P1067 the Hawker Hunter prototype in 5mm XPS

[alan p]

You have definitly got the elegance of the hunter?

[GrumpyGnome]

Good as new!

[Simon Chaddock]

The Hunter did fly at the Ashbourne Scale Day although it tool a bit of rather drastic repair work to achieve it.

After the skin repairs were completed testing revealed the duct was cracked in several places which caused the inlet area of the duct to collapse inward at full power.

https://www.youtube.com/watch?v=kboAuM1_RJQ

This required 3 separate local areas of the fuselage skin to be cut out, both top and bottom, to gain access the duct. Each duct crack had to be reinforced with one or more a printed patch repairs. This was the largest cut out.

All together I used 16 patches before I was confident the duct would hold shape under full power. At the same time several areas of the duct had to be re glued to the supporting fuselage formers.

With 100/100 hindsight the damage would have justified scrapping the whole airframe and doing a new build.

 

Not that I have any intention of building another at the moment I have spent some time redesigning the inlet portion of the duct to significantly improve the improve the airflow and thus reduce the suction forces.

Perhaps I should do a late mark version that had all the aerodynamic 'bells and whistles' added.

You never know. ?

Edited October 20, 2021 by Simon Chaddock

[Simon Chaddock]

This is a CAD image of the inlet section of the Hunter duct as built.

The problem results from the sudden change in shape from the triangular inlets to the circular portion that leads up to the 50 mm EDF which is a light press fit into the opening. The joggle endure the duct exactly matches the 50 mm inside diameter of the EDF body.

Although the combined area of the inlets is more than the FSA (actually 169%!) the sudden change creates a disturbance and thus extra reduced inlet pressure. Although the duct was capable of resisting it obviously it was loosing a bit of thrust. Obviously when it was damaged after the crash it just locally collapsed virtually destroying all the thrust.

After some hours of trial and error I managed to create this 'improved' inlet duct.

  

The object was to transition as smoothly as possible from the combined duct inlet area to the 50 mm circular inlet of the EDF. The problem is my CAD package only uses mathematically generated shapes.

With the benefit of hindsight the fusege formers will be more closely spaced in this area to provide additional support to the duct.

This is what it looks like in the CURA preview.

It will be 'vase' printed with a single 0.3 mmm wall. The duct has been 'submerged' into the bed to give a flat surface that the individually printed full length inlets will be glued onto. The 3 mm 'brim' is retained for this purpose. The full inlet duct is too tall to be printed in one piece.

At the moment this is just an exercise. ? 

 

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[Dad_flyer]

Does your CAD have a 'Hull' transformation? It makes smooth transitions between your geometrical start and end shapes. Like a ship's hull made from the formers, except that the shapes can be 3D, so in this case a 50 mm cylinder and the inlet triangle. I find it can help with a lot of complicated shapes.

[Simon Chaddock]

Dad_flyer

It appears that the FreeCAD package I am using has its roots in engineering hence its reliance on mathematically generated shapes. In effect it means every surface, even elliptical ones has to be fully dimensioned. To make matters worse some of its auto features like creating bevelled edges and corner radii although depicted in the CAD image are not actually recognised by the CURA slicer so do not appear in a 3D print..

 

At some point I fear I shall have to learn how to use something like 'Blender' which is intended to manipulate meshes to create infinitely variable shapes which are more more suited to aerodynamic.?  

[Dad_flyer]

We use OpenSCAD, which is also an engineering type of package. You effectively write a program to generate your object. It is not based on constraints, but on adding and subtracting simple shapes. Everything can be parametric, so you can make a design and then just change the value for a parameter to make it fit a different fan or servo or screw size. There are a couple of really useful transformations, of which Hull is one, and you can also define modules which provide sub-shapes or combination rules. Blender is not so good for engineering type shapes, it does not have the record of where you started from, you are always manipulating the current version of the shape, not the process needed to build the shape. Child_flyer uses Blender, but more to draw and animate in 3D than to build things.

[Simon Chaddock]

Its not that I have anything else to do but I was not entirely happy with the first improved inlet duct so I had another go or rather modified it buy adding bugle sections to the flat ones so in now translates smoothly from two triangles to a circle.

Out of interest I printed it out as well.

 

I added four stiffeners (top and bottom) along the remaining straight lines between the bulged segments.

I did wonder if it would be practical to actually use this to replace the original inlet part of the duct but it would mean cutting of the fuselage nose and tail sections, the wings and then building a complete new fuselage centre fuselage. As all the wiring go through the centre section too I fear it would much simpler to just build another Hunter.

I will have to think hard about that as I don't have space for the one I have. ?

  

 

[Simon Chaddock]

Just for completeness I did fly & video the Hunter two days ago. I did not realise how strong the gusts were so it was a short rather nervy flight but no damage.

https://www.youtube.com/watch?v=KhtD6oqPC6E&t=3s

I will have to wait for some proper calm weather,

[Simon Chaddock]

After the deluge over the past two days this morning was pretty calm although the field was all squelchy under foot so I took the opportunity to hopefully fly the Hunter properly.

Only a short flight as I didn't want to strain the rather old 1800mAh 3s and a leaky boot meant one of my feet was getting wet! ?

https://www.youtube.com/watch?v=jQhOl8fg2T8

Pale blue is not an ideal colour against such a sky (better in full screen mode) but it all went smoothly thanks in no small part to the gyro assist. ?

Light and aerodynamic means it not only glides well but pretty slowly too. Finals and flair all power off. 

[GrumpyGnome]

Sorry, didn't mean to press the 'sad' icon ........ 

[Simon Chaddock]

Now the inlet part of the duct profile is done things can progress a bit, particularly as the new 50 mm EDF has arrived from China.

The full duct is just like the Prototype Hunter 'silly' long.

By the same token the EDF is 'daft' small for the size of the airframe!

A couple more fuselage formers to add and the EDF wires extended to the cockpit area through the formers then the skin planking can start.

It takes about 22 planks to go all round so its a long and tedious 'fitting' job. It also uses quite a bit of foam safe glue. ?

 

 

Edited November 12, 2021 by Simon Chaddock

[Simon Chaddock]

As you may have guessed I am definitely going to build a complete new Hunter but this time it will be an F6 with the 'saw tooth and drooped' leading edge and the 'Sabrina' cartridge case bins.

The F6 had over 30% more thrust than the prototype so its modest 3s performance is about right but it may require a 4s to give an F6 scale 'oomph'! This will make ensuring  adequate support for the inlet duct all the more important. 

[Simon Chaddock]

Moving slowly on with additional formers and reinforcing top and bottom around the inlet duct as indicated to be necessary after a 4s test run.

 

The inlet trunking outside the fuselage will have additional support when the wing root fairing is added.

It will have added maybe 10g (1%?) but it should mean the duct will not collapse!   

[Simon Chaddock]

Slowly progressing but then the fuselage is by far the most complex bit so 'slow' is to be expected.

The wing roots on.

I suspect the next task is to install the ESC as this will require some solder and heat shrink work ominously close to the foam. ?

Once that is out of the way, and the EDF tested again, start building the wings which by comparison are simple indeed. 

 

 

[Simon Chaddock]

The 40A ESC with its deep fingered heat sink mounted just ahead of the RH intake.

 

The  50 mm EDF takes just under 30A on 3s so on 4 it will be close to 40 so good cooling.

As with my other XPS planes it means the battery will have to be offset to the left a bit to counteract it weight.  

[Simon Chaddock]

The wings are built in the same way as before but include the extended and slightly drooped outer leading edge.

I don't yet have the servos so I may just have to feed a servo wire through the wing so the servo can be installed after the wing is attached.

Obviously such a wing is very light and does flex a bit under load but is surprisingly strong. All the Hawkers built so far using this technique can be looped safely enough.

 

[Simon Chaddock]

The wing go on.

The only thing I can do now is build the cockpit/nose until the servos arrive. 

In the meantime I did run the EDF at full 4s power as a test.

https://www.youtube.com/watch?v=iznMsOgNAdo

I must say the PowerFun EDF is supremely well balanced.

52.5A 752W Wow!

That will give it close to 500w/lb, although not for very long as everything, including the wiring, was getting a touch warm. ? 

  

[Simon Chaddock]

The nose is a straight copy of the one used on the Hunter prototype.

This time I was able to build the battery box in at the same time so it became part of the structure rather than added.

The 1800mAh 4s battery is still inserted through the cockpit (just!) but is slid forward up to the nose bulkhead.

If necessary the battery can be moved about 2" to achieve a good CofG.

The radio will go on the cockpit floor.

Apart from glueing the nose in place I am now stuck until the servos arrive as I will need all the current access to run the servo wires.

   

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[Simon Chaddock]

With the nose added I really have gone as far as possible until the servos arrive.

Compared to the previous 4 Hawkers built like this out of XPS this one has gone very slowly. In my defence I had not intended to build it so no parts were ordered in advance. In addition there is no rush, it is winter and I have no where left to store it when it is finished! ? 

[Simon Chaddock]

After a long delay (6 weeks!) for the 3.7 g servos things can at last start again.

The aileron servos let in to the wing underside.

In this sort of application the servo is glued into the skin so it actually becomes part of the load bearing structure.

I had already run a servo wire through the left hand wing. The other was a bit harder but as the wing has no ribs it went well enough. The extended servo wires will eventually be fed through to the cockpit area.

The elevator servos are similar but rather easier as the tail plane is not yet fixed in place yet. ?

As I will be setting it up to use a Lemon 'stab' receiver there is only a single stab elevator port so they have to be on a Y lead which means a servo reverser. No problem except I am waiting for one of those now. ☹️

Still there is no rush.   

 

 

[Simon Chaddock]

Slowly completing the airframe.

The servo wires for each elevator half pass down through the fin into the fuselage where one is fed into a servo reverser.

The servo reverser output is combined with the other servo via a Y lead. The single wire is then fed inside the fuselage spine to the cockpit. The fuselage planking can then be made good.

Canopy still to do and quite a few 'scale bits' need to added to the underside.

It will be painted black overall in the style of the Black Arrows of 111 squadron.

 

[Simon Chaddock]

Talking of the Black Arrows.

The record breaking 22 plane loop at the 1958 Farnborough show. I was there as a 12 year old with my Dad.

In fact this routine involved doing 2 loops one after the other.

This picture was actually taken during earlier practise. It had to be restored as the original was hanging in the mess and had been slightly cropped.

Interesting is the differences in the aircraft. The 14 black are identical but some of the others have wing pylon stubs, other do not.

Although 111 was the official RAF aerobatic team it was still an operational squadron so the planes carried their guns, technically only the barrels, as the Hunter had the gun breeches and ammo in a quick change pack. For the display this pack was exchanged for a dummy which contained a 40 gallon diesel tank to make the smoke.

It says something of the quality of the pilots in the squadron that there were at least 22 suitably trained. Their 'normal' display used 16 planes.     

[Simon Chaddock]

The scale underside bits. 

 

All made  in 5 mm XPS.

The "Sabrinas" are surprisingly big but then each had to hold 300 30mm cartridge cases.

The underside air brake looks (and was) a bit of an afterthought literally mounted on the fuselage underside. It obviously worked well enough as it was never changed in all the 1900+ Hunters that were built.

Still waiting for the paint! 

[alan p]

Aircraft need sabrinas

Looking good Simon