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Big plane - smalr servos

quorneng

Active member
#1
Big plane - small servos

On my light weight versions of the 40" Wing Dragon I ended up using cheap and cheerful 3.7g micro servos.
Many forum contributors advised strongly against this saying they were weak, unreliable and would fail after a short time.
To me the advantage of these tiny light servos is that they can be installed close to the control surface allowing a light and efficient linkage.
I have had much bigger servos strip their nylon gears but after analysing what had happened it was clear that it was the sudden retardation resulting from in the crash that had caused the mass of the long (metal) servo to horn link to overload the servo.

I decide to build a bigger plane using the Depron pod & boom pusher layout I had developed from the Wing Dragon but retaining the same 3.7g micro servos.
The wing is made from 3mm Depron with balsa flanges to the spar.
Ribscmplt.jpg
The skin is 2mm Depron.
The 3.7g aileron servo is almost 'lost' in the wing.
Smlservo.jpg
60" one piece wing weighs 6oz.
Joined.jpg
The fuselage is made of 6mm Depron with balsa reinforcement. The wing is carried on a tall pylon to give clearance for the 10" pusher prop.
Fusepylon2.JPG
The elevator servo in the fin. There is no rudder but the ailerons have quite a positive differential action.
Infinservo.JPG
Completed.
Complete.JPG
The motor is rated at 220W. With a 3000mAh 3s it weighs 23oz.
How well would such small servos manage?
[video=vimeo;44835119]https://vimeo.com/44835119[/video]
Almost straight up and down!
This video was made in 2010. It has had several serious crashes and rebuilds since then yet it still flies with the 3 original 3.7g servos!

I am sure small light efficient linkages and hinges is the answer.
 
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quorneng

Active member
#3
Yes it flies very nicely.
It has a rocket full power climb but in truth it is probably more of a glider than a power plane although it can loop (easily) and roll (with difficulty).
Not a very exciting video but boy! was the air smooth on a freezing cold November day. It was just gliding there is no hill lift or anything.
If you stick with it you will see I gave up before the ground got even close (no gloves!).
[video=vimeo;51868828]https://vimeo.com/51868828[/video]
Now if I fitted a heading hold gyro.......
 
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quorneng

Active member
#4
Big plane - small servos

One advantage of having the wing retained by good old rubber bands is the ease with which you can compare different wings.
The 60" 'Big Dragon' flew and glided well with docile handling and ample stability so it begged the question could it handle a bigger wing?
I also wanted to take the balsa/Depron construction technique a stage further by using thin (0.8mm) but wide (40mm) spar flange as this makes most efficient structural use of the material.
Epplersection.jpg
Although this diagram an Eppler section I chickened out and reverted to an easy to build Clark Y!
The wing under construction.
19ribs.JPG
Despite it full 2m span it still uses 3.9g aileron servos.
ailservos.JPG
The 60" and 2m wings.
BigWings2.JPG
The bigger one is actually slightly lighter!
I needed to make sure the new construction technique and extra span had not unduly weakened the wing so it was load tested. My rule of thumb is that supporting a wing by its wing tips will generate a bending load roughly equivalent to pulling 4g.
The 2m wing carrying a centre load of 18oz, about 10% more than the total weight of the fuselage/motor/tail/battery.
18ozLoadTest.jpg
Not wanting to risk breaking it I did not go any further.
With this wing it is definitely more of a glider than a power plane!
BigComplt.JPG
 
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quorneng

Active member
#5
Due to a couple of crashes the big 2m wing was broken into 3 pieces with one break rather close to the root.
Its complex lightweight structure makes it very difficult to repair and maintain both the alignment and avoid adding significant weight.
However there was enough remaining to create a 'cut down' wing using the outer 22" of the wing with a new centre join.
CutDown.JPG
Retaining the same ailerons does mean this will be more of an aerobatic wing but I have no suitable fuselage to add it too - yet!