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Old Aug 06, 2008, 12:17 AM
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GWS A4 Skyhawk w/mech retracts using 1 servo

I kept trying to talk myself out of doing this. As I kept looking at it, the layout of this plane looked like it would be possible to do with a single servo, the plane's large enough to take retracts, and the more I thought it through, the more the mechanics seemed to fall in place. Probably the best thing is that I thought about it for a good while before rushing into it, which tends to help avoid problems and afterthought with mechanisms like retracts.

First, the airframe was braced with CF sparring, which also is located such that it ties in the gear/servo mounts for added rigidity. Since reading is boring and pics are worth a thousand words, here they are:

Bill
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Old Aug 11, 2008, 02:13 AM
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Works perfectly.
Worth the effort of working out all the throws, to use a single retract servo.

Gear used:
The Waypoint micro retract servo worked well, and fills a niche for this size of plane, as there are not many smaller retract servos on the market. Used the second outermost hole for all 3 E-Z link attachments to the retract servo. This works well, as the throws for the Robart and Great Planes retracts are nearly identical.

Nose gear-Micro Robart Steerable
Main gear-Great Planes .10 size

This setup would be good for any low wing jets, where the linkage can all be routed to the single servo on relatively the same plane (plane meaning flat surface in space, not airplane).

Bill
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Old Aug 12, 2008, 11:36 PM
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Covered the wing bottom with Econokote to clean up the appearance and aerodynamics. Figured I'll make a retract servo cover if I need to get in there.

The GWS Type15 connectors seem to work well, but I don't fully trust them. The covering also ensures that the wings will not come loose. Also adds a degree of assurance to stick-on servo attachements, as the servos are covered. If I build another one of these GWS jets, I've learned that the locking connector blocks need to be mounted such that they protrude maybe 0.010" from the wing/fuse joining surfaces. Mounted flush with the surfaces, these things are dang tight to lock. I had to shave a small amount from the hook edge on one of these Type15 "glad hand" connectors, to get it to latch.

BTW, the GWS canopy latch is strong enough to lift Roseanne Barr. Could anyone imagine someone trying to get those latches to adhere to the foam canopy/fuse with GWS glue?
I didn't bother to file down the canopy latch pin to reduce grip strength, and just used rare earth magnets instead. I've saved 2 of the latches so far, and may just use them as a heavy duty hinge for something someday.

Bill
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Old Aug 16, 2008, 01:21 PM
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I generally only like camo on Luft planes, but a brown camo scheme struck me as I've seen so many Skyhawks with solid colors. The subject I came across on the web is a single seater, but the colors seemed to work well, so I found close matches with Model Masters enamels.

Amazingly (well at least I think so) this plane is lighter than expected, with a heavy JS1400 and EVO Lite 4s-2500. AUW at 40.5oz. I was expecting closter to the 48oz AUW of my Monokote covered GWS15 with stock motors and 3s-2100.

GWS appears to be referencing their CG measurment from the top leading edge of the intakes, which are further forward than the bottom leading edge. The probably should clearly state that in the manual, as it is a difference of maybe 4mm. From that point the CG is roughly 162mm. Considering that I usually seem to lean toward the forward CG settings, it will probably be fine without requiring any lead, which is always a good thing.

Bill
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Old Aug 16, 2008, 04:01 PM
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Quote:
Originally Posted by EDF30
GWS appears to be referencing their CG measurment from the top leading edge of the intakes, which are further forward than the bottom leading edge. The probably should clearly state that in the manual, as it is a difference of maybe 4mm. From that point the CG is roughly 162mm. Considering that I usually seem to lean toward the forward CG settings, it will probably be fine without requiring any lead, which is always a good thing.

Bill
It's kind of hard to tell from the manual. One of the pictures looks like it's from the top of the intake and another looks like the middle or bottom. Here's a picture of where I balance mine. I just use the intersections of a couple of the panel lines when the plane is balanced upside down. It's nice and stable for me at this point.

Andy
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Old Aug 17, 2008, 12:40 AM
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Originally Posted by ajreynolds
It's kind of hard to tell from the manual. One of the pictures looks like it's from the top of the intake and another looks like the middle or bottom. Here's a picture of where I balance mine. I just use the intersections of a couple of the panel lines when the plane is balanced upside down. It's nice and stable for me at this point.

Andy
Sounds good. I'm about 2mm back from that line.

They should just state the CG as being on that line, other than ambiguously referenced measurments. It is a bit forward of the reccomended range, but I tend to always be there anyways. I believe that a reccomended CG should be as conservatively forward as possible, without being a lawn dart. Gives flyers of different skill levels a better chance of success.

As for swept wings and deltas, I've noticed that the swept angle does not push the CG back as far as the calclulations would entail. I'm sure there are reasons for this.

Bill
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Old Aug 17, 2008, 01:44 AM
what it do everybody?
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The hatch hinge on mine loosened up after about 6 flights. It's just right now.

I've run various dual packs in mine and it still holds cg at 170mm.
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Old Aug 17, 2008, 02:01 AM
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Originally Posted by zaskarkid
The hatch hinge on mine loosened up after about 6 flights. It's just right now.

I've run various dual packs in mine and it still holds cg at 170mm.
Quote:
Originally Posted by EDF30
BTW, the GWS canopy latch is strong enough to lift Roseanne Barr.
So I guess it won't do the job anymore.

I used the 3/8" dia HL rare earth magnets, installed in GWS nut mount plates, after drilling out the nut recess area to enlarge it a bit.

Bill
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Old Aug 17, 2008, 02:28 AM
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very cool! what fan are you using?
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Old Aug 17, 2008, 08:53 AM
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Originally Posted by corsair nut
very cool! what fan are you using?
Thanks. It's an overpowered SF fan. I made an outlet duct to reduce to around 53-54mm.
I've ran it at 450W and the JS1400 on 4s is barely getting started at that level. I'll probably get an HW 505 rotor for it, if I find the desire to go past that.
As of now, I'm guessing that this motor has enough torque to easily drive the rotor with 6 blades, and not require stripping to 3 blades.

I did have to make a faring to reduce the intake diameter inside the fan cover. Probably a bit draggy as it's fairly steep, but still better than not blending it in at all.

If you do another, the retract setup is nice in that it doesn't add much weight with the smaller Waypoint servo.
I know when you glass/fill the entire plane as you did, you can't afford to throw much weight after it afterwards. This plane is about about 40.5oz after painting. Lighter than my iron-on covered GWS15 by a good 8oz.
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Old Aug 17, 2008, 09:34 AM
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youll want about a 2" outlet for the SF fan
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Old Aug 17, 2008, 10:27 AM
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Originally Posted by corsair nut
youll want about a 2" outlet for the SF fan
Sounds about right. I did a 90% FSA calc, and I believe it was around 52mm off top of head.
2" is 51mm. I went downstairs to measure the outlet exactly and it is at 53mm. Close enough.

I've heard people say anything from 52mm to larger than 55mm here so far. I thought above 55mm would definitely be too large, especially considering my reduced FSA with the JS1400.

Bill
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Old Aug 17, 2008, 05:05 PM
what it do everybody?
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At least you won't have to worry about the air not getting to the motor

A tube turned my a-4 cow into a jet with decent top end, but at the expense of some vertical, b/c it takes quite a bit of run way to get my 1200g bird up. Blows away stock phase 3 f-16 in sound and would make for a good drag race.
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Old Aug 18, 2008, 03:55 AM
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Quote:
Originally Posted by zaskarkid
At least you won't have to worry about the air not getting to the motor

A tube turned my a-4 cow into a jet with decent top end, but at the expense of some vertical, b/c it takes quite a bit of run way to get my 1200g bird up. Blows away stock phase 3 f-16 in sound and would make for a good drag race.
What size fan are you using? Stock?
If stock, I imagine it would be faster with a smaller outlet. The stock size is huge, and probably used to optimize thrust from the GWS motors, versus speed. With better motors, it would probably do better if a bit smaller.

------------
Did a bit of detailing to the Skyhawk. Looks a bit less plain now. Wish they had given white stars-and-bars in the decal set.

Bill
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