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Feb 02, 2009, 04:46 PM
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GWS J-10, pusher-prop

I like pusher-propjet models. This will be my fourth. The GWS J-10 is big, sleek-looking, and has canards that actually function -- instead of just being a glued-on/fixed decoration (like so many models of planes with canards). How cool is that?! And it has a stick mount option for pusher-prop power. So it was just a matter of time till I got around to building one.

Here's my setup:

GWS J-10 NPS kit (white/unpainted)
Scorpion 2215-1860 motor (outrunner, 1860 Kv)
APC 7x5 Sport prop
Pentium 40 ESC
Epyaya LiPo battery, 3S/2200 mAh
GWS and Hitec servos
Spektrum AR6000 receiver

The motor is one I've been anxious to try out. It's one of two Scorpion motors that is specifically suited to pusher-propjets. See this announcement.

I'm also trying out a Pentium ESC from Don's that has a built-in switching BEC, instead of my usual Thunderbird ESC with Dimension Engineering BEC. It's more simple (less wiring rat's-nest, less soldering), and cheaper. Hope it does the job as well.

I'm getting close to finishing the build. I'll post more photos when I get a chance. For now, here's a photo of the kit and some of the components.

[edit: more photos and maiden report added, in "Comments"]
Last edited by herk1; Feb 08, 2009 at 06:06 PM.
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Feb 03, 2009, 07:26 PM
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Additional photos added.

This is my first time doing fiberglassing. I just wanted to protect the plane from minor "incidents" and hangar-rash. Toward that end, I glassed the most vulnerable the very thin outer wings, the elevons, and the front of the plane from the nose cone back to the forward wing root (including the canards). I used BSI 20-minute epoxy finish-cure, and 3/4 ounce cloth.

I got some help on a nice technique for painting the clear-plastic canopy. Link here, with additional photos (thanks, Ken).

I used music wire to make a set of "handles" to open the battery hatch (photo)...a tip borrowed from a recent article on the GWS F-15 in the AMA magazine.

This kit gives two options for control of the canards -- a one-servo method with a "Rube Goldberg"-worthy linkage with many parts, and a two-servo method. I used the two-servo method because it leaves open a huge length of floorspace for the battery to be shifted back-and-forth for CG. For the two-servo method, the manual calls for the GWS "naro pro" I bought a couple of those. The molded pockets for the canard servos (either side of the fuse) are a glove-perfect fit for these servos.

My final weight, including the 6.5-ounce battery, but before paint, is 31.2 ounces. I expect paint to add about 1.5 ounces, give or take. So if my motor actually gives me the 37.5 oz of thrust as per the article that I linked to, I should have a pretty lively plane!
Last edited by herk1; Feb 05, 2009 at 10:53 AM.
Feb 05, 2009, 05:07 AM
Living the dream
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Looking good. Is it just me or do you have your prop on backwards. Writing should face the front?
Feb 05, 2009, 07:57 AM
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Wow good catch, it was on backwards! I think what happened was I had slapped that on as a "temp" install just to do the balance...and then forgot about it. Thanks!
Feb 05, 2009, 04:59 PM
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Programming the radio for a delta-wing with canards

Today I got all the electronics hooked up and programmed the radio. I use a Spektrum DX6 radio. I wanted my canards to respond to elevator inputs, but not to respond to aileron inputs. I also wanted to be able to set the canard throws independently of the elevon throws. Here's how I set it up:

First, I plugged the elevon servos into the AR6000 receiver's aileron and elevator channels, then set the DX6 to activate Delta mixing (manual, p. 35). The Delta mixing is always a chore to get working right, and the manual doesn't tell you what you need to know to do it. It always seems to take some combination of telling the radio to reverse one or both servos, and/or swapping the channels that the two servo leads are physically hooked up to. In this case I needed to tell the radio to reverse both servos.

Once I got Delta mixing working, I plugged the nosewheel steering servo into the rudder channel, and the canard servos into the gear and aux (flap) channels. The AR6000 is now completely full -- all six channels used up! I had bought a reversing Y-cable, with my original plan being to run both canards off of one channel in the AR6000...but the problem with that is that there would be no way to independently fine-tune the neutral position of each canard, because they are physically splined right to the servos (no pushrod to adjust), and if on the same channel, you wouldn't be able to give sub-trim instructions to them separately.

So I then programmed the sub-trim (DX6 manual p. 41) for the gear and flap channels to set each canard to neutral (canard faired with the shallow molded extension of the canard on the fuse).

I then set up two separate mixes -- one for each canard -- using the "Programmable Mixing (A, B, C)" capability of the DX6 (manual, p. 49). Using the "A" mix, I slaved the right canard (gear channel) to the elevator, with a negative-value "mixing value adjustment" in each direction. Then, using the "B" mix, I slaved the left canard (flap channel) to the elevator with a positive-value "mixing value adjustment" in each direction. I had to use a negative value on one side and positive on the other because of the servos' physical orientation being mirror-imaged. I found that a magnitude of about 15 percent "mixing value adjustment" gave me the amount of canard deflection that I wanted.

Then I set up my dual rates to achieve the desired control throws for high and low rate on the elevons. Then I set exponential to 25 percent on both channels and both rates.

Something to note: one of the quirks/limitations of the DX6 radio is that with the programmable mixes (the A/B/C ones), the slaved flight control will respond to the desired control stick, but it will NOT respond to the trim switch for that channel. So my canards respond to elevator stick movements, but NOT to elevator trim changes.

Another, more significant quirk: flipping the gear switch results in the respective canard going to extreme, 90-degree deflection! I taped the gear switch to avoid accidental flipping. I really don't understand why they design these radios to still respond to a switch after you've programmed it to slave to a control stick. That's an accident waiting to happen.

I also installed a battery strap and velcro floor and rechecked the CG; the plane is now in a state where it could be maidened, but still unpainted. I ran up the motor on a half-charged battery (winter storage charge of about 11.4V) and the wattmeter read 24 amps, 235 watts.
Last edited by herk1; Feb 05, 2009 at 05:04 PM.
Feb 07, 2009, 01:33 PM
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maiden success!

I couldn't pass up this unseasonably beautiful Saturday for a maiden, so I loaded up the new J-10 and went out to the club before the wind kicked up. Photos attached of the J-10 on the runway this morning (still unpainted).

I had read more than one horror-story about maiden crashes with this plane due to oversensitive ailerons and/or not enough elevator authority, so instead of the throws recommended in the manual, I went with a consensus of the recommendations that I had read on RCG. That was plus-or-minus 4mm throws for the aileron function of the elevons, plus-or-minus 12mm throws for the elevator function of the elevons, and plus-or-minus 10mm for the canards (programmed for elevator function only). There seems to be a wide variation in what people have been using for canard throws, so I was ambivalent about how to set those. When I got out on the runway and checked my flight-controls for proper direction, that aileron throw seemed so miniscule that I bumped it up a bit...maybe to 5mm. Of course, I had higher-rate throws on tap with my dual-rate switches, just in case. I double-checked, triple-checked, and quadruple-checked that my canards were operating opposite to the elevons! Then I started taxiing and realized that, with all my fretting over the elevons and canards, I had never bothered checking my nosewheel servo for proper needed reversed!

I fixed that and taxied into position facing a mild bit of wind, about 3 mph, and as I goosed the power, I could tell that the Scorpion motor with the 7x5 was giving me very lively thrust/acceleration, so it wasn't going to take much runway to get up to a good takeoff speed. And it didn't take much...but I held it on the runway a little extra to be safe. Off we go! Right away after getting airborne I could feel the plane responding very nicely to control inputs, and I was greatly relieved that the control throws I set were not only safe, they were darn near perfect. Even that small amount of aileron throw made it plenty sensitive in the roll axis, but not excessively so. I got some altitude and trimmed it up. I flew a circuit or two, then tried a stall. Wow! This plane doesn't drop a wing, it doesn't drop the nose, it just maintains rock-solid level attitude and seamlessly transitions into a soft downward float. I've never seen that before...I guess it's the canards that keep the nose from dropping. It certainly should not have been due to an aft-CG, because my CG was set to exactly the 80mm called for in the manual (you can see the taped-on CG marks on the photos). I took it around and tried the stall again and it was just the same.

Most of my flying had been at about 60 percent power, which gave a comfortable sedate cruise speed. I pushed the throttle to max to do a level "high-speed" pass to see what it could do. It wasn't as fast as I thought it might be. I would say that the top speed is in the ballpark of my stock Parkzone Trojan.

I maneuvered downwind to set up a good long final approach to land. On final, jockeying the power and getting a feel for establishing a landing attitude felt a little wierd...just like with the stall test, the plane kind of has a tendency to want to maintain that same "space shuttle" pitch attitude. I'm sure it'll just take some getting used to. It definitely responds to throttle reductions with healthy increased descent rate...this plane is not a "floater." My landing was a bit firm (pulled back the throttle too much), but no harm done other than a slightly bent nose gear wire.

I look forward to my next flight, where I will put this plane through it's paces, to see how much vertical it has, how many rolls-per-second it can do on high-rate aileron, and things like that. But first I need to paint's a keeper!
Last edited by herk1; Feb 07, 2009 at 01:41 PM.
Feb 07, 2009, 05:03 PM
Houng-wen Lin
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Thanks to herk1 did such a great job for test and writting this GWS J-10 pusher type review.
Feb 23, 2009, 05:56 PM
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It's painted

I used brush-on acrylic craft paint...2 coats most areas, some areas 3 or 4 coats. Applied decals and sealed the decals with Minwax water-based Polycrylic ("WBPU"). Topside is pale green, bottom side is bright flame-red-orange for contrast.

The paint only added about an ounce of weight, but it shifted the cg back that I had to shift the battery over an inch forward to compensate. The battery is almost full forward now. And that meant adding another battery strap. I guess that's something to watch out for with delta-wing planes...they have much more paintable surface area aft of the cg than forward of it.

Mr. Lin - Thanks for the visit, and the kind words. I now have a GWS-15 pusher-prop, and a J-10 pusher-prop. When the GWS F-4 comes out, if it has a stick-mount option, I'll probably do that one as a pusher-prop too!
Mar 04, 2009, 06:37 PM
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Cheater hole cover

Well, a pusher-prop plane doesn't have much use for a cheater hole, so I cut and shaped a cover for the hole from a piece of foam cut from a shipping carton. One of the photos in message #2 shows the foam piece and the hole in the plane it needs to cover; below are in-progress (after gluing, spackling, and sanding) and finished photos. Should get a few more mph with that drag out of the way, and it looks nicer.
Mar 14, 2009, 01:29 PM
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I flew two batteries thru the J-10 today, finally giving it a good workout. The main two questions I wanted to answer are as follows:

1. Unlimited vertical? Yes!

2. How fast is the roll rate on the high-rate switch with throws dialed up? Drill-bit fast! Too fast to even count the rolls-per-second! I need to dial down the high-rate throws to something less's too easy to lose orientation with that thing spinning like a top. I can really see why people have crashed this plane on maiden when having the aileron throws set too high.

I noticed significant speed improvement compared to the maiden flight...probably as a combination of having sealed off the cheater hole, and using some brand-new 25C batteries (EVO25's). I did a wattmeter test yesterday with a battery charged to 12.2V, and got 28.5 amps, 300 watts, and a voltage drop to 10.84V. I also did a crude thrust measurement by taxiing into my scale held on its side, and measured 37 ounces of thrust. Which agrees with the numbers posted by Lucien from Scorpion Motors. Gotta love it when a company has the integrity to publish realistic performance numbers. It also agrees with the actual flight performance that I got -- enough surplus thrust for unlimited vertical, but just barely.

GWS J-10 walkaround (1 min 5 sec)
Apr 05, 2009, 10:34 AM
Lori, hey, you're home early
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Very nice. I'm glad I found this one. I destroyed my first one due to too much control throw! Live and learn. I have another in the box.

Apr 11, 2009, 09:10 PM
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Mike - I was reading thru the Sapac JAS-39 thread a few weeks ago and seriously considered getting one and going with your idea to build it pusher. It seems like a good candidate for a pusher conversion, since everyone talks about problems with inherent nose-heavy CG hanging a motor on the back end might work out nicely without having to put dead weight in the nose like with some pusher conversions (e.g. my GWS-15 pusher). The only thing really stopping me is, why didn't they design the plane with detachable wings? For a plane that big, you really need to be able to break it down for storage and transport. I like how big the JAS-39 is, but I don't think I have anyplace that I could store it, and it would be troublesome to get into the car securely, if it would fit at all. It's too bad they didn't design it with a nice, secure detachable wing system like on GWS's J-10 and F-15.

I think it might be possible to build it without the prop-strike issue that you had, by mounting the motor up at the top of the exhaust nozzle like on my J-10.
Nov 10, 2009, 05:22 PM
Doing it in the Lateral Axis
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sweet! I did not know that this plane could be equipped with a pusher motor!
cool. I will be getting one now as I really like the looks of this plane and like the functioning canards as well.
Nov 10, 2009, 08:29 PM
Doing it in the Lateral Axis
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Just went and picked one up at the LHS herk..
Last edited by modfly; Nov 14, 2009 at 01:03 PM.
Nov 14, 2009, 10:20 AM
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Welcome to the J-10 clan Modfly! My J-10 needs a little bit of repair work before I can fly it again. The last time I flew it, I was maneuvering out a bit far on a downwind leg for landing approach, and as I turned to base leg, I got into some orientation confusion. I was unable to get oriented again in time to save it from disappearing behind a steep slope off one end of the club field. When I found it, it had "landed" in a tree about 15 feet up. I managed to get it down by poking it with a PVC pole that the club keeps handy for that purpose. The plane was virtually undamaged except for the damage I did to it poking it with the pole, and dragging against branches on the way down. So now it has some dings that need filled, and a torn-out elevon that needs reinstalled. I think that the fiberglassing work that I did on it really helped to keep it from getting badly gouged/shredded.

The orientation difficulty is the only negative thing that I can say about the model. That sleek, narrow delta wing makes it darn hard to stay orientated on it inflight...even with the highly contrasting paint/tape job that I did on it. It looks like a dart in the air!

What do you plan on powering it with? I will say that I'm real happy with the Scorpion motor that I put in it; it's a perfect match. As long as you're not looking for blistering speed (and you might be able to get that too, if you went to 4S with the motor).

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