Convair YF2Y-1 Sea Dart

[Alex Jones]

Over the last year we have put together a new website started some large product developments and had those personal issues that we all go through at some point in our lives. In amongst all of that we have been working very hard on the SeaDart and can now see the light. The last push has begun so we can finally get it into the water for the first time, hopefully in the next 3/4 months. The problems have been many and varied but we now have a prototype that I'm confident in moving forward with. I'm looking forward to updating this thread over the next few weeks.

There is some information on our website which can be found here:

www.littlejetco.com

Just a few pictures to start off then I'll go through the build to date.

_DSC4619 by Alex Jones, on Flickr

_DSC4620-2 by Alex Jones, on Flickr

Most of my following build pictures are just done on my phone but should provide plenty of information even if the quality isn't there.

I hope you are all having a good weekend.

Cheers, Alex

[Alex Jones]

I think its worth talking about the engine setup and how we have designed the internal structure around the two P300RXG’s

I've built plenty of jets with twin setups but all with turbines in the 140 - 160N range. This will be the first with 300N turbines which shouldn't be that much different, however, the fuel draw on the larger engines is significant at full thrust. We don't want to find out that we have any positioning problems with the tanks, air-traps and thrust tubes once the model is built as this would be a huge problem to resolve on the completed model. With this in mind we decided to build a test stand using the aircraft internal structure so everything is in exactly the same place. This effectively tests all the components in situ but with full access. The other benefit is with a short documentary being filmed on the development of this model the test stand allows an easy way to explain things to those who don’t have any background in what we do.

The general arrangement per side is an 8 litre kevlar tank running to an FCT air-trap then onto the pump, filter, shutoff valve and finally the turbine. All the tubing is 6mm OD Festo which steps down to 4mm OD on the last run to the turbine. The turbines are housed in a water tight bypass ducting. This will allow any water coming into the intake or up the exhaust to flow freely through and not enter the fuselage. The ducting has recesses for O-ring seals designed into the tooling. The total weight of the dry tanks, tubing, turbines and ancillaries is 8.4kgs (18.5lbs), when we add 16 litres of fuel the full system weight is just over 21kgs (46.3lbs). Obviously this is significant but not unexpected.

We started the test stand by beefing up the internal structure of the model and adding two end plates which allowed it to stand. With a combined maximum thrust of 60kgs (132lbs) I wanted to keep the stand low to the ground and have attachment points for anchoring and tethering the stand. The stand is aluminium which is bonded together with Hysol.

The turbines need to be in place while we bonded the structure to ensure a perfect fit. The positioning has been designed into the structure so its just a matter of bolting them in place.

IMG_2217 2 by Alex Jones, on Flickr

IMG_2211 by Alex Jones, on Flickr

IMG_2184 2 by Alex Jones, on Flickr

IMG_2208 2 by Alex Jones, on Flickr

IMG_2221 2 by Alex Jones, on Flickr

[Alex Jones]

The finished test stand with all the fuel and turbine ancillaries fitted.

_DSC3924 by Alex Jones, on Flickr

_DSC3912 by Alex Jones, on Flickr

[Alex Jones]

The test stand uses the same electronics as the model. Wanting multiple redundancy we have decided to use a Jeti DC24. This has two 2.4 GHz receivers totalling four antennas and a backup 900MHz receiver with two antennas. All of this runs through a CB400, we have full turbine telemetry using the new digitech CTU’s along with, IAS, altitude, heading, accelerometer and a multitude of other flight parameters that will be recoded and analysed after flight. The model won’t be taken out of the water as a matter of routine on a normal flying day (after the testing) so an RC switch is used to turn the model on and off. Refuelling will be accessed through a panel where water ingress isn’t a concern.

41900304_1091585980991542_8912605116897427456_n by Alex Jones, on Flickr

IMG_2246 2 by Alex Jones, on Flickr

I’ll do short video to explain the setup to those interested, we have used 4 over-center switches on the transmitter to allow the turbines to have an ‘idle detent’ disabling pilot control. This allows us to start each turbine independently of the other and also help with asymmetric thrust in the water if required.

[Alex Jones]

Onto the actual build...

The first task was to cut the apertures for the cockpit and ski wells and install the two main formers that join the rear half of the fuselage to the front section. We have a glass pre-preg fuselage structure with Nomex core and monolithic carbon pre-preg ski wells. The fuselage structure is incredibly strong... I'll demonstrate this by hitting some of the off-cuts with a hammer, it barely makes a scratch! I'll upload the video in the next few days.

To mark out the apertures I dry fitted all the internal formers and marked the position of the ski well on the inside. I then marked it out again moving inside by 20mm and at each angle change I drilled a hole. Finally joining the dots on the outside and making the cut. Once the ski wells are bonded in I'll cut back the overhang and seal the cut.

Photos are just from my a phone so apologies for the quality in places.

IMG_2086 by Alex Jones, on Flickr

The school holidays is always a challenge... what could possibly go wrong?

Screenshot 2018-12-12 at 21.10.27 by Alex Jones, on Flickr

The final aperture cut to size.

IMG_0136 by Alex Jones, on Flickr

[Alex Jones]

The job of installing the rear main former was a carful affair as it's essential this is positioned correctly. The forward fuselage former has a recess for a rubber seal. The two halves are fixed in position with large tridair anti vibration fittings allowing easy disassembly of the model for storage or transport.

IMG_2088 by Alex Jones, on Flickr

The forward ski retraction unit is mounted on a CNC aluminium former. This needs to be aligned exactly or the ski retraction system when tied to the aft mechanism will not work. Lots of dry fitting and jig building before any bonding takes place! The below photo shows the forward fuselage formers dry fitted with the forward ski wells in place.

IMG_0133 by Alex Jones, on Flickr

[Alex Jones]

The formers in the cockpit area are dry fitted at the same time. All the components are tied together through either the Keel beam or aligned with carbon tubes so they need to all be fitted at once. Some trimming was required mainly at the base due to the composite being slightly thicker than specified in this area. I also had a new keel beam cut due to accommodate the increase in thickness. Once this was done the parts slotted in well.

The large painted former you see is a 4mm thick carbon pre-preg component that creates the wet area for the forward ski retraction mechanism. I had made the wet side of this the 'A' side meaning the the molded edge which is smooth is visible when looking into the ski well. This leaves the 'B' side which is rough and can be seen in the below picture. This needs to mimic a Stainless Steel former so I filled then cut back the B face until smooth.

IMG_0134 by Alex Jones, on Flickr

IMG_0135 by Alex Jones, on Flickr

[Alex Jones]

With all the parts dry fitted and fettled where required it was time to bond everything in place. As everything is interconnected it wasn't really possible to do this in stages, a few of the minor formers were left out as these could be installed afterwards but everything else had to be done in one go, so full commitment with the Hysol 9462!

A nervous 24 hours passed but thankfully all went to plan, although, I won't know if the forward ski mechansim is exactly aligned until we get the skis going up and down. The ski mechanisms have caused plenty of sleepless nights and is by far the biggest challenge as we are operating them exactly like the original aircraft....anyway I digress, back to the forward fuselage. The formers bonded in place.

IMG_0138 by Alex Jones, on Flickr

The cockpit area will have three cameras installed for in flight video so there is nowhere to hide with regard to detail so I have bonded on 0.2mm plasticard over areas which will be visible to the cameras as a base for further detail.

IMG_0147 by Alex Jones, on Flickr

[Alex Jones]

The forward ski mechanism had to be in place during the bonding process to ensure alignment. This mechanism was put in the deployed position on a sheet of toughened glass and the fuselage was jigged level while resting on the forward mechanism. The mechanism itself is made up of Titanium, Stainless Steel and Aluminium. A picture of the finished parts assembled for the first time.

TLJC FWD SKI MECH View 2 by Alex Jones, on Flickr

It seemed a shame to paint parts but they were sent up to FighterAces for painting regardless. Once back I removed any paint that had found its way into the pivots and re-assembled ready to fit on the former. The actuator is a custom length Bimba product (from the United States) which puts out around 250lbs of force on the upstroke. You wouldn't want to get your fingers trapped during a cycle!

The mechanism is mounted on a removable circular plate which bolts onto the main former and seals with a large O-Ring.

IMG_2196 by Alex Jones, on Flickr

IMG_2197 by Alex Jones, on Flickr

IMG_2198 by Alex Jones, on Flickr

This shows the position required before bonding, the right hand carbon ski well hasn't been installed at this stage.

IMG_2202 by Alex Jones, on Flickr

[Alex Jones]

With everything bonded in I can spray all the internals zinc green which we believe would have been on the original.

IMG_0418 by Alex Jones, on Flickr

[Alex Jones]

I forget to mention that the colours in the photos aren't accurate but I'm sure you all know the colour we've gone for.

Onto the canopy ... This has been a major pain! Mainly because of the a design oversight in that the canopy is the exactly the same size at the fuselage, maybe even slightly larger due to making a carbon pre-preg part in a glass tool. We couldn't make pre-preg tooling as the pressure in the clave would have destroyed our wooden pattern so it's a limitation of our process more than anything. Ideally you would always want the tooling to made from the same material as the component. In our future projects the tooling will be done a little differently so we can get better quality tooling although for a one off project what we have is adequate.

The canopy needs to rotate down into the fuselage so lots of carful trimming was required! I got it to stage that I was happy to bond the hinge formers in with canopy which is what you see here.

IMG_0051 by Alex Jones, on Flickr

[Alex Jones]

The canopy was originally moulded with a return but as the canopy was slightly oversized I decided to completely remove this. I could then trim the canopy with ease. I trimmed it to sit slightly lower than the fuselage at the rear to allow the backward rotation. With the fuselage being constructed from Pre-Preg this allowed me to trim the rear of the aperture to a fine edge allowing the rearward rotation of the canopy without compromising the strength of the trimmed edge. I replaced the canopy return with glass sheet bonded in place with Hysol.

IMG_0144 by Alex Jones, on Flickr

This shows the rearward rotation of the canopy and the edge of the aperture which is around 0.3mm in the area where the canopy needs to miss. It's only a small segment because the rotation immediately presents a smaller part of the canopy to the aperture.

IMG_0139 by Alex Jones, on Flickr

The hinge structure in the rear of the canopy.

IMG_0143 by Alex Jones, on Flickr

This is a large canopy and to show some mechanical sympathy to the servo that will operate it we have to counter balance. To do this was pure trial and error. I ordered lots of springs and fitted each until I found the perfect balance. It now takes very little effort to move the canopy in any position.

The springs temporally fixed in place.

IMG_0146 by Alex Jones, on Flickr

After a lot of work we finally have a working canopy that doesn't bind and is counterbalanced nicely. Now I have to work out how to seal it! We're thinking along the lines of an inflatable seal? More thought required...

IMG_0148 by Alex Jones, on Flickr

IMG_0150 by Alex Jones, on Flickr

[Alex Jones]

While stuck back at the computer doing some design work I thought I'd make use of a nice winters Monday to test the engines. After the problems with one of the engines (more accurately the fuel pump) in the last test I swapped the engine, pump and ECU for one of our spares. I found as the pumps have been sitting around for a long time they needed around 20 liters of fuel put through them at 6v to loosen them. With this done the engines started nicely... I could have upped the initial voltage during start to the pump but if I had done that eventually I would have had a wet start so better to start with not enough fuel rather than too much during the initial ramp up. I also discovered a fuel leak around the output nipple of the pump on engine No2. I'll replace the tubing and wire lock this in place and hopefully that will solve the problem. It has certainly been beneficial using a test stand as if all this was installed into the aircraft it would have been a massive PITA to sort out.

With both turbines running I could only get to 75% thrust before the rear of the stand started to lift. The stand was pegged and tethered but I still didn't want to risk the full thrust of both. One turbine at a time for full thrust was fine. The model will certainly have a tendency to pitch down with an increase in thrust. The initial water based testing will be interesting!

A link to a very short video on Flickr or the following YouTube link.

YouTube Twin P300RXG Test

[Alex Jones]

With the forward fuselage structurally complete I can move onto the rear. The purpose is to install only what is required initially to allow the ski retraction to actuate. We still don’t know how successful the ski retraction will be and with this part of the project taking a high proportion of the entire design and manufacture time I’m nervous that it won’t work…. Very nervous…

The first task was to dry fit all the formers and do the inevitable fettling to get them all to fit snugly in place. I’m primarily concerned with the first two formers in the aft section. These were fitted with the carbon ski wells in place. In conjunction with the keel beam slots, moulded return of the ski wells and formers including the main pivot block it all essentially self jigs into place. I bonded the first former and ski wells into place but left the main pivot block and second former which takes the load from the skis as I’m unsure if this part will require a re-design if the skis don’t retract.

IMG_0141 by Alex Jones, on Flickr

I’m now at a stage where I can fit the prototype parts of the ski mechanisms and see if the skis will push up into place by hand. It’s a different matter to actuate it prototypically but at least I can see if the mechanism folds away as designed and the ski seats correctly on the side of the fuselage.

IMG_0213 by Alex Jones, on Flickr

Did I mention earlyier I was nervous? This is where all my years of being around aircraft, models, engineering, all my academic and practical experience in aerospace for the last 30 years was bundled together and drop kicked out of my brain…

In my excitement on receiving the machined aluminium pivot block and stainless steel pivot pins I immediately installed them without any lubrication, without even checking the machined parts or removing any burrs and not installing the PTFE bushings (these were the raw machined parts with no finish post applied) After about three turns the whole thing seized inside the pivot block. The next three days were spent trying to remove the pivot block without damaging the composite fuselage or stainless steel pivot pin. After much head banging and profanity the only option left to me was to cut the block out with an angle grinder sacrificing the machined parts in order to save the fuselage. I had to remove a 2 inch square section from the carbon ski well to allow the external part of the pivot arm to fit through the ski well. I then set about destroying the pivot block taking care not to damage any of the hull composite.

It would have been unfair and just not right on my client to have charged for any of this time or replacement parts so I was rather grumpy with myself to say the least at having lost an entire week due to not engaging my brain! I did, however, manage to remove the block without compromising the composite hull.

IMG_0216 by Alex Jones, on Flickr

Another week past and the new replacement components arrived. These were then inspected, polished to the specified RA, bushings and O-rings installed, greased and finally installed into the pivot block. This of course is what I should have done the first time...Let us never mention this again!

I can now see if the ski would at least push into place. The below video shows my initial thoughts.

https://www.youtube.com/watch?v=Uyhr-2MBVqI

Yay… although actuating it on what is a very short moment is still making me nervous, less so than before though…

[Alex Jones]

Onto assembling the oleos... This was a relatively simple affair. I cleaned up the machined components and polished all the surfaces to the specified finish which should help the stanchion move freely in the oleo. The seals were installed and the entire assembly greased. They were tested overnight with no pressure loss seen.

IMG_0266 by Alex Jones, on Flickr

IMG_0267 by Alex Jones, on Flickr

IMG_0342 by Alex Jones, on Flickr

IMG_1112 by Alex Jones, on Flickr

IMG_0263 by Alex Jones, on Flickr

[Alex Jones]

With the oleos assembled and installed we could move on with getting a better understanding of how our ski retraction system works. The first task was to add pressure to the rear actuation while moving the front of the ski by hand. We’re using relatively high pressures (for a model) with large pneumatic rams some capable of large linear forces that would happily crush your hand. With this in mind we started off just adding a little pressure with a hand pump to the extension stroke of the ram.

We designed the gear to retract on ram extension as you get the most force from the ram on this stroke and we knew that we needed all we could get due the short moment involved. Initially I had chosen a set of large Bimba actuators for the rear retraction but on reflection this was the wrong choice as they were very industrial and on the heavy side. I had squeezed the largest actuators possible into the space available and really it was too tight for these. I changed the design to accommodate two Festo DNSU-25-60-PPV-A actuators. These have a bore of 25mm and a force of 29kg on the extension stroke at 6 bar (87psi). These actuators are light, well made and being off the shelf readily avialable.

Our first attempt captured on my phone.

https://www.youtube.com/watch?v=aK0xz7Uha8g

The main pivot block is only dry fitted but as you can see in the above video the ski retracts but not in the correct position. It seats too high in the ski well hitting the top edge of the well aperture. This isn’t entirely unexpected… Ourselves and the engineers we worked with on the ski mechanisms could never get it to function in CAD as we would have liked. We built in ways of moving the geometry to try and achieve a scale retraction. The twist in the ski you saw was one of these variables moving to help the retraction. This is not scale but does help us get closer to the desired actuation.

We are able to move the geometry of the two most crucial elements in the system. The first is the position of the main slider bar as seen in the below picture. Once the ideal position is found I’ll have a carbon part made which will plug all the gaps and make this section watertight.

IMG_0305 by Alex Jones, on Flickr

The other variable geometry change we can make is the rear ski attachment point. This is a crucial angle which effects how the ski retracts and how it sits in the ski well.

IMG_0303 by Alex Jones, on Flickr

[Alex Jones]

With the ski in the correct position when retracted it binds on extension and just won't move if we have it extended and try to retract it. There is a balance somewhere in the geometry that will make it work but we now have to make small steps from the extended to retracted position to try and hone in on the correct balance of geometry's.

Incidentally through our research we don't believe the original worked that well to begin with. The two video clips we have show a rather staggered affair. Also the two clips show different sequence of retraction. Reading between the lines in some of the test pilot notes indicates that perhaps this was the case.

You can see the skis retract on this footage from critical past.

https://www.youtube.com/watch?v=GG0B7vAdou0

[Alex Jones]

After much fettling with angles and changing the offset claw on the main oleo to be central we got a ski that retracts into the ski well. I’d say we were about halfway there as it is still a very modular retraction. The sequence works slightly better with a different actuation order of the front and rear rams but I didn’t film this.

I’m happy so far in that it all seems to be going in the right direction but I do have some redesign work as there are clearly a few issues which I’ll detail in the next posts. Nothing insurmountable but I’m confident I can get a really smooth scale retraction with a little more work.

https://www.youtube.com/watch?v=O7JzLUar1K8

[David Berry]

Amazing stuff!
This is a project and a half
I suspect a lot of people are watching your progress but, like me are lost for words.

[John Greenfield]

Hi Alex

A really interesting project, thanks for sharing it with us.
One thought though, with so much carbon and metal in the structure have you thought about where to put the radio aerials. Given the shape and size of the airframe, there is serious potential for blanking of the radio signals by the carbon airframe and internal structure.

John