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Old Feb 18, 2009, 07:13 PM
aejr sucks little black ones
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United States, TX, San Antonio
Joined Nov 2007
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Hi Matt, thanks for the pictures thats a beautiful looking sailplane.
Ahh, one of these days I might be able to join the ranks...
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Old Feb 18, 2009, 08:17 PM
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St. Louis, MO
Joined Mar 2005
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think i'm missing something on the 15 degree rule... i'm up in the kitchen playing with it, and it doesn't seem to work right... what am i doing wrong?

i made a triange with the 3 angles, 90, 15, 75. placed the 15 degree side on the CG mark, with the 90 forward, and the 75 pointing up (with the wing upside down), and i get a tow hook location WAY far forward... and with this method it would seem to varry the tow hook location by how big your fuse is... so i think i'm WAY off here... what am i doing wrong?

thanks jack,
matt


atomsteve - your welcome about the pics... i have learned SO much from looking at so many other peoples photos here on RC Groups, so when i can, i try to post what i can, to give a visual to others!
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Old Feb 18, 2009, 08:37 PM
Hot Dawg Glider Pilot
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United States, TX, Weatherford
Joined Nov 2002
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Oh No!!!

AAAHHHH!!!! You did it just like I said!!! Sorry, I goofed. The 75-degree angle goes aft and the 15-degree angle goes up... That makes it 15 degrees from vertical... I'll get better... gimme a chance...

That angle works well for a starting point regardless of how fat the fuselage is. That's why I gave it to you in degrees instead of a measurement.

Again, you did it just like I wrote it... pre-contest jitters, I guess...

SWC bound at 4:00 AM... I'll bet the stars will be nice out around Mountainair.

Jack
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Old Feb 20, 2009, 11:48 PM
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United States, AZ, Arizona City
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I saw Jack and his Yardbird II at SWC today. Nice flying bird and a really friendly guy!
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Old Feb 23, 2009, 06:52 PM
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Granby, CT, USA
Joined Jan 2001
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fifteen degrees

I think that hook position as an angle forward of the CG is a wonderful idea. It yields up consistent results no matter how tall the fuselage is.

And where might that CG be? I don't think we want to measure from the arbitrary place on the plan that the plan's author/draftsman put the crosshatched circle denoting 'cg'. It identifies the balance plane of the aircraft, but not the internal center of gravity.

In an attempt to be consistent, I've measured the angular displacement from the balance point in the plane of the wingrod, guessing that the airplane will be pivoting around the balance point in that general region regardless of the depth of the fuselage. The taller the fuselage, the towhook will be farther forward in inches as compared to a slimmer fuselage.

Does this sound reasonable?

Thanks, folks
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Old Feb 23, 2009, 08:20 PM
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United States, TX, Weatherford
Joined Nov 2002
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Peter,

The actual center of gravity, or true center of mass, if that sounds better to you, may well be above the wing of some models. You are correct that the angle measurement will move the hook forward with the vertical thickness of the fuselage. However, what I've described is just a consistent starting point, and it will usually be moved, though, usually not too much. It would be easy to over think this...

Enjoy...

Jack
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Old Feb 23, 2009, 08:57 PM
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St. Louis, MO
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it all sounds good to me! i got the tow hook mounted according to the 15 degree rule, and after doing it correct, it seems to be just about right! now working on covering the ship. it's going to be BRIGHT!!! hopefully great at altitude when this ship is a spec in the sky! i'll post pics when more done!

matt
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Old Feb 23, 2009, 09:07 PM
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Well.... some people keep peacocks in the yard...
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Old Feb 23, 2009, 11:19 PM
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USA, AZ, Tucson
Joined Apr 2002
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Quote:
Originally Posted by jmfitzgerald
it all sounds good to me! i got the tow hook mounted according to the 15 degree rule, and after doing it correct, it seems to be just about right! now working on covering the ship. it's going to be BRIGHT!!! hopefully great at altitude when this ship is a spec in the sky! i'll post pics when more done!

matt
Can't wait to see it!
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Old Feb 27, 2009, 07:26 PM
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St. Louis, MO
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it's coming along.. here is a sneak peak at it! the photos don't really do it justice... i went with the neon yellow monokote on the LE, and boy is it bright!!!

just got to finish up the spoilers, glue on the boom, run pushrods, and complete the ballast stuff... then hopefully i'll get her in the air next week???!!!

this build has taken me a long while, with all of my travels, but it sure is a rewarding build, now getting to this point!!! i can't even imagine how cool it will be when i get it to the field! this thing feels SO strong! this thread has been HUGE help, thanks to all!

matt
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Old Feb 27, 2009, 09:08 PM
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Albuquerque, NM
Joined Aug 2002
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Matt,

Is that a Bubble Dancer horizontal tail or the tail from Jack's plans? Reason I ask is that I just finished the BD horizontal tail for my Yardbird, and it looks kind of big.

Also, what is the gap between the wing trailing edge and the leading edge of the horizontal and vertical tails? I'm using a Supra pod and Bubble Dancer boom and the fuselage is extermely long.

Thanks, and good luck with the maiden flight, you did an excellent job!

John Ihlein
Albuquerque, NM
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Old Feb 28, 2009, 01:04 AM
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United States, TX, Weatherford
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Matt, BEAUTIFUL!!! I'd love to see that fly...

John, that's a scaled down BD tail, not mine. The BD has more area nad span than the YB.
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Old Feb 28, 2009, 08:15 AM
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Albuquerque, NM
Joined Aug 2002
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Hi Jack,

Hoping for some warmer weather so I can get this project going again.

If I were to mount the Yardbird stab from the plan you sent me on the BD v mount, would I place the pivot point under the rear joiner tube since you use the front tube to drive the stab?

John
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Old Feb 28, 2009, 11:00 AM
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John,

That's where I would put it. The BD uses a Drela airfoil for the tails that has the thickest point in the airfoil well forward. The NACA-008 that I used has it at about 30% chord. My Xplorer uses the Drela tail airfoils, or something similar, and has the pivot point pretty far forward. However with the NACA series, the thickest point should be the pivot, with the balance point of the surface as far forward as possible to help prevent flutter.

JW
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Old Feb 28, 2009, 01:20 PM
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St. Louis, MO
Joined Mar 2005
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well actually that IS the bubble dancer stab.... i compared what was on the yardbird plan and the plan for the BD stab, and whats on the yardbird plans is actually bigger, it measures about a 28" span vs the BD's 23.5"... oops, after looking more into it, the online plans show a span of about 24.5", my printer must of scaled it down just a bit, but only about an inch in span. but, yes that is a BD stab, to what i thought was the right dimensions... he he

also looking at the 2 planes compared to each other, i think the yardbird is a bit bigger all around. YB 126" span vs BD 117" span. i rough calculated a wing area of 1050 sq. in vs BD 1014 sq. in. but i sure didn't keep the weight as low as BD specs! as it sits now, it weighs in at 42.5 oz, with the finishing touches (pushrods, ballast tube, spoilers and rigging, and probably some nose weight) i'm hoping for an AUW of 45-46 oz. which should give me a great wing loading of about 6.17oz/sq ft!

and as fas as distances between surfaces... i measure 36" from wing LE to stab LE, and 41.5" from wing LE to rudder LE. i think it all looks pretty proportionate. and i think the size of the stab will be about right! as mentioned earlier, i used an art hobby high aspect pod and boom, and fin. comparing them to a topaz, this is about an inch longer to the nose and about another inch to the rudder. i guess we'll see here soon how it all works out!

matt
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