Thursday, 29 May 2014

Red Sloper

Here are the specs for the Red Sloper fuselage, designed and built especially for the 1.5m depron 10% chord wing detailed in the previous post. 

Aileron servos - TGY 9018MG metal gear micros
Elevator and rudder servos - HXT900 9g
Receiver - Hobby King 6ch
BEC - Hobby King 5A

The 750mm fuselage is a square tube 50 x 50mm outer size constructed using Flite Test style joins rather than ExAir style bends. 

A 15mm x 350mm tapered slice is removed from each side, and the bottom bent up and re-glued to close the gap. That gives the tapered rear half.

Horizontal stabiliser is 440mm x 130mm, tapered to 100mm at the ends, and the 50mm elevator is cut from that. 

Vertical stabiliser is 200mm high, 120mm at the base (for a longer glue join) tapered to 40mm at the top. Rudder is 50mm tapering to 20mm at the top. 

The control surface tapers are just for looks, but looks are very important. A daggy tail ruins the overall impression of the plane.

Elevator servo is stuck inside the fuselage near the rudder servo. 

There is no need to have the tail servos mounted forward with long push rods. The amount of weight saving is not significant for a speedy slope soarer. In my normal 10-15kn flying conditions more weight is often needed.

The nose is soft EPP foam with a tongue extending into the fuselage. It holds the battery, nose weight (4 large washers), BEC and receiver. 

Nose is held on with 2 velcro tabs.

Wing tie-downs are 4mm carbon tubes glued in and reinforced with ID card plastic 

Here's the maiden flight of this excellent glider, probably my best design and build so far.

Tuesday, 20 May 2014

Better airfoil wing build

Following on from the Bending Better Depron Airoils article, this is the complete wing build.

It's basically an Experimental Airlines Armin wing with a few tweaks for the 6mm paperless foamboard and a nicely shaped airfoil. I'll mainly cover the steps that differ from Ed's instructions so check out his superior videos for more in-depth coverage.
This wing will be 1500mm (59") wingspan made from two 750mm (29 1/2") halves, joined with a 7.6mm Skyshark P4X wrapped carbon fibre tube spar. 

Airfoil section design
% thickness
I wanted a speedy slope soarer so Max thickness will be a slim 10% of total chord.
Using the information from my previous tests:
No extra former on top of the spar = 19mm thick airfoil, so a total chord of 190mm gives a 10% section.
3mm former on top of the spar = 23mm thick airfoil, so a total chord of 230mm gives a 10% section.
6mm former on top of the spar = 26mm thick airfoil, so a total chord of 260mm gives a 10% section.
For this wing I decided on the narrowest option - 19mm thick x 190mm chord section.

Max thickness location
30% back from the leading edge is what I'm aiming for and ailerons are included in the total chord measurement. 30% of 190mm = 57mm, so the spar will be positioned 57mm back from the leading edge.

Aileron size
40mm ailerons give sufficient stiffness and some aerobatic performance so now we have the overall airfoil dimensions.
Total chord = 190mm with a 150mm main airfoil and 40mm ailerons. Thats roughly a 6" main wing section with 1.5" ailerons giving 7.5" total chord.

Wing build
For this 190mm chord airfoil we need 150mm for the bottom surface, 150mm for the top surface, 40mm for the ailerons PLUS 10mm extra for the leading edge bend. That gives 150 + 150 + 40 + 10 = 350mm

Cut out the wing panels - two 350mm x 750mm rectangles

Cover one side with packing tape and use a plastic gift card edge to smooth it down. Watch Ed's tape covering video for complete instructions.

 Mark the leading edge fold line 150mm in from one of the long edges.

Draw a line 25mm in from the same edge for the bottom layer bevel.

Draw a line along the edge 2mm up from the taped surface. The idea is to leave a bit of thickness here so that the sub-trailing edge ends up 8mm thick. This allows the ailerons to form part of the overall airfoil shape.

Slice and sand the bevel back to the marked lines.

The finished bevel with 2mm thickness left at the sub-trailing edge. 

Make a dent along the leading edge fold line. I'm using my specially designed tool, a rounded off piece of 10mm dowell. I want a reasonably sharp leading edge so I'm making a deepish dent here, maybe 3/4 of the way through. This is the only dent I make, the slimmer airfoil section means the depron doesn't have to curve too much over the top, so kinking is not a problem. 

Clamp down a stiff ruler at the fold line and, using another full length piece of something stiff, bend the depron up to 90 degrees.

Remove the ruler and bend the depron right over, squashing down hard on the leading edge.

Mark the spar position. 30% back from the leading edge is 57mm for this wing. 

Position the spar and formers and mark the positions. The formers are 20mm wide and only one layer with nothing on top of the spar for this wing.

Glue the formers to the bottom layer then apply glue along the top.

Fold the top layer over and hold down to set with a long stiff ruler.
Depending on how hard you push down, more or less bottom curve will be formed. For a flat bottomed wing push down hard on the leading and trailing edges. For a more symmetrical airfoil you can let the leading edge curve up a little. As there is no paper to stiffen the bottom layer, depron naturally tends to produce some bottom curve.

Squirt glue into the sub-trailing edge join and clamp down to set. Again you can decide how much bottom curve you want by holding it down flat or letting it lift up a bit.

To finish the ailerons score down the sub-trailing edge but dont go right through to the tape.

Crack the aileron hinge free over a straight edge then slice off a 45 degree bevel and sand smooth to allow the aileron to swing down. 

Slice off the aileron trailing edge bevel and sand smooth. I draw a line 20mm in and bevel back to that and all the way down to the tape on the trailing edge. 

Here's it is just about done. See how the aileron forms part of the overall airfoil shape, very nice.

Add more tape to form the aileron hinge and make a sharp trailing edge. Check out Ed's Kissing tape technique for more detail.

To finish the wing completely, tape over the open outer ends, fit servos and control horns then tape the two halves together. As this wing has a curved bottom it may not sit flat on the fuselage. Some packing under the leading and trailing edges may be needed to hold it level.

Tape on two icypole sticks where the rubber bands pass over the wing edges.

I'll make another fuselage to match this wing but this one will do for the moment.

1.5m depron wing build video

Maiden flight

Links for materials and information
Experimental Airlines for designs and techniques
Tradewarehouse for bulk depron
Kitesandfunthings for CF spars
My website for underwater photography, kite aerial photography and RC flight

Friday, 2 May 2014

Better airfoils using 6mm depron

After the poor performance of my first 2m motor glider and great performance of the slim wing and symmetrical wing I though it was time to investigate airfoils more thoroughly.

My standard building materials are 6mm depron and 7.5mm Skyshark P4X spars while Experimental Airlines and Flite Test use 5mm Dollar Tree foam so I needed to tweak their methods for my designs.

For inspiration I looked at the Bixler 2 wing which glides beautifully, and the widely used Clark Y airfoil.

The key features are ... maximum thickness is around 11.7% of the total chord and positioned 30% back from the leading edge.

My mission was to come up with a technique to replicate these features using my materials.

Wing section construction

My standard wing building method is to cover one side of the 6mm depron with packing tape. I have used PPS and Scotch tapes and prefer the cheaper PPS.

The inside of the bottom trailing edge is tapered to 1" to conform with the top layer for gluing. I found it was better not to sand all the way down to the covering tape but to leave about 2mm thickness at the trailing edge. This allows the aileron to better blend in to the overall airfoil shape.

I make a slight dent line (with the other end of a pencil) along the inside of the leading edge position to give an accurate fold. Don't make the dent too deep or you will have a sharp leading edge.

I made up airfoil sections ranging in chord length (including ailerons) from 6.5" to 9.5", spar placement 20 to 30% back from the leading edge line with 20mm formers glued either side, and zero, half or a full thickness former on top of the spar.

The combinations that most closely matched the Clark Y airfoil were:

A. 7.0" chord (5.5" plus 1.5" aileron) Spar 53mm back with no extra former
B. 8.0" chord (6.5" plus 1.5" aileron) Spar 61mm back with an extra 3mm former layer
C. 9.5" chord (7.5 plus 2" aileron) Spar 76mm back with an extra 6mm former layer

To my eye these sections looked much nicer than my previous builds and much closer to commercially produced RC gliders and planes. Depron will kink rather than bend smoothly over the formers if the curve is too great but it's not a problem with these slimmer sections.

These have become my standard wing chord designs now. I would add larger 2" ailerons for aerobatic models, especially slope soarers. Varying degrees of airfoil section symmetry can be built-in by varying how much you press the wing down flat while gluing up.

A symmetrical section gives better inverted and aerobatic performance while a flat bottomed section gives more lift and better gliding performance.

I used the B section in my next project, the wonderful 2m slope soarer.