Fokker V25

Nearly 60 years ago when at university the library had a big aeronautical section. Not that surprising as the aeronautics department was the biggest after athletics.
I spent some time reading about Fokker and the design of his WW1 fighter designs. What surprised me was that although a ruthless business man and a gifted pilot he had little influence on a planes design. That was handled almost exclusively by his chief designer Reinhold Platz and he couldn't even fly but he did have a good understanding of structures and the strength of materials.
It appears that Fokker would outline the features that would make a good fighter and then Platz would set about creating it in the structurally most efficient way with the materials available.
The story goes that Fokker wanted light weight and maneuverability over sheer speed so Platz designed the Triplane to give adequate wing area from a short wing span. At the same time Platz used an unbraced wing with a built up spar and a wind tunnel tested wing section. This was coupled with a fuselage frame constructed using welded steel tube. At the time construction methods unique to Fokker.
Platz then persuaded Fokker that the unbraced wing design of the Triplane would greatly benefit a biplane. Any loss in maneuverability would be more than made up for by the  reduction in drag and the increase in speed. A number of biplane derivatives of the triplane were built but none could match the Mercedes powered DVII then in production. 
With a deteriorating war situation in 1918 Platz suggested a monoplane would benefit even more from the cantilever plywood covered unbraced wing. The unanswered question was a low, medium or high wing. With a new fighter competition imminent Fokker directed a prototype of each be built using the proven Triplane fuselage, the V17 mid wing, V25 low wing and V28 high wing on struts. A Mercedes powered version of each were also built at the same time.
All the Mercedes versions were rejected as all the limited engine production was reserved for the Fokker DVII. Only the high wing V28 was accepted for production as the DVIII as the parasol wing was close to the pilots eye line so giving the greatest all round visibility although the low wing V25 was the fastest. The actual selection did not really bother Fokker as he already had the controlling interest in the Oberursel engine company which all the Triplane fuselage prototypes used anyway!
A period photograph of the V25 prototype.
 
So close to what eventually became the "standard" fighter configuration but not for 20 years.
I did build a rather small 26" (690mm) span sub 250g Depron V25.
 
It has a scale(ish) rotary inside the cowling that goes round with the prop!
 
It did fly but was very "twitchy" hence the reason for this post. I want to build a 50% bigger 40" span version but with a gyro!           

Last time I was at the flying field someone had a Fokker Eindecker which was about 50" span. It looked quite cumbersome but I was impressed by how well it flew. It lifted off & landed easily & was stable in the air. I wonder if it had a gyro.

Outerzone have the plans for a 49" span one.
 

My bigger Fokker V25 will have a scale Oberursel U-II going round inside the cowling so the first thing is to make a start on it.
The printed crankcase next to a picture of a commercial model.
 
Note the 18 crankcase blot heads!
The commercial model is actually quite a bit bigger than what I want so the picture was been reduced to the required size.
The crankcase over a front view of the the V25 at the intended size.
 
A scale prop will be 12" diameter and on just a 36" span! 
Still an awful long wat to go.
 

Lets try bolt heads!
They are tiny hexagons. 1.3mm AF and 0.6mm high.
 
 

The beginnings of the Uberursel Rotary.
 
the crankcase is clamped to the prop driver along with the prop.
Take the prop off and the whole rotary will then slide off the motor bell.
So far so good.
Now I have got to make 9 cylinders, cylinder heads, induction pipes and push rods. Only 9 rods as each was a push/pull and worked both valves. No valve overlap so not very efficient giving just 110hp from 15 litres = 7.3hp per litre!   

A bit more progress on the rotary. Five cylinders on the crankcase!
 
Note the cylinder heads show the rocker to work the inlet and exhaust valves.
 
The inlet pipes and the push/pull rods are still to be designed.
 

Rotaries were slow speed high torque engines so they turned huge props for the power of the plane. My "electric" rotary will be turning a 12" prop with a modest 80W motor on 2s.
A scale prop with scale power as well!

Nearly 3 days designing and printing but the Oberursel UR-II is now complete.
 
It weighs 14g, not bad as the intended motor and 11" prop weighs 76g!

I have made a mistake! The rotary I have printed is 20% too small for the size of the Fokker V25 I had intended. 
So two options:-
Either build a 20% smaller plane or print a 20% bigger rotary.
Guess which I have started. 
 
Nor quit as daft as it sounds as quite a number of the parts can be simply "resized" by 120% in CURA. There are only two bits that have to be redesigned in CAD as they exactly fit  around the motor.  

More printing.
 
4 done, 5 to go!

A first test run of the big Oberursel UR-II.
www.youtube.com/shorts/roqPMTxWeM4
It is turning a 10.75" printed prop. Scale would be 12".
2212 1000kV motor on a 2s. 
Not yet run at full power!!
 

Now the glue has had time to harden a run a full power with an 11x7 e prop from an 1800mAh 2s.
www.youtube.com/shorts/4Qc1VQdy8Qc
At peak it draws 9.1A and 51.1W. Modest but adequate for a light weight.
 
It appears You Tube now automatically insist that a brief video is automatically listed under "Shorts" and displayed as if recorded on a phone!