A small RC Sopwith Pup.

Despite appearances I do not always build using Depron.
My small (18") 'Round the pole' scale Fokker Triplane

and its companion Sopwith Pup

Did make me wonder if such a plane could be made to fly RC.
The Sopwith had a bigger and probably more efficient wing area for its span so the Pup was chosen and scaled up slightly to 24" span.
The object would be to build the structure to scale - as I did with the Triplane - but to be a light as possible.
To stand a chance of achieving the correct CofG everything, the motor, battery, ESC and servos would have to concentrated right at the front.
A true scale fuselage was not likely to be to much of a problem as the full size was built like a model anyway!
The wings would be a bit trickier, very thin and slightly under cambered.

There would be no doubt the rigging would have to be structural.
So much as Andrew has done I built a 'test' wing.

All balsa. Ribs and riblets soft 0.8 mm, the trailing edge hard 0.8 mm square and the spars hard 3/32 thick
Although it was certainly strong enough it felt rather heavy as the spars made up nearly 50% of the total wing weight.
The full size spars were actually machined (spindled) to make a one piece "I" section. At this scale I could not follow that route so I chose to build up each spar out of 0.8 mm balsa to make a "U" section but lying on its side.
The ribs are in 3 pieces. The front section and the riblets are glued to the face of the spar. The middle sections have their ends "formed" to fit into the "U", the rear section is glued onto the back face of the rear spar.
The first "working" wing.

All four wings. Note the 'tubes' for the scale single loop pull/pull control cables to work the ailerons.


The Pup (officially the Scout) although it had excellent flying characteristics (for the time!) had a relatively short operational career. With only 80 hp it had neither the speed nor could carry the armament to match later German types.
It did find service in the home defence squadrons with the more powerful 110 hp Monosoupape which significantly improved its climb rate to intercept German bombers. To squeeze in the Monosoupape the cowling was modified with a squarer section "horse shoe" type (and thus easier to model) than the more rounded full cowling used on the Le Rhone.

As expected the fuselage is basically quite simple although some of the detail gets a bit fiddly at this scale.

The fuselage has all the scale bracing using very fine mono filament fishing line.
As I did with both the RTP Triplane and Pup I decided to give it a scale 'rotating' rotary but with the out runner bell forming the crankcase with the cylinders glued onto it.

The Emax 1600kV 2805 is completely hidden and its 'drum' mounting fits unobtrusively through the scale fire wall.

To be perfectly honest this build was becoming more of a 'construction exercise' than likely to produce a plane that would be able to survive any sort of flight.

Looks good and the extra weight up front should prove a saving in overall weight.

With such a delicate structure actually mounting the servos could add weight so they were first glued together to form a servo 'block' which meant only a single mounting would be needed.
The batteries (1s 500mAh) are saddle mounted on either side of the servos with the rx on the back of the fire wall.

The ESC is positioned above the servos.
This compact arrangement ensures no part of the RC gear is behind the CofG.

All the struts are hard balsa but flexibly joined by small strips of acetate sheet slotted into the end of the strut.
These are the cabane struts but all the others, including the undercarriage and lower wing to fuselage, are done in the same way.

As the acetate strip is 'sandwiched' between the balsa it allows the joint to handle quite high tensile forces and also gives a degree of 'bendability' to make setup easier. At this scale this type of joint behaves in much the same way as the bolted pin joints of the original.

Hmmmmmm....I do like the idea of joining the struts to the structure with acetate strips. In principle, it appears to be better than my normal method of very fine wire, so I'll try it on my next lightweight.

Is this the first time you've used this method Q, or is it tried and tested?

Look absolutely lovely...very neat indeed. Does the dummy engine rotate as well?

martynk
Yes it does rotate - it has to - it is all part of the out runner!


Andy
It is tried and tested as far as the Pup goes but to be fair it was specifically evolved for it being the smallest lightest balsa RC plane I have built so far.
I did use this technique to fix the fully operational balsa wing struts on my Depron Super Cub.
On the Pup I used a fine knife to in effect split the balsa to insert the acetate strip. It really needs a very fine (0.1 mm?) saw to actually 'cut away' the balsa to make a proper mortice.

Brilliant

By far the trickiest bit of this Pup is its scale aileron control system as it follows the full size using a single closed loop pull/pull cable from the fuselage servo, out inside the lower wing, around the lower aileron, up to the top one, back right across inside the upper wing, around the upper aileron, down to the lower and finally back inside the lower wing to the servo. By using tiny polyurethane tubes built into the wings it was just possible to rig it all after the wings were covered.
The elevator pull/pull cables run virtually in a straight line (as they do on the full size) servo to elevator horn through small holes in the fuselage covering.
All control cables are 5 lb mono filament nylon line. As there are only two 3.7g servos the rudder is fixed.

Sopwith ran the control cables over the horns and down to the surface trailing edge.

This meant I could cheat a bit and use a single length of line from servo to the surface and back again without actually fixing the surface in its final position. With all the cables in place and under suitable tension the control surfaces can then individually positioned and secured with a drop a glue where the line runs through the control surface.

I did begin to seriously wonder if it could ever realistically be flown out doors.

Nice Job Q.

- Small PTFE tubes - will that not add some unnecessary weight?

- I didn't see the rudder move on your test. Is it, and/or will it be connected.

- You'll be fine outside on a calm evening. I've flown small RC models that were lighter than a Vapor outdoors successfully (a 10gm three channel Hangar Rat). My lightest FF model flown outside successfully was a 0.35gm Living Room Stick! The lightest scale model was a 3.0gm 8" span rubber powered Lacey M-10.

Andy
The tubes are the innards of a ball point pens (dissolving out the ink is messy!) which with a bit of heat can be formed into a smooth right angle bend.

The end of the tube is cut so it is flush with the surface of the wing and so provides a 'lead out' for the line.
The nylon mono filament line runs very freely within the polyurethane.
This was about the only way I could run (and thread) the control lines inside such a thin wing.

So far I have had little trouble flying bank and yank without a rudder but in this case the bigger concern is the torque from the relatively huge scale diameter prop. The amount of aileron deflection required to counter the torque and the associated adverse yaw could well mean turns to the right will be difficult.

Still concerned at the potential flight characteristics of what is a very delicate and 'time valuable' plane so the answer is to build an aerodynamic test prototype.
No point in going half size - it plenty small enough already - and with the same control system (but with all the control cables run externally) so cheap (in man hours!) and cheerful and you guessed it made of Depron!
Each wing is very simple - top and bottom 2 mm Depron skins with 5 ribs.

The wing section is the same as the balsa Pup so it should have the same flight characteristics - more or less!

Of course this could all prove to be a 'folly' if it takes too much effort to get it to fly.

That should work O.K.

The Depron "aerodynamically similar" test Pup showing its CofG.

The controls use exactly the same mono filament closed loop system but for simplicity all the lines run externally.

The RC installation is rather simpler with a single 500mAh 2s.

And it does fly (sorry no video) but slowly and is crazy sensitive to almost any disturbance and that included the controls!
With all that drag it does not really glide, more of a controlled (just!) descent, and as feared it turns much better one way than the other.
As the rigging is fully operational it is actually very rigid so the benefit of being made of Depron is actually lost in any sort of hard landing.
So does it give any confidence that he balsa version will fly?
Well I suppose it does but more than once? - I am not so sure.