Fundamental basics of lift

[Nao]

Sorry guys, im kind of lost here, does the

And 100% of the lift corresponds to deflection. Newton's 3rd, etc. What you can say is that majority of the deflected air is not swept by the wing directly. Again, see the illustrations of flow patterns around the wing to see how much air is actually being deflected in the flow.

means we need to deflect the (100%) of the plane mass in air mass downward to stay afloat? If so wouldn't that mean that a plane without propulsion would deaccelerate at (at least) 9.81m/s in level flight? That is assuming the kinetic energy of air deflected downward is coming from the plane, regardless of how it actually happens (drag, pressure, deflection, etc) the outcome sounds just wrong. High performance gliders can have level flight deacceleration as low as 0.15m/s, and that includes drag losses too.

[Bill Phil]

Sorry guys, im kind of lost here, does the

means we need to deflect the (100%) of the plane mass in air mass downward to stay afloat? If so wouldn't that mean that a plane without propulsion would deaccelerate at (at least) 9.81m/s in level flight? That is assuming the kinetic energy of air deflected downward is coming from the plane, regardless of how it actually happens (drag, pressure, deflection, etc) the outcome sounds just wrong. High performance gliders can have level flight deacceleration as low as 0.15m/s, and that includes drag losses too.

No, the force produced by moving air down must equal the force of gravity on the plane.

[Nao]

No, the force produced by moving air down must equal the force of gravity on the plane.

And that's whats doesn't add up for me, if the requirement is >moving< the mass (that is changing it's velocity) it means it works like a rocket (actually a jet engine as the reactionary mass (air) comes from outside)... and we know how efficient that is to hover.

Not trying to reinvent the wheel here as we all know wings aren't rockets, but the idea of lift coming 100% from moving the air mass downward to suspend plane on constant altitude kind of is saying just that. Wings are jetpacks!

I agree,that Bernoulli effect is not good explanation of lifting force, but i think most of it actually comes from pressures and not air movement (kinetic energy).

In the end both air pressure and its downward movement are means to transfer the energy and it can be said that the force supporting the plane in air has it's reaction on the ground itself through interaction with air. So i wonder if we should not consider what happens in a whole column of air the plane is suspended in with the foot print of wing area, and not just frontal area times chord, or a cylindrical shape around the wing.

Edited February 8, 2015 by Nao
fixed a typo, expect more missed :P

[K^2]

And that's whats doesn't add up for me, if the requirement is >moving< the mass (that is changing it's velocity) it means it works like a rocket (actually a jet engine as the reactionary mass (air) comes from outside)... and we know how efficient that is to hover.

Because rocket moves very small amount of mass at high speeds. An airfoil moves a very large mass of air at a very small speed, which is proportionally more efficient.

[cpast]

And that's whats doesn't add up for me, if the requirement is >moving< the mass (that is changing it's velocity) it means it works like a rocket (actually a jet engine as the reactionary mass (air) comes from outside)... and we know how efficient that is to hover.

In any motion, momentum is conserved. This isn't unique to rockets -- if you walk down the street, you're pushing the entire planet backwards by a tiny, tiny amount to push yourself forwards. The reason it's inefficient to hover with a rocket is that you have to carry that reaction mass on board; in order to hold against gravity, you either have to accelerate it to a really high speed, or you have to go through the fuel very quickly (and also expel it to fairly high speed). For jets, it's also normally inefficient to hover; that's because a jet generally can't pull that much air through, which means you again have to accelerate it to really high speed. With wings (and with helicopters), you have a lot of mass you're pushing down; remember, while momentum is linear with velocity, energy goes with the square of velocity. If you have to provide a fixed amount of momentum to your reaction mass, then if you double the mass you halve the energy required.

[Nao]

Because rocket moves very small amount of mass at high speeds. An airfoil moves a very large mass of air at a very small speed, which is proportionally more efficient.

But here is the thing, if we consider a conservation of momentum would the wing not loose as much energy as the air has gained? Not even counting any resistances, just reactions between wing and air, it would mean that to keep any plane afloat the wings would be slowed by deflected air by at least 9,81 m/s. Which does not happen for any plane, maybe except for space shuttle.

To put it in different units, Concordia glider, 533kg of plane mass, needs ~2kW of power to fly level, how can you imagine air mass moved by 5.228kN of force by less than 2kW of energy (not even counting drag). It just doesn't add up if we just consider lift comes from air that is moving downwards after plane has passed.

Not saying there is no air movement, but just that reasoning the lift force with air movement does not make much sense. Because the air that went downward will partially return upward as the difference pressures caused by plane start equalizing again. The pressure would be the cause of the momentary downward movement of air (although idk if it would even manage to add up to 100% of the plane weight in a momentary flow), while end result would have only a small amount of downward moving air.

edit: (im slow) @cpast i mentioned in my previous post that jet engines (at least the turbofans) use AIR as reaction mass... im calling it a "rocket" because this is KSP forum

edit2: (ill actually take time and respond in a moment to your post properly @cpast)

Edited February 8, 2015 by Nao

[Bryce Ring]

The 100% figure (to me) was referring to all air affected directly (from bottom of wing) and indirectly from air above the wing.

the air above the wing was caused to move down due to the pressure gradient which was caused by the wings interaction, but the wing did not need to interact in a direct Newtonian sense with that air above the wing.

I have a feeling (I know it is for sure with helicopters) that the kinetic energy of the air that is moving down is more than the weight of the aircraft. I tested a helicopter hovering over a scale and the force on the scales was more than the weight of the helicopter at rest, For the helicopter I think this was due to induced flow.

[K^2]

But here is the thing, if we consider a conservation of momentum would the wing not loose as much energy as the air has gained? Not even counting any resistances, just reactions between wing and air, it would mean that to keep any plane afloat the wings would be slowed by deflected air by at least 9,81 m/s. Which does not happen for any plane, maybe except for space shuttle.

You are confusing forces, acceleration, momentum, and energy all into one single mess.

Incoming air gets deflected, which means momentum is being transferred from wings to air. That means there is force acting on the wings, which is lift. It also slows the air down. And that's drag. There is no lift without drag. But drag is not, usually, equal to lift. For a good airplane, drag is about 10 times less. For a high quality sailplane, it can be over 70 times less.

The reason it all works out in terms of energy is because air is already moving at high speeds relative to the airplane, and energy is proportional to the square of the velocity, while momentum is linear with velocity.

[Nao]

@K^2, @cpast Yeah that was a pretty smelly brainfart, thought of energy conservation like it was momentum conservation... not the first and probably not the last time that happened. Sigh me and the stupid basics blurring out after some time. Idk how i actually manage to get more advanced math/physics right sometimes.

At least it was refreshing to plow through old equations to find where was the error~~. Thanks for the help! And please carry on

[Mattasmack]

This might be a cliche, but I still haven't found a better text on fluid dynamics that deals with general cases, rather than simplify it all down to incompressible/inviscid.

Landau Lifshitz: Fluid Mechanics - Not familiar with this particular translation/edition, though.

Thanks! I'll look for a local copy (of that or any edition).

Hi Matt.

big side note here. Just wondering, been wondering this for a long long time actually.

would it be better to have a CPU GPU fan blow air over a heat sink radiator, or to have the fan turned around so that it allows air to accelerate over the heat sink radiator on its way to the fan where the fan then blows the air away.

PS no doubt a new heat sink radiator design which takes advantage of this would need to be used.

Your thoughts ?

Curiously yours

Bryce.

If you have a tight-fitting duct arranged so that you can be sure all the air going to/from the fan must pass through the heat sink, you shouldn't see any dramatic difference in heat transfer from the heat sink between sucking the air through or blowing it through. But there are other considerations:

The air that my ~4 year old laptop exhausts can get pretty hot. If the fan were arranged so that it was pulling the air through the heat sink, the fan itself would be in that hot airstream. The fan bearings and motor would then operate in a higher-temperature environment and might fail sooner. The fan would also be dealing with slightly lower-density, higher-kinematic-viscosity air, so it would probably move a slightly lower mass of air through the heatsink.

Also in this laptop scenario, the duct itself would become hot and would heat up all the laptop components around it. Blowing the air through the heatsink makes it possible to put the heatsink right at the edge of the laptop so that the hot air is immediately exhausted to the surroundings and doesn't heat anything else up.

If you hold a running computer fan in the open air, on the intake side it pulls in air from all directions. That means the air velocity on the intake side stays pretty low right up until you get to the intake of the fan itself. If you place an obstruction such as a heat sink near the intake, the air would also come to the fan by the path of least resistance -- the air would preferentially flow around the heatsink through the largest gaps it could find and a significant amount of air would go through the passages of the heatsink itself only if all other larger gaps were blocked. On the other hand, on the exhaust side of the fan the air exits with the momentum imparted by the fan, which will carry it some distance as a jet. The effect isn't so strong for computer fans, I think, because the air exits with significant radial and/or angular velocity so it forms a weak cone-shaped jet. But if you put a heat sink in front of the exhaust side of the fan, you can count on that momentum of the flow to at least carry some air into the heatsink and do some good, even if there are gaps around it.

Anyway, those are my thoughts!

[cantab]

I don't know of any cooler that has the fins solely on the intake side, but there are certainly models that put the fan in the middle with fins both sides, and models that have two or more fans in a push-pull arrangement.

[Bryce Ring]

thanks matt

I stuck a coke bottle over an older CPU cooler I had (stock cooler for intel CPU) on my desktop PC, this made sure the air had to come in from the part of the heatsink radiator furthest from the fan and then pass through all of the fan. same could be done with tape.

can not see this being done to easily to a laptop.

the air was then drawn off with a vacume cleaner, got good results, but dam was it noisy

I did not manage at the time to run the stock cooler fan with this set up, changes thought to just pointing my air con vent towards my PC. then I just thought, its about time for a new PC. then new PC is crammed with fans and the CPU cooler has a push pull (Blow on blow away setup) , I have had no problem with the fan working in the Blow away set up, It is still working well despite being in the hot side of the flow (Knock on wood).

[Iskierka]

My CPU cooler is a dedicated watercooler block with a single 120mm radiator. The recommended way to configure it is to fit the system to the normal exhaust case slot, in reverse so it pulls cold air over the radiator, and to have the fan placed in before the radiator, though this is more for simplicity as then only four screws are required, rather than eight. Looking at various configuration comparisons for the cooler online, the intake setup is unsurprisingly more effective, but it really makes no useful difference whether the fan goes in first; in fact even push-pull fan combos don't help much. So long as the radiator block has sufficient sealing round the edges so flow doesn't leak, the difference in cooling is negligible. Even with a traditional block cooler, with open sides that allows flow around, it's quite likely that it'll be a maximum of a couple degrees difference, for a huge block with lots of open leaks.

Laptops actually do a similar thing to your impromptu duct, though they use a pusher fan - the parts are all far too distant to be cooled by complex ducting around the case, so any high-power laptop has heatpipes from the hotspots into a single radiator for the entire laptop. (Or two, for the rare fancy dual-fan laptop) The fan pushes air through this one radiator to cool the entire thing - or at least, this is how my laptop and every other I've observed handles cooling.

[Bryce Ring]

I think the greater effect will be the Venturi re design of the heat sink. would swirling help or hinder as the air passed through a constriction, IE Not sure if having many circular holes or long wide constrictions would be better.

Sorry I didn't mention the venturi before, I was forgetting if it was Bernoulli or venturi that noted the temperature drop when air is allowed to accelerate.

why are so many of you all on laptops ? doesn't every one have bulky Desktops ? It was my original thought to mod a desktop, that's where most of the enthusiast PC modding is done. should have said that desktop in the first place, sorry about that.

Wow. This is too much discussion a little of topic, I fear I am Hijacking the thread. Sorry so much Doogey.

Edited February 8, 2015 by Bryce Ring

[Iskierka]

You assume I don't have both. You think a liquid cooler fits in a laptop?

Venturi effect may be a factor, but it's going to be fairly similar either side. You'd also then have to consider whether the pressure change from the fan induces another temperature change that is too unfavourable - a pulling fan might have lower velocity and thus less venturi effect, but it also reduces the air pressure and thus is pulling slightly cooler air through. Conversely a pushing fan will cause a temperature rise. I'm not sure how strong venturi effect is, so it might dwarf the ~1mm H2O pressure that typical PC fans can generate, but there are multiple effects to consider. It may even depend on whether you bought pressure-optimised fans or volume-optimised - though the latter is smarter for the CPU cooler.

Either way, both of these effects are fairly negligible in the overall system of a PC. You'll get considerably better gains just by finding a corner of the room that's cooler and doing a dust clean out. And yes, this is somewhat off-topic now, unless someone wants to merge them and think of an interesting question about jet engine high pressure turbine cooling.

[peadar1987]

I'm another contributor who doesn't know much about supersonic flow, but in a subsonic regime, it's not just the air in the immediate vicinity of the aerofoil that is deflected.

In the region of the aerofoil, the flow will follow the contour of the foil unless it is stalled ("nature abhors a vacuum" and all that, the flow really hates to separate if it can avoid it, it takes a lot of energy to do so). However, this has a knock-on effect on the flow further and further away from the foil.

Diagram here:

Some very rough layman's maths*:

An airbus A320 has a mass of about 40,000kg, and a wingspan of 34m. The density of air is about 1kg/m^3. If we assume the wing has a significant effect on the airflow up to 5m vertically away from the surface of the wing, that means that at its stall speed of about 50 m/s, you have 50*34*10 kg of air entering our "effect box" around the wing. That's 17000kg. F=m*a, so you need to give that air an average acceleration of about 23.5 m/s to give a force of 400,000N and cancel out the weight of the plane. At 50 m/s, that would deflect the airflow by an average of 28 degrees, which is a lot, but not completely physically unreasonable, especially when you consider that the stall speed is pretty much the point of flow separation. At higher speeds, the airflow will be deflected a heck of a lot less.

The fact that this downward velocity is quickly dissipated by viscous effects after the foil passes by doesn't really matter to the foil at all. In the grand scheme of things, the force will eventually find its way to the earth in the form of a very tiny increase in pressure across a huge area, but this doesn't affect our aeroplane or helicopter.

*Apologies if I make anybody cry with the sloppiness of the maths here. It is a pile of assumptions and shoddy approximations for demonstration of the concept only.

[K^2]

747 on approach with flaps fully extended has a glide ratio of about 5. So the deflection angle is going to be much smaller, which suggests that it grabs a lot more air. Which is, most certainly, true.

[peadar1987]

You assume I don't have both. You think a liquid cooler fits in a laptop?

Venturi effect may be a factor, but it's going to be fairly similar either side. You'd also then have to consider whether the pressure change from the fan induces another temperature change that is too unfavourable - a pulling fan might have lower velocity and thus less venturi effect, but it also reduces the air pressure and thus is pulling slightly cooler air through. Conversely a pushing fan will cause a temperature rise. I'm not sure how strong venturi effect is, so it might dwarf the ~1mm H2O pressure that typical PC fans can generate, but there are multiple effects to consider. It may even depend on whether you bought pressure-optimised fans or volume-optimised - though the latter is smarter for the CPU cooler.

Either way, both of these effects are fairly negligible in the overall system of a PC. You'll get considerably better gains just by finding a corner of the room that's cooler and doing a dust clean out. And yes, this is somewhat off-topic now, unless someone wants to merge them and think of an interesting question about jet engine high pressure turbine cooling.

A simple laptop fan isn't going to cause enough of a change in pressure for compressibility effects on temperature to have much impact on the cooling, and heat transfer coefficient changes roughly linearly with Reynolds number, so I doubt that adding a Venturi into the system will increase the heat transfer any, you'd be better off having a straight-walled passage, and using a larger heat exchanger area. Of course, if space or heat exchanger cost is your major design consideration, then a Venturi would be a great way of increasing heat transfer without compromising on these.

[Bryce Ring]

have either of you discussed this on RCGroups. sounds familiar.

http://www.rcgroups.com/forums/showthread.php?t=1539175

http://www.rcgroups.com/forums/showthread.php?t=1610771

http://www.rcgroups.com/forums/showthread.php?t=1479190

some nasty buggers in that group.

Not like here. I like KSP's attitude towards grumpy attitudes etc. though I dislike how it can close a thread.

Here is a nice little short one.

http://www.rcgroups.com/forums/showthread.php?t=1227290

There is a challenge on that first link. Page 92 post #1378

PS Hi Brandano I am guessing your in here somewhere

[Bill Phil]

I forgot this thread...

Ok..

I was rushing earlier. I used Newton's generalization F=ma in conjunction with a few assumptions about how much air would be affected. Must've been a good assumption!

Anyhow, I was rushing( I needed to be somewhere) and I forgot that air is a mass flow in this case. So it is 151 m/s.