RC India

In this thread I have tried/will try to to capture the whole exercise of design, construction/build and trimming of scratchbuilt
Requirements
Wanted a good thermal and a slope airplane… not necessarily aerobatic but agile and could stay in the air for a long time.
Structurally wanted maximum use of locally and easily available material. Should be easily transportable. Easy to build.. span around 48” to 50”. Based on previous experience hot wired EPS(thermocole) covered with paper would be chosen construction method. This keeps the weight down and gives a good airfoil adherence.
Planform
•   Decided to do in for a flying wing on basis of lift/weight characteristics and less drag. Though the “wing” of a wing and tail design would have been more efficient, it was not chosen keeping in view wing area vis-à-vis weight  and having a high aspect ratio when working with local materials i.e. EPS (Thermocole) as the construction material. (balsa with covering was out of scope) . Reynold number is estimated would be around/below 80000
Construction method
•   Based on previous experience hot wired EPS(thermocole) covered with paper would be chosen construction method. This keeps the weight down and gives a good airfoil adherence.
Design
•   Did not want to go with an absolute plank design primarily due to pitch response issues when having a high aspect ratio in a ‘wing’ configuration and without having additional trim elements, and also because of tradeoff of  C_L with drag.
•   Wanted to keep the sweep angle  at the min required, 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 structural / construction issues (wanted to have a single airfoil template construction method instead of two template method)
•   Airfoil, wanted to choose a low Reynold number airfoil which maximises C_L  and which has minimum C_M.  The   C_M aspect comes in as one of the design parameters was to have the basic minimum sweep and having no built-in twist in the wing. 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, btys 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 podded fuselage which would aerodynamically house the motor, esc, bty and Rx. And also provide sufficient leeway in achieving the correct CG, without having to have dead weight, by forward/rearward placement of the bty.
•   The above also gives 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 (wing tip mounted split rudders were ruled out because of complications)

Construction Details
Wing
•   The airfoil being generated using ‘profili-22’

•   The printout pasted on thin formica sheet and cut using scissors. It was reinforced with fibre 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 thermocole 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 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
•   This is a close up of the screw .. note the 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 screw 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 IMP.. 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.

Is there a ready made foam cutter wire set up avilable in India ??

Rahulroy {:)} this is a on hand tutorial for novice like me.
Qn why are you keeping weight on top of sheet?
I saw those Americans just doing without it

•   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/thickness tapering which automatically happens when using single template method of cutting wing cores ... and also some due to kerf due to hotwire

•   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-in.  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

the weights are kept to ensure that the foam block does not move during the entire cutting process.. because if it moves there is a high chance of ruining your whole work

Excellent work Rahul... i see you are not a novice and have a good workbench you've even made a decent hot wire cutter...
May be its your first try at wings but what do you do or make otherwise.

Highly impressed by your work...which plane you are making and do you plan to cover the wings with .... vinyl, paper or monocote...?

Any technique for creating grooves for CF rod... or just the hot wire grove?

@rahulroy31
Excellent. Keep posting progress.
Looked at your profile but did not find your age!
Just curious that's all.
Wonder how many of us know that we are typically flying below 100,000 Re.

@ratssaurabh

By no means is this is first wing cut. He is a seasoned DIY builder and his wing cores come out exceptionally well . I had the honor of owing and flying one of his planes which had a hot wired wing. Flew it without power..on a slope..and it was an amazing feeling.

His creations are something you marvel at.

That is exactly I meant to write his work is exceptional but the topic boss mentioned was misleading.......experiment with scratch built......

Please post your pics of finished crafts....

Beautiful build..
the wings came out so nicely. in pic 19 .its looks perfect airfoil.

It is a own desig, 47" span, root cord 10", Tip cord 5", Sweep 7.25"

The wing will be brushed with thinned down fevicole and covered with wet paper (newspaper to precise) and then painted with acrylic colour

the groove is cut with a soldering iron ... just drawing the soldering iron tip over a strainght line forms a nice groove.. just ensure it does not stay at one place for a long time.. keep it moving constantly

Hello Rahul..Great work as always..

My two cents..The cf rods if placed in an A shape on the wing, give a stronger bond to the entire wing..

C u at the field..

• Covered the wing yesterday ..
• normal newspaper was used for covering (it is more tear resistant..longer fibers, than many kraft papers and hence provides good stiffness and strenght)... glue used is fevicole thinned with 50% water so that it flows easily

• the paper is cut with some overhang on all sides

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• Now comes the messy part... this part requires patience... good kraft skills helps
• 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 it is throughly wetted 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 knowledge helps
• please pardon the background of the pics... since it is messy i do it in a place where it has the least impact and can be cleaned quicky

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• 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

• a pic of the wet covered wing left to dry

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• 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 dure to shrinkage of the paper you would have a natural dihedral... (and shrinkage will always be there.. in fact the covering is taunt and without wrinkles because of the shrinkage)... 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 associated shrinkage)
• ....next stage is fixing the elevons and then painting (acrylic pait would also make it water resistant.... need some information .. how do you make water based acrylics opaque ???...i do not want the newspaper prints to be visible

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excellent work rahul... one query ... will the fevicol stick on the fiber tape on the leading and training edge.

is this the delta Wing plane or you will make fuse and feathers as well....

For me also its the same place to do such work including sanding of thermacol.

good question....I sand the fiber tape very lightly (320 sandpaper)...  and then before sticking the paper swipe it throughly ...even after that it does not stick as well on the tape as it does on the foam.... however since the paper is overlapped to the other side it does not make too much difference.

Right Atul... my previous wing (yellow and black one) had a "A" pattern.... in this one i put a cross cord CF strip on top side of the wing at the apex where the two CFs meet... I could not do a "A" because of shape of the undersurface of the airfoil.. the groove would have to be very deep which i wanted to avoid.. hence had to put it at the apex

Where you get the Carbon fiber rods ?

one of my work ... since it was small and thermacol was strong  enough did cover only at the edges with brown paper.

I get my CFs from Hobbyking / RCBazzar... you are right the paper covering gives great stiffness .. in some of my planes i have used aluminium 'U' channel and some are without spars.. just thermocole reinforced with some spanwise tape and covered with paper

rahul.....really cool. looks like you have had a lot of practice on the foam cutter.

Sorry for the delay in posting the rest of the details

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

here are some pics of the fuse under cosntruction

the wing-tips/winglets are made out of coro.. notice the markings at the base, these help in aligning the elevons and trimming.

the wing is painted with fabric acrylic paint (normal camelin) mixed with fabric medium (camelin) 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. at a 47" span, 10.25" Root cord, 7" tip cord (incl elevon cord) and 7.25" sweep, at 20% CG is at 5.14" ... Notice the markings on the LE which are the CG positions at 15%, 20%, 25%.

The wong with the servos, horns, pushrod and wing-tips fitted... the servo wires are taped up and will... completed

the wing mated with the fuse.. it is tied down with rubberbands

A closeup of the Rx with the servo leads fitted.... notice the velcro stuck on the topside to correctly position the bty

Flew her yesterday

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 vauge 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.

One of the design objective of having a good thermalling plane has been successfully met.. looking forward for some sloapsoaring now... attaching a pictue of her in flight.. you would have to zoom to see her... she is a dot in the sky

wow! Stunning!

Lovely.... Thanks for the great thread...