Scratch Build Twin

[G-JIMG]

Been working on the vertical tail assembly.

It looks pretty horrible at the moment but will hopefully look good when it's sprayed with a top coat.

The photo isn't very good so I've added some lines for clarification. As you can see, the Elevator Snake exits the tail outside the fuselage skin. I did this to keep the bend as shallow as possible and it isn't an issue because the King Air has a Dorsal Fin that will conveniently hide the snake. There is also a smaller diameter tube installed that will carry the lighting wires.

Like the Elevators the Rudder also has a reinforced leading edge, but flush riveted. Again I've simulated this with metal tape. I have also simulated the 3 inspection panels with a thinner metal tape.

The orange lines represent the Rudder torque tube. The snake from the rudder servo will run in a straight line to the bottom and, as the inner snake moves forward and aft, it moves the Rudder left and right. (the same servo arm will also connect to the nose wheel steering).

Jim.

[G-JIMG]

This fibre glassing, filling, and sanding is never ending. Have to keep telling myself it's worth it in the end.

Unfortunately I broke one of the flaps today . The hinge point is near the narrowest, and thinnest, part of the flap and the balsa gave out when I was reinserting the hinge. The flap on the other side is fine so I'm hoping it was just a weak spot in the balsa. Really annoying though, especially as it was pretty much ready for paint.

Decided to do something other than work on the flying surfaces for a while so I've made a mould for the under fuselage pod.

Next step will be to make a silicon mould so I can make a fibre glass part for the model. It will be the perfect size to take two LiPos side by side, an ideal battery pack. However, I still haven't settled on a method of securing it to the model. Ideally I want to be able to easily install and remove it without using screws (not very scale like) but I'm conscious that two LiPos are not the lightest of things.

Any ideas would be more than welcome.

Jim.

 

 

Edited By G-JIMG on 22/04/2020 17:03:34

[G-JIMG]

The mould came out very well.

Jim.

[Nigel R]

Build is looking fabulous Jim.

Have to ask - probably a silly question - if the flap has broken from hinge insertion, will it be strong enough for flight?

Also, I can't quite get my head around the geometry of the elevator torque rods, is there two separate rods and clevis, or are they joined in the middle and you rely on the wire flexing as the elevator moves?

Edited By Nigel R on 23/04/2020 10:24:52

[Martian]

Loving your build Jim and following with admiration

[Chris Walby]

IMO the batteries should be retained with Velcro straps, then the cover can be held on with magnets. If you don't want to rely on just magnets then a dowel/hole at one end is an option.

I like the idea Seagull use with a canopy - it drops inside the cowl then slides back with hooks and a magnet, extremely unlikely to jump off the magnet, overcome wind resistance and pop out, but needs no tools for easy access

[G-JIMG]

Nigel,

I rechecked the broken flap and realised the hole for the hinge pin was at the wrong angle, it came too close to the top surface. Consequently, when I pushed the hinge back in, it burst through the thin wall. I've since made a new flap and everything is fine.

Regarding Elevator operation:
There is one metal rod bent at right angles either end. These are the ends that are glued into the Elevators.
The rod has an 'actuator' arm soldered in the middle. The clevis for the Servo snake is attached to the end of this arm.
There are two short lengths of brass tubing on the rod, one either side of the arm, these are glued to the tail.
The brass tubes are aligned with the Elevator hinge pins so they have the same rotational axis. When the arm is moved up the rod rotates, moving the right angled parts of the rod downwards, which pushes the Elevators down. Likewise. if the arm is pulled downwards, the rod rotates within the tubes pushing the Elevator up.

Jim.

[G-JIMG]

Made the under fuselage pod, and its aerodynamic fairing, using the same method I used making the engine cowls. Very pleased with result. Very sturdy but also very light.

This is where I'd ideally like to install the batteries.

As you can see, there's plenty of room for the batteries and I can install foam and straps to stop them moving around. The problem I have is attaching the complete 'battery pack' to the underside of the aircraft.

Alternatively, I could manufacture a battery 'cradle' on the underside of the model and simply use the pod as a cover, attaching it by hooks and magnets as suggested by Chris. However, the pod spans the width of the wing and a small part of the fuselage so I will have to see if it's feasible to have the entire cradle on the underside of the wing.

Jim.

[G-JIMG]

I think I have a solution for the battery problem. Really, I should have had this all planned before I started the build but in every other build I've only had to slot in a fuel tank, not ESC's and a relatively hefty battery.

Anyway, I can make the battery cradle (Red) a permanent part of the wing (Blue) and secure both to the airframe via 4 screws (Green), two aft and two forward. The cover can then be located by small spigots and held by magnets (Orange).

Jim.

[G-JIMG]

I've designed and built the battery cradle, and all appears fine.

The cradle will form part of the underside of the wing (sorry the photo's are upside down). I'll install it when I join the two wing halves together.

The base of the battery bays will form part of the wing surface, with the added advantage of strengthening the connection between the two wing halves.

The forward portion of the cradle will blend into the lower fuselage and have wing hold down screws.

The batteries (which will be hanging upside down) will be secured in place by straps.

The entire installation will then be covered by the fibre glass pod.

Jim.

[G-JIMG]

With any model there is always the question of what to do with the interface between the control surfaces and the wing/tail.

The easiest option is just to leave it as is and have a small, but visible gap between the two. Another option is to concave the trailing edge so the shaped control surface 'slots in'.

Unfortunately neither of these options will work on this model. Leaving a gap for the ailerons and flaps would not be very scale like and the TE, especially near the wing tip, is too narrow to concave without weakening it.

So, we come to option three, covering the gap.

To do this I use a little tool I made myself. It's simply a piece of right angled wood with a piece of glass paper glued to it. The glass paper is the width I want to remove from the model and the thickness of the paper is perfect for the depth I need.

I simply press the tool against the trailing edge and run it backwards and forwards until it bottoms out. When it does, the model has the perfect step to accept the cover.

I then glue in a strip of 1/64" wood.

The aileron then installs exactly as before but the gap is now invisible.

For the ailerons I will install a cover top and bottom but only a top cover for the flaps, installing a lower one would restrict their movement way too much.

Jim.

Edited By G-JIMG on 27/04/2020 11:04:46

[Martian]

Excellent solution one that a few use I personally have used carbon tape as well on some wings

[G-JIMG]

Not much to report I'm afraid. I've reached the stage with the wings where there's a lot of effort for little visible gain.

The Port wing is now fully assembled and operational, although I won't glue in the control surface hinges until after it's all painted. It just needs a final sanding with 400 grit paper before being primered (I used to use 600 grit but found the paint did not key as well).

Each wing has split flaps, which are at a slight angle to each other.

To drive the outer flap I have araldited a carbon fibre tab in the end of the inboard flap. This fits, very snugly, into a corresponding slot in the outboard flap. When the inner flap is driven it forces the outer flap to follow and the tab, being free to move fore/aft in the outer flap, allows for the slight difference in angle. It's a configuration that has proven successful in the past, on the Corsair which has three flaps each side.

When lowered the flaps also take the aerodynamic fairing that forms part of the nacelle fuel tanks with them.

The starboard wing is about two days away from being in the same condition.

Jim.

[G-JIMG]

Progress has slowed, mainly due to SWMBO's list of jobs! However, the wings are nearing completion. Today I managed to add the wing dressing (LE 'fin' just inboard of the wing tip lights) and the aileron dressing (reinforced TE section near the centre).

I had previously made an exhaust mould and have been making the 4 exhausts I'll eventually need to attach once the model is painted. I used my small airbrush to paint them and they've come out quite well. The finish looks more metallic than it appears in the picture.

Jim.

[Simon Chaddock]

G-JIMG

Impressive problem solving.

Eliminating a control surface gap is always an issue.

Physically covering it in such a way that involves sliding over the surface does have risks. You just have to hope it will never cause a jamb.

[G-JIMG]

Posted by Simon Chaddock on 05/05/2020 18:47:33:

G-JIMG

Impressive problem solving.

Eliminating a control surface gap is always an issue.

Physically covering it in such a way that involves sliding over the surface does have risks. You just have to hope it will never cause a jamb.

Simon,

The covering, being so thin, is flexible so will move rather than jam. The trick is to make it wide enough so that it can flex, if it's short and stubby it could indeed cause a problem. Additionally, I gently run glass paper along the underside edge to feather it, so there's no sharp edge to catch on the control surface.

I guess there is a risk of something getting into the gap and jamming the aileron but it's not happened so far and I always do a full movement check before I taxi.

However, fate and sod's law being what it is, you know what's going to happen first flight!

Jim.

[G-JIMG]

I forgot to add, it's important to have the wood grain running the length of the cover. This helps it to flex if it needs to.

Jim.

[G-JIMG]

The two wing halves are very nearly complete but I've only just realised I only had enough primer to paint one of them.

Jim.

[G-JIMG]

While I wait for the paint to be delivered, I thought I'd make a start on the fuselage.

The hardest part was calculating the correct angle, and relative height, of the Nose Wheel Retract box. Fortunately it appears to be correct.

Jim.

[G-JIMG]

Now I have the fuselage partially built, I've taken the opportunity to make the shaped fibre glass sheet that will eventually become the nose undercarriage bay doors.

I temporarily sheeted the underside of the nose by tacking in a piece of scrap sheet balsa. I then ironed on a piece of gloss covering (fibre glass doesn't stick to it).

The area covered far exceeds that required by the actual doors.

I then applied the fibre glass and resin.

Finally, when the resin was still a bit tacky, I applied a liberal coating of a tooling gel coat. The gel coat isn't essential but I find it sands easier than the resin.

Once fully cured, the sheet is removed.

I've roughly trimmed and sanded it before putting it to one side for now. When the time comes to make and fit the doors they will (hopefully) perfectly match the contours of the nose.

Jim.

[Martian]

fantastic work Jim enjoying your work

[G-JIMG]

Thanks Martian.

Enjoying your Hovercraft blog.

[Martian]

Well thank you Jim.

[Rob Fairweather 1]

It's a masterclass for me

[G-JIMG]

The paint has arrived, so I was able to primer the other wing.

Typically I use Halfords, or Craft Shop, primer. However, as both were difficult, or impossible to get, I used Wilco Grey Primer this time. It's almost half the price of the others and I must confess I'm impressed with it. It covers well and, a real bonus for me, it's near enough the exact colour of the real aircraft. So, it will be used as the primer and the top coat.

Before spraying I do the usual vacuuming, brushing down and wiping with a dry cloth but it always surprises me how much still comes off when you giving the part a final wipe with a tack cloth.

For large parts, such as the wing, I make a small spray booth in my garage with drop cloths hanging from the beams. When I'm finished they just get furled up onto the beams, until I need to drop them again.

Jim.