1940’s Thermic 70 flight report - RC Groups
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Oct 03, 2009, 04:33 PM
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nick_cool's Avatar

1940’s Thermic 70 flight report

First things first: why building a plane from a kit instead of buying an RTF one?

There are several reasons about why this time I wanted to build from a kit an old-timer free flight model.

The first is because several senior pilots have similar models in the field, and I have been always fascinated with their flying and their look. They have this kind of magic only achieved by hand made models.

The second reason is that I have been building planes since I was 11 years old (long time ago), and for me it is a relaxing and healthy activity that makes me feel a kid again.

Nostalgia apart, an electric power plant and RC equipment updated the Thermic 70, making it more practical and enjoyable. Sorry to the purist, but I believe that planes are to be flown.

The building log is here:

Last edited by nick_cool; Oct 03, 2009 at 05:09 PM.
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Oct 03, 2009, 05:18 PM
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nick_cool's Avatar
The day was gusty, with average wing of 9km/hr, certainly not the best conditions to maiden this fragile model.

So I check everything and metered the power plant that sweetly amazed me with these results:

AXI 2212-20
Jeti Spin 22
Polyquest XQ 11.1v 1200mAhr 30C or 700mAhr
Prop : Robbe 8x5 folder

The results:
Voltage 11,4v
Current 9,4A
Power 107W
RPM 12.300 !!!

Ready to flight the model is bellow 500g, so the power is more than enough for a the sailplane, and it is at the most efficient point for the power plant.

Here is the blog about the power plant selection:

Last edited by nick_cool; Oct 03, 2009 at 06:54 PM.
Oct 03, 2009, 06:49 PM
Registered User
Very cool....I too like these old freeflights and recently built the Aerocente Thermic 50 for electric power....Flys great too.....Good luck
Oct 03, 2009, 07:11 PM
Registered User
Buran's Avatar
A huge Ziac fan here; your model looks great.
Oct 03, 2009, 07:38 PM
starzak's Avatar
" The day was gusty, with average wing of 9km/hr, certainly not the best conditions to maiden this fragile model. "

Well ......... Don't keep it a secret. How did it fly ? ? ?
Oct 03, 2009, 07:44 PM
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nick_cool's Avatar
Wind went worst with the time and I wanted to maid this bird, so two alternatives were evaluated. To launch it as sailplane and let it sail from 2m to see the behavior, or to launch it with full motor and flight against the wind.
A friend told me to choose the second option because of the wing, and I did it.

My friend ran letting the Thermic abandon his hand, the +100W were sweet but powerful!

Sustaining a gently pull, the plane started to climb at 25° facing the wind, very stable. I pushed the stick to level the model and put the power at 60%. No trim in elevator was needed. Then I gave some rudder to see its effectiveness. The response was very good.

So I started to turn with some concern, because the wind speed increased and I did not know if I could let it out of the downstream. But the behavior of the Thermic couldn’t be better, it turned first to downstream, I let it take some speed, and then to upstream perfectly.

Despite the wind, I turned the motor off and I realized that again no elevator trim was needed to pass from power on to power off, cool!

My Futaba 7UAF Super plus the 2.4Ghz Assan module were great too providing a strong link.

But the wings were not passing a great moment, the flexed a lot, very elegantly, but a lot. So I decide to land.
I made a turn, trim 3 clicks down the elevator to keep the nose down, and got the runaway smoothly ending at 2m of me feet. Mission accomplished!

End of the day. The wind got even worst, so I have to put the model inside my car, I did the same with my Extra 330 that did not leave the land, and returned to home to drop these lines.

The wing incidence was 1°, the CG at 35% and motor at +5°. In a better day I will fine adjust the CG to soar
Last edited by nick_cool; Oct 04, 2009 at 11:01 AM.
Oct 03, 2009, 07:45 PM
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Leadchucker's Avatar
Beautiful model....Well done!! My you and her have many happy hours in the sky together!!
Oct 11, 2009, 07:11 PM
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nick_cool's Avatar
I did the second flight with the wing at 2° and the CG at 40%, and I needed 5 clicks down to trim the model.
Again, with variable wind the wings flexed a lot.

So, I moved back to 1° and 35%, and the flight was faster but smoother.
Oct 18, 2009, 12:02 PM
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nick_cool's Avatar
The third day I confirm that the incidence al 1° makes the Thermic go faster and more steady, at least in windy conditions, it produces less stress to the structure, making the flight more enjoyable. The model can be flown with steady wind or no wind, but variable wind makes it suffer, maybe because the huge dihedral.

Hand launch is not an issue for this slow plane, the best way to climb is to let it advance “fast” instead of pulling the stick and making it loose speed at a high AoA.

I am happy to have the motor in the nose, and not in a cabana, it makes the transition from powered/no powered condition very natural. With the Polyquest 1200mAhr the autonomy is near 20 minutes with motor, so in calm days the 700mAhr pack will be perfect.

The Assan module is great, I am using the small Rx with short antenna, and it has no glitches even flying far and with other 2.4Ghz radios working at the same time. It feels as PCM and it let me use the friendly programming of the Futaba 7UAF.
Oct 19, 2009, 12:21 AM
Registered User

It looks like there is spar damage inside your wing. The dihedral angle at the wing root should not be that large in level flight. You might want to remove some of the wing covering and look for damage to the spars. If you don't fix it soon, the wing will break! It must be difficult to fly with so much extra dihedral. Your model is too nice to crash because of a damaged wing!

Oct 19, 2009, 12:54 PM
starzak's Avatar
Originally Posted by mlbco

It looks like there is spar damage inside your wing. The dihedral angle at the wing root should not be that large in level flight.

Definitely a lot less dihedral in posts #1 & #2. Steve is right Nick.

Be careful !

Oct 20, 2009, 07:55 AM
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nick_cool's Avatar
Thanks for the advice. I will take a look again.

I tested the wing without the covering putting a 400g in the center and it did not break.

The bending is distributed along the wing, and not concentrated in a single point. The reinforcement is stepped, meaning stronger in the center, and lighter in the tips.

So next time I will use harder balsa in the spars and maybe C.F. too.
Oct 28, 2009, 05:55 PM
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nick_cool's Avatar
This plane turns the birds crazy because they see it as a fellow bird flying over their “homes”. I have to deal with tero’s angry at every landing.

I prefer the CG between 35% and 40%, it is windy here. With the cg at 40~43% I mixed throttle with elevator (down) 10%.
If you are used to flight with ailerons (I am used to use both coordinated, rudder ands ailerons), it is friendlier to mix ailerons with rudder.

To keep the plane in land, I tied the winds the way I show; it avoids stressing the stab because even light wind makes the Thermic roll.

With the structure reinforced (wings and fuse), this plane is ideal to learn to flight with a minor help. Fact that it is not surprising been a free flight model.

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