My Motor Analysis - Am I Doing This Right? - RC Groups
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Mar 16, 2012, 10:56 PM
Crash Test Engineer

My Motor Analysis - Am I Doing This Right?

Hello All,

I am completing a scratch build on a fairly large $Tree Foamie from the video series Ed at Experimental Airlines has on You Tube (and here on RCG too). I am trying to figure out the best motor for the plane between the two that I have.

The two motors are the Grayson Hobbies Microjet V3 and a Turnigy Aerodrive 3530-1100. Due to the fact the plane is a pusher plane, I can only get a 7 inch prop on it (without putting the motor up quite bit over the wing line).

From my analysis (see below) the Micro jet motor seems to be the best (only) choice for the plane. Please take a look at the approach (in the pdf) and let me know if I am on the right track. Any other considerations you recommend would be appreciated greatly as I am a newbie and this is only the 3rd plane I have built (with the other two not lasting much longer than 20 seconds (see video link for a laugh at my newbie flight achivements )

The plane is a high wing type with a 60" wing span and a 42" fuselage. The tail feathers are a little larger than Ed's plan.

Thanks in advance!

RC Dad

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Mar 16, 2012, 11:28 PM
Registered User
Dr Kiwi's Avatar
That analysis for the MicroJet seems good... except that I cannot believe the figures for aircraft weight: I really cannot imagine how anyone can build a 60" span plane with a dry weight of only 20oz!!!!!!!

What do you estimate the AUW of the plane will come out at... with a complete power system in it.... maybe 3lb-4lb??? You need to think in terms of ~100W/lb... so maybe 300W-400W.

A 3530-1100 won't get you enough power with only a 7" prop.

A Microjet v3 won't survive a 7" prop on 3s (it draws 20A/210W with only a 6x4)... and I can't imagine that it would have enough grunt for a 60" span plane anyway. If you limit the MicroJet to the quoted 170W you'll be running a 5" prop and getting only about 1lb of thrust.

I have just tested a Grayson Hobby MonsterJet P21 (96g, ~ 2235Kv) ... now this is probably too high a Kv for your purpose, but it will give you an idea.....with a 7x5 APC E that gets you this:

10.4v, 43.70A, 456W, 16050rpm, 76.0mph, 1208g, 42.54oz, 2.65g/W

That might be too much, but use a 6x4 APC E and it gets you this:

11.1v, 25.60A, 280W, 20040rpm, 75.9mph, 794g, 27.96oz, 2.84g/W
Last edited by Dr Kiwi; Mar 16, 2012 at 11:39 PM.
Mar 16, 2012, 11:41 PM
Crash Test Engineer
Thanks for the response. I was not clear in the first post, in that with the Grayson Microjet, I would only use a 6X4 prop. The 7 inch would be on the Aerodrive. I know the right answer is to put a 3 bladed prop on the aerodrive, but the local hobby store did not have the right size and am trying to get out and fly in this great 70 degree weather (unusual for March in MN).

I will check the weights again on the scale, but that is the weights it gave me. The plane's fuse is a hollow tube so not much weight there. I think the whole plane took less than 3 sheets of the $ Tree foam board. However, given the size of the span, it may have been resting off the scale (i used the local post office scale).
Mar 21, 2012, 08:34 PM
Crash Test Engineer
Ok, I confirmed the weights. In fact, the new scale actually shows the plane coming in slightly less at 2.12 pounds flight weight (full motor train & battery).

I have come up with the following two motor options. Would appreciate advice on which would be better - either from pure motor specs or from your experience with quality, etc.. Here are the two motors I think will work well:

1. Heads Up RC - 3536-16 1600 KV

500 watts and 36 oz of thrust with 7x6.

2. BP A2814-6 1410 KV

440 Watts, don't have thrust estimate for a 7x6, but makes 37.14 oz with a 9X6.

My thoughts is to go with the Heads Up motor as it may even work with a 6 inch prop. I will see if I can find any test figures for this motor and prop.

Any more guidance and advice you guys can provide would be greatly appreciated.

On a side note, we are going to use the Turnigy Sk3530-1100 mentioned above on a Super Cub upgrade to brushless. Think that will be a perfect motor for the plane.

RC Dad
Mar 21, 2012, 08:48 PM
Registered User
Dr Kiwi's Avatar
The Heads Up motor is decent choice, but with a 6" prop on 3s is likely to draw not much more than 20A... I doubt that will be enough power for a 2lb+ plane. You could go to 4s if you must use a prop as small as 6".
Mar 23, 2012, 11:07 PM
Crash Test Engineer
Thank you sir. I am going to see if I can make a 7x6 work if not, I will try the 4S option. Your assistance is much appreciated.

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