A Hawker Sea Hawk in 5 mm underlay.

[Simon Chaddock]

The Hawker Sea Hawk always was a very elegant jet.

By bifurcating both the inlet and exhaust Sidney Camm was able to install substantial fuel tanks either side of the rather portly RR Nene in an era when jets suffered short range and endurance compared to the previous generation of piston powered planes.

Due to its size and weight it lacked performance so the RAF rejected it (the Meteor was faster) however the Navy liked its duration and load carrying ability so was ordered into production. It only had a short service career as the much more capable De Havilland Sea Vixen soon appeared.

 

I am electing to use the rather soft 5 mm underlay foam as it is all I have access to in quantity!

A fully bifurcated EDF is always a challenge. The ducting becomes the heart of the plane as the rest of the airframe is pleasantly simple and aerodynamically efficient.

When I built my Folland Gnat I printed the entire duct and then built the airframe around it.

The shoulder mounted wing meantthere was no issue with the spar crossing the duct. This will certainly not be the case with the Sea Hawk where its spar will have to go round the EDF. ☹️.

On the plus side the Sea Hawk duct although a fairly complex shape will be short so keeping the weight down.

First a half size test print of the exhaust duct.

You can see why it was nicknamed "the trousers" at the Factory.

Next to try to print it twice the size and to the exact dimensions to fit the plane. 

 

 

       

Edited May 8, 2021 by Simon Chaddock

[Colin Leighfield]

It’s got all of the right ingredients, as long as you can minimise those duct losses. 

[alan p]

This is going to be interesting, 

[Gary Manuel]

I had to google "bifurcating " but I'm with you now. Watching.

[GrumpyGnome]

Looking forward to another construction masterclass..

[Simon Chaddock]

After a lot of printing and head scratching a 'big' trouser.

I eventually had to print it in 3 pieces as when the head travelled between the legs it tended to touch the print. No problem one way as the print would simply flex but the other way with such a long lever arm it simply lifted the print from the bed. The unfortunate part was it was an hour into the print before the problem arose! 

After two attempts I elected to print the 'legs' individually. Using a brim on each print gave a nice area to glue them together as well as stiffening the very thin walled structure.

Overall this part of the duct has come out better than I dared hope and is amazingly rigid. It weighs 13.5 g

A quick check shows this is going to be quite a big plane for a 55 mm EDF on a 1800 mAh 3s. It will have a wing span of 42" (1066 mm).

It will have to end up seriously light. ?

  

Edited May 9, 2021 by Simon Chaddock

[ken anderson.]

as always simon,fascinating,i admire your skill.....well done..

 

ken anderson...ne..1....skill dept.

[alan p]

Obivously the trouser designer was a horse  rider with a spread like that?

Will the bifurcation on the exhaust effect the resultant thrust?

[Simon Chaddock]

alan p

Doing almost anything to high speed low pressure airflow causes losses. There are several issues with bifurcating the exhaust. Obviously the act of splitting the air flow causes a loss and made worse by the fact that two circular ducts with the same area as one has four times the surface area and the air friction from the wall surface is more or les proportional to the surface area. ☹️ 

Having used energy to speed up the air the best thing you can do with it is the let it out the back as quickly as possible. ?

 

Compared to the exhaust where at least everything is kept circular the inlet duct is more of a problem as is goes from two rather narrow angle triangles to a circle. With a single fan inlet losses can be as serious as exhaust losses. The only positive is when flying the forward motion can generate dynamic pressure which can help to over come any inlet losses.

After much trial and error I have arrived at a 'workable' inlet duct here sitting ontop of the EDF and its trousers exhaust.

By comparison to the structurally efficient circular section of the exhaust the inlet with large area of flat? panels is almost twice as heavy.

It seems to fit reasonably well in the space available.

 

That EDF will look almost lost in the substantial fuselage. 

 

  

 

[Colin Leighfield]

That's a work of art Simon. Did I read somewhere that the plenum chambers used on the Harrier to enable multiple/variable nozzle ejection actually increased thrust from the Pegasus and if so, what is the trick in achieving that? There's some similarity between the "trousers" here and that. I know they intended to use plenum chamber burning on the P1154 for supersonic flight, but it was not a feature on the Harrier. 

[Simon Chaddock]

Colin

I was aware of the same claim but I do wonder if in reality it was not an actual increase in thrust but perhaps that the plenum chamber proved more efficient than using individual ducts to the nozzles. 

I understand plenum chamber burning intended for the 1154 was tested by RR but it was a solution that kept the nozzles short and provided equal thrust to both nozzles. I am not sure if the intense heat it created effectively completely inside the fuselage was fully resolved.

 

This picture rather emphasises just how small the fan is.

It would not look out of place as the generator propeller on the nose of a ME163!

 

I am currently working on a new shorter inlet design that will place the EDF exactly on the wing spar as done like that it will provide a secure mount for it.

I will have to judge and hopefully test whether any losses for the more convoluted inlet path are worth the construction benefits.

Once I have the duct 'sorted' the actual airframe construction should be relatively straight forward. Well that's what I keep telling myself.

 

 

[Simon Chaddock]

The new shorter inlet and matching longer trouser in place on the wing plan.

Note the EDF exactly spans the spar line.

 

It is all glued together so it is possible to do a nose down thrust test.

On a 3s at 'storage' charge it delivered 310 g of thrust. More to the point the thrust was increasing right up to full throttle so it can be expected that the extra voltage from a fully charged 3s should create still more thrust.

Of note is the combined weight of the duct including the EDF, the ESC and the intended 1800 mAh 3s is just under 300g so on this basis as long as the bare airframe is in the region of 250 g it should fly ok. No 'rocket ship' but then neither was the full size.     

 

[Simon Chaddock]

Next the centre section of the fuselage that will hold the duct.

After much thinking chose to make it as a half shell over the plan.

This would mean with the formers suitably cut away the complete duct would 'sit' in the half. Once sufficient planking was in place it could be lifted and with suitble adjustment to the formers the duct was glued in.

Planking in 5 mm soft foam underlay is rather more difficult than in 3 mm Depron but it looks like it will be plenty strong and rigid enough.

Next is the other half of the formers and a lot more planking.

 

[Dwain Dibley.]

Clever stuff Simon, I am watching with great interest Mate.

D.D.

[Mike Chantler]

Like the build ?

What do you glue the planking on with? Looks pinned as well? (I can't seem to find a glue that doesn't end up really hard compared to foam when sanding.)

[Simon Chaddock]

15 hours ago, Mike Chantler said:

Like the build ?

What do you glue the planking on with? Looks pinned as well? (I can't seem to find a glue that doesn't end up really hard compared to foam when sanding.)

Mike

I am using a polyvinyl adhesive called Technicqll (Not a typo) made in Poland rather than the more normal UHU POR.

I have found the problem with POR is that it remains 'rubbery' for weeks if not months which makes it impossible to sand. Technicqll on the other hand does dry hard, like Foam2Foam glue, which allows it to be sanded with a block and fine paper.

5 mm foam is really to thick to plank at this size so both tape and pins are required, as well as 'finger forming' to get the parts to hold to shape.  

This particular foam has a really soft surface so I have yet to find out what sort of surface I end up with but 'high gloss' it will never be. ?

 

[Mike Chantler]

Many thanks - I spent ages trying to find a glue that sticks and sands, so really good to know. 

 

Will order some. It's the Technicqll.pl (polyvinyl acetate) as below? 

 

PS Have you seen the 6mm white XPS in B&Q "Diall Polystyrene Insulation board (L)0.8m (W)0.6m (T)6mm)" ?

Good price for 8 sheets. Quite stiff, so good for formers, also in 3mm, according to website, but I've only found the 6mm in my local store.

[Mike Chantler]

Ah - also found this stuff:

[Simon Chaddock]

That yellow tube version is the one that I am using.

I am using 5 mm XPS underlay board simply because my daughter had 13 sheets left over so it was free!

It would be easier to plank the required double curves in 3 mm although the skin would require more support. 

[Simon Chaddock]

With the other half of the formers added work can proceed on 'skinning' the centre section.

Note the motor leads have been extended so the ESC can be positioned on the cockpit wall ahead of the RH inlet.

The root fairing is the next major step.

I will have to leave some of the underside 'open' to be able to run the aileron and elevator servo wires through to the cockpit area.

Slow but it looks like it will be plenty strong and rigid enough. 

 

 

[Simon Chaddock]

It is slow going making small thick bits of plank fit but one side is more or less complete.

 

No visible but a part of the root underside is open to give access to run the servo wires.

I have used a small bit of 2mm Depron to surround the tail end of the jet pipe. I will also use some 6 mm Depron sanded to shape to give a small bell mouth radius on the inlet.

Depron is much harder than the XPS foam so should give a bit more 'ding' resistance.

Will probably do the wing panels next. 

 

[Simon Chaddock]

Although simple in shape making the top and bottom wing skins each in a single piece of 5 mm XPS is proving to be quite troublesome.

In theory the wing is made up of just the skins drawn over a pair of XPS shear webs to give the required section but apart from the substantial amount that has to sanded away to give a relatively thin trailing edge the force required to bend the sheet is such that it has to be held in place until the glue is fully set otherwise it simply springs open.

Giving XPS sheet a permanent bend is difficult as it is quite brittle so cracks on the tension side very easily.

The RH wing all clamped up.

It has to be left for 24 hours and I only have 8 clamps so I can't make more than one wing at a time.?

The LH wing complete.

The aileron simply cut out, suitable pieces glued in to fill any gaps between the skins. The aileron is top tape hinged and the servo glued in place so it is flush with the lower skin.

The final oddity will be that the wing will be simply glued onto the centre section. I believe a well prepared butt glue joint will be as strong as the XPS foam so will give about the lightest possible structure. 

 

[Mike Chantler]

Looks like you got nice TE / LE shapes. Did you use the Technicqll to glue it all together?

 

Mike

 

(PS was in B&Q yesterday and bought some of their 3mm XPS, no good for formers but might be good for skins)

[Mike Chantler]

really liking the build btw, lots of interesting techniques that I plan to have a go at ?

 

[Simon Chaddock]

Yes Mike

Its all been glued with the yellow tube Technicqll.

It takes a good 24 hours to go truly hard but until then it has limited adhesion. POR on the other hand dries 'firm' much quicker but takes a lot longer to gain full strength and even then it is not hard like Technicqll.

The other disadvantage of XPS is the surface is very soft. You only have to catch it with something and it can leave a big mark.

 

Although the wing is adequately stiff I felt the ailerons had very little torsional stiffness so I covered them in ordinary tissue stuck down with Eze dope. Now about 3 times as stiff for very little extra weight. ?

 

I shall have to have a look in B&Q for some of the 3 mm stuff.