Greetings,

I am about to build a balsa trainer kit that was designed for gas, but I plan to build it for electric. Are there any good guides for converting a gas design to handle electric? Since It is just a pile of blasa, I can make modifications up front.

Thanks,
Bilz

Good question, but, unfortunitly there is currently not a good answer.

Your best start is to remember the rule of 100 watts per pound for sport planes. Subtract 25-50watts/lb for Cub-like flyers and add 25-50 watts/lb for 3D and high performance planes.

Good luck!

Bilz,

Can you provide more specifics to the exact plane you have in mind? That will allow us to give you more useful information.

Specifically, this is the plane: A Super Decathalon. http://www2.towerhobbies.com/cgi-bin...?&I=LXNC96&P=0

The guy at the hobby store told me that the modifications for electric would be pretty straight forward.

Brian

Bilzmoude,

Looks like a good candidate for a conversion. Nice light wing loading.

Keep Greg's advice in mind for selecting a power system.

Once you select one you like you should select a mount for it...that will help you decide on how you will configure the firewall.

If you can understand how it tends to balance you can decide where the battery will go because this will be (besides flow versus electric motor mounting) the other big possible modification. You will want to get the CG right and you might want a hatch for conveniently getting the battery in and out.

Agree, nice plane Love the Bellanca; acro, scale, high wings and really cute.
It should suit using an AXI 2212/26 at least, but I would have chosen the Nippy 1812/100 with 8x3,8 draining just 12 Amps. Thus being able to use some of the cheap 1500-1600 3S LiPo packs. If you later need more power just add a propeller like 10x4,7 increasing thrust by more than 60% (thrust above 1,1 kilo...), but also closing up to 22Amps! Then you have a motor both for the Bellanca and also for JumpingJack and other planes in the 20-25oz range being able to do 3D
Just an example...

This reply goes out to Greg or anyone else who has more experience with this than me (been doing this for about 2 months). I was noticing the "watts/lb" guidelines that were placed here. Now, I know that they are guidelines, so it is not cast in concrete.

I was just wondering whether this was just opinion or a more widespread agreed-upon rule of thumb. The reason I ask is because of a book I purchased ("Getting Started in Backyard Flying") and their recommendations seem to be much less than this (only 1.5 watts/oz or 24 watts/lb for docile flyers and 2 watts/oz or 32 watts/lb for sports flyers).

Are the numbers from the book for smaller park-sized flyers mentioned within the book (never going above size 400 motors in the publication), or are they just off-base ? Personally, I like the numbers posted here because they look to put you toward the higher-end of the spectrum and look to say that it is better to have a touch-more power when you need it (and would not have it) than to have lower power and want it (but not have it).

Keep up the good posts and decent recommendations. I look to this site to help me make intelligent choices on what planes might suit my "fun level" and what i would need to make them behave in a manner that would be less likely to frustrate me.

the watts/lb is something more acurate than you think.
100W/lb +- 25% is a good rule. You can't get wrong using this.
The second point is , once you know the watts required, to choose the right motor . That's more diffcicult as you need to find a motor with a good efficiency % at the level of power required. You need to choose between brushed, brushless, outrunner, gb, prop size.....
there you have to check things such as motor Kv, amps,....or you can ask for prior experience of others to pick the right combo motor/prop.
A whattmeter or equivalent is useful to determine amps and watts used.

I swear by the watts per pound. I'm an advanced pilot and I personally find that I like a minimum of 100 watts per pound unless its a cub type. I generally look for at least 125 to 150 watts per pound. Keep in mind that most of the folks that comment about my planes say that they are well powered. I've flown other planes in the 75-80 watt per pound range and they do fly OK but lack the aerobatic proformance that I like.

I look at the empty weight of the plane to get an idea of the wattage of the system I need, then I look at the battery that I'll need to carry to provide the current draw and duration, and then I look at the motor/prop/gearbox combo to get the most out of the battery.

John

OK, remember that I am a newbie at this and I'm trying to catch on to this game. I found a program called P-CALC (from someone called DMA - Diversity Model Aircraft. I decided to punch in a few figures to see how my stock Hobbico Superstar EP w/ailerons fared. Using Sanyo 2000SCR as the battery (it's actually a 2100 mAh, but that's close enough for me), the Graupner Speed 600 8.4V motor and the 1/1 gear (direct-drive) with an APC 9x5.5 and what I got was a draw of 25.4 A, an output of 99.2 W, in-flight thrust of 18.9 oz and a full-throttle duration of 4:43.

First of all, does this look kosher ? If I read it right, the flight time for the battery sucks (which is why I will buy something better, it just ticks me that this is what they recommend online), but it says 99.2 W output (although I'm not sure if this is "per lb" or overall). If that is overall, then with a loaded plane weight of 2.7 lbs, we're talking only 36.7 W/lb (sounds like it wouldn't even get into the air).

I then dropped the prop down to APC 8x4 and got better duration (6:29) because of lower motor draw (18.5 A), but only 87.6 W output however the in-flight thrust drop was just to 18.2 oz. So, lastly, I went a different direction and tried it again using a 2.5/1 gearbox with a Master Airscrew 10x8 prop, figuring I could extend the duration. It did that (up to 10:50) and the motor/gearbox/prop combo only draws 11.1 A, but the power really sucked, providing 57.76 W output and only 12.3 oz in-flight thrust.

So, it sounds like the stock version (the direct-drive motor and 9x5.5 prop) gives me the best power at 99.2 W(unless that output is not per lb., in which case I'm nowhere close at under 37 W/lb) and I should just increase the battery to give me a bit more kick and longer flight time (for the heck of it, I punched in a 3300 mAh battery and got 25.9 A draw, a nice 102.9 W output, in-flight thrust of 19.3 oz and a flight time of 7:38 full throttle).

Am I using this correctly ? I plan on trying MotoCalc when I get home (trying all of this at work right now) to look at the numbers again. If I am using it right, then would the key fields on which to focus be the system output and flight time, while still keeping a lesser eye on motor draw and in-flight thrust ?

Thanks for any help and/or advice on this !

Watts/lb is always figured and calculated on Watts IN, not Watts out. You can easily measure Watts in. Watts out depends on so many factors that it's not humanly possible to predict it. Even the computer programs only make a rough guesstimate based on ideal scenarios.

That 99.2 Watts out figure is probably the result of 150-180 Watts in. Roughly 60 Watts/lb, which fits right in with the performance of the airplane as I've experienced it.

Keep in mind that calc programs are not all that accurate due to the countless variables that they'd need to account for to get a 100% accurate calculation.

The Watts/lb rules were originally developed by electric "guru" Keith Shaw, based on Astro Cobalt series motors, back before brushless technology was widely available and Astro Cobalts were the only alternative to car motors. Cobalt motors were/are roughly 60% efficient, somewhat better than cheap can motors, but not as good as a brushless, obviously. For your 100 Watts/lb, you can expect decent performance with a can motor, better with a cobalt, and "WOAH BABY!" with a brushless.

Pay little heed to the "full throttle flight time" figures. That's FULL THROTTLE flight times. You won't be flying full throttle the entire time, unless you seriously underpowered your plane. Fellows in my club regularly keep their Superstar EPs in the air for 20+ minutes with the stock power system catching thermals and soaring around. For regular flying, expect 8-10 minutes, right up there with a typical glow flight.

Matt

Thanks for the reply and the guidelines. I guess as I get further into the sport and get more experience, this will become second-nature.

As I sit here at work, I just had my Watt's Up wattmeter delivered to my desk. Now, I hope that I can get more definitive stats based on real-time study and I will continue to use P-Calc and MotoCalc to give me ideas for upgrades (in addition to the great ideas I get online).

Ok, I'm not trying to be a pain here, I just want to make sure I'm heading down the correct path.

Took the meter home and hooked it up to my still-being-built Slinger. The battery that I have for it is an 1100mAh 8.4V NiCad (I'm planning on going to the 1800mAh NiMH once I buy the DuraTrax Intellicharger), the flying weight of the wing is supposed to be 27 oz (let's use that for now), the ESC/receiver combo is a 30A/5ch and I still have the stock Speed 400 motor that comes with the wing; that should take care of the basics.

I fired up the system and watched the meter as the motor ran full-throttle for a few minutes. I did not know how long to run it, but I hoped that this was long enough. The 2 key figures that I got were that the motor drew a max of almost 10A (9.96Ap) and the power looked to be just a touch under 70W. The battery was less than the 1100mAh to start, but I'll stick with 1100 since I did not write that number down.

So, if I am doing my math right, knowing the motor draw (at least its max), the battery (1.1A) should last me just over 6.6 minutes (I know that's full-throttle, so if I fly 1/2 to 2/3 top speed, I might get 10 minutes or so). And, if I divide the weight of the wing into the power, I'm looking at 41.48 W/lb (70W / 1.68 lbs).

First of all, am I calculating these items correctly based on the readings ?

Secondly, if so, then I think I need to look into getting a better motor that can get me up to, say, 120W which would get me over a 70W/lb rating (which might allow me to something other than fly in a circle...boring !)

Thirdly, is the ESC/receiver combo overkill or a moot point ? The next lower (at least in Electrifly) is a 4ch receiver, but not an ESC combo so I'd need to get a separate ESC (say 20A) and wouldn't that add weight ? And, if I decide to go to a better motor, I figure I would need the extra amps on the ESC anyway.

I think the keys to me sound like the battery (for duration and power) and the motor (more power). I think the ESC should be something that comes afterwards after I know the power needs of the other two. And, I know I can work a different prop, but I don'tr believe any prop will really boost the stock setup enough, right ?

Thanks for helping a fledgling earn more of his wings.....