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Old Nov 08, 2010, 02:48 PM
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Originally Posted by aclinml View Post
Thanks, Nofly.....I have a followup question. For comparison, the plane I am looking at is the Great Planes Electrifly DeHavilland Tiger Moth.

They recommend their own 1750Kv motor. If I plug that into WOC, most of the numbers stay the same except for the prop values. It looks like the prop that is closes to a 1:1 gear ratio, would actually have a bit less thrust (6.8) compared to a 970Kv motor. Are there any other advantages in choosing their motor over the 970 you recommend?

Thanks again.
Sometimes people want to preserve the aspect too. I guess a 970kv motor will led to a much bigger prop that will not look right or even don't fit.
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Old Nov 08, 2010, 07:02 PM
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Originally Posted by wjbite View Post
Great thread!
WOC is nicely done, too.
Here is my WOC output in the attached photo - it is based on what I have - except the props available in WOC don't seem to include mine. Mine are mostly lower pitch props.

Here is what I have:
A desire to get my 5 oz. camera aloft.
A Honda Civic with a trunk opening of 42".>> Trainer One style wing 42"
A four cell A123 battery 11 ounces - four cells allows me to trick my cheap ESCs into thinking that they see a three cell LiPo for a better LVC.
Two BP-21 brushless motors - Kv=1400 -- two because I want the camera in the center pointing forward.

So I put that data in WOC and it gives me props that I think should be more like 10 x 3.8. None get close to a gear ratio of one. And I think that using the 8x4 is way to whimpy. Looks to me like a GWS 8 x 4 would probably be better than the APC 8 x 4 but maybe that isn't in the data base.
Playing around with the parameters I have not been able to find any props that have a pitch/diameter ration of less than 0.5.
Is there any way to see what props are in the data base for WOC.
Adjusting the max current doesn't help and I am stuck with the Kv of my BP-21s.
Any suggestions?

Walt
Hi Walt,

Indeed there is a list of props available in WebOCalc. One only needs to search through the .js file to see them. WOC is open source software and all the world can see the code.

And if I'm not mistaken, there is in fact a 10x3.8 in the database of props. Can I ask a question? why do you need two 10" props on a 41" wingspan plane? Just curious.
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Old Nov 08, 2010, 07:06 PM
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Quote:
Originally Posted by aclinml View Post
Thanks, Nofly.....I have a followup question. For comparison, the plane I am looking at is the Great Planes Electrifly DeHavilland Tiger Moth.

They recommend their own 1750Kv motor. If I plug that into WOC, most of the numbers stay the same except for the prop values. It looks like the prop that is closes to a 1:1 gear ratio, would actually have a bit less thrust (6.8) compared to a 970Kv motor. Are there any other advantages in choosing their motor over the 970 you recommend?

Thanks again.
Do you happen to know which prop is recommended for your bipe with a 1750 kV motor? I can't see any advantages in using a motor with that high of a kV value. Do you have a link to the plane? I'll go have a look at what they recommend.
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Old Nov 08, 2010, 07:20 PM
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Originally Posted by wjbite View Post
Great thread!
WOC is nicely done, too.
Here is my WOC output in the attached photo - it is based on what I have - except the props available in WOC don't seem to include mine. Mine are mostly lower pitch props.

Here is what I have:
A desire to get my 5 oz. camera aloft.
A Honda Civic with a trunk opening of 42".>> Trainer One style wing 42"
A four cell A123 battery 11 ounces - four cells allows me to trick my cheap ESCs into thinking that they see a three cell LiPo for a better LVC.
Two BP-21 brushless motors - Kv=1400 -- two because I want the camera in the center pointing forward.

So I put that data in WOC and it gives me props that I think should be more like 10 x 3.8. None get close to a gear ratio of one. And I think that using the 8x4 is way to whimpy. Looks to me like a GWS 8 x 4 would probably be better than the APC 8 x 4 but maybe that isn't in the data base.
Playing around with the parameters I have not been able to find any props that have a pitch/diameter ration of less than 0.5.
Is there any way to see what props are in the data base for WOC.
Adjusting the max current doesn't help and I am stuck with the Kv of my BP-21s.
Any suggestions?

Walt
Walt,

Check out what an APC 7x5E prop does for you. A combined thrust of over 42 ounces on your 32 ounce plane.

Imagine what the amp draw would be if you used two 10" diameter props.

Chuck
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Old Nov 08, 2010, 09:48 PM
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Thanks NoFly
It didn't occur to me to reduce the max. prop dia.
I just naively put in the rule of thumb value equal to 1/4 of the wingspan. I thought the WOC would consider all props less than that - WRONG!
Seeing what you came up with is good for me - got lots of 7 and 8 inch props
I guess I will have to break down and boot into WinDoze to look at the code. I was using the web version on my Linux system. I stay away from MS products as much as possible.
Yeah, even though my A123 battery wouldn't complain, I'm sure the BP21 would hate those big props even if they were very low pitch, eh?

Thanks again,
Walt
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Old Nov 09, 2010, 04:10 AM
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Originally Posted by wjbite View Post
Thanks NoFly
It didn't occur to me to reduce the max. prop dia.
I just naively put in the rule of thumb value equal to 1/4 of the wingspan. I thought the WOC would consider all props less than that - WRONG!
Hi Walt,

Actually, WOC does indeed consider all props less than what you enter as a prop diameter. The problem here was that you didn't hit the "Run Prop Size Wizard" icon, which would have told you that the recommended prop maximum size is around 7" for a dual motor set up.

By ignoring the recommended max prop diameter and forcing it to look at 10" props and under, WOC simply tried to make you happy by coming close to what you told it to look for.

Chuck
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Old Nov 09, 2010, 07:40 AM
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Originally Posted by NoFlyZone View Post
Do you happen to know which prop is recommended for your bipe with a 1750 kV motor? I can't see any advantages in using a motor with that high of a kV value. Do you have a link to the plane? I'll go have a look at what they recommend.
Here you go, Nofly...

http://www.electrifly.com/parkflyers/gpma1134.html

At the bottom of the page, there is a link on the right to 'Download the manual'. On page 3 they list the additional items, including an 8 x 6 prop.
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Old Nov 09, 2010, 08:49 AM
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Originally Posted by NoFlyZone View Post
Hi Walt,

Actually, WOC does indeed consider all props less than what you enter as a prop diameter. The problem here was that you didn't hit the "Run Prop Size Wizard" icon, which would have told you that the recommended prop maximum size is around 7" for a dual motor set up.

By ignoring the recommended max prop diameter and forcing it to look at 10" props and under, WOC simply tried to make you happy by coming close to what you told it to look for.

Chuck
Ha!
No matter how well a programmer thinks out all the possible scenarios and programs to cover them, some bozo (like me) will come along and make some assumption he/she didn't think of. I retired from teaching microcontroller interfacing and programming six years ago. For thirty years it always amazed me how the students could come up with possibilities I hadn't thought of - loved it!
Thanks Chuck.

Walt
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Old Nov 09, 2010, 09:03 AM
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Originally Posted by aclinml View Post
Here you go, Nofly...

http://www.electrifly.com/parkflyers/gpma1134.html

At the bottom of the page, there is a link on the right to 'Download the manual'. On page 3 they list the additional items, including an 8 x 6 prop.
What a GREAT opportunity to explore something and learn from it! I want to take the time to explain this in a way that makes sense, but there is actually a reason why they are using such a ridiculously high (and terribly inefficient) motor as that 1800 kV unit.

Give me some time and I'll have a nice answer for you today...

Chuck
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Old Nov 09, 2010, 09:05 AM
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Ha!
No matter how well a programmer thinks out all the possible scenarios and programs to cover them, some bozo (like me) will come along and make some assumption he/she didn't think of. I retired from teaching microcontroller interfacing and programming six years ago. For thirty years it always amazed me how the students could come up with possibilities I hadn't thought of - loved it!
Thanks Chuck.

Walt
I've had the pleasure of beta testing a couple versions of WOC, and believe me, I know exactly what you mean... lol

Chuck
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Old Nov 10, 2010, 11:26 AM
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Originally Posted by NoFlyZone View Post
What a GREAT opportunity to explore something and learn from it! I want to take the time to explain this in a way that makes sense, but there is actually a reason why they are using such a ridiculously high (and terribly inefficient) motor as that 1800 kV unit.

Give me some time and I'll have a nice answer for you today...

Chuck
OK, Nofly. While we wait on a discussion on the above biplane issue, I have another biplane "oddity" to discuss.....This time it is the Eflite Jenny JN-4 described here...

http://www.hobbyzone.com/rc_planes_e...jenny_jn-4.htm

Using WOC and plugging in the numbers from the bottom of the product page, WOC says that there NO suitable props for that setup! They do have it using a 6 in. prop. If I change it to an 8 in. prop, it does put out some numbers that can be worked with.

Now, I am just getting into airplanes for the first time, but I have owned Eflite helicoptors, and generally their stock configuration on their helicopters work fine.
Why is there a problem getting WOC to work with a 6 in prop?

Thanks again.....this is all interesting (if not a little bit over my head!)
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Old Nov 10, 2010, 12:05 PM
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I am thinking about putting some of my props into WOC.
I have found the files with the prop descriptions in them.
Could someone point me to the info that will let me calculate the constant (the third number) for the prop description.
I have generated lots of thrust vs.rpm plots for my props - is there a way to measure the constant from those?

Walt
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Old Nov 12, 2010, 01:33 PM
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Originally Posted by aclinml View Post

http://www.hobbyzone.com/rc_planes_e...jenny_jn-4.htm

Using WOC and plugging in the numbers from the bottom of the product page, WOC says that there NO suitable props for that setup!

Why is there a problem getting WOC to work with a 6 in prop?

Thanks again.....this is all interesting (if not a little bit over my head!)
Let's look at what the setup is.

E-Flite Park 250 2200 kV motor, with a rated continuous operating current of about 7 amps max. A GWS RS 6x5 prop. Now, using a 2s Li-Po putting out about 7.2 volts for the duration of the flight, let's examine this, and we will find something VERY interesting!

Our little motor is an average motor, and therefore about 75% efficient. Let's do a rough little calculation here....

2200 kV x 7.2 volts = 15,840 rpm
Since our motor is 75% efficient, that works out to about 12,000 rpm that our GWS RS 6x5 will be turning at under full throttle. It'll actually be turning faster than that at the start of our flight when the battery is fully charged. But 12,000 rpm is what we can expect on average for the flight.

GWS props come in two flavors. RS and DD (sometimes called HD). RS stands for "Reduction Series" and DD stands for "Direct Drive". Reduction series means they are to be used with gear box and slowed down to a safe operating rpm. How do we figure the safe operating level? Simple. We take the diameter of the prop and divide it into 50,000.

50,000 / 6 = 8300 The maximum SAFE operating rpm for this prop is 8300 rpm.

WHAT????? Why would we be trying to turn it at that 12,000 rpm that it will be spinning at then????? We'd be downright foolish to do it.

WebOCalc knows that 12,000 rpm is too fast for this 8300 rpm prop, and guess what it does? It refuses to even show the prop...

More to come...

Make sense a little?

Chuck
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Old Nov 12, 2010, 03:24 PM
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OK, your numbers make sense. What doesn't make sense is to why they are saying to use the 6 in. prop in the first place??? I looked through (and searched) the plane's manual and they also did not say anything about a gearbox... I would think that a lot of people purchased the plane and ALL the recommended set up components. So shouldn't a lot of people be having some sort of problems using this prop, and why in the world would E-flite be recommending it???

Also, please look at post # 219. I would still like to know why they are requesting people to use the motor they are recomending instead of one that WOC recomends....

I've been going over 'wing loading' and motor sizes and watts and amps all the other numbers that WOC uses and I'm starting to feel like I'm back in my 5th grade math class! (I think I'm ready for recess now, too.....)

Thanks, teacher!
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Old Nov 12, 2010, 03:43 PM
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OK, your numbers make sense. What doesn't make sense is to why they are saying to use the 6 in. prop in the first place??? I looked through (and searched) the plane's manual and they also did not say anything about a gearbox... I would think that a lot of people purchased the plane and ALL the recommended set up components. So shouldn't a lot of people be having some sort of problems using this prop, and why in the world would E-flite be recommending it???

Also, please look at post # 219. I would still like to know why they are requesting people to use the motor they are recomending instead of one that WOC recomends....

I've been going over 'wing loading' and motor sizes and watts and amps all the other numbers that WOC uses and I'm starting to feel like I'm back in my 5th grade math class! (I think I'm ready for recess now, too.....)

Thanks, teacher!
The reason they recommend this prop and motor is that they know people won't be trying to fly it at WOT throttle very much. In fact, I have this exact same plane, with that motor in it, and it flies sort of nice, but the motor is so fast, that the prop wash flowing back over the wings (even at moderate speeds) makes for jumpy, erratic movements of the plane. What we basically have is the exact opposite of what WebOCalc is telling us makes for a good flying plane, which is to use the largest diameter prop and spin it JUST fast enough to give us a top speed of about 3 times stall speed. The top speed of this little GWS 6x5 RS prop and that motor is about 5 times stall speed. Very, very, inefficient. It flies terribly at top speed, and not all that good at slow speed either.

Is there such an animal as a small lightweight 8 ounce plane that is able to use the correct sized prop, and with the correct top speed? Yeppp, there is. And the way it does this is to use a gearbox. I have a GWS Pico Tiger Moth that uses a big honking 9 inch prop, and a high kV motor that is geared down to about 900-1000 rpm. And it flies perfectly at slow speeds, and perfectly at high speeds. And it does it all on about 2 amps of current draw from the battery.
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