Need advice building a plane. - RC Groups
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Mar 09, 2012, 06:59 PM
Registered User

Need advice building a plane.

I've wanted a Sbach ever since I fist saw one and am trying to decide between these two:
The 39" "BX"

The 40" "SC"

I originally wanted the "bx" because it looked a little more durable but I don't think that is the case any more and would rather have the more scale looking "SC".

I already have my servos ( ordered. Those servos look comparable to the HS-65MG's but a lot cheaper and are digital so I should be able to get more than 90 degrees of travel out of them.

I am probably going with a 4A esc ( The 40A is probably way more than needed but it seems to be the smallest size with a built in 6V BEC. Or are there any other smaller switching ESC's with a built in 6 volt BEC. The HobbyWing I linked is switchable between 5.25 and 6V but I think 4.8 and 6V would be better. Then I could run on 4.8V to conserve battery when faster throws and higher torque isn't needed. Or would a smaller more appropriate sized ESC and a seperate BEC be better?

Batteries will be Gens Ace 25C in either 1000 or 1300mah.

Choosing a motor has been the most difficult for me so far. Matching kv's to props, etc... The recommended motor is the Dualsky XM2834CA-9 980Kv which weighs 70g. So I'm looking for other motors in the 70g range which are the:
Hengli W2834-1100kv
Twisted Hobbys 1250kv Telink motor
I believe the same 1250kv motor but a lot cheaper.

After doing some research and playing with WebOcalc I think I've determined a APC 11x5.5 E would be the most efficient prop for this plane.

Is 1.4 to 1 about the best I can do with a 70g motor? I'd like around 2-2.5 to 1 I can't seem to get any slow fly props on WebOcalc but it's my understanding that you should only use them when you can't fit a larger prop. And that they'll make more thrust but at a higher amp draw.
Last edited by itsmillertime; Mar 10, 2012 at 02:10 PM.
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Mar 09, 2012, 07:25 PM
Registered User

This is with a 1100kv motor and 10 more amps.
Mar 09, 2012, 08:31 PM
Lost but making good time

Try 61.7 tip speed....46 thrust...35 amps...1000 Kv ...2.1/1 thrust


Last edited by hankg; Mar 09, 2012 at 09:02 PM.
Mar 09, 2012, 09:02 PM
Registered User
I don't have or know of any 1000kv motors though. The test with a 980 and 1100 are motors that are readily available.

*Edit* I also can't find any 1000kv motors in a 70g range that can handle 35A.
Last edited by itsmillertime; Mar 09, 2012 at 09:13 PM.
Mar 09, 2012, 09:14 PM
Lost but making good time

I know I may catch it from electronics experts when I say this but I think given two motors, turning the same props, the lower Kv motor will give you the same torque with less amp draw. The difference in amps will be proportional to the difference in Kv...i.e. a 500 Kv motor vs a 1000Kv motor will have a higher amp difference than a 900 Kv motor and a 1000Kv motor, everything else being the same. If this is in fact true, I would go with the 980 because of efficiency.



Post Script...Have you looked at for motors?
Last edited by hankg; Mar 09, 2012 at 10:05 PM.
Mar 10, 2012, 03:04 AM
Shore Huckin'
KE Spin's Avatar
Well, I'm not an electronics expert but let's look at couple things.

For our example let's say that we have two motors that are the same size but wound to differing Kv ratings.

The first motor is wound to a lower Kv rating that will give less rpm but deliver higher torque so it can turn a larger diameter prop.

The other motor is wound to a higher Kv so it will deliver higher rpm but needs a smaller diameter prop to do it.

In 3d flying you may want to use a larger prop to keep the most air flowing over the control surfaces so the lower Kv motor may be the ticket.

However, you may want to mix in some pattern flying with your 3d so you may want more speed and quicker prop response so the higher Kv motor may fit these requirements.

When sizing a motor to your plane you need to consider the power that it can supply, how much the motor weighs and the size prop you would like to use or that is needed.

1. You want your motor to supply thrust that is 1.5 to 2 times the all up weight (AUW) of the plane.
Calculation: AUW (oz) x 1.5 or 2 = Thrust (oz) that the motor needs to deliver.

2. Or, when the motor specs list power in watts then you are looking for 150 to 200 watts per pound of plane.
Calculation: Motor Spec (Watts) divided by the AUW (lbs) = Watts per pound

If you follow rules 1 and/or 2 then the motor weight will generally be correct as they are designed with these numbers in mind.

Now that you have determined some basics regarding the size motor that you need, you will have to decide on the prop you want to use or that is supported by that motor.
Calculation: Wingspan (Inches) divided by 4 = min prop diameter
Check the motor specs to see if they list a prop in that diameter. If so then that is likely the motor you want.

However, you may want to use a larger diameter prop to maximize airflow and make hovering a little easier so then you will likely need a lower Kv motor in the same size range. Verify the larger prop diameter that you would like against specs and select the motor that meets your requirements....try not to go up in motor weight just to gain a larger prop..this can result in a nose heavy plane with cg issues.

Yes, it takes some calculations and a little research but becomes easy after you have done it a couple times.
Last edited by KE Spin; Mar 10, 2012 at 03:16 AM.
Mar 10, 2012, 06:04 AM
Lost but making good time
Sound advice...well thought out.


Mar 10, 2012, 10:05 AM
Registered User
Agree with KE Spin... if you can't find the motor kV you need, you can go a bit higher kV and go one size smaller prop (or go a bit lower kV and go one size larger prop).

With regards to finding a 70 gram motor that handles 35 amps... a rough rule of thumb is for each gram of motor you can safely run 3 watts of continuous power or 4.5 watts for a 60 second burst. These are non-scientific ideas I have seen people post based on their experiences. For example, if I run my 37 gram motor at 13 amps (140 watts or 4 watts per gram), it gets too hot to touch.

Personally, I would not trust an ad for a 70 gram motor with a 35 amp rating. Make sure you Google the motor and find some reviewers who confirm the motor takes that much power without failing.

Overall, your numbers look good... 150 - 200 watts per pound of 3D airplane. This should let you hover and start vertical climb at about 1/2 throttle.
Mar 10, 2012, 02:07 PM
Shore Huckin'
KE Spin's Avatar
Thanks Major and Hank.

Here is a little more info to use when sizing an electric motor for a 3d plane: The weight of the motor should be about 1/10 the AUW of the plane.
Calculation: AUW (oz) divided by 10 = Motor Weight (oz)
Then: Multiply the motor weight (oz) by 28.2 = motor weight in grams

Here is a link to a motor that I use and like in planes of this size:
Super Tigre .10 (69g):

The motors that you have listed all look pretty good...just watch the specs and select based on your prop preference and what props are recommended for use with the motor.

Enjoy your flying!
Last edited by KE Spin; Mar 10, 2012 at 02:27 PM.
Mar 10, 2012, 02:35 PM
Registered User
Here's the WebOcalc for the Super Tigre .10:

I'm waiting on Twisted Hobbys to get back to me on how they managed to get 443 watts from a 70g motor without frying it. ( That's over 6 watts per gram, double what is considered a safe PEAK watts per pound. If it's indeed made by intergalactic aliens with superior technology then I'll get it. If not, that Tigre .10 is looking to be the best motor in the 70g range I can find.
Mar 10, 2012, 02:38 PM
Registered User
If you notice though. I increased my auw to 28oz from 26 and went down to a 10x5 prop from a 11x5.5
Mar 10, 2012, 03:04 PM
Shore Huckin'
KE Spin's Avatar
The props listed are the APC TE. The TE stands for thin electric. You may want to try APC SF (slow flyer props)...I'm currently using a 10 x 7 apc sf my 39" Wingspan Hokusei (Skywing) Sbach. The slow flyer props are lighter and require less torque than the thin electrics so they don't eat as much power or build up quite as much heat.

It was windy the last time I flew it so I wanted the 7 pitch for a little extra punch. On calmer days I might try a 10 x 4.7 or 10 x 6. Only had a couple flights on the little Sbach so I also want to try 11 x 4.7 slow fly prop as well.

Yeah, the Telink wattage looks impressive but I'm a little skeptical of the numbers posted. I should get one and give it a try.
Mar 10, 2012, 03:26 PM
Registered User
I thought the slow fly props were less efficient and mostly used when you can't fit a larger diameter prop. Say if a 11x5.5 was the recommended prop but you can only fit a 9" then the 9x4.7 would produce nearly the same thrust but at a higher amp draw.
Mar 10, 2012, 05:56 PM
Shore Huckin'
KE Spin's Avatar
Here are some pros and cons of slow flyer props:

Maximize thrust
Lighter than other types of props
Can allow more watts from motor

Less rigid so model weight can be a factor
Sacrifices speed

I like sf props on my 3d planes that are in the 16 to 30 oz weight range but I move to thin electric props on 3d models over 30 oz.
Mar 10, 2012, 06:02 PM
Registered User
It's too bad no slow fly props are in WebOcalc...

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