Printing some streamlining bits..

 Acroduster  Comments Off on Printing some streamlining bits..
Aug 102019
 

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.

not moving..

Streamline tubing

 Acroduster  Comments Off on Streamline tubing
Aug 072019
 

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.

Most streamline tubing has a more rounded TE, we need to fix that!

So this would be glued to the back of a round tube. I’m sure it will look right 😉

Designing and building the motor mount.

 Acroduster  Comments Off on Designing and building the motor mount.
Aug 032019
 

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.!)

plates to mount the tubing for brazing.
..bottom view
side view
Top view, those bushings for the bolts will be shortened and I will add an inside tube to prevent buckling. All in good time!

And we have a Valach

 Acroduster  Comments Off on And we have a Valach
Jul 242019
 
test fitting the bit at the front.

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..

Center of Gravity

 Acroduster  Comments Off on Center of Gravity
Jul 232019
 

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!

23.5″ avg, scaled down locates the CG at 200 mm. TLAR as they say.
And of course some heavy metal to balance everything 😉

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!

Sandblasting

 Acroduster  Comments Off on Sandblasting
Jul 232019
 

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.

Tubes everywhere..

 Acroduster  Comments Off on Tubes everywhere..
Jul 222019
 

After a few weeks of putting bits of tubing together, I am at the point where things start to look like something you would recognize as a fuselage.

This morning I cleaned up the fuse with a diy sandblaster, much to my surprise this went really well. It’s the usual bucket of dry sand, empty bottle and some compressed air contraption. Needless to say, do wear a decent respirator. For the rest, the neighbors are building an extension to their house, I blame all the extra sand in the air on that!

The brazing is getting easier as we go along, however, the further the construction progresses, the more time it takes to make the parts. (When it looks 80 complete, you have 80% of the work ahead of you 😉

Anyways, below some pics from the various stages of construction. I was at this point still struggling getting the right flame from my torch. After suffering from empty bottles, I have now switched to a decent propane tank (still using the oxy-bottles) and a .6 or .7 mm needle as a welding tip. When I am all finished I will be an expert and write a nice story about the pitfalls of silver soldering.

Alu foil works great for protection of the ply when soldering. 4 layers works fine.
..next time I’ll do it different.
it almost looks real!
these are the templates for rudder/stab/elevator.
as it says: rudder horn.

Stolp SA750

 Acroduster  Comments Off on Stolp SA750
Jun 092019
 
All good things take time to mature. From a time many, many moons ago I wanted to build one, but from idea to first metal took a bit of time. I started off with the original pans that still can be had from Aircraft Spruce. On a separate website I gathered all and everything I could find about building metal fuses. In the end, just get your metal hot and start is the best method to learn. My biggest stumbling block was an old pot of Silversolder powder with an activating point of 850C, when my soldering rods (the usual yellow ones) melt at a lower temp. Why I did not notice earlier? Anyway, I now have a good combo from Silberlote with the proper paste. And all of a sudden the soldering is easy.. Notes for soldering: getting the right flame is still a bit of hit and miss. Today I learned form an expert welder that I overheated the bar where it turned black. You need no more then a yellow flame, blue is too hot! Lesson learned. Previous joints were made with a tiny needle like tip, but I did not quite get it right all the time, the valves on my rig are too sensitive, just moving the torch changes the amount of gas. The joints made with the needle tip are nice, easy to clean, but I could use just a little bit more heat. The number 6 tip supplied, is too much, or I need to reduce the O2. By the way, I am building things to scale, within the limits imposed by the availability of the tubing: 1″ tubing ends up 9 mm, which looks about right. Strength wise it is overkill, but I don’t want a skinny looking frame. Weight will not be an issue.
Station 12, Landing gear bungee cord supports
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