Minus Thermic, Precision Products (TopModel)

[Konrad]

Well, it looks like the only real tedious job is cutting the trough for the wing rods. I used a ball mill in my Dremel tool to take out the bulk of the material. I then used the end of the receptacle tube to bring everything to final size. and last I ran the tube to burnish the trough to make sure it was a uniform feature (No high spots that might interfere with the lamination of the two wing sheets). Well, I wish the instructions would have mentioned that the trough is not centered between the laminations, as one needs to take into account the dihedral. So it looks like I’ll need to make some clearance (slop) to allow for some dihedral. I’ll need to fill this up with epoxy and micro-balloons.

That's another deviation in the kit. The parts list, calls out this tube as aluminum. My kit was supplied with a brass tube. I wonder if this was done out of concern for the corrosion issues associated with aluminum in contact with carbon? (You might recall from your high school chemistry that aluminum and carbon are at the extreme ends of the galvanic corrosion scale.) Or was this a strength/ weight issue to aid on the slope?

 

[Hank GB Z]

Hey Konrad,


Yeah I was thinking this would be a hard part of the build. The only place i saw the call out about the location of the tube is in the airfoil picture. But it's not mentioned in the steps where it shows the work.

If you make the clearance would it be better to fill in with wedges of balsa, have more grain running lenght wise over just epoxy and filler?

Hank

 

[Konrad]

As the void that will need to be filled is less than 0.25mm at ether end of the trough I think epoxy with filler would be fine.
My set of instruction make some mention about cutting between the lines. But state nothing about taking into account the dihedral. I also see a trap when they talk about epoxying in the tubes, again no mention of dihedral. Dihedral isn't mentioned until closing the top sheeting with CA.

Another issue with the instructions. They state to sink the retention tongues 1mm into the wing blank. With the Minus Thermic this will not place the the end rib and wing blank in proper relationship to each other. The instructions should state to use the wing rib to define the location (depth) of the tongue. The problem with setting the tongue deeper than the thickness of the tongue is that when sanding the airfoil this will leave a void above the tongue mount. I wonder how many folks have had to use a lot of aileron trim as a result of this geometry issue. That is using the flat bottom of the wing blank as the datum when sanding the airfoil.

I think this is a geometry oversight when designing the Thermic from the Acro. With a symmetrical airfoil of the Acro this isn't an issue as the wings are sanded along the part line of the two wing blanks, not the flat bottom of the wing blanks as with the Thermic.

 

[Hank GB Z]

Konrad,

Good point on the size of the void. I have the Acro so that’s what was on my mind. It has more dihedral at the root than the thermic.

Good catch on the tongue placement even though the Acro won’t have the same issue it’s good to be aware of surprises.

Hank

 

[Konrad]

Hank here's a heads up for the Acro.

Just received my order of servos from Aloft, thanks Victor!

To lower the cost I got a Minus Acro with the order (Aloft Hobbies combo deals). I can see that TopModels is having an issue with both the laser operator and the cut files. As you can see it looks like the operator did not adjust the laser power to make etch lines. As a result the wing rod trough was cut clean through! So was the fir tree feature for the wing mounting tongue. The power setting might also explain the burned ribs.

Now there is an engineering issue with the cut file. Both plywood ribs are cut the same. You can see this by stacking them on top of each other. This will result in a set of wing that are twisted relative to each other as the tongues over lap in the fuselage. One of the tongue slots needs to be moved to allow this overlap without causing wing twist.

 

[Hank GB Z]

Konrad,

Good attention to detail. I'll go look at the cut wood in my kit tonight to see if my Acro is like your's or more like your Thermic.

Hank

 

[Konrad]

As the wing to the Thermic is in need of some major geometric rework I thought I'd take the opportunity to make my Minus thermic into a long winged Acro. So I cut out a set of ribs using the one from the Acro kit as a master. You might have noticed that these ribs have a bit longer chord. This was done to widen the ailerons of the Thermic to match those of the Acro more closely.

I still need to use that #11 light guided cutter to make the fillers for the laser cut voids in the Acro set of wing blanks.

 

[Konrad]

As these ribs really are just assembly fixtures (sand to lines)I thought that the need for continuous fibers wasn't needed. So I just cut a new slot and used the waste to fill in the area improperly cut by the cut file.

All the best,
Konrad

 

[Konrad]

Here is how I extended the ailerons. I split the aileron span wise and added a 10mm piece of wood at the root. I then made a filler wedge for out near the tip.

Sometimes kits just have issues! This kit has a piece of wood that had internal stress void in it. You really couldn't see it in the surface. But you can see the voids in the laser cut line. This will be an issue with TE strength.

I filled the voids with balsa dust then saturated the area with thin CA glue. We will see what issue this may cause when sanding the trailing edge to shape.

 

[Konrad]

I have to admit that making the wing bigger while keeping the V-tail the same size does scare me. I fear that I may be making my version of the Destiny glider. This plane is notorious for it's poor handling!

SoaringUSA > 1.5 Meter > Destiny Sailplane

Destiny Sailplane,Destiny, Destiny 54, Valenta Destiny,

www.soaringusa.com

Remember that the Thermic is an outgrowth of the Acro. Both ships have the same size stabs. This would lead one to think that the Thermic V-tail is on the small side. Not so if built per the plans, as the thermic has added dihedral.

Now I've removed this added dihedral and added more wing area with the extended ailerons. So I will be doing much the same thing to the stabs as I did to the ailerons to get a bit more area in the V-tail

 

[Hank GB Z]

Well I opened my Acro kit last night. The wing tubes are brass, the wing panels are etched marking where the tubes go not cut like yours. Also the plywood ribs are identical so I fixed it the have the offset like you did.

I also assembled the tail feathers. For my Acro I will keep the stock V tail to start and if I don’t like how it flies I will mod as needed probably going to a conventional T tail.

Oh and the holes in the formers were to small too so I drilled them out while still on the parts tree.

I’m in the middle of covering the Taylor craft so I’m not gonna go nuts just yet on the Acro but it’s coming.

Hank

 

[Konrad]

Good to know your kit is from a better production run. Other than taking care of the manufacturing issue in my kit I think I'll leave my Acro stock. Well, I'm planning to install the aileron servo with the output forward. This is to give the aileron push rods more of a straight line push. I will relieve the doubler to allow the servo arm to move inside the fuselage. The kit has us use far too long a servo arm. I like short servo arms and the use of full servo travel in most of my radio set ups.

I really like V-tail on these small ships as weeds tear off low stabs.

Oh. I'm going to add carbon tow in the space under and ahead of the V-tail. This looks like it will fail on the first cartwheel.

 

[Konrad]

Hit a snag, I don’t know how to test that the wings are square to the fuselage while the wing rods are being glued in place with the needed dihedral. On my first attempt I had assumed that the etch lines were good enough. Well they weren’t. I had to free the epoxied tubes and just free hand the wings against the fuselage. (I hate building like that). She looks square I just hope she flies true.

On the Acro I think I’ll draw some datum lines on a large sheet of paper and hope that the wings will fit against the wing mounting holes in the fuselage.

As I’m making my Thermic into a large Acro I added some glass fiber across the laser cut line for the polyhedral joint. I’m always impressed how much durability adding fibers across a joint adds.

 

[Hank GB Z]

Konrad,


Looking good. I see you've done all this work before shaping the air foil. Smart, keeps dings and dents to a minumum. Also all the finger prints will come off during the shaping process. I'm thinking you'll strike a pretty good balance between the two designs with your long wings and symetrical air foil.

Hank

 

[Konrad]

Believe it or not but I am trying to follow the instruction sheet. While the kit is of modern technologies (laser cut. It really is a throw back to the good old days where the instructions said glue "A" to "B" and fit "C". And then the last part of the construction instruction said; Sand away anything that doesn't look like an airplane!

All the best,

Konrad

 

[Konrad]

Oops, I've been having some issues with this site. I lost the text of this post.

While these are simple models they aren't easy to build. The sanding of the airfoil take a bit of skill, read is an art. One needs to be aware that every piece of balsa has different densities. Heck this density changes even along the same piece of would. You also need to be aware that your arm is biased. This means take it slowly. I thought I had a lot of material to remove on the TE so I just whaled down on the sanding block. Oops,I find I had sanded in a low spot. No big deal I thought it only looks like I missed by .25 mm. Well, looking at the TE one can see a lot of undulation. So I built the void back up with light spackle. It now looks good. I just hope the TE is stable enough. BTW: the stress tears I found earlier sanded out for the most part.

Even though there is a lot of sanding take your time. Sand a little, look at your work critically! Make adjustments and repeat!

Again as part of my mod. I chose to only use one rib at the Thermic polyhedral joint. I did this so as to not draw the eye to the cuts. Oops, staying with my motto "form follows function" I removed the rib to allow for faster rolls as a result of lower inertia. (That's my story and I'm sticking with it)!

All the best,
Konrad

 

[Konrad]

What more can I say.

 

[Konrad]

When I was a manufacturing engineer, one of the things we strived for was consistency in our processes. I fear this isn’t what they meant! One rib was broken by catching the sand paper. the other was broken by snagging my finger.

What you see here is about 3 hours of work with an 80- grit sanding block. Sanding a wing out of a block of balsa is a slow tedious process. But it is far better than wasting an afternoon watching a game of football. Both wings are with 0.5 a gram of each other and around 25 grams each. I use mass to help me determine which wing still needs some work. That is I use the light wing as the master and and adjust features on the heavy wing to try to match the light wing. I’m now moving to 150 grit sand paper in preparation for covering. OK, I first need to replace the broken ribs and check for loose glue joints.

 

[Konrad]

Well, that’s close enough for me. Everywhere I can measure along the wing, both wings are showing the same thickness within 0.15mm (0.006”).That's the thickness of 2 human hairs!

I’m now ready to separate the aileron. Anybody have any input on which style of hinge line I should use. I’m liking the Acro if for no other reason that there is a few more square mm to the aileron. I don’t know how well the actually hinging will work with the then TE or if there are durability issues.

 

[Hank GB Z]

Konrad,


I'd say go Acro, 1: it results in a bigger aileron 2: It's easier (only one cut really) 3: i'm pretty sure the Thermic style is a result of the poly hedral which you removed right? The Acro style still ends before the wing tip minimizing the chance of getting damaged in a cart wheel.

Hank