DBono99

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Hi Everyone I'm Building a Sig Kobra for Electric flight.. I would like to put some lightening holes, to make It some what lighter..Is there any system/sequence, or what ever for the use of these holes..Or should I just use good old common sense.

Thanks Dan

GarySS

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Recommendations:
replace any hard balsa in the kit that isn't critical to strength
Use light weight coverings such as Coverite Microlite
Scratch build a balsa turtle deck to replace the plastic unit.
Maybe build a stick frame tail.
Use light weight servos and of course LIPO battery. The Berg RX is also lightweight.

DBono99

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Gary There really isn't any heavy wood in the kit, except for the firewall..The firewall is 2 pcs of 3/32 lite ply..The only other lite ply is the 3 former's..All the balsa looks and feels soft, not to much grain or color in the balsa..So out side of some lightening holes, there is not much I can do. I will only 1 of 3/32 piece for the firewall.. Maybe cross grain a pice of the 1/16 balsa for added strenght..

Dan

GarySS

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Hi Dan,
Sounds like you have good wood. You might be able to shave a little on those ply formers. Just go easy with the glue (use CA) and easy on the wing joiner area. I think the best weight saving area would be in the radio system. Try for servos no heavier than 1 ounce. I used Hitec 225's, with nylon gear. Some say these break, but in a small light plane like the Kobra you might be OK. You might also think about making it a tail dragger to eliminate that nose gear (see picture). Nice fly'in plane. Mines nitro tho....

DBono99

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Hi Gary
Nice plane is that a Kobra?? Tail dragger was in the plans already, servo's (3) Hi Tec 81's, Hi Tec light weight reciever.. Maybe replace the 3/32 sheeting with 1/16.

I was really amazed with lightness of the wood, even the landing gear block, which usually weighs a ton, feels light..It's like it was designed for E flight..

Here's a pic of my Kougar/gasser..I had to add yellow to the fin and turtle..Ikept on losing sight of it..LOL

Thanks Dan

DBono99

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Gary Did you have any trouble moving the landing gear up front..How did you get to install the tail wheel..I was thinking of a plastic tube to the rudder with the tail wheell going thru, put the bottom of the fuselage.bent back under the rudder..

Dan

GarySS

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Great minds think alike. My Kougar was also dark metalic blue, also hard to see on its first and last flight........lost due to radio failure.
It wasn't hard to move the Kobra's gear forward, I just planned ahead to have at least half of the main wheel appearing out from the leading edge while viewing straight down from above. I had to renotch the wing ribs, and reinforce that area with a little thin ply. I think now maybe the whole wheel should be seen. I increased the wire to 5/32 to stiffen it up on our grass field.......adds weight tho[:'(].
I used a plastic tail wheel bracket and bent my own landing gear wire; glued into a lateral hole in the rudder. I would recommend not doing that, just adapt as light and simple a design as possible. Just a wire through a nylon bearing "plate". That should be pinned into the fuselage with short lengths of music wire. Or your idea sounds good too!

OT: I'm starting my first electric project; a Model Airplane Engineering Nemisis with a Westport 20-turn CD-ROM Brushless Outrunner Motor, BLue BIrd radio system and 700MAH 2cell LIPO. Good luck on you project!

DBono99

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HiGary I'm in the process of notching out the wing ribs for the main gear.. I have question in the manual it tells you to cap the top spar amd top of the ribs..Do you remember if you put cap strips on the bottom also..I sent Sig an E-mail, and they said "THANKS" for the heads up next time the manual is printed they would note it..All the wings I'vebuilt always had cap strips top and bottom??

Thanks sDan

PS Your goona lov E flight

GarySS

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Dan,
My ribs are capped on the bottom, but I modified the wing to give it full sheeting all of the way to the main spar/ top and bottom. That was the way I was used to building wings. I had to shave away wood on the bottom to get it to fit as I think the design was capless on the bottom. For electric purposes I would just do it per design although capping it on the bottom does create a very stiff wing. I saw somewhere in RCU that the Kougar was discontinued. Hope the Kobra isn't also done in, too![&o]

DBono99

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Gary
To cap strip only the top seems strange to me..I've never built a wing where only the top gets cap striped..Where does the covering adhere on the bottom then.?Just the rib top surface? Plus the cap strips add strenght..I think I'l just put them on, will help with covering...LOL

I just checked Sig's Web Site, the Kougar is still, listed there think the Kougar is one of my all time favorites love the look of it when it's flying..LOL I was thinking of building another for electric but it's kinda on the heavy side..That why I'm building it's little brother the Kobra..

Thanks Dan

GarySS

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Hi Dan, Yeah, adding cap strips on the bottom won't add much weight. You will need to shave the edge of each bottom rib to make room for the strips. The Kougar weighed a ton for it's size, but handled the wind very well.

DBono99

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Hi Gary
I once was thinking of making the tail feathers out of sticks..I weighed the 2 sheets with the tail feathers on in and it came out to only around 2 oz. now see No need for the stick feathers.

My motor weighs in at almost 6 oz ( Hyperion Z 3025/08),so I'm probably going to need some weight in the rear for the CG..I'm putting the elevator & rudder servo's in a hatch behind the TE of the wing..Clear up some of the space in the fuselage for battery placement.. Worked out great in my 1st E plane

My hatch will be going from the firewall all the back to the truttle deck or the end of the canopy.

Later Dan
PS Here's a pic of my 1st E Plane..It's called the Apache by Aerocraft..AXI 2820/10/Jeti 40 esc/TP 3s2100/APC e 9x4.5..Flew great got me holled on E flight

GarySS

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Hi Dan,
My Kobra was tail heavy with the larger rudder that I made (fully sheeted) tail wheel bracket, and slightly deeper fuselage that I fashioned when I was cutting out the fuse sides from those printed sheets. This was even with an 11 ounce engine. I was able to avoid adding lead by moving the flat nicad battery under the fuel tank. Since your brushless motor is so much lighter I would think you would need to watch any of the weight factors aft of the CG?

What is your target all-up weight for the E-Kobra?

Nice looking Apache! I'll bet it does nice rolling manuevers.

DBono99

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Hi Gary
I really don't have a "target" weight for the Kobra. Just trying to keep IT as light as possible..The battery pack should come in at around 9 oz..That will be placed in front of the CG..So I don't think I'll have a problem with the aft being tail heavy..One never knows till I reach that point..

The motor that I going to use should put out 300-400 watts.. So I think I'll have power to spare, if I come out a little bit on the heavy side.. This is only 3rd attempt at E flight, so most of it guess work at this point...LOL

Question about the bottom cap strips, if the bottom of the wing has 1/16" LE & TE sheeting, the cap strips should be even with the sheeting? If I have to take any thing off the rib bottoms I'll fore go the cap strips..too much work messing around with the ribs..LOL

The Apache flew great. I only have a few minutes on it.. I'm not into 3D, I just like to fly the pattern.. I was just looking for an electric plane that didn't look like one..If that makes any sense..LOL
I was really impressed with It and E flight..

Thanks Dan

GarySS

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Dan, I don't remember the kit's wing sheeting thickness and I've misplaced the plans. Yes, you would want the cap strips to come up flush with any sheeting to provide a smooth transition and preserve the original airfoil.

OT: Have you had any of your LIPO's puff up or "vent with flames"? I think I will keep mine in a waterproof container under a tree in my backyard once I start charging and using my two 700 mah'ers.

DBono99

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Gary I did have a problem with one pack of Lipo's..Brand new only had 2 charges put thru it.. It was my fault..I had the Apche on a cradle and was testing the rudder and elevator servo's..by accident I hit the throttle stick, the motor started and the plane flew off the cradle..It was a mess the firewall was broken wires wraped around the motor.. Put ever thing back together got noithing. boy was I pissed!!!! I did damage the crysta in the RX....The battery came out of
the plane and hit the ground, putting a puncture in 1 of the cells..No leakage , I charged 2 more tmes seem to be fine, but I didn't want to take chance with it..I called Tunder Power up and got a replacement at 1/2 the cost..But to answer your question I haven't had any problems.. I try to be very careful with the Lipo's..I think the horror stores you read or hear abut come from mis use of the battery..Ie..Running them too hot/charging at the wrong settings..Etc.

The LE & TE sheeting on the Kobra is 1/16 so they match the cap strips..Don't see a problem there..So your Kobra was tail heavy. I have to watch out for that..I was assuming the battery being 9 oz would take care of the CG, but can only move up too F2, unless I open up the firewall..I haven't put any time in on the Kobra lately..I got some catching up to do..LOL

Dan

GarySS

My Feedback: (13)

Started on my Nemesis Kit: The framework is light and "airry". I'm shooting for about 6 ounces all up weight (including a full battery). This is a factory picture of the completed airframe, ready for covering. There is a landing gear option which I will add.

fwman1

My Feedback: (22)

I have built three of these for higher power glow engines. Cap strip top and bottom of wing. You can put a U shape in a wire leading from the rudder "around" the elevator pushrod to a tailwheel unit directly below the rudder LE. Works like a charm. The HS-225 are a great choice. If you anticipate excess speed, then clip the strip ailerons about 3" in from the wingtips. This will avoid a flutter problem with the full length ones.

DBono99

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fwman 1
This is going to be an Electric Kobra..I'm going to use a Hitec HS 81's for the servo's, trying to save as much weight as possible.. Only a single wing servo.. Can you tell me more about your tail wheel set up..Sounds easy enough, if I can figure out what you meant..LOL

Gary
6 oz wow.. I thought my last E plane was light came it at 16 oz AUW/"all up weight"

Thanks Guys Dan

fwman1

My Feedback: (22)

If your setup has the power of a .20 engine, I'd stay with the HS-225s. The Kobra is a fast little ship. I have zero experience with electrics, but speed is speed. The ailerons will flutter at high speeds without sealing the gaps and shortening the ailerons. At least this was my experience. My current Kobra has a TT.36 on a Mac's mini pipe with a 9-7 three blade prop. It could be propped for more speed, but is a bit faster than my reflexes until I have flown it a couple times. It is faster than all but the local delta speed jobs, and one Magnum ARF from England that was so fast it stripped the covering off the wings.

GarySS

My Feedback: (13)

I agree the Kobra is a fast smooth ship. I also had a Thunder Tiger .36 with stock but unrestricted muffler, 10x6 prop (it would rev over 13K on 10% nitro). I've not experienced any flutter with my full span ailerons, but I rarely went full throttle on the level. Are your 225's metal or nylon gear?

Dan, my Nemesis is really a small model with only a 27 inch wing span, 200 square inches so a 6 ounce weight is needed for good performance.

DBono99

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So the Kobra is a fast little aircraft.. Looks like I'll be flying at 1/2 throttle..LOL The wing for the Kobra is a very strange building one..I've built a dozen of wings, even a 2 with out plans just pic's and E-mail's with a guy from British Columbia..
The last rib has a piece of 1/4" TE stock which is a (right angle),yet the ailerons are regular 1/4" tapered aileron/elevator stock..I'm guessing I didn't sand down the right angle enough one side just doesn't look right..LOL.. I just might remove both pieces and replace them with 1/4 tapered aileron/elevator stock..Then sand the trailing edge of the aileron to match the last bay..The center ribs are put in after wings are glued together..Also NO where on the plans or the manual it tell what to use for the Elevator.
I realize this is and old kit, but maybe it time Sig maybe should updated it, or reprinted the manual.

Thanks for Listening
Dan
PS This what I flew today..AUW/all up weight is 16oz

fwman1

My Feedback: (22)

The current Kobra I'm flying uses metal gears on the ailerons, and nylon on the elevator and rudder. The rudder servo is inside the turtledeck (with the battery pack) This allows a short straight run to the rudder and helps balance out the plane as even with no nosegear, things get a bit noseheavy with the TT.36. There is no dead weight on my plane. Have fun!

DBono99

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fwman1 Your tail wheel set up sounds easy enough, but I'm not sure what you meant..Can you run it by me again..A pic will help greatly.LOL

As far as servo location goes. I was thinking of making a small hatch right behind TE of the wing, then it will be a short run for the pushrods to the elevator and rudder..Might even use carbon fiber pushrods to keep the weight down..As far as weight goes,I'd like to stay as close as I can to 3 lbs. The motor & battery should be close to 1 lb.

Thanks Dan

DBono99

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fwman1/GarySS Hi Guys Did any of you come across a problem with the fillet blocks..Its a block that measures 1/2 x 1 x 9, now out of this I'm supposed to get (2) blocks for the fillet that measure 1/2 x 5/8 x 6, now I'm not a math whiz, but if you add up 5/8+5/8 I do believe that comes out too 1 1/4".. Now according to Sig that's the piece to use?? Even after shaping looking at the plans it still comes out to 9/16..

Thanks Dan