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Old Jan 16, 2012, 06:13 PM
Feeling the THRALL
Jackson Stone's Avatar
USA, CO, Parker
Joined Dec 2009
815 Posts
Hi Tomy. I think my comments were misunderstood. I didn't mean to suggest that putting lots of power on the Minimoa airframe meant the pilot had crappy flying skills, only that it was not necessary to put lots of power on. I have thoroughly documented my mod in the thread with photos already, just look it up. I do stand by the comment that the Minimoa is a plane that requires a certain finesse to fly well. I like having an electric motor to get this plane in the air and put it into lift, but after that I am interested in flying it as a sailplane, not a sport plane. All of this is a matter of personal preference, so if your objective is different then go for it dude!

Cheers, Jackson
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Old Jan 16, 2012, 07:04 PM
low tech high tech
vtdiy's Avatar
Southern Vermont
Joined Feb 2007
3,028 Posts
As I said earlier, one re-powering mount that I like because of its simplicity was the one by vasp in the video in post #736 in this thread.

What he did was mount a motor backwards (can facing aft) behind a bulkhead he glued to the front of the nose. The shaft extends out the rear of the motor. This rig has a great set of advantages:

1.) simplicity
2.) no cutting of the plane -- only adding a bulkhead.
3 .) easy access to the motor -- easy to remove or replace
4. ) motor cooling -- motor is in the breeze
5.) moves the motor weight forward, reducing the need for ballast and therefore reducing AUW.

The HobbyKing reviews of the particular motor used were unfortunately bad, but the owner seemed to have good luck with it.

I searched for an 850 kV motor and found a 2830-09 at HeadsupRC with a similar mount which looks like it may possibly fit in the nose. It's well recommended for 11" props, and the outfit is a trustworthy supplier.

Specs:

http://www.headsuprc.com/servlet/the....8oz%2C/Detail

No guarantees about fit, but I'd consider this one myself with a little more examination of the nose of the Minimoa and info about the cross-mount from the supplier. It would definitely put more power into the plane.
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Old Jan 16, 2012, 07:34 PM
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FlyWW1's Avatar
United States, VA, Winchester
Joined Nov 2010
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Here are some more options out of Leaderhobby's LA warehouse. So you can get them cheap and fast. They are the same motor that headsuprc sells just cheaper. I have 3 motors from Leader already and plan on buying all my motors from them from now on. The 2836 will of coarse need a modified mount but the 2830 is probably possible to use with the stock mount with a little creative tweaking.

http://leaderhobby.com/product.asp?I...48&Location=LA
http://leaderhobby.com/product.asp?I...49&Location=LA
http://leaderhobby.com/product.asp?I...58&Location=LA
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Old Jan 16, 2012, 09:16 PM
low tech high tech
vtdiy's Avatar
Southern Vermont
Joined Feb 2007
3,028 Posts
I was looking at the leader hobby specs myself before you wrote. But the EMP 2830s have really high resistance -- when plugged into motocalc they don't produce anything like the thrust figures HeadsupRC shows, and they also exhibit very poor efficiency. Their projected performance on an 11x7 was worse than my standard Minimoa motor performance on 2C and 11x7 prop!

Unfortunately the HeadsupRC doesn't list resistance, just thrust test values. Either the EMP resistance figures are way off, or the HURC motors are not the same.

On the other hand, the EMP 2836, which I also checked, seems to have one third of the shorter motor's resistance, and so checks out in motocalc as similar to the performance of the 2830 that HURC shows. So if it was me, I'd go with this one.

http://leaderhobby.com/product.asp?I...61&Location=LA
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Old Jan 16, 2012, 09:54 PM
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United States, VA, Winchester
Joined Nov 2010
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Yeah I'm not sure how much stock to put into their resistance numbers. They seem to be all over the place and sometimes the same motor just in different warehouses have different numbers. LOL. I just checked a motor I bought for my Rare Bear and it's number was 175! My Rare Bear does 111mph with a 6x5.5prop and the motor comes down cool. So who knows. Honestly I've never even noticed the resistance number. I've always just gone by KV, Watts, Amps, and prop size if they offer it. Maybe it's not the right way but it's always worked out great for me.
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Old Jan 16, 2012, 11:32 PM
Feeling the THRALL
Jackson Stone's Avatar
USA, CO, Parker
Joined Dec 2009
815 Posts
Quote:
Originally Posted by vtdiy View Post
As I said earlier, one re-powering mount that I like because of its simplicity was the one by vasp in the video in post #736 in this thread.

What he did was mount a motor backwards (can facing aft) behind a bulkhead he glued to the front of the nose. The shaft extends out the rear of the motor. This rig has a great set of advantages:

1.) simplicity
2.) no cutting of the plane -- only adding a bulkhead.
3 .) easy access to the motor -- easy to remove or replace
4. ) motor cooling -- motor is in the breeze
5.) moves the motor weight forward, reducing the need for ballast and therefore reducing AUW.

The HobbyKing reviews of the particular motor used were unfortunately bad, but the owner seemed to have good luck with it.

I searched for an 850 kV motor and found a 2830-09 at HeadsupRC with a similar mount which looks like it may possibly fit in the nose. It's well recommended for 11" props, and the outfit is a trustworthy supplier.

Specs:

http://www.headsuprc.com/servlet/the....8oz%2C/Detail

No guarantees about fit, but I'd consider this one myself with a little more examination of the nose of the Minimoa and info about the cross-mount from the supplier. It would definitely put more power into the plane.
Vermont, this is almost exactly what I did to install the motor, and I am using the HeadsUpRc motor you refer to. You can find the details and photos in my post 1363. I checked the post and found that it was drawing 145 watts before changing back to the original 11x8 inch props supplied by Art Tech. With the Art Tech props I believe I am pulling closer to 200 watts. It is late here in Colorado and I don't want to disturb the household by testing the motor, but I will test next time I am getting ready to fly and report the watt and amp draw at full power. BTW, the motor and ESC get warm if I use full power alot, however this setup is much cooler than the original motor (which melted the red plastic motor mount on my Minimoa).

From my experience flying with this setup (about 20 flights to date) I can easily fly 13-15 minutes on a 11.1 2200 mAh battery with lots of motor usage. The plane climbs out with good speed at about a 15 degree angle. This seems appropriate to me, and the time to climb to a good altitude is quite reasonable. As a counterpoint, I have a hotliner from HobbyKing called a GL-Speedy. Haven't built it yet, but am planning on using a 500 watt motor w 1500 kv driving a 7x6 prop on 4S. This will surely give me vertical climb ability and lots of speed, which is appropriate for a hotliner. For the Minimoa, however, I think a solid 15 degree climb fits the character of this classic glider. I suspect Martin Schempp and Wolf Hirth, who designed the Göppingen Gö 3 Minimoa in 1934 and went on to design many famous high-performance gliders, would concur with this assessment. A stately classic airframe deserves to be flown in a stately manner IMHO.

Just as a comparison, I recall that when flying full-size gliders we thought a climb rate on tow of 500 feet per minute was excellent. As a comparison, in a strong thermal I have pegged the variometer at over 1200 feet per minute climb rate. Since the Art Tech Minimoa is a 8.5 to 1 scale (the original has a wingspan of 17 meters), this means a scale climb rate simulating being on tow would be 58 feet per minute. I suspect my actual climb rate is four to five times that. Frankly, at a scale climb rate the Minimoa model would look pretty anemic. So, I think the climb rate I achieve with the current setup is a reasonable compromise between performance that is scale-like and performance that allows me to quickly climb and set up for my next ridge pass.

Cheers, Jackson
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Old Jan 17, 2012, 08:27 AM
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vtdiy's Avatar
Southern Vermont
Joined Feb 2007
3,028 Posts
Great info, Jackson, as always. Enjoy your posts!

FlyWW1, the resistance determines how much current, and so power, the motor will draw. The higher the resistance the lower the power for a specific propeller. It isn't the only factor, just as kV isn't the only factor as well.Motocalc requires 3 factors to determine motor performance,kV, idle current, and resistance.

Two equivalent can size motors (say 28mm. as in a "2830) with equivalent kV using the same prop and the same number of cells can produce very different RPM, thrust and amperage draw if their resistance is different.

As I said, if the LeaderHobby EMP 2830s resistance is accurately presented in the specs of around 300 milliohms, the performance in my motocalc model shows it less efficient and lower performance than the stock motor on the Minimoa.

I don't know if their specs for these are a mistake, but neither does anyone else unless they buy and test one. I'd be very glad to hear otherwise, but at present there is no reason for me to create different specs than the ones they publish. If you buy one and test it, please let us know what the actual performance is. It would be a good economical alternative.

As I said their specs for the 2836 are good, and it is the same price. The slight extra weight in a backwards nose mount is actually an advantage and the length is no problem.
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Old Jan 17, 2012, 08:28 PM
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Tarasdad's Avatar
USA, CA, California City
Joined Jul 2009
164 Posts
How would these do as replacements? Reverse mounted, of course.

http://www.hobbypartz.com/86ma70-2215-800kv.html
http://www.hobbypartz.com/86ma13-2220-840kv.html
http://www.hobbypartz.com/86ma73-2220-890kv.html
http://www.hobbypartz.com/86ma74-2220-910kv.html

Still haven't flown my Minimoa yet but want to be prepared when the inevitable happens. I've shortened one set of prop blades by 1" per side (8.5" dia.), added vent holes to the spinner backplate, enlarged the prop slots on the spinner and cut an air exit hole on the bottom of the fuselage behind the wheel but still don't trust h stock electronics. The batteries I got for it are 3s 2200mAh LiPo but I didn't notice when I got them that they're 40C rated. 'Sokay, I can always find a use for them somewhere if they don't work for this.
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Old Jan 17, 2012, 11:20 PM
low tech high tech
vtdiy's Avatar
Southern Vermont
Joined Feb 2007
3,028 Posts
I like the first one the best, in Motocalc using the model I have so far, I get 105 watts on 3 cells WOT with the stock 11x7 and about 28 watts waste heat.

The rest don't do as well, and have higher resistance.

The Leader Hobby 2836 referenced earlier is hotter, more efficient and cheaper.

But a cut down prop will reduce performance with any of these to a point where the plane probably wouldn't fly at all. Low kV motors need big props.

Suggestion, fly with the stock motor and stock 11" prop and limit your throttle so you're putting out 8 amps from the battery. You can't burn out the motor mount with that, and you don't have to buy anything. Then if you don't like the performance, buy a replacement motor.


Also there's nothing wrong with 40C batteries! The Minimoa just doesn't need their added capacity to supply current since you are using less than 10C with the stock motor if you limit throttle within reason. 10C is 22 amps!

But just because a battery CAN supply 40C (88 amps in this case) doesn't mean it WILL supply it. It is just capacity. You're fine. You just have a high quality battery.
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Old Jan 17, 2012, 11:20 PM
Feeling the THRALL
Jackson Stone's Avatar
USA, CO, Parker
Joined Dec 2009
815 Posts
Minimoa Motor Mod Amp and Watt Draw

Vermont,

Checked the Minimoa watt and amp draw at full throttle with the HeadsUpRC 28-36 900 kv motor. She is drawing 13.5 amps and producing 160 watts using the original Art-Tech blades on a metal spinner. I think the blades have been determined to be 11x8.

As mentioned in my previous post, I find the aircraft to be quite capable at 160 watts. I can understand the desire of some Minimoa owners for more power in order to get to altitude more quickly. However the trade off is shorter flight duration. You may recall that I am flying at 6300 feet of elevation here in Colorado. At sea level I believe this setup would yield 20% greater effective thrust due to greater air density.

I am looking forward to warm Spring days when the thermals are popping and I can use the electric motor to sniff out some lift and hitch a ride to cloud base.

Best wishes from Colorado,

Jackson
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Old Jan 18, 2012, 01:39 AM
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Česká republika, Hlavní město Praha, Praha
Joined Nov 2011
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Quote:
Originally Posted by Tarasdad View Post
Still haven't flown my Minimoa yet but want to be prepared when the inevitable happens. I've shortened one set of prop blades by 1" per side (8.5" dia.), added vent holes to the spinner backplate, enlarged the prop slots on the spinner and cut an air exit hole on the bottom of the fuselage behind the wheel but still don't trust h stock electronics. The batteries I got for it are 3s 2200mAh LiPo but I didn't notice when I got them that they're 40C rated. 'Sokay, I can always find a use for them somewhere if they don't work for this.
The plane (current HK version at least) flies out of the box as it has 200 Watt motor. It's not an hotliner but it climbs on its own power, which is the only thing one should expect from a scale glider. There are absolutely no overheating issues (flown 8 times with quite hard use of throttle). The temperature inside the fusselage without any aditionaly made holes did not exceed 28deg C (outside temp 4deg C), measured during the flight. The fusselage was quite full of the battery, inline current sensor, quantum telemetry tx...)

I was as well ready to mod the plane, but it's definitely not broken (yet).

The only thing that distrubs me is the non existant downthrust. The violent pitching under WOT together with the limited elevator authority was something new to me. On the few first takeoffs where I pushed the stick later after applying wot and throwing the plane I was quite near to a disaster as the plane shoot up reaching almost 90deg of pitch and the response to the elevator was really sluggish. I'm now overcoming this with a throttle to elevator mix but I still don't like it...
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Last edited by amalka; Jan 18, 2012 at 01:46 AM.
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Old Jan 18, 2012, 07:53 AM
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Česká republika, Hlavní město Praha, Praha
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EDIT: So at WOT in flight the max. reached current was 14 amps (on 3S). Is it possible, that static WOT drains more than WOT when in flight? Or did I made some mistake when measuring?
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Old Jan 18, 2012, 08:51 AM
low tech high tech
vtdiy's Avatar
Southern Vermont
Joined Feb 2007
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Quote:
Originally Posted by amalka View Post
EDIT: So at WOT in flight the max. reached current was 14 amps (on 3S). Is it possible, that static WOT drains more than WOT when in flight? Or did I made some mistake when measuring?
Yes.

Also,a 24C degree temperature rise at 4C won't melt plastic. At an ambient 30 C with plenty of foam insulation lower cooling temp differential, and no vents it may be a different matter.

60-65 degrees C is well within the softening and melting range of most plastics. Even softening will enlarge mounting holes. Let us know how your plane does at WOT in summer after sitting on the ground in the sun.

I'm not sure there is, or is not, a "new" motor, and my HK Minimoa was purchased in December. I'm pretty sure you aren't using the 11 inch prop, correct?
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Old Jan 18, 2012, 09:33 AM
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Quote:
Originally Posted by vtdiy View Post
I'm pretty sure you aren't using the 11 inch prop, correct?
Correct. Mine was supplied with 9x5. So far happy with it.
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Old Jan 18, 2012, 04:27 PM
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vtdiy's Avatar
Southern Vermont
Joined Feb 2007
3,028 Posts
w/ 9x5 and your earlier reported 17 amps, looks like about 39 watts of waste heat -- maybe with a little venting it will be okay in hot weather, too.
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