A 53" Cessna O-2, scale and in Depron!
05-05-2014, 09:47 AM
#1
Thread Starter
A 53" Cessna O-2, scale and in Depron!
Many people class the use of Depron in a plane as just a 'foamy'.
This rather ignores the fact that polystyrene from which Depron is made is an 'engineering' plastic with recognised load bearing capabilities.
Sheet Depron has quite a sophisticated make-up having a lightweight core with a denser 'skin' on both sides which gives it reasonable 'structural' properties in both tension and compression. As Depron is only about 1/10 the specific density of balsa it does mean its actual strength to weight ratio is not that different provided you make effective use of it properties..
I wanted to build a twin and an in-line with counter rotating props would eliminate any motor torque problems in what was likely to be a light weight plane.
The Cessna Skymaster filled the bill nicely and the military O-2 version in these markings looks pretty cool too.
I found this 3 view that gave a range of fuselage cross sections.
The O-2 used a NACA 2412 wing section.
I fly from a rough grass field so a hand launch/belly land is really the only option and it saves the complexity and weight of an under carriage!
So I now had all the information required to build my true scale Depron O-2.
.
This rather ignores the fact that polystyrene from which Depron is made is an 'engineering' plastic with recognised load bearing capabilities.
Sheet Depron has quite a sophisticated make-up having a lightweight core with a denser 'skin' on both sides which gives it reasonable 'structural' properties in both tension and compression. As Depron is only about 1/10 the specific density of balsa it does mean its actual strength to weight ratio is not that different provided you make effective use of it properties..
I wanted to build a twin and an in-line with counter rotating props would eliminate any motor torque problems in what was likely to be a light weight plane.
The Cessna Skymaster filled the bill nicely and the military O-2 version in these markings looks pretty cool too.
I found this 3 view that gave a range of fuselage cross sections.
The O-2 used a NACA 2412 wing section.
I fly from a rough grass field so a hand launch/belly land is really the only option and it saves the complexity and weight of an under carriage!
So I now had all the information required to build my true scale Depron O-2.
.
05-05-2014, 12:58 PM
#2
Thread Starter
I had to interrupt my first post.
Using Depron as the planes primary structural material means where ever possible the skin will be taking most, if not all, the load.
The only exception is in the wing where Depron alone would not have sufficient rigidity so it needs some reinforcement.
Whilst it would be simple to insert a carbon fibre tube spar this is by no means the lightest solution. Carbon is so much stiffer (and heavier) than Depron that it would carry virtually all the load leaving the Depron there just keep the air out.
My solution is to in effect build the entire wing in Depron but add strength and stiffness only where it is required in the form of balsa spar flanges. Balsa sticks very well to Depron and it flexes enough to ensure the Depron still takes a significant share of the load.
For maximum possible rigidity the 3mm balsa wing spar flange is part of the 3mm Depron wing skin.
With the ribs and spar shear webs added the leading edge skin incorporating the top spar flange is glued on.
At this point the wing is rigid enough to be handled and the tiny aileron servo can be added.
The tail booms are hollow 3mm Depron structures.
The lower wing skin is cut out and the boom glued in. Note the leading edge of the tail boom overlaps the top and bottom of the wing spar
The elevator servo extension wire is run inside the LH tail boom.
The whole process is duplicated for the other wing half so the wings can be joined.
The tailplane has a symmetrical section with 3mm skins and contains the elevator servo. The fins are single 3mm sheets sanded to a streamline profile.
There is no moveable rudder as this plane will be 'bank and yank'.
m
With the aileron and elevator servos wires extended and tested the wing top skin is completed.
The ailerons are tape top hinged with an angled leading edge to allow 30 degrees down movement.
It does seem rather strange to build and test the complete control system without even making a start on the fuselage!
Using Depron as the planes primary structural material means where ever possible the skin will be taking most, if not all, the load.
The only exception is in the wing where Depron alone would not have sufficient rigidity so it needs some reinforcement.
Whilst it would be simple to insert a carbon fibre tube spar this is by no means the lightest solution. Carbon is so much stiffer (and heavier) than Depron that it would carry virtually all the load leaving the Depron there just keep the air out.
My solution is to in effect build the entire wing in Depron but add strength and stiffness only where it is required in the form of balsa spar flanges. Balsa sticks very well to Depron and it flexes enough to ensure the Depron still takes a significant share of the load.
For maximum possible rigidity the 3mm balsa wing spar flange is part of the 3mm Depron wing skin.
With the ribs and spar shear webs added the leading edge skin incorporating the top spar flange is glued on.
At this point the wing is rigid enough to be handled and the tiny aileron servo can be added.
The tail booms are hollow 3mm Depron structures.
The lower wing skin is cut out and the boom glued in. Note the leading edge of the tail boom overlaps the top and bottom of the wing spar
The elevator servo extension wire is run inside the LH tail boom.
The whole process is duplicated for the other wing half so the wings can be joined.
The tailplane has a symmetrical section with 3mm skins and contains the elevator servo. The fins are single 3mm sheets sanded to a streamline profile.
There is no moveable rudder as this plane will be 'bank and yank'.
m
With the aileron and elevator servos wires extended and tested the wing top skin is completed.
The ailerons are tape top hinged with an angled leading edge to allow 30 degrees down movement.
It does seem rather strange to build and test the complete control system without even making a start on the fuselage!
05-06-2014, 06:01 AM
#3
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Nice subject. Not often modeled. I think the only kit I have seen of this aircraft was from Royal. Your build from a 3 view is outstanding. I am starting to make use
of various types of foam myself. The way to go if you want light large E-powered models.
Nice work looking forward to completion
of various types of foam myself. The way to go if you want light large E-powered models.
Nice work looking forward to completion
05-06-2014, 10:24 AM
#4
Just for the record in a push pull aircraft both motors or engines must turn the same direction and not counter rotate. The only difference is the rear engine will require a pusher prop installed in the proper fashion. The selection of pusher props seems to be getting thin. The requirement for a pusher prop of course is probably why push/pulls or pushers have never been to popular.
By the way wonderful project, very cool
John
05-06-2014, 02:22 PM
#5
Thread Starter
John
A bit surprised at the comment "in a push pull aircraft both motor or engines must turn in the same direction and not counter rotate".
They certainly do on the full size Skymaster/O-2 directly as a result of mounting the rear engine as a pusher but is does mean a 'left hand' prop is required.
I actually set the overall size of my O-2 specifically to match the counter rotating prop pair that was available.
Of course for an electric single motor plane a normal prop can be used as a pusher as the motor direction is easily reversed.
.
A bit surprised at the comment "in a push pull aircraft both motor or engines must turn in the same direction and not counter rotate".
They certainly do on the full size Skymaster/O-2 directly as a result of mounting the rear engine as a pusher but is does mean a 'left hand' prop is required.
I actually set the overall size of my O-2 specifically to match the counter rotating prop pair that was available.
Of course for an electric single motor plane a normal prop can be used as a pusher as the motor direction is easily reversed.
.
05-06-2014, 02:50 PM
#6
Both engines or motors still turn in their normal direction this is not altered. The only difference is the rear engine/motor must use a pusher prop. These are labeled with a 'P' after the pitch marking.
You can easily demonstrate this to yourself simply by holding a engine/motor in you hands with a tractor prop and just turn the engine by hand - now turn that engine/motor around 180 degrees but keep rotating the propeller the same direction. Note the tractor prop is now wrong but install a 'P' prop and everything comes together.
Also note this cancels any torque.
John
Edit: Here is a picture of a sorta wanna be 337 That I helped this gentleman with maybe fifteen years ago I am the gympy one, just a Kit Bash ship that flew better than it had any right to, I think it started life as a Malibu Trainer and had a pair of KB 40 both stock direction of course and a 'P' prop on the rear.
Your post conjured up a lot of wonderful memories of the early seventies when I worked for NBC news in LA flying both old Bell 47's and a single T337 for several thousand hours. I just loved flying that ship. Just loved shutting down the front to save fuel but watching the reactions of camera crews.
By the way that full scale ships big continentals both turned the same direction but did utilize a pusher prop in back just like the models. There were differences between the engines that was mostly due to the fact that rear was fan cooled and of course the ducting.
Thanks for the memories
John
You can easily demonstrate this to yourself simply by holding a engine/motor in you hands with a tractor prop and just turn the engine by hand - now turn that engine/motor around 180 degrees but keep rotating the propeller the same direction. Note the tractor prop is now wrong but install a 'P' prop and everything comes together.
Also note this cancels any torque.
John
Edit: Here is a picture of a sorta wanna be 337 That I helped this gentleman with maybe fifteen years ago I am the gympy one, just a Kit Bash ship that flew better than it had any right to, I think it started life as a Malibu Trainer and had a pair of KB 40 both stock direction of course and a 'P' prop on the rear.
Your post conjured up a lot of wonderful memories of the early seventies when I worked for NBC news in LA flying both old Bell 47's and a single T337 for several thousand hours. I just loved flying that ship. Just loved shutting down the front to save fuel but watching the reactions of camera crews.
By the way that full scale ships big continentals both turned the same direction but did utilize a pusher prop in back just like the models. There were differences between the engines that was mostly due to the fact that rear was fan cooled and of course the ducting.
Thanks for the memories
John
Last edited by JohnBuckner; 05-06-2014 at 03:28 PM.
05-07-2014, 07:14 AM
#7
Thread Starter
I believe on the initial models the cooling of the rear engine was marginal and was only really cured when they added the substantial rear inlet duct.
The fuselage of the Cessna O-2 is quite substantial so apart from the motor bulkheads I judged it could be made entirely from 3mm Depron as a true stressed skin structure.
Due to the significant amount of double curvature the easiest way to do this would be as a 'planked' half shell built over the plan.
The first planks go on to locate the formers.
The planking progresses.
The rapid change in fuselage cross section particularly at the rear requires many carefully shaped planks.
Once the planking has progressed sufficiently the half shell can be lifted from the plan and the other half of the formers added.
Note the front motor mounting Depron 'tube' to spread the forces between the first two fuselage formers. The actual plywood motor mount will be glued inside the tube at the correct point to position the prop to clear the nose block.
The motors are 1200kV Emax 2822s.
The RH planking is started.
However before the fuselage planking progresses too far the motors & the 30A ESCs have to be installed and the battery box built inside the cockpit area.
The fuselage of the Cessna O-2 is quite substantial so apart from the motor bulkheads I judged it could be made entirely from 3mm Depron as a true stressed skin structure.
Due to the significant amount of double curvature the easiest way to do this would be as a 'planked' half shell built over the plan.
The first planks go on to locate the formers.
The planking progresses.
The rapid change in fuselage cross section particularly at the rear requires many carefully shaped planks.
Once the planking has progressed sufficiently the half shell can be lifted from the plan and the other half of the formers added.
Note the front motor mounting Depron 'tube' to spread the forces between the first two fuselage formers. The actual plywood motor mount will be glued inside the tube at the correct point to position the prop to clear the nose block.
The motors are 1200kV Emax 2822s.
The RH planking is started.
However before the fuselage planking progresses too far the motors & the 30A ESCs have to be installed and the battery box built inside the cockpit area.
05-08-2014, 03:21 PM
#8
Thread Starter
The front ESC mounted between the fuselage skin and the Depronbattery box.
The ESC has a finned heat sink that will protrude through the Depron skin into the airflow.
The front Emax 2812 fixed to the ply bulkhead in the motor tube.
The rear ESC also has the same type of heat sink.
The rear motor is mounted on a ply disc inserted into the Depron bulkhead.
The rear air scoop will be functional to feed air to the motor.
The huge battery box.
This is actually a structural part of the fuselage its enclosed side cheeks act as torsion boxes to maintain the fuselage's strength and rigidity.
The final position of the battery will be determined to achieve the required CofG.
It should be able to fly adequately on one so I will have to work out how to programme the mixes on the DX6i to cut the front motor with the gear switch.
The ESC has a finned heat sink that will protrude through the Depron skin into the airflow.
The front Emax 2812 fixed to the ply bulkhead in the motor tube.
The rear ESC also has the same type of heat sink.
The rear motor is mounted on a ply disc inserted into the Depron bulkhead.
The rear air scoop will be functional to feed air to the motor.
The huge battery box.
This is actually a structural part of the fuselage its enclosed side cheeks act as torsion boxes to maintain the fuselage's strength and rigidity.
The final position of the battery will be determined to achieve the required CofG.
It should be able to fly adequately on one so I will have to work out how to programme the mixes on the DX6i to cut the front motor with the gear switch.
05-08-2014, 03:59 PM
#9
Quorneng.... Neat project. What adhesive are you using to bond the material? I have no familiarity with it but it sure looks like it forms beautifully to compound curves. Going to have to look into Depron. The Sky Master is a classic design as well and congrats on how well it looks.
05-09-2014, 08:45 AM
#10
Thread Starter
thailazer
I am using UHU POR. A foam safe glue especially formulated for polystyrene. It is basically an impact adhesive so if used as a simple 'glue' it takes some time (hours) to reach full strength. However if used as a true impact adhesive (a thin film on both surfaces and left to dry for a few minutes) the bond is instant.
Depron itself is a form of expanded polystyrene but unlike the stuff used for packaging it has a denser skin and is 'engineer' moulded into big sheets normally 2,3 or 6mm thick.
If its skin can be retained and not sanded away for its weight Depron has quite reasonable mechanical properties and it is of course completely waterproof.
I am using UHU POR. A foam safe glue especially formulated for polystyrene. It is basically an impact adhesive so if used as a simple 'glue' it takes some time (hours) to reach full strength. However if used as a true impact adhesive (a thin film on both surfaces and left to dry for a few minutes) the bond is instant.
Depron itself is a form of expanded polystyrene but unlike the stuff used for packaging it has a denser skin and is 'engineer' moulded into big sheets normally 2,3 or 6mm thick.
If its skin can be retained and not sanded away for its weight Depron has quite reasonable mechanical properties and it is of course completely waterproof.
05-11-2014, 02:10 AM
#11
Thread Starter
With the main electrics in place the wing 'assembly' can be glued to the fuselage. This will be a 'one piece' plane!
The rear air scoop is planked up.
Rather fiddly and as you can see it needed a bit of lightweight filler to achieve the tight radius curves. The internal formers have been cut away so the airflow reaches the motor.
The rest if the fuselage skinning can now be completed.
The removable cockpit 'canopy' is built in situ.
When complete it is basically a big box. There is plenty of room below it for the battery and radio.
The rear is held down by the wing overhang, the front by small magnets.
This is a really lightweight plane so to keep the weight down all the cockpit glazing with be painted on - but first it has to be black overall.
Just emulsion. It is ever so black!
When complete it will be given a light spray of 'special' clear lacquer to give a more durable finish.
The rear air scoop is planked up.
Rather fiddly and as you can see it needed a bit of lightweight filler to achieve the tight radius curves. The internal formers have been cut away so the airflow reaches the motor.
The rest if the fuselage skinning can now be completed.
The removable cockpit 'canopy' is built in situ.
When complete it is basically a big box. There is plenty of room below it for the battery and radio.
The rear is held down by the wing overhang, the front by small magnets.
This is a really lightweight plane so to keep the weight down all the cockpit glazing with be painted on - but first it has to be black overall.
Just emulsion. It is ever so black!
When complete it will be given a light spray of 'special' clear lacquer to give a more durable finish.
05-14-2014, 04:25 PM
#12
Thread Starter
Finally comes the 'decoration'.
The US 'single colour' marking were stencilled onto sticky backed plastic.
Upper wing surface.
The painted cabin glazing and the fuselage star.
Although not really required for strength a scale stressed wing strut is added but designed to break off easily on impact rather than damage the wing.
Complete.
It weighs 22.3oz with a 2200mAh 3s. With close to 250W at full power it has 170 W/lb but needs only 10C from the battery.
To my surprise it flies extraordinarily well. Plenty of power and very forgiving handling characteristics. The complete freedom of torque gives the confidence to undertake some pretty scary low level manoeuvres.
http://vimeo.com/67734020 It loops freely but the small scale ailerons limit it to a slow barrel roll. The full power climb is practically vertical.
It also flies quite adequately on just the rear motor and glides pretty well so landings are always 'dead stick' to protect the props.
On an endurance flight using just the rear motor it managed just a few seconds short of 45 minutes!
The US 'single colour' marking were stencilled onto sticky backed plastic.
Upper wing surface.
The painted cabin glazing and the fuselage star.
Although not really required for strength a scale stressed wing strut is added but designed to break off easily on impact rather than damage the wing.
Complete.
It weighs 22.3oz with a 2200mAh 3s. With close to 250W at full power it has 170 W/lb but needs only 10C from the battery.
To my surprise it flies extraordinarily well. Plenty of power and very forgiving handling characteristics. The complete freedom of torque gives the confidence to undertake some pretty scary low level manoeuvres.
http://vimeo.com/67734020 It loops freely but the small scale ailerons limit it to a slow barrel roll. The full power climb is practically vertical.
It also flies quite adequately on just the rear motor and glides pretty well so landings are always 'dead stick' to protect the props.
On an endurance flight using just the rear motor it managed just a few seconds short of 45 minutes!
Last edited by Quorneng; 05-14-2014 at 04:30 PM.
