Minus Thermic, Precision Products (TopModel)

[Konrad]

Many of you might not know this, but if you purchase an aircraft with your sales order many of the electrical components in that order will have a 10% discount from Aloft Hobbies.
This doesn't need to be a high dollar aircraft!

To that end these little Precision Products (TopModel) gliders can pay for themselves!

It is not unusual for me to spend $500 on servos, motors and ESC and other radio support equipment every time I step into Aloft's warehouse. So I've effectively gotten these gliders for free!

This little minus Thermic caught my eye, and for $50 looked to be a fun novelty built. I chose the minus Thermic over the minus Acro as I want a little more wing area for those light lift days.
https://alofthobbies.com/top-model-minus-thermic.html

What is real nice about the kit is that the plans show and give instruction for both models. This can also be a problem in that you really need to study the plans (TopModels has tried to highlight what is different between the models).

In this thread I will document my build of the minus Thermic. This should not be thought of as a kit review! What I really like about kits is that it is often much easier to customize the model to your own need than starting out with an ARF.

Looking over the model I really like the laser cutting. The plans and instruction are written in both French and English which make it easer for me to follow than some of the same types of models we can get from Asia.
What I really like about the minus Acro is that the airfoil is symmetrical and the ailerons are a bit wider than what we find on the minus Thermic.

What I don't like about both models is that the V Tail angle is the ubiquitous Beech 110°. And that the V tail cord and area look to be a bit small.
I'll look into changing the V tail angle and chord based on my limited understanding of how to size and set up a V tail. It is my understanding that the V tail angle is highly dependent on the aspect ratio of the wing and the fuselage moment arm. The chord is critical with these super small models as the Reynolds number are really working against us.

On my minus Thermic I hope to widen the aileron chord for more low speed roll authority. I'm also looking into adding a second aileron servo to allow camber changing and spoileron control. I think I can make all these modification with the wood supplied in the kit.

While the kit is called a Minus Thermic I hope to have a light lift slope ship when I'm done.

 

[purview]

hi Konrad,
mixing is somekind a key to success...
and yes it only allows benefits within the frame. How is the progress of your building?
(I also like to know if the given dihedral of the main wing will suit to your changes.
or how do you judge a plane by looking on it and saying what I don't like? There must be a goal kept in mind u wanna achieve?)
Hints and tips outside the plans are very welcome.
friendly regards
Chris

 

[Konrad]

Good questions.

I base my modification from experience and formal training. In this case it is heavily based on experience. With this model I'm lucky in that the designer of the Minus has offered us two variants, the Minus Acro and the Minus Thermic. I also have experience with this type of construction as I have a Pilot Models, Mint 2.

As I stated this is a novelty item and was purchased as a way to lower the cost on a order of electrical components from Aloft Hobbies.

What I want is a light lift micro aerobatic ship. The Minus Acro is such a ship but I wanted a ship to perform well at even lighter lift conditions. So I choose the Minus Thermic for its added wing area. But plan to steal some of the design elements from the Minus Acro.

Now in most slope and aerobatic aircraft dihedral is to be minimized, if not avoided all together, as it induces a roll component to the aircraft as the aircraft yaws.

On the slope I like to fly 90° to the wind, while going back and forth along the face of the slope. No matter how fast I fly this results a large yaw component from the relative wind that the aircraft sees. To keep the model from rolling onto its back or crabbing at a huge angle we remove dihedral from model that we intend to fly on the slope.

Now when a designer is aiming to make a thermal hunter he has a concern with avoiding spiral instability so he adds dihedral to lessen the pilot load as the model is often difficult to see at altitude or at a great distance. This usually isn't a problem on the slope.

So I plan to remove the dihedral (polyhedral) from the outer panels of the Minus Thermic and stay with only the 4mm (per side) of the Minus Thermic's straight inner panels. Please note this results in my model having a lot less dihedral than the shorter wing Minus Acro, which lists the dihedral at 6mm for a shorter span model.

This is because I plane to enlarge the V-tail area greatly. What I haven't mentioned is that there is a strong relationship between the vertical (effective) area and dihedral in controlling spiral stability. Generally the less dihedral a design uses the more vertical area is needed for yaw stability.

Now my experience is that I don't like V-tails! Well, most V-tails as they are designed by the false notion of projected area. That is area of the V-tail is the same as that of a conventional tail with the vertical tail area projected on the area of the stab. This often results in there not being enough wetted area for the V-tail to offer good control. This really shows up in turbulent air. V-tails usually need more wetted area than a conventional tail for the same control. Then comes the V-tail angle. Far too many designer fall back on what they read (see) from the Beechcraft V-tail's 110°. Many designers think this 110° is written in stone. What they don't seem to understand is that the V-tail angle is highly dependent on the aspect ratio of the wing. The higher the aspect ratio the closer to 90° the V tail junction angle should be.

Now we all love high aspect ratios for controlling tip losses. But on a V-tail we actually want a broader chord (lower aspect ratio) to allow the V-tail to work in more favorable reynolds number near or at the stall speed of the wing. So I'll be changing the shape and area of my V-tail.

V-tails do NOT offer less drag than conventional tails for the same amount of control!

The best, by far, V-tail I have flown is the one designed by Don Stackhouse on his Chrysalis lite (FAI-F3RES). Love the large area, broad chord surfaces, hinge line very far forward and the 90° junction angle on the low aspect ratio wing.
https://www.djaerotech.com/chrysalis-lite-f3-res/

No I haven't started this model as I spend most of my hobby time ether repairing or doing maintenance chores on my flying fleet. I hope to start this model in 2 to 3 weeks.

 

[purview]

Thanks Konrad,

it impedes that changing one parameter has one or more effects on other design components. For me it is now easier to understand the way how you can get to a light lift (aerobatic) ship. With a desired flying behaviour in mind, based on your experience, the plan is to avoid effects of instability and/or wrong perceptions of the pilot during some specific tasks (for example circles, upwind, downwind, legs or a looping). So to control the behaviour of the plane, on one side with mechanical properties (angles, Re's, velocity and stall) and on the other hand to avoid misconceptions (sight, bank and correct yaw) of the pilot, there must be something utterly true within the coming Minus Thermic. I wish your project the best. The main area of concern I see is the aerobatic performance (due to the effect of up side down and vice versa turns into ... act) with less weight.

I saw light weight ships that stole some design elements from free flight planes. It works quite good but the problem there are the aerobatic capabilities.

Do you have a short answer (...not to capture your topic and convert it into fundamental basics) on the NOT available lesser drag between V and X or T? Is it just a property of the leading edges, or what the f** is going on beyond?

Many regards,
Chris

 

[Konrad]

Yes, the Minus Thermic has a cambered airfoil this helps with the light lift at the expense of a bit more pilot load for inverted flight. The great Dream Flight Ahi (a light air aerobat) also uses semi symmetrical (cambered) airfoils.
https://dream-flight.com/products/ahi-kit

The V-tail offers no net drag reduction. What some folks think is that since the V-tail removes one surface junction (and the resultant angles are usually greater than 90°) that the interference drag is less for the V-tail. This is very likely true. But for a V-tail to have the same control force as a conventional tail, the V-tail needs more surface area. This adds surface drag (skin drag, wetted area).
https://en.wikipedia.org/wiki/Parasitic_drag

Most model designers used the theory of projected area to size their V-tails. This almost alway results in undersized V-tail and the resultant poor control authority we often hear about with V-tail.

Again a properly designed V-tail offers no reduction in drag. Now what it does do is keep the tail out of the weeds, making for a more durable model. It can also result in a lighter tail section, but that getting real deep in to the design aspects, and often not seen in practice.

By Free Flight (FF) design elements I assume you are looking at somrthing like this.

Trotter HLG KIT

The Trotter HLG is a traditional hand launch glider and has been designed and constructed by a Croatian hand launch glider team. The team has successfully campaigned the Trotter and have taken home numerous medals as a result. The wing construction utilizes a carbon-kevlar D-Box (the kit...

alofthobbies.com

There is no reason this type of FF construction can't be made very aerobatic.

All the best,
Konrad

Edit: Found better link

 

[Konrad]

Staying with low smell building (Wife is home with me) I’m building the Minus. Here are some rough points in the instructions. First I had a heck of a time finding F-10. It was not on Sheet “F”. You will find these on the 4mm thick Sheet “A”. Also F-10 is glued on to the top of the sheet. They are not glued on the top of the fuselage (note triangle sticking above F-10 in the photo). F-10 needs to be notched (1mm x12mm) to clear the ply doubler. There is a difference between the left and right sides of the fuselage, make sure that the doublers reflect that (note the offset for the wing hold down tabs). When building the wings verify these offsets for the wing hold down tabs.

All the best,
Konrad

 

[Hank GB Z]

Konrad,


Nice to see you working on this glider. I pulled my Top Model Minus Acro kit down the other day to look at. I haven't started it yet but maybe if i can get the Pirat wing assembled over the weekend i'll make room on the bench for the Acro. It'd be cool to be able to fly them side by side.

Hank

 

[Konrad]

Ah, the Pirat! I should finish mine. I'll go up in the attic and look for her. That should be a "fast" build, yeah right!

All the best,
Konrad

 

[Hank GB Z]

Yeah, I thought the same thing about many models. Someday it'll be true then it'll be a boring build i suppose...


Hank

 

[Konrad]

We could follow the manual or build shoddy sloppy models, but that isn't our style.

 

[Hank GB Z]

Yeah the last model I built totally stock was a Stevens Aero C-140 micro kit. It died in a mid air with a helicopter. I often wonder if it was the aviation gods smiting me..

 

[Konrad]

A few more minor issues (notes) with the instruction.
Please note that the nose block parts are not symmetrical. Fit all four to find the best shape for the nose block prior to making up the laminated nose block.

Other issues noted in photo.

 

[Konrad]

First real issue with the kit. The laser cut holes in the bulkhead appear to be cut for 1.6mm PTFE tubing. In my production run the kit is supplied with 2mm tubing. I should have check the fit between the bulkhead holes and the tubing before I installed the bulkheads. Drilling the holes in situ was a chore.

I’m still not comfortable building “in the air" with these snap together type laser kits. I'm use to datum lines and fixtures. It was a bit difficult keeping things straight as the fuselage transitions from a rectangular cross section to a polygon. As a result I do have a slight twist in my fuselage. Nothing that a little steam and block sanding can’t take care of. In fact it is so slight most would think it was just distortion from the lens if I tried to photograph it.

I’m also concerned that there is no mention of how to prep the V-tail mounting area for the V tails.

 

[Konrad]

A face that only a father could love!

While I was sanding the fuselage the wife came into my work area and said “Ooh that looks boxy and ugly". I was shocked, I thought the fuse was looking cute. She said "Nah, just because something is small doesn’t mean it is cute". And then she points to the virus, small but not pretty!

OK, this is the issue with a lot of laser cut kits. The laser really only work in 2D. Also because the laser can control the tolerances so well most laser kits are snap together. As a result things do end up boxy. I’d like to see more kits made with rounder bulk heads and pre cut sheeting to give these models more shape. Rounding corners and nose blocks is fine. But there is a lot more potential to be had in today's laser cut kits for curves.

 

[Hank GB Z]

I think it’s looking good. I hear ya on the build in place method. For the Mint 2 I drew a line in a piece of paper to use as a datum when pulling the sides together. I still ended up with a twist but it did help ease any banana shapes happening.

I hear ya on the sheeted curves though. The SA cap 232 had great cut parts for the turtle deck. If you can do it for a turtle deck you can do it for the rest of the fuse.

Hank

 

[Konrad]

Ah, the older SA models. I have an Edge 540. The downside is it gets a lot of its shape defined by the plastic cowl. And the gear is week for my style of landing.

I also have the very round Aerolock Models Pitts M12. Lots of round bulk heads laser cut 1/32 sheet and lamination to make the round cowl. change the LG to carbon and she is my kind of ship!

 

[Wayne]

I received some bad news last week. The Minus have been discontinued. So if you got to have one, we have a few Acros left in stock. I think icare in Canada might have some too?

 

[Konrad]

With this shelter in place order I'm sure many of you guys are burning through your inventory of spare servos. With the sad news about the Minus line I just placed a large order for servos. Adding an Acros almost paid for itself with the discount.

I can't say how these birds will fly. But I can say they build nice. Great fit and well thought out kits. Yes, there is a bit too much sanding to get the airfoiled wings. But as the wings are small this really isn't too bad.

So get your Minus while you still can!

All the best,
Konrad

 

[Wayne]

Konrad you are right.. We are getting cleaned out of a lot of gear. We have open orders with all of our major manufacturers as well as our less majors and a number of new suppliers. I don't think I have ever had this many open invoices. Covid is really slowing the supply chains. Luckily some of the orders are starting to roll in.

 

[Konrad]

Good to see thing starting to fill in the pipe line.

I was talking about your customer's own private inventories. I was hinting that they might want to bring these back up to regular levels as it will take time for importers like you to get things back to normal. I'm suggesting that while placing their orders for servos that they get a Minus Acro. If the order is large enough the kit will pay for itself, with Aloft's combo deal.