Frog Wren

[quorneng]

The Frog Wren was originally a simple but pretty 25" span, twin fin, balsa and tissue tow line free flight glider, like this.

I wanted to make a powered 3ch version and built entirely out of Depron.

The 3.7g elevator and rudder servos are glued together as a block and mounted just behind the wing.

The control surfaces are driven by closed loop mono filament fishing line.
To operate the twin rudders the loop passes down over the top of the fuselage

out along the tail plane leading edge

Round the front of the fin, over the rudder horn to the trailing edge

Across to the other fin and back in the same way to the other servo arm.

It weighs almost nothing and yet is surprisingly free moving.
The elevator loop is just direct from the servo arm to the elevator horn.

The tiny 7g motor drives a 5x3 prop.

Initial testing showed a serious lack of directional stability so the twin fins were replaced by a big single one.

It now flew well enough to get the CofG and the main plane incidence at the best position.
The single fin was reduced in staged , then bigger, taller twin fins added along with a fixed central fin

and finally with just the taller twin fins.

Although the directional stability is now just about adequate it does fly remarkably well. With a 500mAh 2s it weighs 3.25oz.

This altimeter recording show just how well it goes.

47 minutes 30 seconds
The green line is the temperature.
It may look like I was using a 'climb and glide' technique but actually it was just the power difference of one click on the throttle stick!
Not bad from a 500mAh 2s with power on the whole time!

[quorneng]

The next development, to see if I can extend the endurance to a full hour, is to use an under cambered wing.
Of exactly the same size and built the same way with top & bottom skins of 2mm Depron but each skin is in one piece tip to tip for strength.

The under camber 'washes out' towards the tips to improve the handling.
The dihedral is elliptical.

Formed by supporting the wing by its tips and before the gue set hard adding weights along the wing to create the required shape.

It flies very nicely and a bit slower but it needs a clam day to test if it is really any more efficient.

[BradT]

Nice build, Q. I like the fact that you've included your progression of mods to show how you've fine tuned the model. You're getting outstanding duration with her!

Cheers,
Brad

[quorneng]

I have been asked what such long flights prove and I would have to agree that flying at minimum power for 47 minutes is not very exciting - to say the least.

However what is does show is that with attention to detail to keep weight (but not the strength) to a minimum you can do almost anything with it for an 'average' 15 minute flight without even worrying about how much is left in the battery!

[quorneng]

The under cambered wing although stronger did not show any significant improvement in efficiency so what to do with it?

I happen to have a pair of tiny brushed motors with 2.5" diam 'handed' props salvaged from a toy plane.

So a Twin Wren!

The ESC and radio go under the wing with the rudder and elevator servos just behind as in the earlier Wren.

The battery goes in the nose in the cockpit area.
Can you have a 'brace' of Wrens?

Too windy to test at the moment but it makes an impressive noise.
https://vimeo.com/63571129
As the motors are not particularly well matched they are placed as close to the fuselage as possible and just to be safe it has a big fin and rudder as well.
With the same 500mah 2s at 3.75oz the Twin weighs just 1/2oz more than the brushless single.

[quorneng]

It does fly and considering it has brushed motors and tiny props it does so quite well.
Edited down from a 7 minute flight to include those bit where you could actually see it!
https://vimeo.com/63830883
Needs a bit of tweaking to get the best out of it but it was really only built as bit of fun.