How ducting a propeller increases efficiency and thrust

g'day and welcome back to RC model reviews today I'm talking about ducts propeller ducts I receive these a little while ago did a video on them came from Australia 3d printed propeller ducts for mini quads or anything really and a lot of people ask well how do these work how you're going to increase performance of a propeller by putting it in a duct and it's a very good question and it brings quite a number of elements of physics and aerodynamics into play to understand how these ducts can actually improve the performance of a propeller and a motor combination and they'll improve it in theory by giving you more thrust for the same amount of power or requiring less power to achieve the same amount of thrust the bottom line is your battery should last longer and you should get longer flight times out of your flying machine whatever it may be so what I'll do now is just look at some of the principles the basic principles and look at how they apply to these ducts now the first of these theories we need to look at is one proposed by mr. bernouli it says theorem and it's about assaulted with air pressure and this assume we've got a flat sheet of anything here flat plate I've drawn this line it's a very accurate representation down to the finest detail of a flat plate and a normal situation if this flip the plate had no weight for example we just sit there in midair I wouldn't go up or down we just sit there assuming the air was completely calm because the forces pushing on the bottom of this plate would be equal to the forces pushing on the top with air pressure the air pressure is balanced all around and so hey that's fairly straightforward but now what happens what would happen if we actually directed a flow of air across the top of this plate like this anybody got any ideas I know lots of people do lots of people already know this stuff for the benefit of those who don't I can tell you the plate would rise up and would rise up because banila Bernoulli's theorem says that we're here or a fluid is moving the pressure is lower the pressure is large where the fluid is moving so because we've got air moving across this way the pressure being exerted outwards is lower and that means the pressure underneath the normal air pressure on would push our plate up and it would rise and I can give you a practical demonstration of that actually and this is a you may even have seen this before but I gotta here's my flat plate as you can see any problem is this one isn't weight but so when I let go one end it oops okay now what I'm going to do is I'm going to blow across the top of this piece of paper and if what I've proposed here and what mr. bernouli says is correct and by having air flowing across here it'll produce a low-pressure area their pressure underneath will then push the paper up into that low-pressure area so the paper should rise let's see if it really happens takes a bit of skill does so hold on what do you see that exactly what was predicted happens where you have air traveling or any fluid and air is a fluid anything traveling the faster it travels the lower the pressure the lower the pressure at right angles to the flow so you go that's basically how wings work it's what's one of the theories what's one of the reason that wings work because we have aerodynamic air foils that cause air to flow more quickly over the top from the bottom so they get left because the faster the flow the lower the pressure so if you've got a flow that's really fast over the top and the low flow over the bottom then the pressure will still be higher underneath than on the top okay but nor Lee's theorem got that out of the way so now it's time to look at our propeller and a tube and a duct like this now this is what it looks like motor propeller the tube we've got a cross-section obviously here of what's going on and what I'm going to show you now is first of all talk about how propellers work because that's pretty important if you don't know how propeller works this isn't going to make any sense to you but I'll draw a diagram over here we look at the cross-section of a propeller let's do the hub like this you'll know that it's got some two blades aerofoil there's one on the other side as well now these blades are as an inclined angle because we draw another diagram without looking down on it because we get the plan and the elevation here say is that there's our propeller looking down on the top and the propeller of course will rotate and you can rotate neither direction depending on the way we want the thrust to come out in the way the propeller blades are organized so there we go propeller rotates and in doing so these blades move through the air like a wing moves through the year and they work on exactly the same principle so if we look at how our wing works I'm going to be a quick revision for those who don't know here this have a wing and there's an aerofoil section like this here we go and let's have some air I should use blue for the air when a propeller is when a airfoil is moving through the air some of the air comes along and it gets deflected down here and some of it goes up over the top and as ginn directed down so you have two flows which splits the flow and how two wings work I mean I've already done a video on this on my extra channel I'm going to tell you again quickly there are two reasons that wind creates left first reason is Newton's law Newton's law scissors the third law country may even or might be the first I can remember the order but anyway Newton's laws is that for every action there is an opposite and equal reaction you know so when you push someone know that you actually feel yourself being pushed back at the same time what happens here is got air coming along it gets deflected down so what happens if the reaction is the wing is deflected upwards simple and this is how flat plate air force work you know you make a paper plane it hasn't got a cube Durov or it's got a piece of paper we make a thin balsa wood glider it's tough let boss all those styrofoam guides you press out of a pledget they don't have air force you have a flat plate those wings work solely by causing the air to be deflected down that's producing a reactive force pushing or giving left to the wing but there's another factor that comes in here this is a bit debated Ashley some people said aw no there doesn't work but I think if you've just watched my demonstration with the Bernoulli effect you'll agree this has to play some effect on the whole thing and that is that the air traveling over the top here if we were to measure the length of these lines from say here to here you can see that this line is a lot shorter than that line so what happens is in order for the air to get right over the top and down here because of the curve has to travel more quickly and in traveling more quickly it mr. balloon Bernoulli says that if this is traveling faster than area what is the pressure going to look like on this propeller well I will use my red pen to draw what we call a bubble of low pressure now this area here which I'm cross-hatching is a bubble of low pressure caused by the Bernoulli effect the air traveling quickly of the top is reduced the pressure up there so what you then have is the high pressure below low pressure above again you're getting left you're getting left so there's two factors creating the left and it's the same on our propeller our propeller is just one of these oils attached to a hub so we can spin it round so we're gonna have low pressure on the top of the propeller and high pressure on the bottom and little basic one of the fundamentals of fluid dynamics is that where you have high pressure and low pressure if there's no nothing to obstruct it there will be a flow from the high to the low it's simple you know it's it's just the way things go you know if you've got high pressure one area low pressure or another you get wind or whatever you get flow from the high to the low that's what hat tries to happen on the propeller obviously the low pressure on the top and the high pressure on the bottom means that this high pressure is always trying to get up the top to fill in the low pressure as nature bores you know differentials it tries to cancel everything out likes you know likes to have everything level and even so it likes to neutralize the pressures but what it can happen of course is it can't just go through there cuz you gotta you can prepare in the way propeller blade it blocks the flow of air from top to bottom however what also happens is that remember a propeller is spinning spinning around around around another piece of physics now where's my little demonstration of repeat earlier here it is and it may look like an ax in motor to you on some wires but trust me it's not it's a piece of physics equipment again like that piece of paper this ax in motors hangs down right doesn't go anywhere I can make it do this though by spinning it if I spin that you see that now suddenly the wire is horizontal but Y is pretty much horizontal why is that what what is the force that's making this motor go from hanging straight down to going up like that well it's centrifugal force it's another nice things that you find in physics and basically what it means is that when you spin something around there is a force exerted that tries to move things to the outside of the circle in which you're spending it and of course got a propeller spinning here it's spinning in a circle so what happens with propeller is that we've also got this high pressure low pressure set up so we have ear actually traveling along the propeller because this is spinning around the ear naturally tends to get flung to the outside so some of the ear travels along and we've got our high pressure bubble on the top the pride pressure bubble actually gets flung to the edge so now you can see the problem we've got high pressure with nothing to separate it from the low sorry low pressure on the top nothing separating it from the high pressure underneath so we get a flow get a flow here from the bottom the propeller to the top of the propeller so that basically what happens is let me draw another diagram here if I can find my black sharpie running out of pen so ever done lost it I'm always losing there is this draw another one here here's our propeller and here's one blade the other blade so we have our low-pressure bubble on the top here and we have high-pressure underneath so the high pressure is being flung out towards the edge and the low pressure has also come back towards it so where they join you get this big vortex the es curls around because the bottom is trying to reach the top of propeller and in doing so it goes into a circle and creates a vortex and that basically a vortex just means lost energy wasted energy the energy that goes into creating this vortex at the tip there where the high and low pressures meet is simply turned into noise and heat it heats the air they are very very small amout but the air gets heated up and it makes a hell of a lot of noise because of all the molecular movement of the air and that's pressure waves making sound so that's what happens now obviously obviously if you've got a propeller operating like that and it's drawn this propeller up here we've got our low-pressure area here and high pressure underneath how can we stop this vortex this tip vortex and the answer is really simple as you can see if we just basically put a fence up here it stops the high pressure being able to reach the low pressure it blocks it it's it's physical barrier to the movement of high pressure air to the low pressure area so suddenly well you can't get this vortex anymore so the energy that previously went into creating their vortex is actually going to be turned into left because we're not cancelling out this low-pressure area on the top of this vortex flow so that becomes stronger because it's not being nullified by the flow over the tip so you're getting more lift from the propeller and you haven't had to put any more power on in fact you need less power because this power is being wasted in this vortex so when we put our propeller and a measure inner tube the tip vortex is greatly diminished it won't go away because there's always a little bit of gap between the propeller in to end the thing so it's always a little bit of leakage about it dramatically reduces the effect of that tip vortex from the propeller so we get much more efficiency and that's why a propeller and a DUT in theory is going to be more efficient than propeller just operating in free space but now if you thought that was good if you thought it was cool that we got something for nothing by putting our propeller and a duct you can be even more impressed with what comes next because there are two ways that a propeller duct can improve the performance the efficiency of a propeller and the next way is actually really really cunning and you've probably seen it didn't realize it now at the moment we've just got a parallel sided tubes if the tube just achieve it peace pipe and we'd get that improvement but if we do this something really interesting is going to happen and I'm drawing a nice big curve looked at quite a large radius curved lip on the edge of our duct so basically we're taking that children would just roll the edge around all the way around so it's rolled outwards and that's going to dramatically improve the amount of lift this setup creates how's it going to do that well I'm going to draw another line in here so you can actually see something here let's draw a line in there and a line in there and I want you to see if you can spot something else on the board that looks pretty similar to that shape oh look a wing yeah what we have here is a wing honestly it's an annular wing around the top of air duct and it's going to produce lift how well let's draw some lines let's draw some air flows now obviously our duct is going to be drawing air in the front blowing it out the back it has a lip or not right so that's pretty fundamental but what it's also going to do and when we've got special got a slip on here is it's going to draw air around the side here and into the duct and around the side and to the duct now what do you notice about what's going on here well we have a flow of air over a surface and as we did right at the beginning of this video when you have a flow of air the pressure drops the pressure Falls the faster the flow the lower the pressure now this obviously if we were to draw that lift bubble to show where that low-pressure area is going to be it's going to be all the way around here right and all the way around here on this draw the crosshatch so we can see this is an area of low pressure caused by the flow of air over that letter and interesting enough that's a co ended up I'll talk about CO and erfect in a moment but suffice to say suddenly we've got low pressure up here and remember that we also have no flow over the back of this lip I mean there's no air flow here so we're going to have esthetic pressure just the normal air pressure pushing on here normally your pressure on there so high pressure behind the lip low pressure in front of the lip what do you get or do you get a force you get thrust you get the lip look trying to move up getting a thrust created by this the wing at the nick of the of the duct creating forward forward force so if this is hovering you're getting more left because this bubble here is helping lift up the whole duct if this was tucked over it was on a plane so like a ducted fan and an a cell or something then this is going to be producing if your tip us all over its going to be producing forward thrust its contributing to the thrust generated by the motor itself and hey the interesting thing is doesn't cost you anything there's no extra drag created in fact there's less drag in the setup than just having a tube so you're actually getting more free energy or not free energy but you're getting more efficiency for nothing just having the slip on there it's called a Co n the maximum to what's called a Co n deduct it's a co ender lip and I'll tell you a little bit about the co and ER effect in another video if you're like I think I've done one on my extension I think I did a video on Co n terrific to Mike's jet channel can't remember if they've look suffice to say that a combination of this lip which is an annular wing and the ducting itself can produce in theory quite significant improvements and performance and that's what I'll do in the next video I'm going to take that that duct that was sent to me one of the ducts that was sent to me and the hobbyking thrust stand and I'm going to help this up we're going to test propellers without the duct and we're going to test them with the duct and see what effect of improvement this makes now as I say in theory it should make quite an improvement but there are a number of factors which can greatly influence the amount of performance increase you're getting I'm going to go through those and explain what those are and perhaps in another video as well one of the key things is the clearance between the propeller and the edge of this duct obvious if you have too much gap in here then too much of the high pressure air can get around into the low-pressure and you you get up with it turbulence in the loss of power this has to be a very very close fit because as your nose through the smallest gap you can blow air you know put your fingers like that still blow through them you really have to tether get very small and we're talking microscopic in some cases in large gas turbine engines that gap is incredibly small as a percentage of the total diameter you know we're talking just a couple of millimeters on something that may be like six hundred millimeters in diameter and it may be you know even just half a millimeter in some cases and if they'd get gets too wide all your efficiency goes to hell in a handbasket if you fly gas turbine model guess too jet engines then you will know possibly sir experience that if the gap between the turbine wheel and the and the housing which holds the nozzle guide vanes if that gap is even just half a millimeter too big then your engine runs like crap it has little power at overheats and it doesn't struggle very well because too much of the high pressure air is getting to the low pressure side and energy is just being lost and you need the energy that goes through the turbine wheel to drive your compressor to compress the air before it goes into the combustion chamber so it really has a big effect anyway that's how your ducting works it's how you get all the wonderful things we've got the Bernoulli effect we've got centrifugal force we've got the co and ER effect on the look there all these things combined to give us wonderful efficiency so you might ask well why aren't all propellers putting in shrouds and ducts well obviously there's practical reasons against that you've got the weight of this assembly the weight of the duct the weight of the shroud that produces a downside you've got to carry the extra weight and if you're only getting say you had a setup here and this weighed you know 100 grams you were getting 95 grams improved thrust and it's a waste of time because you're still down 5 grams so that's why we have to do practical tests to see just how much practical difference it makes and there's one other thing one other very important thing especially for multirotors and if people want to know about I'll talk about it in a future video it's about what happens when you start tilting these and moving forward and why that effectively caused the failure of a very promising project in the US military about 50 55 years ago 50 years ago there was a product a thing that was built around these ducts and it looked really promising until they actually flew it and found oh my goodness we didn't count on net effect and I'll tell you what they're defectors and what happens if you're interested just asking the comments so that I know otherwise I just wasting your time it's a another whiteboard video from RC model reviews thank you for watching if you've got comments you've got questions critiques anything like that just stick them in the section below the description that YouTube kindly provides for this purpose and now I've got a heap of stuff to do gotta get back to the editing suite because I mean so much footage is piled up there to be edited and so but this is really quick whiteboard video it's really quick to read it it's just talk to camera put it on YouTube excellent thanks for watching time to go catch you later

Channel: RCModelReviews

Views: 1,126,514

Rating: 4.8837996 out of 5

Keywords: aviation, aerodynamics, propellers, ducts, Bernouilli, lift, thrust, efficiency, power, theory, physics, white-board, coanda effect

Id: Cew5JF8q6eY

Length: 18min 17sec (1097 seconds)

Published: Wed Jul 22 2015

Reddit Comments

[u/Mac_O-]

At 15:25 he mentions the clearance between propeller and duct needs to be microscopic, if not, any effeciency gains are lost

[u/Bulldogskin]

What a fantastic teacher. Gets right to the point clearly and quickly. His enthusiasm is contagious. More please!

[u/atpeters]

That was very informative and really well put together I feel like. Great presenter. Thanks for the share.

[u/Owlmerlyn]

I have seen his videos before and they really good. To top it off they come free with an Aussie accent :)

[u/dexter-sinister]

Never seen this guy before, but I love it.

[u/futureroboticist]

Did he have the test video?

[u/falafely]

I thought that was Hide the Pain Harold from the thumbnail.