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Old Apr 26, 2007, 05:45 PM
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Maryland, USA
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Star Cobra 3F

I had contemplated building this one over 20 years ago! Some may even recognize a Star Cobra having been featured in RCM back in the very early 80's. Originally design by 3 fellas (E. D. Bridges, Ken Willard & N. L. Williamson), they intended it to be built out of FoamCore with power being an Axiflo RK20 fan/K&B 3.5cc tuned pipe equipped engine combo. I purchased the plans, started build, got sidetracked, and ultimately took a break from R/C for the next 18 years.

Getting back into the hobby just last year and discovering FFF foam and electrics got me thinking this old design would be a prime candidate for such treatment. I dusted off the plans and set to work scaling her down to 70%. While doing this I envisioned a number of mods but figured we'll just "KISS" this one and save the mostly aesthetic revisions for subsequent versions. That said, I will admit that there were a number of changes made in the area of flight control. Since this was designed pre computer-radio the original plans called for a Dubro mixer and an extra servo and this still only allowed elevator function of the fore-plane surfaces. Now simplicity abounds...fewer parts and less weight and more control options!

This build in not quite completed but she is air-worthy. What remains is to finalize the fuselages "bridge" area, aft turtle deck, and add the landing gear. Have to order 3mm C/F tubing prior to finishing but just had to put it in the air for proof-of-concept. I've been flying it with just one of the cabin formers installed and a skid temporarily attached to the motor boom. I must say that with all the horizontal moving surfaces in split mode this thing loops and rolls in an eye blink! Couldn't really judge slow speed characteristics as it was a bit windy, steady 10+mph along with gusts hitting 15mph or more! Those winds combined with an AUW of 7.8oz and a wing area of 403sq/in means a calmer day is required for a more complete wringing out.

Chose to power this one with a propped 20mm CD-Rom motor I recently built even thought this design is just screaming to be an EDF. As soon as I get a fan and buy/build a suitable motor you can be sure it has a home ready and waiting. In EDF form the battery and ESC would be placed in the bridge to achieve the correct CG, this will also help to tidy up the airframe.

Now that I am comfortable she's a keeper I'll finish up the bridge and aft turtle deck while I await the C/F. Will post more pics when it's completed.

Cheers,
golem
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Old Apr 26, 2007, 06:18 PM
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Fort Smith Ar
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Golem

That is cool. I had the same idea a few months ago. I built one full size out of coro a coupleof years ago. I thought it would be great in foam using electric. Here is the thread of it. http://spadworld.net/forum/viewtopic...asc&highlight=
I really like it. Good luck and keep us posted.
Russ
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Old Apr 26, 2007, 09:20 PM
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Maryland, USA
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Wow Russ! Not only did you build one...you built TWO!

The most amazing part of this is my assuming the design had melted into obscurity. All along (well, at least a couple of years) you have been enjoying this solid design. I followed your link to both the 100% and 50% versions. That vid of the 50%'r gives a good display of the very high roll rate she delivers.

I noticed on yours the rear control surfaces are set up as elevons and the fore wing has only elevator actuation (as initially designed). That actually may be the correct option for this plane. As it was really windy on my test flight it was hard to tell but I seem to feel it was experiencing a very slight amount of corkscrewing during aileron rolls. Thinking about this it seems probable considering each wing being on opposite sides of neutral center. Will have to keep an eye out for that.

Must also comment that you have a fine portfolio there. No one could ever accuse you of not being diverse in choice of craft to build.

Cheers,
golem
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Old Apr 27, 2007, 06:36 PM
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Fort Smith Ar
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Thanks Golem. I do like the challenge of odd planes

I have the elevator and elevons mixed together but I can turn them off. It flies good with just elevator of front and ailerons on the rear. With the mixing on it will do a much tighter loop and fly hi alfa quite nicely. The landings are nose high and very jetlike.

Russ
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Old Apr 28, 2007, 07:44 PM
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Maryland, USA
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Thanks for that additional info Russ!

I've now had a chance to fly her twice in still air. All I can say is... ! This has got to be the most gentle flying plane I have ever laid my hands on. I've never flown a canard before so the flight characteristics might be the norm for this type design.

What is so sweet about this one, which is different from all others I've built even at this same wing loading, is the fact that it will fly at near walking pace while maintaining level flight! It has proven so perfect for gentle elevator style spot landings that I've even decided to nix any plans for landing gear. This also allowed me to finalize and add the bridge as seen in the pics below.

I didn't overpower this by any means as it weighs in at just under 8oz while my 20mm CD-Rom motor provides about 10oz thrust using a GWS 7x3.5HD on 3s-620 (<4a). Wing loading works out to right around 2.9oz sq/ft.

Flying her gives very much the feel and confidence of a trainer. The only thing keeping it from being such a plane is the lack of self-righting. Slow or fast she tracks straight and true, very predictable, and no hunting whatsoever. Loops can be very tight (1.5 times length) and are quite unique looking, almost cobra like. Rolls can also be fast and are, thankfully not like my miss-perceived concern, perfectly axial. The most fun and eye-catching maneuver it excels in would be hovering at 80-85 degrees and just slowly rotating keeping the outer main elevon point locked in space. She will also happily keep this attitude while gaining or reducing altitude.

This is all together one very impressive design, sure wish I could claim credit but alas I tip my hat to the 3 gentleman that put pen to paper back in 1980!

Cheers,
golem
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Old Apr 28, 2007, 09:18 PM
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Melbourne,Australia
Joined Sep 2006
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Hi golem,
Is there anyway of getting hold of plans for this build? I've been wanting to try a canard type plane, but have shyed away as they all seem to be fast and difficult to handle. I am a pilot of only modist skill . This build maybe what I need to experiance canard flight, and progress my skills.
Cheers,
Andrew.
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Old Apr 28, 2007, 09:50 PM
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Maryland, USA
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Quote:
Originally Posted by Kombatant
Hi golem,
Is there anyway of getting hold of plans for this build? I've been wanting to try a canard type plane, but have shyed away as they all seem to be fast and difficult to handle. I am a pilot of only modist skill . This build maybe what I need to experiance canard flight, and progress my skills.
Cheers,
Andrew.
Hi Andrew!

Without doubt, if you can fly an aileron equipped plane you will have NO problem keeping this one on the air. Flying can be done in slow motion as long as the weight is kept reasonable.

I don't have plans to post but since everything is basically straight lines I can post a sketch with dimensions. I have at hand calcs for 100% and 70% (which is what mine is) and can provide either with little effort. If you let me know which you would prefer I'll do my best to get it posted tomorrow.

Cheers,
golem


*EDIT* - I'll just post the 70% dimensions as I know this works very well. Not everything is 1-to-1.
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Last edited by golem; Apr 28, 2007 at 10:34 PM.
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Old Apr 28, 2007, 10:00 PM
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United States, FL, Alachua
Joined Jul 2004
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Golem

What a great looking airplane! It must look great flying with the different profile. Is that Protection Board you are using? It sure would make a nice model to have plans and a build guide. How large is the plane?

Oh yes, I am getting ahead of myself.....first I need to know a lot more about building and flying! This would be something to look ahead for building in the future.
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Old Apr 28, 2007, 10:31 PM
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Melbourne,Australia
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Golem,
Cool! Either will be fine, but, seeing as how your 70% modle is tested and flys well, those dimentions will be fine. What is the WS and length?
Thanks in advance,
Andrew.
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Old Apr 28, 2007, 10:45 PM
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Maryland, USA
Joined Mar 2006
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Quote:
Originally Posted by FlaArtGlass
What a great looking airplane! It must look great flying with the different profile. Is that Protection Board you are using?
You are correct, it is ProtectionBoard with the skin peeled off. Saved nearly an ounce and it still has all the strength it requires using 5mm x .8mm carbon strip at the trailing edges of the main and canard.


Quote:
Originally Posted by Kombatant
Golem,
Cool! Either will be fine, but, seeing as how your 70% modle is tested and flys well, those dimentions will be fine. What is the WS and length?
Thanks in advance,
Andrew.
To funny! You posted at the same time I upped an edit to my previous post. I think we are in concurrence.

At 70% she is 25.5" x 34" x 6".

Cheers,
golem
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Old Apr 29, 2007, 12:21 AM
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Joined Jan 2006
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Hey Golem -

Thanks for the link to this great plane from the "Little Go Bipe" thread. From what you've posted, I'm DEFINITELY interested in trying a 50 or 60% size version of this plane. It sounds perfect for that "small backyard flyer" application.

Looking forward to the plans when you get a chance!
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Old Apr 29, 2007, 02:30 AM
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For those interested I've done a quick-n-dirty page of dimensions. Since everything is straight lines and geometric a set of cut-out plans is really not necessary. I have yet to provide the bridge or motor mount details. The bridge is basically just aesthetics and not an integral part of the structure so personal tastes can flow freely, mine is a bit of a departure from the original design.

A couple of things I can think of right now to keep in mind are the following:

1. Prop clearance - I mounted my motor on the bottom of the stick mount which puts it on the center of the main wing plane. With a 7" prop this still leaves me with less than 1/4" of prop to canard clearance.

2. Balance - Even with my final built-out bridge installed it has required almost 1/2 oz of weight in the nose for balance. My initial intent was to place the battery in the bridge but it fits all to nicely where it is now behind the motor. To alleviate this I would opt for another change which would have the added benefit of providing better linkage geometry...move the servos up and forward. Right now the canard linkages are routed below the surfaces which required lots of fancy bending and less than optimum actuating angles.

3. Structural Stiffeners - The only requirement would be to stiffen the main wing span wise. I chose to do this with a full span 5mm x .8mm strip of CF along the trailing edge. Although it wasn't necessary because of my motor mount design I also added a CF strip to the trailing edge of the canard wing. It was a piece I had lying around, was 3" short of full span but I figured why not put it to use.


I'll provide a bit more detail on my current bridge and motor mount dimensions tomorrow. I can also take a bunch of detail pics which should be able to answer many questions about part placement and registration. Will upload them to one of the photo dumps. If I have left anything a gray area just let me know and I'll do my best to fill in the blanks.

Cheers,
golem
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Last edited by golem; Apr 29, 2007 at 02:58 AM. Reason: Changed attachment. Error in canard length dimension.
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Old Apr 29, 2007, 03:01 AM
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*Notice* - Dimensions plan in previous post had the canard length improper. Attachment has been updated.

Cheers,
golem
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Old Apr 29, 2007, 05:12 AM
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Melbourne,Australia
Joined Sep 2006
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Quote:
Originally Posted by golem
To funny! You posted at the same time I upped an edit to my previous post. I think we are in concurrence.

At 70% she is 25.5" x 34" x 6".

Cheers,
golem
A tad coincidental. As they say "great minds think alike", rules me out, but, your obviously a smart cookie

BTW those spec's look good. Just the kind of size I had in mind. Now I've got 1 plane flying, 1 waiting for gear (motor & batt), 1 under construction and two (or three ) to start. I think I'll drop the others down the list to get this one done. Like I said , I've been hanging out for something like this to come along. I just couldn't find one that had me inspired to build.
Cheers,
Andrew.
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Old Apr 29, 2007, 05:39 AM
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Melbourne,Australia
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One last question before I sign off.

I've built a Komodo KH278 with 10 magnetts at 13 turns. with a 7*3.5 it'll pull 16oz thrust to 47mph at 6.8amps. I'm dying to give it a home.

Just how much speed do you think the Cobra will handle?
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