Mold Redux

I’ve actually had this done for a week or so, but forgot to post it. I made a mold for the carbon fiber glove box top that I want to make. I’ll take this down to the guy that’s going to make the part to see if this will work. If it looks good then I’ll make one for the throttle console as well. The mold came out OK, but its not a glossy as I thought it would be and I had some issues with getting the gel coat to the correct thickness. This is supposed to be really good gel coat, but for some reason I’m having a lot of problems with it. The manufacture says it needs to be sprayed, but I see a lot of other people brushing on the gel coat and it seems to come out really nice. I’ll have to look around to see if I can find another manufacture for the gel coat.

Part is ready to go. I filled all the holes with expose resin and sanded them flat.I greatly simplified the mold prep from the last time and also used a lot less wax on the seams.

Gel coat not going on so well

I was able to get a second coat on without it bubbling, but still not great.

Used some epoxy resin and talc to reinforce the corners

Two coat of resin and fiber mat. Now I need to add the wood bracing.

Wood bracing is in and now just need to add another 3-4 layers of mat. This makes a sizable mess.

Finished mold. It came out OK. I was hoping it would be a bit more glossy for a better finish on the final part. I think I can polish it so I need to look into that.

 

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More Elevator Trim

Time: 1.0 hr

Today I mounted the elevator trim servo in the elevator. I was going to use some washers and locking nuts on the inside to secure the servo to the elevator, but it proved to be way too difficult to get them onto the screws. So instead I just followed what the manual says to do and so I installed M3 rivets on the trim servo case. There was no issue with the rivnuts cracking the trim servo case like I had worried about. Once that was done mounting the servo in the elevator was was pretty trivial.

rivnuts installed (with high strength thread lock), connector on and cotter pin installed on the clevis. Also put medium thread lock on the clevis bolt.

Trim servo installed with 4 M3 screws and medium thread lock. I’m wondering if I need to secure the connector some how. I’m thinking it may bump around and dent the skin. I wish I could have use the smaller connectors, but the smaller ones only go up to 4 pins and this needs 5 pins.

I also tried to install the new hinge pin. I was hoping I could put in a little longer pin and just put 90 degree bends on the ends. Well the hinge pin is pretty strong and there’s not much room on the left side of the trim tab to get large plyers in there so it’s pretty much impossible to make the bend. I also tried to flatten out the end using a hammer and some thin blocks of metal, but once again the hinge pin is just too strong to be able to do that. So it’s looking like I’ll either have to come up with some other creative way to secure the hinge pin or I’ll have to remove the hinge and put in a new one. To do this is a lot of work. It means removing about 50 rivets, cutting the new hinge, drill new holes in the new hinge, priming the hinge and then installing it back which needs about 30 shortened rivets to be made. It’ll probably be 4-6 hours of work, but worth it to have the piece of mind that the pin isn’t going to fall out in flight. Though I guess it hasn’t happened yet to the other Slings that were built per the manual (with no means of securing the hinge pin).

Elevator Trim Servo

Time: 1.5 hrs

Well it’s been over a week since I worked on the Sling. It’s been a combination of being busy with other things and the lack to cash to put towards the plane. Too many things going on financially… wife needed a new car, paid off the Grumman Tiger, estimated taxes :-(, and working being a bit slow. In any case I’ve been trying to get to the low hanging fruit as far as cost is concerned to at least keep working on the build.

The trim servo seemed like a good option. The Sling uses the Ray Allen T2-7A servo and the RC8-7 clevis so I ordered those from Aircraft Spruce. I also purchased a new trim tab hinge since I want to replace the hinge pin with a longer one that I can bend the ends on. I’m not able to drill a hole for a security wire like the other builders have done unless I want to unrivet and install a whole new hinge. As a side note I know that you can just purchase the hinge pin for Aircraft Spruce, but it’s a 6ft piece they’re charging over $25 to ship it (for a $4 part). I found if I buy a 3ft hinge the shipping cost is minimal and the price difference in the part is only $1. So I’ll just remove the pin from the hinge and use that. Thanks to Pascal for the tip on the alternate approach to securing the hinge pin.

Wiring the trim servo was a bit more time consuming then expected. The instructions that shipped with the servo only showed it being installed with the Ray Allen switch and LED position indicator (no of which I’m using) so it didn’t call out what each wire actually did. I referred to the VPX manual which did list out the what the wires were for, but just to double check I referred to the installation of MGL avionics and that didn’t seem to match up colorwise to what the VPX manual showed. The only way to sort it out was to do a test (I posted a short video of that). What it comes down to is that the White/Green is the position and then the White/Orange and White/Blue can technically be swapped around since all that it’s doing is putting power through a potentiometer so ground and supply can be swapped around and it doesn’t matter (unless you use the Ray Allen LED indicator).

Here’s a few photos of the test

At full extension the position resistance is at around 20 ohms.

Fully retracted is around 5k ohms

So centered should be around 2.5k ohms

And the mess of clip leads connecting to the switches. I didn’t happen to have a momentary DPDT switch so I used 2 DPDT latching switches, hence the larger mess of wires. Basically you need to connect one motor wire to 12V power and the other to Ground. You do this with a double pole switch (one side ground, one side power). This gets you to move the servo one way. If you want to go the other way you need another switch that flips the ground and power so the motor runs backwards. If you have a monetary DP switch the it will sit in a no connect state, when you press on the top of the switch then it comments the common (shared) terminals to the position 1 terminals and when you press on the bottom then it connects the common terminals to the position 2 terminus on the switch. I just had to use 2 separate switch to accomplish that.

With the testing and verification of the wires down I wanted to pin it up and install it. The manual says to drill out the mounting hole sin the servo for M3 rivnuts. I’m thinking though the the M3 screws fit nice trough the skin and the mounting holes and then I can just use locking nuts rather then the rivnuts and thread lock. I’m worried that the rivnut is going to crack the plastic mount when I install it and I like using the locking nuts rather then rivnuts anyways. The problem is I only have 2 locking nuts and now M3 washers so I can’t install it today. But with the wiring done and the push rod installed on the servo it will go quick once I get them.

You need to install the pushrod and clevis pin on the servo side before mounting the servo. With the servo set to approximately center I checked that the pushrod is screwed in enough to allow enough adjusting on the trim tab side. With that checked I think it’s safe to thread lock the retaining nut on the clevis (on the servo side).

I put a short piece of wire sleeve and heat shrink on the wires. Also writers are labeled with heat shrink labels and the connector is on on. The last bit is just to wrap the area where the back shell strain relief with some silicon tape and screw on the back shell.

 

 

Filling and Sanding Rivets (Elevator)

Time: 3 hrs

Filled the rest of the Elevator rivets and sanding all of them. I need to order the trim servo so I can put that in and the connector ends should be here in a day or two so I can finish up the wiring. I wanted to drill the hole in the trim tab piano hinge for the safety wire, but it seems like my hinge is a bit different the other builders. I don’t really have much room to drill the hole. I might be able to drill the end of the wire that sticks out past the end of the hinge and then another hole somewhere close, then I can string a short wire between that and the hole in the hinge wire.. Not the proper way to do it, but it would at least prevent the hinge wire from falling completely out. If anyone has any ideas about how to secure the hinge wire please let me know. Some photos are below that show what I’m talking about

Elevator with rivets all sanded, now just need to finish up the electrical work.

Here’s the right side of the piano hinge. That small triangle bit of aluminum is where I’d have to put the hole for the safety wire. Although it doesn’t need to be a very large hole… a little more the 1/32″ I believe, I still don’t think there’s enough material there.

And this is the left side. There’s a little more material on this side, but it’s in the side that moves so I can’t put it there either. Maybe I can do two holes on one of those larger areas and loop it through… Hmm that might work.

The Making of a Mold

I tried my hand at making a mold of the “glove box” top so that I can have someone make a carbon fiber copy of it. Well it didn’t come out terrible, but it didn’t come out great either. Probably not good enough to use for the infusion process. A few things I learned and need to do different. First I used polyester epoxy gel coat which is better then the regular epoxy gel coat, however this stuff really needs to be sprayed on rather then brushed on. Since I don’t want to spend a bunch of money on a spray, I think I’ll try regular epoxy gel coat which I believe is a bit more forgiving when brushed on. I tried to use Bondo as a backing for the mold, but it’s still too flexible. I suppose I could use a lot more and a build up a thicker mold, but I really don’t like the process of using the Bondo anyways so I’ll just abort this idea and use the layered fiberglass and resin method instead. The last thing is that I covered the part with 3M wrap to hide the holes, but the gel coat gets so hot that it softened the wrap and the holes still showed through so I’ll instead fill the holes with epoxy or something.

The mold ready for the mold release then gel coat.

Sprayed the mold release onto the mold. I need a better sprayer though. It supposed to just be misted on, this spray got it a bit too wet.

Well I forgot to take a photo after I put on the gel coat… probably because I was frustrated that it didn’t go on so well. When that got tacky I put on the Bondo. Looks like a tasty cake, but doesn’t smell so good.

I let that all dry over night and inn the morning after carefully prying it free from the board and other mold parts this is what I ended up with. You can see that the 3M wrap didn’t work so well because the holes are still quite visible. The mold also cracked a little when I was prying it off because the bondo was too flexible. Overall though it didn’t come out too bad. I was hoping to get it right on the first try, but I think the second will be the one. I need to order some epoxy gel coat and I’ll try again.

Son of Return of the Elevator

Time: 2.5hrs

Worked on the elevator today. The primer had dried for a few days on the right elevator skin so I was able to rivet it onto the skeleton. I used the technique from Pete C’s site (and Peter V,s) to make sure the elevator lined up with the HS. Seems like it lines up very lol now maybe very slightly off (1/32″ or less). I also started to fill the rivets. I’ll be riding the Trim Servo (Ray Allen T2-7A) so I can finalize the HS and Elevators. 

Also used took this time to get the trim tab moving a little better  

Clamped the elevator to the HS… all ready for the right skin to go on. 

All finished. The new skin lined up very well and the clamps helped with the HS alignment. Now just need the trim servo and the connector ends to finish these up… oh and also need to fill and sand the rivets. 

Elevator Redux

Time: 1.5 hrs

I went down to Torrance TAF yesterday and got my new right elevator skin. So today I de-burred and primed the skin. I used the old epoxy primer so I need to let it dry for a few days. I’ll probably try to put the skin on this weekend since it should be dry by then.

In other news I talked to Jean about the 915. Let me just say there are a quite a few changes that are going to be made to the airframe in order to be able to fly at full power. I guess I shouldn’t really disclose all the changes since they are still preliminary and should wait for an official announcement from TAF. One that I should report one though is that it seems like they are going to move the parachute box back to aft of the bulkhead. TAF had moved it forward into the luggage area for better CG with the 914. However the 915 is around 40lbs heavier then the 914 so that will move the CG a bit more forward. I mention this because having the correct CG for the 915 is the one necessary item from the list of changes. The other items are really only if you want to take advantage of the higher cruise speed that the 915 will give you. For me the problem is that I already put on the top rear fuselage skins so if want to be able to use the 915 I have 4 a few choices:

1. Remove the top rear fuselage skins and install the new parachute box aft of the bulk head. I have all (or at least) most of the parts so that should be an issue. But it means removing hundreds of rivets and prepping all the alternate parts. The plus side is that I’d be able to install the new magnetometer bracket much more easily and I’ll have all the luggage are space back. However then I’m absolutely committed to using the 915.
2. Don’t install a parachute. The more I think about it the more of a pain the parachute will be. It needs to be repacked every 6 years or so and that means drilling out rivets to remove the skin that covers the parachute opening and sending the parachute in to be repacked… not sure if the rocket needs to be replaced as well. I guess it’s still easier then the older Cirrus models, but still an added cost and hassle. But I suppose it could be worth it if you find yourself in the position were you have to use it. If I don’t install a parachute I believe I can remove the box as well and gain the luggage space back, but I’m not exactly sure what happens with the “big ass” cables that tie into the fuselage.
3. Don’t use the 915 and just stick with the 914. There’s nothing wrong with the 914. When I flew the Sling for the first time I was amazed at how it climbed out and performance seemed very good. So it’s easy to lose that and get caught up in the “latest and greatest” syndrome. The 915 is quite a bit more expensive, heavier, and the performance gains are pretty minimal (once again I saw the preliminary numbers so I don’t want to say anything). The 914 is fairly rock solid, unlike the 912is which has been plagued with problems (and if the 915 is based on the 912is then well…). I think with some time TAF will have a great Sling 4 using the 915, but I think it will take a while to work out the issues. Also while I think Rotax did their work in testing the 915 you never really know until you get a few out in the field to see what the problems will be.

So well there it is, my conundrum. Do I build the plane with all these uncertainties just to use the newest engine and hope that it’s really all Rotax is saying it is or do I stick to what works? It would be nice to get a little better climb out performance, lower fuel consumption, and easier integration (the 915 uses Can Bus for most of the sensors). On the other hand I’d like to fly and enjoy my airplane when it’s done. I don’t want to be dealing with ADs and making costly repairs both to the engine and airframe. I guess I’m leaning towards playing it a bit safe and just build the plane for the 914 (also my wife said she won’t fly in it if I use the 915 because she’s worked there will be problems, but that’s why there;’s a parachute 🙂 ). If the 915 does work out well then I suppose I could always build another Sling using the 915. By that time it would probably be a few years down the road and the engine and airframe will have all the bugs worked out. I also need to consider that the parachute location is just one of a multitude of other changes needed to take advantage of the higher cruise speed. I can’t really justify spending quite a bit more money just for a little better climb and slightly better fuel consumption. So if I use the 915 then I want to do it right and make all the necessary changes to take full advantage of the 915.