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Twin Rudders - The Frog Wren


Active member
The Frog Wren was a pretty 25" span free flight tissue and balsa kit with twin fins.
This was going to be a powered 3ch version but made entirely in 2mm Depron.
The wings are built up with an upper and lower skin to form a Clarke Y section but with no structural spar, all the load being taken by the skin.
The two servos are glued together to form a block and are mounted just behind the wing.
Mono filament fishing line in a closed loop is used to connect the rudder and elevator.
It weighs almost nothing and it works.
The 7g motor drivies a 5x3 prop.
To start with it had serious stability issues. Initially it had to be rebuilt with a big single fin to get it to fly well enough to sort out all the various problems but eventually it was returned to the original configuration albeit with slightly taller bigger fins and rudders.
The completed Depron Wren.
With a 500mAh 2s it weigh 3.25oz and flies really quite well.
With a minute Hobby King recording altimeter on board I wanted to see just how long it could stay up.
47 minutes 30 seconds
It may have looked like I was using a climb and glide technique but actually its just the difference in power of one 'click' of the throttle stick! The motor was running all the time.
315ft may not sound that high but I can assure you that a 25" white plane against a streaky white sky is pretty hard to see particularly when edge on!
Not bad for a 500mA 2s. :)

Now can I get it to 1 hour?


Nice build. You can get 1 hour with strong thermals!

How do you like the HK altimeter? I've seen mixed reviews on it. What is its max sample rate? I could use one of these for various activities for my students.
Thanks for the nice comments.
Actually built like this in 2mm Depron it is actually a bit lighter, rather stronger and a good bit more crash resistant than if built in balsa and tissue.
Fishing line closed loop controls actually work very well indeed on a small light plane whether its made of Depron or not.
I used the same technique on this scale balsa and tissue 24" Sopwith Pup. All four ailerons are operated by a single servo in the fuselage via a scale closed loop system running inside the leading edge of both wings. The elevator uses scale lines to each elevator half.
I would agree that as a technique it is not really suited to larger faster planes.

The HK altimeter works well and is very small and light. It also records temperature.
The sample rate can be set from 8Hz to 1Hz. At 1Hz it can record for 70 hours!
Its biggest drawback is the rather old software needed on the PC to read it. It is not particularly user or Windows friendly to install but is fine once it is.
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I believe in these very small sizes a small amount of under camber can be advantageous but the wing still has to very thin.
So to test this I have made a wing of the same plan form and built the same way with 2mm top and bottom skins but incorporating modest under camber.
The skins are in one piece tip to tip to eliminate joints at the centre for added strength.
It has elliptical dihedral.
Technically flat sheets cannot be formed into such a shape but Depron is just about flexible enough by supporting the wing by its tips and before the glue set hard adding weights to the wing until it had about the right dihedral.

Now to see if it will fly for the full hour! :cool:
The new under cambered wing is rather nicer to fly that the flat bottomed Clark Y version. I suspect it is because the under camber is 'washed out' towards the tips improving its 'wing drop' manners considerably. The one piece skins also mean it can be looped 'as tight as you like' without worry.

In far from ideal conditions (wind almost equal to air speed and very turbulent) I managed a 40 minute flight.
It is possible that in milder conditions I might get close to an hour.


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Just for interest I did an endurance comparison on the same day between the two wings.
There was very little difference between the endurance obtained other than running the battery right down to LVC repeatedly does not do it any good as the maximum times are slowly coming down.

Nevertheless 2hr 31mins 'stick time' in just 4 flights is not bad going. :)
After a few flights I decided that the under cambered wing although stronger and flew a bit slower but otherwise gave no measurable improvement.
I happened to have 2 very small brushed motors with handed 2.5" props.
So use the under cambered wing on a Twin Wren.
The motors as close a possible to the fuselage as I don't know how well matched they are and just to make sure it has a big single fin and rudder.
The basic Wren fuselage is deep enough to protect the props in a belly landing.
The two Wrens.
Too windy to fly at the moment but it makes an interesting noise.
It certainly flies well enough and considering it uses brushed motors it has a reasonable endurance.
Only really built to make use of the motors but at least it does not take up much room to store!