A4 Skyhawk - minimal build blog

[John A H]

As most people are well along with their builds of the PSS A4 Skyhawk I thought it was probably better if I only posted any bits of the build that I have done differently or not covered by others.

[John A H]

Here's what I've done with the Fin - similar to others with slight differences.

Build fin with rudder out of 1/8th balsa

Then created a template out of card for a one piece rudder.

Cut 2 out of 1/18th Balsa (keep the "hinge" cut outs for later).

Then glue either side of central rudder

Front edge is "radiused" to create dummy rudder line and "ribs" are sanded down at the same time that the fin trailing edge is shaped.

finally the "hinge bits" are added back in after been sanded to ensure they show up.

The challenge is going to be not loosing the detail when I apply the tissue/dope (or glass) finish. I might be able to use the card templates to cut the shapes out of the tissue before applying it.

[John A H]

I wanted to be able to get at the elevator snake incase it ever needed "servicing".

I created a balsa plug that was slightly thinner than the rudder trailing edge and then using glass cloth created a fairing to slide into the gap below the rudder, around the elevator horn and sprung to fit in the tail supports.

 

 

The piece of wood is there to spring the sides out so that when slid between the lower tail supports it will hold itself in with friction.

How it looks without the fairing

 

 

and with it slid into place

 

 

Its not a perfect fit but it will do the job and it seems to be a good friction fit so hopefully should not fall off in the air.

Edited By John A H on 27/07/2016 01:18:14

Edited By John A H on 27/07/2016 01:21:36

[Phil Cooke]

Some really nice touches in there John, excellent! Looking forward to seeing this master crafted example soon. You settled on a scheme or is it classified?

[John A H]

As usual dithering over a colour scheme. Did wonder about the "clansman" markings with the tartan stipes but with not many weeks left I think the amount of work for that will rule it out.

I always forget that building the basic airframe is not the time consuming bit - it's all the fiddly bits, final fitting and preparing for painting that takes up time.

[John A H]

I decided that I fancied having drop tanks mounted under the wings. The problem is that as the wing is mounted low down they will get ripped off on a slope landing. Attaching them with magnets would still mean they would probably damage the wing as they detached on landing.

So they need to be releasable before landing, this actually has the bonus in that it means the Skyhawk will be able to be used in the bomb drop antics.

I'm using the same method as on the Jim Strain PSS Victor; the tanks are attached through a slot in the wing by a piano wire that runs to a central servo.

I've created the slot by adding two extra half ribs into each wing (faced with ply) that the piano wire passes through

The piano wire will be bent at the servo end once I line up both wings and decide the position of the servo

[Andy Meade]

Really nice as always John. Another award winner in the making

[John A H]

Top wing skin on, bomb release servo mount added and release wire bent to shape

Release servo is located in port wing - once wings are joined I will know if the starboard release wire matches up to the servo horn.

The drop tanks will be "built" around this ply keel. The tab at the top locates in the wing slot

Under the wing showing locating slot

and with drop tank keel in place

As we are running short of time I might be turning up in september without the actual drop tanks and end up dropping these pieces of bare ply sheet on the bombing range. I need to concentrate on getting the airframe finished first before faffing around creating drop tanks.

[John A H]

I'm now at that stage when the completed balsa parts start to magically attract dints and dings every time I pick them up and put them down. Today the fuselage tipped over and knocked a piece out of my carefully crafted tailpipe shroud - so that ment an extra 3 hours moulding and replacing the damaged section with fibre glass.

[Phil Cooke]

nice work John, looks like a great solution for the drop tank and pylon - Very neat.

I too know the hangar rash feeling I tend to end each session now with the filler correcting 'dings of the day' and start the next morning with a little 'corrective' light sanding before the proper work begins! The sooner the glass cloth is on, the better!

[graham Gilliver]

Hi, I really like the glass fairing around the elevator snake/horn coupling can I use that one ? you must be happy about the amount of lift you expect to get to be fitting drop tanks I am fretting something awful about how heavy mine is getting , and while I still hope to fly it at Manilla slope fest in Sept I am some way behind you and my lack of skills means catching up may take some time. you keep up the great work in the meantime as I am sure I am not the only one benefitting from your interesting solutions to things. looking forward to seeing the fibreglass process. cheers, G

[John H. Rood]

John, a fantastic build underway. Q: Is the action for your drop release simply the piano wire being pulled away from the slot that you created with those half-ribs?

Me, I've now closed-up my wing -- but the mad scientist Dr. Strangeglove within -- he barks that it ain't too late to add a centerline NUKE release.

Don't take my word for it, here's an A4D-1 (A-4A) in test in California mid-late 1950s with a nuke! (Happy Doomsday Sunday!)

May the gods have NO mercy on my enemies, real or imagined! Actually, I'd best just shut up and build, and keep mine SIMPLE.

Edited By John_Rood on 31/07/2016 19:12:52

[Chris Barlow]

Nice build John. Different approach for the rudder ribs too.

Releived to see I'm not the only one playing with the idea of drop tanks too. The Orme could be littered with then in September!

[John A H]

Graham - on a good day at the Great Orme the drops tanks would probably soar by themselves

John - yes the wire slides through holes in the ribs that make up the slot (the inside edges of the balsa ribs are faced with 1/64th ply to stop the wire "sawing" through the balsa ribs over time). The servo travel is then setup so that the wire pulls enough to clear the slot, the carbon tube is there to make sure the wire is supported and pulls straight. Where the wire goes through RIB 2 1/64th ply has been added both sides.

The slot was created by sandwiching 1/8th light ply between the two ribs, the drop tank keel is also out of 1/8th ply with the tab that inserts into the slot given a slight sanding on each side so that it is a loose fit ( having to land with the drop tanks still stuck on could be interesting).

I thought about also having a central tank and the outboard weapons pilons but chickened out. I think if you went for a central tank could maybe have it attached with a simple wire loop on the tank mount that fits in a slot just to one side of the central ribs so that the release wire can run along side RIB 1. To keep it in the correct position you could then have a small bit of dowel at the front and back of the mount that located in holes under the wing.

[John H. Rood]

John, thanks for taking the time to answer my "slow student" question there, and for sharing all that good info.

Another little idea floating in my head is to make wing tanks that could, say, stay put in flight but can literally be a cushion on landing ---if we're landing on a patch of relatively flat terrain, of course. (Dream on, dude!) 

Fabricate, say, a pair of fixed or removable or even pop-off wing pylons for the two standard wing tank stores stations; make a ply base for each each pylon that spreads the load out a bit along the wing underside, and that base can be held on with small magnets (or even velcro?) to the wing underside. The under-half of the standard Douglas 175 or 300 gallon drop tanks could be made of a relatively hardy but soft foam, e.g. EPP, so they serve as shock-absorbers; their fins could be of soft thin plastic with plenty of give.

Yes, that whole wacky little fantasy sprang from seeing this insanely fun photo ...

Edited By John_Rood on 02/08/2016 19:15:16

[John A H]

Wings now joined together - I rushed the process and ended up with epoxy oozing out of everywhere.

Now the moment of truth - do the drop tank release wires match up with the servo

Servo sat in position and set to "hold" drop tanks

And by luck there is enough movement to pull the wire into the correct "drop" position

In that last photo you can see my attempt at creating the leading edge fairing that houses the guns. I've had 3 goes at making the fuselage section (where the guns would sit) to match up to the wing but non look right so far. I think to create them to scale would require the front of rib R2 to not curve down as much. At the moment matching the curve of R2 makes the fairing look "droopy".

I'm nearly at the point of not bothering and shaping the wing leading edge straight, as shown on the plans - time is ticking by.

[Phil Cooke]

John I've tried too and couldn't get them right - like you say its a function of our glider wing aerofoil - the full size wing looks completely flat on the underside too so there is no transition from wing to fus - it must almost be co planar - our model cant recreate this with the wing as designed.

Our saviour (in plan view at least) is that the gun flash shields stick out and cover this region (I drew them on the plan) which means you'd never see the wing LE curve in plan view - of course that doesn't work from all angles... maybe a representative 'drooping' or thinned down LE curve is still better than none...?

Edited By Phil Cooke on 03/08/2016 07:20:19

[John A H]

Formed front wing fairing from 1/16th sheet. Cut root of leading edge off and inserted shaped blocks ready for the dreaded gun shrouds.

Also wasn't keen on the wing bolt plate sitting proud of the bottom of the wing so blended it into the wing/fuselage using a sheet of 1/8th balsa. It's not scale (wing should be "flat" at this point) but we'll see how it looks when painted.

And how the wing looks with them both attached.

[Mark Kettle 1]

Looking good John, I like the flared look.

[John A H]

Also had a play with the jet exhaust. Decided I needed a bigger pipe than provided by Phil's skincare product.

Found a cardboard tube of the required radius ( I knew it was worth collecting all the various tubes from rolls of cling film, tin foil, solarfilm, vinyl etc. ). Covered it in sellotape and release agent.

Then wrapped 4 layers of fiberglass round it . Once dry peeled the cardboard off it and was left with a tube that, once it was glued into fuselage, was strong enough for me to sand right up to it to get shape of jet nozzle (gave it a quick spray of primer before putting it in. The yellow colour on the fairing is because I've already fiberglassed it as I kept breaking bits off while handling fuselage.

And this is where I am up to - the filler shows how many dints I've inflicted on it while in the workshop - still needs some more first aid.

Still to sort out gun fairings and the undercarriage housings. Might then be ready to start the covering/painting.

[John A H]

Skids/undercarriage pods carved and fiberglassed.

Stuck them onto some balsa sticks and held them in the workbench. Attach pegs around the edge of the glass cloth to keep the cloth pulled onto the pod

Used roller to apply resin. just need to trim near to the edge and then a sand around the edge to clean them up.

[John A H]

Started to cover the bit of the airframe that are complete. Might use fiberglass on fuselage but rest is to be tissue and dope.

After Peter reporting loosing the washout in the wings I though I better be careful when covering the ailerons in case the dope "changed" the twist in them that matches the wing. I clamped it into place on the wing and then applied a coat of 50/50 shrinking dope/thinners the up to where they were clamped. Once dry unclamped them and they still matched the wings. I then doped the ends that had been under the clamps.

After a light sanding I then applied a layer of thin tissue using the 50/50 dope/thinners mix, again clamping it to keep the correct shape. After this dried I checked again and ailerons were still matching the wing washout.

I have used the same process to apply tissue to the tailplane, and elevators.

I was able to apply tissue to the rudder up to the dummy hinge line with it only attached to the "riblets" at the rear. Once dry I could then cut down the sides of the "riblets" and then by flooding with thinners attach/push the tissue into the "gaps" between them. I then covered the rest of the fin from the front of the hinge line.

To get the tissue into the intricate parts you can "prod" it into place using a small paint brush loaded with thinners, this reactivated the pre-applied dope. This also works on any bits that need a bit of persuasion to stick or are not lying flat.

[John A H]

I've also bodged up the gun fairings on the fuselage/wing join. They are not great (or symmetrical ) but they will do.

There is a carbon fiber tube inserted into them. Cartoon (or is that "Comedy scale" guns are made of plastic tube with a carbon tube muzzle. These are just a push fit so I need to make some spares for when I loose them..

How they look from below . Maybe they should be a bit shorter ...... then again the amount of time I've spent messing about to make them they need to be obvious

You might be able to tell from this shot that I drilled the gun holes lower on one side so they don't match.

and how the back of them look with the wing removed - filler as far as the eye can see !

[Phil Cooke]

Congrats on the fairings and guns John, they look great, I'm not sure there will be too many modelled with that detail so great job in sticking with it! I think they stand a very good chance of surviving all your landings in place with them being pushed in and through the ply bulkhead like that!

[John A H]

Created a basic ejection seat

and cobbled together a refuelling probe - early model straight version.

With it extending beyond the nose the front straw section is a push fit and should just bend on less than perfect arrivals.