Minus Thermic, Precision Products (TopModel)

[Hank GB Z]

I started my fuse last night. I got up to the point of putting the tubes in. I was having a hard time following the instructions so I figured I was tired and stopped. One thing I noticed was that my fuse sides weren’t identical. The important stuff was correct for the wing tubes and hold down tongues. But the heights were off. I just cut the high one down to match the smaller one. Didn’t seem to affect the build any.

My allergies have been pretty bad lately so I’m gonna sharpen my block plane to do the majority of the wing shaping.

Hank

 

[Konrad]

That's why we have sanding blocks.
Hum, my Minus Acro sides are the same other that the tongue slot. I wonder if it was just stresses in the wood that were released during the cut.

Yep, the razor block plane has been very helpful. Here is what I just did this morning with my 35 year old German razor plane.

All the best,
konrad

 

[Konrad]

Ok, now I see the issue! I have no idea how the lazer would skip like that. Maybe there was a piece of tape (foil) on the the wood when it was being cut.

Yep, It is looking like Top models was/is having some QA issues.

 

[Hank GB Z]

Yeah I wondered about the stresses too but considering how both my wings were cut differently than yours I think it’s safe to conclude there were multiple cut files out there. Like you said it why we have sand paper.

Hank

 

[Hank GB Z]

Ok the fuse is almost done. I was sanding it when I realized that the hatch for the aileron servo should be shaped too. That’s whe I noticed that it’s a few mm short. I’ll just make a new one but yeah I think there are different revisions of the cut file out there each with their own quirks. But I’m liking the look of the plane.

Hank

 

[Konrad]

Looks like you had some issues fitting "F7" and looks a bit too short. My "F7" is 29mm long. Your canopy parts look to fit much like mine did. So I don't think it was a problem with the nose.

I haven't done this yet. But I will be adding some carbon roving inside the fuselage extending about 12mm ether side of the stab LE. I think Top Models went a bit too far back there when it came to thinning the fuselage.

 

[Hank GB Z]

Yeah my F7 had the slot in the wrong place. Adding roving back in the tail area is a good idea. That’s a weak point on the Mint 2 also especially on the side where the elevator push rod comes out.

Hank

 

[Jock]

Hi there, i found this forum through a google search for the Minus. My kit is on it’s way so I will be following with intrest and hope I can ask you guys some questions? First off have you been flying the Minus already? I am very interested in how the ‘flat’ version of the larger wing is working out. And also, using CA for the built would low viscosty be best? Thank you. Jock

 

[Hank GB Z]

@Jock

No I haven’t finished mine. Hope to do some more work on the wings today.

Thin ca works well but get a capillary tube or cap the control the flow.

Hank

 

[Jock]

Hi All,
I have started the built. One thing that is unclear to me is where, as per instructions, “to beavel where the control plastic tubes go through” Where precisely should this be?
Thanks Jock

 

[Konrad]

I hope this helps

 

[Jock]

Thank you Konrad. The picture is precisely what I was looking for and I’ll gladly take your suggestion to bevel the top.
I am considering to built the thermik wing flat (wing tips) like yours because I will only fly this on the dunes and no thermik flying. How did the ‘flat’ wing work out for you?
Thanks again
Jock

 

[Konrad]

The straight wing is the least of my modification. I think the airfoil will be the one I notice the most. Now if lowering the wing tips do think about adding some area to the V-tail. Here is what I did to balance the controls a bit. Note that I added a bit of a wedge (in red) to add the extra area.

Sorry I haven't finished mine.

 

[Jock]

I understand there is more to it than simply lowering the wing tips that is way beyond my expertise so I decided to stick to the original plan. Thanks

 

[Konrad]

Based on where you fly, slope, I'd drop the wing tips. Mainly because you will be spending most of your time flying cross wind. Adding a little area to the stab is my fix to what I think MIGHT be an issue with longitudinal stability. This is done at the risk of adding spiral instability issues. You will note that this was done keeping the root of the V-tail the same as stock. I did this so as to not interfere with mounting the V-tail on the fuselage. The wedge and ribs were all made from scrap balsa found in the kit.

So with all the work you will need to do to build this model I recommend you try a few simple mods to make it fit your flying style and site. No real expertise needed beyond the building skills needed to build the model stock.

All the best,
Konrad

 

[Jock]

Thanks for the advise Konrad. I’ll look into it

 

[Jock]

For me to learn and understand: I googled the longitudinal stability and spiral instabilies and understand what you say. What changes on the main wing when talking out the angle at the tips that we would like to compensate with a larger tail v wing? I am trying to understand this since the short wing version uses the same fuselage and tail wing. Thank you

 

[Konrad]

The effective dihedral angle changes when the tips are dropped. To gain back some directional control (longitudinal stability) we often add area to the vertical fin. With a V-tail we can narrow the V angle and/or add some area. A lot of this is discussed by guys that actually do this kind of work on the Charles River RC site. I like to read Dr. Drela's and Don Stackhouse's articles.

https://charlesriverrc.org/articles/design-and-construction/aircraft-design/

For our models I'm assuming that the Minus Acro has adequate stability. (To my eye the tail looks to be a bit on the small side). When Top Models added another version of the model, the Minus Thermic, they added some wing area but kept the tail the same. So to gain back some stability they added some dihedral to the wing.

Now if we remove that dihedral we might want to add back some stability in the way of vertical (V- tail) area. With most RC models I'd rather error on the side of spiral instability (too much vertical area) rather that directional instability (too little vertical area).

Remember engineering is the science of compromise.

All the best,
Konrad

 

[Jock]

Hi Konrad, thank you very much for explaining. I understand. Much appreciated. Jock

 

[Jock]

work in progress