Replies to This Discussion

Permalink Reply by Jake Stew on July 11, 2012 at 8:23am

I've read that 100w/kg is a reasonable figure.

The way I do my holes for the wing CF tubes is to use a metal tube, heat up the tip, and run it through the wing.  You need to make a jig to hold the tubes right.  Take a table or your bench and screw down or clamp a board across the back to act as a fence.  Then use/make some wing ribs for patterns and drill the tube holes in a block of wood sized similar to your rib pattern.  Fix that to your table, insert the metal tubes, heat the end with a torch then run your wing along the fence stabbing the tubes into the EPS.  You may have to do this more than once depending on your wing length.  You can also use the ribs to help support the tubes and you should glue one on the end of your wing facing the tubes to help with alignment.

Make sure everything is right and your tubes are secure and level.  It's not hard to do if you get everything aligned right and secure.  Hollow aluminum or copper tubes are usually not hard to find at the hardware store.  Copper will bend if you're not careful and aluminum doesn't hold up as well to the heat.  Both conduct heat very well, so it can be a trick to get everything to the right temp.  The heat flows away from the tip very fast, so you have to heat gently at the tip for a bit to get everything somewhat hot, then heat harder to get the tip hot, then slide your wings down quickly at a medium pressure.

It should go without saying to do this outside or in a well ventilated area.  Burning EPS stinks and is probably not the best for you.

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Permalink Reply by Richard on July 11, 2012 at 10:26am

Hi Jake, cheers for the info, I am going to need to make 2 holes in the wing for the tubes, one 1/3rd from the leading edge, and one 1/3rd from the trailing edge. I guess i will need a hole all the way through the center wing section with a tube in it as well. How far do you think i will need to go into the outer wing panels to ensure a good strong join? I am thinking around 200mm?

Tubes laid into outer wing panels = 10mm outer dia, 8mm inner dia. 1mm wall thickness

8mm external diameter tube 1000mm long slides through entire center wing section and 200mm into each outer wing panel

Visual showing tubes in wing... 1px = 1mm (when viewed full size.)

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Permalink Reply by Jake Stew on July 11, 2012 at 11:25am

I would put your tubes all the way through the wings.  Many designs I've seen use tube(s) about 75% of the wing length.  However, since you're going to use multiple tubes I think it would work well to go all the way through and put a cap on the outer ends of the wing.  That's just my idea from a design I'm working on at the moment.

A more traditional way would be to go about 75% of the way into the outer wings and have them meet in the middle.  Essentially reverse what you have in the drawing and put the tubes in the wings rather than the center.

There's lots of ways to do things.  You could also use larger tubes in the middle and have tubes from the wings slide into them.

Since your flaperons/ailerons are right at the end you're going to have a lot of force at the ends of the wings.  EPS isn't very strong so you will need some sort of spar or support or they'll just break off in flight when you try a maneuver that puts force on them.

I'd also consider moving your ailerons in a bit.  You don't want the tips to stall before the wing does.  When that happens you roll very quickly towards the stalled wing.  

If you're trying to roll right the left wing is trying to provide more lift and will stall before the middle or right wing.  Once it stalls and loses lift you will roll left (instead of right).  That's why flaps are always closest to the middle.  You don't want your control surfaces stalling, you want the wing to stall before the tips and/or control surfaces.  When that happens the whole wing is stalled and your control input will still work how you expect, which makes for easier recovery.

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Permalink Reply by Richard on July 11, 2012 at 12:16pm

Jake, cheers for the ongoing help, it is really valuable to me! as i have no experience in scratch building airframe's like this.

I only need the tubes for strength in the join between theouter panels and the center wing section. The Carbon strip should provide strength the whole way along the wing. It is in the picture below. Or do you still think i need 2 full width tubes as well end to end? The longest i can get is 2m long tubes, @ £60 each! was hoping to keep this build below the price of a Borjet maja. 

Do you think the carbon strip is not enough to stop the wings from snapping?

I have also added flaps to my design, and moved the ailerons in a bit from the tips, to prevent tip stall.

My only concern now is that i will not have enough aileron authority as i have reduced the size of the ailerons, what do you think?

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Permalink Reply by Jake Stew on July 11, 2012 at 1:27pm

I don't know much about the strip strength.  But it looks reasonable to me.

I would bring your ailerons in a bit more and combine the ailerons and flaps into flaperons.  That should give you good control authority.  I'm no expert on control surface size ratios, tail moments, or the like, so I usually try to do something similar to existing designs and then tune from there.

If you find later that you need more flaps or control authority you could also make the ailerons larger chord-wise.

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Permalink Reply by Richard on July 13, 2012 at 9:18am

Hi Jake, cheers. Yeah i may unify the surface area and make flaperons, less to go wrong then, If I only have a single servo per wing. 

I noticed the rq 200 shadow uav has large flaperons, and they do not go to the wing tips. It also has some stabilizers on the underside of the outer wings. I dont think I will need those, I plan on using a beefy motor to get out of trouble!

R

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Permalink Reply by Richard on July 17, 2012 at 12:36pm

After a bit more research, i have yet again refined the measurements - and rounded the front of the fuse to give more aerodynamic performance. I have also aquired the alloy tubes for the wing. (Am using a EPS wing I have already reinforced with spruce from a previous project).

New measurements (click on picture to read measurements)

The fuse for this build will be 150mm wide, not 170mm as in the measurements drawing. 

And some pics. the alloy tubes will run 200mm into the wing panels from the center wing - 8mm Carbon rids 1000mm in length will run right through the center section and into the outer panels to form a join. Still unsure how to fix the wings on for flight.

Fuse has been reduced in height by 30mm..

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Permalink Reply by Richard on July 18, 2012 at 1:26am

Slots cut in wings, and 10mm x 10mm Alloy tube ready to be cut and glued into wings..

i was thinking of laying 2 thin bits of ply over the alloy outer wing tubes to sandwich them in and connect them to the Spruce spa.

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Permalink Reply by Marcelo Gomes on July 18, 2012 at 5:31am

Wow, it's beautiful.

Following your project worklog.

Good luck!

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Permalink Reply by R. D. Starwalt on July 18, 2012 at 9:42am

Regarding fueslage mounting: Could you cut a rectagular section from the fuse sides, incorporate the cut part into the wing (cutting out the airfoil profile) and then reinsert the assembly back into the fuse?

This way you could use nylon bolts entering from the top, through the fuse edge (reinforced section of course), through the fuse/wing section, and finally into the lower portion of a reinforced fuse.

Insertion of the wing/fuse assembly would have to be from the rear, above the motor.

Definitely something to sort out before committing too much effort into the fuselage.

Good work!

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Permalink Reply by Richard on July 19, 2012 at 6:12am

Hi R.D., I thought about doing that, but i want to keep the central wing section as strong as possible, so have opted to insert it right the way through the fuselage. That way I will only have a single join on each wing panel. 

It would be good to make the entire left and right wing removable though, and the extra fuselage space would be good.

I am currently struggling with wood work! I cant seem to use a coping saw to cut wing profiles. And am considering getting a small band saw or a scroll saw to do this - does anyone have any experience with these power tools for cutting 4mm ply wing profiles? until then I am a bit stalled on the project :-( 

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Permalink Reply by Martin Poller on July 19, 2012 at 6:30am

Hi Rich,

What about using thinner ply, which you could cut with craft knife a la balsa formers, and then double or triple them up with pva, clamped overnight maybe.

Cheer

M.

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