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Scratch building an Extra...

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Old 01-29-2011, 12:07 PM
  #1  
RC Fan chris
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Default Scratch building an Extra...

So I got a little ambitious the last few months and I started to build an electric sport plane based on the Extra. Its made of blue Dow Styrofoam and some Elmers' foam board. So far, I built most of the fuselage including the tail, and hotwired the wings out. For the cowl and canopy, I tried to do the pop bottle deal, but I ended up making both out of blue foam. I got the motor (950 kV brushless) and some hitec HS 55 servos. And by the way, I was wondering if these servos are okay or if I'll have to get bigger ones...I already have a receiver and transmitter for this, so I'll just need a suitable ESC and battery. I have a 30 amp ESC and a pretty hefty Li Po, which will probably need a bigger ESC, but since my dad and brother are working on another plane they could use em if I cant. So, I'm just a little iffy on what kind of battery, ESC, and servos I need. The plane is about 36 inches long with a 40-42 inch wingspan (subject to change). How heavy should this plane be? I saw that the PZ Extra 300 is about the same size, while weighing in at like 36 oz or so, I think.
LOL, thats a lot of questions!
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Old 01-29-2011, 09:16 PM
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Default RE: Scratch building an Extra...

I am interested because I know nothing about electrics but would like to learn, I will be watching this.

I cant see your pictures, there not showing up for me.
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Old 01-30-2011, 09:23 AM
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Default RE: Scratch building an Extra...

I'll reattach the pics, plus more new ones

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Old 01-30-2011, 03:41 PM
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Default RE: Scratch building an Extra...

Pics. Hopefully they show up.
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Old 01-31-2011, 05:01 AM
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Default RE: Scratch building an Extra...

Yeah, they showed up, looks really good!!!

I'll be keeping my eye on this build.
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Old 01-31-2011, 04:25 PM
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Default RE: Scratch building an Extra...

Thanks! The main things I'm worried about are the servos and attaching the landing gear. I dont have a paved runway, though I am blessed with a huge yard so I dont have to go anywhere to fly my planes. Anyway, the landing gear on my other planes tend to get bent (even ripped right out o.0) even when I land pretty gently, imo. I dont want the same happening to this one. The servos, the hs55's, are they too small? I want to save on the weight... Oh yeah, btw, I've yet to cut out the ailerons on the wings.
thoughts?
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Old 01-31-2011, 09:11 PM
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Default RE: Scratch building an Extra...

i would like to see the detail build process pictures. what lipo battery are you using for this model?
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Old 02-01-2011, 05:18 AM
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Default RE: Scratch building an Extra...

I think I have a 2S 25C 2200mAh lipo, but I'll get back to you on if thats accurate.  Not sure if the numbers are correct.. 
Oh yeah, I'll be sure to keep taking pics.

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Old 02-01-2011, 10:47 AM
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Default RE: Scratch building an Extra...

As with any hot aerobatic flying model the lighter the better. A good target weight for a 42 inch model would be in the high 20's to low 30's oz range. A lot depends on the wing area. For a model in this size range that you want to fly like an Extra should fly you'll want to keep the wing loading down to 10'ish or less oz/square foot. Find the wing area in sq inches, divide by 144 to find sq feet then multiply that by 10 to find what your target weight should be. Try to keep it under that much.

A 2S pack on a Kv=950 motor may require too large a prop to pull the required amps to achieve the optimum watts of power to the prop. You may well find that you want to switch to a 3S pack to allow you to use a better size of prop in the 9 to 11 inch diameter range.

It's not the size of the battery pack that determines the rating of the ESC. It's the amount of power the motor will draw. Also you want to over rate the ESC so it isn't operating too close to it's limit. If your motor can be proped for 20 amps it's a good idea to buy at least a 28amp or higher capacity ESC for this reason. It's also a good idea to place it in the cowl or at least in an open area of the fuselage instead of wrapping it in packing which would insulate it and prevent proper heat dissapation. They DO get warm.

For smaller models I think you're going to find that you need to go with a smaller pack to keep the weight under control. In keeping with the 3S pack suggestion above I'd suggest something more like a 3S1200mah option or similar. With throttle control that would still give you lots of flying time per flight. Also keep in mind that if the model comes in at 32'ish oz that's 2 lbs. And with the requirement to use around 150 watts per lb or even morewith such models that means the motor needs to put out 300 watts of power. At the 11.2 volts from a 3S pack that is 300/11.2 = 27 amps target current for the motor. To keep it from operating too close to it's limit I'd want that motor to have a rating of 30 amps continuous.

HS55's will be a bit on the small side for this model unless you keep the flying speed down. HS65's or something similar would be a bit more of a proper matchup.

Foam is not my material of choice but you've done a superb looking job on this model with it.. I hope when you start checking the weight that it's coming in on target for you. It would be a shame to see all that work result in anything but a great flying model.
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Old 02-01-2011, 05:49 PM
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Default RE: Scratch building an Extra...

Sorry, the battery specs should actually read: Tenergy 25C 2200mAh 3 cell 11.1 volt LiPo. Motor is a Suppo BL -2217/9, see link for details: http://www.bphobbies.com/view.asp?id...7&pid=B2632605
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Old 02-01-2011, 08:48 PM
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Default RE: Scratch building an Extra...



That's a lot of battery for this size of model.  If you're having trouble keeping the weight down then by all means by a smaller pack or two to dump a few ounces.

A few ounces may not seem like much but on small models the difference is very noticable.

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Old 02-05-2011, 04:23 PM
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Default RE: Scratch building an Extra...

Duly noted. by the way, what do you guys suggest on how to attach the wing together and how to do the landing gear? I have a few ideas, but I'd like to know what you guys do so I don't do something stupid... [:@]
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Old 05-26-2011, 12:52 PM
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Default RE: Scratch building an Extra...

Progress has been a bit slow, with school and everything, but work has picked up again. I hollowed out some of the fuse to save weight, and so far, the fuse plus the motor is coming in at around 12 oz. Pics coming up shortly...
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Old 06-01-2011, 05:59 AM
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Default RE: Scratch building an Extra...

Ok, here are the pics
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Old 09-03-2011, 01:12 PM
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Default RE: Scratch building an Extra...

pics. covered with 21st century film, canopy is yet to be painted black. some misc. work needs to be done to it yet, like hooking up all the servos, motor testing, etc.
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Old 10-01-2011, 04:02 PM
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Default RE: Scratch building an Extra...

Hi

This is kinda of off topic but i have a question related to the aspects of building/repairing !


The plane is a 40 size Cap fairly aerobatic plane and it has some cross braces broken/missing from the formers, 2 to be exact. Each former has 2 cross braces one on the top and on the bottom, the fuse of the plane itself is sheeted. How much do u think this will hurt the integrety of the airframe for general flying purposes ?

Iam debating if i should repair this or save weight and leave it be ?

Any help would be appreciated


Thanks Scott
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Old 07-21-2012, 07:37 PM
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Default RE: Scratch building an Extra...


Bump.
I'm pretty much finished with this plane, finally. But I have a slight problem. The cg is too far back, and I really don't want to add weight in the nose because the plane weighs 37 oz. I don't know where exactly the cg should be though. 1/4 of the way from the LE, or 1/3 of the way? Right now, it's around an inch back of 1/3 from the LE. I held the plane at the expected cg and put the tail wheel on a scale, and th scale read 2.5 oz. where should I have the cg, and how would be the best way to move the current cg up?
One of the pics above has a top view of the plane, btw.

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Old 07-21-2012, 10:12 PM
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Default RE: Scratch building an Extra...

I am thinking that the CG will be at the thickest point of the wing cord. To move the CG forward, start by moving the battery pack and receiver forward. You might get lucky and not have to add any weight, but getting to the battery will be difficult.
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Old 07-22-2012, 06:28 AM
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Default RE: Scratch building an Extra...

I put the battery up to the firewall, but now I plan to get a smaller one and put it in the cowl. The ESC is now where the battery used to be. I'll measure where the thickest part is also.
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