beginners guide to motor and prop selection

8/13/2019 Beginners Guide to Motor and Prop Selection

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Beginners Guide to Motor and Prop Selection

One of the most often asked questions on RC Groups is from people who want to know whichmotor 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 motorand prop for our planes. As it turns out, there is verylittle 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.


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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 thatplane 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 ofwing 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 widthof 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 notrocket science! Inthe next post, we'll see how to calculate the stall speed of our plane.... using it's wing loading

only.


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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 theinformation 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 veryimportant in helping us select a motor and prop for ourplane!

So how do we determine or calculate the stall speed? It's simplicity at it's finest. We simply useGoogle 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 toGoogleand 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 donow 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 allthe

math we will ever use. Period. We're on our way...
http://www.google.com/http://www.google.com/http://www.google.com/http://www.google.com/


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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 atop speed of about 40-48 mph.

Why? Because we find that if we give our plane a top speed that is toohigh, it has terrible flightcharacteristics 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 enoughthrust 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 ofslow 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 sametime 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 propour 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 timesit'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 responsiblefor in our planes.

1) It's a measurement of how hard we can climb. Given enough thrust, we can actually make ourplane 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 situationswhere 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.
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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 needfor 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 thrusttables.

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 goingto 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 ourplane'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 betrue. 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 justfast enough toget you your desired top speed.

That's it, we've learned all the background knowledge we'll ever need to know. There is no moreto 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...

Part II

Ok, now that we know the background, and the driving forces that steer our decisions in pickingmotors 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.


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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, WebOCalcto 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 suggestionsfor us. It'll put us smack dab inside the ballpark, and we can then fine tune the results to pick justthe right setup, without any hassle, and without any more learning on our part.

You can runWebOCalc 1.5.2totally 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 48Total 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 Wizardwhich 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 Calculatebutton, and WebOCal goes to work.... showing us the following screen.
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Now we'll take a look at what WebOCalc is showing us about our plane.

It's saying that with an APC 10x7 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 readon 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


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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 pullon 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.

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 possiblecandidates 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 10x7 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.

beginners guide to motor and prop selection

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