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Not another Super Cub!

Old 07-21-2013, 12:00 AM
  #1  
quorneng
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Default Not another Super Cub!

I do not have a scale 'conventional' plane so I thought I had better build one - out of Depron!
Like many high wing light aircraft the Super Cub relies entirely on its wing struts for strength so I would do the same.
At 40" span the wing halves would be rigid enough to need no spar simply relying on the 2mm Depron as a stressed skin to resist the bending and twisting loads.
Hopefully the end result will be very light.
First a 3 view.
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The Super Cub uses a USA35b wing section.
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As the wing skin is relatively thin the wing ribs a quite closely spaced, 13 on each side.
The wing ribs glued onto the lower skin with a single Depron spacer.
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The dark blobs are the Depron wing strut attachment points.
The 3.7g aileron servos are placed outboard of the wing struts.
The RH wing top skin added.
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The aileron is tape top hinged and the leading edge is a strip of 3mm Depron carefully sanded to shape.
There is no attachment mechanism at the root as it will simply be glued directly to the side of the fuselage.

The result is a very smooth exact scale wing made entirely of Depron.
Each half weighs just under 1oz and that includes the aileron servo.
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Old 07-21-2013, 12:40 AM
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CHELLIE
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subscribed I always build a wing to survive 10 Gs, its easy to load up a wing to 10 Gs just by just doing a simple fast loop, I know your a Expert Builder, but seeing no spar in the wing makes me cringe a little I iknow the struts will help a lot to support the wing too, Good luck on the build, Chellie
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Old 07-21-2013, 10:39 PM
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quorneng
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Chellie
The way I look at it a fully sheeted wing is a box spar. A funny shape maybe but a box spar none the less.
My simple strength test is to pick up a ready to fly plane by its wing tips. This produces bending equivalent to a 4g manoeuvre - a gentle loop?
I would expect my Cub wing will be able to carry twice its own weight and still be picked up by its tips.

My original intention was to build the fuselage as two half shells so I cut out half formers.
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I then realised the fuselage could be just as easily be assembled using a more normal construction type build.
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It also meant I could leave the top open to install the all the electrics.
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To save weight there is no cockpit glass. It will be painted on.
I copied the motor mounting method I used on my Cessna O-2 with a Depron tube glued between the first two formers.
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This provides a very light mounting and allows the circular motor bulkhead with the motor attached to be glued in exactly the right place to position the prop.
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In this installation the bell of the 1500kv Emax 2805 runs very close to the motor tube!

For no reason other than I was advised that flying a Super Cub would be very difficult without a rudder mine will be bank and yank!
The 3.7g elevator servo mounted right under the tail plane.
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The wing and fuselage laid out to give me an idea of its size.
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As the motor only takes 8A on a two cell I should be able to use an 800mAh battery which should give an all up weight below 10oz and a wing loading of under 7oz/sqft.
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Old 07-23-2013, 02:55 PM
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quorneng
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With such a lightweight build how to fix the undercarriage needed some thought.
I eventually setted on a non scale spring steel type rather than a bungee arrangement.
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The U/C legs are located by a plastic 'plate' glued to the side of the fuselage.
It has the benefit of chunky but very light 1.75" diam by 1" wheels. They are made of Depron!
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The tail wheel is mounted using a similar technique.
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The complete airframe.
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As in the original the wing struts are located by the rear fuselage mounting.
A top hatch provides access to the radio, ESC and battery.
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The windscreen is built up using Depron planks.
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With an 800mAh 2s it weighs just 7.5oz.
The video of its actual maiden, still unpainted.
https://vimeo.com/70623236
Needs a bit of adjustment but it looks like it is going to have to be a calm day only flier!
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Old 07-24-2013, 12:11 AM
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dgjessing
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Originally Posted by quorneng View Post
With an 800mAh 2s it weighs just 7.5oz.
The video of its actual maiden, still unpainted.
https://vimeo.com/70623236
Needs a bit of adjustment but it looks like it is going to have to be a calm day only flier!
Wow - that's light!
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Old 07-24-2013, 12:53 AM
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quorneng
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I decided on a simple contrasting colour scheme - yellow and black!
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Flies well enough but still needs a bit more tweaking to fly really well.
https://vimeo.com/70767262
Later I actually managed to demonstrate its STOL capability and achieve a complete take off and landing sequence from rather rough and long grass.
https://vimeo.com/70798549
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Old 09-24-2013, 09:48 PM
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DavidR8
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Default Compared to the Champ

Originally Posted by dgjessing View Post
Wow - that's light!
It's a veritable stone. The Champ flying weight is a mere 1.3 oz or 38 grams.

http://secure.hobbyzone.com/HBZ4900.html
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Old 09-24-2013, 11:50 PM
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quorneng
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DavidR8
It may be 'stone' compared to the Champ but then the Champ doesn't have:-
a wing span of 40"
or ailerons
or carry a 800mAh 2s LiPo
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Old 09-25-2013, 02:51 AM
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Originally Posted by quorneng View Post
DavidR8
It may be 'stone' compared to the Champ but then the Champ doesn't have:-
a wing span of 40"
or ailerons
or carry a 800mAh 2s LiPo
I missed that spec somewhere along the line. My bad
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Old 11-17-2013, 07:53 PM
  #10  
quorneng
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An update on the Super Cub.
It now flies with a 1500mAh 2s which fives it a cruise duration approaching an hour.

To achieve a reasonable bank & yank turn required 100% aileron differential. This obviously reduced the roll rate so I added a rudder but controlled by a pull/pull system from the ailerons.

The mono filament line is attached to the aileron horn and passes through an eye to travel down the wing.
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Close to fuselage another eye is used to turn the cable towards the tail.
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The cable passes directly to the double sided rudder horn.
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And similarly back to the other aileron horn.
It works.

This is really just a lash up to test the principle.
The mark two version will steer the tail wheel as well.
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Old 11-20-2013, 02:17 AM
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quorneng
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The combined rudder /aileron actually works very.
Even with all the programmed differential removed there is little or no adverse yaw and of course the roll rate is dramatically improved indeed it is now quite pleasant to fly.
As my calculations suggested it should be able to cruise for nearly an hour I thought I had better try and see.
The HK altimeter trace from this morning.
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50 minutes 11 seconds to be exact.
It was not at the LVC and the 1500mAh battery only needed 960mAh to recharge it so taken right down to the limit well over an hour should be possible.
Using 960mAh from a 2s in 50 minutes means its average consumption was 8.5W and 15.3W/lb.
Not bad for a 40" scale plane with wing struts and balloon tires on its undercarriage.
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Old 11-20-2013, 03:46 AM
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kyleservicetech
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Originally Posted by CHELLIE View Post
subscribed I always build a wing to survive 10 Gs, its easy to load up a wing to 10 Gs just by just doing a simple fast loop, I know your a Expert Builder, but seeing no spar in the wing makes me cringe a little I iknow the struts will help a lot to support the wing too, Good luck on the build, Chellie
Yeah, add me to the readers that like building a strong wing. Four years ago, I did some tests on "G" loads on a model during a simple loop. One of my models hit 9.5 G's on exactly that type of a loop.

Take a look:
http://www.wattflyer.com/forums/showthread.php?t=47288
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Old 11-20-2013, 05:44 PM
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quorneng
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Hmmm.
My rule of thumb is:-

If you can pick up a plane that is ready to fly by its wing tips it generates bending loads equivalent to pulling 4g.
If you can do the same with the total planes weight added in the centre of the wing it is 8g.
If you can do it with twice the planes weight added you are at 12g.

Each of these block of steel weighs 9oz (just over the Super Cubs flying weight)
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The Super Cub (with its flight battery installed) carrying this load supported only by its wing tips.
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The wing may be spar-less but I think this demonstrates that it is very unlikely to break with any load it can generate in flight!
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