Large scale of the Sig Kobra - RC Groups
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Sep 27, 2017, 01:59 AM
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Discussion

Large scale of the Sig Kobra


Has anyone ever considered building a large scale Kobra (Say in the 80" wingspan neighborhood) for a gas powered version? I would love to hear from other members on their thoughts.
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Sep 27, 2017, 02:04 AM
Scale Builder
As a matter of fact I have considered it and even went so far as to track down a copy of the plans. Unfortunately that is as far as I have managed to get. Without a doubt a giant size Kougar or Kobra would make an outstanding project. Retracts would be an absolute must have item on mine!
Sep 27, 2017, 02:14 AM
Registered User
I guess it would be better to start with the King Kobra plans since the horizontal stab is airfoiled.
Sep 27, 2017, 01:22 PM
Registered User
The only advantage of using the kobra plans instead of the Kougar or King Kobra is that the Kobra has a built up wing. The other two are foam. Maybe blend the enlarged plans from two of them?


carl
Sep 27, 2017, 01:48 PM
Scale Builder
My intention was to use the Sig plans as a basis for the outline only. The internal structure would be completely redesigned to suit the larger scale of the model. I've got very little interest in foam so my wing would/will be of all balsa construction and I would not even consider flat tail feathers on an airplane this size. Airfoiled looks better, flies better, and is much stronger. I might consider a glass turtledeck to replace the vac formed unit offered with the kits but it would not be too difficult to build up that area as well. Have to consider that one a bit more. I prefer the outline of the Kougar the best, although the King Kobra is very similar. I'm not as much a fan of the vertical fin and canopy shape of the standard size Kobra.
Sep 27, 2017, 09:08 PM
Registered User
I have flown the Kobra for several flights until one of the rudder hinges gave up the ghost and I am scratch building another now. Anyway I just got to thinking how well the plan flies from take off to landing ( Impossible to bounce it on landing). The airframe really has a way of making you look like you are better than you really are! I can only imagine that one in the 80" wingspan neighborhood would fly equally well. Oh, for the turtle deck I would probably use a bow to cut one from foam and cover with balsa.
Sep 28, 2017, 01:08 PM
Registered User
Check out "TomClark" threads in builder's workshop for great examples and build threads for light weight building of expanded sizes of funfly/basic pattern planes; here's one of his threads:

https://www.rcgroups.com/forums/show...0#post34242370

Michael in Ontario, Canada
Sep 28, 2017, 02:30 PM
Registered User
Quote:
Originally Posted by Chad Veich
My intention was to use the Sig plans as a basis for the outline only. The internal structure would be completely redesigned to suit the larger scale of the model. I've got very little interest in foam so my wing would/will be of all balsa construction and I would not even consider flat tail feathers on an airplane this size. Airfoiled looks better, flies better, and is much stronger. I might consider a glass turtledeck to replace the vac formed unit offered with the kits but it would not be too difficult to build up that area as well. Have to consider that one a bit more. I prefer the outline of the Kougar the best, although the King Kobra is very similar. I'm not as much a fan of the vertical fin and canopy shape of the standard size Kobra.
That kobra came with an option for two different tail styles. A T-38 style and something else. You might take a peek. I thought I read once that the king Kobra wasn't near as aerobatic as the other two. I had a Kougar for years until some idiot dumb thumbed it into the dirt. It flew fantastic. Now that I think of it, there are no full size plans for the Kougar. Might not be any for the King Kobra either since it's foam. You might be stuck with the Kobra regardless.

carl
Sep 29, 2017, 11:31 AM
Registered User
Quote:
Originally Posted by carlgrover
........ Now that I think of it, there are no full size plans for the Kougar. Might not be any for the King Kobra either since it's foam..........

carl
Both the King Kobra and Kougar kits came with full size plans. My recollection is that the plans show the root airfoil but not the tip airfoil. Btw, both models used the same plastic cowl.

Michael in Ontario, Canada
Sep 29, 2017, 12:24 PM
B for Bruce
BMatthews's Avatar
Things that would concern me if I were building an enlarged version and running it with a gas engine are:
  • The strip ailerons are often the source of flutter due to poor stiffness to length ratio. I'd want to "box" in these controls with 1/2oz cloth and resin or carbon veil and resin from raw to greatly increase the stiffness. Even to the point where if a woven cloth were used that the cloth would be cut and wrapped on the bias so the fibers are all at roughly 45. I'd also change things and run each aileron with a separate servo mounted out at mid wing instead of the normal method for strip ailerons of using one servo in the center and running torque rods for "control horns" to the root ends. On the original size this might have been considered as optional. But on an enlarged version I'd say it's closer to mandatory.
  • The power to weight ratio of the engine for the large version would need to be considered. Few gasoline engines have the same power to weight as the nitro engines used on the original designs. By the time the model is powered suitably you may find that it's nearly impossible to fit the engine into the cowl.... or rather that there won't be much cowl left. If this is the case then a re-design of the fuselage to fatten it up might be required. Or you'd need to stick with a more compact and properly powerful glow nitro engine.

Also since performance on such a model is the whole key to its success I'd have to second Chad's idea of "outlines only" and designing the structure to suit the new size from the ground up.
Sep 29, 2017, 02:00 PM
AndyKunz's Avatar
+1.

There's plenty of lightness available to add to the Kougar and Kobra vs. building them stock. Shoot, I wouldn't even build them per the plans at the original sizes.

Andy
Sep 30, 2017, 06:45 PM
Registered User
Quote:
Originally Posted by 2michaely
Both the King Kobra and Kougar kits came with full size plans. My recollection is that the plans show the root airfoil but not the tip airfoil. Btw, both models used the same plastic cowl.

Michael in Ontario, Canada

My Kougar didn't come with full size plans.

carl
Oct 01, 2017, 04:09 AM
Registered User
Roguedog's Avatar
The only thing about the Kobra, Kougar and King Kobra was they all had washout.

I think you can remove it in the Kobra by cutting the build tabs off.

The Kougar and King Kobra had wash in built into the foam wings so not much could be done about unless you cut new cores.

I did enlarge the Kobra plans to 40 and sixty sizes at blue print place back in 1982 but the resulting plans an 1/8 inch lines on the 40 size and almost 1/4 inch lines on the 60 size.

My advice would be to have the plans scanned to a 400dpi black and white TIF file format then us photoshop or GIMP to scale it. The printer used the scaling feature of the scanner and the scaled up plans ended up excessively thick lines.
Last edited by Roguedog; Oct 02, 2017 at 02:29 PM. Reason: corrected error should be washout instead of wash in.
Oct 02, 2017, 01:15 PM
Registered User
Carl,

My error; the Kougar kit does not come with any large format full scale plans in the way that the King Kobra did.

The center fold pages of the manual for my mark II Kougar kit does have some full scale templates: for the fuselage formers, a top view of the fuselage back from the wing leading edge (for correctly positioning the fuselage during the build), and the fin and half of the stab (to show the paint pattern). There is only a half size side view of the fuselage from spinner to just rear of the wing to show placement of items. There is no full size pattern for the root (or tip) wing airfoil.

Roguedog,

I think you mean washout, not wash-in.

Wash-in would make a plane more prone to stalling than washout (or a neutral wingtip airfoil).

I have an unbuilt Kougar mark II kit and checked both the foam cores/their carriers and the manual and can confirm washout.
I measured the distance of the bottom of the tip and root wing cores from the edges of their carriers and my measures show two or three millimeters of washout.
The manual on page 5 explicitly states that the wing cores are cut with 1/8" of washout.
Further, the control movements section of the manual also shows a diagram in which it is suggested that for "test flying" that aileron neutral be set slightly above - 1/16" to 5/64" by my measure" - above the rest of the trailing edge, adding a bit more washout for initial flights.

Nothing like actually checking/reading the plans and manual; sorry for the earlier incorrect information.

Michael in Ontario, Canada
Oct 02, 2017, 02:30 PM
Registered User
Roguedog's Avatar
Yes your right my error. Funny thing is I read wash out in the Kobra manual right before i posted.

Corrected the error. Thanks for pointing it out.


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