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PatriciaJB
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Posted: Sun Nov 30th, 2008 02:02 pm |
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Message from D Davis [moved here from 'Manned Ornithopter Updates' thread]
I found it quite awkward to find all I wanted through the above mentioned reference… (Journal of experimental biology) but simply Googling the subject quickly led me to several pages that probably say enough.
‘Wingbeat frequency of birds’ brought up an article by C.J. Pennycuick, and also a wonderful page (translated from the German) ‘How Ornithopters fly,’ from the original ‘Wie Ornithopters fliegen.’
This informative and highly readable page encapsulates several others, including ‘Calculation Tools.’
Other pages of interest for those investigating ornithopters based on pterosaur flight include ‘High lift function of the pteroid bone and forewing of pterosaurs’ –which indicates that the membrane ahead of the wings centre section controlled by the pteroid bone makes quite a big difference to the flying characteristics of the wing, a conclusion also supported by a modeller whose page I found several years ago who was playing one night with balsa and tissue.
Though this membrane, the ‘propatagium…’ does not cover very much of the span, nor does the alula of birds… the feathered ‘thumb’ that in their case operates as a leading edge slat. Birds evidently find its effect quite useful, despite the small size relative to the span of their wing.
Another informative page is ‘Sailing the skies: the improbable aeronautical success of the pterosaurs’ by Mathew T Wilkinson. This was featured in the ‘Journal of Experimental Biology’ and mostly discusses the membrane of the wings. (the ‘cheiropatagium’)
Particularly, it covers some experiments attempting to establish the behaviour of the membrane under flight loads. Though not very thorough… (they did not try to tension the membrane) it offers plenty of scope for musing.
Even without calculating everything out fully, it is clear that the slower the flapping frequency at cruising speed, the less twist a wing needs to adopt. In fact, going by Wilkinson’s study, the natural twisting under load of a membrane wing with its centre of lift behind the main spar, may well be all that I need for a craft that is powered by rowing. Rather than try to build the ability to twist into it, I am more likely to be limiting it as much as I possibly can.
Though recent fossil evidence suggests that a trailing edge tendon existed, this was not included in the model Wilkinson used in his study. This would seem a rather strange omission, as it is his page that says recent fossil evidence suggests that it existed. He also says that the membrane does in fact appear to connect to the lower legs in probably all pterosaurs. This will have provided them with plenty of control over the tension of their wings. (There has long been some uncertainty about this with the larger species.)
I would certainly be using a trailing edge ‘tendon’… and I suspect that simply by varying its tension I can have all the control over wing twist through both up and down strokes that I need.
The best we can do is still likely to be well behind the control enjoyed by the living animal however… as it is now known that they employed muscle fibres running through the membrane and also that they had stiffeners (aktinofibrils) that probably served a similar function to the batons in the wings of hang gliders.
I hope to be in a position this year to get on with building the ornithopter that so far exists only in my head and on various scraps of paper, and will be concentrating on flying-efficiency in cruising flight. This is I believe, the main area we need to prove first (for a human-powered ornithopter) before worrying about taking off from the ground, with requirements for a substantial undercarriage and more complex wing movements. Engine-powered ornithopters might be able to afford the luxury, but considering human-powered have so much going against them I’d rather concentrate on getting the flying right first.
Another reason I will be planning on winch launches, is I believe the only safe way of testing is to prove the craft in gliding control and performance at altitude first, before even thinking about flapping the wings. I have no intention of adding myself to the long list of those who have broken bones and worse, in the pursuit of this dream.
I am looking forward to the day a human-powered ornithopter flies across the English Channel in a time comparable to that of the Gossamer Albatross or perhaps even better. This is what I believe should be possible for a craft of the right design, and is what I would see as success.
David.
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