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Old Mar 18, 2006, 08:05 AM
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Quote:
Originally Posted by Treetop
...

Additionally using the motor calculator at Diversity:

http://brantuas.com/ezcalc/dma1.asp
...
Discuss.

tt
Hi guys,

Lets stick with reality rather than using motor calculators which are even worse than MotoCalc.

Jürgen
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Old Mar 18, 2006, 11:11 AM
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Quote:
Originally Posted by Jurgen Heilig
You still don't get it.
This has been gone over before, I don't think I'm the one who still doesn't get it.

Tree Top, yes indeed, if you run a cheap can motor on a relatively high voltage (3S lipo or more) and gear it down to spin a prop chosen to match the airframe, you can sometimes get a respectable 65% - 75% efficiency out of it - Vintage1's thread "Can motors on 3S lipo" that I linked to earlier is entirely about this concept.

At that point most inexpensive brushless motors do not give you much efficiency gain: what they do often give you is reduced weight, and increased life.

It is interesting, too, where that extra bit of efficiency and weight savings comes from: much of it is due to the better magnets in most brushless motors (rare earth, vs cheap ferrite magnets in can motors), some of it is due to ball bearings rather than bronze bushings, and only a little comes from reduced motor resistance and friction losses due to eliminating the actual brushes. This, of course, is why the old Astro and similar brushed motors with ball-bearings and cobalt magnets were about as efficient as many of todays crop of cheap outrunners.

My 5.1-oz (145 gm) EPP mini flying wing (a Little Scrappy) was built using the concepts Vintage1 sketched out in his thread. I used a GWS IPS motor in an S2 ratio IPS gearbox, run on a 3S lipo pack. Prop and gearing were selected to keep current around the 2 A mark, as much as the IPS motor can handle for decent motor life, and to match the pitch speed to the wing. The motor had a heatsink on it to help keep it cool.

The result was an inexpensive little wing that had unlimited vertical with a cheap can motor, and all the speed necessary for continuous loops, rolls, inverted flight and what have you. (Since there was no rudder, the wing would eventually fall off to one side or the other on long vertical uplines - but there was enough power and thrust to sustain vertical.) The motor died eventually after many hours in the air, as all IPS motors do. It probably didn't help that my ESC malfunctioned and locked up at full throttle, and I had to fly the little wing for about 15 minutes at full power until the battery finally hit cut-off and I could land it - the motor died not long after that incident.

-Flieslikeabeagle
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Old Mar 18, 2006, 11:59 AM
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flieslikeabeagl

That funny thing once occured to me also. I was flying the MPX Twinjet with dual Speed 400 motors when the Esc decided to lock on full speed. Not funny to fly a Twinjet on full throttle for 15 minutes.
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Old Mar 18, 2006, 09:53 PM
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Quote:
Originally Posted by iceman3
Not funny to fly a Twinjet on full throttle for 15 minutes.
I'll say!

I was with two flying buddies when my wing's ESC went nuts. We passed the Tx back and forth a couple of times while I waited for the battery to dicharge far enough for the motor to cut off.

-Flieslikeabeagle
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Old Mar 19, 2006, 10:37 PM
Hold my beer and watch this!
Jeremy Z's Avatar
Northern IL
Joined Oct 2005
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Photos of my brushless mod

Sorry to link to another forum, but I don't want to invest the time to attach the pictures all over again. (I'm dial-up crippled)

http://www.wattflyer.com/forums/show...ed=1#post54282

This thing flies like there's no tomorrow. It is faster at half throttle than the brushed version was at full throttle. Result? I fly at 1/2 throttle most of the time.

My maiden flight was 31 minutes on the 8x3600 mAh sub-C NiMH pack. It is VERY smooth, and the control surfaces do a lot more work with the higher speed and more prop wash. I have the low rates set to 50% on my Tx now, and that is plenty.

The spoilerons are a thing of beauty to get this fast, long-gliding plane down in a reasonable length of field. (much better than flaperons; try it!) They slow it down, and give it a bit of nose-up, instead of making it dive.

I'll post video when I get either a pinch-hitter pilot (whom I trust) or a competent videographer.

It is just amazing how much better this plane becomes with more power. (especially the thrust) I can now climb out without so much planning.

Cheers!
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Old Mar 19, 2006, 11:34 PM
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Quote:
Originally Posted by Jeremy Z
...

This thing flies like there's no tomorrow. It is faster at half throttle than the brushed version was at full throttle. Result? I fly at 1/2 throttle most of the time.

My maiden flight was 31 minutes on the 8x3600 mAh sub-C NiMH pack.
...
It is just amazing how much better this plane becomes with more power. (especially the thrust) I can now climb out without so much planning.

Cheers!
Hi Jeremy,

Are you sure that you are not exaggerating a bit here?

Your average amps at your maiden were approx. 7A (for both motors) at your maiden, provided that you flew your pack until it was empty.

Faster at half throttle than with the brushed motors at full throttle? Brushless motors are more efficient - but not that much more.

But the main thing is ... that you enjoy your model. Wtach out if your wings are bending. You use a very heavy battery pack, and you have weakend your wing by putting the ESC in a high stress area.

Jürgen
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Old Mar 20, 2006, 12:10 AM
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In the MiniMag thread, you can find video clips of the model with standard Permax 400 and Align 400LF brushless. Judge for yourself.

The MiniMag is similar to the TwinStar II, but only half the weight - and half the power.

Jürgen
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Old Mar 20, 2006, 01:35 AM
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Quote:
Originally Posted by Jurgen Heilig
Hi Jeremy,
Are you sure that you are not exaggerating a bit here?
Great, so anyone who has an opinion that differs from yours must be exaggerating?
Quote:
Originally Posted by Jurgen Heilig
Faster at half throttle than with the brushed motors at full throttle? Brushless motors are more efficient - but not that much more.
It's not hard to be much more efficient than a 45% efficient motor driving a tiny, inefficient prop at high rpm. I've been saying this for quite a long time on this very thread. Try running some figures through MotoCalc yourself - you might learn something from the exercise.

Since the carbon tube spar carries most of the loads on the wing, I have my doubts that carving those small cavities for the ESC's has done anything to perceptibly diminish the wings strength.

-Flieslikeabeagle
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Old Mar 20, 2006, 03:35 AM
SlopeHead
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The carbon tube spar DOES not carry the load, the Elapor around it does. I is possible to damage the wings with high G forces. I've managed (on the so inefficient stock setup that doesn't even fly) to bend the wings at the end of the spar boxes, and landed with the carbon spar protruding out of both wings.

This just goes to show the quality of information available to people who keep making comments without having flown the plane.


Congratulations on your maiden, Jeremy!
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Old Mar 20, 2006, 08:05 AM
Ahhgh! Not ANOTHER new plane!
Robert Stinson's Avatar
San Diego, CA
Joined Dec 2001
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Man, you guys are really contentious! I'm un-subscribing to this thread! Chill out, it's just a hobby.
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Old Mar 20, 2006, 08:47 AM
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Joined Dec 2005
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Hi everybody!!! I got my MPX Twinstar II after flying it for about 3 months on the AFPD and after reading (a lot of this thread) and plawner's review. Just one thing I think is missing from this beautiful model. Landing gear! I have already created the front gear but I am having troubles puting the back ones. I am using landing gear from an electirc Piper which has the perfect size and it is not heavy. I thought of glueing a piece of wood inside the fuselage and puting screws to tighten the landing gear. The fuselage is still open because eventhough it would be easy to put the first screw when i glue the two pieces of the fuselage together I would not have access to the woodpiece. Any ideas? I have no experience in model building so photos would be most welcome...
Thanx in advance,
Dionysios
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Old Mar 20, 2006, 12:24 PM
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Dionysious - congratulations!

Have you considered glueing the piece of wood on the underside of the fuselage (rather than inside the model)? If you use fairly thin ply (1/8" or 3mm or thereabouts) it would not be hard to recess the foam slightly so the wood would be flush with the surface. The wood would accept the screws that hold your landing gear, and spread the load from them over the entire area that is glued to the foam. I suggest the wood piece be several inches long in the front-back direction, as that is the direction in which the biggest shock loads (from landing) will be.

Earlier in this thread, Franck constructed a very ingenious landing gear setup using carbon fibre rods, eventually covered with Depron fairings.

-Flieslikeabeagle
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Old Mar 20, 2006, 12:36 PM
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Quote:
Originally Posted by flieslikeabeagl
Great, so anyone who has an opinion that differs from yours must be exaggerating?
...
-Flieslikeabeagle
I am not talking about opinions, but facts:

Have a look at this post:

http://www.rcgroups.com/forums/showp...&postcount=220

Flight time of the stock TS II with a 2s1p Kokam 3200mAh was 20 minutes which equals to an average current of just 4.5A per motor.

Assuming an average voltage of 7V, I was flying the TS II on 7Vx9A=54W

Jeremys brushless version flew for 31 minutes on an 8 cell 3600mAh pack, which equals to an average current of 3.5A per motor.

Assuming an average voltage of 8.8V, he was flying the TS II on 8.8x7A=61.6W

Despite using brushless power, he actually consumed more battery power per flight minute (Don't forget that my model is a lot lighter).

We measured the Top Speed of a TwinStar II on 8 cells at 79km/h in horizontal flight with a radar gun. That's close to 50mph.

I measured 8.8V - 25.6A - 14520rpm on the stock props. That equals a pitch speed of 26.62m/s or 59.6mph (static). That is a pretty nice pitch speed to achieve approx. 50mph in flight.

Jeremy used two brushless outrunners and APC 7x5" props. Using the data found at www.allerc.com:

8.82V - 24.6A - 10560rpm. That equals a pitch speed of 22m/s or 49.2mph (static).

There is no way you can fly approx. 50 mph at half throttle with this setup.

Jürgen
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Old Mar 20, 2006, 12:39 PM
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Quote:
Originally Posted by jcosta
The carbon tube spar DOES not carry the load, the Elapor around it does.
...
Yes and No.

First of all, MPX does not use carbon tubes. It is just black GRP.

The spar carries most of the load, provided that the spar covers are properly glued into position.

Jürgen
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Old Mar 20, 2006, 01:05 PM
SlopeHead
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Lisboa, Portugal
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Quote:
Originally Posted by Jurgen Heilig
Yes and No.

First of all, MPX does not use carbon tubes. It is just black GRP.

The spar carries most of the load, provided that the spar covers are properly glued into position.

Jürgen
Sorry, you are absolutely right and I know it's GRP, but was mislead by the original post I quoted.
In my case, after a full throttle bunt, the tips of the spar pierced the spar covers... It was easily solved with some CA and some fiber tape, but I'll never do a full throttle bunt again...
If it weren't for the convenience of transport I'd glue both wings and the spar together...
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