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Cessna O-2


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The Cessna O-2 was the military version of the Skymaster.

Skymaster O-2

That's a rather cool colour scheme!

From a scale point of view the O-2 used 2 bladed props rather than 3 on the civilian version.

A tandem twin has the advantage that an engine shut in flight has no asymmetric thrust issues - so how about arranging that this can actually be done in flight?

A tandem motor test rig and a bit of fiddling with the mixes on my DX6i.

It seems the capability of the mixes is limited. So far I have only been able to arrange for one motor to be shut down.

A 3 view - my usual starting point and it has the advantage of fuselage sections.

O-2 three view

Scaling up from a pair of 'handed' 9" props gives a span of 53".

One advantage of a twin tail boom layout means you can build and complete all the flying surface before even starting the fuselage. wink 2

 

 

Edited By Simon Chaddock on 03/05/2013 15:53:24

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I built a Skymaster, about 50" span in balsa from a French kit about 15 years ago, I remember that I had to alter a lot of the construction and made the wings, boom and tail in one piece rather than have detachable outer wings, far too complicated.. There also was not much room for the rear fuel tank. The front motor was a Super Tigre 0.34 and the rear a Thunder Tiger 0.25. It was finished in silver Oracover because I had been given 10 metres of it.

When I was running up the engines for the first time I was very aware that I had to be careful about keeping my hands safe, I kept telling myself 'be careful'. Of course I put my left thumb in the front airscrew and took about 2mm off the tip, oh did it bleed and hurt. face 14

It flew very fast, far too fast even at half throttle..

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Thanks for the offers but I should be able to build directly from the 3 view as I can tile print it to any size - almost!

At 53" this will be a 'one piece' plane as it will just fit in my car. wink 2

The wings boom and tail will be built first and then the fuselage added. It will be a hand launch belly lander.

In Depron my target weigh is under 20oz. Given that each motor produced nearly that much thrust it should fly ok with one shut down!

First the wings. It will be scale NACA2412 at the root to 2409 at the tip using a flush balsa/Depron/balsa spar with 3mm Depron skin. Grey Depron in this case as it is going to be painted black.

The bottom skin with the 3mm hard balsa spar inserted.

LH bottom skin

As the NACA24xx series has a cambered underside the wing skin is first 'formed' to approximately the correct profile and then the leading and trailing edges supported on Depron blocks.

The spar complete with the leading edge skin. The wing is then rigid enough to be safely handled.

LH spar

The full span but not yet joined.

Left and right wing

Next the tail booms.

Edited By Simon Chaddock on 04/05/2013 14:04:51

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I had to think quite a bit about the tail booms.

My previous booms have always been glass fibre tubes which compared to the rest of a Depron structure are really too strong and thus heavy. The Cessna booms have quite a big so the question was could they too be made of Depron?

They are quite a tall 'oval' in section. i decided this could formed from a simple oblong tube with bulging sides with top and bottom sanded to shape.

Tail boom1.

The centre spacer down the middle stops the sides being squashed in.

The lower wing skin is carefully cut away and the boom glued in.

Tail boom 2

The leading edge of the boom is stepped to pick up the profile of 'I' beam spar.

The completed boom if very light and felt adequately stiff. If necessary this could always be improved by covering it with tissue stuck on with PVA.

Tail boom 3

The aileron has been cut out and a 3.7g micro aileron servo installed.

As the elevator servo will be in the tail plane its servo wire will have to built in to the other boom.

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The fuselage is built as a half shell, planks over formers.

Fuse Plank 1

The formers are white simply because it is translucent so they can be traced out holding the sheet of Depron over the plan up against a window.

I have to admit some of the design features are made up as I go along but one thing now fixed is that the complete cockpit section will be removable for battery access.

With its full compliment of formers the planking continues but it is a rather slow process.

Fuse Plank 2

Unlike the Libelle and the V-1 which a simple flowing lines the Cessna fuselage is quite a complex shape so the plank shape gets complex as well.

Although this method of construction is very 'wobbly' to start with it does get stiffer and stiffer the further it progresses and is very light.

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Posted by Simon Chaddock on 08/05/2013 00:19:29:

Although this method of construction is very 'wobbly' to start with it does get stiffer and stiffer the further it progresses and is very light.

Simon - almost had to report that one

Looking really good, envious of the time to do such a lovely job!

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The first half complete and the formers for the other side added.

Fuse Plank 3

Planking started on the other side. Quite an area of the fuselage is flat so the initial planks are specifically cut to size but the rest has sharp radii and double curves!

Fuse Plank 5

The formers in the cockpit area have been cut away to make a huge battery compartment. The cockpit will be fully lined in 3mm Depron so will add significant torsional stiffness.

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With e cockpit area fully lined the front motor and ESC have to go in before the skin can be completed.

Front motor ESC

The unusual tube motor mount spanning between two formers.

Front motor mount

The actual motor mounting board is glued inside the tube about 1/2" back from the front bulkhead.

Emax 2212 front

The rear motor can be left a bit longer as there is sufficient access until the wing is glued on.

Now this is what I call black, very black!Very black

It will be sprayed with a clear lacquer - more to keep your fingers clean than anything else!

Edited By Simon Chaddock on 10/05/2013 15:29:29

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With the basic fuselage skin complete the rear ESC goes in.

Rear ESC

Not exactly short of space!

The Emax 2822 pusher.

Emax 2822 rear

The air scoop above the wing will be functional to provide air to cool the motor but until the upper skin is in place the mounting is not really rigid enough to risk running at full power.

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I got tired of just looking at the boat shaped fuselage so I balanced the wing on to see what it looked like.

Wing & fuselage

Although it looks quite a substantial plane it is very light so its power requirement are modest. The Emax 2822 motors are only rated at 125W (12A on 3s) each but at 20oz all up that still means it has 100W/lb with one shut down!

The 1800mAh 3s battery is almost lost in the huge battery compartment.

Battery box

Oh! I forgot the radio has to go in there as well. wink 2

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With the wing glued in place the air scoop and rear decking can be completed.

Air scoop


Still 3mm Depron and fully functional delivering air to the rear motor. It took along time to do!

As I don't need anything like the full access for the battery I have taken the nose top decking back to the windscreen line.

Cockpit to windscreen

At this point I realised I had made a mistake! sad

I originally sized the Cessna for 9" props but I just found out the handed pair I have are actually 8x4.5.

Not that it really matters as there is still plenty of push from the smaller props with a thrust to weight ratio close to 1:1 with just one motor! smile

 

 

Edited By Simon Chaddock on 14/05/2013 14:38:42

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Just about complete enough for a ground run.

It has a ferocious level of thrust so I dared not use full power with both running.
I thought quite a bit about wether or not to add glazing in the cockpit.
For weight reasons, this is after all a light weight model, I have come down of the side of solid glazing in other words the canopy will be made Depron like the rest of the airframe.
However the underlying structure does allow for full glazing to be added if solid looks too strange.
.
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