Beginners Guide to Motor and Prop Selection One of the most often asked questions on RC Groups is from peoplewho want to know which motor and prop to select for their particular plane. Even the most experienced and knowledgeable flyers among us are often confused by this seemingly dark and mysterious process of choosing the right motor and prop. It's my contention that with a little bit of background knowledge, and the right tools, this whole confusing process can be laid out in the open and exposed for what it really is; which is nothing more than child's play - so easy that even I can do it. With that in mind then, the goal of this mini-tutorial will be as follows: 1) Make it so that anyone can pick the right motor and prop for their plane. 2) Do it in a way that eliminates long and complex formulas and other nonsense. 3) Use real world examples along the way so that we can apply what we've learned. A little background... The first thing we'll do is look at the amount of information we need in order to choose a motor and prop for our planes. As it turns out, there is very little we actually need in order to do this! Well over 90-95 percent of the typical planes we fly can be successfully outfitted with the correct motor and prop by simply knowing the following information: 1) The all up weight of the plane. 2) The amount of ground clearance we have available for our prop. 3) The wingspan and wing area of our plane. Items 1 and 2 are pretty much self explanatory, so what we'll do is start by exploring why item number 3 is so important, and what it allows us to understand about our plane. The wing... is the thing As it turns out, the biggest secret to unlocking the mystery of proper motor and prop selection is contained in the most obvious of places... the wing itself. Here's what the wing can tell us: 1) It can tell us what the stall speed of our plane is. Stall speed is the lowest speed our plane can fly at. Going below that speed will make our plane fall from the sky. 2) It can tell us the diameter of the prop we want to use on our plane. 3) It can tell us how fast we should aim for when propping our plane. Step 2) Find out how many square feet this is by dividing our answer by 144. we can calculate the stall speed... the technical term for 'average wing width' is the word 'chord'.. 10.4 square feet. And so we might hear wing loading expressed like (for example). This is not rocket science! In the next post. or even own. and we'll see it's not really all that hard to do. and ready to take off is 25 ounces. That's it. We always use ounces per square foot in countries that use the Imperial system of measurement.. Making the wing give up it's secrets.. using it's wing loading only. The way we talk about this wing loading is to speak about it in terms of weight per square foot of wing area. .4 ounces.4 ounces per square foot. By the way. as it sits on the runway. Step 3) Find the wing loading by dividing the weight of the plane by the wing area in square feet. A perfect time to stop and see an example would be right now. 345 / 144 = about 2. Step 1) Multiply the wingspan of 48" x 7 3/16" 48" x 7. battery installed. What we'll do now is figure out the wing loading of this plane. To begin with.. and the stall speed will automatically tell us how fast we want the plane to go when it comes times to pick a motor and prop. Notice I use the word 'about' a lot.Let's look at #1 above and unlock some mysteries. we'll see how to calculate the stall speed of our plane. We often hear about this plane or that plane having a certain 'wing loading'. and the average width of the wing is about 7 3/16"... we can measure the wingspan from wingtip to wingtip and find that it's pretty close to 48".4 square feet = about 10. This is the only time we will ever get into any arithmetic.. In the pic below.. or whatever it's calculated to be. 25 ounces / 2. And once we know what the wing loading is. or maybe 15 ounces per square foot. Let's take a typical park flyer type plane and examine it's wing. The weight of the plane below.187" = about 345 square inches. so bear with me here.. we know that the wing itself supports the entire weight of the plane when it's flying. This is just a way of expressing how much weight our wing is carrying around. 6 ounces per square foot..4 ounces per square foot. where we will calculate the wing loading of a typical plane we might see at the park. That's the wing loading of our pictured plane. It tells us that each square foot of wing area has to carry 10.. Remember that wing loading is simply the weight of the plane in ounces divided by the area of the wing in square feet. That's because there's no need at all to be super accurate and exact. ..2 . This. we calculated the wing loading of our plane.So all we have to do now is multiply this answer by 5. .. We're on our way.. which happened to be 10...4). In the last post. And we remember the stall speed as the slowest possible speed we can fly our plane without having it fall to the ground. a bare minimum.2 x 5 = 16 mph. of 16 mph.. by itself. We simply use Google to tell us the square root of our wing loading (10. Go to Google and type in the following. But it does give us all the information we now need to calculate the stall speed of our plane. Our plane has a stall speed. Google shows us the answer is 3. square root of 10. Knowing this stall speed will be very important in helping us select a motor and prop for our plane! So how do we determine or calculate the stall speed? It's simplicity at it's finest. That's all there is to it.. and then we multiply the answer by 5. doesn't tell us much of anything.4 ounces per square foot. And that is all the math we will ever use. and we have the stall speed of our plane! 3.. Period..4 Now hit the Search button and presto.Calculating the stall speed of our plane. then we only need to have a motor/prop combo that develops about 75% of our plane's weight in thrust. We might also find that it takes a rather long time for our plane to get airborne.5 to 3 times it's stall speed will most often always give us a great flight envelope for our planes. or the Mustang P-51.. it all boils down to what kind of flying we are going to be doing. because it doesn't have enough thrust to give us nice strong climbouts. we have to decide on how much 'thrust' we would like our plane to have. Guys who like to fly 3D planes pick combos that give them lots and lots of thrust. we find that it behaves erratically. A plane that will handle very well at it's top speed. planes behave remarkably well when we select a top speed of about 2. . We can make the plane fly like a rocket ship at the expense of slow speed handling. but propping a plane for 2. Now that we know what the stall speed of our example plane is (16 mph).. when we slow down our plane for some lazy cruising around the sky. scale like manner. Basically. which were known to be extremely fast in real life. In other words.. or we can wisely choose a top speed that will give us fantastic performance all through it's speed range. we have a choice. Given enough thrust. The rule isn't written in stone. it has terrible flight characteristics in the lower part of it's flight envelope. or with a little more thrust.5 times their stall speed (this will affect our slow speed handling to a degree). So the 2... For our plane.. we can actually make our plane hover in one spot. Why? Because we find that if we give our plane a top speed that is too high. Thrust. and at the same time will handle extremely well when it's flying along just above it's stall speed. we want a top speed of about 40-48 mph. then. and need a lot of power (thrust) to quickly get out of the predicament. And we may find that it doesn't slow down very well for landings either. Let's take a look at what thrust is responsible for in our planes.5 to 3 times the plane's stall speed. Can we go above this range? Sure. we can select the speed range we want our plane to fly at. 1) It's a measurement of how hard we can climb.. As it turns out.. with some mild aerobatics. For the uninitiated (and that's who this tutorial is really for). and it has some latitude. In other words.Stall speed begat top speed. not having enough power and thrust to give us nice crisp responses to our stick movements. we might want to prop them for 4 or 4. gentle. send it into an unlimited vertical climb. On planes like the Spitfire. an uplifting experience Now that we have successfully calculated what the top speed of our plane should be. thrust is quite simply the climbing ability of a plane. It's measured in ounces in the Imperial system of measurement. If all we're after is to fly our 25 ounce plane in a nice.5 to 3 times stall speed 'rule of thumb' turns out to be an extremely efficient way to prop our average run of the mill planes. 2) Thrust is also the thing responsible for giving us quick response from our plane in situations where we may get into trouble. and that gave us a good ballpark figure for the top speed of our plane.. There is absolutely no need for you to remember charts and graphs of what kind of flying requires what kind of thrust. it's time to start getting some hands on experience to see for ourselves just how easy this is with the right tools. Let's recap what we've done so far. this means we want to pick a prop that is about 12" in diameter (1/4 of our 48" wingspan). I said early on that this was going to be child's play. measured it with a tape. As it turns out. Based on that. and maximizes the potential of our planes.. and all the other confusing nonsense that's thrown at beginners when they ask how to pick motor/prop combos for their planes. props. That's what we will shoot for whenever we pick a motor/prop combo for our plane. boring math. There is no more to learn. In our example plane above. all you really need to know is what thrust is. Part II Ok. Now we get to start having fun with a piece of software that will make us all experts at picking combos. And this is going to determine just how big a prop we can stick on our plane. discussions of motor kV. We did it without fancy equations.5 to 3 times your plane's stall speed. And now. And now. . I don't know about you... we want a prop with a diameter that is about 1/4 the wingspan of our plane. Why? Because we want the largest diameter prop that will fit on our plane! Ideally.. This "1/4" rule gives us the greatest system efficiency. so I know good and well I'd never be able to memorize thrust tables. The only thing left for us to do now is measure how much ground clearance we have for our plane's prop.. we've learned all the background knowledge we'll ever need to know. though. we came up with the plane's stall speed. but I can't remember what I did yesterday. many years of testing and real world experience and fancy theories have shown this to be true. 2) Pick the biggest diameter prop possible (up to 25% wingspan) and spin it just fast enough to get you your desired top speed. and the driving forces that steer our decisions in picking motors and props. At this point. and it is. We've taken an average plane from the shelf. watts per pound. we'll wrap up everything we've learned: 1) Pick a top speed of about 2. and weighed it. before we start having fun with a special piece of software I'll introduce you to. We then learned that the manner in which we want to fly our plane is going to determine how much thrust we will need. now that we know the background. That's it.18-19 ounces of thrust would be perfect for this type of flying. Then we click on Suggest Top Speed.. Then we are going to tell WebOCalc what kind of flying we want to do. and we ourself will type in '730' in the Motor kV box.. Once I closed down the Voltage Wizard.And the right tool is a software program written by a very respected member of our RC Groups forum.6" to 14" in diameter.. we'll select "Slow Sport Aerobatics" as our Flight Mission.. and enter the three things it needs: Ready-to-fly Weight 25 Wingspan 48 Total Wing Area 345 Then we let WebOCalc suggest a prop size for us clicking on the Run Prop Size Wizard. complicated formulas and algorithms to help us pick very nice combos for our planes. We'll be using his program. So now we'll pick as our working example. All we are going to do is enter the AUW of our plane. Then we hit the Calculate button.. without any hassle. WebOCalc to run all kinds of behind the scenes.6 amps. so I simply clicked the little radio button next to the 3s Li-Po option. which will then tell us that the most efficient prop would be in the range of 8. Next. We will now be asked what kind of Li-Po or A123 battery we want to use. all we are going to do is next click on the Run Voltage Wizard. and WOC will enter 25 ounces of thrust for us. that HobbyZone Super Cub that was pictured earlier in the thread. I'll end this post. and it also filled in the current we'll be pulling from our battery. We'll close the kV Wizard down. First we'll open up WebOCalc. and we can then fine tune the results to pick just the right setup. showing us the following screen. . At this point. and without any more learning on our part. of 12. and WebOCal goes to work. The last thing we click on is the Run kV Wizard which shows us that the most efficient motor is one with a kV of between 580 and 730. and it'll pop out some suggestions for us. You can run WebOCalc 1.5.. At this point. we notice that WebOCalc automatically entered the 3s Li-Po. and start another so the posts don't get so hard to read. So now we'll close the Prop Size Wizard and select 14" as our Maximum Prop Size. It'll put us smack dab inside the ballpark.. We do the same thing with Suggest Thrust. it's wingspan. FliesLikeABeagle. and it's wing area. I have a lot of 3s Li-Pos.2 totally free or download a free copy to your computer. and WebOCalc will automatically enter 44 mph for us. Watts per pound is a joke. Of . 29 ounces of thrust on a 25 ounce plane is a very good output! It means we'll be able to hover. Here's something interesting for you to consider. It means absolutely nothing! We often read on RC Groups that a plane needs certain power to weight ratios to accomplish things like hovering and unlimited vertical and certain speeds. and is totally useless as an indicator of a plane's performance. and developing about 29 ounces of thrust at full throttle. we have a plane that is a powerful aerobatic performer and can go vertical on only 87 watts per pound. our plane will be developing a top speed of around 40 mph. and even have enough power to go vertical to some degree. It's saying that with an APC 10x7 SF prop. An often repeated formula is 100 watts per pound to do decent aerobatics and 150 watts per pound to go vertical. Shown also is our 'power to weight ratio'. It's also showing us our stall speed of about 16 mph. This "power to weight ratio" is about as useless a term as there ever was. which is displayed as 87 watts per pound. WOC is also showing us that we'll need an area of about 900' x 640' to fly our plane comfortably. As you'll see though.Now we'll take a look at what WebOCalc is showing us about our plane. The best prop was given to us as an APC 10x7 SF prop on a gearbox ratio of . than our static amp draw. you can safely assume that if the Gear Ratio is between .99 gear ratio.course. getting it to give us a perfect 1. so now we are going to learn how to play with WOC to show us the figures for that prop with a 1. .99. which simply means that it will pull about 10% less. A gear ratio of 1. We also see that when we have our plane at WOT (Wide Open Throttle). This is what is called a 'static' amp draw. In real life. that those particular props deserve some consideration as possible contenders. Pay them no mind. But you can see there are four more contenders that we can consider. As it turns out. This is the part I like the best about working with WOC...00 represents a direct drive motor.6 amps from our battery. we all know that the forums are rife with clowns who brag that they can fly their planes in much less space. I've found after extensive use of WOC that the closest prop to a 1. on average.00 (direct drive) ratio. we should stop and take a look at that last picture and notice that WOC is actually giving us a few different possible candidates for props. If you look at the last column where it says "Approximate Gear Ratio". And some go so far as to claim that they can fly their plane in a phone booth if need be. our closest match was a .. and they can.00 gear ratio. Before I go on to the next lesson. our motor will be pulling around 12.05. a prop 'unloads' while it's actually flying.00 gear ratio is the only one I ever need to consider for my planes. or the amps it would pull on our test bench at WOT. which is pretty close to a perfect 1. Here's a little tip for you. Use WOC's realistic estimate of the space you'll need to fly your plane.95 and 1.00.
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