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Old 04-29-2013, 03:22 PM   #1
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Default General Electric Conversion Questions

I have just ordered my first laser cut kit and will probably build a couple more.
But this is just to get my building chops before I start some long term projects.
I plan on converting a couple of classic kits to electric. It's not that I am apposed to glow power but am comfortable with electric.
I have scoured this forum, and many others and have learned a great deal about the conversion process but still a have a question or two.

I keep reading to shave as much weight off these older glow designs as possible due to them being built much more robustly, particularly in the firewall / fuselage area, to deal with the vibrations of the motor. But these designs where magnificent performers at their designed weight. Why not just lighten the tail to assist the cg when using a lighter electric powerplant?
I, for one, would appreciate the extra weight due to constantly flying in wind.
Would this added strength in the forward fuselage also not help when adding a hatch for a battery compartment?

At this time I haven't decided on what type of battery pack to use. I may got with a sealed into the plane A123 pack, the extra weight would greatly help with cg issues, or standard lipo packs for the versatility of using them in multiple applications.

Any input is greatly appreciated. I am very anal about doing as much research as possible before starting a major project.

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Dumas Ecroupe, MM EVA, E-flite L4 Grasshopper, Sig Kougar, Sig Kobra, Top Fight Contender, top flite mini contender, Carl Goldberg Skylark mark II, M&H Sportster 40.
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Old 04-29-2013, 03:47 PM   #2
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Well it would help if we knew what bird(s) you plan on building.
That way we could maybe give some additional tips and such.

When I die, I want to go like my Grandfather did, in his sleep...... Not screaming like the passengers in his plane.
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Old 04-29-2013, 03:51 PM   #3
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I would think that with the powerful electric motor and batteries we have available now trying to save weight probably isn't as critical as it was a few years ago. No you just run a bigger motor and or more voltage. If your not exceeding the original weight specs I dont see a problem but we've only converted 1 glow plane to electric so far and it was a built plane when we decided to try electric
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Old 04-29-2013, 03:57 PM   #4
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You don't need to make any changes if you don't want. However electric from glow rarely is equal. Electric is generally heavier especially when using a123 cells.

A hatch is really a must but most kits just won't need much strengthening for that (maybe just a top rail).

Is this for the Kougar you picked up?
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Old 04-29-2013, 04:15 PM   #5
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Originally Posted by rcers View Post
You don't need to make any changes if you don't want. However electric from glow rarely is equal. Electric is generally heavier especially when using a123 cells.

A hatch is really a must but most kits just won't need much strengthening for that (maybe just a top rail).

Is this for the Kougar you picked up?
I haven't picked up the Kougar. I took the good advice and started with a Mountain Models kit instead.

But the three I'm am looking at long term are the Sig Kougar MKII, the Sig Kobra, and the Top Flite Contender. I realize there is a scaled down Mini Contender available as a EP ARF for a very good price at that, but it's not as much about flying these birds as it is about the build process.
the styles are very similar but I have always loved the look. The twelve year old boy inside my head always feels it's the type of plane Steve Austin would have flown. They have that wonderful early seventies military feel to them.

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Old 04-29-2013, 04:22 PM   #6
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The Kougar is my pick of that group. It is a wonderful bird. I do miss my vintage Contender. I have the small EP ARF and love it.
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Old 04-29-2013, 04:33 PM   #7
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Originally Posted by rcers View Post
The Kougar is my pick of that group. It is a wonderful bird. I do miss my vintage Contender. I have the small EP ARF and love it.
That's my first pick as well. But it also seems the most difficult build for a new builder.
As for the conversion, I was thinking along the lines of installing the Turnigy G45 as it's a direct match for a .45 glow. That with a 60 amp esc and a bec coupled with a 5s 4000 lipo i should be good for power. The biggest hurdle seem to be prop size. Older conversion I have read up on seem to indicate the need for a larger prop than with a glow setup. But that leads to mowing the grass with the tricycle undercarriage. I'm leaning towards using a three or possibly a four bladed prop if possible.
I'm also toying with the idea of retracts but won't know for sure till I have the kit in hand and really know what I'm working with. The foam core wing does lend itself to easy modification for retracts in the wings but a nose retract will be more difficult I suspect.

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Old 04-29-2013, 04:38 PM   #8
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Kobra... some lightening holes in the fuselage and tailplanes. Drill the ribs so you can run servo wires and go to dual aileron servos. Move a couple of bulkheads to accommodate batteries and possibly move firewall forward.
Top hatch would be from a couple of inches back from firewall to the back of the canopy. Balsa rails added to the hatch sheeting.

Kougar would be similar (its just a bigger version of the same plane with foam core wings) except I'd do some planning to cut lightening holes in the wings (clear through sheeting and cores, all behind the "spar line" (See Kobra for its spar line) You can save maybe 1/5 to 1/4 of the weight of the wings here. Using a different adhesive than listed in the instructions for sheeting will save weight too.

None of these changes would be visible on the completed model without careful inspection... except the hatch latching method. I would NOT use magnets for the hatches on these models.

As noted elsewhere... I have a King Kobra waiting its turn for space on my building table. I've been thinking about how to do it for a while.
I'll stick in an Astro 4120/18 (because I have it) on 6S. 4000 (because I have 4X 3S 4000 used in series for 12S already)
I may put in retracts... probably not. I do have appropriate retracts sitting in a box though.
I expect my final weight, including LiPos to be lower than the expected weight on the box.

*********

Prop selection... you'll probably go to a 3 blade for ground clearance.

Note that the King Kobra plans show how to do the retract installation. Same method would work for the Kougar with modern electric retracts.
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Old 04-29-2013, 04:49 PM   #9
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Originally Posted by fhhuber View Post
Kobra... some lightening holes in the fuselage and tailplanes. Drill the ribs so you can run servo wires and go to dual aileron servos. Move a couple of bulkheads to accommodate batteries and possibly move firewall forward.
Top hatch would be from a couple of inches back from firewall to the back of the canopy. Balsa rails added to the hatch sheeting.

Kougar would be similar (its just a bigger version of the same plane with foam core wings) except I'd do some planning to cut lightening holes in the wings (clear through sheeting and cores, all behind the "spar line" (See Kobra for its spar line) You can save maybe 1/5 to 1/4 of the weight of the wings here. Using a different adhesive than listed in the instructions for sheeting will save weight too.

None of these changes would be visible on the completed model without careful inspection... except the hatch latching method. I would NOT use magnets for the hatches on these models.

As noted elsewhere... I have a King Kobra waiting its turn for space on my building table. I've been thinking about how to do it for a while.
I'll stick in an Astro 4120/18 (because I have it) on 6S. 4000 (because I have 4X 3S 4000 used in series for 12S already)
I may put in retracts... probably not. I do have appropriate retracts sitting in a box though.
I expect my final weight, including LiPos to be lower than the expected weight on the box.

*********

Prop selection... you'll probably go to a 3 blade for ground clearance.

Note that the King Kobra plans show how to do the retract installation. Same method would work for the Kougar with modern electric retracts.
Great tips there, especially on the Kobra. Thanks.
But again I am wondering why all the effort to shave some weight of a design that flies tremendously well at its designed weight.
I understand the concept of weight is your enemy, but in windy, gusty, conditions it can be your friend as well. There's something i'm probably just not getting or understanding on this point.

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Old 04-29-2013, 06:00 PM   #10
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Saving weight in general makes planes fly even better.

Trim unnecessary weight from a plane and make no other changes...

Lower stress on the airframe in high G maneuvers.
Lower drag from having to create the lift to carry the weight and thus higher top speed.
Longer flight times at the same level of performance.
Lower power needed for any aerobatic maneuver.
Lower minimum speed to maintain level flight (often called "stall speed", but you can stall at any speed)

You can trade that structural weight for battery weight for more duration at the same performance as original...

Kougars especially tended to come out heavy (people liked to fiberglass them on top of the already heavy structure) and required high minimum speed for landing approach. You can save a LOT of weight and make it a much better airplane.

**********

I'm considering another modification that I have done to other models, sheeting the King Kobra foam core wing with aircraft ply. It comes out lighter due to lower glue absorption. Thinner wing by use of 1/64 ply instead of 1/16 balsa for less drag (not a lot...) and the ply has no glue joints to interfere with getting a very smooth finish.
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Old 04-29-2013, 07:13 PM   #11
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Originally Posted by fhhuber View Post
Saving weight in general makes planes fly even better.

Trim unnecessary weight from a plane and make no other changes...

Lower stress on the airframe in high G maneuvers.
Lower drag from having to create the lift to carry the weight and thus higher top speed.
Longer flight times at the same level of performance.
Lower power needed for any aerobatic maneuver.
Lower minimum speed to maintain level flight (often called "stall speed", but you can stall at any speed)

You can trade that structural weight for battery weight for more duration at the same performance as original...

Kougars especially tended to come out heavy (people liked to fiberglass them on top of the already heavy structure) and required high minimum speed for landing approach. You can save a LOT of weight and make it a much better airplane.

**********

I'm considering another modification that I have done to other models, sheeting the King Kobra foam core wing with aircraft ply. It comes out lighter due to lower glue absorption. Thinner wing by use of 1/64 ply instead of 1/16 balsa for less drag (not a lot...) and the ply has no glue joints to interfere with getting a very smooth finish.
Big big thanks for the info. Every thread on glow conversions mentions shaving weight but no-one ever says why. I guess the posts are geared for those with more experience than me and it assumed the reader knows what going on.

About the sheeting method you mention. How much more difficult is it to form the ply around the core than the balsa sheeting. One method recommended to me with the balsa was to mist it with windex which will allow it to be formed with less effort. Never thought windex could be used for that.

----------------------------------------------------------
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Old 04-29-2013, 07:39 PM   #12
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Originally Posted by Wrongway-Feldman View Post
I have just ordered my first laser cut kit and will probably build a couple more.
But this is just to get my building chops before I start some long term projects.
I plan on converting a couple of classic kits to electric. It's not that I am apposed to glow power but am comfortable with electric.
I have scoured this forum, and many others and have learned a great deal about the conversion process but still a have a question or two.

I keep reading to shave as much weight off these older glow designs as possible due to them being built much more robustly, particularly in the firewall / fuselage area, to deal with the vibrations of the motor. But these designs where magnificent performers at their designed weight. Why not just lighten the tail to assist the cg when using a lighter electric powerplant?
I, for one, would appreciate the extra weight due to constantly flying in wind.
Would this added strength in the forward fuselage also not help when adding a hatch for a battery compartment?

At this time I haven't decided on what type of battery pack to use. I may got with a sealed into the plane A123 pack, the extra weight would greatly help with cg issues, or standard lipo packs for the versatility of using them in multiple applications.

Any input is greatly appreciated. I am very anal about doing as much research as possible before starting a major project.
Yeah
Weight was a real issue with electric conversions of glow type models in the past years. Problem was with the anemic (Compared to the current brushless motors) geared brush motor, and the heavy Nicad battery packs that were used. Also have had some very light weight models that flew OK, but had a real tendency to flip upside down while doing the taxi back to the pit area.

Now, we've got the high power, light weight LiPo batteries, and the very high power brushless motors that have really changed the game. One issue with the LiPo batteries, they need access for removal during the charging process. That is not an issue with the 35% heavier A123 battery packs. All of my A123's are a permanent part of the model. With the Cellpro Powerlab 8 charger, I'm recharging these A123's in 15 minutes.

Right now, I'm finishing up on a Hanger 9 Twist 40 model designed for a 4 stroke 60 sized glow engine. Power is a Hacker A40-10L and a 6S1P A123 battery pack. That battery pack is right up against the back side of the firewall. And the model is way tail heavy, needing about 6 ounces of lead in the nose.

So, the rudder servo was relocated from the tail to the middle of the fuse with nyrods to the rudder. (Yeah I know nyrods and temperature changes, but shouldn't affect rudder to much, hopefully.) That got the cg pretty close.

This model is nearly done, only thing left is to provide external access to the charge and balance connectors.

It isn't hard to electrify a glow model and have it outperform the glow version. Only problem is shorter flying times between charging.

As for conversions, take a look:

Thread on 70 size glow engine conversion to electric
http://www.wattflyer.com/forums/showthread.php?t=45222

Hacker 6S2P A123 powered Models
http://www.wattflyer.com/forums/showthread.php?t=44686

Hangar 9 Kantana Model
http://www.wattflyer.com/forums/showthread.php?t=68844

AEAJR's Site on Electric Power
http://www.wattflyer.com/forums/showthread.php?t=18521

BEC Linear Current Rating
http://www.wattflyer.com/forums/showthread.php?t=63497

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Old 04-29-2013, 08:00 PM   #13
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Originally Posted by kyleservicetech View Post
Yeah
Weight was a real issue with electric conversions of glow type models in the past years. Problem was with the anemic (Compared to the current brushless motors) geared brush motor, and the heavy Nicad battery packs that were used. Also have had some very light weight models that flew OK, but had a real tendency to flip upside down while doing the taxi back to the pit area.

Now, we've got the high power, light weight LiPo batteries, and the very high power brushless motors that have really changed the game. One issue with the LiPo batteries, they need access for removal during the charging process. That is not an issue with the 35% heavier A123 battery packs. All of my A123's are a permanent part of the model. With the Cellpro Powerlab 8 charger, I'm recharging these A123's in 15 minutes.

Right now, I'm finishing up on a Hanger 9 Twist 40 model designed for a 4 stroke 60 sized glow engine. Power is a Hacker A40-10L and a 6S1P A123 battery pack. That battery pack is right up against the back side of the firewall. And the model is way tail heavy, needing about 6 ounces of lead in the nose.

So, the rudder servo was relocated from the tail to the middle of the fuse with nyrods to the rudder. (Yeah I know nyrods and temperature changes, but shouldn't affect rudder to much, hopefully.) That got the cg pretty close.

This model is nearly done, only thing left is to provide external access to the charge and balance connectors.

It isn't hard to electrify a glow model and have it outperform the glow version. Only problem is shorter flying times between charging.

As for conversions, take a look:

Thread on 70 size glow engine conversion to electric
http://www.wattflyer.com/forums/showthread.php?t=45222

Hacker 6S2P A123 powered Models
http://www.wattflyer.com/forums/showthread.php?t=44686

Hangar 9 Kantana Model
http://www.wattflyer.com/forums/showthread.php?t=68844

AEAJR's Site on Electric Power
http://www.wattflyer.com/forums/showthread.php?t=18521

BEC Linear Current Rating
http://www.wattflyer.com/forums/showthread.php?t=63497
That makes sense on the issue of older electrics. Many of the threads I've been reading to get info from are from nearly 10 years ago. Not alot to read on the planes I'm looking to convert. The most recent post seem to be from around 2006 or so but even in those posts they are talking about achieving power ratings that are laughably easy to achieve today, and at a fraction of the cost. One poster was talking about his 5sp 4000mah lipo pack and how he got a great deal at just under $200. it's amazing how much cheaper it has become in such a short time.
Plus, many of the conversions were being done with a geared system and I much preffer a direct drive outrunner.

You mention the A123 packs, and I have been toying with the idea of utilizing them in one of my builds. But the cost and, if I incorporate the pack into the build, not being able to use them in other planes may keep me from using them.
I usually go the field with 3 or 4 planes. I try to build planes that tend to use the same batteries. That way I arrive with 6 to 8 ready to go packs that are interchangeable between the planes. It's very cost effective and I'm never waiting for a pack to charge when I should be flying

----------------------------------------------------------
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Old 04-29-2013, 08:03 PM   #14
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The thin (1/64 and 1/32 inch thick) aircraft ply forms around the wing core more easily than the balsa

Its a little more expensive than the balsa, especially since the balsa came in the kit.

*************

Windex contains ammonia. The ammonia softens the "glue" in the wood that holds the fibers together. This allows you to bend the wood more easily.

If you soak 1/16 balsa in 10% ammonia solution (strong concentration! Heavy "dishwashing" gloves, goggles and good ventilation required.) you can almost vacuum form it (not as good as hot forming plastic...). It becomes very flexible and will do some seemingly impossible compound curves from flat sheet stock.
Then when it dries it stays in the new shape.
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Old 04-29-2013, 08:57 PM   #15
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Originally Posted by Wrongway-Feldman View Post
You mention the A123 packs, and I have been toying with the idea of utilizing them in one of my builds. But the cost and, if I incorporate the pack into the build, not being able to use them in other planes may keep me from using them.
I usually go the field with 3 or 4 planes. I try to build planes that tend to use the same batteries. That way I arrive with 6 to 8 ready to go packs that are interchangeable between the planes. It's very cost effective and I'm never waiting for a pack to charge when I should be flying
Yeah
In the USA, this is a very good source of those A123 cells, for $9.99 each. (Oops, $12.00, after several years they raised their prices!)
http://www.voltmanbatteries.com/serv...1-dsh-A/Detail. But you've got to build up your own packs.

Most of my A123 packs are configured as 6S2P, for a weight of 30 ounces.

I've got five year old A123 packs that still perform as new.

(A few minutes ago, I just ordered four more A123's to be used as receiver batteries in my giant scale models.)

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Old 04-29-2013, 09:26 PM   #16
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Originally Posted by kyleservicetech View Post
Yeah
In the USA, this is a very good source of those A123 cells, for $9.99 each. (Oops, $12.00, after several years they raised their prices!)
http://www.voltmanbatteries.com/serv...1-dsh-A/Detail. But you've got to build up your own packs.

Most of my A123 packs are configured as 6S2P, for a weight of 30 ounces.

I've got five year old A123 packs that still perform as new.

(A few minutes ago, I just ordered four more A123's to be used as receiver batteries in my giant scale models.)
That's very interesting. If I read the info correctly, to achieve the same output as my planned 5s 4000 I would need 12 of the A123 cells you linked to. Then build a 6s2p 4600.
As I understand the technolgy, A123 cells can be charged quickly and safely. But what would be the discharge rating? I'm trying to do a cost benefit analysis for myself. Bear with me.
For example, I can purchase a 5s4000 lipo with a 60C rating for $70 to $100 depending on sales discount codes etc. And then the cost of shipping to Canada on top of that.
To achieve similar performance from A123 cells I would need to buy 12 for $120 plus the cost of shipping, which would be significantly more due to the excess weight.
It seems like there is a large premium to be paid for not much benefit. At least that's the way it seems to me.

----------------------------------------------------------
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Old 04-29-2013, 10:12 PM   #17
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Originally Posted by Wrongway-Feldman View Post
That's very interesting. If I read the info correctly, to achieve the same output as my planned 5s 4000 I would need 12 of the A123 cells you linked to. Then build a 6s2p 4600.
As I understand the technolgy, A123 cells can be charged quickly and safely. But what would be the discharge rating? I'm trying to do a cost benefit analysis for myself. Bear with me.
For example, I can purchase a 5s4000 lipo with a 60C rating for $70 to $100 depending on sales discount codes etc. And then the cost of shipping to Canada on top of that.
To achieve similar performance from A123 cells I would need to buy 12 for $120 plus the cost of shipping, which would be significantly more due to the excess weight.
It seems like there is a large premium to be paid for not much benefit. At least that's the way it seems to me.
Agreed:
These A123 cells so far will never match the power/weight capabilities of the LiPo batteries. And they do cost more.

On the other hand, they are "premium" type batteries, so they'd more be similar to the $$$$ Thunderpower batteries, than the lower cost units. they have zero fire hazard, and from my experience, they pretty much last the life of the model.

I've been running my 6S2P A123 packs at about 60-70 Amps, which represents about 16C or so.

As for the 60C rating, I'd read that with a bit of caution. That 60C rating would be at 240 Amps, and that Lipo would be run flat in less than 60 seconds at 60C. Not to mention how many cycles that LiPo would last if run at 60C.

In fact, another LiPo supplier claimed 150C on their 8000 Mah battery, where their battery lead was only #12 wire. That 150 C represents 1200 Amps on that battery, #12 wire melts at about 230 Amps. (http://www.wattflyer.com/forums/show...highlight=150c)

So, if you'd like 6 minute flights, that represents an average of about 10C on your battery.

Take a look:

http://www.wattflyer.com/forums/showthread.php?t=65869

DennyV
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Old 04-29-2013, 10:33 PM   #18
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Definitely look at the C rating as just a measure of burst (very brief) power available.

I normally aim for 10 to 15 min duration capability from my models.

Even with the lighter LiPos sometimes the flight time is not possible. This is common with EDFs as the EDF is really a very inefficient way to drive a model.

I use Sky Lipo 20C rated LiPos in all of my electric models. They generally have 3 years capability to deliver current for 6C average demand. Then they have the ability to deliver 4C average (at appx 80% of original capacity) for another 2 years.

Not too bad getting 5 years use from a pack (replacing many gallons of glow fuel for the cost of 2 gallons).

My 4 year old 20C 4S 4400 mah packs are doing great in a "Commander EP 40 (calls for 4S 3300 mah and 37 amps peak) for 12 min per charge. I demoted them from my 12S (3X4S in series) 5000 watt (105 amps at peak charge) EDF where they were giving 4.5 min per charge.

I still have a 6 year old 1800 mah 3S that will fly my small Tiger Moth for 27 minutes.
But that is rare for a LiPo to survive that long.
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Old 04-29-2013, 10:46 PM   #19
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Originally Posted by kyleservicetech View Post
Agreed:
These A123 cells so far will never match the power/weight capabilities of the LiPo batteries. And they do cost more.

On the other hand, they are "premium" type batteries, so they'd more be similar to the $$$$ Thunderpower batteries, than the lower cost units. they have zero fire hazard, and from my experience, they pretty much last the life of the model.

I've been running my 6S2P A123 packs at about 60-70 Amps, which represents about 16C or so.

As for the 60C rating, I'd read that with a bit of caution. That 60C rating would be at 240 Amps, and that Lipo would be run flat in less than 60 seconds at 60C. Not to mention how many cycles that LiPo would last if run at 60C.

In fact, another LiPo supplier claimed 150C on their 8000 Mah battery, where their battery lead was only #12 wire. That 150 C represents 1200 Amps on that battery, #12 wire melts at about 350 Amps.

So, if you'd like 6 minute flights, that represents an average of about 10C on your battery.

Take a look:

http://www.wattflyer.com/forums/showthread.php?t=65869
you are correct on the mis-advertised C ratings on many packs out there. much of it is due to new people to the hobby jumping on the bigger is better bandwagon.
The motor I'm planning to use in the Kougar is this one.
http://www.headsuphobby.com/Emax-GT4...otor-F-211.htm

I've read posts about people mowing the grass with a 11 inch prop and my club field only has grass so I'm mating it with a master airscrew 10X7X3. this rates the setup at a peak of 1050 watts on 6s. Ive had great success with the turnigy nanotec packs and will use a 6s5000 45-90c and the recommended Hifei 80A ESC.
Probably much much more than the plane requires. But it puts me at a power to weight ratio I'm comfortable with and the robust airframe can more than handle it. This should end up being a very fun plane to fly for a total cost to me of less than $300 including the kit itself. Thats the price of a Parkzone BNF model at the local shop.

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Dumas Ecroupe, MM EVA, E-flite L4 Grasshopper, Sig Kougar, Sig Kobra, Top Fight Contender, top flite mini contender, Carl Goldberg Skylark mark II, M&H Sportster 40.
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Old 04-29-2013, 10:53 PM   #20
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sounds like you have a good handle on this one.
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Old 04-29-2013, 11:02 PM   #21
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Originally Posted by fhhuber View Post
sounds like you have a good handle on this one.
Thanks
I've become very comfortable with EP and read and research every idea or question I may have.
Hopefully I get the 1KW badge once she's built.

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Dumas Ecroupe, MM EVA, E-flite L4 Grasshopper, Sig Kougar, Sig Kobra, Top Fight Contender, top flite mini contender, Carl Goldberg Skylark mark II, M&H Sportster 40.
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Old 05-03-2013, 04:31 PM   #22
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Default Light Wing Loading Is Good

Keeping overall weight/wing loading to a minimum is good for any RC model if your goals include ability to take off, fly and land in a scale-like manner. Lightly loaded models generally are less stall-prone and less subject to landing gear and other damage during landings. Locating power and RC gear forward to avoid need for nose ballast is a good thing. Choosing an adequate but light as practical motor/battery combo is good. (Don't compensate for tail heaviness with a heavy motor and battery combo.)

Use light RC gear and servos. Building aft fuselage and tail structure as light as practical, is good ( avoid heavy tail wheel assembly). Use light weight covering materials and paint. Older IC-powered model designs generally have heavier duty wood and ply around engine and landing gear structure as well as heavier afte fuselage stringers and longerons. You can lighten this to some extent substituting thick with thinner ply, etc. There is a specific forum for IC to electric conversions on RC Groups that may give you some good ideas.
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