Topic: Scratch built 1/6 Hunter FGA 9

After a couple of months with this project being stalled, I have rekindled my interest and made some good progress with the CAD Drawings - I reckon there are several hundred man hours of work in the drawings - too many, as with a drawing you can tweak away, which then alters everything!  I am at the point where I am setting up the sheets ready for CnC cutting, and with any luck I should have a kit of parts to start putting together soon!

Wing Drawing:

EEEK!

There's nothing more dangerous than a fast jet pilot with a CAD system...



Looking good Pat. A couple of points:

1. How are you intending to do the spar carry-through structure?
2. Given that tyou have the CAD resource are you going to try to area-profile the intake ducts?
3. Are you going for a follow-up tail or a fully-powered one?
4. Will the flaps be stressed as combat flaps?

PDR

This should be nice,think you have the numbers still mixed up Mr Barnes even after previous discussion.
FGA11 in a proper colour
John

Is it the prettiest jet fighter of all? Certainly one of them. Keep promising myself to build an EDF version of the one sitting at the end of a local farmers drive coordinates 52.387125,0.503268. So I shall be watching this with a great deal of interest Pat, look forward to seeing how it turns out.

An excellent project Pat       and I'm sure we're all hoping that you'll do a build thread.

Where will you source the retracts and legs?

Cheers

Gordon

USAS says there are still several sets in stock at Shawbury...

PDR

Marvelous, looking forward to this. Are we to assume it'll be finished in FRADU colours ?

 

I was assuming you'd be looking to do a T8-M (SHAR radar development) variant...

PDR

EEEK!

There's nothing more dangerous than a fast jet pilot with a CAD system...



Looking good Pat. A couple of points:

1. How are you intending to do the spar carry-through structure?
2. Given that tyou have the CAD resource are you going to try to area-profile the intake ducts?
3. Are you going for a follow-up tail or a fully-powered one?
4. Will the flaps be stressed as combat flaps?

PDR

Hi Pete

1.  Here are the centre formers, F7 and F8 - will be made of 6mm Ply, reinforced with Carbon.  I think this should carry the load, but any suggestions from an engineering perspective will be welcome.  (I knew that second year tech drawing would come in useful one day!)



2. Not at the moment.  I have a mate who can do some 3D modelling, so may engage his assistance in the future....

3. Yes!

4. Anything is possible!

As to the colours:



Just ordered wing tubes and wing skinning stuff.  Build thread will follow! 

AFAICS you're resolving the lift loads into what amounts to a "shear" between F7 and F8, so lift loads go though the tube and try to lift F8 and push down on F7. Therefore I'd be looking for some bracing between the two to stabilise these two frames against that "shear" so that the two frames form a stabilised subframe assembly, and then I'd tie the fuselage longerons into that assembly.

Being a swept wing you might also have some concerns about CP movement as the wing flexes under vertical loads - as the lift load increases the sweep induces a washout towards the tip (ie the wing will twist), which biases the lift distribution inwards (and therefore forewards), which moves the CP forward and thus increases the pitch-up, which increases the lift-load and so on. It's a structural instability which can turn a small elevator input into a full runaway until the wing either stalls or folds! So I would want to build in a significant amount of torsional stiffness - probably ad some diagonal webbing between each rib at the mainspar out to the next rib at the false TE (they don't need to be full-height half-ribs and can just be trapezoid shapes, but half-ribs bonded to the wing skins would be neater and stiffer).

I would also brace the wing tube to the fuselage longerons where the tubes pass through both F7 and F8.

Well you did ask...



PDR

Is it the prettiest jet fighter of all?

I think the question mark is superfluous - that should have been a statement

Alan

Can we ever have too many rhetorical questions?



PDR

AFAICS you're resolving the lift loads into what amounts to a "shear" between F7 and F8, so lift loads go though the tube and try to lift F8 and push down on F7. Therefore I'd be looking for some bracing between the two to stabilise these two frames against that "shear" so that the two frames form a stabilised subframe assembly, and then I'd tie the fuselage longerons into that assembly.

Being a swept wing you might also have some concerns about CP movement as the wing flexes under vertical loads - as the lift load increases the sweep induces a washout towards the tip (ie the wing will twist), which biases the lift distribution inwards (and therefore forewards), which moves the CP forward and thus increases the pitch-up, which increases the lift-load and so on. It's a structural instability which can turn a small elevator input into a full runaway until the wing either stalls or folds! So I would want to build in a significant amount of torsional stiffness - probably ad some diagonal webbing between each rib at the mainspar out to the next rib at the false TE (they don't need to be full-height half-ribs and can just be trapezoid shapes, but half-ribs bonded to the wing skins would be neater and stiffer).

I would also brace the wing tube to the fuselage longerons where the tubes pass through both F7 and F8.

Well you did ask...



PDR

Hi Pete



Glad I did!  If you look at the part of the drawing shown, the diagonal structure at 45 degrees is a 6mm ply brace which has lugs locating in the square holes in each former.  This ties the two formers together as you describe, I hope, making the desired rigid box structure.  The wing tube routes through this brace as well for additional support.

The wing is still a work in progress.  I'm planning to use Pro-Skin to skin the wing, which adds a significant amount of stiffness,  but the diagonal half ribs are an excellent idea.  Oh good, another several man hours ahead as I interpolate the rib shapes!   

Hi Pat, it’s nice to think of your Hunter flying in formation with my 1:6scale scratch built Dh110.

Below is a picture of the basic structure which is ¼” end grain balsa skinned with carbon, very light and very strong.

I had a try with Pro skin personally I couldn’t get on with it, compound curves are a no no and I also found it temperature unstable, my preference for a model of this size is 3/32” balsa glassed a well established and more forgiving material.

m

Hi Mick
I haven't used Pro-Skin before, so time will tell... What caused the temperature instability?  It should be quite stable as from my understanding, it's a glass resin sheet? If it all goes horribly wrong, I'll use Balsa/Glass skins as I'm familiar with this method.  The Fuselage will be skinned with Balsa/Glass regardless.
Formation... sounds good! 

It is a decision you will have to stick with as the material is vastly different in thickness.

m

Another early picture to demonsrate how simple the basic structure is, the carbon poles are an obvious godsend.

m

I'd have to do some thinking, but I think that brace would be more effective if aligned parallel to the fore-aft datum, and I think there should be more of them (as a minimum I'd suggest one where the tube pases through F7 and another where it passes through F8, but a few more between these two would be nice). That will transfer the lift loads from the wings into these frames, where the load from each wing is reacted by the load from the other one. But this will put a large stress concentration into the load-path through the middle of the frames which, as it is a curved load path (to clear the intake ducting) will have a strong tendency to buckle upwards at the top (positive G case - negative G case it would be the bottom and downwards, obviously). So the next area for attention is this load bath which probably needs something stiff from a unidirectional material (carbon tow, flanged rolled steel, prestressed unobtainium etc).

£6.16 supplied...

PDR

Now you've got me thinking...  The brace is at an angle to allow the ducting a way into the fuselage centre.

I wanted the spar and tube running along the max thickness of the wing, as the CP will act largely along this line, reducing the twisting tendency of the wing compared with a spar at 90 degrees to the fuselage.  As I understand it, this will make the wing torsionally stiffer, but obviously causes some unusual stresses in the fuselage structure, with the tubes coming in at an angle.  If you can draw lines on the JPEG, Pete, I'd be grateful for your suggestions (picture painting and all that...)

As you say, the load path is curved through F8, so I'm reluctant to cut any more holes in the former - I guess I will have to bond the braces in and reinforce with Carbon.

Pat,

Good luck with the Hunter

FWIW, I'd be looking at something similar to the fullsize with the way the wings attach, (I'm not a big fan of 'joiner tubes').  As for the tail, I'd be wary of an all flying tail, (from spending 5-1/2 years working on them and dealing with UTP debriefs), and would either go with fixed stab and elevators, or - if you're feeling really ambitious - a follow-up tail

Mark

I was only taking the P on the follow-up tail. Its role in the full size was to aid supersonic dive recovery before the fully-powered tail mod was implemented. I don't think it has any significant role to play in a model, other than to add a few extra failure modes!

Pat - I'll try to do some sketches to indicate what I'm thinking of, but I'm a bit busy with family stuff over the next couple of days so don't hold your breath,

PDR

Nice work Pat 
I am currently doing a Buckaneer in CAD for twin Wrens, have to agree its a long old process

I was only taking the P on the follow-up tail. Its role in the full size was to aid supersonic dive recovery before the fully-powered tail mod was implemented. I don't think it has any significant role to play in a model, other than to add a few extra failure modes!

Oh I don't know, I suppose Pat could knock up a pantograph to operate it

Pat,

Good luck with the Hunter

FWIW, I'd be looking at something similar to the fullsize with the way the wings attach, (I'm not a big fan of 'joiner tubes').  As for the tail, I'd be wary of an all flying tail, (from spending 5-1/2 years working on them and dealing with UTP debriefs), and would either go with fixed stab and elevators, or - if you're feeling really ambitious - a follow-up tail

Mark

Cheers Mark

IMO, The tubes are a simple way of providing the strength required without over-complicating the build.  With the limited space in the fuselage, it seemed the best option - here's hoping my solution works!  I'm going for a fixed tailplane and conventional elevators, also in the interest of simplicity!

Pete - No rush mate. Still drawing, not cutting yet!

Got the first 'bit'! 

Canopy arrived today - now there is something tangible, and not just a bunch of 1's and 0's stored on electronic media!    The quality is OK - not perfect, but better than I could have done and great to see that Mick Reeves and my measurements at 1/6 scale are within a few millimetres!

Where dd all your pictures go Pat? 

Hi Jamie

I figured I'd move them into a dedicated album before there get to be too many!  Here they are again:

Fuselage:


Wing:


Colour Scheme:


Centre Section - load bearing formers:


Chees - Pat

While tweaking my drawings, I had a disaster and corrupted the main Fuselage plan, losing the lot!
Fortunately I had exported 95% of the formers, so could reconstitute the plan from those - more time drawing!! It did highlight a few minor errors, which are now corrected, so not a complete waste of time. Sent my drawings off to my friendly CnC operator today!  Fingers crossed I should have some parts soon!

While tweaking my drawings, I had a disaster and corrupted the main Fuselage plan, losing the lot!
Fortunately I had exported 95% of the formers, so could reconstitute the plan from those - more time drawing!! It did highlight a few minor errors, which are now corrected, so not a complete waste of time. Sent my drawings off to my friendly CnC operator today!  Fingers crossed I should have some parts soon!

Unable to get Hotmail at work so I can't check the quality of your work, but assuming it is up to standard    I have arranged to use the machine on Friday whilst the place is shut.  It will be nice and quiet in there with Guinness Dog, so there should be no problems getting it all done.  As the weather looks a bit suspect for Sunday at MF there are 2 options:
1. If you want to take a drive down the 303 you can collect (will send directions) and then start building over the w/e, or
2. Risk I'll be able to come up on Sunday.
Up to you matey, off to sea now so send a txt and I'll get it later.
 
Dizz

Looks rather tasty!

Spent a day working with Dizz on this - CnC machine was working solidly for around 6 hours!  Result is I have six of seven sheets cut, with another few parts still to be cut as the build goes along.  Many thanks Dizz for all your help mate!    Just one sheet that we couldn't resolve for the cutter, so been back to the drawing board tonight to try and get rid of any random artifacts...  Fingers crossed the latest drawing has resolved any problems.  I'll wait for the 'Eureka' shout from Dizz, when he finally gets the software to code up the tooling paths! 

The pic shows three 6mm ply sheets and three 2mm ply sheets.  To give you an idea, each sheet is 4ft x 1ft!  We also managed to print off some construction plans for the build - seeing the drawing at 1:1 scale for the first time was awesome!  This is going to be a BIG model! 

After all that work, I had to test fit a couple of pieces! 

Top rear fuselage section, from transportation joint backwards.  This section is 88cm (2ft 11in in old money) long.  The forward section of the fuselage will be 141cm, making the fuselage 2.3m long (7 feet 6.5inches)...  I need a bigger car!  :



Happy with that! Had to resist the temptation to tack it together with Cyano.  Need to drill holes for wiring to elevator and rudder servos first.  Funny the things you think of after the CnCing is done!

Eureka!

The left side only had one hiding duplicate, but the smaller right side put up the same fight as this afternoon.  I took time out and glassed the 1091 then re-attacked.  I found some overlapping splines and a solitary dot loafing under a line.  The .nc files are now done and I am going to have a beer     
I'll get the last sheet cut on Tuesday..............hurry up and get the fin done and I'll cut that then too!

That comes from your accurate CAD drawings mate.  It sure is going to be big, but you know you really need a bigger turbine 

Early days yet I'm sure, but I reckon there's at least a few folk who would happily pay for the parts and plans.

Still watching with great interest - proper aeromodelling this 

Glad to see the Eureka!  Happy days!    I was having too much fun cutting out bits and sanding them to do any drawing on the fin, Dizz.  I now have a kit of fuselage parts ready to go, and have drilled the formers in the top rear section for cabling and the bolts that will hold it together at the transportation joint.  Can't believe I forgot to put those holes on the drawings before CnC cutting!!  Adapt and overcome - I'm sure there will be lots of fettling required as I go. 

Now that I'm sitting in front of my laptop, time to do some drawing on the fin!!

If the weather looks ok for next Sunday then MF will be on and I will bring the final fuz sheet with me.
I have got the 1091 ready for the flow coat now, but first am going to make some noise for 2 x 90 second bursts to do some power tests..............happy days indeed   

After putting off my least favourite job, I decided I had to get on and just do it!  First bit of planking done: 



Been tweaking the drawings as I find things, so hopefully they will be correct when the model is finished!    Airbrake and actuation is now done, but this requires some minor mods to the lower fuselage structure - there will be some parts that are hand cut, rather than CnC'd! 

Good to see your winter project has started early too