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Old Jan 25, 2015, 02:01 PM
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Joel K. Scholz's Avatar
Kingsland, Texas USA
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Are there alternatives to a vertical stabilizer?

I have built a model Harris Hawk that utilizes elevator and flaperons on a plank airfoil. I have experimented with downward pointing legs as a substitute for the normal vertical stabilizer. It works okay in a glide, but insufficient under power. I can put a clear vertical for stability, but would prefer something that doesn't detract from the bird profile. Any ideas?
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Old Jan 25, 2015, 02:45 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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I can't see anything of a passive nature. If you don't want to make it a "V" tail or use a clear plastic fin then I think you're looking at some form of drag rudders and onboard active stability control similar to those used on the B-2.

I'm not surprised that the leg fins didn't work. They are behind the CG but not by a whole lot. And they don't have a whole lot of surface area in any event.
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Old Jan 25, 2015, 03:33 PM
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richard hanson's Avatar
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Consider putting legs n claws aft- not down - for better stability
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Old Jan 25, 2015, 04:23 PM
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Some specific combinations of wing sweep and washout are directionally stable, but if you have to replicate a bird wing shape that's probably not an available avenue.
You could use split ailerons with a small sensing vane as drag rudders, I imagine
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Old Jan 25, 2015, 05:17 PM
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Zurich
Joined Apr 2006
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why not do as they do?

Raptors, and most birds, use their "horizontal tails" for yaw .... by rotating it so that the vector has a sideways yaw component. Birds' CG is generally far aft, which helps the physics of their complex flight stroke, and the soaring raptors take further advantage of this by sweeping their wings forward when soaring [not for attacking]. To counter that and add even more lift they push their tails down to provide even MORE lift.

In this way, with the tail producing lift by being at a positive AoA [their wings in this mode have washin like the Grumman X-29], as the soaring raptor shown below makes a left turn [head level!!], the tail is providing left yaw plus lift. Important to note that the bird is also using "right" aileron [left wing at greater AoA], normal for when they make a tight circle in which the turn radius starts to order-of-magnitude approach the size of the wingspan.

Of course the drag on the left wing helps the yaw as does the tilted tail. All of this can be done with your model with minimal cutting [see below photos of "I'm Fenice" RC ornithopter with effective rotating "horizontal" tail, but very un-birdlike because CG is model-like forward using a simple up/down flapping and NOT a real bird flight-stroke, so tail is at a negative AoA: lift-producing positive AoA tail with washed-in wings is much better!].

Also attached is a photo of my little bird spreading his downturned tail while making an abrupt maneuver while flapping.
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Old Jan 25, 2015, 08:59 PM
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Joel K. Scholz's Avatar
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I have tried them in several positions, but full back is the least effective. Here is a video of a flight with no vertical stabilizer.Don't watch this video if you are allergic to swear words. The video is not all blurry so bear with it. https://vimeo.com/116112878[/url]
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Old Jan 25, 2015, 09:04 PM
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Kingsland, Texas USA
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Legs full back was the least effective. Here is a flight without a vertical stabilizer. Don't watch if your allergic to swear words.
yard Harris Hawk (1 min 42 sec)
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Old Jan 25, 2015, 09:10 PM
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Here is a longer video of the maiden flight with several different configurations.
Final Harris Hawk Movie (10 min 57 sec)
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Old Jan 26, 2015, 01:24 AM
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Zurich
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I still suggest you consider modeling an actual bird's soaring configuration by first making a small FF model with fwd-swept mean chord-line, wings washed IN and tail depressed at positive AoA. Find CG after setting all AoA's + wing-twist correct [extra positive AoA on wingtips, incl. aileron position, compensates for positive tail AoA]. Then play with amount of tail twist rotation + "reverse" aileron in circling turn. Since you still need to use tail as elevator, in the worst case you could pre-set and couple reversing aileron and tail AoA in Tx when you want to slow down and circle in thermals.
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Old Jan 26, 2015, 11:10 AM
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Replace the legs with Lexan fins that go up to the horizontal tail all the way back to the elevator. Then paint the leg and feet detail on the fins.
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Old Jan 26, 2015, 02:44 PM
Canadian Bacon
flypaper 2's Avatar
Kingston, Canada
Joined Jun 2004
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Are there any plans or drawings for this type of bird??

Gord.
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Old Jan 26, 2015, 02:47 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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In the air the 'V' tail isn't particularly noticeable, (picture below of my Eagle)..

A gyro may be useful -- (though a rear prop can I believe give a similar effect to a vertical fin.).

Wii MotionPlus keeps flying wing level in high wind (1 min 16 sec)


though a rear prop can I believe give a similar effect to a vertical fin.
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Old Jan 26, 2015, 02:52 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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Quote:
Originally Posted by flypaper 2 View Post
Are there any plans or drawings for this type of bird??

Gord.
Here's a very big thread on 'bird' type models --

EPP Peregrine, FFF-Eagle, Dragons and other Bird Planes
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Old Jan 26, 2015, 02:57 PM
internet gadfly
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Colorado
Joined Aug 2006
2,203 Posts
Hi, Joel--

You need a source of drag at the tips. A little delta wing fin like this might help. I've marked a couple of places where it would be fairly inconspicuous. Herk Stokely managed to get a plank to fly fairy well with split drag rudders coupled to the elevons but incorporating his system looks like it would be a lot of work.

--------.~.
--------/V\
------//----\\
-----/(------)\
----(^^)---(^^) --Norm
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Old Jan 26, 2015, 03:37 PM
Canadian Bacon
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Kingston, Canada
Joined Jun 2004
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Thanks Ray. You're a lifesaver ......................again
My next project when I get caught up.
We have turkey vultures over our field and I would love to join them.

Gord.
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