HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jan 04, 2013, 06:26 AM
C'mon more Energy
Swoopdown's Avatar
Melbourne, Australia
Joined Mar 2005
3,166 Posts
Ouch. nasty at the end there. I think anyone who has DSed has done the same thing many times before. Dont ask me how I know

I think the 2 biggest constraints with a large chevron/Horton wing DSing are:

1. As the wing is swept then it will be hard to build a spar in the centre strong enough to withstand big speeds, this can be compensated with span loading. I would consider making ballast tubes in the tips that you can put in for bigger days, noting that it will alter your CG so you will have to add more lead up front to compensate. Some of the early span loading DS models tested had wings bending down during hard turns!

2. Control surface flutter. Definatly need to make them stiff (aluminium tube behind the surface can help alot) and make sure you have good servos with good linkage geometry and no slop.

Good luck Im looking forward to seeing how you go. Do a build thread! I need insperation to build a few of my big Chevron wings. In particular my 3.0m monster based on a CO7
Swoopdown is online now Find More Posts by Swoopdown
Reply With Quote
Sign up now
to remove ads between posts
Old Jan 04, 2013, 07:23 PM
internet gadfly
nmasters's Avatar
Colorado
Joined Aug 2006
2,145 Posts
Span loading may cause a few problems:

1 Tumbling is basically an inertial coupling phenomenon caused by high inertia on the lateral axis and low pitch damping ie heavy wings and no tail.

2 Body freedom flutter is usually thought of as due to a lack of stiffness but it's also a stiffness to mass ratio problem. You need the spar to be stiff but light

3 Adding mass to any point on the wing other than the spar bending nodes will amplify the tendency to flutter. The nodes are the points that remain stationary during flutter. The farther a mass is from the node the farther it moves during flutter ie the amplitude is greater the farther away from the nodes you get

4 Span loading would also increase the inertia resisting starting and stopping a roll

--Norm
nmasters is offline Find More Posts by nmasters
Reply With Quote
Old Jan 05, 2013, 10:03 PM
C'mon more Energy
Swoopdown's Avatar
Melbourne, Australia
Joined Mar 2005
3,166 Posts
Thanks Norm.

Can you just elaborate on point 1? Are you saying that if span loading the tips (which will be behind the CG then under heavy G loads it will become tail heavy?) or are you saying that the model will be prone to boomerang? If its the latter than I assume wingtips should stop this. Though wing tips of the ends of elevons I think would be structurally disasterous under DS condtions approaching any real speed (though a very cool idea in normal slope or thermal conditions)

I understand point 2 and 3 and agree if it were a thermal or HLG but I think for a DS style wing stopping the flutter/flex/oscillation in the firt point is more important than managing it after it starts. Even if flutter does set up a heavier more solid wing should dampen it.
I think that the stiffness and lightness would be a bigger concern in the elevons as in my experience flutter for chevrons always seems to start with the elevons.

I think for a DS wing that is 2.0m span I would be looking at 2Kg min weight with the potential to load it up towards 4Kg and for a 2.8m wing I would be thinking 3-4kg with allowance for ballast so no need to build anything too light.

Point 4. Yes you are right and that is a good thing if your DSing.

Suppose it all depends on Uwe's speed ambitions. Designing a wing for 100mph is very differnt from designing for 300mph.

Sure would be interesting to see results either way. Span loading is common with conventional DS models would be interesting to see how real testing goes.
Swoopdown is online now Find More Posts by Swoopdown
Reply With Quote
Old Jan 07, 2013, 12:56 AM
internet gadfly
nmasters's Avatar
Colorado
Joined Aug 2006
2,145 Posts
Quote:
Originally Posted by Swoopdown View Post
Can you just elaborate on point 1? Are you saying that if span loading the tips (which will be behind the CG then under heavy G loads it will become tail heavy?)
When you pull up all parts of the plane are accelerated equally so the CG will not change from that. I've seen two papers on tumbling, actually one NACA report and Irv Culver's hand written notes published in the TWITT newsletter. IIRC Irv came up with a simple crotch depth to MAC ratio that determines if there's enough pitch damping to overcome the inertial tendency to go over backwards. On the other hand NACA studied the root cause which is inertial. Basically tailed aircraft usually can't tumble because the weight is concentrated in the fuselage and distributed along the longitudinal axis, and the tail has a strong influence toward keeping it pointed forward. When a plane goes out of control it finds an axis to spin around. As I understand it in the absence of good aerodynamic damping the heaviest axis becomes the major axis of rotation and thus NACA came to the conclusion that heavy wings cause tumbling. I was going to write more of this stuff down here but upon researching my old posts on the subject I've decided that I really don't need to: Here's a link to a post with a link to an NACA tech report.



Quote:
Sure would be interesting to see results either way. Span loading is common with conventional DS models would be interesting to see how real testing goes.
Span loading doesn't cause the same problem for straight winged, tailed, airplanes because spar bending isn't coupled to pitch, as it is for swept 'wings.

--Norm
nmasters is offline Find More Posts by nmasters
Reply With Quote
Old Jan 07, 2013, 05:27 AM
C'mon more Energy
Swoopdown's Avatar
Melbourne, Australia
Joined Mar 2005
3,166 Posts
Quote:
Originally Posted by nmasters View Post
When you pull up all parts of the plane are accelerated equally so the CG will not change from that.
Figured as much. I thought I was going to undergo a physics lesson in another dimension.

Though that tumbling paper by NACA need some serious looking at. It will take me to another dimension! Thanks it certainly learn me a few things. Thanks for posting I will certainly try to educate myself on some new terms and principles.

I have had 2 models tumble on me on 3 occasions and they were both planks (straight wing no sweep, JW60 and M60) and they all happened when the tail blew off at about 80-120mph. Was crazy. Tail blew out plank instantly went into a tumble. 100mph to 0mph in 2 metres! Still trying to get my head around loss of yaw stability leading to pitch instability.... But that seems to be the proof in the pudding.

"absence of good aerodynamic damping the heaviest axis becomes the major axis of rotation" certainly a line that will fly around my brain for a while.

Would span loading a swept wing increase the lateral weight distributed along the longitudinal axis therefore decreasing its tendency to flip out? Sorry I got to print and read the report in full. Might answer some questions.

I have seen conventional models "flick" and frisbee or "ninja star" (high speed stall leading to a awesome flat spin) around its CG but never seen a wing "ninja star" only "tumble" which might back the high speed stall leading to tumbleing though I have yet to see it on anything but a tailless plank undergoing a good lot of angle of attack.

Quote:
Originally Posted by nmasters View Post
Span loading doesn't cause the same problem for straight winged, tailed, airplanes because spar bending isn't coupled to pitch, as it is for swept 'wings.
Agreed 100%. Definitely while putting a chevron/horton through big G turns I would think that through structural,span loading or other means that you would aim for as close to 0 bending/twisting as possible.

Fastest model flying wing I know of (plank)
http://www.rcspeeds.com/aircraftdetails.aspx?AC=309

Fastest Chevron I know of (foam)
http://www.rcspeeds.com/aircraftdetails.aspx?AC=410

This guy has a few runs on the board having a hand in the design of fast models and this looks like its his take on a fast Chevron.
http://cybermodelle.com/Projects/Mod...ovispoint.html

Either way I love wings and would love some smarter nuts than me to think of solutions to overcoming some of the structural and aerodynamic hurdles that need to be overcome to take it to the next level.

Here is my favourite at the moment of taking things to the next level with wings.

http://www.rcgroups.com/forums/showthread.php?t=1781085
Swoopdown is online now Find More Posts by Swoopdown
Reply With Quote
Old Jan 07, 2013, 08:46 AM
I don't like your altitude
Stupot46's Avatar
Joined Sep 2011
2,946 Posts
Thanks for posting the links,some good stuff in there.I've been following the Gizmo build.
The Horten wing is interesting,shall have to follow it's progress.
Stupot46 is offline Find More Posts by Stupot46
Reply With Quote
Old Oct 13, 2013, 04:12 AM
a wing is enough
Tail Saw's Avatar
Germany
Joined Aug 2010
93 Posts
Soft DS with Horti

Next try dizzing with a Horten, this time with my Horti.
No winglets, no ballast, but less turbulences at backside of the DS spot at my vacation slope.

I never thought that DS with my light thermal Horti is possible. It was not easy to fly because of the low stability of the aircraft around the vertical axis, but it was great fun

Horti Soft-DS am Gardasee (2 min 32 sec)


Regards,

Uwe.
Tail Saw is offline Find More Posts by Tail Saw
Reply With Quote
Old Oct 13, 2013, 11:05 AM
Registered User
United States, CA, Lancaster
Joined May 2013
161 Posts
Awesome Uwe!
Cable is online now Find More Posts by Cable
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Alert New dynamic soaring speed record!! Tappet Sailplane Talk 4 Mar 08, 2012 11:17 AM
Dynamic soaring with foamies Vector Foamies (Kits) 16 Mar 09, 2010 08:31 PM
Idea Urban dynamic soaring..... Possible? Magnum9 Australia 13 Oct 11, 2006 07:55 PM
Dynamic soaring with foamies Vector Electric Sailplanes 1 Feb 02, 2002 01:08 PM