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Old Mar 08, 2006, 11:26 PM
Got shenpa?
flieslikeabeagle's Avatar
Los Angeles
Joined May 2004
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Quote:
Originally Posted by jcosta
I paid $30 for a 3000 NiMh 8 cell pack. It weights 170 grams more than the Kokam 3200 MAh pack.
The lipo pack I'm using (Apex 2000 mAh 3S, 10C) weighs right about 170 gm, and cost $33. So the NiMH pack weighs twice as much, and costs about the same. I believe you referred to lipo packs costing two to three times as much as lipo packs in your earlier post - here the difference is more like 10%.
Quote:
Originally Posted by jcosta
So you keep talking about something you don't know except from theory
Hardly. I have experiencing flying a model or two with direct drive Speed 400 motors (including the EasyStar) and have had a chance to see how dismally they perform. I have chosen powerplants for several of my models using the exact same calculations that led me to the 400XT motors for the Twinstar, and they all fly pretty much as I had expected them to.

Since Multiplex models obey the same equations of physics as other models, I have no reason to expect the Twinstar will be any different.

Quote:
Originally Posted by jcosta
My Twinstar 2 has, in addition to the stock motors and props, a $35 37A Esc plus the above mentioned $30 3000MAh 8 cell pack. Are you telling me that between Lipos, brushless motors and 2 ESCs you spent less than $130???
Not less, but only an insignificant $3 more. I bought two Esskay 400XT motors on sale from Hobby Lobby at $20 each ($40 total). I bought two Thunderbird 18 ESC's on sale from Todds Models at $30 each ($60 total). I bought my Apex lipo pack on sale at $33 from GWS Expert. Total cost for two motors, two ESC's, and 1 lipo pack was $133.

For the extra $3, I get an efficiency increase from the stock 45% to maybe 75%, the use of bigger and more efficient props, and a pitch speed better matched to the model.

Currently the 400XT motors are going for $25 each. TB 18's often sell for $35 each. Bought at these prices, the whole caboodle would cost about $153 - perhaps 20 bucks more than the heavy, inefficient direct drive Speed 400/NiMH setup.

For what it's worth, I already had the battery as I'd been using it on other models. So my cash outlay was about $100 total.

In the interest of full disclosure, I have previously bought Esskay 400XT motors at $30 each from the same source. I no longer remember which ones I installed in the Twinstar wings - the ones I paid $20 for, or the ones I paid $30 for.
Quote:
Originally Posted by jcosta
So you are still talking about theoretical calculations and not reality.
Most of Einsteins work wasn't experimentally proved till after his death. Should we discredit all the work he did during his lifetime because it was "only" theory?

I'm no Einstein, but the simple classical mechanics equations being used to study how these little RC planes fly is not General Relativity, either. And Newtonian mechanics is a theory that has proven to be rock solid until you're dealing with speeds comparable to the speed of light.
Quote:
Originally Posted by jcosta
I would suggest until you get real experience instead of theories based on a motor manufacturer's publicity, that you refrain from mocking others who do have practical experience.
I don't think I'm the one doing the mocking. And, as I've already explained in a previous post, I have used no "theories based on a motor manufacturers publicity". I have used a combination of simple classical mechanics, motor data posted on RC Groups by Dr Kiwi and others who have measured the 400XT motor, and some well known concepts such as stall speed and cubic wing loading.

While I do not have decades of experience in RC, I've been flying for two years, and have flown a good fistful of models in that time. It's not as though I have no experience selecting electric powerplants for planes around this size.

I do agree with you that all this posting and counter-posting is no longer productive. So I sincerely apologize to you for any hurt feelings I've caused. We'll never agree on this issue, so let it go. Other readers can decide for themselves.

-Flieslikeabeagle
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Old Mar 08, 2006, 11:55 PM
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Jurgen Heilig's Avatar
53859 Niederkassel, Germany
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Quote:
Originally Posted by flieslikeabeagl
...
For the extra $3, I get an efficiency increase from the stock 45% to maybe 75%, the use of bigger and more efficient props, and a pitch speed better matched to the model.
...
-Flieslikeabeagle
You are talking about worst case here, i.e. static conditions.

When you just cruise around with the TwinStar II (that's what the model does best anyway), the efficiency of the standard motors is closer to 70% and as you are flying at half throttle, the pitch speed is pretty well matched too. When you fly WOT at speeds close to 50mph, the pitch speed of the props is also very well matched.

That is also one of the reasons Multiplex is not really happy with their recommended tuning setup. It costs a lot more money, provides very little advantages, and can be tricky when one motor cuts off prematurely.

Jürgen
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Old Mar 09, 2006, 03:00 AM
SlopeHead
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Lisboa, Portugal
Joined Jan 2004
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Quote:
Originally Posted by flieslikeabeagl

Hardly. I have experiencing flying a model or two with direct drive Speed 400 motors (including the EasyStar) and have had a chance to see how dismally they perform.
OK, now I see... If you find the performance of the EasyStar dismal then everything is clear for me: you have a very serious problem either in your setups or in the way you fly. I suggest you check http://www.plawner.net and read their independent review of the EasyStar.

And this is definitely my last dialogue with you on the subject.
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Old Mar 09, 2006, 03:03 AM
SlopeHead
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Lisboa, Portugal
Joined Jan 2004
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Quote:
Originally Posted by Jurgen Heilig
When you just cruise around with the TwinStar II (that's what the model does best anyway), the efficiency of the standard motors is closer to 70% and as you are flying at half throttle, the pitch speed is pretty well matched too. When you fly WOT at speeds close to 50mph, the pitch speed of the props is also very well matched.
Jürgen, he had problems flying an EasyStar with the stock motor. That has only one explanation. As someone who was able to fly it with a Speed 300 (I wish I had enough skill for that) you know what the explanations is...
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Old Mar 09, 2006, 04:55 AM
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Norway
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Anyone

I posted a little question (1449) about a brushed setup for the TSI. As I have many 3cell lipo packs but no Nimh packs and want to use them but not buying 2 BL motors and 2 Esc what do you guys suggest? I have thought of using either two Permax 480 or two Permax 450 Turbo. Would these handle a 3 cell lipo and not get burned out within a few hours of flying? What kind of prop would be the most efficient for this?
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Old Mar 09, 2006, 05:33 AM
SlopeHead
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Lisboa, Portugal
Joined Jan 2004
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Quote:
Originally Posted by iceman3
Anyone

I posted a little question (1449) about a brushed setup for the TSI. As I have many 3cell lipo packs but no Nimh packs and want to use them but not buying 2 BL motors and 2 Esc what do you guys suggest? I have thought of using either two Permax 480 or two Permax 450 Turbo. Would these handle a 3 cell lipo and not get burned out within a few hours of flying? What kind of prop would be the most efficient for this?
Go for either Permax 400 or 480 but 7.2V so they will not burn with the higher voltage. Those will work ok with the stock propeller or equivalent.
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Old Mar 09, 2006, 07:09 AM
Hold my beer and watch this!
Jeremy Z's Avatar
Northern IL
Joined Oct 2005
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Quote:
Originally Posted by jcosta
Go for either Permax 400 or 480 but 7.2V so they will not burn with the higher voltage. Those will work ok with the stock propeller or equivalent.
They will NOT work OK with the stock battery pack though. Forgive me for not going back 100 pages and checking which battery you're using, could you remind us here?

For a given input voltage, a 7.2 V motor will turn fewer RPMs at a given input voltage.

Also, we should note here that it is not the higher voltage that kills a motor; it is the higher current pushed by that voltage. It is not the same thing, because if you have a smallish 9.6V 8 cell pack, it is not going to have the ampacity to push a high enough current to damage the 6V stock motors.

Now I have just yanked the stock speed 400s from my TS2. After taking one apart and examining the armature windings, I see that two windings are discolored compared to the third one. They got hot enough to cook the varnish a bit. (it was immediately noticeable in flight. I could hardly climb at all. My setup? 5x5 props and an 8 cell 3.6 Ah pack. The stock props are 4.9x4.3, but the buggers wouldn't stay on for me. I almost lost one during flight. So the increased load from the props was one factor. The 8-cell, high-Ah pack had plenty of ampacity to supply enough current to damage the motors' windings.

Based on this, and the fact that I have never really been happy with the thrust of the stock setup, I went with a twin brushless setup. (PM me if you want to know which one) Jürgen's concern that one motor could cut out before the other does not worry me. You do get some warning. On my brushless ESCs, I know it's time to land when I goose the throttle and it hesitates. I can still fly at 3/4 or 1/2 throttle with no problems. That's plenty good enough to land. Just don't try gutsy maneuvers after you've already been flying 17 minutes or so.

If I were going to stay brushed, I go with a pair of 7.2V speed 480s, and use enough prop to really utilize the capability of the motor. Remember that it's going to spin at a lower RPM than your 6V motors, but it has more torque. Since it is thrust that the TwinStar 2 is lacking and not speed, start with 6x3 props and measure the current draw. Full load, you're looking for no more than 12A per motor. So if it is much under 12A, either try a 6x4 or a 7x3.5.

jcosta, just because iceman3 and I are not happy with the thrust of the stock setup does not mean that we do not have good flying skills. It just means that we like to have more than "adequate" thrust. This is especially important if one flies in smaller areas, where one needs the ability to climb quickly if one is to utilize the area well. Also, if one is more accustomed to flying conventional planes, some with a LOT of thrust, it is a bitter pill to swallow to have such a nice-flying plane lacking thrust. The fact that iceman3 is Norweigian and not American disproves your theory that it is because Americans always like to have "too much". Possibly, he has windy conditions like I do? Possibly he does not have a huge area to fly in, so he needs to be able to climb quickly? But I doubt it is that he and I are lesser pilots than yourself and Jürgen.
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Old Mar 09, 2006, 08:01 AM
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Norway
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Jeremy

You are absolutely right about that my regular field is somewhat limited. I can not fly through the trees surrounding the field, but that I must do if the plane lacks the thrust and we all know what that means. I do not need a TS rocket, but I need to be comfortable with the fact that I have the power to climb out if necessary. The other fact is that I dont intend on buying Nimhs as stated earlier since I have got plenty of 3cell lipos. The Speed 400 will burn out quickly with those packs, so I need motors that can cope with 3cell packs. I have a MPX Twinjet with original Speed400 motors and I have learned the lesson with lipos on that one. They survived just about 3 flights.
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Old Mar 09, 2006, 09:08 AM
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United Kingdom, Bracknell
Joined Nov 2000
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Quote:
Originally Posted by Jeremy Z
They got hot enough to cook the varnish a bit. (it was immediately noticeable in flight. I could hardly climb at all. My setup? 5x5 props and an 8 cell 3.6 Ah pack.
Did you measure the current? From memory I got around 34A with CAM 5x5s on 6V S400s with 8x2400 (Twin Jet). 17A per motor is pushing it, just a touch
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Old Mar 09, 2006, 09:21 AM
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Twin-Jet-Crash's Avatar
Norway
Joined May 2004
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Just a little question here.

Took the motors from my old Twin Jet, two P.480. Thought that i could use these motors on my Twin Star, together with 3S LiPo.

I do now have Graupner CAM Slim 6,6x3 on the S400's, and i'm planning to use 2S LiPo for this.

The question: is the P480 going to be toast with those props on 3S LiPo, and how much current would the pull?

Thanks
Jan-O
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Old Mar 09, 2006, 09:34 AM
Wheeeeee!
Birdchased's Avatar
VAFB, California
Joined Oct 2005
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And why hasnt anyone bashed one of these into a flying boat yet??
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Old Mar 09, 2006, 10:55 AM
Hold my beer and watch this!
Jeremy Z's Avatar
Northern IL
Joined Oct 2005
2,380 Posts
Quote:
Originally Posted by Twin-Jet-Crash
The question: is the P480 going to be toast with those props on 3S LiPo, and how much current would the pull?
It depends on the LiPos. The first and second generation ones have a higher internal resistance. This means it has less ampacity. (it cannot provide as many amps) So it may be OK if yours are older. Also, they are 7.2V motors, so they will surely last longer than 6V speed 400s. A few people have done it successfully with good throttle management, but I don't think I'd do it unless I took some measurements first.

Bill Glover: Yes, that would explain the short lifespan of my poor little speed 400s, hehehe. Live & learn, right? I'll try to save others from this mistake. When I was shopping for replacment props at the local shop, they didn't have 4.9x4.3 props so I thought 5x5 wouldn't be that much of a difference. Wrong-O! The fact that my max flight times went down from 27 minutes to about 21 minute should have clued me in, but I was a noob and kind of brushed it off. I'm getting smarter all the time, but it is expensive sometimes.

Offhand, do you know the continuous current limits of the following?
6V speed 400, 7.2V speed 400, 7.2V speed 480 (aka "long can 400")? I think they're all around 12A, but I'm not certain. Those are just values I've seen in posts.
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Old Mar 09, 2006, 10:56 AM
Hold my beer and watch this!
Jeremy Z's Avatar
Northern IL
Joined Oct 2005
2,380 Posts
Quote:
Originally Posted by Birdchased
And why hasnt anyone bashed one of these into a flying boat yet??
They have. Look at http://plawner.net --> Reviews --> Foamies --> TwinStar 2 and look around. Also, check his video gallery, as there are a couple good videos of this mod. The TS 2 is a natural for this.
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Old Mar 09, 2006, 11:19 AM
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53859 Niederkassel, Germany
Joined Sep 2000
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Quote:
Originally Posted by Jeremy Z
It depends on the LiPos. The first and second generation ones have a higher internal resistance. This means it has less ampacity. (it cannot provide as many amps) So it may be OK if yours are older. Also, they are 7.2V motors, so they will surely last longer than 6V speed 400s. A few people have done it successfully with good throttle management, but I don't think I'd do it unless I took some measurements first.
...
So far we only know that Iceman wants to use 3 Lipos. We neither know the capacity nor the brand. Running a setup were the max. current is limited by the inner resistance of the battery pack is nonsense and may cause damage to the battery or worse.

Using props small enough to keep the current within motor specs and avoiding WOT all the time should be sufficient to achieve decent lifespans.

The props may need to be rather small, i.e. APC 5.7x3"

The next problem will be the CofG. The old TwinStar needs a fair bit of weight in the nose. I would hate having to add lead to an electric model.
In this case, the heavier Permax 450 Turbo may help here.

Jürgen

P.S.: If the flying area is big enough to land a TwinStar, there should always be plenty of space to take off - even on stock power.
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Old Mar 09, 2006, 12:18 PM
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Norway
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Jurgen Heilig

I am planning on using either a PQ 3cell 3100Mah battery or a Maxamp 3cell 4000Mah. I have checked the MPX Motor calculator but I am not sure how far I can push the Speed480 or the 450 turbo motors when it comes to the props. So far it seems likely that I might use the Apc 6x3 or 6x4 props since the efficiency seems to be over 70% with those.

In my regular flying field I have got limited space for both takeoffs and landings because of surrounding trees. That is why I usually need to use spoilerons and down elevator for landing my bigger planes. How do this work on the TW? Any comments?
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