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Old Jun 02, 2007, 09:19 AM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
1,952 Posts
Cool
Stephens Akro aerobatic profile foamie (not Stevens Aero)

Well, I'm waiting for some electronic gear to arrive before I can continue work on my semi-profile F14, so I decided to build something with clean lines that would go together quickly, be extremely aerobatic and move like a bat out of . So after a perusal of Logical Psycho's massive 3 view thread (thanks LP!), and copying about 9 different plane views, I settled on the Stephens Akro - though it probably helped that I also found an article about the real plane, which was essentially a rave review of it as an aerobatic performer.

Scaled the plans to be a 36" wingspan, which puts the length at almost 29". There's not much weight to it - the foam parts are only 2 1/8 ounces. I figure by the time I add a 2410-08Y motor, 3 cell Lipo and radio (and landing gear) I should be around 10 ounces, but be able to produce 15-18 ounces of thrust!!

I'm pretty sure it'll be a screamer....

I'll be working on it today - if there's any interest in plans, I can clean up my CAD drawing and post it too, though I do want to test the 36" version first, to decide if it really warrants a 48" wingspan instead. I'll post some pictures soon.

EDIT: Updated because it's actually 36" wingspan. Noted incorrectly before.

EDIT: The FREE, tiled PLANS are in PDF format down in post #11.
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Old Jun 02, 2007, 03:30 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
1,952 Posts
Pictures attached - not bad for a quickie, two-day project, eh? Radio still isn't on it yet, but that shouldn't take too long, once I manage to find enough servos!!!

Hope to maiden her this evening.
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Old Jun 02, 2007, 06:34 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
1,952 Posts
Maiden flight report

In a word: FUN!

I have often read the phrase "goes where you point it" but never really had a plane yet that actually DID that - well, until now. I got to the high school field, did my pre-flight check, and tried a couple short ROG hops. I had the 7.4V lipo on for now, instead of the 11.1 I was planning on using, to keep things relatively under-control for the time being. I also went with my little blue 24g Hextronic brushless motor, and a relatively small 8 x 3.8 APC prop.

The wind direction wasn't exactly co-operating with me in that regard, since the driveway runs east-west and the wind was more out of the SSW. Finally, I looked at the landing gear and realized that one of my epoxied wires to the CF gear had gotten loose, so the wheel was essentially useless for takeoff attempts.

I decided to give her a throw, rather than go home (besides, dinner was soon anyway!).

So, I walked aways out into the field, checked the wind one more time, decided, "Eh, it's only a 10-12 mph wind...", gave it full throttle and a good shove towards the south west. Now keep in mind that this motor / prop / battery combination only puts out about 7 oz of thrust while drawing a tiny 3.5 amps. The plane is about 9 oz with my 800 mAh 7.4V Lipo, so plenty of ooomph for a calm day, but a little suspect with the wind today.

But it did pretty well, actually.

It would climb out without much effort. Turns were far more predictable than I had expected for an aerobatic plane. Rolls were quick - though I wouldn't define them as "snap" rolls by any means. Loops were tight, and it would quite easily flip from upright to inverted flight and not complain in either orientation.

Now, I noticed that on some of my more dramatic loop attempts that the wing was under considerable stress, as the wingtips came up quite a bit at the bottom of loops. I must have had it up about 100 or 150 feet (I'm not a good judge of height, but it was definitely up a little ways), and was pulling out of a rather badly executed inverted maneuver, when it happened:

CRACK!



Ooops.

The main spar snapped on the left wing right where the aileraon starts near the root.

So, here I am, Mr. Dumb , standing there (never having snapped a wing before) with full throttle on, watching the plane dive rather unceremoniously towards the ground. Fortunately, after a second or so (easily half the height of the fall), I realized that I needed to KILL THE THROTTLE!!!! Hoping to slow it's descent, I also had the presence of mind to pull full-back on the elevator to provide some additional drag.

And of course, it's always a sickening sight watching a plane crash, but even more so when it's inevitable because you have little / no control over the plane...


WHACK!

Nose-first, into the ground. Damage wasn't nearly as bad as I thought it might be - it's really only bad on the wing where it cracked, and of course the (too small) spar I used which I will be replacing with a carbon fiber rod thank-you-very-much. Nose was bent a little, but easily bent back into shape, and the prop looked like it might have been stressed a little, but not cracked at all. Oh, and the landing gear popped off, but that needed to be fixed anyway.

All in all, this is going to be a SUPER-FUN plane when I actually put an 11.1V Lipo on it and give her full throttle! Thrust with that setup is 15 ounces, which will be almost 160 percent of the AUW (yes, it will be right around 150 watts per pound). So, a little epoxy, a new spar, and I'll be back at it!

Any interest for plans? This is a super-simple, quick build.
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Old Jun 02, 2007, 08:28 PM
John Cock...tos...ton
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LaGrange, GA
Joined Oct 2006
117 Posts
Hey Magic,

Another beautiful build and wonderful description of your maiden. I snapped the wings on one of the first trainers I built. So I know that feeling.

But anyway....beautiful plane.

Stephan
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Old Jun 04, 2007, 08:24 AM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
1,952 Posts
Thanks, Stephan! I appreciate the kind words.

I did get to try the 11.1V pack for a few minutes on Sunday. It was sunny overhead, but dark clouds were building to the west, so I didn't have a lot of time. Standing in an open field with a 3 foot metal rod in front of oneself is usually not the brightest idea when possible thunderstorms are imminent!!

In any case, with the sun still peeking through the clouds, I popped on the 800 mAh, 11.1V pack, checked for proper CG, and gave her a toss into the wind.

HOLY COW!!! This plane is FAST!!!

The CF spar I used to fix the previous snapped wing CLEARLY made an enormous difference. Almost no detectable wing-flex when looping. Rolls were authoritative, and the plane was still just as comfortable up-side-down as right-side-up. Downwind passes at full throttle were FAST, yet completely predictable, with no bad tendencies at any throttle from 1/4 to WOT. She would climb straight up with no complaints at all.

I wish I'd had more time to try the higher voltage pack, but the sun went behind the clouds, and the dark clouds were rolling in. Of course, this meant higher winds, but you wouldn't have known that from the way this plane flew in them. She hardly knew the winds were there!!

So, am I going to have to post a video before I can generate any interest in plans? It's a simple, quick build, and everything is put together straight - wing, elevator and motor are all parallel. One piece of fan-fold foam, or a large piece of Depron is all it takes. I even took the landing gear off and went with a bamboo skewer underneath for simple belly-landings.
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Old Jun 04, 2007, 09:40 AM
Registered User
Lake In The Hills, Il
Joined Jun 2005
359 Posts
Hey Magic man,
Sign me up for some of those plans! It always nice to have a simple bird or two hanging around!
Of course a video or two wouldn't hurt, just to compare planes. HAHAHA
thanks,
windy city
P.S. Just somewhere NW of Chicago,,,
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Old Jun 04, 2007, 11:57 AM
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Joined Jul 2006
298 Posts
Nice Job

I built one based on plans from Flying Models. 34" span and a e-flite Park 370 outrunner on 2 cells and a 9x6 prop. Needs to fly in good light as it gets out there rather fast and then you have to guess real quick which way is up. Real fun to fly and I am geting to the point where I am not spending all my time trying to catch up with it....

Have a blast.
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Old Jun 04, 2007, 03:10 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
1,952 Posts
Windy City (fellow denizen of the windy Midwest!) - I'll post the plans as soon as I clean them up so they're easy to print. And I'll work on that video - I'm still not all that great at aerobatics, and I'm especially leery of performing them a tad too near to the camera-operator (i.e. - my wife!) in order to get really good close-ups. But I will work on that - hopefully I can get some video of it done soon... perhaps next weekend.

kf2qd - I discovered very early on when first flying planes that it helped immensely to decorate the top of the wing / elevator not only differently, but in a different color than the underside, ESPECIALLY on smaller planes. Makes for vastly quicker up/down identification (which saved me from a crash just yesterday, too!). This plane has black "racing" stripes on the underside of the wing - parallel to the fuse. Quite different from the red diagonal on the top.

And yes, it does get small quick!! I'm thinking about making a 48" wingspan version because of that. But I think I'll get as much useful life as I can from this one first, then move up in size, perhaps even opting for a full-fuse version.

For anyone interested, here's the article I read about this plane which sparked my interest.
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Old Jun 04, 2007, 03:50 PM
Sticks & Stones
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Kinston, North Carolina
Joined Dec 2006
214 Posts
Really like the lines of the Akro and the Chipmunk . Takes me back guite a few years. Nice start on a great plane. Just curious, what size carbon spar did you snap and what size did you replace it with? Going to miss the landing gear

Thanks, Strick
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Old Jun 04, 2007, 04:00 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
1,952 Posts
Quote:
Originally Posted by Strick
Really like the lines of the Akro and the Chipmunk . Takes me back guite a few years. Nice start on a great plane. Just curious, what size carbon spar did you snap and what size did you replace it with? Going to miss the landing gear

Thanks, Strick
Actually, I was trying to be cheap with the plane, and didn't use carbon fiber the first time - I used a rather spindly 1/8" lightweight dowel I had laying around from Michaels. Bad idea!!! That was replaced (after the folded wing) by a 2.5mm carbon fiber tube, and a few layers of 3M Long Term Storage Tape. No flex now.

I miss the gear too, but the epoxy kept popping off from the carbon fiber rods I was using to attach the .047" wires. Plus, even with couple bamboo skewer strengthening braces, the foam was still giving way around the area where it was attached to the fuse. I need to figure out a better way to attach landing gear to a profile plane.

Thanks for the kind words!
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Old Jun 04, 2007, 07:28 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
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Plans

Okay, here's the plans. Pretty simple, so I'm not sure there's much I can add to them, except to say that the COG that worked for me is 3.625" back from the LE of the wing at the root. Gear is optional - if you opt to go without it, put a bamboo skewer on the bottom of the fuse with some packing tape to save the fuse. I used a 28" long, 2.5mm OD carbon fiber spar just ahead of the CG location. The rest is epoxy and packing tape, with a .375" square motor mount.

Oh, and I only provided half a wing and half a horizontal stabilizer on the plans - it's easy enough to just flip the half over and trace the other half. Saves paper, and downloads faster that way.

Any questions, just ask!!

EDIT: Note that the top of the rudder is found on page 10, even though the aft section of the side-view fuselage is on page 4. Be sure to make the full vertical stabilizer!
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Old Jun 05, 2007, 12:43 AM
Registered User
Lake In The Hills, Il
Joined Jun 2005
359 Posts
"Windy City (fellow denizen of the windy Midwest!) - I'll post the plans as soon as I clean them up so they're easy to print. And I'll work on that video - I'm still not all that great at aerobatics, and I'm especially leery of performing them a tad too near to the camera-operator (i.e. - my wife!) in order to get really good close-ups. But I will work on that - hopefully I can get some video of it done soon... perhaps next weekend."

Magic Man got the plans thank you!
It is great that the wife will go out and handle the camera for you! So Please don't risk that camera operator with the close in flying. Give one of your buddys a few beers and the camera. That way if it does go real ugly, the home life won't change..................
windy city
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Old Jun 08, 2007, 09:51 AM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
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Quote:
Originally Posted by windy city
Magic Man got the plans thank you!
You're welcome!
Quote:
Originally Posted by windy city
It is great that the wife will go out and handle the camera for you! So Please don't risk that camera operator with the close in flying. Give one of your buddys a few beers and the camera. That way if it does go real ugly, the home life won't change..................
windy city
Hehe - my wife is my flying buddy (not that she does much actual flying). Besides, she's the only one I trust to operate the camera WELL.
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Old Sep 04, 2007, 05:30 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
1,952 Posts
Any interest in a (slightly more) full fuselage version of this plane? I was thinking about a rounded-square type fuselage that could be built out of anything from thin Depron to FFF.

Basically, I think the radii required to make a full-fuse at this size would be too tight to do anything other than a 2mm or 3mm Depron skin, but if I make the fuselage somewhat more square, but retain some of the rounded-ness of the fuse, then it should be easy to build for most everyone, and have a nice full body.

Any interest? If not, I'll just make a stick-built version and leave it at that.
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Old Sep 10, 2007, 08:20 AM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
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Seeing as there was no interest in the full-fuselage foam version, I went ahead scaled up the profile version of this plane 33% to the 48" wingspan size I intend to eventually stick build, except that I doubled up the fuselage pieces, AND used a KFm airfoil for the wing. I used the bottom-step version.

I was hoping for a bit more extensive test yesterday, but I had some radio and ESC issues yesterday evening, and daylight was waning. But I did fly it with a 7.4V battery to test basic flight characteristics. Interesting, and rather impressive in flight, airfoil design. The single 2.3mm x 5.7mm carbon fiber spar seemed to keep the wing quite stiff. Didn't really get up to any really high speed given the battery voltage, but the plane was very stable in flight, both right-side-up and inverted. And landings were VERY simple - the plane would come it quite slowly and was very maneuverable during approach and touchdown.

I admit I was quite hesitant about the KFm airfoil, but have to say I'm quite impressed so far. I'm looking forward to trying it at higher speeds and more aerobatic situations.
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