Beginners Guide to Motor and Prop Selection

Beginners Guide to Motor and Prop Selection
One of the most often asked questions on RC Groups is from people who 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.

Let’s look at #1 above and unlock some mysteries. To begin with, we know that the wing itself supports the entire weight of the plane when it’s flying. We often hear about this plane or that plane having a certain ‘wing loading’. This is just a way of expressing how much weight our wing is carrying around.

The way we talk about this wing loading is to speak about it in terms of weight per square foot of wing area. We always use ounces per square foot in countries that use the Imperial system of measurement. And so we might hear wing loading expressed like (for example)… 6 ounces per square foot… or maybe 15 ounces per square foot… or whatever it’s calculated to be.

A perfect time to stop and see an example would be right now… where we will calculate the wing loading of a typical plane we might see at the park, or even own.

Making the wing give up it’s secrets…
Let’s take a typical park flyer type plane and examine it’s wing. In the pic below, we can measure the wingspan from wingtip to wingtip and find that it’s pretty close to 48″, and the average width of the wing is about 7 3/16″. By the way, the technical term for ‘average wing width’ is the word ‘chord’. The weight of the plane below, as it sits on the runway, battery installed, and ready to take off is 25 ounces.

What we’ll do now is figure out the wing loading of this plane. And once we know what the wing loading is, we can calculate the stall speed, 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.

This is the only time we will ever get into any arithmetic, so bear with me here, and we’ll see it’s not really all that hard to do. Remember that wing loading is simply the weight of the plane in ounces divided by the area of the wing in square feet.

Step 1) Multiply the wingspan of 48″ x 7 3/16″
48″ x 7.187″ = about 345 square inches.

Step 2) Find out how many square feet this is by dividing our answer by 144.
345 / 144 = about 2.4 square feet.

Step 3) Find the wing loading by dividing the weight of the plane by the wing area in square feet.
25 ounces / 2.4 square feet = about 10.4 ounces per square foot.

That’s it. That’s the wing loading of our pictured plane. 10.4 ounces per square foot. It tells us that each square foot of wing area has to carry 10.4 ounces. Notice I use the word ‘about’ a lot. That’s because there’s no need at all to be super accurate and exact. This is not rocket science! In the next post, we’ll see how to calculate the stall speed of our plane…. using it’s wing loading only.

Calculating the stall speed of our plane…
In the last post, we calculated the wing loading of our plane, which happened to be 10.4 ounces per square foot. This, by itself, doesn’t tell us much of anything. But it does give us all the information we now need to calculate the stall speed of our plane. And we remember the stall speed as the slowest possible speed we can fly our plane without having it fall to the ground. 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. We simply use Google to tell us the square root of our wing loading (10.4), and then we multiply the answer by 5. That’s all there is to it.

Go to Google and type in the following…[square root of 10.4]. Now hit the Search button and presto… Google shows us the answer is 3.2 – So all we have to do now is multiply this answer by 5, and we have the stall speed of our plane!

3.2 x 5 = 16 mph. Our plane has a stall speed, a bare minimum… of 16 mph. And that is all the math we will ever use. Period. We’re on our way…

Stall speed begat top speed…
Now that we know what the stall speed of our example plane is (16 mph), we can select the speed range we want our plane to fly at. As it turns out, planes behave remarkably well when we select a top speed of about 2.5 to 3 times the plane’s stall speed. For our plane, then, we want a top speed of about 40-48 mph.

Why? Because we find that if we give our plane a top speed that is too high, it has terrible flight characteristics in the lower part of it’s flight envelope. In other words, when we slow down our plane for some lazy cruising around the sky, we find that it behaves erratically… not having enough power and thrust to give us nice crisp responses to our stick movements. We might also find that it takes a rather long time for our plane to get airborne, because it doesn’t have enough thrust to give us nice strong climbouts. And we may find that it doesn’t slow down very well for landings either.

In other words, we have a choice. We can make the plane fly like a rocket ship at the expense of slow speed handling, or we can wisely choose a top speed that will give us fantastic performance all through it’s speed range. A plane that will handle very well at it’s top speed, and at the same time will handle extremely well when it’s flying along just above it’s stall speed.

So the 2.5 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. Can we go above this range? Sure. On planes like the Spitfire, or the Mustang P-51, which were known to be extremely fast in real life, we might want to prop them for 4 or 4.5 times their stall speed (this will affect our slow speed handling to a degree). The rule isn’t written in stone, and it has some latitude… but propping a plane for 2.5 to 3 times it’s stall speed will most often always give us a great flight envelope for our planes.

Thrust… an uplifting experience
Now that we have successfully calculated what the top speed of our plane should be, we have to decide on how much ‘thrust’ we would like our plane to have. For the uninitiated (and that’s who this tutorial is really for), thrust is quite simply the climbing ability of a plane. It’s measured in ounces in the Imperial system of measurement. Let’s take a look at what thrust is responsible for in our planes.

1) It’s a measurement of how hard we can climb. Given enough thrust, we can actually make our plane hover in one spot, or with a little more thrust, send it into an unlimited vertical climb. Guys who like to fly 3D planes pick combos that give them lots and lots of thrust.
2) Thrust is also the thing responsible for giving us quick response from our plane in situations where we may get into trouble, and need a lot of power (thrust) to quickly get out of the predicament.

Basically, it all boils down to what kind of flying we are going to be doing. If all we’re after is to fly our 25 ounce plane in a nice, gentle, scale like manner, with some mild aerobatics, then we only need to have a motor/prop combo that develops about 75% of our plane’s weight in thrust. 18-19 ounces of thrust would be perfect for this type of flying.

At this point, though, all you really need to know is what thrust is. There is absolutely no need for you to remember charts and graphs of what kind of flying requires what kind of thrust. I said early on that this was going to be child’s play, and it is. I don’t know about you, but I can’t remember what I did yesterday, so I know good and well I’d never be able to memorize thrust tables.

And now, props…
Let’s recap what we’ve done so far.

We’ve taken an average plane from the shelf, measured it with a tape, and weighed it. Based on that, we came up with the plane’s stall speed, and that gave us a good ballpark figure for the top speed of our plane. We then learned that the manner in which we want to fly our plane is going to determine how much thrust we will need.

The only thing left for us to do now is measure how much ground clearance we have for our plane’s prop. And this is going to determine just how big a prop we can stick on our plane. Why? Because we want the largest diameter prop that will fit on our plane! Ideally, we want a prop with a diameter that is about 1/4 the wingspan of our plane. In our example plane above, this means we want to pick a prop that is about 12″ in diameter (1/4 of our 48″ wingspan).

That’s what we will shoot for whenever we pick a motor/prop combo for our plane. As it turns out, many years of testing and real world experience and fancy theories have shown this to be true. This “1/4″ rule gives us the greatest system efficiency, and maximizes the potential of our planes. And now, before we start having fun with a special piece of software I’ll introduce you to, we’ll wrap up everything we’ve learned:

1) Pick a top speed of about 2.5 to 3 times your plane’s stall speed.
2) Pick the biggest diameter prop possible (up to 25% wingspan) and spin it just fast enough to get you your desired top speed.

That’s it, we’ve learned all the background knowledge we’ll ever need to know. There is no more to learn. We did it without fancy equations, boring math, discussions of motor kV, watts per pound, and all the other confusing nonsense that’s thrown at beginners when they ask how to pick motor/prop combos for their planes.

Now we get to start having fun with a piece of software that will make us all experts at picking combos…
The software “Webocalc” is an amazing art and a blessing to any newbie to RC Planes. Check it out here .

Part II
Ok, now that we know the background, and the driving forces that steer our decisions in picking motors and props… it’s time to start getting some hands on experience to see for ourselves just how easy this is with the right tools.

And the right tool is a software program written by a very respected member of our RC Groups forum… FliesLikeABeagle. We’ll be using his program, WebOCalc to run all kinds of behind the scenes, complicated formulas and algorithms to help us pick very nice combos for our planes.

All we are going to do is enter the AUW of our plane, it’s wingspan, and it’s wing area. Then we are going to tell WebOCalc what kind of flying we want to do, and it’ll pop out some suggestions for us. It’ll put us smack dab inside the ballpark, and we can then fine tune the results to pick just the right setup, without any hassle, and without any more learning on our part.

You can run WebOCalc 1.5.2 totally free or download a free copy to your computer.

So now we’ll pick as our working example, that HobbyZone Super Cub that was pictured earlier in the thread.

First we’ll open up WebOCalc, 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, which will then tell us that the most efficient prop would be in the range of 8.6″ to 14″ in diameter. So now we’ll close the Prop Size Wizard and select 14″ as our Maximum Prop Size.

Next, we’ll select “Slow Sport Aerobatics” as our Flight Mission.

Then we click on Suggest Top Speed, and WebOCalc will automatically enter 44 mph for us. We do the same thing with Suggest Thrust, and WOC will enter 25 ounces of thrust for us.

At this point, I’ll end this post… and start another so the posts don’t get so hard to read.

At this point, all we are going to do is next click on the Run Voltage Wizard. We will now be asked what kind of Li-Po or A123 battery we want to use.

I have a lot of 3s Li-Pos, so I simply clicked the little radio button next to the 3s Li-Po option. Once I closed down the Voltage Wizard, we notice that WebOCalc automatically entered the 3s Li-Po, and it also filled in the current we’ll be pulling from our battery, of 12.6 amps.

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.

We’ll close the kV Wizard down, and we ourself will type in ‘730’ in the Motor kV box.

Then we hit the Calculate button, and WebOCal goes to work…. showing us the following screen.

Now we’ll take a look at what WebOCalc is showing us about our plane.

It’s saying that with an APC 10×7 SF prop, our plane will be developing a top speed of around 40 mph, and developing about 29 ounces of thrust at full throttle. 29 ounces of thrust on a 25 ounce plane is a very good output! It means we’ll be able to hover, and even have enough power to go vertical to some degree.

It’s also showing us our stall speed of about 16 mph. Shown also is our ‘power to weight ratio’, which is displayed as 87 watts per pound. Here’s something interesting for you to consider. This “power to weight ratio” is about as useless a term as there ever was. 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. An often repeated formula is 100 watts per pound to do decent aerobatics and 150 watts per pound to go vertical. As you’ll see though, we have a plane that is a powerful aerobatic performer and can go vertical on only 87 watts per pound. Watts per pound is a joke, and is totally useless as an indicator of a plane’s performance.

WOC is also showing us that we’ll need an area of about 900′ x 640′ to fly our plane comfortably. Of course, we all know that the forums are rife with clowns who brag that they can fly their planes in much less space, and they can. And some go so far as to claim that they can fly their plane in a phone booth if need be. Pay them no mind. Use WOC’s realistic estimate of the space you’ll need to fly your plane.

We also see that when we have our plane at WOT (Wide Open Throttle), our motor will be pulling around 12.6 amps from our battery. This is what is called a ‘static’ amp draw, or the amps it would pull on our test bench at WOT. In real life, a prop ‘unloads’ while it’s actually flying…. which simply means that it will pull about 10% less, on average, than our static amp draw.

WebOCalc shows us a list of candidate props
Before I go on to the next lesson, 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”, you can safely assume that if the Gear Ratio is between .95 and 1.05, that those particular props deserve some consideration as possible contenders. As it turns out, our closest match was a .99 gear ratio. But you can see there are four more contenders that we can consider. Here’s a little tip for you, I’ve found after extensive use of WOC that the closest prop to a 1.00 gear ratio is the only one I ever need to consider for my planes.

A gear ratio of 1.00 represents a direct drive motor. The best prop was given to us as an APC 10×7 SF prop on a gearbox ratio of .99, which is pretty close to a perfect 1.00, so now we are going to learn how to play with WOC to show us the figures for that prop with a 1.00 (direct drive) ratio.

This is the part I like the best about working with WOC; getting it to give us a perfect 1.00 gear ratio.

But what if I don’t have a 730 kV motor???
We have seen so far that for the HobbyZone Super Cub (our example plane), a great all around motor that will let us fly it very nicely has the following characteristics.

1) It will have a constant current rating of around 12.2 amps.
2) It will have a kV of about 730 kV.

But let’s face it, we might not have this exact motor in our collection of motors, and we might not be able to find it at the store either. So what do we do? We try to find something as close to it as possible. So let’s see what we have in our box of motors we might be able to use.

Now, I just happen to have a motor with a constant current rating of around 14 amps, and it’s kV value is 890, which isn’t too far from the 730 kV that WebOCalc recommends, so let’s plug my motor into WebOCalc and see what we can do with it.

Many times, motors that we buy aren’t rated by the manufacturer for their constant current rating. All motors really should have these specs published, but they aren’t. So what do we do? Easy! We apply a little rule of thumb to help us along the way. This will allow us to get a good approximation of the constant current rating of motors that are sold cheaply on places like HobbyKing.

Multiply the weight of the motor by 2-3 to find a safe power level in watts for the motor.

Looking at the 60 gram 750 kV motor you linked to:
60 x 2 = 120 watts This is the power level we can run it at all day without overheating it. Our constant current rating, if you will.

60 x 3 = 180 watts This is the power level we can run it at for short periods of time only. Our burst current rating, if you will.

Since the average voltage of a 3s Li-Po over the course of a flight is about 10.8 volts, we can take 120 watts and divide it by 10.8 volts and the answer is about 11 amps. We can safely say that this motor then, has a constant current rating of about 11 amps. So can we use it if we wanted a constant current rating of 12 amps? Heck yes, because 11 is pretty darn close to 12, and our rule of thumb we used has a nice safe margin of error built into it.

Without going into all kinds of scientific hocus pocus about why we even put things like down/right thrust into planes (after all this is a tutorial about motor and prop selection), we can safely say that the only reason we use it is to get rid of behavior from our plane that we don’t like or want. So, the bottom line is that if our plane is not exhibiting these tendencies to begin with, then we don’t need the corrective down/right thust angles, period.

And the only way to tell if it needs it, is to fly it. If it needs it, start experimenting with the thrust angles to take out the aberant behavior. There is absolutely no formula that tells us how much down/right we need.

Content taken from: Chuck from (Discussion link)

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