An RC Depron Douglas X3 Stiletto

[Simon Chaddock]

Reprinted the inlet part of the duct meaning I can actually start on the airframe.

I origianlly intended to build a half shell over the plan, insert the complete duct exactly half way into the shell and then complete the other side. This was the process I used on my series of Hawker EDFs.

However I had second thoughts as the X3 duct is both a complex shape and made of LW-PLA. I felt the duct was just too delicate for this process so plan B is to add the formers to the duct one at a time and very carefully plank the complete fuselage section. This would allow each former to be made a better fit around the duct but would require plenty of "eye ball" to ensure no twist was built in.

The process would still be fraught with difficulty but it would be a gentler on the duct.

A modest start. The "exhaust" former.

 

The 3mm Depron former is in two halves, cut together to make sure left and right side are identical with separate pieces inserted between the ducts.  

When the new inlet duct is added there will be 6 more to add..

The tail and nose sections will be built separately and together are actually longer than the centre section. They will only be glued on when the centre section is fully  complete.

This could all take some time. 

 

[Trevor]

Remarkable work as always. But that definitely does look like a case of ‘the wrong trousers’!😀

[Futura57]

An ambitious project Simon. I wish you success 👍

[Simon Chaddock]

Trevor

At the Hawker factory when they were making the Sea Hawk the bifurcated exhaust duct was apparently referred to as "the trousers" by shop floor!

 

[Shaun Walsh]

1 hour ago, Simon Chaddock said:

Trevor

At the Hawker factory when they were making the Sea Hawk the bifurcated exhaust duct was apparently referred to as "the trousers" by shop floor!

 

Was the point where the exhaust duct bifurcated referred to as "the flies".

[Simon Chaddock]

The EDF & Inlet formers made and fitted.

They are the easy ones!

After good few hours over a period of two days the full set of fuselage centre section formers.

Delicate but can be safely handled. 

One advantage of working in Depron is each former can be easily "adjusted" by sanding or adding pieces so the profile of each former follows smoothly on from it neighbour on either side. It need plenty of "eyeball" and imagination to ensure the ultimate shape will come out close to that of the plastic kit

Now to start planking but keeping in mind the how and when the wing and the "permanent" electrics need to be installed.

The battery hatch will be cut out once the battery position is determined and that won't be able to happen for quite some time.  

[Simon Chaddock]

Planking underway, the simpler almost flat areas first primarily to give the formers some rigidity.

 You need a lot of pins!

This the underside. The really complex bits are when there are rapid changes of shape leading to the inlets and exhausts. It requires individually shape and twisted planks to get a good join to its neighbour.

Overall it is rather a time consuming process as you can only do so many planks and then you have to let the glue hardenm 

[PDB]

Very interesting construction method.

 

What material are you using for your planks?

Edited February 11 by PDB

[Simon Chaddock]

PDB

The planks are all 3mm Depron as are the formers.

Over the flatter areas each plank is a generous 28 mm wide. At the more complex shapes each plank will be no more than half that width They will also be shaped, the edges bevelled and the plank hand formed twisted as required so it lies as a good fit in its natural state. That way the loads to make it fit "snugly" are within holding power of the pins.

When the skin is complete and after a light sand the pin holes are small enough to be completely filled by a light "wash" coat of thinned lightweight filler.

The X3 was white overall so it will only take a light coat of white paint over the white Depron.

Resulting visibility as an RC plane?

Terrible! Unless over head or very low against the greenery!    

[Simon Chaddock]

After many hours the planking on the centre section is nearly complete. The only bits left open are where the one piece wing will slide through.

At this point the slim chance that this project succeeding is brought in to focus by sitting it on the plan for the wings.

There is about 2' to be added on the rear and about the same on the front

No matter how light the complete fuselage might be that is a seriously small wing area, let alone achieve a hand launch!

I suppose it is a case of "Nil desperandum" and carry on regardless.

[J D 8]

     More of a horizontal launch rocket than an aeroplane. 

     Catapult launch rail like the Wright brothers or something like Fireball XL5.😀

[Simon Chaddock]

Started to think about the wing structure.

The original used a very thin supersonic section with a sharp leading edge. As will need all the aerodynamic lift I can get my intention is to retain a scale "thin" wing, 12mm thick at the root but use a more conventional flat bottom section with a radiused leading edge to hopefully to be able maintain control at larger angles of attack. As a belly lander it really will need to land nose high.

The middle 103mm of the wing chord which includes the spar will have to be fed in between formers 4 & 5.  

Once the wing is secured in position the leading and trailing edges can then be "made up" to the full 230 mm root chord.

As slow speed control will be important I may have to increase the area of the elevator and ailerons Undecided whether a rudder is worth the extra servo weight.

I did say the X3 was likely to take some time. 😉

 

  

[Simon Chaddock]

The middle 103mm of the wing chord fed through the fuselage.

 

It is full span but looks plainly ridiculous on such a large structure.r

The swept wing spar is a veneer/Depron/veneer structure with carbon tow glued glued over the centre join.

One advantage of the very modest span is even the scale thin wing should be adequately strong enough.

I can only hope when the full chord if the wing is built up it will look a bit more realistic but I doubt it!

[Futura57]

Respect for persevering. As 'bones' might say, it's an aeroplane Jim, but not as we know it. Sealing the fuselage and filling with helium would save some weight 🤔

[stu knowles]

There was a guy came to our club with a GB ish type racer, from the same era, huge engine and not a lot of wing.  He had built the fus at 1/4 scale and the wing at 1/3rd.  Only someone very familiar with the original would have known and it flew very well.

 

Maybe a moment for a bit of artistic licence?

[Simon Chaddock]

I could easily increase the wing area but point of this build is to try and emulate what looked absolutely ridiculous 1952.

I suspect "fuselage lift" will become significant but I won't be in any way disappointed if it simply cannot be made to fly. However if that proves to be the case it might be interesting to see how much more area is required before it will.  

[J D 8]

  Simon you likely know the full size must have been one of the most marginal aircraft ever, however thought others may be interested in its history.

A take off speed of 260mph was needed to get off the ground.

Expected to be mach 2 capable it was only able to go supersonic in a steep dive, just not enough jet power from engines of the day. [ some thought was given to use rocket power but this did not happen]

  Test pilots Bridgeman, Yeagar and Everest found controls to be very sensitive at high speed and sluggish at low speed prompting Everest to say "one of the most difficult aircraft I have ever flown".

  Pilot used a lift to enter cockpit from below, sort of Thunderbirds style.

[Futura57]

They were brave men those test pilots. I'd take one look and do a John McEnroe.

[Simon Chaddock]

Nice picture! I did not know the cockpit entry was from the underside.

What intrigued me more was what were the two planes in the distance?

Looks like a F102. The white one the Bell X5?

 

[J D 8]

On the left I agree a F102, think the other is the swing wing X5

[J D 8]

A pic of the X3 with other Douglas built research aircraft at Edwards airbase.

[Simon Chaddock]

Just to be sure I load tested the wing using my crude support it by the wing tips to load the wing root the equivalent to pulling 4g.

With the fight battery resting on the fuselage and with the EDF and ESC already built in that accounts for all the heavy bits with no noticeable wing bend .

In total it weighed 356g The nose and tail section will all Depron structures and will add about 60g.

This test seems to show the wing will be adequately strong it proves to be able to fly!

The centre fuselage is now fully planked. I will leave the wing uninstalled and move onto the tail section.  This will of course include the elevator servo. Its wire will have to be run through the now fully planked centre section.😟

 

Just as well I built in a servo plug tube that runs through all the formers to the battery compartment.  

[Bill Southwell]

Hi Simon,

 

    I have always had a thing for X-3. I thought the X-3 looked like a huge leap in design for it's time and even for today. It was a constant sketch in my school note books along with the early jets and WWII aircraft. If engine development had been up to promises a lot of failed programs might not have.  I have always wanted to build an X-3 "one day" but the issue of the fuselage's apparent overabundance of side area forward of the CG  plus the tiny wing and even smaller tail volume has over the years made it seem not worth trying. But.........

 

    Your bravery and obvious skills have made me a lot more optimistic about taking a better look at the design to figure out how they got that thing to fly more than once.

 

     Looking at the issue of what amount of fuselage area ahead of the CG and the tiny tail surfaces. When looking at a 2D side view it looks like it would be a tail swapper for sure. But if you look at the X-3 in pictures and look not at the entire fuselage but break the fuse down to the shapes of the sections you can,

 

      see that the fuselage just forward of the cockpit wind screen transitions to a round cross section and tapers to a rather fine point. The circular shape can't be as strong of a destabilizing force as compared to a flatter vertical fuselage shape would be.  Think of what has more drag- a flat plate or a sphere?  The round nose section has to be less effective in its felt effect for  reaction to yaw loads for lack of better words on my part.

 

  If you look at the fuselage sides from just aft of the air intakes and all along the engines they are more of a flat vertical surface and radiused top edge. This forward area ahead of the CG will have much stronger felt effects or reaction to yaw loads leading to possible yaw instability more so than the nose cone.

 

       The area aft of the CG goes into the positive yaw stability column. The fuselage sides from the CG aft to the exhaust exits are now a positive yaw effect. 

 

     The tails vertical area starts on the top center of the fuselage at about the CG and extends back to the tail cone. The aft fuselage boom if kept as flat on it's sides as you can get away with and make the top curved shape more of a triangle with the top point slightly rounded will make it more like a long vertical strake like D model P-51's and P-47 D's received after cutting down the aft turtle decks for the bubble canopies mods.

 

My gut says it will fly straight. A flat fuselage test glider will probably not be stable in yaw. I think that if you were to add the cross sections of a 3D accurate scale shape and glue some light foam blocks in between the cross sections. Then sand it to shape. I believe the shaped glider would work. Another idea to try, a light filament 3D printed skins sort of like the old vac formed scale model kits. Use a balsa flat keel and apply the left and right halves. 

 

Sorry for being so long winded but your build thread has me amped up. All the best with your build and flight tests. I really appreciate your sharing this cool project with us.   I wonder how big the X-3 would have to be to fit the 2) 80mm fans I have in the shop. More like how long it would be. LOL!  With the super light built up foam models and the modern power systems, edfs and amazing batteries make almost anything possible.

 

Best Regards

Bill,

Iuka, MS

[Simon Chaddock]

Moving on I have started the fuselage tail section.

It has a rather rapidly changing shape so it has been built as a half shell over the plan.

Tapered, twisted and formed planks so rather slow going.

Once lifted the other side of the formers are added and the planking has to start all over again.

 

The one piece tail plane (less elevators) is slotted through the tail. The fin and fixed rudder is slotted in vertically to rest on the tail plane. It is then they are glued into position. The tail temporarily taped onto the centre fuselage to see what it looks like

Note the LW-PLA printed tail cone and the hard balsa reinforcing embedded into the tail plane.

The buried elevator servo and elevators next.

 

The ridiculously long (delicate?) nose and the side area it creates suggest that for the initial flights it would be sensible to use a "stub" nose that terminates just ahead of the cockpit.

In fact truncated like this the proportions of the X-3 start to look almost normal apart that tiny area wing!  

Edited February 22 by Simon Chaddock

[Futura57]

I think the snub nose is an excellent idea Simon, until you have her trimmed out and flying on rails 😉