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A World War One warbird!

The Fokker V25 was a prototype built in the closing stages of WWI and in a configuration that was well ahead of its time.
Only one was built and there are only 2 known published photographs.
A neat low wing monoplane with what was basically a Triplane fuselage mated to a new plywood covered wing.
Although entered for the January 1918 fighter competition and possessed a good performance it was immediately rejected on the grounds of poor downward visibility.
This problem was overcome with the very similar parasol wing Fokker DVIII which won the April competition but as a design it was a bit of a dead end as far as future development was concerned. The RAF in particular retained the braced biplane layout for nearly 20 more years. One can only speculate that if the V25 been accepted the classic low wing monoplane fighter might have been adopted much earlier.
There are no 3 views available but its similarity to the DVIII meant I could 'reverse engineer' a V25 from it.
For practical reason this was going to small, light and made entirely of 2mm Depron.
The all Depron wing.
The top skin added and the aileron cut out just as they did in the original.
The wing section was fairly generous to give the original a reasonably deep spar.
The fuselage is little more than a hollow Depron box with just an engine bulkhead and a single former behind the cockpit. The elevator servo is mounted right up against the bulkhead.
External pull/pull cables connect it to the elevator.
The ailerons are done the same way with a single servo in the wing leading edge.
A closed loop cable runs externally through tiny eyes fixed to the top and bottom of the wing surface.
And connects to the aileron horns.
The battery box is also right up against the bulkhead.
The aileron closed loop cable runs through the drinking straw tube.
It has a scale Oberursel rotary, the crankcase of which is the brushless bell so it will go round with the prop!
Each cylinder is a thin paper tube with a Depron cylinder head. Each weighs 0.1g.
Painted the complete 9 cylinder 'engine' adds just 1.4g to the brushless motor.
A test run of the rotary on a 2s LiPo.
The 'clicking' is interference from the ESC on the camera mic!
This also allows an almost true scale engine mounting on the bulkhead.
Still quite a bit to do.
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creator of virtual planes
Yet again, another amazing build, quorneng. And a nice history lesson to boot.

I wonder if the spinning rotor engine will give you the same problems that bi-wings with that type of engine had. I heard that the rotation of that weight would actually shift their control inputs 90degrees. So when they turn right, their plane would go up. When they turn up, their plane would go left, etc. I'm sure that you're not adding enough weight for centripetal force to do weird things, but it might be more noticeable with this plane. Torque roll will at least be more noticeable.


creator of virtual planes
I learned about the centripetal forces of rotary engine from a PBS show, and here is a clip from that show that thoroughly explains the 90degree change in direction. I highly doubt you've adding enough weight for this to happen, but it might shift things a little bit.

At 1.4g the gyroscopic forces of the 'engine' will be insignificant compared to those of the out runner bell and prop which weigh 30 times as much.
The original Oberursel weighed 323lbs or about 1/4 the weight of the fully loaded plane.
To match that ratio I would have to make the cylinders out of steel and the resulting gyroscopic forces would then be very real!


Dedicated foam bender
Heat is what I would be concerned with since the cooling from airflow through the windings is gone. Looks fantastic though!
There obviously some extra drag from the rotating cylinders but it is a small relatively low kV motor (1500) on a 2S so is turning the scale diameter 8" prop pretty slowly.
The motor only draws 7A (50W) at full power and the Fokker will probably fly on half that so I don't expect cooling to be much of a problem although motor torque might be!
Sanded Depron makes a nice cockpit coaming.
The Oberursel mounted on the bulkhead.
The next question was how to make the cowling.
A stepped ring method looked feasible in Depron.
The outside was sanded to the required radius.
The inside was left stepped with hopefully enough clearance!
The cowling in place on the bulkhead to check for fit.
Still a complete circle at this stage for rigidity. The bottom section could be cut off later.
Only the front undercarriage strut is fixed to give springing and is retained a plastic plate glued to the fuselage.
The rear leg is just a strut retained by a Z bend.
The wire has Depron fairings glued on which are sanded to a streamlined profile. The rather 'rubbery' UHU POR glues Depron to wire very well.
The full size had an undercarriage wing that contained the axle suspension but for the time being it will be left off.
The tail feathers are simple Depron 'plates' with balsa reinforcing at the elevator hinge line.
With balsa struts on the underside.
Now nearing completion the issue is what colour scheme to paint it?
The original appeared to use the standard German pre-printed lozenge fabric on the fuselage and I don't fancy doing that! The wing finish is not so clear. They may have just have been painted.
The simplest scheme I could find was on some Royal Dutch Air Force Fokker DVIIIs. Forest green overall with their simple Orange roundel national insignia. Of course the Dutch never had the Fokker V25 but if it had been adopted by the Germans I am sure Anthony Fokker would have included them in the train loads of planes he smuggled into Holland at the end of the war!


Dedicated foam bender
But the lozenge scheme looks so good! I have to agree though, it's a bear to paint. Not difficult, just tedious as all get out.

This build has me in awe. Fantastic!
Thanks for the kind words but I hope it will be a 'rotary'. ;) Radials are fixed.

Even the wheel is made of Depron with small sheet plastic hub bearings.
So green over all it is. The head rest and cockpit coaming.

As all the cable runs are external the final task is to connect up the closed loop controls with 7lb mono filament fishing line. As each is just a single loop the first concern is to achieve a suitable tension. Once obtained the control surfaces can be moved to their correct positions and 'locked off' with a dab of glue where the line passes through the control horn.
With a 800mAh 2s it weighs 5.9oz (167g).
The wing area is 130sgin giving a wing loading of 6.5 oz/sqft.
This may sound quite modest but in small planes the lift coefficients are always poor so it will be no floater and coupled with a no dihedral low wing it is likely to be a handful to fly. :eek:

The maiden struggling against the wind and a rearward CofG.
Originally the battery was inserted through the cockpit and pushed forward but to move the CofG still further forward it had to be inserted vertically from the top. With the battery in place the final connection is still made through the cockpit.
This made all the difference but the 'landing' damaged the bulkhead.

So it is has plenty of power, glides like a brick and is remarkably tough. This is just as well as the relatively massive motor torque means a left hand spiral is the guaranteed consequence of any sort of control input when close to the stall and the rather high drag airframe means the stall speed arrives pretty quickly!

Perhaps the high wing Fokker DVIII actually did have nicer flying characteristics! :D

A stronger balsa 'ply' bulkhead is required.
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The old bulkhead removed to expose its supporting structure.
Not easy to do without damaging it.
The new 4 ply (4x0.8mm) balsa bulkhead.
The rotary itself was also seriously weakened due to paper cylinder tubes becoming waterlogged by contact with the wet grass.
A new rotary was built all in Depron.
Slightly heavier (added 2g) but more robust and water proof.
The rather rough test flying has put a distinct crease in the left wing (motor torque ensures than in any sort of crash it always hits first!).
Still plenty strong enough so not worth what would be a major repair.
Complete (again!)
It now flies quite well. A short edited video of it doing some aerobatics in the sun. In the rest of the video it was just a tiny spec!
Full power was not used at any time during the flight. :eek:
The long grass ensures a rather untidy 'arrival' but no damage sustained.

Overall quite successful but its biggest drawback is its rather small size. It would be interesting to make a bigger one, 1.5 or even twice the size but built in more or less the same way.