully loaded takeoff weight, so I suspect it could fly in some fashion *without* the wings... :-)

A 1:1 thrust/weight ratio doesn't mean crap when you're slowing down on glide-slope to land. The only thing keeping that aircraft in the air was the lifting force generated by the fuselage itself. It was a staggering feat of airmanship! But yeah, having mighty engines and an ejection seat probably helped his confidence.

That's true. It was amazing flying.

Haha, I opened the forum wondering what discussion would spur 29 replies in one day, but yeah airplane wings'll do that. Personally even as a physics major, I decided years ago that I was comfortable not understanding them.

Next let's do "does 0.999... equal 1?", or maybe the puzzle where one envelope contains twice as much money as the other and should you switch?

Coldstream wrote:

Bernoulli is relatively weak per square cm of wing, which is why wings need to be so damned big if you're relying on it as a major lift factor (as I'm sure you're aware), so I really can't see how the small surface area of a light GA prop is able to generate sufficient horizontal lift (thrust) via that mechanism to suck a 2500lb aircraft forward fast enough to fly.

The only thing that gives me pause about this is the difference in magnitude of typical lift forces vs thrust forces for an airplane.

E.g. You can push a Cessna 172 by hand, but you can't lift it over your head. It's making something like 500 lbs thrust at takeoff power, with a gross weight of 2500 lbs (which means your lift forces are going to be bigger than that during any climb). So that's a factor of "more than 5" difference, then we factor in that the aerodynamic operating range of a propeller is narrower than a wing, so it can be more tightly optimized for that condition, so we can't assume a 1-to-1 comparison of wing area to prop blade area.

Likewise, a 787 at max-rated takeoff is making 152,000 lbs thrust, but it weighs 400,000- 500,000 lbs, so that's a not too dissimilar ratio of lift to thrust.

Coldstream wrote:

To move that amount of air back, you must be deflecting it. If you're able to explain the physics otherwise, I'd love to hear your thoughts. I just don't see how a prop moves air backwards or an aircraft flies on knife-edge (with the wings contributing zero upward lift component) without the deflection explanation.

Short version of is that classical Bernoulli involves significant downward deflection of the airstream flowing over it. But honestly, I'm way out of my wheelhouse at this point, and I should defer to wikipedia.

That said, I can say that when it comes to a turbofan, turbulent flow through the engine means stall or surge, so I'm more confident that when it comes to jet propulsion, you're talking about smooth, laminar Bernoullii-esque flow past fan, compressor and turbine blades.

Coldstream wrote:

I'm such an aviation dork. :D

You're in good company then.

Modern planes have lift to drag ratios of well over 10 and closer to 20. The space shuttle is somewhere around 5 or 6. Typical lift coefficients, based on planform area of the wing, are usually in the tenths or ones. While typical drag coefficients are in the hundredths. Meaning the thrust needed to pull and airplane through the air is an order of magnitude less that what it takes to keep it in the air.

I'm intrigued that the flow through an engine would be laminar. I would have expected it to be highly turbulent. TIL I guess. On the aero side you generally want turbulent flow over the wings, unless you're a glider. Turbulent flow over a wing is less likely to cause abrupt stall. And at the kind of Reynolds numbers typical airplanes fly at it's almost certainly turbulent. Hell, most airliners I've seen have vortex generators along the wings specifically to cause boundary layer tripping and turbulent flow.

The discussion about Bernoulli's principle is getting a little out of hand. The existence of its effects are not in question. It is not some magical device that adds to lift or something, it is a simplified version of THE way to describe and quantify how flow moves around a wing, completely described by the Navier-Stokes equations. This is not an opinion. This is a fact. If one were to make a number of simplifying assumptions you can derive Bernoulli's principle from those equations. But it's not as if lift on a wing or a propeller blade is 10% "Bernoulli" and 90% "flow turning", that's not how this works. The Bernoulli principle accurately quantifies the pressure around a wing or propeller blade in the subset of flows for which it is applicable.

Lift as a physical phenomena, it's really just a single force and Lift and Drag are just arbitrary names we give to the two components of it, is a surface pressure integral around the wing. There is no other way for air to interact with the wing, except friction which is a separate topic. As Jonman has stated, this is a discussion of how to describe it and what are the mechanisms through which this happens. Pressure differences due to both accelerated flow or flow turning are mechanisms through which the static and dynamic pressures change over the entire wing surface to generate forces.

IMAGE(https://www.grc.nasa.gov/WWW/k-12/airplane/Images/cp.gif)

As a matter of fact, if you were to take one more step and zoom out. Aerodynamic lift and buoyancy are the exact same phenomena. You can see this in the slightly expanded version of the Bernoulli principle, which has a rho*g*h part to the total pressure sum. We just usually neglect it in discussions of airplanes because the density of air is so small and the difference in height between the top and bottom surfaces is tiny.

Note: Here lift and thruster are interchangeable and wing and blade are also interchangeable. A wing and a propeller blade are essentially the same thing in this context.

Edit: VSTOL concept airplanes are my guilty pleasure.
The VSTOL Wheel
IMAGE(https://vertipedia-legacy.vtol.org/vstol/images/VSTOLWheel/pics/16.jpg)

fenomas wrote:

Haha, I opened the forum wondering what discussion would spur 29 replies in one day, but yeah airplane wings'll do that. Personally even as a physics major, I decided years ago that I was comfortable not understanding them.

Yeah, I've come to the conclusion that I should probably just stick to flying the damned things and let the physics take care of itself.

I have learned a lot in the last few pages of the thread though, so I'm happy!

WHERE IS YOUR GOD NOW?!??

IMAGE(https://i.imgur.com/u3s54Rn.png)

And here we see a mother airplane carrying its young on its back. Soon enough the young plane will grow strong enough to fly on its own.
IMAGE(https://www.nasa.gov/sites/default/files/images/273242main_EC01-0129-17_full_full.jpg)

The best part about that picture is the bird totally owning the airspace between the helicopter and the airplane.

Jonman wrote:

I'm not 100% on this (not an aero guy), but I think you're entirely wrong here. At least with prop and fan blades, we're entirely talking Bernoulli. The last thing you want on a commercial application is to smack the air with the blades - that'll be draggy as f*ck, and drag is the enemy when your prime design consideration is efficiency.

I was reading about a year ago on this subject, and it turns out that they didn't entirely understand why planes fly. The common explanation was the Bernoulli effect, but as it turns out, that's only a small component of the lift generated.

Rather, when a wing is being pulled forward through the air, it tends to assume laminar flow on both sides of the wing. (In fact, if you don't have laminar flow, you are stalling.) It's not Bernoulli that generates the lift, but rather that the laminar flow forces the air that's going over the top of the wing to bend downward, and it gets thrust down below the aircraft. I can't do a diagram in text, but imagine two airflows coming off a wing... the bottom airflow goes more or less straight back, but the airflow from the wing top is coming down at an angle, and thrusts well below the plane as a whole.

In other words, what's actually happening is that the plane is pushing enough air downward, at a high enough velocity, to hold itself up.

This is separate from propulsion. The prop or turbine pushing the air backward moves the plane forward. The wings generate lift, by deflecting air downward over their top surfaces. A few planes, like the Harrier, mix the concepts up and sometimes use the engine thrust to generate lift (for carrier landings, in that specific case), but typically the engine is throwing air out the back to move forward, and the wings are throwing air downward to hold the plane up against gravity.

Malor wrote:

Rather, when a wing is being pulled forward through the air, it tends to assume laminar flow on both sides of the wing. (In fact, if you don't have laminar flow, you are stalling, and probably falling out of the air.) It's not Bernoulli that generates the lift, but rather that the laminar flow forces the air that's going over the top of the wing to bend downward, and it gets thrust down below the aircraft. I can't do a diagram in text, but imagine two airflows coming off a wing... the bottom airflow goes more or less straight back, but the airflow from the wing top is coming down at an angle, and thrusts well below the plane as a whole.

This is entirely inaccurate. Most high Reynolds number flows, like on full scale aircraft, are turbulent rather than laminar. And most airplanes are designed to fly with turbulent flow over the wings precisely because turbulent flow is less prone to sudden detachment and stall.

Think about it, maverickz..... you don't get something for nothing. If you want to hold the plane up in the air, you need to displace enough air downward to compensate for its downward acceleration.

You have to push against something, and when flying, there is only one thing to push against.

I've thought about this for five years and two degrees. And then for another sixteen years of work.

If you have a 5,000lb plane flying level, then you have to displace enough weight of air downward to hold the plane up. Velocity counts, too, so you don't likely need 1:1 in terms of mass, but ultimately it has to be air being forced downward for the plane to go up.

Hmm, you're right.

Malor wrote:

If you have a 5,000lb plane flying level, then you have to displace enough weight of air downward to hold the plane up. Velocity counts, too, so you don't likely need 1:1 in terms of mass, but ultimately it has to be air being forced downward for the plane to go up.

That's Newton's 3rd law. But Bernoulli is also correct - lower pressure above the wing literally sucks the wing upwards. And both of these are strongly affected by velocity. This video was helpful for me to understand that these things happen *simultaneously* - dynamically, not in sequence as we unconsciously expect.

https://www.youtube.com/watch?v=E3i_...

Maverickz, is this guy accurate? Certainly the smoke is highly illustrative.

For my part, I want to make it clear that I'm fully on board with the whole Bernoulli thing as a significant portion of lift generated by airflow over a wing. As maverickz says, the stuff described by Bernoulli is fact as best we can measure it. Where my training diverged from maverickz training (are you an engineer? I'm unclear) is that there are significant other contributing factors, primarily deflection of air downward from the airfoil's AoA to the relative wind. I applied what I had been taught to how I thought a propeller worked, which seems intuitive to me but may well be wrong.

Sounds like better minds than mine still disagree on exactly what physics allow wings to fly, but I think the disagreement is one of degrees, rather than fundamentals. For my part, I like the mental model of Bernoulli + deflection, so I'll go with that since it's everything I need in the cockpit to predict how my aircraft will fly. Great discussion though!

merphle wrote:
Jonman wrote:

"Suck, squeeze, bang, blow", right?

qft

.... golly..

I am not even close to an aerodynamicist, but the same low-pressure/high-pressure dynamic is also used in Formula 1 (and many other open wheel series) to keep the cars stuck to the road. There used to be skirts on the cars that would seal the floor of the car, and the low pressure under the car would almost crushingly suck the car down. The problem with the skirts was that they could bounce around and open up enough on occasion to completely break that seal, and suddenly that suction was gone. Cars would just dramatically fly out of control.

F1 actually still uses this principle, but instead of physical skirts, they use complex aerodynamic structures to create vortices which use air to create that same kind of seal.

Mermaidpirate wrote:
NSMike wrote:

Yeah, I very rarely use ATMs, but in my experience, most don't carry anything larger than $20 bills, nor give you the option of which denomination bills you get. You're probably going to have to go in and ask for one from a teller.

Also, not sure why you need one, but if you're doing it, for example, to get a large denomination bill as a gift for a kid (niece/nephew, young cousin, etc.) because it would be a neat thing, it might be best to check in with the parents first, because you might just be adding an errand to their schedule - some stores are kinda crotchety about taking large denomination bills because of the risk of forgeries and because they don't like to keep large denomination bills in cash registers.

I don't need one, I was just listening to the 99% Invisible episode about US bank notes and I got curious.

Also in the part about (awesome) Australian polymer notes they didn't mention that we got caught bribing foreign officials to licence it from us. Embarrassing.

There's been talk of phasing out the $100 note here but it hasn't gained steam.

If I was going to get one, and for some reason the bank wasn't an option, I'd go to ALDI and see if the cashier there had one, because that's where the older people that take all their money out at a bank counter go I would think.

Before the massive economic crisis in Greece almost six years ago, it was common to see people (especially older people) in Greece at the supermarket using 100s, and even 200s. I think this also fueled a lot of the whole black market/off the tax books commerce in Greece. Once the ATMs ran out of paper money, and during the six months where I had to go to an ATM to get 60 euros/day (if they had 20s) or 50 euros/day (just 50s), this stopped as everyone suddenly got debit cards...and now everything is mostly debit cards (and actual taxes being paid), except places that are having financial trouble who claim "our wifi isn't working" to try to get you to pay in cash, so the government won't see it. On islands (pre-covid) we were happy to pay cash only a lot, cause those folks basically only get a real income for 3 or 4 months.

When I came to Greece back in 2005, I had maybe $20 at most in my wallet ever. Really freaked me out when I had to keep 500 euros in my pocket to pay contractors or whoever was cash only, before all this changed.

Robear wrote:

https://www.youtube.com/watch?v=E3i_...

Maverickz, is this guy accurate? Certainly the smoke is highly illustrative.

This is a really good video! I'm obviously not the final word on the subject, but I think this video illustrates the concept really well.

Coldstream wrote:

For my part, I want to make it clear that I'm fully on board with the whole Bernoulli thing as a significant portion of lift generated by airflow over a wing. As maverickz says, the stuff described by Bernoulli is fact as best we can measure it. Where my training diverged from maverickz training (are you an engineer? I'm unclear) is that there are significant other contributing factors, primarily deflection of air downward from the airfoil's AoA to the relative wind. I applied what I had been taught to how I thought a propeller worked, which seems intuitive to me but may well be wrong.

It's important to separate in our minds what is actually happening and what concepts and theories we use to explain and quantify the effects. What is actually happening is a pressure difference between two sides of the airfoil. How we explain the cause of this pressure difference, the why, is where the differences begin to show. That video Jonman posted talks about this pretty well. One way to explain the difference is using the acceleration of flow over a curved surface. Another way is by using Newton's third law. Both arrive at an explanation of the same physical phenomenon.

The thing with Bernoulli's Principle is that it applies in both cases. If you were to measure the speed of the flow (the dynamic pressure or 1/2*density*speed^2) and the static pressure on both sides of the airfoil, they would absolutely follow what we predict with Bernoulli's Principle. Which, by the way, is a very common experiment done both in universities and in other research. During my education, they focused on the "accelerated flow" explanation with Bernoulli's Principle because it is a good introduction to other more complicated subjects in aerodynamics.

Oh, and yes, I am engineer.

maverickz wrote:

During my education, they focused on the "accelerated flow" explanation with Bernoulli's Principle because it is a good introduction to other more complicated subjects in aerodynamics.

Sure. Like I said, I'm not debating whether Bernoulli is a thing. Where I'm struggling in my mental model is how Bernoulli would explain the blast of air I experience behind a propeller while stationary. The lift vector of the propeller blades is pointed away from me, and I get blasted by wind that's moving backwards from the blade.

Thinking it through, I know that there's relative negative pressure over the front of the propeller disc (i.e. the upper surfaces of the prop blades) and relatively higher pressure behind it. My initial thought is that air should want to move forward through the disc from high pressure to low pressure, but clearly the opposite is happening. This is where it doesn't make sense to me. I guess I could make the supposition that the higher pressure air from behind the disc is driving surrounding air backwards, but that doesn't seem intuitive. Alternatively, the lower pressure air might be pulling in surrounding air from the front of the disc with sufficient force to drive it backwards.

So, my understanding of Bernoulli is insufficient to explain prop-wash, but the combination of Bernoulli and Newton's third law (air deflection) seems to explain both the lift and the prop-wash. Ultimately, as long as I understand AoA, stall characteristics, adverse yaw and so forth, that's sufficient to fly my machine, but it's intellectually interesting to know a little more.

Think of the propeller as a disk with low pressure in front and high behind. The air does indeed try to move from high to low pressure - from back to front. But the propeller disk is in the way! So *it* is pushed forward (and in the process, that high-pressure air is reflected backwards). But that's relative to the movement of the plane, which naturally occurs through a fluid - the air - and so the plane is rushing forward through the air, and thus experiencing air flowing front to back on the other parts of the plane too. It's stronger behind the propeller because that's where the highest pressure differential exists.

Picture it without the propeller. If you just had an area of low pressure in front of the plane, the air would rush from the back to the front, right? So you use the propeller to harness that pushing air and pull the plane along.

But hah!, you say, what about a plane on the ground? No forward motion because it's chocked, but the propellers still produce propwash. Well, what happens when you push a lot of air at an object? A lot of it rebounds because it's *pushing* the object. Remember, it's not that the high pressure air is actually making it *back to front* through the propeller. No, it's shoved backwards because the propeller disk is blocking it. And that, again, produces the force.

Bernoulli's effect produces and maintains the low pressure in front and high in back of the propeller disk. Air movement provides the motive force and the propeller disk is what is pushed on during the air movement from high to low, and that air is also reflected backwards (for the most part).

With a fan, the same thing happens, but the fan is designed so it *won't* generate enough force to move. Although I've had some large fans that were thin enough to fall over when turned to "high" lol.

And now after much reading and checking my brain is bent. I hope I got this right.

What's funny, and embarrassing, about my initial post that disagreed with the "pushing air" thing is I was modeling a thruster as an "actuator disk" just a week ago. (an actuator disk is a stand in model of a propeller that accelerates flow from one speed to another as the flow goes through it and captures the effects without having to model all the specifics of a propeller)

So to set the record straight, or--in the immortal words of Douglas Adams--firmly crooked, I decided to consult the FAA's Pilot's Handbook of Aeronautical Knowledge. You guys can find the section on Aerodynamics here if you're so inclined.

FAA wrote:

To understand the action of a propeller, consider first its motion, which is both rotational and forward. As shown by the vectors of propeller forces in Figure 5-44, each section of a propeller blade moves downward and forward. The angle at which this air (relative wind) strikes the propeller blade is its AOA. The air deflection produced by this angle causes the dynamic pressure at the engine side of the propeller blade to be greater than atmospheric pressure, thus creating thrust.

The shape of the blade also creates thrust because it is cambered like the airfoil shape of a wing. As the air flows past the propeller, the pressure on one side is less than that on the other. As in a wing, a reaction force is produced in the direction of the lesser pressure. The airflow over the wing has less pressure, and the force (lift) is upward. In the case of the propeller, which is mounted in a vertical instead of a horizontal plane, the area of decreased pressure is in front of the propeller, and the force (thrust) is in a forward direction. Aerodynamically, thrust is the result of the propeller shape and the AOA of the blade.

So basically, we're all right. Air is indeed deflected and creates a higher pressure region on the engine side of the disc, while a low-pressure region is created on the front of the disc thanks to our friend Bernoulli. Both contribute to overall thrust, and reassure me that I'm not hallucinating the strong winds behind a stationary propeller or helicopter rotor.

Since the FAA is the one grading me on this stuff for tests, I'm going to accept this as my final answer.

Listen, let's be honest. The only reason airplanes fly is because everyone on board firmly believes that they ought to. The moment everyone on board stops believing, the darn thing falls right down.

maverickz wrote:

Listen, let's be honest. The only reason airplanes fly is because everyone on board firmly believes that they ought to. The moment everyone on board stops believing, the darn thing falls right down.

So you're saying the journey ends when everyone stops believing?