Douglas F4D Skyray

[Simon Chaddock]

I build one of these as one of my first Depron 'jets'.

Built entirely in 3 mm Depron, skin over formers with a single balsa spar.  

Not strictly an EDF but with a 4x4.5 ducted prop squeezed in right at the tail.

It did mean a slight scale sacrifice with over size inlets and exhaust.

Light and simple, only two elevon servos it flew very well with just a 1500 mAh 3s in the cockpit.

 

Having developed a 3D printed casing to suite a 4 blade 3x3.5 prop I wondered if it would allow at least a scale exhaust on a similar overall size F4D.

5 mm XPS sheet is almost the same weight as 3 mm Depron so could I use that as I have some left over from the construction of my 'fleet' of XPS EDF Hawker Sea Hawk/Hunter series.

I do have a limited supply of true 2mm Depron so the intention is to use that to build the duct which would then act as a 'spine' to build the XPS fuselage around it.

The first task was to find a suitable 3 view with fuselage sections.

The was painstaking 'simplified' to remove all the unnecessary surface detail and redraw the lines so the original image 'fuzzy' lines could be blown up to the required size.

 

The printed duct prop casing with the first duct section planked in 2 mm Depron.

 

The next problem is the bifurcated inlet. With such modest power available, about 180W, and particularly low pressure differentials duct efficiency becomes a critical factor.

A printed inlet duct would give the smoothest possible transition from the twin inlets to the circular duct.

Impressive when printed out.

 

But unacceptably heavy at 51g 😟.

in fact virtually equal to the weight of the ducted prop casing including the motor!😲

I was not all sure I could make such a duct out of 2mm Depron sheet and still retain adequate airflow efficiency.

Just as well I hadn't actually started to build the airframe.   

Edited July 30, 2022 by Simon Chaddock

[Simon Chaddock]

To build a bifurcated duct in 2mm Depron planks the design would have to be somewhat simplified with straight arms but the Skyray fuselage is also a constant size back from the intakes so the arms could significantly longer than the printed one which was kept short to limit the weight.

To build each arm required a jig with a printed ring of the correct profile at each end. Each ring had a 2mm wide slot to hold each plank.

 

Once the arm was fully planked it could be cut free from the jig. The rings are for construction only so are remove and replaced by lighter single sided 'joining' rings.

The completed bifurcated section in 2mm Depron.

 

Even with the joining rings at 18g it is only 1/3 the weight of the fully printed equivalent.

Once the duct is fully assembled construction of the fuselage can start.

 

 

  

 

Edited July 31, 2022 by Simon Chaddock

[GrumpyGnome]

Another masterclass.......

[Simon Chaddock]

The final duct configuration and the fuselage formers around it.

Each former is in two halves with a root wing rib joining them together.

The fuselage is the "heart" of the Skyray as everything else is simply attached to it but first it has to be skinned. Planking even modest double curves is not so easy in thick 5 mm XPS.

It becomes a bit of a 'chicken and egg' situation as some skin areas have to be left off to run the elevon servos wires through to the cockpit but the wings cannot be attached until the skin is in place.   

[Simon Chaddock]

after 24 hours and there is a bit of progress.

These Skyray 'trade mark' fairings, although fixed in my case, were actually movable on the full size as huge elevator trim tabs.

By moving upwards they allowed the Skyray to hold a nose high attitude for a carrier landing without much actual stick movement.

Note also the leading edge slats deployed.

[Simon Chaddock]

Working from just a 3 view you not only have to work out what to do next but also the consequences if you do!

So the fuselage underside planked in 5 mm XPS, the fin and fuselage strake added. The small radii involved meant it had to be done in 3 mm Depron.

Note also a strip of the top skin at each wing root to give the full wing section area so the wings can be glued on at the appropriate point.

There will be just about enough of a gap between the duct and the top skin when in place to clear both the elevon and motor wires.

The compact nature of the Skyray fuselage makes it particularly rigid particularly with a whacking great duct running inside.

    

[Simon Chaddock]

Took two days but it now has a cockpit with a printed nose cone,

The nose was built as a 'half shell over the plan' and once lifted the other half of the formers were added and the skin planked, 

If you look closely you can see the three 0.7 mm 'magnet' wires for the motor. They run forward into the cockpit area.

[Simon Chaddock]

Just to prove it actually works in the duct.

https://www.youtube.com/watch?v=QdScpTC6hyg

It is running on a 1000mAh 4s (its intended battery) and is drawing just 6A giving 94W.

It at least blows the door closed.

[Simon Chaddock]

Still rather slow progress.

A bit unusual as the complete canopy is a one piece hatch.

 

The first wing. The Skray will have quite a modest span of 660 mm but there is plenty of area.

The 3.7g servo buried flush with the underside.

When filled and painted the only bit sticking out will be the tip of the servo arm.

The alarming bit is the wing will be simply glued on to the fuselage. No reinforcing or bandages, just glue. 

Edited August 10, 2022 by Simon Chaddock

[Simon Chaddock]

At last the wings glued on.

It has the motor, fan, ESC and servos installed. In this condition it weighs 220g.

The 1000mAh 4s weigh 107g. Add the final bits of skin, the rx and paint say 350g ready to go.

Although the overall dimension are true scale some of the detail profiles are 'stand off' mainly due to the needs of the duct  and the size of the inlets.

In 5mm XPS foam it is extremely rigid so in hindsight I could have made it quite a bit bigger and by retaining the same power plant and battery it would allow true scale fuselage profiles and inlets. Further by keeping exactly the same construction even expanding its dimensions by 1/3 to 880 mm span (67% more area) it would increase the overall weight by no more than 35%.

However first is to complete it "as is" to see how it flies.   

[Colin Leighfield]

I bet it will fly very well. I’ve always liked the Skyray, another great design from Ed Heinemann (I think). 

[Simon Chaddock]

Ed Heinemann indeed.

 A self taught engineer joined Douglas as a draughtsman in 1920. As chief engineer in 1936 he lead the team for the Dauntless Dive bomber, A20 and A26, A-1 Skyraider, A-3 Skywarrior, A-4 SkyHawk. F3D Skynight, F4D Skyray, Skystreak & Skyrocket research aircraft. He later joined General Dynamics as the design vice president and oversaw the development of the F16. Absolutely fantastic!  

[Simon Chaddock]

Made a start on the paint.

It will be a 'stand off' paint scheme.

White under pale grey top side, typical US Navy 'Pacific' colours. The fin, fuselage strake and elevons will be dark blue, more for visibility than scale.

The insignia are pretty big too which will help.  

[Simon Chaddock]

Now complete and decorated.

The paint and battery box has added a bit more weight than I anticipated. It now weighs just over 400g with a 1000mAh 4s on board.

Just need some calm weather to try and see. 😉

[Simon Chaddock]

Well it does fly but is rather underpowered (At 5A full power it is only drawing 5c from the 100mAh  4s!) It also needed some elevon reflex.

After 3 flights I seriously mis judged the final approach and got into a typical delta high alpha attitude which of course the limited thrust could not handle. It resulted in a severe nose drop from 8' into the ground.

Not so much a 'lawn dart' as a 'nose crumple' which the 5mm XPS does quite well!

It can be fixed but it needs a different fan/prop. From previous experience a coarser pitch 3 blade will handle the long inlet duct much better and absorb considerably more power.

The only problem is at the moment I can see no practical way of physically changing the prop. The entire foam structure was built around the motor with the prop already attached. 

[GrumpyGnome]

Ouch! I'm sure you'll solve the conundrum.....

[Simon Chaddock]

Now rebuilt.

Even the prop has been changed. Note the 8mm socket extension, a carbon tube long enough to undo the prop nut via the inlet!

The 4 blade prop itself just came out through the inlet.

The next problem was to mount the 3 blade onto the forward facing motor shaft. Eventually solved by unbolting the motor, manipulating it through 90 degrees and then coaxing the 3 blade onto the shaft. Not easy all 3" up the duct with the motor mount in the way. With the prop nut 'glued' into the .socket it was tightened up via the extension.

This proved to be not quite the end as the three blade proved to be 1 mm bigger diameter. it contacted the duct walls! This meant the whole process had to be repeated to remove. shave 0.5 mm off the prop tips and replace the prop. Just as well I had nothing else to do!

At least the coarser 3 blade does what I hoped. It absorbs 10A instead of 5A and generates considerably more thrust. 🙂

We shall see.    

Edited August 25, 2022 by Simon Chaddock

[Simon Chaddock]

Just to complete the story, well for now at least, this how it looked after its successful flight but uncontrolled landing in the dreaded willow plantation at the DVMFC.

Although only about 20' high but young densely planted willow will always stop a model pretty abruptly. Fortunately being an EDF it was relatively easy to locate with a blip of throttle. Actually getting to it was another matter.

Today, 30th, it is structurally repaired. Just needs a bit of filling, sanding & paint.  

The more serious issue is the impact on the left hand wing tore off the elevon horn and in so doing stripped a tooth in the servo.

No great problem cutting the servo out but the issue will be can enough servo wire be 'extracted' to permit 3 soldered joints and to safely 'shrink' the wraps?

[GrumpyGnome]

I admire your perseverance Simon......... as well as the handiwork leading to this point! 

[Simon Chaddock]

Just to complete the story it is all back together - again.

As it is not the nicest plane to fly I doubt I will repair/rebuild it any more times.😟

[Simon Chaddock]

Just a short up date.

The Skyray does fly reasonably well given that its roll ability is many times that of pitch!. However its overall size, shape and bland colours do make distance judgement a bit harder as I discovered on the approach during its last flight. Flat calm and it glides well so I had to go quite close to the top of that big tree to keep within the field. 😟

Needless to say I well too close and with so little inertia it stops dead so it made its own way down from 40'. Many branches broke the descent so no serious damage but it did leave a few more 'notches' out of the leading edge.

Easy to repair with simple XPS inserts.

There are a couple more on the other side too. Just a bit of paint to finish it off.

I will video its next flight, before it gets any more tatty.😉

 

[Simon Chaddock]

The saga continues.

As promised here is a not very useful video of it flying but it needs a bit of back story. At the field I discovered that at full power it would run for a few second and then cut out however reducing the throttle just a touch it did not. Having got the video camera on I decided to go anyway, particularly as there was no one else there, and kept high so I would have a few seconds for it to reset.

https://www.youtube.com/watch?v=60G4DCldVtU

It flew well enough and even did some basic aeros. It did cut out twice but power returned very quickly. After 3 minutes I considered I had pushed my luck enough so cut the power for my usual a glide landing. Sod's law being what it is it decided to cut out again just as I was turning onto final. As you can see from the video it did not complete the turn and flew into a big tree about 40' up.

I immediately tested everything  and everything worked, the EDF even fluttered the leaves, but it did not budge. I went back for my 9 meter roach pole and with a bit of effort did extract it.

With a stout branch embedded in the wing & in exactly the same spot as a previous repair it rather explains why any amount of EDF thrust made absolutely no difference.

The split damage to the nose is the result of the 40' near vertical drop to the ground!

So apart from a note to self "Don't fly if there is an obvious problem" I now have to try to find out what causes the intermittent cut outs. The ESC, its separate SBEC, a faulty connection, the radio or the battery, not that there is much else in such a 'simple' plane!     

 

  

[Simon Chaddock]

Just a quick up date.

It appears that the problem was simply the ESC getting too warm and triggering the thermal cut out. I had assumed that it uses a small drone motor (just 12A at full power) and a 3 blade 3x3 prop the ESC would not over heat sitting in the big volume cockpit.

After anther crash that virtually destroyed the nose.

I decided the solution was to add a fingered heat sink to the ESC with the fingers protruding though the skin just ahead of the left air intake.

To a degree hidden by a repositioned decal.

Still not exactly a pleasant plane to fly but at least the ESC does not over heat.

Almost shamefully crude airframe construction and really only stand off scale but it is light and was very cheap to make.