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53" Cessna O-2 in Depron

#1
Having lost (literally, see the glider topic) my scale Glasflugel Libelle I wanted something scale to replace it.
The twin Wren was fun to build and fly so a bigger twin.
O-2paint1.jpg
The Cessna O-2 was the military version of the Skymaster and used 2 blade props. That 'night' camouflage looks pretty cool. :cool:

The push/pull arrangement meant one engine could be shut down with no asymmetric forces.
I recently bought a DX6i which has the ability to do 'mixes' so how about arranging a motor shut down in flight?
[video=vimeo;64051797]https://vimeo.com/64051797[/video]
I found this 3 view which gives some of the fuselage cross sections.
O-2sections.jpg
Scaling for 9" props gave a useful span of 53".
As in the full size it will use counter rotating props so with both running there is no torque reaction either.

The 1200kV 2822s I used in the video each give about 20oz thrust at 15A on a 3s. I guestimate as a lightweight belly lander an all up of 20oz so it should have a more than adequate performance on one and on two? Straight up! :)
 

Foam Addict

Squirrel member
#3
Awesome!
But if I may suggest a camo scheme?
I will post a pic soon, there is one at my local airport.
EDIT: hmm, apparently that was a custom scheme, never mind. Black is really cool too.
 
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#4
Using a former and skin technique means there is virtually no internal structure so dedicated plans are not really required, the 3 view blown up to the appropriate size is sufficient.
The 'tile printed' top view to give the wing plan.
PlanTop.JPG
The wing section will be true scale NACA 2412 root reducing to NACA 2409 at the tip.
naca2412.jpg
As I intend a black colour scheme it will built from grey Depron.
Each wing is built from the lower skin. The trailing edge has been sanded down to a thin edge.
LHbottom.JPG
It has no flat bottom so the skin has to be hand formed to the correct shape and the leading and trailing edges supported at the correct level. Note the tapered 3mm hard balsa spar flange is built into the skin.
The ribs are added and Depron shear webs glued in between them.
The top spar and leading edge skin added.
LHdspar.JPG
As this forms a complete 'D' box the wing is rigid enough to be handled.
The tiny 3.7g aileron servo glued into position with the scale aileron cut out.
AilServo.JPG
It seems few people believe such a small servo can ever be adequate. I do use very free running hinges and simple direct linkages to make best use of the force available. I also take the view that if the aerodynamic load on the control surface is so high that the servo stalls then that is fine. It stops the wing from being over loaded! ;)
 
#6
A bit more progress.
I thought quite a bit about the tail booms and eventually decided to take a risk and make them out of Depron as well. I can only hope they will be rigid enough.
They are quite a tall oval in section so could be made from a simple box with bulged sides and the top and bottom sanded to shape.
TailBoom1.JPG
Once the tail boom was complete the wing under surface was carefully cut away.
TailBoom2.JPG
The leading edge of the boom is notched to fit the 'I' section spar.
The wing joined.
WingJoin.JPG
The tailplane is made up of two 3mm Depron skins and a hard balsa leading edge. The 3.7g elevator servo is built into the centre of the tailplane. Its cable runs inside the right hand boom.
It will be a 'bank and yank' so there is no rudder.
The wing skin complete and the ailerons and elevator added with full length 'magic tape' hinges.
WingTail.JPG
Complete with the servos it weighs just 6oz.
It does seem a bit odd to be able to complete all the flying surfaces without even starting the fuselage!
The
 
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earthsciteach

Moderator
Moderator
#7
Looks great, man. Question - how did you settle on the NACA 2412-2409 tapering airfoil? Is this the airfoil the O2 uses?
 
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#8
earthsciteach
Yes those are the full size wing sections.

The relatively bulky fuselage is a former and plank skin construction built over the 'plan' as a half shell.
Fuseplank1.JPG
The planking under way.
FusePlank2.JPG
Once sufficient planking is complete for rigidity it is lifted off the board and the other half of the formers added.
FusePlank3.JPG
And the planking started. The inside of cockpit is fully sheeted and will add considerable stiffness to the cockpit area.
FusePlank5.JPG
Before the planking is complete the front ESC and wiring has to be added.
FrontESC.JPG
Along with the front motor.
FrontMount.JPG
The Depron motor tube spreads the load between two formers and allows the actual motor bulkhead to be glued in position within the tube to give the correct prop location.
The front Emax 2822 in position.
2212Front.JPG
Installation of the rear motor and ESC can wait until the wing is ready to be installed.
I got tired of just looking at the boat shaped fuselage so I balanced on the wing assembly to get an idea of the planes size.
WingFuse1.JPG
I hope it will fit in my car!
 
#10
The final design issue is the cockpit area.
As I was not sure where the CofG would be I allowed for a huge battery compartment. The initial assembly demonstrated that it did not need to be anything like as big.
BattBox.JPG
As a result the nose decking was extended back to the windscreen line.
Cockpit1.JPG
The rear ESC and motor installed.
RearESC.JPG
It is not not exactly short of space!
The rear 2822 installed.
2212Rear.JPG
With the wing glued in place the rear top decking including the air scoop is added. Lightweight filler is used to smooth the joints in the planking.
AirScoop1.JPG
Rather fiddly to do but the air scoop is functional ducting cooling air back to the rear motor.
The complete airframe.
Wingfixed1.JPG
Now just the cockpit canopy to do.
As I hoped with an 1800mAh 3s it weighs just over 19oz.
 

Foam Addict

Squirrel member
#11
Picking my jaw up off the floor...
That is a huge battery bay!
How did you reinforce the rear motor? It doesn't seem to have anything around it but depron. Did you soak the Depron in CA?


If I may say, I am a little worried about those booms.
Don't try any loops on the maiden.;)
 
#12
Foam Addict
Indeed both motors have nothing but Depron around them!
The rear motor is mounted on a modest particle board bulkhead which is inserted into the Depron fuselage former with additional Depron bracing to carry the loads out to the fuselage skin.
The intent is that any impact strong enough to cause the motor to break free would have already crushed much of the fuselage.

To be fair the battery was never intended to fill the bay (you could easily get five 2200 mAh 3s in the space but to allow the battery to be easily positioned as even the 1800mAh 3s is 25% of the weight of the complete plane.
It is located by Depron lugs glued to the cockpit floor and held down by velcro.
BatteryFix.JPG
The small box is the Orange Rx in white Depron so its LED will be clearly visible.

I am reasonably happy with the strength of the booms for flight but any sort of cartwheel landing........
 
#13
Next comes the canopy. More Depron planking.
Canopy1.JPG
It is really just a big hollow Depron box with small magnet on its leading edge.
CanopyOff.JPG
It weigh less than 1/2 oz.
The Cessna is ever so black!
Black2.JPG
The windows with the plan cut out as a guide.
LHwindows.JPG
The full size had a low viz red USAF Star and Bar that was stencilled on.
StencilFuse.JPG
Very fiddly to cut out. It was used a a mask for enamel spray onto sticky backed plastic.
The top decal.
StarTop.JPG
Then I had to wait for some calm weather.
[video=vimeo;66953772]https://vimeo.com/66953772[/video]
This was only its second ever flight.
It flies remarkably well with a remarkable 'straight up' climb with both on full. The complete absence of torque is very comforting.
Note it has not yet got its wing struts fitted.
It also has quite an adequate performance on one so a couple of days later I tried a 'loiter' flight just gently cruising about on one with the HK altimeter on board.
28May13.jpg
The flight was terminated at 29 minutes not because it was at the LVC but because it started pouring with rain!
 

FlyingMonkey

Stuck in Sunny FL
Staff member
Admin
#14
Amazing work! We have a full scale one of these at the local air museum. I'll have to remember to take some pictures and share them here.
 
#15
As the first loiter flight was terminated because of rain and now the weather was superb, clear blue, warm and virtually no wind, I just had to have another go, just using rear motor.
03Jun13.jpg
45 minutes from just a 1800mAh in a scale airframe and lugging round a dead motor with a stationary prop!
The rise in cockpit temperature (green line) was largely due to the black finish doing a good job of absorbing the strong June sunshine!
I did video the whole flight but it would really only be a candidate for the most boring video ever!

When edited down showing the launch and landing the key fob cam on screen clock shows a flight time of 44:45.
[video=vimeo;67550588]https://vimeo.com/67550588[/video]
I am not sure what this proves but it certainly builds up the stick time!
 
#16
The O-2 really does have pretty benign handling characteristics allowing low and slow aerobatic manoeuvres that would spell disaster in some of my other planes.
Of course its extreme thrust to weight helps as well.
[video=vimeo;67734020]https://vimeo.com/67734020[/video]
In truth its roll rate is a bit low for this sort of thing but it nevertheless feels very 'secure' in the air. :)
 
#19
FlyingMonkey
Nice pictures. Those 'clouded' perspex windows even look quite like the grey paint windows on mine!

With some rather nice weather I flew the O-2 a bit harder.
Apart from having some fun I was trying to determine if any flexibility in the tail booms became an issue when flying at speed.
[video=vimeo;67804670]https://vimeo.com/67804670[/video]
Still not quite full power speed but so far so good and a nice bounce on landing!
 
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