Rc Aircraft - Wing - Build log

The whole exercise of design, construction/build and trimming of a Flying Wing.

Requirements
Wanted an aircraft which could fly long in a calm (thermal) or on slopes (windy)… .
Structurally wanted maximum use of locally and easily available material. Should be easily transportable. Easy to build.. span around 48” to 50”. It would be hot wired EPS(thermocole) covered with paper. This keeps the weight down and gives a good airfoil adherence. 

Planform
•   Opted for a flying wing on basis of wing loading  and less drag, and the fact that I would not have to construct a fuselage.   Balsa with covering was out of scope. Reynold number is estimated would be around/below 100000
Construction method
•   Hot wired EPS(thermocole) covered with paper . This keeps the weight down and gives a good airfoil adherence.
Design
•   Did not want to go with an plank design primarily due to pitch response issues  with a high aspect ratio in a ‘wing’ configuration and without having additional trim elements.
•   Wanted to keep the sweep angle  at the minimum, to have the max effective span and  have the least span wise flow
•   Decided to go in for a elevon induced twist instead of doing in built-in twist, due to construction ease (wanted to have a single airfoil template construction method instead of two template method)
•   Wanted a low Reynold number airfoil which maximizes C_L  and which has minimum C_{M .} Based on these,  MH44, MH64, EH2010 & Tsagi were shortlisted. Though MH44 would have been most efficient in terms of penetration, lift, C_M, there is a construction issue when constructing it with a single template method for the given size i.e. 50” span. The trailing edge becomes very thin and there are high chances of flutter. Hence a airfoil with a decent amount of material near the trailing edge was chosen, hence Tsagi.
•   Ideally would want to have a ‘pusher’ design to ensure that the wings ride clean undisturbed air. However keeping in view CG issues with my existing motors, batteries available the option of having a ‘puller’ was also kept open till the last minute… i.e till such time the wing was completely made.
•   The above necessitated a pod which would aerodynamically house the motor, esc, bty and Rx. And also provide sufficient leeway in achieving the correct CG by the way of shifting the battery.
•   It also opens the possibility to convert  from a 3 channel (throttle, elevon ( aileron& elevator)) to a 4 channel (throttle, elevon ( aileron& elevator), and rudder). This would be helpful in thermalling.  Though it would mean of having to go in only for a ‘puller’ design if wing tip mounted split rudders were not used 

So it turned out as

47" span, root cord 10", Tip cord 5", Sweep 7.25", airfoil - Tsagi with no twist built-in twist 

Construction Details
Wing
•   The airfoil being generated using ‘profili-2’.The printout pasted on thin formica sheet and cut using scissors. It was reinforced with fiber tape to prevent splitting of the formica sheet. A leading edge snout (scrap cf) was added to help in lead-in the cutting wire

•   the wing blanks were cut out of foam using hot wire. Note the wire guides which are used to keep the cut vertical. The wire is pulled down by the weight of the cutter itself and the guides ensure it is a straight vertical cut… see the hot wire cutter in the photograph.. it is made in such a way so that it is adjustable to accommodate a 24” or a 36” or a 48” cutting wire (obviously the current being different for different length and controlled through a rheostat.. basically a resistor type fan regulator)

•   Before sticking the templates a datum line needs to be drawn on the foam. Later on when sticking the templates to the foam the cord line of the templates will be aligned to this datum line. This ensures that the templates are correctly aligned on the foam block. The templates were stuck on to the foam block using double sided tape … ensure that the cord line is aligned to the datum line drawn on the foam and after they are stuck there  is no play.. 

•   This is my simple set-up to cut single template wings. It is basically a piece of thick ply with a hole at one end to accommodate a screw. A strong cord is tied to the screw and the other end secured to the cutting wire. The other end of the cutting wire has a handle attached to it. This handle is just a piece of wood which helps in pulling the wire to keep it straight and guiding it over the template. The bolt and nuts allow height adjustment of the cord attachment.. this is important .. the cord should be attached at a height which is equal to the height of the cord line when measured from the base of foam block.

•   The distance between the foam block and the bolt where the wire is attached is such that the screw should be a few mm behind where the line extending the leading edge and the trailing edge meet. .. THIS IS IMPORTANT.. to ensure that the hot wire exits across the trailing edge at the same time. This is vital to ensure that the trailing edge is proper and there is no kerf.

•   The cutting in progress. It starts off at the leading edge, the snout helps in the initial alignment of the hot wire.  Ensure that the hot wire just floats over the profile. Ensure that the wire is straight.. no lags… it should not be pulled…it should be guided.. ensure that the wire does not snag on the template (that is why the template profile needs to be sanded to a very smooth finish). Extra care needs to be taken when the wire is near the edge of the trailing edge. The hotwire SHOULD exit the trailing edge at the same time. Ensure that the hotwire does snag /pulled back at any stage…. The photograph shows me supporting the electrical wire to ensure this

•   I cut the upper and lower surfaces without moving the foam block.. however  this is not necessary and left to individual preference… for me I did not want to go through the process of realigning the blocks again and again

•   The wing cores out of the blocks…. Keep the left over pieces.. they help a lot in acting as a jig for further cutting/grooving the wing cores.
•   The wing cores would require some light sanding to remove rough edges /ridges … notice the difference in the trailing edge thickness at the root and tip... this is due to the hight tapering which automatically happens when using single template method of cutting wing cores ... and also some due to kerf 

•   The wing root is then recut to ensure that when rejoined, the sweep angle is at 18 degrees. Also the tips are recut so that the tips are parallel to the root or 90 degrees to span… in fact there is a 1 degree toe-in to ensure that the tip winglets have a 1 degree toe-out.  this, is done for more yaw stability .. in case tip winglets are used
•   The wing has one 3mm CF as a spar and two small CF strips near the tips .. the extra stiffness near the tips would help in reducing flutter and also prevent aileron reversal.
•   The wing with the spar fitted/ being fitted… I use Araldite TUFF FIX / Fevicol 1K PUR for sticking most of foam surfaces

·         Newspaper was used for covering (it is more tear resistant (longer fibers) than many kraft papers and hence provides better stiffness and strength)... glue used is PVA (fevicole MR) thinned with 50% water.
·         The paper is cut with some overhang on all sides. 
·         Now comes the messy part... this part requires patience... the thinned down glue is brushed on to the foam wing.... I do this in four parts.. one side, one pannel at a time
·         the paper is aligned with the leading/trailing edge and then wetted with a plant watering sprinkler... earlier I used to dip the paper in water to make it wet, however in that method, most of the time the wet paper used to tear during alignment... hence switched to just aligning the dry paper, thereafter thoroughly wetting with the sprinkler before laying it and pasting it onto the foam.... ensure that there are no air pockets..
·         the overhangs are rolled on to the other side and glued down... this would require some trimming/slitting to ensure that it sticks properly across curved surfaces
·         this is like old time paper covering of balsa.... all those skills and experience helps
·         the process is repeated for all the wing panels... and the complete wing covered
·         the wing at this stage would seem very very heavy.... do not worry, that is because of all the water.. as it dries up it will become light and stiff
·         the wing after the over night drying... no wraps.. very light and stiff... and perfect airfoil

a tip.. first cover the top side of the wing.... this is because if there is a wrap due to shrinkage of the paper you would have a natural dihedral... (and shrinkage will always be there)... if you want anhedral cover the bottom first.. this is due to the fact that the surface covered first dries up first and hence also shrinks first. .. also it better to let the wing dry in shade .... i.e. slowly... fast drying could lead to wraps (due to uneven wetting and drying speeds)

The fuse pod is a simple box made out of coro.. the wing will be fixed to it using rubber bands.. the RX, and bty will be housed inside it.. planing to keep the ESC outside due to cooling issues... finally went for a pusher config

The wing-tips/winglets are made out of coro. The wing is painted with fabric acrylic paint mixed with medium in a 60:40 ratio. this gives it a good water resistant finish.the servos are mounted on the underside of the wing.. this ensures that the top side of the wing is absolutely free of any appendages for more efficiency... a hole is cut in the wing and the servo stuck in with double sided tape and then covered from top with fiber tape which is flush with the surface.. the pushrods and horns are connected

the CG is location is calculated with a online CG calculator .. consider the elevon cord also.

Flew her  

weight of airframe with motor, prop and all electronics except bty = 406 g
AUW with 3 cell 1000 mah = 486 g
AUW with 3 cell 2200 mah = 571 g

initially tested it with 1000 mah and at CG at 15%.. was a bit tail heavy.. had to add a dry pencil cell at front  to get the proper balance.. trims OK  .. climbs at 50% throttle.. felt bit vague in yaw at slow speeds.. the folding prop is not folding during gliding.. 
next was flew it with 2200 mah...this time with CG at 20%... this confirmed the sweet spot of the CG at 20% ... 

It was not a windy day with few thermals, and even under thse conditions and with the heavier bty (2200mah and AUW at 571 g) she could stay up without very very less use of power. In thermals she tracked beautifully and could be flown hands-off when trimmed to circle with the thermals, the reduced sweep helps .. outside a thermal the glide ratio is superb and she retains energy well during penetration..... the additional wt of the 2200 mah below the CG also helped in stability............... a lovely bird and a treat to fly..... the podded design also protected the wings during landing by keeping them off the ground.........only issue is that at large distance orientation is difficult.. not sure how this can be bettered. 

looking forward for some slope-soaring now... attaching a picture of her in flight.. you would have to zoom to see her... she is a dot in the sky