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Old Jul 21, 2009, 02:07 AM
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Build Log
Electric Firecracker; The Madness Begins!!!

Hello to all,

I can't say what has precipitated this plunge into madness, but I have decided to attempt an electric control line model aircraft.

Let me just add that I have never flown control line before but this aspect of the sport has forever been lurking in the shadows and is now rearing it's ugly head in the form of a Brian Eather designed Firecracker.

Sanity prevailing, this will not only be the slowest build thread in history but one giant learning curve, and where better to start that curve with building and learing to fly a full house F2B stunt model, and putting electric in it to boot

With a stroke of luck, building of the craft itself should be straight forward and I have a nice big AXI 4120/14 motor, 5S 5300mAh Li-po and CC-85HV ESC currently hauling my 6 3/4lb .60 sized trainer around that should play with a 3 pound control line model.

Learning to fly should take care of itself, how hard can it be? I fly RC

The secondary challenge and most exciting to me is the ESC control and it's here that a microcontroller, 3-axis accelerometer and rpm feedback will hopefully keep that ellusive constant 5.5 second lap speed with extra power in the uplines and a touch of braking in the downlines, if my flying ever progresses beyond half a lap of doing my best Flipper the dolphin impression

Wish me luck people, I'm gonna need it
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Old Jul 21, 2009, 09:39 AM
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Well, all I can say is welcome - and best of luck. Keep us updated.

When you get a little closer to the "Ready to Fly" state, I'm sure that there will be those who urge you to get some help learning and probably to start with something less intricate. You will probably even come to that conclusion on your own.

WRT your equipment choices, it does sound as if your power plant will be adequate although your will probably be surprised at how much power it does require to fly control line as opposed to RC -- I haven't looked up your motor spec's, but if the Kv is around 700, you should be OK.

The controller/accelerometer/closed loop feedback is probably going to be an elusive goal - it has been tried and for various reasons it has been found to be wanting. You may want to join www.stunthanger.com and read up on old threads in the electric power forum - Igor Burger, Bob Hunt and others have tried various RPM control solutions and are still actually using the built in "Governor" mode only. That, however, is in YOUR distant future. I personally am having quite a good time flying without any feedback, using only the power ramp-up built in to my flight managers (Will Hubin is the supplier of these).

My advice for now would be to start building the Firecracker and get your hands on an ARF Flitestreak or similar, then get some help with the initial flying of that, while work on the Firecracker progresses.

And, of course, keep us posted on your progress ...
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Old Jul 21, 2009, 11:00 AM
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Welcome to the world of control line. Since you want to learn and you want to go electric, you might want to consider the electric Super Clown ARF from Brodak Distributing. The Clown is a very good airplane to learn on and you will also get some experience in the electric area.

http://www.brodak.com/

Bill Hodges
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Old Jul 21, 2009, 11:37 AM
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Mike A.
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bjr 93tz --- Forgot to ask -- where are you located?
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Old Jul 21, 2009, 09:21 PM
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Good Luck, and may the electrons be with you.
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Old Jul 22, 2009, 04:09 AM
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Quote:
Originally Posted by bjr_93tz
Learning to fly should take care of itself, how hard can it be? I fly RC
HAW!!! Only a Queenslander would say that
Actually it's quite easy to fly straight and level but you really need someone competent to hold the handle with you so you can get used to the response. I've done this with my own stunter and a 12yo girl who managed to fly it reasonably well after a few minutes. But if you get into trouble then you'll really be in trouble very quickly .

Brian's Firecracker is a very well known (and successful) design so it would fly very well on a proven electric power. I've only seen one stunter (a modified ARF Nobler) fly with electric and it was less than perfect, basically way underpowered, but it's still in the experimental stage. To be honest (and I always am ) I think the Firecracker is too complex and difficult to build to learn on. You'd be better off building a simple profile type model like the Flitestreak that mikeainia mentioned. Call it disposeable

For interest's sake, a CL stunter like the Firecracker only needs about 1/3rd HP for level flight even with a .60 in it so the engines aren't exactly over- stressed .
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Old Jul 22, 2009, 05:37 AM
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Thank you all for your words of support, I've been losing sleep over this thing already..

After lying in bed for 1/2 an hour this morning figuring out how to do plug in tails, I think I've decided to build it one piece and bolt it on in a manner similar to my Drela designed SuperGeeII DLG. But that comes later.......

mikeainia, the motor KV is 660 and it pull 35-40 amps on a 12x6 APC electric prop with the 5S pack. Up to 800W static, who knows when flying? I expect I'll need to use a lower pitch 3 blade prop to at least look the part. A single differential pressure transducer and pitot would be a better choice of sensor arrangement as I figure it's really airspeed you want to keep constant and the accelerometer with rpm sensing is a convoluted way of achieving that, but I have my reasons of which I'll rant about in time.

downunder, too right I'm from QLD but it could be worse..I could be from New Zealand

Looking at the plans for the elevator a couple of things stood out, there are a boatload of ribs so I'm taking the lazy way out and CAD'ing them and gonna' mill the ribs on a little NC machine which rased my first challenge of the build... Which airfoil do I use for the elevator The one drawn on the plan which is has negative camber in the first 20% of the airfoil, or the oversize symmetrical one drawn on the fuse outline Welcome back to the world of hand drawn plans After much deliberation I decided the curves of best fit was a symetrical section based on an eliptical front section with straight tangent lines to the trailing edge. The tip section was sort of symetrical so I figured the root must be as well

Just to add more fun to the mix the root and tip airfoils are different as well In the bad old days you'd use the "sandwich method" to make this lot up but these days I use the root and tip airfoils to make up some solids in CAD, slice them up and offset the outlines to allow for the kerf. Takes much longer than the sandwich method but it allows the same mistake to be repeated on every rib On the other hand, I could have had foam cores cut and laid up in the vaccume bag by now and they would probably turn out lighter stronger and stiffer.

Interestingly enough, the root section is sheeted in 2mm balsa and it isn't shown recessed into the ribs/spar/TE/LE but I don't think it just sits on top either. I'll figure that one out before I finalise the rib outlines.

Cheers
Brett
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Old Jul 27, 2009, 09:32 PM
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Arrrrrgh!!!!!!

Good thing they didn't employ me doing the CAD work for the latest Airbus as it would have turned out a right mess. I'll blame the fact I don't use AutoCAD every day and that version changes every year so it's always a little different.

BUT after a few days of tearing my hair out I have a set of files for the elevator ribs.

Cut a long story short, the outlines in the tip and root templates were sort of ok but the hingeline wasn't so when I placed the rib printouts on the plan view of the elevator both the sweep and taper were wrong by about 4mm. if was "only" 2mm I could have lived with it but I needed to find out where I went wrong, there were other issues which I won't bore you with but with a stroke of luck I'll be milling ribs this afternoon.

Oh yeah, I recessed the 2mm sheeting into the ribs, you might be able to make it out in the pic.......
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Old Jul 29, 2009, 08:00 PM
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Hi Brett,
I understand you are new to E-C/L, but why not ask for help before heading down the wrong path? From your posts so far I can tell you are not going to meet your goals. A 3lb Firecracker using the outlined power system is impossible. The motor/Battery/ESC alone weighs ~35oz!

I don't mean to discourage you! Your project is as feasable as any with the right parts. Also, I would REALLY recommend flying at least one C/L airplane before flying the Firecracker or all your hard work WILL be gone. Everyone crashes repeatedly when learning to fly.



Mike
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Old Jul 29, 2009, 08:06 PM
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YEP! I rolled my first 1/2a this weekend and broke it good! Got home and in 5 minutes she was back together...just not as pretty!
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Old Jul 30, 2009, 12:03 AM
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Thanks Mike and Rodl,

I've really got no idea what weight this thing will turn out at, but it will be heavier than if it was powered by a glow engine for sure.

When I built my Bubble Dancer glider the designer was kind enough to provide target weights for all the bits as well as suggestions for density balsa to be used in different areas so I'm trying to apply what I learned on that build to this one. When I hold up one sheet of 1.5mm balsa which weighs 14 grams and another which weighs 23 grams my heart says use the lighter sheet to cut the ribs out of but my head is saying it's my first model and it's gonna cop a bashing Dilema, dilema.

I don't subscribe to this whole "crashing to learn" business, I didn't apply it to my first planes or my first heli and I certainly didn't apply it when I took up motorcycle road racing. I also don't have any accidental children But I know I'm taking a risk albeit a calculated one, so thank you for the concern and I hope you'll be hanging around when it comes time for you to post the "I told you so's" as I hustle together a 1/2A C/L while my busted up Firecracker sit's in a bin.......

Oh, some pictures of ribs......
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Old Jul 30, 2009, 03:23 AM
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Brian's construction method for the tailplane is just a little overcomplicated with all those half ribs to cut out. The tailplane (and elevators) I use on my stunter has an almost identical airfoil section but built up as in the (very poor ) photo below. I covered the framework with 1mm balsa followed by doped on lightweight tissue which made it very rigid. I'd recommend using 4-40 ball joints on all controls, none of this bent wire stuff .
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Old Jul 30, 2009, 03:55 AM
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Hi Downunder, thanks for the picture. I agree, Brian's method of construction is a tad fiddly. I am tossing up one or two ways to jig this thing so it builds straight as well. Maybe cut some foam beds for it or just build it free hand? That's why I like to do my ribs on the NC machine now as I know they're straight, plus if I want to add a million lightening holes to the next batch it's not too hard.

In your first post you mentioned an electric nobler, I was down at the Control line field at Ipswich last Saturday morning and there were a couple of nice blokes there and one of them mentioned he had seen a e-nobler so it might have been the same one. He had a real sweet WWII lookalike with an OS.46 in it and performed very well.

Ipswich was a whole new experience though, you could almost see people's second head poking up through their shirt collars
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Old Jul 30, 2009, 08:14 AM
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Quote:
Originally Posted by bjr_93tz
Ipswich was a whole new experience though, you could almost see people's second head poking up through their shirt collars
Careful what you say there matey because I belonged to that club back when it was called the Ipswich Lightnings (ILMAC) and was even president for a time . Then again, I was borne in Tassie so......two heads are better than one .
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Old Jul 30, 2009, 04:34 PM
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Zaphod Beeblbrox .........so thats where you got to......................
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