Why is the curvature of pump rotor blades "backwards"? - Page 1
Forum section: electronics / general

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 30, 2021, 05:01:03 am
content: I'm designing a rotor for a small centrifugal oil pump that is to be 3D printed, to circulate mineral oil for cooling electronics. (Before anyone says to just buy one, I haven't found one of the right specs that's anywhere near a reasonable price. It and the rest of the project is also a learning experience.)When it comes to designing the rotor blades, I noticed that most centrifugal pump rotors seem to have the blades curving "opposite" to what I would expect, in that the convex side is driven into the liquid. I would have expected the concave side to be "pushing" so that it would act as a wedge with a changing angle, initially a shallow angle giving more force to get it moving and then steeper angles to continue accelerating the fluid that is already set in motion by the shallow part. Indeed, most fans are built that way. Or looking at it a different way, most airplane wings are concave on the bottom to accelerate the air downwards.Thinking it might be some nonintuitive effect of incompressible liquids as opposed to compressible gases, taking a look at boat propellers seems to disprove that since the concave side is pushing, same as for fan blades.I'm sure there's a good reason why centrifugal pumps use the "backwards" curve, what is it?

author: bdunham7
author_rank: Super Contributor
timestamp: September 30, 2021, 05:32:39 am
content: Does the oil accelerate as it reaches the outside of the pump or does it slow down?

author: Berni
author_rank: Super Contributor
timestamp: September 30, 2021, 05:57:05 am
content: If you look wide enough you can find both directions, some bend forward some bend backward, or some are just straight. It's an optimization for the job the centrifugal blades are designed for.The logic behind the backward bent blades is that the pumped medium has to slow back down from the rotation inside the rotor. This is most easily facilitated by the blades bending backwards since it provides a nice gradual transition from the high speed center to the slowly moving outside collection chamber. Eventually the medium has to go into the output flange and that is moving at the output flow rate, no faster, no slower. So such a rotor doesn't waste mechanical energy spinning the medium where it doesn't want to spin anyway.So then why are not all of them backwards if its so nice and efficient? Well it turns out that if what you want is high flow rate at low pressure (like a lot of air moving fans) the forward bent blades are better. They act as scoops to grab as much air as possible, get it up to speed quickly as possible and fling it out like a shovel. This lets the same size rotor create more flow rate.As for straight blades, it might be that they are designed to spin in both directions. Can also help if the medium has a lot of solid particles since they have a harder time hitting straight blades. Perhaps they actually needed some compromise in between spot between forward or backward bent designs. Or they didn't bother to optimize it, the flat design was easiest to manufacture and worked well enough for the job.

author: beanflying
author_rank: Super Contributor
timestamp: September 30, 2021, 06:44:56 am
content: What type of Oil are you planning to pump? Apart from maybe mineral types of weights seriously look at a vane type of impeller over a centrifugal one in particular as it is likely small. You will likely find the vanes print better than the complex curves.Commercially look at the small weed or ag type spray diaphragm pumps or the Jabsco and Johnson ranges of flexible impeller pumps.Or if you have a known flow, pressure and oil type (or viscosity will do) throw it here and I will see what suits it what I did when I had a real job.

author: armandine2
author_rank: Frequent Contributor
timestamp: September 30, 2021, 06:37:27 pm
content: if taken to the extreme the opposite curvature would not release the liquid - the centrifugal pump isn't a positive displacement device, it accelerates the liquid. Look at the relative linear speeds of points along the curvature.

author: MarkF
author_rank: Super Contributor
timestamp: September 30, 2021, 08:00:44 pm
content: Cavitation

author: IanB
author_rank: Super Contributor
timestamp: September 30, 2021, 08:19:55 pm
content: Here is a technical explanation with some relevant equations:https://youtu.be/cj1edFvGqRk

author: thm_w
author_rank: Super Contributor
timestamp: September 30, 2021, 08:52:34 pm
content: An existing watercooling pump could work for the task, assuming the seals are compatible or changed out to something compatible.https://forum.level1techs.com/t/mineral-oil-in-a-water-cooling-loop/70339/14

author: Whales
author_rank: Super Contributor
timestamp: September 30, 2021, 10:07:23 pm
content: In the world of air impellers my vague understanding is:- backwards curved impeller blades are quieter and more energy efficient- forwards curved impeller blades are more space efficient (ie more pumping power out of a given size impeller and fixed RPM)- straight impeller blades are a compromise between the twoThis may all be completely different for less-compressible or incompressible fluids, I don't know. Cavitation is not a problem for air pumping AFAIK (I wonder if you can pump so hard you cavitate into plasma? Or separate the gases into low-high density?)

author: bdunham7
author_rank: Super Contributor
timestamp: September 30, 2021, 10:30:15 pm
content: Quote from: Whales on September 30, 2021, 10:07:23 pmDespite the obvious difference between air and liquids, the same ideas do apply because even though liquids are mostly incompressible, with air it is still much more efficient to avoid excessive compression. "Cavitation" in air would make a lot of noise, but you really have to work at getting your pump rotating fast enough (transonic speeds).

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: September 30, 2021, 11:49:54 pm
content: The comparison with a fan is not very good as a fan is axial flow whereas a centrifugal pump is radial flow. Also, if you look at the impeller at flow entry whether fan or cent pump the fluid 'sees' forward swept vanes. At the exit the fluid 'sees' reverse swept vanes. I have seen only one example where the reverse was true. I had a dishwasher whose pump had extreme swept impeller blades that upon reversing would actually pump water from the pump casing back through the impeller eye to drain the basin. Forward rotation was trailing blades at impeller exit and pumped water through the wash bars.Generally, straight blade are for high flow and swept blades are for high pressure.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 01, 2021, 12:16:09 am
content: There may be additional factors besides the 'pump theory' ones mentioned before. If the pump is designed with little or no clearance to the blade tips, friction between the blade tip and wall will tend to drag a forward facing blade into the wall, with the reverse for a backward blade. In one case increasing pump power requirements and reducing leakage, while the other case minimizes power at the potential cost of leakage. Chatter could result from the forward configuration. Several other possibilities.

author: beanflying
author_rank: Super Contributor
timestamp: October 01, 2021, 12:38:04 am
content: Some of what has been posted above isn't really correct.Centrifugal Pumps come in a range of types from Axial, Radial and Mixed Flow impellers depending on applicationAlso they can be run in reverse (tips forward for a crude description) but they run very inefficiently with lower flow and reduced pressure (Typically in the 25-30% range).

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: October 01, 2021, 12:47:42 am
content: Centrifugal pumps are by definition radial flow. Fluid may flow through the casing axially, but within the impeller it is only radial.

author: beanflying
author_rank: Super Contributor
timestamp: October 01, 2021, 12:51:41 am
content: Tell that to the industryAxial Flow impellers do not have the fluid flow radially in the casing at all (middle of the picture above). Typical applications are high flow low head applications like flood irrigation or ground dewatering and in some cases longer shaft drives are used. The impeller is a propeller and just because it spins radially doesn't mean the flow past the impeller is.Some more reading herehttps://www.batescrew.com/axial

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: October 01, 2021, 01:01:32 am
content: That is not a centrifugal pump as they say in the first sentence. An axial flow impeller is a screw or propeller impeller as they also describe. Centrifugal force is orthogonal to the axis of rotation and is, again, radial.

author: beanflying
author_rank: Super Contributor
timestamp: October 01, 2021, 01:12:02 am
content: In the industry they fall under one on the THREE types of typical Centrifugal Pump impellers in use. If you want the industry to change because you demand it then good luck with thatThere is also in addition to those the types a bunch of application specific ones I have left out as they are for process, chemical or waste/sewage applications.The bottom line for the OP still is a Centrifugal Pump for oil is not really the best solution before we get to far away from that.

author: amyk
author_rank: Super Contributor
timestamp: October 01, 2021, 01:48:17 am
content: Quote from: beanflying on October 01, 2021, 01:12:02 amThis. There's a reason nearly all oil pumps (except possibly the huge ones in industrial plants) are vane, gear, or gerotor types. If you're 3d-printing one, a gear pump would probably be easiest.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 01, 2021, 01:56:21 am
content: Quote from: beanflying on October 01, 2021, 01:12:02 amMy application needs a good amount of flow but not much pressure, quite a different case from most oil pumps with relatively low flow but high pressure. The oil is farm grade mineral oil (often used to cool amateur radio dummy loads), not specified for viscosity but much thinner than car engine oil.

author: beanflying
author_rank: Super Contributor
timestamp: October 01, 2021, 02:21:42 am
content: Quote from: amyk on October 01, 2021, 01:48:17 amThe reason not to go for a gear pump here is tolerance and wear on the sides and faces of the gear with 3D printed bits. And blip or bump on a surface is a grab point. Also extra complexity with needing a second shaft. Some of this can be overcome with installing very thin shims of maybe Teflon or Brass on the sidewalls for the 3D printed Gears to run against but you still have gear to gear mesh wear.Vane as I offered initially makes a lot of sense and will be fine with mineral types of oil. Use the same thoughts as the gear with a thin shim on both sides of the case. It is also 'normal' on smaller vane pumps to leave the impeller floating on the shaft to align with the cases so depending on the motor a keyway and a slightly loose fit will work.For seals Nitrile lip seals should be ok but maybe Viton if you can source them for not a lot more is a good idea. You can also go a full mechanical seal but it adds to the size and complexity.More width = more flow and diameter is proportional to pressure both will increase with speed is a 'sort of' linear fashion minus some slip.For info

author: IanB
author_rank: Super Contributor
timestamp: October 01, 2021, 05:14:11 am
content: Quote from: NiHaoMike on October 01, 2021, 01:56:21 amFor a general answer to your question, spinning blade (impeller) pumps fall under two broad categories: axial flow pumps and centrifugal pumps. Axial flow pumps are similar to a fan: they move liquid through from one side to the other like a propeller. Centrifugal pumps work a different way: they take liquid in at the center, spin it round with the blades inside a closed casing and push it out at the edge using centrifugal force (hence the name).Axial flow pumps generally favor higher flows with lower pressure rise, whereas centrifugal pumps favor lower flows at higher pressures. So for your application, you might be better with an axial flow design, though this is not guaranteed. For example, garden fountain pumps are usually centrifugal, but they circulate water with a reasonable flow and not all that much pressure. You will need some pressure to move the oil through the various heat exchangers in your cooling circuit (this may not be trivial). Selecting a pump requires knowledge of how much flow and how much pressure rise (head) is needed.

author: Berni
author_rank: Super Contributor
timestamp: October 01, 2021, 05:16:33 am
content: Impellers and centrifugal pumps are great pumps for when you need lots of flow rate at low pressures. They don't mind there output being blocked and will just keep making a steady pressure. They are easy to make because no tight tolerances are needed, nothing actually rubs and the only seal is around the drive shaft. For 3D printing this is the best design since anything in the right shape will work, having really loose tolerances just means that you will have somewhat reduced pumping performance.The reason for positive displacement pumps like gear or vane pumps is that they can produce much higher pressures. Once there output is blocked off they will create as much pressure as you can put force into its drive shaft. For pumping lubricating oil this is usually what you want since you need to squeeze the thick oil trough tight passages into small gaps, at the same time the oil itself lubricates all tight tolerance rubbing surfaces in the pump. This makes most of these pumps not very 3D printer friendly.Pumping mineral oil in cooling should be fine for a centrifugal pump. The stuff is reasonably thin and you typically need mostly flowrate not pressure for cooling. Tho if you had the idea to push mineral oil trough standard PC cooling waterblocks then you might need a bit more pressure (the fins in those are really closely spaced), so for that case you might have to spin your pump at rather high RPM or go for a multistage design that puts 3 or 4 of these pumps in series to build up extra pressure.

author: beanflying
author_rank: Super Contributor
timestamp: October 02, 2021, 02:42:46 am
content: You should never dead head (no flow) any pump regardless of type.If you don't circulate so fluid into and out of the pump body you will see an increase in temperature of that fluid. It is fairly common for example for a water pump left running to turn the casing water into steam vapor and kill impellers and seals.Re Vane pumps without getting into petrochem grade tolerances are generally going to top out pressure wise at 30-60m maximum head (60-90 psi) and I doubt you will get the tolerances on a small 3DP impeller much over 10m without a lot of work post printing. Even with that said nearly all industrially plumbed options will either have an internal and or an external pressure relief valve. The little domestic and fire service jacking pumps generally don't.If you make a low tolerance centrifugal impeller then you will be back into the realms of the junk pond pumps and a few M maximum which may or may not be an issue. Still not a fan of unspecified mineral oil and a centrifugal regardless.

author: jmelson
author_rank: Super Contributor
timestamp: October 03, 2021, 04:45:09 pm
content: My understanding is "forward" facing vanes, meaning concave toward the direction of rotation, are used to impart more energy to the fluid, giving higher pressure differential at lower flow rates. "Backward" facing vanes are for higher flow rates but lower pressure.Jon

author: RJSV
author_rank: Super Contributor
timestamp: October 15, 2021, 12:49:20 am
content: I'm mainly curious (gets me in trouble, fast, often):?: I've thought, as to wing shape, it was speed, that essentially 'lowers' air pressure, although now I'm not convinced, that the reduction is (?) at rt. angles, to air flow direction?So, having a wing 'hump' upwards shape, THATs going to produce a volume with lower air pressure,this regular pressure (underneath) pushes UP, more than the upper surface is getting.Part of that, is an 'attitude'; that 'vacume' doesn't exist,(without real 'pressures' elsewhere).Anyway, no thoughts, about a 'downward directed' air stream, is that real ?

author: IanB
author_rank: Super Contributor
timestamp: October 15, 2021, 01:46:03 am
content: Quote from: RJHayward on October 15, 2021, 12:49:20 amYes, because you cannot defy the laws of physics.You can think about this by choosing a frame of reference. You could have a wing moving through the (still) air, or you could have a stationary wing (in a wind tunnel) and blow a stream of air over it. To understand wings, it is easier to use the wind tunnel example.If the wing in the wind tunnel experiences an upward force, then it of necessity must be "pushing down" against something--Newton's law of action and reaction.The only thing the wing can "push down" against is the air stream, but air will give way if you push on it. So what must happen is that the air receives a downward acceleration from the wing. To accelerate something downwards you have to push on it, which is exactly what the wing does.This principle becomes very clear if you stand beneath a helicopter (which is an airplane with rotating wings). You will get blasted by downdraft. This is exactly the same downdraft produced by a fixed wing aircraft moving through the air. Conversely, it would be impossible for a helicopter to lift off the ground if there were no downdraft.So what about pressure differences, you may ask? What about lower pressure above the wing and higher pressure below the wing? This is true too, but it is simply the other side of the exact same coin. Mathematically, the results are equal, but the formulas are different.

author: beanflying
author_rank: Super Contributor
timestamp: October 15, 2021, 02:13:19 am
content: You also cant really include discussions or a lot of the analogies on compressible fluids and gases in a topic about incompressible ones

author: IanB
author_rank: Super Contributor
timestamp: October 15, 2021, 05:17:45 am
content: Quote from: beanflying on October 15, 2021, 02:13:19 amEven so, the subsonic flow of air over wings, or through simple fans and blowers, behaves in almost the same way as incompressible fluids. As long as the overall change in pressure through the flow path is not that much (say +/- 10%), then the same rules can be applied with minimal error.

author: beanflying
author_rank: Super Contributor
timestamp: October 15, 2021, 05:40:30 am
content: err NO. Reynolds numbers and if we get into smaller impellers then low Reynolds number issues on top of that are not 'minimal'.

author: Berni
author_rank: Super Contributor
timestamp: October 15, 2021, 05:55:45 am
content: Centrifugal pumps don't really use the effect of a areofoil aerodynamic lift, the curve is just there to help shape the flow, not to actually create the pumping action. It's propellers that use this effect, those are basically a wing that is actively forced trough the air. Even then you don't actually need to use wing curvature to make a propeller work, they work just fine with flat straight blades that are turned in the appropriate angle. The wing aerofoil shape just is a more efficient mechanism because it produces less drag, so your propeller can move more air for the same mechanical power input.And yes air can be thought of as a really low viscosity fluid in a lot of cases. Most fans used to just move air around do not produce enough pressure to compress or stretch air in any meaningful amount. But once you move over to things like vacuum cleaners or turbochargers that are actually designed to produce a good deal of pressure then yes air is no longer just a simple incompressible fluid.

author: beanflying
author_rank: Super Contributor
timestamp: October 15, 2021, 06:24:14 am
content: Centrifugal Pump impellers won't 'pump' air worth a damm they are nothing like a fan (for a gas or air) and are in most cases also not a propeller (axial flow case mentioned earlier is an exception).In this case of the OP's topic AIR is not some sort of analogue for a non compressible fluid. There is a reason wind tunnels are not used to test Pump impellers.This is a reasonable description too.Quote

author: Berni
author_rank: Super Contributor
timestamp: October 15, 2021, 06:56:10 am
content: Most vacuum cleaners use it to pump air just fine, here is what one of those looks like:https://www.aliexpress.com/item/4000006766726.htmlThe thing is that air is much lighter than water, so the centrifugal force experienced by the air being spun around in there is a lot smaller, so the static pressure produced is also a lot lower. The workaround for this is to simply spin it a lot faster (especially since the force goes up with the square of the RPM) to increase this force to a point where it does start producing a significant amount of pressure. This is the main reason why vacuum cleaners are so darn loud, they use series would brushed motors to spin those blades at ridiculous speeds. It is also why a centrifugal water pump will basically stop working as soon as air gets in there, they spin fairly slow since that's what is efficient for pumping water, but is way too slow for air (not that the pump would survive running that fast)They do work in reverse too. If you force air into it it will start spinning and producing mechanical power. This is exactly what the exhaust side of a turbocharger does.

author: beanflying
author_rank: Super Contributor
timestamp: October 15, 2021, 07:09:56 am
content: Just because it looks sort of similar doesn't mean it behaves the same. Put that same impeller at the same speed in water and then air and they will behave NOTHING even close to each other for characteristics as the medium passing over it is NOTHING like the same.You seem to be fighting to make a case against basic Fluid Mechanics by adding additional requirements (speed) to try and prove air is the same as a non compressible fluid. This is simply not the case.

author: Berni
author_rank: Super Contributor
timestamp: October 15, 2021, 07:42:39 am
content: I am not trying to say that they are the same.What i am saying is that air does act as a low viscosity fluid when it is not exposed to any significant pressure changes that can compress it. So a lot of predomenently air moving designs that just push air along and not compress it will act much like moving a liquid. Obviously air and water do have different properties hence why propeller blades designed for one or the other have a different shape and speed due to being optimized for the particular application.The thing with centrifugal rotors is that they almost exclusively rely on the density of the medium to impart force on it, so that's the parameter that matters the most. Doubling the density would also rise the produced pressure by roughly double.However centrifugal rotors are usually used because they can produce more static pressure than a fan blade. This means in a lot of cases the air will get inevetably exposed to large pressure differences that cause it to compress and expand, making it NOT act like a fluid.Not that testing centrifugal rotors in a wind tunnel is very useful since the housing shape around the rotor is just as important as the rotor shape itself, so where the action happens is hidden. Since the blades are not operating as aerofoils there won't really be much interesting to see anyway. Most of the usefulness of an air tunnel test would be to optimize the shape of the collection chamber around the rotor to direct the flow to the output as smoothly as possible.

author: beanflying
author_rank: Super Contributor
timestamp: October 15, 2021, 08:49:25 am
content: You are starting with a narrative you require to be true then manipulating select variables to try and fit that narrative and it simply isn't the case as far as Fluid Mechanics works.You are also confusing 'density' in relation to Air as it is a compressible fluid and as such varies with pressure making it fundamentally different to Water which does not. All of this is before we get anywhere near laminar or turbulent flow differences between Air and Water due to flow speeds entering in and exiting an impeller.It is simply a bad comparison.

author: Berni
author_rank: Super Contributor
timestamp: October 15, 2021, 11:51:58 am
content: Yes that is my point that in the special case of having fairly constant pressure everywhere air can act very similar to a fluid. Otherwise do have to treat it as a compressible gas rather than just a fluid. If you move supersonic trough air then you once again need to treat it differently, a wing will act very differently past the speed of sound. Then with water if you move it vigorously enough it will cavitate and again no longer act like a regular fluid.Lots of models have special edge cases like this, but this does not make them completely wrong or useless, they just need to be used within the limits.As for density for that i mean the 1.2kg/m3 that the air weighs at sea level versus the 1000kg/m3 density of water. This has nothing to do with being compressible. Plain simple something heavier needs more centripetal force to go around at the same speed, that force over an area is pressure and this is what a centrifugal pump is producing. This is why a centrifugal fan can't pump water and a centrifugal pump can't pump air.

author: RJSV
author_rank: Super Contributor
timestamp: October 15, 2021, 06:17:37 pm
content: Uhh: Ianb, thanks, let's see if I can repeat your thoughts, that an upwards 'hump' is also going to cause air deflection away, which is up. SO, that looks like the case like you said, that BOTH surfaces actually send an air deflection, away from wing volume, so upside down has similar action.BUT, and here is the struggle, I still am perceiving a mention of the principal, if you throw an 'anvil', wearing dollar skates, you move, opposite direction, (due to conservation of momentum).Do that just means, the upper hump shape will get some DOWNWARD FORCE, a little bit. Why can't I say ' You would crash...downward force could exceed the upward force (supposedly caused by reaction to sending some air downward).Difficult for understanding.

author: beanflying
author_rank: Super Contributor
timestamp: October 15, 2021, 11:24:34 pm
content: Quote from: Berni on October 15, 2021, 11:51:58 amStop claiming 'special case' and just use Water or whatever fluid is required and don't muddy the WATER further and drag in supersonic flow. Air is a compressible GAS period full stop. It is NOTHING like an incompressible liquid!

author: IanB
author_rank: Super Contributor
timestamp: October 15, 2021, 11:37:36 pm
content: Quote from: RJHayward on October 15, 2021, 06:17:37 pmJust pick the important points out of what you said here. If you throw an anvil away from you while wearing skates, you move in the opposite direction, due to conservation of momentum.Now apply this same thought to a wing on an airplane. The wing "throws" the air downwards, which tries to move the wing upwards by conservation of momentum. Same principle. Now the actual air flow over the wing is complicated, it curves this way and that on its journey, but at the very end, it is moving downwards to some extent.In reality, aeronautical engineers use advanced computer design tools with CFD and such to design wings. Modern aircraft need to operate with very high efficiency and wing design is a major part of that. But still, the same basic principle of conservation of momentum is always present.

author: IanB
author_rank: Super Contributor
timestamp: October 15, 2021, 11:39:30 pm
content: Quote from: beanflying on October 15, 2021, 11:24:34 pmBerni has been correct in the posts above. You might try stopping to think about it instead of arguing so much?

author: beanflying
author_rank: Super Contributor
timestamp: October 16, 2021, 12:04:28 am
content: Berni and you if you like initially are fundamentally WRONG. Bending or worse moving the rules to suit a narrative is not Science and or Engineering.And if disagreeing on major FLAWED technical points or claiming 'special case' raised is arguing then you might want to try some basic reading here.https://www.mvorganizing.org/what-is-compressible-and-incompressible-flow/https://www.chegg.com/homework-help/definitions/incompressible-and-compressible-flow-5

author: IanB
author_rank: Super Contributor
timestamp: October 16, 2021, 02:01:50 am
content: Quote from: beanflying on October 15, 2021, 05:40:30 amHere, in fact, you answered your own question, and is why the answer is "yes".The Reynolds number is a dimensionless group, which allows us to apply the principle of similarity between systems that vary in scale and other parameters.If the Reynolds number, and other relevant dimensionless quantities are equal, then two apparently different systems will behave similarly, and the same performance characteristics will apply.Under the principle of similarity, air and water can be treated with the same equations when the variation in air pressure is small, such as is the case with a typical cooling fan.

author: beanflying
author_rank: Super Contributor
timestamp: October 16, 2021, 02:43:38 am
content: Reynolds numbers are dimensionless but when you throw AIR at around 800-1000 less density and viscosities at circa 100 times different at the calculations compared to water it becomes clear that water and air are nothing alike. Even if we take your earlier assertion of +-10% Pressure without doing the numbers that equates to about a 25% change in Density.That is UNLESS you make upconstraints and 'special case' rubbish such as you are continuing to do.

author: IanB
author_rank: Super Contributor
timestamp: October 16, 2021, 03:31:10 am
content: Quote from: beanflying on October 16, 2021, 02:43:38 amIf we do the numbers it is about a 7% change in density. It is always good to do the numbers.

author: RJSV
author_rank: Super Contributor
timestamp: October 17, 2021, 05:35:30 am
content: beanflying: I get your point, about compressible fluids / gases, maybe off OP topic, sorry (I'm at about 1/2 year basic physics expertise).Please see photo, regarding 'pushing' air streams downward, VS. simple lower air pressure, up around the faster moving air over that wing 'hump' shape.Now, easy to accept, that a downward deflected air stream causing a 'lift' force on the wing, but do course textbooks then talk about TWO lifting force types ?My experience, they always just talked about 'pressure' is lower, where the air is moving faster, there.Note the picture features wing totally flat, on bottom, no super curvey.

author: beanflying
author_rank: Super Contributor
timestamp: October 17, 2021, 06:29:21 am
content: Quote from: RJHayward on October 17, 2021, 05:35:30 amDiscussing wings and Aerodynamics is way off topic so start a topic maybe in the Mech Eng. section? Meanwhile add this to your reading it is sort of the R/C bible for low (non full sized or high speed) Reynolds number data and researchhttps://m-selig.ae.illinois.edu/uiuc_lsat/Airfoils-at-Low-Speeds.pdfor the source page herehttps://m-selig.ae.illinois.edu/Also this bootleg copy of Martin's book is well worth a read for basic concepts of Aerodynamics.https://archive.org/details/ModelAircraftAerodynamics/mode/2up

author: beanflying
author_rank: Super Contributor
timestamp: October 17, 2021, 07:43:46 am
content: Quote from: IanB on October 16, 2021, 03:31:10 amYou might want to check your maths unless you are working with air at 200+C and even then it is +-7% at around 20C it is +-12 ish or around 25%https://www.omnicalculator.com/physics/air-density

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2021, 09:18:39 am
content: Quote from: RJHayward on October 17, 2021, 05:35:30 amAs I explained above, it is two sides of thesamecoin. When you do engineering, there are sometimes different ways of calculating the answer to a problem. Just because a textbook shows one way, it doesn't mean other ways do not also exist.If you have doubts, keep thinking about a helicopter. Do you think it would ever be possible for a helicopter to lift off the ground without a downdraft? Have you ever been near to a helicopter that was landing or taking off? It produces an absolute hurricane, which is the only way such a heavy craft could get off the ground.QuoteDon't be misled by pictures. Anyone can draw a picture of anything. What counts is what is observed in real world experiments. Be in no doubt, if a wing is generating lift, then air is being deflected downwards.