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Old Jul 23, 2012, 05:24 PM
B for Bruce
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The 'Wack, BC, Canada
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Once it dries the WBPU is very water proof. And it's feul proof as well, not that you're worried about that fact since your model is electic.

With the varnish applied you can then LIGHTLY sand off any fuzz and spray it with rattle can enamel for a final finish. Just don't go crazy with the rattle can colour paint. The pigments in colour paint are quite heavy. But a light coat along with some trim to make it easier to recognize in the air won't add much at all.

A nicely done job of varnishing on a layer of newsprint will result in a torsionally stiffer wing than if you use any of the plastic coverings.
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Old Jul 25, 2012, 07:44 AM
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Quote:
Originally Posted by BMatthews View Post
Once it dries the WBPU is very water proof. And it's feul proof as well, not that you're worried about that fact since your model is electic.

With the varnish applied you can then LIGHTLY sand off any fuzz and spray it with rattle can enamel for a final finish. Just don't go crazy with the rattle can colour paint. The pigments in colour paint are quite heavy. But a light coat along with some trim to make it easier to recognize in the air won't add much at all.

A nicely done job of varnishing on a layer of newsprint will result in a torsionally stiffer wing than if you use any of the plastic coverings.
That's excellent news - I am certainly going to cover the wings in this way. I have found the below products - this should work do you think?

paper roll

poly urethane varnish or this one in a tin

I am hoping to finish the wing boom joiner boxes this evening, I am also going to weigh all parts, and see what the weight is going to com out at - As I need to order a motor- I don't think the 1000kv 300W motor on a 10 x 7 will do the job (one of the two motors used on this wing before).

R
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Old Jul 27, 2012, 08:39 AM
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Your design looks very similar to a UAV that was designed at MIT in the Aero/Astro capstone project class. I have seen the actual airplane around campus but I cannot find anything online about the design. The class number is 16.821 'flight vehicle development'. Check it out.
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Old Jul 27, 2012, 08:40 AM
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16.821 Engine Test - Elevator Test (0 min 42 sec)


Looks very similar
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Old Jul 27, 2012, 07:18 PM
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Allways nice to see a working engine! Looking foreward to see what comes next
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Old Jul 28, 2012, 02:35 AM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Richie, kraft paper is far thicker than you need. It's more like heavy butcher's paper. All you need to get a good skin on the foam is newsprint.

It's often sold by places that sell packing supplies. It's sold in big sheets or rolls to use for packing china, glass and other fragile items. Oddly enough it's also the same stuff used for newspapers. Hence the "newsprint" name. But when you by it as newspapers it comes "pre printed" Anyway I only offer this last bit of whimsy to let you know the sort of paper you're looking for. It's not at all the same as "kraft" paper.

The varnish you provided the link for is the solvent based stuff. Instead you want the "water based" option. Go back up one level at that web site and select the water based option.

You want to use the water based "emulsion" varnish as it'll soak into and bond with the water soaked newsprint far better.

Keep in mind that the proper procedure is to paint one area with the water based poly urethane (WBPU) then slap on the damp newsprint and brush it down well. Then quickly do the opposite side. You want the two sides to dry at more or less the same time. So haste is needed. If you mess about the one side will begin drying and shrinking and the other side won't be able to catch up and keep the foam wings straight. But done quickly and then hung so both sides have a free flow of air and all will be well.

Once the paper is down and dryed for a good 24 hours you can lightly sand off any fuzz or lumps then put on another coat of the WBPU. And then some colour spray if you so wish.
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Old Jul 29, 2012, 08:47 AM
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Quote:
Originally Posted by mklinker View Post
Your design looks very similar to a UAV that was designed at MIT in the Aero/Astro capstone project class. I have seen the actual airplane around campus but I cannot find anything online about the design. The class number is 16.821 'flight vehicle development'. Check it out.
I'll look it up, and thanks for the video - I will see what specs i can get and if necessary improve my build...
Cheers :-)
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Old Jul 29, 2012, 08:49 AM
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Quote:
Originally Posted by BMatthews View Post
Richie, kraft paper is far thicker than you need. It's more like heavy butcher's paper. All you need to get a good skin on the foam is newsprint.

It's often sold by places that sell packing supplies. It's sold in big sheets or rolls to use for packing china, glass and other fragile items. Oddly enough it's also the same stuff used for newspapers. Hence the "newsprint" name. But when you by it as newspapers it comes "pre printed" Anyway I only offer this last bit of whimsy to let you know the sort of paper you're looking for. It's not at all the same as "kraft" paper.

The varnish you provided the link for is the solvent based stuff. Instead you want the "water based" option. Go back up one level at that web site and select the water based option.

You want to use the water based "emulsion" varnish as it'll soak into and bond with the water soaked newsprint far better.

Keep in mind that the proper procedure is to paint one area with the water based poly urethane (WBPU) then slap on the damp newsprint and brush it down well. Then quickly do the opposite side. You want the two sides to dry at more or less the same time. So haste is needed. If you mess about the one side will begin drying and shrinking and the other side won't be able to catch up and keep the foam wings straight. But done quickly and then hung so both sides have a free flow of air and all will be well.

Once the paper is down and dryed for a good 24 hours you can lightly sand off any fuzz or lumps then put on another coat of the WBPU. And then some colour spray if you so wish.
Cheers for the input - I haven't bought the varnish yet - I will ensure to get the water based emulsion varnish - I will have to do this before maiden, as there is some twist in the wings.

Update to follow...
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Old Jul 29, 2012, 09:15 AM
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Update - More pictures

A bit of an update -

Wing/boom joiners
Progress has been slow - mainly due to the fact I am completely unable to find 16mm ID tubes for the boxes. I have settled with larger pipe and used duct tape to add 1mm around the alloy booms, when the c/f booms arrive I will probably have to do the same thing. they are now dry and test fitted in place.

Wing
I am going to cover the wing as per BMatthews advice with lightweight paper and varnish, I am reluctant to fly her until I have done this. I am not sure how to join the center section to the fuse, I may glue it in place. I wanted to keep this modular so i could replace parts easily, but I think for this initial example, I will need to glue it in place. I will be adding nav lights in the wing tips, along with gps, telemetry and video rx. So i am going to lay some ducting in the underside of the wing to make cable insertion easier.

Fuse
The fuse is fairly week towards the rear section, the top of the wing is near the top of the fuse so no room for any plywood. so i think glueing the center section in is my only option. The nose is shaped, needs a bit of sanding and filling before its finished.

Tail
The tail has not been started yet, now i know the exact distance between the booms I can make a start. I know a lot of people go for the traditional T tail or h tail, but i really want an inverted V, so am going to go with that, and possibly add a small vertical fin on top for initial flights to ensure good yaw performance.

The current AUW including 10ah batteries is 3.23kg. this does not include camers, vtx, esc, ubec, lighting, wiring, wheels, front nose assmbly, tail, wing covering. So i can expect it to go up a bit more, If I can keep it below 5kg I will be happy.

I am still undecided on the electric motor,I think a 14 x 7 / 14 x 8 is the prop size needed for this. id like to get one on order whilst I am still building the airframe.
Unfortunately its looking like i am going to need to buy soem 5s + lipos to power this thing, If anyone can suggest a motor that can use a 3s id like to know!

Next steps - glue wing joiners together, build fastrner mechanisms for attaching wings, and attaching booms...
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Old Jul 30, 2012, 03:56 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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It would not be too late to consider going with twin motors mounted on the wing out at the boom/wing joint. The center section could be paper and varnished with multiple layers to build up a paper mache like skin. This would make the center section quite a bit stronger and torsionally stiff enough to easily handle the load.

To fly with a moderate but solid climb you need about 50 to 60 watts/lb. Given your expected weight of up to around 5kg, which is around 11 lbs, you're looking at motors and props passing a total of 660 watts. From a 5S pack with an output voltage of up around 20 volts at full charge and dropping to around 18 volts for the majority of the flight. So that's a total current draw of 660/18= 36 amps. And if you split this between 2 motors then you're looking at 18 amps each. That might open up some more options. Keep in mind that due to the higher voltage you may need to go with a VERY low Kv value.

Or if you were to go with a twin motor setup switch to a pair of 4S packs and two ESC's. The current will go up to 660/14.4volts= 46 amps or 23 amps per motor but you'll have an easier time of finding two motors with more normal Kv values to turn 12'ish inch props on that sort of voltage.

Having the motors ahead of the CG location may also reduce the amount of nose weight needed. This assumes that the nose will require some nose weight to reach the proper trim.

You would also gain a little effiency by having the props out in front where they meet up with clear and undisturbed air unlike the center pusher position where the prop is working with air that has been split up around the center pod and pushed by the wing.

And then there's a certain "panache" for having TWIN motors vs a boring single motor setup....
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Old Jul 30, 2012, 05:10 PM
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Cheers for the advice on the motors. I think this is where I am going to struggle. I'd like to keep this aircraft as a single prop pusher, although I have to agree with you, twins are pretty cool, I own a multiplex twin star brushless that does sound pretty cool when at full chat.

I am thinking of using 4s packs. With this motor

Apparently with a 4s battery and 14 x 4 prop, it gives 492 watts @ 32A.

This motor also looks like it will do the job -

As does this one from BRc Hobbies

They all seem to be hoveriong around the 700-800kv range. - prop 14 x 7, 4s, 40A continuous.

A little low but it should still fly...hmmmm, I'm going to spend some time tomorrow looking the large list of hobbyking motors and reviews and see what I can realistically afford. The motor I initially had for this project is certainly going to be underpowered!

rich
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Old Jul 31, 2012, 08:21 AM
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Update - After looking around, I have found some guidelines for motor sizing, eFlite recommend 70-90 Watts per lb. If my model will weigh 4.5 - 5kg, thats around 10-11lb (as BMatthews has stated previously),

Therefore 10 lb x 90 watts = 900 watts required. This is a lot. Thinking of using eFlites power 46 motor.

With a 4s battery - 14.8v x 40amps continuous current = 592watts. I think i've got that right. If anyone could recommend one of the eflite motors that would be great...

Also, is my following calculation correct, ...
If i wish for 900 watts of power, and i know my battery voltage, Can I divide 900watts by 14.8v to work out the amps?

900 / 14.8 = 60.81Amps - does this mean that I need a motor that can give 60 amps? Where does prop size come into it? Does a motor generate a different number of watts depending on prop size?

Cheers
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Old Jul 31, 2012, 06:12 PM
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Wings glued into fuse, boom joiners glued in, nav lights installed.... Name: ImageUploadedByTapatalk1343776336.963752.jpg
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Old Aug 02, 2012, 08:42 AM
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Here is the motor I am about to potentially buy - It powers planes upto 4.5kg -

Turnigy g60 400kv brushless motor
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Old Aug 04, 2012, 01:30 PM
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Small update..

Started building the V tail, its coming along okay, ive gone for an all ply, and covered tail - I am going to cover with Solarfilm, never used this build technique before, so need to get the film and the iron and the heat gun! - I was going to order a NACA0012 Tail made of EPP - but decided to make it myself.

The Angle is a little large, but will be increased when the joining block is completed. The Tail surfaces are also going to be made in the same way - I may also add some small fins on the underside of the outer wing panels to help with yaw stability.

Rich
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