Monster 230% Sparrow - BUILD THREAD

[GremlinRC]

In a previous post, I described my 175% super sparrow. I'm totally in love with this airframe design. A couple of weeks back I was flying my super sparrow with a friend. Of course I decided to show off. I floored it downwind and immediately went into a full power loop. A little voice inside my head said this was a bad idea, I should have listened. The wing failed right at the join where the spar ends. As it spiralled to the ground, I was already planning another! So should I just downsize? Nah. Bigger and stronger. Thats the way to go. So just how much bigger. Well after a bit of poking around with illustrator, I figured I could go to 230% and still get each wing on one sheet of A1 only if I made it with ailerons and cut those from a separate piece of foamboard.

So the design goals are;

• 230% and include any strengthening required to ensure no more failures. This makes the wingspan 1660mm (65.3")
• Carbon fibre tube in the wings passing the normal spar ends to ensure my monster sparrow doesn't meet the same fate as my 175% sparrow.
• Fixed landing gear. Throwing a plane that big isn't really an long term option.
• Landing flaps. I'm going to put these on although, I have absolutely no idea how the plane will behave. If I don't like them, I'll disconnect and fix them rigid.
• Removable nose section. In my experience, this is the bit that gets bashed most and I wanted a way to be able to replace the nose without having to rebuild the entire fuse.
• I'll put some lighting in there too just for the cool factor!

I have a full ardupilot kit sitting waiting for the right plane and this may well be it.

Power option is still a bit in the air. My kit box gives me three choices.

1. NTM prop drive 35 36 1800kw. Nice motor and would work with a smaller prop
2. Turnigy aerodrive xp 35/48 900kv. This would seem like a better choice and could turn a bigger prop (heavier than the ntm prop drive)
3. The third option/idea is to make it into a twin with 2 x 35 30 1100kv ntm prop drives mounted on wing nacelles. I could pull the battery back a lot this way and make the COG infinitely adjustable.

So what do you guys think? 1,2,3? Maybe another idea?

So I'll move on with some pics so you can see how the build is going.

Just for scale here's me with the wing


The wing is reinforced in a number of ways. Firstly (but not visible here) are three carbon fibre tubes two go the length of each wing and one more braces the two wing halves. Additionally the normal spar has embedded 2mm carbon fibre rod for extra strength. In the photo above you can see that a douple of pieces of foamboard are glued across the center. As well as providing extra strength, this fits snugly inside the wing preventing it from rotating laterally.

Heres the carbon rod extending out past the spar (where my 175% sparrow failed). You can also see here where I've Laminated (but not yet sanded) the outer edges of the wing. I used 24gm carbon fibre cloth and WBPU. I will almost certainly glass the entire wing and more than likely the whole plane.

Remember I mentioned the removable nose section, by pulling out the carbon rod and the screws at the bottom the nose comes off. The inside to the nose is reinforced with 2mm lite ply formers. They also provide nice solid mounting points to zip tie the undercarraige.

The tail section integrates a steerable tail wheel. The mechanism is primarily made from stiff clothes hangar wire bent and formed. I cut an aluminium disc for a wheel hub on the lathe anda rubber tyre on my cnc machine, though I need to redo that.


Next steps will obviously depend on the power choice. If I go with the twin idea, I'll need to mount nacelles before finishing glassing. Stay tuned for more pics as this progresses and I'll also have dxf plans for all the parts used in this build when its done.

 

[GremlinRC]

Slow progress over the last few days but finally some more work done.



So as you can see, I decided to go with a twin design. Pods and motors have been mounted. 2 x NTM prop drive 35 30 1100kvs installed and the motor wires extended so they allow the escs to be placed inside the fuse for easy access later. With the motors up front, I need stress less about keeping all the weight forward. I can put the elevator servos further back and if additional tail weight is needed I may well add a camera bay where the normal sparrow motor mount goes.



Tail feathers built and reinforced. Three barbie skewers recessed and glued into each side and a section of carbon fibre rod installed along the leading edge. The whole plane will be fibreglassed so the reinforcements will not be visible when complete.




Here's a close ups of one of the motor pods. I plan to rounded filet joints along the right angled joins and then glass over to make it all look like one piece.

Next steps to complete the glassing and add the control surfaces to the wing then I can start adding servos and electronics.

 

[Gryf]

Splendid! Definitely keep the photos coming, this one's going to be amazing! I particularly want to see how you do the glassing.

Gryf

 

[wedgetailaus]

Really cool! Keep it going.

 

[GremlinRC]

Splendid! Definitely keep the photos coming, this one's going to be amazing! I particularly want to see how you do the glassing.

Gryf

Thanks Gryf. The glassing is tedious but surprisingly easy. The quick drying WBPU varnish works well. You don't get a huge amount of strength from it but it does the job of proving a nice water resistant smooth surface to paint.

Here's a a pic of the glassing in progress. on the underside of the wing. I've just applied WBPU to the center secion and starting to work it out to the edges. I found trimming the edges pretty tedious. I also found by letting it dry makes it easier to trim.



Here's the top side with the WBPU wet. A light sanding afterwards gets rid of the brush lines



The other thing i did was added fillet joints around the power pod nacelles. This is just lightweight model filler shaped and sanded to get a reasonable curve then glassed over.


Here's the fuse with the electronics fitted. Using a Spektrum AR636 and satellite.




Now I'm beginning to have a problem. This is the entire plane assembled and pretty much ready to fly. A few things still left but nothing major. All the flight surfaces work as expected. The escs are calibrated and tested. The whole thing balances perfectly. I'm seriously starting to worry that I might have to try fly this thing Don't know about you guys but I get butterflies in my stomach even just thinking about a maiden.



I hooked it all up to a watt meter and the combined power was 550W at full throttle. The whole thing weighs in at 2.4kg (5.3lbs) incl a 4s 4000mah battery. I expect that will be enough power to get it in the air although I don't expect this to be any speed demon.

 

[wedgetailaus]

That's great! I'm interested to hear back after the maiden, even if it is nerve wracking!

 

[WORMSS]

That is looking very good, From pictures without something to set scale, I would swear blind that it was a normal sized Sparrow.

I understand the power limitations and the reasonings behind it, but I am sad to see the pusher prop has gone. But don't let that detract from such an amazing job you are doing.

- Colin

 

[GremlinRC]

That is looking very good, From pictures without something to set scale, I would swear blind that it was a normal sized Sparrow.

- Colin

Cheers Colin,

Take a look at the very first pic which gives a good idea of scale.

I made an attempt to fly it on Sunday, but the landing wheel struts wouldn't hold up on the grass. I decided it was best to leave it for another day until I had made a better fist of the landing struts. I've cut a better bracket from sheet aluminium and am bracing it with a spring to keep the legs from splaying. Also I was worried that the struts were too narrow and it would tip over very easily so I've widened a bit. Pics to follow.

Dave.