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Old May 03, 2010, 04:35 PM
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Dialing in a perfect 1.00 Gear Ratio

Now we'll use our example above to set that gear ratio exactly to 1.00 from it's present value of .99 even though for all practical purposes, we don't really need to, because .99 is almost exactly a 1.00 gear ratio.

We will always use the following steps to accomplish this task, so follow along with me as I do it for you.

1) Change the "Desired Top Speed" box on the left hand side to 39.9, which is what the suggested APC 10x7SF prop is showing as a top speed in the output table on the right.

2) Now, in order to raise the .99 to our desired 1.00, we must lower the "Desired current..." box on the left hand side. So what we'll do is lower the Desired Current a couple of tenths to 12.4 Amps. (We can lower it by a single tenth if we really want to, but from experience, I can say that a couple of tenths is a good starting point).

3) Hit the Calculate button and see our results.

When we hit the Calculate button we still see our .99 in the gear ratio box. No big deal! This just shows we are really close to getting it now. So we will repeat the process over again.

1) Change the "Desired Top Speed" box on the left hand side to 39.6, which is what the suggested APC 10x7SF prop is now showing as a top speed in the output table on the right.

2) Again, since we want to raise the .99 to 1.00, we lower the "Desired current..." box a couple more tenths of an AMP, to 12.4 this time.

3) Hit the Calculate button and BINGO, we have now dialed it in perfectly.
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Old May 03, 2010, 05:36 PM
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Practice exercise for the reader

Ok, we just went and dialed in the APC 10x7SF prop. Now what you may want to do is try your hand at dialing in one of the other props that are close to 1.00 and see how you do. Pick the APC 11x8 TE prop which shows as a gear ratio of 1.03

This time, we want to lower the 1.03 to a 1.00, so we will raise the Desired current a few tenths at a time. Don't forget to keep changing the "Desired top speed" on the left side to reflect what the results chart is showing us.

Any questions? Fire away!
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Old May 03, 2010, 06:16 PM
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Ok, now that you've had a chance to play with dialing in the APC 11x7 TE prop, here's the results. I followed the exact same procedure that we outlined earlier.

Remember: If we want to raise the GearRatio to 1.00, we lower the "Desired Current".

And if we want to lower the GearRatio to 1.00, we raise the "Desired Current".
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Old May 03, 2010, 06:26 PM
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Let's recap

Ok, let's take a second here and look at the results of the two props we picked as possible candidates for our plane.

1) APC 10x7SF
This will give us a top speed of about 39 mph and deliver about 29 ounces of thrust, while it pulls about 12 amps from our battery.

2) APC 11x8TE
This will give us a slightly higher top speed of about 45 mph, and deliver just a hair less thrust of about 28 ounces, while it pulls about 13.5 amps from the battery.

So we get a bit more top speed, and slightly less thrust from the second prop, and we're having to draw about 1.5 amps more from the battery.

My choice would be the first prop, you may decide the extra speed is worth it to you, though. That's where personal flying styles enter the picture.
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Old May 03, 2010, 06:36 PM
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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.
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Old May 05, 2010, 07:04 PM
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Using what I have

In the pic below, we'll see that I have substituted my 14 amp 890 kV motor for the 730 kV that WebOCalc recommended. If I had a 730 kV, I'd use it in a heartbeat, but 890 is as close to it as I've got, so that's what we'll work with.

Once we hit the calculate button, WOC shows us a very good prop to play with in the APC 10x7 TE. It is telling us that it will use 12.6 amps of current with a gear ratio of 1.05 but we're going to use the method outlined above to set that gear ratio at a perfect 1.00 which represents an outrunner motor.
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Old May 05, 2010, 07:13 PM
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By using the 2 step process of adjusting the "Desired top speed", and adjusting the "Desired current" numbers, we finally arrive at the pic below.

Note that since we want to lower the gear ratio to 1.00 (from the 1.05 we started with), we have to raise the "Desired current". Do this a few times, and WebOCalc spits out what our final product will be producing for us.
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Old May 05, 2010, 07:31 PM
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Looking at what WebOCalc is teaching us

Ok, let's go back and compare the results. We'll look at the specs when we use a 730 kV motor, and the specs when we use an 890 kV motor in our Super Cub example plane.

Using a 730 kV:
Prop: APC 10x7SF -- Top speed 39 mph -- 29 ounces of thrust -- 12 amps of current.

Using an 890 kV motor:
Prop: APC 10x7 TE -- Top speed 48 mph -- 28 ounces of thrust -- 14.6 amps of current.

We can see that my little 14 amp 890 kV motor will work fine. It'll be a faster plane, but still very close to the 3 times stall speed we usually aim for. It will develop about 1 ounce less of thrust, which is still fairly good thrust. But it will be pulling about 20% more juice from my battery.

WebOCalc shows us that we are paying a slight penalty for using a higher rated kV than what is ideal. And WOC is showing us that for this 14 amp 890 kV motor, our best bet will be the APC 10x7 TE prop. In other words it picked a prop that will maximize my equipment...

Now, I have one of these Super Cubs, and I used WebOCalc to help me pick the right motor and prop for it. It works verrrry nicely with the prop WebOCalc picked.
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Old May 06, 2010, 11:56 PM
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Finally, I can understand what all those numbers mean! And this will definately cut down on guessing!
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Old May 07, 2010, 04:33 AM
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this is absolutley amazing.... thank you for this thread!!! I really appreciate the time you took to explain all this!!

Whe you want to make a pusher plane, do you simply turn the motor arround and propeller arround? or do you need a special setup? or am I way off base..... I am new to flying and i would like to make a pusher plane to protect my prop and motor from crashes, but i am not sure how to do it.


I am using brushless outrunner motors
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Old May 07, 2010, 05:38 AM
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Hi Adam,

Thankl you for the nice compliment! I'm adding more example planes to work with for the beginner so that you all can see just how easy this whole thing is...

Standing behind the plane (on a regular plane), you will notice that the prop is turning exactly like you would turn a screwdriver, when driving screws. There's no difference between a screw, and a conventional prop. In fact, we often refer to airplane props and boat props and submarine props as "screws".

Turn your motor around, and then switch any two wires on your motor to make it turn in the correct direction.

Then turn the prop around to make it face forward just like it faced before (the front of the plane). All done...

Chuck
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Old May 07, 2010, 09:04 AM
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Hi Chuck,

Thank you for the very detailed, and well laid out discussion that I am sure will help a lot of beginners.

I am beginner in flying RC planes myself. I purchased the Super Cub LP and I am just starting to get comfortable with the plane, however there is still a lot for me to learn.

Anyway, I will be looking to maybe upgrade the plane in the very near future and this info has been very helpful. As far as I know the Super Cub (used in your example) and the Super Cub LP have the same specifications regarding:

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.

I have identified a Brushless 750kv outrunner motor. Just a little above the estimated 730kv. My queston is how do I identify whether there is a constant current rating of around 12.2 amps. I found this motor on HobbyKing, here is the link:

http://www.hobbyking.com/hobbyking/s...0_750Kv_/_245W

This particular model does not seem to favored based on the reviews I just saw, but perhaps you could just point me to what I should be looking at.

Or there is this 800kv model

http://www.hobbyking.com/hobbyking/s...r_Eq:_AXi_2826

Well, thanks for the info and looking forward to hearing back from you.
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Old May 07, 2010, 10:05 AM
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Quote:
Originally Posted by TouroLouco View Post
I have identified a Brushless 750kv outrunner motor. Just a little above the estimated 730kv. My queston is how do I identify whether there is a constant current rating of around 12.2 amps. I found this motor on HobbyKing, here is the link:
Hi Touro,

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.

Does this help?
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Old May 07, 2010, 12:10 PM
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Noflyzone, thank you for helping me figure out the pusher prop set up. I knew it had to be something simple like that, there was a little doubt though, so instead of posibly frying my motor I thought it would be best to ask.

I was also wondering... when you mount a motor conventionally (on the front of the plane) they say you need to give it a little down and right angle.... do i need to place the pusher prop setup at an angle as well? I am thinking there is no need for it when it is a pusher, but again, thought i would ask.
Thanks
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Old May 07, 2010, 12:54 PM
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Quote:
Originally Posted by adams0259 View Post
Noflyzone, thank you for helping me figure out the pusher prop set up. I knew it had to be something simple like that, there was a little doubt though, so instead of posibly frying my motor I thought it would be best to ask.

I was also wondering... when you mount a motor conventionally (on the front of the plane) they say you need to give it a little down and right angle.... do i need to place the pusher prop setup at an angle as well? I am thinking there is no need for it when it is a pusher, but again, thought i would ask.
Thanks
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.

Chuck
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