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Old Feb 02, 2015, 02:55 AM
Parkesflyer
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United Kingdom, Ryde
Joined May 2006
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Stiffening flying surfaces

I expect other people use a similar method of stiffening flying surfaces but I thought it might be worthwhile showing the system I currently use as it has had positive results.

Sometime ago I noted that slope folk were stiffening their foam wing ailerons by covering them with light weight glass cloth using foam safe CF glue. I adopted this approach on my own design slope soarers and found it really helped. I also used it for all the moving surfaces of the light weight foam based ¼ scale models I have been building recently.

I then added a cross on the top and bottom of each surface using 12k flat carbon fibre tow under the glass cloth and found that this provided another level of stiffening.

I tease the tow to make it as wide and as flat as possible then stick it to the foam before covering all with the 25g glass cloth.
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Old Feb 02, 2015, 02:57 AM
Parkesflyer
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United Kingdom, Ryde
Joined May 2006
793 Posts
The wind and rain this weekend prevented any flying so I used the ailerons of a 4.5lb ¼ scale Vans RV14 that I am currently building to test how much stiffer the ailerons actually were. The wings are hot wired from some 15kg/cum foam.

The 800mm long x 88mm foam aileron weighed 10g, and this was increased to 26g when stiffened and covered.
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Old Feb 02, 2015, 03:00 AM
Parkesflyer
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United Kingdom, Ryde
Joined May 2006
793 Posts
The foam only aileron was clamped at one end and loaded with a 50 g battery pack mounted on the end of a steel ruler, this provided a total of 125gm with a 300mm lever.

The same test was repeated after the aileron had been covered with 4 carbon carbon tows and 25g glass cloth using Eze-Kote. I have also used foam safe CF and epoxy applied sparingly with similar results. I often use laminating film to stiffen foam structures including powered gliders and slope soarers, and provided the carbon tow is glued to the foam using CF first, the aileron or elevator is stiffened in the same way.

The twist angle was measured and recorded on a photo.
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Old Feb 02, 2015, 03:05 AM
Parkesflyer
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United Kingdom, Ryde
Joined May 2006
793 Posts
The results

Aileron length 800mm 31 1/2"
Aileron chord 88mm 3 1/2"
Airfoil section S8035
Chord width 360mm 14.2"

Foam only weight 10g 0.35 oz
Reinforced weight 26g 0.92oz

Foam only twist height and angle 44mm 25.3deg
Reinforced twist height and angle 8mm 5.1 deg

The aileron twist with the reinforcement was reduced to about 1/5 of that of the original foam only and with very little weight penalty. This can of course be aided in flight by positioning the control horn half way along the aileron span.

Previous tests with glass only had resulted in some twist reduction, but the carbon tow contributes considerably more.

This technique works as well with Depron & EPP sheet surfaces, and in this case I usually use laminating film ironed on over the carbon tow once it has been glued in place with CF.

For sometime I have reinforced the elevator and rudder as well using the same technique and the models are definitely crisper and more reactive. I even applied tow to the fuselage of a pattern biplane with good results.
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