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DDavis
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Posted: Thu Apr 17th, 2008 01:44 pm |
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Thank you Jon and Murray for the above explanations! Allowing the rotating wings to freewheel makes all the difference...
I can follow concepts easily when they are in words, and your explanations are wonderfully clear. Not being a trained engineer I am unfamiliar with formulae, but with a background in programming NC lathes (ISO code) have little trouble visualising. (I have a CAD machine in my head, and always knew where those tool tips were and what they were doing.)
Those rotating wings remind me of the extreme pitch of the TU-95 Bear's propellors, the fastest propellor-driven aircraft around. Nobody else at the time even guessed such performance was possible. Very clever people those Ruskies... (by which I mean, ingenious!)
I'll be watching how this design develops with much interest!
Cheers, David Davis.
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PatriciaJB
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Posted: Tue May 13th, 2008 02:53 pm |
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The Nightingale Manned Ornithopter Chronicles cont'd
Nightingale's wings are hybrid flexible/membrane wings. They have a spar with a carbon fibre bid shear web and graphlite microcarbon rod spar caps. It has a shaped LE 'D' section and fairing aft of the spar extending to approx' 1/4 to 1/3 chord. There are several articulations including the 'metacarpal bend' as seen in Quetzalcoatlus northropi. These articulations can be locked in any position throughout the [limited] range of movement allowed and will all be locked for initial testing. After the initial testing has produced reference data the articulations will be unlocked one by one and results recorded.
The wing membrane aft of the fairing has several pterosaur- like features including some actinofibril-like structures.
Tests will be conducted with both 4 wing and 2 wing configurations.
To be cont'd 
Cheers,
Patricia
Last edited on Tue May 13th, 2008 02:55 pm by PatriciaJB
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Sigurd
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Posted: Wed Dec 17th, 2008 10:13 am |
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DDavis wrote:
A conventional aircraft with its tail at the rear works by generating negative lift at the back to keep everything in balance. This means that the wings have to carry the weight of the tail on top of everthing else. A forward canard wing develops positive lift and so it should be possible to build the whole aircraft lighter.
I found an old quote by Tom Speer at boatdesign.net but it is about hydrofoils.
"Whether the negative lift on the aft foil contributes drag or not depends on the net spanwise lift distribution from all the foils. The lift from all the foils should result in a uniform downwash left behind in the wake. If that is produced by more lift on the middle of the main foil and some negative lift on the aft foil, that's OK."
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Sigurd
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Posted: Sun Jan 25th, 2009 06:37 am |
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I thought that if the aft parts of the upper and lower skins are unsupported by ribs, then they can be bent by a rope from bottom skin (50% chord or some) to the top skin trailing edge. no hinge and more smooth. Besides, twisting the flap might be easier.
I tried it with two battens and it looks fine except they need a distance limiting rope or else the bottom skin is too straight while the top skin bends.
Might be a problem with the ropes running back and forth through pulleys each cycle being worn fast. What do you think?Attached Image (viewed 177 times):
 Last edited on Sun Jan 25th, 2009 06:38 am by Sigurd
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murray
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Posted: Mon Jan 26th, 2009 04:13 am |
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Hi Sigurd,
I have been also looking at flex ribs. My approach was to have the whole rib flexible (maybe fibreglass moulding) with a slot near the top of the profile. The idea was that if you had to split one surface it might as well be the top one so the high camber, high Cl case would have slightly more area. Also you could get the slot to open a bit for a reattachment jet. As for how to bend the rib, you string idea might work but I was looking for other mechanisms linked to the the local attack angle for an aero-mechanical pitch servo. You can dream up lots of possibilities but I am bogged down in establishing which ones would be practical at full (human carrying) scale.
Sigurd wrote:
I thought that if the aft parts of the upper and lower skins are unsupported by ribs, then they can be bent by a rope from bottom skin
Regards, Murray.
Attached Image (viewed 172 times):
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Sigurd
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Posted: Mon Jan 26th, 2009 08:41 am |
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I didn't see that jet there before. Does it work? has it been tried?
When you say "rib" do you mean the chordwise profiles or the spanwise ones?
I saw one cambering model which had the aft part of the wing filled with a soft foam/bog. Behind the spar there was no gap I think, so it got a bit odd shape when flaps were down. It also had a rotating LE.
How will you sense local alpha then?
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murray
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Posted: Mon Jan 26th, 2009 10:07 pm |
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Sigurd wrote: I didn't see that jet there before. Does it work? has it been tried?
When you say "rib" do you mean the chordwise profiles or the spanwise ones?
I saw one cambering model which had the aft part of the wing filled with a soft foam/bog. Behind the spar there was no gap I think, so it got a bit odd shape when flaps were down. It also had a rotating LE.
How will you sense local alpha then?
Hi Sigurd,
So far I have only played with structural models (glued wood veneer or crudely moulded fibreglass) for the (chordwise) flexible split ladder-ribs. In these models both LE and TE sections rotate and bend, the camber line being smoothed by elasticity of the rib structure. The awkward structural feature is that the ladder rungs should hinge freely on the streamwise battens.
It would be nice to have each rib section flexing independantly to self- stabilise local pitch, not only allowing the wing to adapt optimally to local changes in attack angle due to turbulence and flapping but also relieving the structure of torsion loads. No, it hasn't been tried experimentally to my knowledge.. I have spent a lot of time however using Profili software to model the pressure distributions for a series of aerofoils with varying camber at different angles of attack. From this data I integrated moment coefficients for fwd and aft sections, looking for conditions in which the ribs would tend to bend automatically, camber increasing with alpha, to maintain a constant C of P. You get two sets of conditions, one (level flight) in which airspeed varies to maintain constant lift, the other (manoever) in which airspeed remains constant and lift varies. The level flight case produces weaker changes in moments and is not very promising for self-stabilisation.
I have now retreated to consider the easier case of bird-like articulation of the wing so as to stabilise its pitch against overall alpha. What makes this easier is that with a non-elliptical planform the lift and induced drag distributions change with alpha, so sweep articulation driven by aerodynamic forces can be linked to camber change to achieve the desired average stabilisation. The result would be nicely suggestive of bird wing articulation.. wrist forward, tips back at high speed where profile drag dominates, straight wing at low speed when tip upwash pulls the local lift vector forward and inboard downwash to pushes back the wrist with greater induced drag.
Lots of thought-experiments to do before investing time and money in models, wind-tunnels etc. for verification. Meanwhile I also want to go hang-gliding.
Cheers, Murray.
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Sigurd
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Posted: Tue Jan 27th, 2009 03:09 am |
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The awkward structural feature is that the ladder rungs should hinge freely on the streamwise battens.
That is why I suggested using limiting ropes instead of ribs.
You could use the glitch in the upper skin for BL suction if the jet doesn't work.
This is the way I see dragonfy wings, but I'm not sure: The membrane is more supported at LE and at some line close to TE, so that the pressure between the supports will induce camber. Thus if they beat harder or with more AoA, more camber will come.
The relation between fore and aft "support line" stiffness is also partly responsible for the AoA twist, but this can be modified by wing torsional stiffness too. Maybe they are using hydraulic pressure in the "bones" to change properties of the wing.
Camber control could be the reason most insects changed to two wings, the two support lines now being able to be moved independently?
This is a simplification of the wing structure:Attached Image (viewed 164 times):
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murray
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Posted: Tue Jan 27th, 2009 06:07 am |
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Sigurd wrote:
That is why I suggested using limiting ropes instead of ribs. ........ This is a simplification of the wing structure:
Is your diagram a plan view? Is there a compression member to counter tension in the "support lines"?
Sigurd wrote:
This is the way I see dragonfy wings, but I'm not sure: The membrane is more supported at LE and at some line close to TE, so that the pressure between the supports will induce camber. Thus if they beat harder or with more AoA, more camber will come.
That description implies a camber dependant on lift force rather than AoA, the distinction being that V2 airspeed factor. We need to stabilise pitch and optimise profile drag by camber change even at low airspeed when the rib-bending forces are much reduced. This is the same problem of "level flight" vs. "manoever" cases I mentioned in the previous post.
Murray.
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Sigurd
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Posted: Tue Jan 27th, 2009 06:51 am |
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The drawing with the camber trim line is viewed from the wingtip. the battens (if soft membrane is used) or the "hard skin" (possibly reinforced with chordwise stringers) are the compression members against the trim line tension.
The last drawing, the illustration of a dragonfly wing, is a top view.
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Sigurd
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Posted: Tue Jan 27th, 2009 07:12 am |
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That description implies a camber dependant on lift force rather than AoA, the distinction being that V2 airspeed factor.
I'm not sure the distinction is important to the odonata, since their forward speed to wingtip up/down speed is no greater than 1. EDIT: but ofcourse it is possible that at the beginning of the stroke (low lift) they have less than ideal camber.
We need to stabilise pitch
Are you not going to have a tail or four wings then?
and optimise profile drag by camber change even at low airspeed when the rib-bending forces are much reduced. This is the same problem of "level flight" vs. "manoever" cases I mentioned in the previous post.
And thus I cannot see how the structure can be made to autoadjust camber after freestream AoA without an independent sensor.
Maybe it is possible.
However it would be possible to link camber control (ie my rope scheme above) to the main wing pitch control _relative to the fuselage_.Last edited on Tue Jan 27th, 2009 07:17 am by Sigurd
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Sigurd
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Posted: Tue Feb 17th, 2009 05:55 pm |
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I found this neat camber mechanism.
http://www.boatdesign.net/forums/boat-design/shapeable-airfoil-wingmast-ice-boat-25193.html
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Sigurd
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Posted: Sat Apr 17th, 2010 01:16 am |
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| Oi, here is another old camber mechanism I found today. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091138_1993091138.pdf
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