Optimizing Wing Aerofoil for UAV long range flight

Vince Hogg December 21, 2012 at 11:27am

Once again Joe, lots of info, thanks. I have been laminating models and full size for many years and have even had a demonstration by Burt Rutan at Mojave back in the 80s. I still see new ways that are an improvement on what Ive been doing. Peel ply hinges are a new one. It must be easy to accidently cut through them, unlike Kevlar that will stop a chainsaw? Even thin Kevlar is thick :/ and leaves a step in the surface. Will try that idea. Do you try to stop the resin soaking in to the hinge line? How do you cut the skin and not the ply?

My recent cores are cut using gravity with weights and pulleys and am getting good results. I used to resin the cores but dont anymore. When I try to pull the skin of a crashed wing, even the driest layup will take the foam with it. I just leave the cloth very slightly wetter now not painting the core. Regarding my carbon tail. Its not at all dry but if you catch it at the right angle you can see a slight dull weave pattern, not a big deal but when it is first separated its like a mirror.

Cutting cloth – my workshop is such a mess, it takes an hour to clean the table for cloth cutting – but once clear I suppose I should cut more. I guess you cut bigger than the mylar and then trim once wet? Probably a silly question, I don’t know any other way.

Regarding the motor current. Beer mat calculations – L/D about 10, therefore drag about 5N x 15m/s =75w. A =W/V 75/18 = 4A 50%effcy –about 8.

Sort of makes sense and is backed up with brief data.

I have just posted some motocalc stuff on my blog about the new motor. Also check out my new fuselage idea. Im always suspicious of motocalc because it gives lift at various speeds, like its doing some sort of constant AOA calcs. I wish they would use lift =weight.

http://diydrones.com/profiles/blogs/development-of-a-composite-twin...

Gisela & Joe Noci December 21, 2012 at 7:19am

Thanks for the info Vince. I will look at that motor and compare to my Hacker A40. I must admit I am surprised at the low amps (40) with the 16X10 and 5 cells. The Hacker A40 ( 430KV) draws 58amps with a 13X8 Aeronaut CAM folding prop - cant remember the static thrust, although static thrust not mean much. I do know the prop type makes a huge difference - I used some of the APC electric props and they drew 56amps, with only two thirds of the static thrust of the same size CAM props. It is not easy to compare like that - you have to fly and fly in steady state calm conditions to compare.

I note you are flying at 15m/s - only problem I guess is that your landing speeds are a bit high at that weight? somewhere around 11m/s? Similar to mine with the Clark-Z.

Vince, why do you find it difficult to cut the glass/carbon cloth? I use a melamine table top ( these tops are about 900mm wide and 4meters long and form the work tops...), lay the glass on it of the roll and lay a straight edge along the line I wish to cut - I use a piece of aluminium flat bar, 50mm wide, 5mm thick and 3m long. I then use one of those OLFA ( maybe even STANLEY make them) rotary blade cutters - just put the blade up against the straight edge and roll the cutter firmly along - the cloth is cut very neatly. You have to replace the top after hundreds of cuts though - the cloth does not cut so well after a while as the fibers hide from the cutter blade in all the nicks in the table top..

I am not sure I follow re the fibres showing through - If actual loose fibres begin to show then there is severe resin starvation, and as the resin breaks away from the fibres they start showing. Far to little resin then..If I do a carbon layup, I only finish with a glass surface tissue if the carbon is a heavy cloth, eg, 300grams maybe, and if the surface is not cosmetic, ie, it is going to be painted. If carbon is to be left unpainted, a glass layer tends to leave the finish slightly pale or milky looking- it lacks that 3D carbon finish.

And again, dont bother with kevlar - it is a pain to work with if its not needed...I use peel ply for hinges - have never had one fail on me. Very light and easy to implement.

And as a final, the way I do my 'best' layup process - Sand your cores with 200grade sandpaper. Make a wooden block, about 140mm X 100mm X 30mm. Fit a piece of 6mm felt to the underside ( the 140X100 surface.)

Take the sheet of sandpaper - it is of a standard size so that quartering it fit one quarter nicely to said block - with two 1cm strip folded up along the long edge for finger grips. Grab this sanding block and LIGHTLY sand the core - just use the weight of the block to do the work, do not press or force it. Wear a dust mask...

When the core is SMOOTH, clean it of all dust.

Then with your glass in the mylar sandwich, wet the glass with resin - wet it such that there are no visible dry spots - they appear white instead of translucent. Set aside.

Now, take a paint brush and wet the core with resin, till it just shines - do one side only. Now go back the the mylar sandwich - place flat rolled out paper towel on the wet glass, and using a laminating roller, gently, firmly roll over the towel, soaking up any excess resin from the glass. Remove the towel and repeat once more.

Now take fresh towel and lay it on the wet resin on the core, roll it down gently, and remove. Do the same for the other side of the core. The core should have no shiny resin spots visible. When all done, slip the core in the sandwich and process as usual. this will give you the lightest structure possible. Resin only adds weight not strength. BUT with this method you must use a surface tissue on the outer layers, else the pinhole Gods will frown upon you...

I would like very much to ditch my wheels - 280grams and lots of drag...but how...

Joe

Gisela & Joe Noci December 21, 2012 at 6:39am

Martin,

No, what we call tissue is a plain weave, sort of 1 strand under 1 over style, we actually refer to it as 'surface tissue' . It is very thin and conforms very easily to compound shapes, and looks like fine silk. Gives a very good surface finish with very little resin content. I guess tissue is misleading as tissue paper has no weave as such..

Joe

Vince Hogg December 21, 2012 at 6:21am

Jo, this is my new motor, in the 460kv model

http://pulsosystem.com/motor/4120.html

With ESC

70A+

http://pulsosystem.com/esc/a2.html

all with a 16x10 prop. If I remember correctly it gave 4.2kg static thrust 40A at 10,000rpm (cant find my figures right now, it may be on my blog(sons blog Leo Hogg)).

I have had problems getting it to track nicely so haven’t done any long 'missions' with nice straight legs. I think Ive got to the bottom of it now using HIL simulation to 'practice' with the PIDs.(probably nav roll too low - was 0.4, now 1.1). Therefore my data is not great for cruise amps as its just zig-zags around my field.

I have set a cruise of 15m/s which seems to average about 8 amps but its hard to tell really. I wish Mission planner had a function to highlight a graph section and get average values. I know 8 amps on a 5ah battery doesn't last an hour but not far off. Also my ebay battery, claimed 5.5ah - measured 4.2 - pants. Hoping for a break in the weather to get some better data soon. I would be better off with Arduboat.

One of the real chores of making a wing (for me) is cutting cloth. I guess Im being lazy going for fewer heavy layers instead of many lighter ones.

One other question- My tail-planes were made by vacuum bagging carbon/foam with my mylar substitute. Great finish first of all but with handling the weave starts to show. Im guessing its because the epoxy is only a few atoms thick in places. Do you get that, or always finish with a glass layer? The only place I will have Kevlar now is in the hinges. I use 45deg braded tape. Seems to work great but am wondering about its fatigue life.

I know you would have considered it, but if your scratching around for more range, leaving the wheels behind would work wonders. However I have seen the surface you have to land on! Catch net?

I dream of aluminium wing moulds.

Vince

Martin December 21, 2012 at 6:07am

Joe, by tissue do you mean a non-woven fabric that has fibres aligned in random directions?

Gisela & Joe Noci December 20, 2012 at 9:14pm

Hi Vince,

1.8kg for the wing is a lot. However, your wing is structural as well, supporting the tails booms and tail, so reinforcement is unavoidable there. I think a 160g top and bottom first , then a 100gram from root to tail boom mounts, covering the boom mount hard points, and then a 46g layer top and bottom - with a carbon LE strip under is all. This makeup will be strong enough and should cut 500g to 700g from your wing...

Carbon is great for wing surfaces - it is far more resilient to finger dents, but it is less forgiving under impact than glass - it shatters more easily. A combination is usually a good way to go.

I never use kevlar for these sort of structures - it is to costly and the strength is rarely needed for such applications - good for high G gliders though! It is just far to difficult to work the material after lamination.

Vince, tell me more about your setup please -

What sort of motor, what is its RPM/Volt rating, what size and type of prop, etc.

At 5kg AUW, with a 5cell 5AH pack, half charge left after a half hour flight is VERY good!

I use 2 X 4cell 5AH packs, a Hacker A30-12XL V2 motor ( 700kv) and AeroNaut CAM folding prop - 12X8-

At sea level, cool air, I manage 40minutes (just...) at 4.6kg.

Are you flying with autopilot? Do you know your throttle PWM value ( in milliseconds) during normal straight and level cruise?

There is very little user info on these elements - people do not seem keen to share info on motor and prop combinations, battery choices, performance, flight duration, etc...

I am about to try the Hacker A40 motor - a 430kv motor, and want to try larger props - around 13X8, up to 14X10 - larger slower props are more efficient, but at what stage ( size, rpm) does is actually begin to make a worthwhile difference...

Joe

Vince Hogg December 20, 2012 at 3:03pm

Joe, thanks for all that.

You are not wrong about the extra joints etc for a twin boom. My wing 'saddles' that carry the booms took a many hours to finish, and the bolt pads in the wings add weight.

I am rather embarrassed to say that I used 2 layers of 160g glass for the main covering with lots of carbon/Kevlar reinforcement in other places bringing the whole 2.2m x 30cm wing to 1.8kg. At least I can go surfing with it. The fin and stab are 290g carbon on shaped 6mm foam which turned out very nice. I dont think its worth airfoil tails at these Re. My AUW with a 5Ah 5s lipo is 5kg. Ive had 30min flights with half the charge still remaining, after getting a new motor - check out Pulso, very pleased with it.

I still favour keeping the nose clear so the other option for mk2 is a twin motor. My blog has a solidworks image of that. I think it could be simple and light. It would use the same fuselage pod but use a single 25mm carbon tube. I hope to get a fuselage plug 3d printed soon.

I make a big mistake with fuselage no1, constructing it with lost foam method, knowing full well it would need sanding on the outside and I used Kevlar. That stuff just fluffs up and never goes smooth.

If your mylar is a bit milky and slightly green, Id say you have the real thing.

Vince

Gisela & Joe Noci December 20, 2012 at 2:15pm

Vince,

The Clark Z seemed to have slightly less drag at RE-400K and Cl around 0.35 but I think they are so similar that either would do.

As you agonized over wings, so did I over the A/C type - I would have preferred a twin tail boom , inverted V tail, pusher prop. I have a few applications where a camera in the nose would be better, but I have been quite unable to get the weight and complexity of such a format down to acceptable levels. There are just too many mountings, screws to lose in the desert sand, brackets, tubes etc...And the wings need a lots more work to build in hard points to mount the tail booms to. Packaging the bits in a neat package is also complicated = the inverted V tail has complex mountings to the tail boom at well, and then there are servo connectors from each V tail half, to each tail boom, to the wing, then to the fuselage, and then to the autopilot.far to many extra bits and time to makes them, with added weight. So I have a gimballed ball that protrudes below when I need such an application.

Regarding the Mylar - I believe it is 'mylar' but I cannot tell you more - I purchased it from the composites supply company in South Africa, where I get all my cloth. They sell it specifically for this task. I am surprised you cannot find it in the UK - I think South Africa is at best a little behind the UK in that arena? The Mylar I have is a non translucent milky colour, maybe with a green tinge to it. - very shiny on one side, and slightly less on the other. Have you tried a drafting supply house?

Regarding weight of carbon versus glass - You can easily obtain a very light weight construction with Glass.

The lightest wings I have made are with glass -

a 46g ( per Sq meter) tissue wedge at the root , top and bottom- for a 1000mm span and 300mm chord, the wedge is 300mm long at the leading edge, and 150mm at the trailing edge - about 12grams glass and 6 grams resin

a 90gram carbon strip around the LE, overlapping 40mm top and bottom- about 10grams carbon and 6grams resin

A 46gram plain weave tissue layer top and bottom of wing on the outer layer - about 26grams glass and 10grams resin

A 100gram twill layer cut at 45deg, top and bottom - about 70grams glass and 50grams resin

A strip of 90g carbon, the width of the aileron, on the top of the aileron - 8 grams and 5grams resin.

Thats about 200 to 230grams for covering. The foam core for such a wing , in Clark Z, is about 110grams. End caps add another 15grams, so around 280grams tops. And this gives a very stiff and finger dent tolerant wing.

I also use a fibreglass tube, half span in length, as a 'spar' but more as the tube into which the wing joiner tube slides. This tube is bonded into a hole in the wing span, at about 1/3 chord from LE. This tube is around 40grams.

Doing this all in carbon -

You would need to use 100gram or even 120gram carbon to have a surface not to sensitive to finger denting.

You would need only 1 layer top and bottom, no wedge. You still need the LE and aileron carbon strips.

The down side is that without the 46g Tissue, the surface can suffer badly from pin-holes, which makes painting a huge problem - you have to fill the pin holes and that adds huge weight.

So, either you add the 46g Tissue, in which case you may as well stick with all glass, or you add more resin to prevent the pin-holes. The problem is that the foam core surface, especially after sanding smooth, is slightly porous. When the wing is bagged, the resin soaks from the cloth into the core surface under vacuum and pinholes result. The glass tissue is very fine, and the resin fills the weave easily, leaving few, if any, pinholes.

The carbon wings I just made, are overweight - I used some spare 140g carbon twill, with 70% resin by weight, so that is heavy, and they are way to strong. 100g carbon and 60 to 70% resin by weight would be about correct. The extra 20 to 30% resin is less than the weight you would add by painting after bagging. If you can paint the mylars before bagging, that will save lots of weight, and the pinholes will not be so visible, so you can use less resin. It is really a mixed bag - I would like to use carbon and leave the carbon finish unpainted - I believe the weight saved by not painting makes it worthwhile - my problem is that the black surface in our 40deg C plus summers becomes very hot in the sun, and will delaminate..

Paint adds a huge amount of weight! If you have to fill pinholes with filler, prime with primer, and then paint, bank on adding another 20% of the glass/resin weight, in paint. Also, do not use these high coverage grey primer spray can primers - they cover very well, but as they are mostly made from stone - grey slate in fact, they are HEAVY!

Good Luck!

Joe

Stéphane Van der Merwe December 20, 2012 at 1:04pm

I'm very interested in what your final conclusions are going to be! (will start building a twin boom myself soon, can't decide between rcgroups and here to put my build blog hehehe)

Gustav Kuhn December 20, 2012 at 11:25am

Vince, I only have one "boom and pusher" type foamie, so only personal "feel" on this.

It sucks.

Since I'm too stupid to write my own code for the APM I'm using, I'd rather stick with airframes and tuning, that is proven.

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Optimizing Wing Aerofoil for UAV long range flight

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