SteveOBHave
Senior Member
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.
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.