|Oct 12, 2012, 04:12 PM|
Joined Feb 2007
I was looking for a plane to follow on from my Depron ducted prop Douglas Skyray.
Many have never have heard of the P1121 which is not that surprising as although the prototype was actually started it was cancelled by the infamous 1957 white paper.
There was a full scale mock up.
A big and impressive machine. The attraction from my point of view was that it was intended to use the massive DH Gyron turbo jet which meant it had a substantial fuselage diameter giving sufficient room for a ducted prop.
To give a direct comparison to the Skyray it was going to use the same motor and prop. Although the duct would be nearly twice as long I hoped the extra duct losses would be compensated by the more efficient conventional wing and tailplane layout.
The intent is to build it like the Skyray almost entirely from 3mm Depron with just minor reinforcing in the critical areas.
First the wing. No ribs, just the top and bottom Depron skins drawn over a single spar and 4 additional Depron spacers.
The main spar is built up with 0.8mm hardwood flanges over a Depron web.
It has a scale symmetrical sharp edged supersonic wing section just 6% thick
The P1121 was a mid wing design so the real challenge is how to transfer the wing loads to the fuselage around a huge duct occupying nearly 90% of the fuselage diameter.
|Oct 13, 2012, 06:04 PM|
Joined Feb 2007
The fuselage was going to be complex.
It was 'area ruled' and the 'bulge' just after the wing meant it was possible to mount a 4.5" prop but the duct forward has a varying profile.
As is common with EDFs a scale inlet is too small but with a 'chin' type increasing its size would really spoil the line of the fuselage. The answer was to use the large air to air rocket bays either side of the fuselage as 'cheat' holes. They would add about 40% to the inlet area without spoiling the fuselage line.
The fuselage in the 3 sections. The centre and tail sections had to be built 'inside out' that is duct first and skinned later.
The LH centre half.
Half duct complete.
The RH duct half under construction with the LH wing attached.
The tail section was built in the same way and carried the motor at the oint of maximum diameter.
The nose section was built vertically
Skinning complete with the chin and cheat ducts.
Finally the 3 sections could be joined together
Although only 36" span the fuselage was over 60" long.
This was becoming a very complex build.
|Oct 15, 2012, 06:51 AM|
Joined Feb 2007
To keep things as light as possible I chose to use the all moving tailplane as elevons as well.
The large tailplane dihedral and lack of space made a very cramped installation with inly just enough room for the 3.7g micro servos.
The tailplane rotates about a glass fibre tube spar.
The RH servo ad linkage.
With the motor well aft long wires to the ESC would be required. The weight of appropriate silicon insulated wire became significant so a single strand enamelled wire was used instead.
This type of construction requires virtually all the components have to be fitted before the outer skin can be added.
Of course when it is fully skinned access is virtually non existent!
|Oct 16, 2012, 10:35 AM|
Joined Feb 2007
As Depron is an excellent heat insulator and the ESC is below the surface it has a finned heat sink added which will be in the slip stream.
The motor is in high speed airflow so is carefully faired although the bell surface is left exposed for cooling
With all the electrics installed the battery position is determined to achieve a suitable CofG.
This was 'guessed' from the main leg U/C position.
Completing the outer skin, plank by plank, is a slow process.
On the plus side the rigidity of the whole airframe increases considerably as it progresses.
The completed cockpit.
The cockpit canopy and part of the spine are removable to allow final positioning of the battery after flight tests.
To keep the weight down I decided to paint it silver overall with just a red spine and fin.
As the prototype was never completed who is to say what colour it would have been!
The mock up was apparently a pale green.
|Oct 17, 2012, 08:21 AM|
Joined Feb 2007
Completed witha 1500mAh 3s it weighs 16oz exactly the same as the Skyray but as predicted the longer duct reduces the static thrust by 1 oz to 10oz.
The maiden flight was a success in some ways - it flew quite nicely - but a disaster in others - the elevons proved completely inadequate with virtually no roll power at all to the extend I was only just about able to keep it straight.
The maiden flight finished in the next field.
Clearly ailerons were required. Not easy with such a thin wing and with everything built permanently in!
The scale ailerons cut out.
The wing is so thin that even a 3.7g micro servo has to lie flush with the surface.
Running the servo wires was a bit of a nightmare requiring many holes to be made in the skin.
I was not happy with the tailplane spar so I took the opportunity to install a short but bigger diameter one with a spring loaded 'pip' to retain the tailplane.
With the new servo wires installed the holes were simply filled with inserted patches.
A bit of lightweight filler and paint and you can't even see the joins!
The aileron servos have added 1/2oz to the total weight.
For a ducted 'fan' it is still very light for its size so it flies slowly however unlike fans a ducted prop continues to develop significant thrust at reduced power so even with the small battery it has a considerable 10 minute+ endurance. It even glides quite well!
An edited video of its first two flights.
The only real problems with it are being so light it needs really calm weather and the very low set tailplane is easily damaged in the belly landing.
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