September 19, 2018

F-3H Demon.

Here are a few pictures of the genesis our new project at 1/7th scale.

This was entirely CAD designed with Siemens NX12.

The following design considerations were observed:

1. The plane must be true to scale. We used Mc Donnell blueprints and pictures of the real plane taken with a very long zoom and barrel software geometric corrections. The cad views were overlayed on the pictures to validate the shapes.

2. The plane must fly well. The aerodynamics configuration was trialed on XFRL5 . Many different wing/ stabilizer airfoils configurations were tested at low Reynolds numbers, to obtain a nice flight characteristic at low speed. It was then validated on Star CCM +

The airfoil was chosen to be very close to the original one is terms of thickness and curve, but adapted to the low Reynolds numbers seen on this model.
The wing being quite large on this plane, the loading will be very reasonable. Simulations show very nice flight characteristics at high angle of attack/ low speed, similar to our 1/7 scale Crusader.

3. The plane must be very easy to ship and transport.

The fuselage was designed be split in 3 parts. Wings, fin, and stabilizers are removable.
The fuselage breaks into: nose section, cut at the main panel line in front of the wing, middle section excluding the beaver tail, and rear section ( essentially the beaver tail itself ). The rear section splits just at the tailpipe.
This will enable us to ship the kit in a very compact box. We are expecting reasonable rates, even with Fedex Intl priority.

4. The plane must be strong and simple but easy to service.
The wing will have two 30 mm carbon tubes. The rear main tube will be a single 3 foot piece ( that passes just below the thrust tube ). The carbon sleeve will be ceramic coated and aluminum shielded to avoid heat transfer from the engine section.
The front wing tube will stop at the engine bypass section.
The fin will have two carbon fiber tubes
The full flying stabilizer will be rotating on a 1/2" Alcoa Al 2024 tube on quadruple needle bearings.
The stabilizer bearing assembly and servos as well as rudder torque rod and servo will be easily accessible from a large ceramic coated hatch which is located at the bottom of the beaver tails and serves as a jet efflux heat deflector.
A large engine hatch will be available at the top of the fuselage center section, as well as a large canopy opening. The engine hatch has been dimensioned to allow fitting/ removal of the engine thrust tube and single piece large center fuel tank directly.
There will be enough room for a 2 liter smoke system tank in front of the main fuel tank.
The nose dome will be removable to access the batteries. This will allow to remove them very easily for charge/ storage. No risk of on-board fire!

The outer shape was first designed as surfaces.

Then, the surfaces were thickened and internals added to suit the design requirements above.

The engine and fuel tank got located exactly on the CG.
The plane was designed for a 120-140 class powerplant and idimensioned for 15 g load at 16 kgs takeoff weight.
Tank size was made at 5 liters. A smaller/ lighter plasma bag option will be available for 13.5 kg class scale competition.

The flight controls are:
2 stabilizers, 1 rudder, 2 slats, 2 flaps, 2 ailerons, 1 steering. or 10 servos.
The flaps, ailerons and slats will be live hinged, like our Crusader, with our Gorillahinge system ( a polymer infused fabric that sustains 200 kg.cm of tear force ). They include an aerodynamic seal surface, like the Crusader.
All control links are hidden and true to scale. Including the rudder link.

This rendering shows the 3 parts fuselage design as well as the wing and center section internals.

The tail section will connect to the center section with 4 screws and 3 carbon tubes to ensure perfect rigidity and pitch/ yaw precision.
Al large bearing system will be set this section, between two carbon fiber bulkhead.
The two stabilizer servos will be relatively close to the shafts.
The rudder servo will located at the center, in front of the stabilizer servos.

The tail section ( beaver tail ) will be removable and include the fin support, stabilizer support and fin/ stabilizers servos.
A large hatch will be located at the top of the beaver tail to give access to the 3 servos.
The stabilizer and rudder controls links will be hidden.
The rudder will be controlled with a torque rod system that slides in a slot at the bottom of the surface.

The stabilizer shafts will be made of Alcoa Al 2028 12 mm rods. The will have a key system to permanently lock the arms.
The control arms will be keyed/ clamped with a M3 socket head screw.
The shafts will be supported by two ultra high tolerance 12 mm needle bearings each. These sustain 500 lbs each.
The bearing block is a billet milled Al 7075 unit that is sandwiched between two carbon fiber bulkheads.

Here are a few pictures of the raw plug cutting process before surface finish.

Plates of milling material are aligned on our fast processing router, vacuum clamped and cut in slices that will be glued together at a later stage.

Here is an example on the female plug wing mold. The hollow shape allows for a reduction of material cost, mold weight and cutting time of about 30%.

For the flying surfaces, we process negative molds to make a composite positive plug. This allows us to work safely on thin trailing edges without braking them, as we have an extensive surface work going on with these parts.

Here are the negative molds completely cut out of the Kuka robot.

Another shot of the male fuselage plug shape being milled on the Kuka. These are the half beaver tail parts. These specific parts are milled from two faces. The negative exhaust radius has too much angle for the water cooled 30 KW exo bearing milling monster head to do its job in one setting without collisions.

We have two Kuka milling robots at the factory. One two ton KR240 on a fixed base and one 4 ton KR320 on a 30 meter long rail. We exclusively use the KR240 at Enata Aerospace as it allows a milling precision of 1/15 mm.

Here are a few more steps about the wing molds manufacture. The tooling board is cut 0.5 mm wider than the finished shape. The mold is then pressure gun sprayed with a milling fairing compound at 1 mm thickness. After curing, the shape is finished to final dimension with thin passes.

The fuselage plug was cut as a positive shape from tooling boards stack.

Here are a few pictures of the cutting process on our Kuka robot.

Here is a detail of the relatively intricate beaver tail shape. Although it looks relatively simple, it is not at all. This part presents many negative draft angles and non parallel faces, with a complex transition to the fuselage elliptical section.

The fuselage was similarly cut -0.5 mm and pressure gun gun faired at +1.0 mm with milling compound.

It was then finished on the robot at final dimension with thin passes.

Here is a quick video showing one of the hard part about milling the Demon fuselage: the negative beaver tail shape.

Demon beaver tail negative milling from Oli Ni on Vimeo.

Here are some details about the F-3H Demon fuselage plug assembly.
The Fuselage plug inner core was milled with matching alignment holes. These holes allow the insertion of M10 threaded rods that serve two purposes: aligning the two halves and pressing them together at the gluing stage.
Before gluing, the halves are aligned and measurements are taken to verify the width of the plug. It is not uncommon that milling deviations create an offset in the parting plan, thickening the two halves . This is easily detected by verifying the plug width once assembled. Here are a few pictures of the plug after gluing.

Here is a view from the front that shows the alignment of the seam line.

The same view front the back, that shows the alignment of the beaver tail flat surfaces.

And a side view of the nose area. The threaded rod that presses the front halves is visible here, as well as the flatness of the canopy surface.

Here is a short fuselage plug walkaround.

F-3H Demon fuselage plug walkaround from Oli Ni on Vimeo.

Here is a picture of the wing plug molds ready for lamination.

The lamination process:

And the wing plugs out of the molds!

The Demon wings are now on the plug.

The airplane reference surfaces have been verified and trued.

These are the front canopy horizontal surface and the top of the beaver tail.

Both surfaces are parallel and symmetrical as well as centered with the plane symmetry plan. This might seem simple and insignificant, but it is in fact essential and not so obvious. These surfaces are designed to be used many times during the airplane building process to reference to alignment elements.

Here is a picture showing the beaver tail top referenced to the wing axis.