A scale WWI rotary

[quorneng]

Many WWI planes used a "rotary" engine where the whole engine went round with the prop. Such a feature is very rarely modelled usually using just a few dummy fixed cylinders and then only those that are visible.
A very long time ago I built a couple of "small round the pole" (RTP) WWI planes (Fokker Triplane & Sopwith Pup) using geared slot car motors driving big scale props that also had a scale(ish) rotary fixed to the prop shaft.

Both planes actually flew well but despite my attention to scale detail I carved a conventional CCW prop which is wrong for an Oberursel rotary as it rotates the other way. Ooops!

With the advert of powerful light brushless motors with LiPo batteries and light foam construction I wondered if this concept could be used on a scale RC plane.
For the first attempt I modelled the little known Fokker V25 prototype.

For the time, mid 1918, it was a remarkably advanced concept using a Triplane fuselage and a plywood covered low wing. A layout for fighters that was not adopted by others for about 15 years. It was however rejected as it was felt the low wing unduly restricted pilot downward visibility. Fokker solved that issue by mounting the wing on struts above the fuselage. This layout was accepted and put into production as the Fokker DVIII in the last couple of months of the war.
My RC Fokker V25.

Small and light (sub 250g) built entirely from 2mm thick Depron sheet the 2805 motor is the rotary's crankcase with the cylinders simply stuck on to the motor bell.

It used a 8x3.8 prop but still with an incorrect CCW!
Hand launched it flew OK but was extremely sensitive and every landing resulted in a nose over.

With the advent of 3D printing I considered trying a bigger version of the V25 with a true scale Oberursel UR-2 rotary.
First make the rotary as true scale as possible.

The A2212 1000kV is buried in inside the crankcase.

The rotary is clamped to the motor by the prop driver. This arrangement does allow it all to be assembled & disassembled.

The rotary, its LW-LA cowling and a 12x6 prop mounted on a test stand.

Running at full power on a 2200mAh 2s.

And nothing came off!
Interesting that with the cowl in place the "windage drag" from the spinning cylinders is significantly reduced giving a noticeable increase in rpm.
Maybe not every ones "cup of tea" but it does look impressive.
Now all I have to do is build the plane to fit it on!

 

[Mr Man]

It does look cool!

 

[bisco]

watching, well done!

 

[quorneng]

My first cowling had a small nose radius to clear the rotary. To get anything like a scale cowling the rotary has to be further back in the cowling. It needed quite a bit of the rotary to be reprinted as well as a new cowl that had the lower edges extended to match the fuselage.

So time to start on the fuselage.

Very light. All 3mm Depron apart from the motor bulkhead which is 3mm fibre board.
The whole airframe will have to be light because of the low power (2s LiPo) that will be used to keep the rotary's RPM within practical limits.
So far so good but still a long way to go.

 

[quorneng]

Next the fuselage fairings. They are printed in LW-PLA.
They are single wall with internal stiffening ribs.
The top fairing is in 3 pieces and includes the cockpit.

The single piece side fairing

.
Note the "dummy" wing centre section to ensure the wing will fit properly.
"Tail feathers" next.

 

[quorneng]

The tail and fin.

Just 3mm Depron sheet. It will be flown AET so no rudder.
The wing is 3mm foam sheet top and bottom over printed ribs.

There is no spar the wing skin takes all the wing loads. The ailerons are simply cut out.

The wing and tail in position but not yet glued on.

All the servos are inserted and their wires run through the wing or fuselage before the flying surfaces are glued in place.
Slow progress but getting there.

 

[Mr Man]

Cool! Excited to see if this’ll work! 👍

 

[quorneng]

The Fokker V25 with a "rotating" rotary is now complete.

Very non scale colours but it is visible!
The battery 2000mAh 2s is inserted through the cockpit. Ready to go it weighs 444g.
As hoped the extra weight of the scale Oberursel puts the CG in about the right place despite its very short nose.
The CCW prop although scale diameter is wrong it should be CW but I am more used handling the effects of what is likely to be considerable "roll left" motor torque!
Terrible weather so not yet flown.

 

[Piotrsko]

Would be interesting to test whether or not all that rotating mass makes it biased in rudder turns like the original. Some sort of comment like the fastest way to turn left was to turn right 270 degrees. In your case substitute right for left in the quote if it still rotates reversed

 

[quorneng]

Whilst there is more rotating mass than a conventional layout the printed rotary is proportionally far lighter than the original. Mine is largely hollow like the original but it is also made of plastic not steel thus is about 7 times lighter!
I am sure some gyroscopic precession will be present but at a much reduced level compare to the original.
I am more concerned at the torque effects from that big scale prop on what is a light weight airframe.

 

[MadMonkey]

it flew OK but was extremely sensitive and every landing resulted in a nose over.

A WWI fighter nosing over? Shocking 🤣

Looks great, actual rotary engines fascinate me. The torque must have been interesting.

 

[LitterBug]

A WWI fighter nosing over? Shocking 🤣

Looks great, actual rotary engines fascinate me. The torque must have been interesting.

Every time I see the olde rotary planes at the museum, I just think to myself rotating mass = bad.... what were they thinking???

 

[quorneng]

A rotary made sense at the time as cooling was a real problem on air cooled engines. Early engines were pretty inefficient so there was alot of heat to get rid of. An engine could over heat in just a few minutes just running at low power on the ground.
A rotary on the other hand was self cooling indeed it was soon discovered that providing there was a sufficient cut outs air was drawn in from forward facing holes in the cowling. The hot air and exhaust was thrown out (including the unburnt oil!) from the underside much to the satisfaction of the pilot!
The huge rotating inertia also made controlling engine speed by repeatedly cutting the ignition a bit less dramatic.
Eventually W O Bentley (of car fame) did a proper engineering job and boosted the both the power and reliability of the Bentley rotaries to unheard of levels. It gave the last of the WW1 biplanes a spectacular performance for the period but it did nothing for the gyroscopic effects.

 

[bisco]

i think it looks fantastic, and i love the colors!

 

[Piotrsko]

Other side effect to rotary engines: up side they used castor oil as lube because it wouldn't burn. Don't want to be flinging semi burning lube on a nitrate covered surface. Down side was pilots were exiting the craft in a hurry after landing. People not exposed to the effects of castor oil ingestion will not understand the reference. Ever wonder why model airplanes used castor oil? Hint: some people after WW1 used 50 wt oil in their rotaries, for a while, until it caught fire.