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STH8 - Cobra Delta - Foam Core and Fiber Glass - With Maiden Video.

#1
So after the mission that was STH7, I decided that I needed an easy win. I took the same airfoil (Martin Hepperle MH 61) that I had used on STH7 (Kahu) and applied it to a delta design.

Within the design I also applied a couple of aerodynamic 'common wisdom' additions. The first was to cut the airfoil as stipulated by the designer, no sweep alterations after hotwiring. The second was to apply around 2.5 degrees of twist at the wingtips for stall stability.

Cores cut, sanded and glued


Glassed and bagged - for the record the ridge down the root chord is BAD. This is because the strength in glass is achieved through keeping the fibres flat and straight. The core is probably strong enough on its own so at this scale it's not a biggie but it's bad practice to make a rookie mistake like that.


Mocked up - the winglets are only cosmetic ot this point.


You can just see a couple of darker patches in the wing near the motor mount and a long one tip to tip. This is unidirectional glass, it's top and bottom and acts like a spar but still allows me to cut into the foam without compromising the structural integrity.


The Cobra should have about a 400-420W output and approximately 1.3 - 1.5kg of thrust - AUW should be in the 1kg range so it's going to be a real hotrod.

We took it out to the field the weekend just gone and gave it a biff. I fooled myself initially into thinking that it was a bit tail heavy due to seeing it flapping around. I suspect this was actually thrust getting vectored by the elevons because once I got it up to speed it was more nose down requiring all my up trim and a bit of back stick to keep it level.

On the plus side the Cobra flies awesomely, I have way too much control surface but in light of my mistakes in the Kahu design I gave myself a great deal more scope for winding it back without resorting to dual rates. Because it was a bit nose heavy I kept the speed up on landing but when I got over to it I found that the motor shaft had sheared off. DOH!

Here's the vid:
http://youtu.be/e_0YTFdKfjU

It's flying on a 3 cell 2200mah with a 1350KV motor and an 8x8 prop. Sorry about the state of all the tape and stuff, it's just the test flight stage.
 
#2
The next step is to get a cowling on it and make some nice glass wingtip fences. I've also replaced the 8x8 with an 11x7 folding prop so as to avoid the damage on landing again.

With the Cowl I want it to look a little different so I carved up a mould and set to with glassing it then into the bag.

Carved:




Glassed and bagged:


Out of the bag:




And onto the plane!








I need to move it forward a bit as it's too far back on the motor and doesn't look quite as I'm gunning for but otherwise it's looking pretty much as I imagined it.

I've got next weekend away from building so nothing for a couple of weeks I'm afraid but the following week should be a bit of fun!
 
#4
Thanks fella :D yeah I like the aesthetic of it as well. I need to work on it a little more and I'm really not too sure how to attach it to the wing. Any ideas? I'm using tape at the moment but once I paint it I won't want to be putting tape all over it.
 
#7
Magnets. Tighter fit. But make sure they are strong magnets, well stuck in the FG!
Hmm, it could be possible to embed the magnets in the wing and then put a very thin layer glass over them so that they don't pull out - the tricky part is attaching them to the cowling so that the edges still sit down against the wing skin. Not a bad idea at all.