Minus Thermic, Precision Products (TopModel)

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Konrad

Active member
Jan 23, 2018
705
41
San Francisco
#1
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.
 
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purview

New member
Nov 5, 2018
7
0
#2
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

Active member
Jan 23, 2018
705
41
San Francisco
#3
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 lower the aspect ratio the closer to 90° the V tail 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° 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.
 
Nov 5, 2018
7
0
#4
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
 
Jan 23, 2018
705
41
San Francisco
#5
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 use 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.
https://www.clm-pro.com/models/kits/hlg-trotter-kit-hybrid-d-box

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

All the best,
Konrad
 
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