Closeups of my handywork. After sandblasting I ‘ll reheat the joints to make them flow a bit more and fill any voids leftover. This will be for cosmetical reasons only.
The plans call for square tubing for mounting the wings, but you guessed it, not available in the size I want, not even close. Here I have to deviate from the plans and make something that will look the same, but is something I can make. At stations 31 and 51 I added the normal tubing cross bracing. Then I made some solid steel supports that look like the original. I use short pieces of steel. Later I will put a false ‘square’ cover over the visible bits in the cockpit area.
I’ve always been reluctant to start milling steel, but actually, if you take it slow, it goes quite well! (surprising myself here.) This stuff is plain cheap ironbar, I will have to try stainless steel as well, but that is probably a different game. Time will tell. (Note to self: This was done with a 2 mm bit, @300 mm/min, 0,1mm layers. Probably can go larger on the layers, but I need to do something about cooling, since WD40 is not that great as cooling liquid.) Looking at the parts you see that the toolbit vibrates, it creates an uneven surface. However, using a 2 mm bit is really pushing things. I will need to study this subject a bit more!
While I was at it, I also made the support for the LG-front attachment point. By using 2 mm parts I get enough strength, and I can easily cover the gap between the supports with a 0.2mm strip. The rears support consists of triangular gussets. Again a milled part will be the real support, gussets will/ might be added to make it look nice.
As you can see, the new shipment of tubes has arrived. Visible progress is made again today. Overall I’m happy with the result, there are always little things that do not quite turn out as intended, but since this is all about learn as you build, it’s not a disaster. (So far I have not had to scrap anything)
The outline of the rudder is not 100% accurate, I went more along the route of TLAR. I might have a couple of square inches more rudder area, which never hurts. Most of the rudder was build freehand. I pre-bend the outline of the TE as much as I could, then stuck the front of the tube in the top rudder tube, soldered the main post, and added the ribs as I went, keeping a nice smooth shape as I progressed.
Again, 2 bottles of sand were blasted into the atmosphere. At 8 bar air pressure, the ribs do deform, better use 5-6 bar and take longer. Nothing drastic, but keep it in mind.
I found out that stress was present in the construction (surprise) which causes a slight bow in the rudder post. The balancing part of the rudder is now to close to the rudder part. Nothing that can’t be fixed, but it goes to show how much tension develops in the parts after cooling down. I could of course stick everything in an oven @600C for 24 hrs. Only problem is I don’t have one.
Just a relative quick job tonight: making the fin. The only noteworthy item would be that after installing the ribs, I still had to drill a hole for the bracing rods. I decided to simply cut a slit with the Dremel and enlarge it to fit the tube. Seems to work well, and since it will be hidden, it’s good enough for now. Consider this a practice run for the elevators.
Given that it is difficult to find the right size streamline tubing, I better go ahead an make some. With the open-scad script I found I made a handful test pieces. The idea at the moment is to attach the plastic with a layer or 2 of 25 gram glass cloth. That should fill the little grooves in the printing and provide enough strength. Obviously this has to be done after the brazing.
I also tested the deflection in the motor mount once it is attached to the frame. Can’t move it. This is a comforting thought.
I did say undercarriage? A whole new can of worms just opened. I had hoped to be able to use a normal streamline tube as offered by various vendors. However, can’t find anything close enough to what I want.
I could make my own from round tube, just like friend Achim , but that’s a major effort. Many say that deforming a round tube introduces cracks, and therefore failure waiting to happen. Don’t need that.
How about adding some plastic and cover with some glass, a bit of sanding and no one will see? Time for a 3D print job.
A quick search for airfoil scad scripts turn up something useful. This is the first idea. Ideally there should not be a round nose, but given the rather short length of tubing and the ample power of the engine, I think I can forget about the added drag of not having a perfect airfoil shape.
So this would be glued to the back of a round tube. I’m sure it will look right 😉
Once it is all finished it looks easy. At the start it’s more like ‘how am I going to tackle this? I decided to use a 3 mm alu plate as a mount for the short tubes that hold everything together. This way I could mount some 5 mm wire studs to hold the motor, and line things up as I think they should be. The frame bolts are actually mounted in the frame, which makes everything line up perfectly. Mind you, this is probably version 1, when I get further along I might need to change things because of balancing, place for the exhaust, servo’s etc. The mounting tubes are still oversized, but better a bit longer then too short. The top cross bracing went OK, when I started adding the bottom ones, I found there is no space for them because of the Walbro carb. In theory the supports on 3 sides would be enough. I also have to think of a place for the exhaust, might have to make something special, since I really don’t want the hot exhaust next to the carb.
I ran out of tubes, so next stop is to figure out what I need, place the order and wait.
The mount weighs nothing of course, I did not weigh it, but I would guess around 90 grams.
For the clever ones: no sidetrust in the mount? Based on good advice I decide to use a throttle to rudder mix to keep things lined up. I can always change it! I also might use this mount for running in. (There is no need for it, I know, but I want to see how it holds mechanically before letting go into the big blue yonder.!)
It fits, no worries there! It’s gonna be jus’ perfect! I am sure I can’t resist building the engine mount next. It might not be the correct size, because depending on the CG I will move the engine forward/back. But it will be a good exercise, and it makes things look real..
It’s never too early to add that famous mark. Of course this is the full size one, and I might not like it, but it is a good enough place to start!
Probably overpowers the plane, but it has the right sound, and I have seen the engine long enough in use to know it is going to be a happy marriage! Hey, this thing is called an Acroduster, the more oomph, the better!
Last year I had some fine nice sand left over from doing some garden-diy-ing. ‘Just in case’ I stored it in a nice dry place, knowing that one day, dry sand would come in handy.
I knew from the onset that I needed to find a way to clean up my brazed parts. Up to now I used the 3M-scotchbrite pads and lots of elbow grease. However, you never get into the nooks and crannies. I already had a friend offer his help, but of course there is always the thought ”I could do this myself”. Lots of googling and you know sandblasting can be dangerous for your health, so please only do this if you are a responsible adult, and know how to take the proper precautions!
The mechanics do not appear to be complicated, as shown by the many DIY sandblasting contraptions on utoob. After playing around with some fittings and getting lots of sand in the wrong places, I thought I’d try the most basic version: the plastic bottle version.
So, I got an empty bottle, drilled 2 holes in the neck, modified an airpistol, filled up the bottle with some dry sand, squeezed the trigger, and wow, this works!!
The only downside and need for further work is that my compressor does not really have enough oomph to supply a lot of air. So I blast 10 secs, wait 20 sec etc. I might try with a smaller tube, less air, less sand but more continuous blasting. I did not build an enclosure, I just use the ferns at the edge of the garden as my shield, and they do a good job at catching all the sand!
Yes, I know, the surface is not evenly blasted, there are shiny parts etc. However, the main thing I wanted to achieve was to get rid of all the gunge, and that worked 100%. I still need to add a gazillion bits of tubing and supports, so there will be more opportunities to hit every part. For now, I am happy with the result!
Oh, by the way, so far the fuse weighs in at 973 gram, which is pretty good going. I only used 30 grams silver to solder everything so far.