Ed Anderson Posted March 31, 2010 Share Posted March 31, 2010 Unless you are flying a glider, the propeller may be the most important part of your airplane. The most beautiful fuselage, the most modern airfoil, the best servos have nothing to do if the propeller isn't doing its job. And that fancy motor you just mounted is just ballast without the right propeller to give that motor some go! So let's talk about propellers. What do you know about them? How do you select a propeller? Are there different kinds? How about materials? Wood? Plastic? Carbon? Why? Two blade? Three blades? Four Blades? How about one blades????? Can the right propeller make a major difference in how your plane flies? Why do glider and 3D pilots select the same kinds of propellers? What about pylon racers? Why kind or props do they need? Do electric pilots and fuel pilots choose propellers the same way? Can they trade propellers? Share what you know, or ask your questions. Let's talk propellers. BTW, if you pretty new to all this propeller stuff, then take a look at this thread were you can learn about the basics. Propeller Basics Then come on back and join the discussion. OK, who will be first?Edited By Ed Anderson on 31/03/2010 03:55:54 Quote Link to comment Share on other sites More sharing options...
Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 Since no one else cares to get his discussion started I will kick it off. I don't fly glow or gas so what follows is from the interpertation of an electric airplane and electric glider pilot. Hopefully one of the glow pilots will provide some needed input. Propellers - basics As the prop on a boat moves water, so the prop on a plane moves air, another fluid. And since water and air have mass, as we move them in one direction we create a force in the opposite direction. We measure this force in terms of thrust, and thrust is what moves the airplane forward. If you look at a blade of your prop you will notice that it is not flat but has shape. Like a wing, the prop has an airfoil shape that both creates thrust and lift. Different shapes have different characteristics, thus not all 10X6 props have the same thrust characteristics. If you are flying a glow plane, then your prop has to be built strong enough to handle the bang, bang, bang of the power strokes of your engine. This puts a lot of stress on the structure of the prop. Thus glow props tend to be built stronger and heavier than electric props, especially in the hub area. You can us a glow prop on an electric motor, but you are just adding weight that you don't need. Electric props can be made lighter because of the smooth power flow from the electric motor. There is no "jack hammer" pulsing as there is with a glow engine. Note that, because of the lighter design of electric props, you should never use an electric prop on a glow engine. The results could be .... bad! Selecting Propellers Glow pilots talk of "loading down the motor" in order to get to an ideal power zone. I will let a glow pilots talk more about that. Electric pilots speak of matching the prop to the motor to get the motor to draw enough amps to hit the power, the amps and Watts ( Volts X Amps) that are desired. From this point on I am going to focus on electric motors and the selection of the propeller. PROP vs. AMPS Your electric motor draws a certain amount of energy to do its job, which is to turn the propeller. With no prop attached it draws very little energy, very few amps. If you put a big prop on the motor it draws a lot of energy. This is similar to pulling a boat trailer behind your car. The car might get 20 mpg normally, but put a boat on a trailer behind the car and mileage will drop off to perhaps 15 mpg because the motor is using more energy just to maintain the same speed and travel the same distance. However as long as the boat and trailer are not too heavy, no real damage occurs, you just use more gas. If you put too big a trailer behind your car, something will break. The motor may fail, the transmission may fail, or something else. That is because you are asking the drive train to produce more work, use more energy then it was built to handle. Fuel mileage goes way down and then something breaks. You have over stressed things. Back to your plane. Your electric motor needs to "draw" a certain amount of energy in order to turn a given propeller at a given speed to produce a given amount of thrust. Let's use a speed 400 motor as an example and let's say you are using a 6X5 prop. That means the propeller is 6" across and has a pitch of 5" per revolution. Pitch indicates how far the prop would move forward through the air if there was no slippage. As either of these numbers go up, the motor is asked to do more work. Now let's apply some numbers. These are made up numbers for illustration only. Don't assume that these are accurate for your motor in your plane turning your prop. Let's say that, to turn that 6X5 prop your speed 400 motor draws 6 amps of electricity using a battery that delivers 10 volts, just to make the math simple. That would be 60 watts of energy that the motor consumes to turn that prop. (6 amps X 10 Volts) If we go to a larger prop, say 7 inches and keep the pitch the same 5 inches, the draw might go up to 8 amps at 10 volts or 80 watts. The wider prop will move more air, creating more thrust. Likewise if we went to a 7X6 prop, the draw would go up again, say to 9 amps or 90 watts. The steeper pitch, the 6, indicates that the prop is trying to move throught the air further on each rotation. This is what produces air speed. A deeper pitched prop will produce a higher air speed at the same RPMs. ( continued) <font s Quote Link to comment Share on other sites More sharing options... Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 In each case we are increasing the amount of work the motor has to do to turn the prop to produce the thrust we want. The harder it works the more electricity it draws. This is also placing an increasing amount of stress on the motor causing it to generate heat and placing more pressure on the bearings. If we push it too far, the motor will be unable to turn the prop fast enough to be useful in flying the plane and/or it will fail from stress, just like the car example above with the trailer that is too big. What we are trying to do is to get the best balance of propeller and amp draw so that the motor operates efficiently without being over stressed. Likewise if you have that same speed 400 motor and keep the prop at 6X5 but increase the electric pressure, volts, to 12 volts it will force more amps into the motor. This would be like putting a supercharger on your car's motor which forces more fuel/air mix into the car's engine. It will produce more power so it can do more work. However if we exceed the amount of power it was designed to handle, it will fail. It might not fail right away, but over a very short time it will start to degrade, perform badly and perhaps suddenly fail all together. If we push the voltage up too high or the amp draw too high, we will over stress the motor and damage it. The goal is to get a good balance of propeller and power draw. What follows are some tools that can help you select the best combinations of motor/battery/prop for your particular plane and the performance you desire. RESOURCES Understanding Electric Motors - How they work and details of how to read the specs provided by motor manufacturers. MotoCalc - Fee A commercial tool that will tell you everything you need to know: Amps, Volts, Watts, RPM, Thrust, Rate of Climb, and much more! It is a popular tool for predicting the proper motor, prop, battery pack for electric planes. There is a fee but if you are going to do a lot of this kind of work, it is well worth it. Electricalc - Fee Like Motcalc, this is a commercial tool that is very extensive. Well worth the money. WebOcalc - Free A free calculator that can help you get close to the proper design. You put in certain paramaters and it will give you the proper prop combo for that motor set-up. Not as feature rich as the others, but it can be useful for some basic modeling of power systems. Quote Link to comment Share on other sites More sharing options... Barry W Posted March 31, 2010 Share Posted March 31, 2010 Wow Ed this is all great stuff at least for me. You must be the American version of Timbo or perhaps you live in a Holiday Inn Express! I have a question for you do you have any documents that explain in some detail how the LIPO battery charging and equalizer works? I would like to understand how the equalizer arranges for each cell to be charged to the same capacity. Quote Link to comment Share on other sites More sharing options... Tim Mackey Posted March 31, 2010 Share Posted March 31, 2010 Thats off topic Barry.Try over HERE Quote Link to comment Share on other sites More sharing options... James40 Posted March 31, 2010 Share Posted March 31, 2010 Do we have the same sort of info for IC engines and props ? Why is it 3 or 4 blade props are never used on IC engines ? Quote Link to comment Share on other sites More sharing options... Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 Posted by Barry W on 31/03/2010 15:15:16: Wow Ed this is all great stuff at least for me. You must be the American version of Timbo or perhaps you live in a Holiday Inn Express! I have a question for you do you have any documents that explain in some detail how the LIPO battery charging and equalizer works? I would like to understand how the equalizer arranges for each cell to be charged to the same capacity. Comparing me to Timbo is probably one of the nicest things that anyone has ever said to me. I think it will be a quite a while before I can earn that kind of comparison. I have read his posts and Timbo is truly a wealth of knowledge. While I could comment on Lipo balancing, that would not be within the topic of this thread. I think Timbo has pointed you to the right place. Back on Topic However, getting back to this topic, my goal is not for me to talk about propellers but to invite all the experienced people on this forum to share their knowledge and their insights. The goal is to make this discussion a resource on propellers, beyond the introductory thread that Timbo has posted. So, anyone out there have anything to contribute? How do YOU select a propeller for your electric or glow plane?Edited By Ed Anderson on 31/03/2010 17:56:10 Quote Link to comment Share on other sites More sharing options... Ultymate Posted March 31, 2010 Share Posted March 31, 2010 Posted by James40 on 31/03/2010 17:47:06: Do we have the same sort of info for IC engines and props ? Why is it 3 or 4 blade props are never used on IC engines ? There is plenty of info around for IC props. Three and four blade props are used, usually when diameter is a problem for ground clearance or for scale effect. The multi blade props are generally less efficient than their two blade counterparts, the single bladed conterbalanced props being the most efficient but generally only found in specialist disciplines such as control line speed. Another reason for using multibladed props is to keep noise down, on some of the large petrol models on a large diameter 2 blader the tips of the prop will go supersonic and produce what is known as the "ripping" noise. If a smaller diameter 3 blader is fitted it will load the engine to the same RPM but the tip speed will be reduced at WOT thus reducing noise "ripping". Quote Link to comment Share on other sites More sharing options... Tim Mackey Posted March 31, 2010 Share Posted March 31, 2010 As Ulty says...theres loads of info on IC prop choice - we have a chart or two floating about.There is a PDF 2/3rds way down the home page James Quote Link to comment Share on other sites More sharing options... James40 Posted March 31, 2010 Share Posted March 31, 2010 Thanks and sorry for my ignorance Quote Link to comment Share on other sites More sharing options... Stephen Grigg Posted March 31, 2010 Share Posted March 31, 2010 How do I choose ED,I look at the little book that comes with my IC engines or ask the shop to send a suitable prop with the motor,Ignorance is Bliss.I know the prop sizes.. and get a great deal of help from my club mates.I can tell if Ive over propped or under propped from the IC sound and using a watt meter worked out the prop size for a GPS Texan electric,was fun and surprised me how the watts changed. Quote Link to comment Share on other sites More sharing options... Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 Posted by James40 on 31/03/2010 19:09:28:Thanks and sorry for my ignorance Don't feel bad, I didn't know it was there. Unfortunately it is only for glow engines and it provides no information about how or why you would select these props. Here is the link: Glow prop table I presume from the "plenty of info around" post earlier that there may be other resoruces somewhere as well. Maybe even someting for electric pilots. If you find them, post a link here so others can find them too. Perhaps IC engines only have a limited range of choices. Electric motors have a very wide range of prop choices so maybe we should focus on electric motors and props and not worry about the glow side. Edited By Ed Anderson on 31/03/2010 19:21:15 Quote Link to comment Share on other sites More sharing options... Ultymate Posted March 31, 2010 Share Posted March 31, 2010 James it's not ignorance my mate, if you don't ask you don't find out. Final choice on propellers at the end of the day comes down to a personal choice. Manufacturers and charts can point you roughly in the right direction but at the end of the day one usually has to do some experimentation to suit the individual pilots needs. It can vary with flying style, type of use and fuel used with glows (nitro content). Some of the props I use cost around the £70 mark so I try and keep my experimentation to a minimum and I'm always keen to be friendly towards people with large selections of props I can borrow and test .Edited By Ultymate on 31/03/2010 19:21:43 Quote Link to comment Share on other sites More sharing options... Simon Chaddock Posted March 31, 2010 Share Posted March 31, 2010 To contribute a bit - primarily for electric flight but the principles apply equally for IC. Simply loading up the motor to draw the desired amps is not quite the full story.You might for instance draw 10 amps with a 7x6 or an 8x4 so either would be using 100 watts on a 10 volt battery. The difference is what happens as the plane starts to fly. Say in the above example the motor was turning both props at 8000 rpm when static, then as the plane speeds up the prop will speed up as well. The result is the motor draws less current and thus generates less power. At some point the decline in motor power will exactly match the power required to fly the plane and it will go no faster. Now the 7x6 has a coarser pitch than the 8x4 so it will speed up less quickly with airspeed so all things being equal it should fly the plane a bit faster before the declining motor power matches the power to fly. So why consider the 8x4? Well the thrust from a prop is proportional to the square of its diameter. In the above example as both props are going round at the same speed and using the same power the 8" will produce 64/49 (8*8/7*7) or 1.3 times more thrust. The plane will accelerate faster & take off quicker. Yes the 8x4 will speed up and motor power decline quicker as the plane speeds up so at some point the smaller coarser pitch prop will actually generate a bit more thrust. The final part of the equation is the power to fly. Most types of drag build up as the square of the airspeed (double the airspeed means four times the drag & thus 4 times the power to fly). As the drag rises so fast the extra speed from the smaller coarser pitch prop in the above example might only be a few miles per hour but at the expense of significant take off thrust. Motors, models and props all have different characteristics so prop selection is always going to be something of a compromise and personal choice. The trick is to start from a known combination, try to understand what is likely to be going on, make an appropriate adjustment, and then carefully analyse the effect. It is unlikely you will get the optimum set up in one go! Well that's my 2p worth. Quote Link to comment Share on other sites More sharing options... Phil Brooks Posted March 31, 2010 Share Posted March 31, 2010 Posted by Simon Chaddock on 31/03/2010 20:34:32: Say in the above example the motor was turning both props at 8000 rpm when static, then as the plane speeds up the prop will speed up as well. The result is the motor draws less current and thus generates less power. Sorry Simon, I know this may seem like nit-picking, but I think it's relevant. If a motor draws less current for a given voltage, then it is consuming less power, not generating it. What it is generating is torque, which the propeller converts to thrust. I am very much a beginner with electric flight, still trying to come to terms with the differences between i/c and electric. One of the most obvious is the relatively enormous range of props that a suitable motor/battery/esc will tolerate in comparison to an i/c engine. It's a whole new world, and this forum and its' contributors are giving me lots of useful information. Thanks. On a different note, there is one aspect of multi-blade props for i/c that doesn't seem to get a mention. The heavier the prop, the greater the flywheel effect. This can help establish a far more reliable tick-over than a light two-blader. I have a Wot trainer with an Evo 46 driving a three blade prop. The engine and prop are (sadly) all that is left of a Hangar 9 Mustang PTS, I don't know the pitch, it came with the kit and is intended to match the engine. As well as a smooth, slow tick-over the prop also makes a fairly efficient brake when throttled back, both of which are a great help when I'm trying to land in the same field I took off from! Quote Link to comment Share on other sites More sharing options... Stephen Grigg Posted March 31, 2010 Share Posted March 31, 2010 In a less knowlegeable person like myself when using the Wattmeter,I know we are looking at amps within the limits of the speed controller but are we wanting to achieve the highest watts possible by choosing the correct prop for this purpose Quote Link to comment Share on other sites More sharing options... Simon Chaddock Posted March 31, 2010 Share Posted March 31, 2010 PhilIts all in the terminology but an electric motor does generate (mechanical) power by converting electrical energy. You are quite right about the differences between IC and electric motors. With a constant applied EMF( i.e a battery) electric motors have a wide power curve which peaks at approximately half the free motor speed. This sort of power curve makes prop selection less critical provided the motor is kept within its designed amps limit. IC engines on the other hand (and 2 strokes in particular) have a steeply rising power curve over the working rev range but with rapid fall off above a specific speed so the prop must be correctly matched to the engine and model to obtain maximum power in flight. Of course all the above is changed if part throttle performance is considered. As you suggest propeller inertia can be quite important in a single cylinder engine (and even more so in 4 strokes). I understand tests have shown that cyclic speed variation can significantly reduce a props efficiency. It is thus quite possible that a heavier 3 blade could actually be as efficient as a lighter 2 blade one. Quote Link to comment Share on other sites More sharing options... Tim Mackey Posted March 31, 2010 Share Posted March 31, 2010 Posted by Stephen Grigg on 31/03/2010 23:06:12:In a less knowlegeable person like myself when using the Wattmeter,I know we are looking at amps within the limits of the speed controller but are we wanting to achieve the highest watts possible by choosing the correct prop for this purpose Its best to look ( and note down ) 3 three main figures as whole picture Stephen.Sure you may be trying ( in simplistic terms ) to get maximum power ( watts ) but then you also need to check that the current is not exceeding the rating of the ESC or motor, or battery. Observing the battery volatge under load is also a good indicator of how well its holding up. I normally look for a minimum of 3.5V per cell, but many will exceed that depending on the load imposed on them.For simple maths, 10V from a 3s LiPo for example is reasonable.If the voltage drops off below this, then its a sign that the battery is being loaded retty heavily, or could be simply tired out and in need of replacing.Anyway, we are drifting off topic here Stephen Quote Link to comment Share on other sites More sharing options... Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 Ah, now this discussion is getting interesing. Thanks everyone for jumping in. Let's talk about span vs. pitch. and the impact of voltage. Simon touched on this a bit. I will add an actual example. First let's touch voltage. All electric motors have a kV rating. This means that for every volt applied, the motor will try to turn a certain number times. A 600 kV motor will turn about 600 reveloutions per minute, rpm, per volt applied. If I apply 7 volts it will try to turn 4200 rpms. If I apply 10 volts it will try to turn 6000 rpms. Whether I have a small prop or a big prop, the motor will still seek that same rpm rate at full throttle. It will draw more and more current (amps) in order to have enough power (watts = voltsXamps) in order to be able to turn that prop at that speed. The wider or the deeper the pitch of the prop the greater the load on the motor. By adjusting the width of the prop, or the pitch, I can match the propeller to the motor and the load I want to place on the motor in order to get the best performance while making sure I don't overload the motor. For example, I have this motor in one of my 2M electric gliders. Since this is a glider, I am optimizing for climb angle rather than speed. Rimfire 35-30-1250 Brushless Outrunner Electric MotorRimfire Brushless Outrunner Motor Diameter: 1.38" (35mm)Motor Length: 1.18" (30mm)Shaft Diameter: .16" (4mm)Shaft Length: .65" (16.5mm)Lead Length: 3" (76mm)Connectors: Bullet type, .14" (3.5mm) diameterMax. Constant Current: 30AMax. Surge Current: 35AMax. Constant Watts: 333WMax. Surge Watts: 390WNo Load Current: 1.2AInput Voltage: 11.1VRPM/V (kV Rating): 1250Weight: 2.5oz (71g)Suggested Propeller Size: 10x7EI have bolded the key items that will come into play here. Let's see how the motor behaves as we vary the voltage of the battery pack and the size and pitch of the prop. If I run it on a 10X7 prop with a 3S lipo, it cranks about 290 watts at about 28 amps, as measured on my wattmeter. This would vary somewhat based on brand and design of the prop. As I said earlier, different props have different air foils so they produce different results. This is what I got with the prop I am suing. This is the recommended size prop for this motor on a 3S pack, so we are in a good zone for this motor. This takes my plane ( 42 oz) almost straight up. If I put an 11X8 prop on the motor, using the same 3S lipo it will go to over 45 amps, almost 500 watts. This is way too much for this motor. It would burn it out. BTW it would also burn out the 35 amp ESC I have in this plane. Let's change the voltage. If I go to a 2S lipo pack I have dropped the voltage by 1/3, the motor will spin at about 2/3 of the speed it turns on a 3S pack. That 10X7 prop that worked so well on a 3S pack now only draws about 20 amps for about 145 watts. On the 3S pack this prop would take my plane straight up. On the 2S pack it will barely climb at 40 degrees. So, if I want a steeper climb on a 2S pack, for weight reasons, or because that is all that will fit in the plane, I need to change the prop. I go to a 2S lip on an 11X8 prop ( 1 inch wider and 1 inch deeper) it will run at about 200watts, around 28 amps. This gives me about a 55 degree climb. For me, this works fine and I can use my 2S packs. So, as you can see, I can tune the load to the kind of flying I want to do and the kind of battery pack I want to use. All work with the same motor. What if I stayed on that same 2S pack but went to a 12X6 prop? That would take the motor to 35 amps, 252 watts. While I am within my watt limt, I am at my amp limit so I can't got any larger. And I could only hold full throttle for about 30 seconds ( max surge current ) Then I would have to throttle back. But I would get about a 70 degree climb. That last combo might be good for a contest glider where I want a steep climb but the lightest weight and I only need to run the motor for 30 seconds to hit the height I want. the 2S pack is about 2.2 ounces lighter than the 3S pack. In some planes, 2 ounces saved could be important. How do I know all this? I put a wattmeter on the motor and measured what it was doing with the different props and different battery packs. The last one, the 12X6 I did not actually try. That result came from WebOcalc, one of the power system modeling tools I mentioned earlier. I have a 12X6 on order. Edited By Ed Anderson on 31/03/2010 23:59:33 Quote Link to comment Share on other sites More sharing options... Stephen Grigg Posted April 1, 2010 Share Posted April 1, 2010 Thanks Timbo although I would respectfully that I was not drifting off topic as we are talking propellars which would be in my question,choosing the correct prop to get the best out of an E- motor when matched with the most efficient prop for the set up.I now know fully what Im looking for and the result I need to be looking for,I thankyou Quote Link to comment Share on other sites More sharing options... Ed Anderson Posted April 1, 2010 Author Share Posted April 1, 2010 Stephen, You raise a good point. While we are talking propellers, we really need to look at the power source, motor/engine and prop as a system. If any part is out of balance then the other parts of the system suffer. While I am not a glow pilot, as I understand it, the glow guys use three factors in judging if their motor/fuel/propeller combination is well balanced: Sound RPMs Heat They listen for some specific range of sounds that their trained ears know indicate a smooth running engine. If the engine is not running smoothly, if the mix isn't right, they really can't judge if the prop is right. I don't know what impact the fuel blend has on this but I know there are different fuel blends so I presume there is a reason. They use a tachomoter to measure the RPMs. I presume they are looking for some optimum RPM range though I don't know if this varies by motor or displacement. I have often seen glow pilots pull out a tach to read the engine speed. Then some knowing sounds are made, and an exchange of grunts and facial gestures between pilots are made, then a go/no go is made for the flight. In some fashion they are judging that the motor is not overloaded which would lead to overheating. And I think the oil mix in the fuel is a factor here. Hopefully the glow guys will chime in with more details. When judging whether an electric power system in in balance we look at: Volts Amps Watts Heat This is why the Wattmeter is so valuable. Voltage - All ESC and motors have a rated voltage range. If your ESC says 2 or 3 cell lipos, don't run it on 4 or you are inviting a failure. Same goes for the motor. Amps - Each ESC, motor have max amp ratings. Battery packs (at least lithium based battery packs) have rated maximums expressed in C. It is usually best to operate a little below the maximums if you want long component life, but many will give you "burst" maximums which tell you can can abuse this component to this level but only for a short time. Heat is usually the issue here. The motor and ESC can only shed heat so fast before build up will start to do damage. For the battery, internal resistance generates heat. Get that pack too hot and you can do permanent damage. BTW, air flow over the motor, battery and ESC can be an important factor here. Regardless of the ratings, if you don't provide enough cooling air you can burn things up even if you are within the stated ranges. Watts - This is the product of Volts X Amps. While the whole system can be measured in watts, we usually focus this on the motor itself. This product of the voltage of the battery and the amps going into the motor should not exceed the design of the motor. Since every ESC also has a stated voltage range, sometimes stated in cell counts, Watts applies here too. And Lipos carry C ratings which indicate the maximum amperage which ultimately translates to maximum watts. Note that watts is also a measure of heat. Heat - in all of this heat is the issue. If you get the motor too hot, the insulation on the windings can break down causing a short. Heat can also swell bearings and shafts which will cause binding, and more heat and wear. This can eventually lead to a seized motor and/or very loose shaft/bearing margins. Back to the prop But whether we look at electric or at glow, it is the propeller that is putting the load on the system. So we have to choose wisely in order to insure we do not overload any part of the system. I think of the new pilot with his RTF. He posts a cry for help because the magic smoke came out of his plane. After 10 or 15 posts we finally learn that his plane was intended to run on a 7 cell Nixx pack of 8.4V. He changed to a 3 cell Lipo because it would give him longer flights at the same weight. But he had no understanding that this higher voltage would spin that prop faster, casuing the motor to draw more amps, eventually cooking the ESC. It is all a question of balance to get the best out of the power system and picking the right prop is critical to that balance. Edited By Ed Anderson on 01/04/2010 14:39:36 Quote Link to comment Share on other sites More sharing options... Roberto Gava Posted April 1, 2010 Share Posted April 1, 2010 Today I've been making my first electrification calculations with Propeller eMotor Calculator for my Eurofighter project. I'm a little confused about propeller calculations. Until now, with glow engines I've been using one software called: "Extended Prop Selector" The point is, that those two software give me quite different results for the same prop dimensions. I.E: with a 8x8E@14665rpm eMotor tell me Static thrust of 4.75 lb. with a power consumption of 371 W But Prop Selector gives me 3.71 lb and 521 W respectivelly. Does anyone has an idea of what is happening? Quote Link to comment Share on other sites More sharing options... Ed Anderson Posted April 2, 2010 Author Share Posted April 2, 2010 When you said "those two software" gave you very different results, did you mean the electric calculator and the glow calculator? 14,555 RPM? Boy, that is one fast spinning prop. Any chance you got your thrust and watts mixed up. I would expect the higher thrust to also report the higher watts. For the electric motor: What kV did you enter for the motor? What voltage for the battery pack? I would like to run it through my own calculator. Clearly there is no gearbox involved. Quote Link to comment Share on other sites More sharing options... Roberto Gava Posted April 2, 2010 Share Posted April 2, 2010 Ed, I entered a 920 kV motor with a 5S 2600 mA battery and an APC 8x8 propeller with an ES of 40 A. This gave me 25 A max consumption. But what I'm seing is from the prop side. This program tell me that motor is developing 370 W. A asume this power is being consumed by the prop. So, this software gives the 8x8 prop flipping at 14665 rpm with those 370 W of power imput and developing 4.75 lb of static thrust. The other software is only for propeller calculations. Imputing that same 14665 rpm in an 8x8 prop results in more power consumed by prop (521 W) and less thrust (just 3.71 lb) Quote Link to comment Share on other sites More sharing options... David Ashby - Moderator Posted April 2, 2010 Share Posted April 2, 2010 Ed, just use the chain link icon to shorten your html links. Pasted html may also distort the page. I've had to trim the thread I'm afraid. Post again please. Edited By David Ashby - RCME Administrator on 02/04/2010 18:38:32 Quote Link to comment Share on other sites More sharing options... Prev 1 2 3 Next Page 1 of 3 Join the conversation You can post now and register later. If you have an account, sign in now to post with your account. Note: Your post will require moderator approval before it will be visible. Reply to this topic... × Pasted as rich text. Paste as plain text instead Only 75 emoji are allowed. × Your link has been automatically embedded. Display as a link instead × Your previous content has been restored. 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Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 In each case we are increasing the amount of work the motor has to do to turn the prop to produce the thrust we want. The harder it works the more electricity it draws. This is also placing an increasing amount of stress on the motor causing it to generate heat and placing more pressure on the bearings. If we push it too far, the motor will be unable to turn the prop fast enough to be useful in flying the plane and/or it will fail from stress, just like the car example above with the trailer that is too big. What we are trying to do is to get the best balance of propeller and amp draw so that the motor operates efficiently without being over stressed. Likewise if you have that same speed 400 motor and keep the prop at 6X5 but increase the electric pressure, volts, to 12 volts it will force more amps into the motor. This would be like putting a supercharger on your car's motor which forces more fuel/air mix into the car's engine. It will produce more power so it can do more work. However if we exceed the amount of power it was designed to handle, it will fail. It might not fail right away, but over a very short time it will start to degrade, perform badly and perhaps suddenly fail all together. If we push the voltage up too high or the amp draw too high, we will over stress the motor and damage it. The goal is to get a good balance of propeller and power draw. What follows are some tools that can help you select the best combinations of motor/battery/prop for your particular plane and the performance you desire. RESOURCES Understanding Electric Motors - How they work and details of how to read the specs provided by motor manufacturers. MotoCalc - Fee A commercial tool that will tell you everything you need to know: Amps, Volts, Watts, RPM, Thrust, Rate of Climb, and much more! It is a popular tool for predicting the proper motor, prop, battery pack for electric planes. There is a fee but if you are going to do a lot of this kind of work, it is well worth it. Electricalc - Fee Like Motcalc, this is a commercial tool that is very extensive. Well worth the money. WebOcalc - Free A free calculator that can help you get close to the proper design. You put in certain paramaters and it will give you the proper prop combo for that motor set-up. Not as feature rich as the others, but it can be useful for some basic modeling of power systems. Quote Link to comment Share on other sites More sharing options...
Barry W Posted March 31, 2010 Share Posted March 31, 2010 Wow Ed this is all great stuff at least for me. You must be the American version of Timbo or perhaps you live in a Holiday Inn Express! I have a question for you do you have any documents that explain in some detail how the LIPO battery charging and equalizer works? I would like to understand how the equalizer arranges for each cell to be charged to the same capacity. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted March 31, 2010 Share Posted March 31, 2010 Thats off topic Barry.Try over HERE Quote Link to comment Share on other sites More sharing options...
James40 Posted March 31, 2010 Share Posted March 31, 2010 Do we have the same sort of info for IC engines and props ? Why is it 3 or 4 blade props are never used on IC engines ? Quote Link to comment Share on other sites More sharing options...
Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 Posted by Barry W on 31/03/2010 15:15:16: Wow Ed this is all great stuff at least for me. You must be the American version of Timbo or perhaps you live in a Holiday Inn Express! I have a question for you do you have any documents that explain in some detail how the LIPO battery charging and equalizer works? I would like to understand how the equalizer arranges for each cell to be charged to the same capacity. Comparing me to Timbo is probably one of the nicest things that anyone has ever said to me. I think it will be a quite a while before I can earn that kind of comparison. I have read his posts and Timbo is truly a wealth of knowledge. While I could comment on Lipo balancing, that would not be within the topic of this thread. I think Timbo has pointed you to the right place. Back on Topic However, getting back to this topic, my goal is not for me to talk about propellers but to invite all the experienced people on this forum to share their knowledge and their insights. The goal is to make this discussion a resource on propellers, beyond the introductory thread that Timbo has posted. So, anyone out there have anything to contribute? How do YOU select a propeller for your electric or glow plane?Edited By Ed Anderson on 31/03/2010 17:56:10 Quote Link to comment Share on other sites More sharing options...
Ultymate Posted March 31, 2010 Share Posted March 31, 2010 Posted by James40 on 31/03/2010 17:47:06: Do we have the same sort of info for IC engines and props ? Why is it 3 or 4 blade props are never used on IC engines ? There is plenty of info around for IC props. Three and four blade props are used, usually when diameter is a problem for ground clearance or for scale effect. The multi blade props are generally less efficient than their two blade counterparts, the single bladed conterbalanced props being the most efficient but generally only found in specialist disciplines such as control line speed. Another reason for using multibladed props is to keep noise down, on some of the large petrol models on a large diameter 2 blader the tips of the prop will go supersonic and produce what is known as the "ripping" noise. If a smaller diameter 3 blader is fitted it will load the engine to the same RPM but the tip speed will be reduced at WOT thus reducing noise "ripping". Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted March 31, 2010 Share Posted March 31, 2010 As Ulty says...theres loads of info on IC prop choice - we have a chart or two floating about.There is a PDF 2/3rds way down the home page James Quote Link to comment Share on other sites More sharing options...
James40 Posted March 31, 2010 Share Posted March 31, 2010 Thanks and sorry for my ignorance Quote Link to comment Share on other sites More sharing options...
Stephen Grigg Posted March 31, 2010 Share Posted March 31, 2010 How do I choose ED,I look at the little book that comes with my IC engines or ask the shop to send a suitable prop with the motor,Ignorance is Bliss.I know the prop sizes.. and get a great deal of help from my club mates.I can tell if Ive over propped or under propped from the IC sound and using a watt meter worked out the prop size for a GPS Texan electric,was fun and surprised me how the watts changed. Quote Link to comment Share on other sites More sharing options...
Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 Posted by James40 on 31/03/2010 19:09:28:Thanks and sorry for my ignorance Don't feel bad, I didn't know it was there. Unfortunately it is only for glow engines and it provides no information about how or why you would select these props. Here is the link: Glow prop table I presume from the "plenty of info around" post earlier that there may be other resoruces somewhere as well. Maybe even someting for electric pilots. If you find them, post a link here so others can find them too. Perhaps IC engines only have a limited range of choices. Electric motors have a very wide range of prop choices so maybe we should focus on electric motors and props and not worry about the glow side. Edited By Ed Anderson on 31/03/2010 19:21:15 Quote Link to comment Share on other sites More sharing options...
Ultymate Posted March 31, 2010 Share Posted March 31, 2010 James it's not ignorance my mate, if you don't ask you don't find out. Final choice on propellers at the end of the day comes down to a personal choice. Manufacturers and charts can point you roughly in the right direction but at the end of the day one usually has to do some experimentation to suit the individual pilots needs. It can vary with flying style, type of use and fuel used with glows (nitro content). Some of the props I use cost around the £70 mark so I try and keep my experimentation to a minimum and I'm always keen to be friendly towards people with large selections of props I can borrow and test .Edited By Ultymate on 31/03/2010 19:21:43 Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted March 31, 2010 Share Posted March 31, 2010 To contribute a bit - primarily for electric flight but the principles apply equally for IC. Simply loading up the motor to draw the desired amps is not quite the full story.You might for instance draw 10 amps with a 7x6 or an 8x4 so either would be using 100 watts on a 10 volt battery. The difference is what happens as the plane starts to fly. Say in the above example the motor was turning both props at 8000 rpm when static, then as the plane speeds up the prop will speed up as well. The result is the motor draws less current and thus generates less power. At some point the decline in motor power will exactly match the power required to fly the plane and it will go no faster. Now the 7x6 has a coarser pitch than the 8x4 so it will speed up less quickly with airspeed so all things being equal it should fly the plane a bit faster before the declining motor power matches the power to fly. So why consider the 8x4? Well the thrust from a prop is proportional to the square of its diameter. In the above example as both props are going round at the same speed and using the same power the 8" will produce 64/49 (8*8/7*7) or 1.3 times more thrust. The plane will accelerate faster & take off quicker. Yes the 8x4 will speed up and motor power decline quicker as the plane speeds up so at some point the smaller coarser pitch prop will actually generate a bit more thrust. The final part of the equation is the power to fly. Most types of drag build up as the square of the airspeed (double the airspeed means four times the drag & thus 4 times the power to fly). As the drag rises so fast the extra speed from the smaller coarser pitch prop in the above example might only be a few miles per hour but at the expense of significant take off thrust. Motors, models and props all have different characteristics so prop selection is always going to be something of a compromise and personal choice. The trick is to start from a known combination, try to understand what is likely to be going on, make an appropriate adjustment, and then carefully analyse the effect. It is unlikely you will get the optimum set up in one go! Well that's my 2p worth. Quote Link to comment Share on other sites More sharing options...
Phil Brooks Posted March 31, 2010 Share Posted March 31, 2010 Posted by Simon Chaddock on 31/03/2010 20:34:32: Say in the above example the motor was turning both props at 8000 rpm when static, then as the plane speeds up the prop will speed up as well. The result is the motor draws less current and thus generates less power. Sorry Simon, I know this may seem like nit-picking, but I think it's relevant. If a motor draws less current for a given voltage, then it is consuming less power, not generating it. What it is generating is torque, which the propeller converts to thrust. I am very much a beginner with electric flight, still trying to come to terms with the differences between i/c and electric. One of the most obvious is the relatively enormous range of props that a suitable motor/battery/esc will tolerate in comparison to an i/c engine. It's a whole new world, and this forum and its' contributors are giving me lots of useful information. Thanks. On a different note, there is one aspect of multi-blade props for i/c that doesn't seem to get a mention. The heavier the prop, the greater the flywheel effect. This can help establish a far more reliable tick-over than a light two-blader. I have a Wot trainer with an Evo 46 driving a three blade prop. The engine and prop are (sadly) all that is left of a Hangar 9 Mustang PTS, I don't know the pitch, it came with the kit and is intended to match the engine. As well as a smooth, slow tick-over the prop also makes a fairly efficient brake when throttled back, both of which are a great help when I'm trying to land in the same field I took off from! Quote Link to comment Share on other sites More sharing options...
Stephen Grigg Posted March 31, 2010 Share Posted March 31, 2010 In a less knowlegeable person like myself when using the Wattmeter,I know we are looking at amps within the limits of the speed controller but are we wanting to achieve the highest watts possible by choosing the correct prop for this purpose Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted March 31, 2010 Share Posted March 31, 2010 PhilIts all in the terminology but an electric motor does generate (mechanical) power by converting electrical energy. You are quite right about the differences between IC and electric motors. With a constant applied EMF( i.e a battery) electric motors have a wide power curve which peaks at approximately half the free motor speed. This sort of power curve makes prop selection less critical provided the motor is kept within its designed amps limit. IC engines on the other hand (and 2 strokes in particular) have a steeply rising power curve over the working rev range but with rapid fall off above a specific speed so the prop must be correctly matched to the engine and model to obtain maximum power in flight. Of course all the above is changed if part throttle performance is considered. As you suggest propeller inertia can be quite important in a single cylinder engine (and even more so in 4 strokes). I understand tests have shown that cyclic speed variation can significantly reduce a props efficiency. It is thus quite possible that a heavier 3 blade could actually be as efficient as a lighter 2 blade one. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted March 31, 2010 Share Posted March 31, 2010 Posted by Stephen Grigg on 31/03/2010 23:06:12:In a less knowlegeable person like myself when using the Wattmeter,I know we are looking at amps within the limits of the speed controller but are we wanting to achieve the highest watts possible by choosing the correct prop for this purpose Its best to look ( and note down ) 3 three main figures as whole picture Stephen.Sure you may be trying ( in simplistic terms ) to get maximum power ( watts ) but then you also need to check that the current is not exceeding the rating of the ESC or motor, or battery. Observing the battery volatge under load is also a good indicator of how well its holding up. I normally look for a minimum of 3.5V per cell, but many will exceed that depending on the load imposed on them.For simple maths, 10V from a 3s LiPo for example is reasonable.If the voltage drops off below this, then its a sign that the battery is being loaded retty heavily, or could be simply tired out and in need of replacing.Anyway, we are drifting off topic here Stephen Quote Link to comment Share on other sites More sharing options...
Ed Anderson Posted March 31, 2010 Author Share Posted March 31, 2010 Ah, now this discussion is getting interesing. Thanks everyone for jumping in. Let's talk about span vs. pitch. and the impact of voltage. Simon touched on this a bit. I will add an actual example. First let's touch voltage. All electric motors have a kV rating. This means that for every volt applied, the motor will try to turn a certain number times. A 600 kV motor will turn about 600 reveloutions per minute, rpm, per volt applied. If I apply 7 volts it will try to turn 4200 rpms. If I apply 10 volts it will try to turn 6000 rpms. Whether I have a small prop or a big prop, the motor will still seek that same rpm rate at full throttle. It will draw more and more current (amps) in order to have enough power (watts = voltsXamps) in order to be able to turn that prop at that speed. The wider or the deeper the pitch of the prop the greater the load on the motor. By adjusting the width of the prop, or the pitch, I can match the propeller to the motor and the load I want to place on the motor in order to get the best performance while making sure I don't overload the motor. For example, I have this motor in one of my 2M electric gliders. Since this is a glider, I am optimizing for climb angle rather than speed. Rimfire 35-30-1250 Brushless Outrunner Electric MotorRimfire Brushless Outrunner Motor Diameter: 1.38" (35mm)Motor Length: 1.18" (30mm)Shaft Diameter: .16" (4mm)Shaft Length: .65" (16.5mm)Lead Length: 3" (76mm)Connectors: Bullet type, .14" (3.5mm) diameterMax. Constant Current: 30AMax. Surge Current: 35AMax. Constant Watts: 333WMax. Surge Watts: 390WNo Load Current: 1.2AInput Voltage: 11.1VRPM/V (kV Rating): 1250Weight: 2.5oz (71g)Suggested Propeller Size: 10x7EI have bolded the key items that will come into play here. Let's see how the motor behaves as we vary the voltage of the battery pack and the size and pitch of the prop. If I run it on a 10X7 prop with a 3S lipo, it cranks about 290 watts at about 28 amps, as measured on my wattmeter. This would vary somewhat based on brand and design of the prop. As I said earlier, different props have different air foils so they produce different results. This is what I got with the prop I am suing. This is the recommended size prop for this motor on a 3S pack, so we are in a good zone for this motor. This takes my plane ( 42 oz) almost straight up. If I put an 11X8 prop on the motor, using the same 3S lipo it will go to over 45 amps, almost 500 watts. This is way too much for this motor. It would burn it out. BTW it would also burn out the 35 amp ESC I have in this plane. Let's change the voltage. If I go to a 2S lipo pack I have dropped the voltage by 1/3, the motor will spin at about 2/3 of the speed it turns on a 3S pack. That 10X7 prop that worked so well on a 3S pack now only draws about 20 amps for about 145 watts. On the 3S pack this prop would take my plane straight up. On the 2S pack it will barely climb at 40 degrees. So, if I want a steeper climb on a 2S pack, for weight reasons, or because that is all that will fit in the plane, I need to change the prop. I go to a 2S lip on an 11X8 prop ( 1 inch wider and 1 inch deeper) it will run at about 200watts, around 28 amps. This gives me about a 55 degree climb. For me, this works fine and I can use my 2S packs. So, as you can see, I can tune the load to the kind of flying I want to do and the kind of battery pack I want to use. All work with the same motor. What if I stayed on that same 2S pack but went to a 12X6 prop? That would take the motor to 35 amps, 252 watts. While I am within my watt limt, I am at my amp limit so I can't got any larger. And I could only hold full throttle for about 30 seconds ( max surge current ) Then I would have to throttle back. But I would get about a 70 degree climb. That last combo might be good for a contest glider where I want a steep climb but the lightest weight and I only need to run the motor for 30 seconds to hit the height I want. the 2S pack is about 2.2 ounces lighter than the 3S pack. In some planes, 2 ounces saved could be important. How do I know all this? I put a wattmeter on the motor and measured what it was doing with the different props and different battery packs. The last one, the 12X6 I did not actually try. That result came from WebOcalc, one of the power system modeling tools I mentioned earlier. I have a 12X6 on order. Edited By Ed Anderson on 31/03/2010 23:59:33 Quote Link to comment Share on other sites More sharing options...
Stephen Grigg Posted April 1, 2010 Share Posted April 1, 2010 Thanks Timbo although I would respectfully that I was not drifting off topic as we are talking propellars which would be in my question,choosing the correct prop to get the best out of an E- motor when matched with the most efficient prop for the set up.I now know fully what Im looking for and the result I need to be looking for,I thankyou Quote Link to comment Share on other sites More sharing options...
Ed Anderson Posted April 1, 2010 Author Share Posted April 1, 2010 Stephen, You raise a good point. While we are talking propellers, we really need to look at the power source, motor/engine and prop as a system. If any part is out of balance then the other parts of the system suffer. While I am not a glow pilot, as I understand it, the glow guys use three factors in judging if their motor/fuel/propeller combination is well balanced: Sound RPMs Heat They listen for some specific range of sounds that their trained ears know indicate a smooth running engine. If the engine is not running smoothly, if the mix isn't right, they really can't judge if the prop is right. I don't know what impact the fuel blend has on this but I know there are different fuel blends so I presume there is a reason. They use a tachomoter to measure the RPMs. I presume they are looking for some optimum RPM range though I don't know if this varies by motor or displacement. I have often seen glow pilots pull out a tach to read the engine speed. Then some knowing sounds are made, and an exchange of grunts and facial gestures between pilots are made, then a go/no go is made for the flight. In some fashion they are judging that the motor is not overloaded which would lead to overheating. And I think the oil mix in the fuel is a factor here. Hopefully the glow guys will chime in with more details. When judging whether an electric power system in in balance we look at: Volts Amps Watts Heat This is why the Wattmeter is so valuable. Voltage - All ESC and motors have a rated voltage range. If your ESC says 2 or 3 cell lipos, don't run it on 4 or you are inviting a failure. Same goes for the motor. Amps - Each ESC, motor have max amp ratings. Battery packs (at least lithium based battery packs) have rated maximums expressed in C. It is usually best to operate a little below the maximums if you want long component life, but many will give you "burst" maximums which tell you can can abuse this component to this level but only for a short time. Heat is usually the issue here. The motor and ESC can only shed heat so fast before build up will start to do damage. For the battery, internal resistance generates heat. Get that pack too hot and you can do permanent damage. BTW, air flow over the motor, battery and ESC can be an important factor here. Regardless of the ratings, if you don't provide enough cooling air you can burn things up even if you are within the stated ranges. Watts - This is the product of Volts X Amps. While the whole system can be measured in watts, we usually focus this on the motor itself. This product of the voltage of the battery and the amps going into the motor should not exceed the design of the motor. Since every ESC also has a stated voltage range, sometimes stated in cell counts, Watts applies here too. And Lipos carry C ratings which indicate the maximum amperage which ultimately translates to maximum watts. Note that watts is also a measure of heat. Heat - in all of this heat is the issue. If you get the motor too hot, the insulation on the windings can break down causing a short. Heat can also swell bearings and shafts which will cause binding, and more heat and wear. This can eventually lead to a seized motor and/or very loose shaft/bearing margins. Back to the prop But whether we look at electric or at glow, it is the propeller that is putting the load on the system. So we have to choose wisely in order to insure we do not overload any part of the system. I think of the new pilot with his RTF. He posts a cry for help because the magic smoke came out of his plane. After 10 or 15 posts we finally learn that his plane was intended to run on a 7 cell Nixx pack of 8.4V. He changed to a 3 cell Lipo because it would give him longer flights at the same weight. But he had no understanding that this higher voltage would spin that prop faster, casuing the motor to draw more amps, eventually cooking the ESC. It is all a question of balance to get the best out of the power system and picking the right prop is critical to that balance. Edited By Ed Anderson on 01/04/2010 14:39:36 Quote Link to comment Share on other sites More sharing options...
Roberto Gava Posted April 1, 2010 Share Posted April 1, 2010 Today I've been making my first electrification calculations with Propeller eMotor Calculator for my Eurofighter project. I'm a little confused about propeller calculations. Until now, with glow engines I've been using one software called: "Extended Prop Selector" The point is, that those two software give me quite different results for the same prop dimensions. I.E: with a 8x8E@14665rpm eMotor tell me Static thrust of 4.75 lb. with a power consumption of 371 W But Prop Selector gives me 3.71 lb and 521 W respectivelly. Does anyone has an idea of what is happening? Quote Link to comment Share on other sites More sharing options...
Ed Anderson Posted April 2, 2010 Author Share Posted April 2, 2010 When you said "those two software" gave you very different results, did you mean the electric calculator and the glow calculator? 14,555 RPM? Boy, that is one fast spinning prop. Any chance you got your thrust and watts mixed up. I would expect the higher thrust to also report the higher watts. For the electric motor: What kV did you enter for the motor? What voltage for the battery pack? I would like to run it through my own calculator. Clearly there is no gearbox involved. Quote Link to comment Share on other sites More sharing options...
Roberto Gava Posted April 2, 2010 Share Posted April 2, 2010 Ed, I entered a 920 kV motor with a 5S 2600 mA battery and an APC 8x8 propeller with an ES of 40 A. This gave me 25 A max consumption. But what I'm seing is from the prop side. This program tell me that motor is developing 370 W. A asume this power is being consumed by the prop. So, this software gives the 8x8 prop flipping at 14665 rpm with those 370 W of power imput and developing 4.75 lb of static thrust. The other software is only for propeller calculations. Imputing that same 14665 rpm in an 8x8 prop results in more power consumed by prop (521 W) and less thrust (just 3.71 lb) Quote Link to comment Share on other sites More sharing options...
David Ashby - Moderator Posted April 2, 2010 Share Posted April 2, 2010 Ed, just use the chain link icon to shorten your html links. Pasted html may also distort the page. I've had to trim the thread I'm afraid. Post again please. Edited By David Ashby - RCME Administrator on 02/04/2010 18:38:32 Quote Link to comment Share on other sites More sharing options...
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