Depron Tailplane. How do you build one?

[David Davis]

Every year I compete in a daft competition for a three-channel French trainer. It is known as La Coupe Des Barons or the Barons' Cup in English. You are allowed to make alterations to the structure and dimensions of the original model providing that they remain within 10% of the original design. I have two Barons, Boris in Ukrainian markings and Bertie with the British roundels. If I were to crash one shortly before the event then I'd have a reserve aircraft. This actually happened last year! This year I built a new wing for Bertie, using depron for the wing ribs. The completed model is fully 4 ozs (113 grammes) lighter than my Ukrainian Baron which is stock and built from a kit.

 

For the 2025 competition I am thinking of building a new lighter fuselage and tailplane for Bertie using depron for the tailsurfaces in order to save yet more weight. The stock tailplane is made up of 1/4" (6mm) balsa. The model will be powered by a 52 fourstroke because I always compete in the fourstroke class and have given a prize to the leading pilot flying a four-stroke Baron for the last few years. I guess that I am quite famous being the only British entrant and a fourstroke devoté to boot!

 

My question is, "How do you make a tailplane, fin and rudder out of depron sufficiently robust to withstand the flight stresses ofr a Baron in competition when powered by a 52 four-stroke?"

 

Pictures below of the construction of a stock tailplane and the depron wing, and of Bertie and Boris ready for La Coupe.

 

My dog is such a diva that she just had to get into the act!

[Futura57]

For such a powered model I'm not entirely sure if strong enough Depron tail feathers would save much weight compared to an open balsa structure. I'd be inclined to try a 'solid' Depron (or similar foam) outline, edged with thin balsa for ding resistance. Add more substantial balsa 'posts' at the hinge lines and to give strength and bending stiffness. Don't over do the balsa otherwise you are effectively in-filling the structure with Depron where there was previously thin air, if you follow my meaning. Then cover with brown paper (and PVA) to really ramp up the the torsional and bending stiffness. Cutting lightening holes in the foam would rule out brown paper covering for a more conventional covering material, though I doubt would save much weight. And it may not be as stiff. Brown paper can form a surprisingly stiff edge on a curved leading edge, so thin balsa edging could be omitted if you were striving to save fractions of grammes and you were careful to avoid hangar rash.

 

Here is a picture of the twin fins and rudders for my current EDF model using the foam and brown paper approach. The thicker balsa post on the rudder is because I shall be using a long double sided horn (similar to on a closed loop rudder) to transmit rudder movement on to a vectored thrust unit. Personally, I would happily use this construction for a 52 IC powered model without fear of flutter ruining my day. I'd be happy enough to weigh and measure the areas of my fins to give you an idea.

 

 

Edited March 28 by Futura57

[Futura57]

1 hour ago, Futura57 said:

For such a powered model I'm not entirely sure if strong enough Depron tail feathers would save much weight compared to an open balsa structure. I'd be inclined to try a 'solid' Depron (or similar foam) outline, edged with thin balsa for ding resistance. Add more substantial balsa 'posts' at the hinge lines and to give strength and bending stiffness. Don't over do the balsa otherwise you are effectively in-filling the structure with Depron where there was previously thin air, if you follow my meaning. Then cover with brown paper (and PVA) to really ramp up the the torsional and bending stiffness. Cutting lightening holes in the foam would rule out brown paper covering for a more conventional covering material, though I doubt would save much weight. And it may not be as stiff. Brown paper can form a surprisingly stiff edge on a curved leading edge, so thin balsa edging could be omitted if you were striving to save fractions of grammes and you were careful to avoid hangar rash.

 

Here is a picture of the twin fins and rudders for my current EDF model using the foam and brown paper approach. The thicker balsa post on the rudder is because I shall be using a long double sided horn (similar to on a closed loop rudder) to transmit rudder movement on to a vectored thrust unit. Personally, I would happily use this construction for a 52 IC powered model without fear of flutter ruining my day. I'd be happy enough to weigh and measure the areas of my fins to give you an idea.

 

 

FYI: Combined fin and rudder covered in brown paper and sealed ready to paint: weight = 14.5g, area = 220cm2

==> 660gM2 ballpark  That's for 6mm XPS foam with 1/4" med balsa hinge posts (10mm and 20mm wide) and 3/32" sq hard balsa edges with foam sanded to slight profile.

I think I've done that correctly.

I believe Depron is slightly lighter than XPS foam sheet, though not as nice to sand.

Edited March 28 by Futura57

[Simon Chaddock]

The way to think of using Depron i.e. thin sheet XPS foam, is to consider it as a stressed skin with an internal supporting structure. This internal structure could include carbon rod(s) for extra stiffness.  

It is likely the resulting structure will be thicker that a simple balsa 'frame' but it can be a streamlined aerofoil to compensate. It is quite possible that a good streamline shape can be twice as thick as a balsa frame structure and still create less drag. It is also likely to be aerodynamically more effective.

Fully covering a Depron structure in some way will increase the stiffness but it can add significant weight. 

How much weight you can save will very much depend on how stiff you want the Depron structure to be. 

If you use Depron as a single slab structure it is likely to be too flexible even with leading and trailing edge reinforcement.

Depron substitution in a load bearing structure is likely to require some experimentation to achieve a weight saving with sufficient strength.

 

As an example the tail plane on my Bombardier Q400. It sits on top of the fin.

All 3mm Depron except for the tapered balsa spar flanges are set flush with the Depron skin to give the maximum possible spar stiffness for minimum weight.

Not exactly a 'simple' structure but strong enough for its purpose as a twin prop airliner and that includes doing a loop!😮