| Updates to Cobra Consruction, June 27th, three nights till the Marathon before Muncie 2004 |
Two of my first three Cobras made it through Havoc Over Hamilton, Rumble on the Cumberland, and the Dixie Nationals. One
is still going strong. The first batch of Cobras served as a good learning exercise on what to improve.
The Cobra, designed by Lee Liddle, is offered as both a short and full kit by Roy Appleton. Please check his website
www.texascombat.com for more information. I talked at length with Roy about changes to the Cobra and utilized many his
ideas shown on this page.
is still going strong. The first batch of Cobras served as a good learning exercise on what to improve.
The Cobra, designed by Lee Liddle, is offered as both a short and full kit by Roy Appleton. Please check his website
www.texascombat.com for more information. I talked at length with Roy about changes to the Cobra and utilized many his
ideas shown on this page.
The first change in construction was to use different glues. On the next three Cobras the EPP
leading edge was glued to the spar with Elmer Ultimate Polyurethane glue. This bonds much
stronger. The blue foam rear wing half is also glued with this glue. To cover the plane the
wing was misted with 3M 77 and allowed to dry completely to the touch. The tape was then
put on the wing and ironed down with a setting of 225 degrees. This reactivates the contact
glue and bonds the tape adhesive to it tightly. Shown on the right is the car carrier that Roy
designed to transport his Cobras. Following Roy's dimensions mine is made from 1/2 PVC pipe.
leading edge was glued to the spar with Elmer Ultimate Polyurethane glue. This bonds much
stronger. The blue foam rear wing half is also glued with this glue. To cover the plane the
wing was misted with 3M 77 and allowed to dry completely to the touch. The tape was then
put on the wing and ironed down with a setting of 225 degrees. This reactivates the contact
glue and bonds the tape adhesive to it tightly. Shown on the right is the car carrier that Roy
designed to transport his Cobras. Following Roy's dimensions mine is made from 1/2 PVC pipe.
After a year and a half of not breaking a needle valve off an OS LA backplate I broke four off
in Nashville at one contest, and lost three at Dixie. This meant I had to either buy OS LA
backplates or try the FP metal backplate and FP type needle and spraybar. I opted for the
latter. But I am not happy so far. Shown on the right is how I mounted the needle valve to
the Cobra. The mount is simply a large control horn. The thickness of the horn covers the
threads on the intake side of the spraybar so I soldered a brass (not copper which is brittle)
tube over it to make it easy to keep the fuel line secure. My dissatisfaction with the OS FP
needle is that it leaks. It allows air to get into the fuel stream. Notice the piece of silicon
tubing over the needle and spraybar. Without this tubing air is somehow drawn in and gets
into the fuel line between the needle and carburetor. After the Marathon I intend to go back
to the OS black backplate and needle. It would be great if you could rotate it so that the
needle was vertical behind the cylinder. This would protect it from damage.
Also notice the nylon bolts holding the HDPE motor plate onto the wood tongue. In a hard
collision with another plane or the ground the forces of the motor plate, if mounted with metal
screws, are transferred into the wing spar and foam. I wanted the motor plate to break away
in hard impact and therefore used the 1/4-20 Nylon bolts and nuts to secure it.
in Nashville at one contest, and lost three at Dixie. This meant I had to either buy OS LA
backplates or try the FP metal backplate and FP type needle and spraybar. I opted for the
latter. But I am not happy so far. Shown on the right is how I mounted the needle valve to
the Cobra. The mount is simply a large control horn. The thickness of the horn covers the
threads on the intake side of the spraybar so I soldered a brass (not copper which is brittle)
tube over it to make it easy to keep the fuel line secure. My dissatisfaction with the OS FP
needle is that it leaks. It allows air to get into the fuel stream. Notice the piece of silicon
tubing over the needle and spraybar. Without this tubing air is somehow drawn in and gets
into the fuel line between the needle and carburetor. After the Marathon I intend to go back
to the OS black backplate and needle. It would be great if you could rotate it so that the
needle was vertical behind the cylinder. This would protect it from damage.
Also notice the nylon bolts holding the HDPE motor plate onto the wood tongue. In a hard
collision with another plane or the ground the forces of the motor plate, if mounted with metal
screws, are transferred into the wing spar and foam. I wanted the motor plate to break away
in hard impact and therefore used the 1/4-20 Nylon bolts and nuts to secure it.
Shown in the photo at the right is the tether which links the HDPE motor plate to the wooden
motor support plate. The tether is stranded steel line normally used for pull-pull systems.
Small eyebolts are screwed into both the wood and HDPE and the cable is looped through
them. The ends of the cable are passed through a short length of brass tubing and inserted in
again through the other end. The brass tube is then crimped to tether the engine to the plane in
case the nylon bolts are sheared off in a collision.
motor support plate. The tether is stranded steel line normally used for pull-pull systems.
Small eyebolts are screwed into both the wood and HDPE and the cable is looped through
them. The ends of the cable are passed through a short length of brass tubing and inserted in
again through the other end. The brass tube is then crimped to tether the engine to the plane in
case the nylon bolts are sheared off in a collision.
Launching the Cobra is often harder than flying it. Roy Appleton gave me the idea to use 3/4"
aluminum angle stock to form a mount. Shown at right is the way I mounted it on the Cobra.
It extends back over the CG so the plane is held at its balance point. Also note the non-slid
tape that is on the aluminum. It makes holding and launching the plane very easy.
aluminum angle stock to form a mount. Shown at right is the way I mounted it on the Cobra.
It extends back over the CG so the plane is held at its balance point. Also note the non-slid
tape that is on the aluminum. It makes holding and launching the plane very easy.