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n/a
26 Posts |
 Posted - 10/31/2009 : 09:45:33
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Human powered Aircraft have been around in largely the same form for decades now, but they still suffer from limited performance. Certainly a long way from achieving the “fly like a bird” dream humanity is meant to lust over. This perplexes me somewhat, as the tools nessasery to achieve it are clearly in place. Our engineering a computational analysis are more than equal to the task. So what are we waiting for? Is it just the imagination and innovation lack to realise the dream.
I’m starting this thread to raise the prospect of a discussion of the practical form for a Human Powered Ornithopter
Clearly this would be the forum to raise this idea, but are there any people who share the desire to set about the practical design of a HPO?
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Don S
USA
88 Posts |
Posted - 11/02/2009 : 19:41:14
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Timing is rather poor but human powered flight has fascintated me since long before my involvement with hpvs. A recent article on bat wing technology and the U tube videos of human gliders are fascinating as well.
Don S
"it's important to understand what makes them fast. It's more important to understand what keeps them from going faster." DS |
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Talos
United Kingdom
32 Posts |
Posted - 01/11/2010 : 06:03:56
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| Yes, Countrymike. There are lots of "people who share the desire to set about the practical design of a HPO". And more than that, they are actually working on the designs as I write. But like any design, a thorough understanding of the multi-disciplines and theories (old and NEW) are needed. I have found that "the tools necessary to achieve it are NOT in place". There are aspects of ornithopter theory and design that are unique, and many others which are poorly understood by most people. You are right in asking the question "Is it just the imagination and innovation 'lack' to realise the dream", and the answer, for most people (including some 'experts') I'm afraid, is YES. |
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26 Posts |
Posted - 01/14/2010 : 13:49:06
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I have spent quite some time researching this over the last week.
I should speculate that the apparent errors between projection and results for foil performance do indeed show an error in our understanding. My GUESS is that it centres on compressibility and dynamic stress, and kinetic energy.
I have to say i was quite surprised at the size of the schism in the field of aerodynamics and quite how fundamental it is.
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Edited by - Talos on 10/02/2012 15:19:33 |
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Talos
United Kingdom
32 Posts |
Posted - 01/16/2010 : 06:14:04
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I'm glad to see that you have spent some time recently researching the possibility of mankind's oldest dream; human ornithopters. I just wish more people would do likewise.
I do agree with you that the key to unravelling the secrets of human ornithopter flight centres on compressibility (air), dynamic stress (in the structure of the aircraft), and kinetic energy; but also on the understanding and replication of the mechanics for flapping wings.
In closing, I noted the number of visitors to the forum was 341 when I first posted, and it is now, as I write, 374. It would appear that the forum is attracting the attention that is needed.
So, 'congratulations to you' is in order. |
Edited by - Talos on 10/02/2012 15:23:25 |
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26 Posts |
Posted - 01/16/2010 : 14:40:04
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It is unlikely that i shall have to time or space to build my own HPO. I would be more than happy to share my thought with the world (once i have them down on paper) and offer what help i can. To which end:
I’m a composite materials engineer, i can advise on a very large number of construction techniques, materials chooses, as well as the design calculations.
Anyone with any projects feel free to ask......
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Edited by - Talos on 10/02/2012 15:25:31 |
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Talos
United Kingdom
32 Posts |
Posted - 04/10/2010 : 05:44:42
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I have found that there is a prize of £100,000 for a human powered Sports Aircraft waiting to be won, under the auspices of The Royal Aeronautical Society.
Unfortunately, it is only open to fixed wing aircraft.
I'd be interested to know what potential ornithopter enthusiasts think of this. It seems to me, to be a bit incongruous, since The Royal Aeronatical Society have an ornithopter (bird) in their logo (but not a fixed wing aircraft)? |
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Talos
United Kingdom
32 Posts |
Posted - 05/19/2010 : 11:48:29
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| As you were. I have been in contact with an RAE representative and I now understand that they are considering allowing ornithopters to compete for the Sports Aircraft prize. I'll keep you posted. |
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Talos
United Kingdom
32 Posts |
Posted - 09/24/2010 : 03:25:22
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The University of Toronto claims to have flown the first human powered ornithopter on the 22nd of August 2010.
Having watched a video of their claim I am not convinced. They used a tow in order to launch the aircraft, thus imparting it with enough kinetic energy to convert to potential energy in lifting off the ground. The potential energy thus acquired enabled it to glide for most of the flight, with some energy inputs from the pilot as he flapped the outer part of the wings. These energy inputs had to go somewhere (energy cannot be destroyed), and they would to some extent counter the drag, with a possible supplementation in propelling the aircraft, thus extending the glide. This is hardly proof that they have succeeded in producing the first human powered ornithopter.
The very fact that they used a tow to launch the aircraft disqualifies it from being the first human powered ornithopter. Anyone can flap a suitably designed aircraft’s wings in a glide, and claim to be powering it, but no one should be taken in by it.
See for yourselves - http://vimeo.com/album/1199179
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15 Posts |
Posted - 09/25/2010 : 10:19:37
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It's hard to except a glider with floppy wing tips as a real ornithopter. Is there a formal definition of an ornithopter? I like the dictionary's.
"Ornithopter: A hypothetical aircraft HELD ALOFT and propelled
by wing movements."
Harry \../ |
Edited by - n/a on 09/25/2010 10:23:48 |
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Talos
United Kingdom
32 Posts |
Posted - 09/25/2010 : 12:13:06
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Harry, my Oxford dictionary defines an ornithopter as 'a machine designed to achieve flight by means of flapping wings'.
This is very different from acheiving flight using external means such as a tow, and then flapping the wings.
I can’t help wondering why they didn’t mount it on a lightweight free wheeling trolley in order to prove conclusively that the flapping wings were really propelling the aircraft (and trolley) with enough kinetic energy (velocity) to convert into potential energy (lift). If they had, the aircraft would have lifted off the trolley when the kinetic energy was sufficient to convert (just as conventional aircraft do when they lift off the ground).
Alternatively, they could have set it on a frozen lake (especially in Canada) where the ice offers a very slippery surface, thereby by-passing the weight of a trolley and an uneven field surface. There would also be no question of gliding to a lower level in a field surface, in order to claim an increased flying distance.
The amplitude of the wing flaps were well clear of the lowest part of the aircraft and the ground at the bottom of the flaps, so there would be no risk of damage to them.
Until they do this, their claim will never be accepted (except perhaps in Canada). |
Edited by - Talos on 10/02/2012 15:30:49 |
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26 Posts |
Posted - 10/08/2010 : 02:59:37
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I agree with you talos, it does appear that a greater portion of the energy need to sustain flight is imparted by the tow. Though an indication of the air speed would have been enough, or better still a GPS data logger (just a wrist watch these days) rather than incredible extremes and contrivances.
The film reminds me of a large bird “feathering” its wings in the final approach to land. The purpose is to lose KE by incrementally converting it to PE, and so landing at a safe speed. In this respect it is in the Orni form.... as such I’m not sure that it’s fair to condemn it as being a non ornithopter; perhaps it is just the next step in the evolution of the technological form of the human powered ornithopter.
Certainly my understanding has been furthered by watching this craft struggling through the air. It has informed my understanding of the wing form and the effect of pitch and energy recovery during the up stoke.
As only the forth human powered craft to have been flown using a flapping wing technology, i welcome it and hope that they can improve on their successes.
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Edited by - Talos on 10/02/2012 15:32:11 |
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26 Posts |
Posted - 11/17/2010 : 02:50:21
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As i’m sure i’ve said before; Ornithopters are so complex and have so many variables that their final form will most likely evolve through many different iteration of slowly improving forms. As such the more discussion the better the understanding will be.
Bernoulli’s is a misleading and incorrectly applied theory and formula. A classic pieces of imperial research hammered in to a “Law”of fluid dynamics. It has provided invaluable service in understanding fluid flow in pipes, it is a case of “if it did not exist it would have been nessasery to invent it”. However it is massively floored, and a can be shown to be so by an A-level maths student. Perhaps the most important understanding that arose from Bernoulli’s work was the simple understanding that “you can only put in to a system what that system can output.”
The modern approach to Fluid Dynamics problems is to use a computational fluid dynamics analysis. These don’t use Bernoulli’s. They rely on creating a mesh of very small volumes and creating equation to describe the flow of the fluid though these imaginary volumes. From memory these equation only have two components Density and Momentum. With these two properties it is possible to determine mass, velocity, pressure and all the other important factors. This process has become very accurate, and continues to improve.
Still air is different in nature to moving air. Most aerodynamicist don’t worry about this as at higher speeds it really doesn’t matter. However at HPO speeds it makes a massive difference. In short, still air has no Kinetic energy, so it is not possible for a foiling device to gain energy from that air. If the air is moving it has energy that can be “foiled” in to giving up energy.
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Edited by - Talos on 10/02/2012 15:41:11 |
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10 Posts |
Posted - 11/17/2010 : 13:10:42
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Hey All
As someone involved in the building and flying of the ornithopter, but not its aero-design, i would like to clear out a few misunderstandings.
What we have achieved is "sustained flight" meaning that after the tow was released, we are able to keep both air-speed and altitude constant for about 20 seconds. The pilot was too exhausted after this to supply enough power to the airplane, and the field was too short to allow for much longer flights.
We documented this in various ways, for a number of flights. We used GPS, cameras and Flight Data Recorders. the video on youtube is NOT the only data/proof we have. A GPS graph showing altitude and speed is posted on the website http://hpo.ornithopter.net/?q=content/media.
In total we had 30+ flights, with about 6 showing sustained flight. There is a big difference between the glided flights and the ones where we sustained flight.
Taking off under wing power is a whole different problem for ornithopters. Even if the wings do not hit the ground, the intermittent lift, together with the fact that the body moves up and down a lot during take off makes the control of the airplane very difficult. Our other project is a human piloted, gas powered ornithopter, and if you look up videos of it, you can see how violent the plane hits the ground during take off attempts. This was and still is the main hurdle for the powered machine.
cheers
Victor Ragusila |
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26 Posts |
Posted - 11/18/2010 : 09:26:18
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Well my suggestion would be to find a better way of taking off.... like foot launch... or some of that fancy load/ rate dependent suspension used in aircraft landing gear (of all places).
Also glad to hear that you used a GPS tracker. Maybe it not enough for a Guinness world record, but in reality it provides far more useful data.
Incidentally, i had a chance meeting with a professor of sporting aerodynamics this morning. In short Bernoulli’s work is wrong, as is much of the early aerodynamic theory’s and formulas. Certainly it means there is potential for better and more tailored forms for all HPV. Just need a concerted effort to get the true info in the HPV domain.... Dedicated HPV wiki any one?
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Talos
United Kingdom
32 Posts |
Posted - 11/19/2010 : 06:43:30
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Countrymike:
I was interested to hear of the reference to computational fluid dynamics analysis, where a small mesh of very small volumes are used. My background is Electrical Engineering, and I recall as a student way back in the 1970’s that we learnt a similar technique in determining the electrical stress around conductors in space. It goes by the name of: curvilinear squares (Higher Electrical Engineering—Shepherd, Morton and Spence, Pitman Press). There is also an electrical analogy process for determining the streamlines in water (The Design of Sailing Yachts- Pierre Gutelle, Nautical Books) which could possibly be applied to the air.
Your last point regarding the lack of kinetic energy in still air is spot on. However, a foil can and does impart down-going kinetic energy into the air as it slices through, causing the air to acquire a downward momentum known as the ‘downwash’, and it is the energy in the downwash that is converted into potential energy in the aircraft's wings enabling the aircraft to acquire lift. This phenomenon is more a case of Newton’s 3rd Law of equal and opposite reaction, than Bernoulli’s Law.
Victor:
Welcome to the forum, and thank you for your input. I have tried the link in your post but for some reason the page was unavailable.
You claim the aircraft’s speed and height were held for about 20 seconds. Well, you would wouldn’t you?
Unfortunately, until your ornithopter becomes airborne from the ground up I’m afraid your claim will never be truly accepted by the vast majority of us. A flight from the ground up is the only way you will convince the world that you have succeeded in flying a true ornithopter.
I agree with you that taking off from the ground is more difficult, but I believe this is because (in your case) your aircraft is simply too big. The wingspan alone incurs a high Moment of Inertia that prohibits all attempts to flap the wings in an efficient manner. Also, the amplitude of the flap needs to be much greater (and faster) in order to generate enough thrust for the inner part of the wing to acquire the required lift.
Here is an interesting website that you could take a look at, although I distance myself from some of the ideas contained therein: www.dcgeorge.com.
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Edited by - Talos on 10/02/2012 15:47:01 |
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10 Posts |
Posted - 11/22/2010 : 09:41:20
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http://hpo.ornithopter.net/ -> Media -> "The official flight record document submitted to the FAI is available", together with other technical documents.
Please, argue after you see the evidences presented. We did not do this attempt to have a cool video about ornithopters. Foot launch, falling off a rock or running really fast would all be artificial ways of launching the ornithopter, no different than our chosen way with the car tow, only less reliable and less safe. on the same note, i have seen NO ornithopter able to take off by itself. All the RC models are hand-launched.
Despite the lack of respect shown to this attempt, i will explain some more technical details:
1) wingspan: Like any human powered airplane, one must be able to fly slowly to conserve power. Given the inherently less optimal thrust due to wing power, we had to have a very efficient wing, meaning huge wingspan and light structure, similar to sailplanes. The wing is able to lift the airplane with very little windspeed (20km/h is above stall if i recall correctly).
2) flapping efficiency. The flapping of this airplane is not efficient in itself, but, for a human powered plane, is close to the optimal (of course, this is according to our calculations). The human rowing motion that powers the plane has an inherent optimal frequency. Any attempt to change that would either result in less optimal rowing power, or a more complicated and heavier transmission system. The wing also has a twist period which needs to be matched with the flapping frequency. Building a wing that twists fast, while being able to support the weight of the plane is a huge challenge. The engine powered machine achieves 1Hz flapping frequency using a twist mechanism which would simply be too heavy for the human powered machine.
3) weight. All human powered airplanes build so far try to achieve light weight and wing efficiency at the expense of large size and no maneuverability. Unfortunately we saw no way around this, and build a similar plane to the existing HPAs, only powered by its wing. Having a landing gear with suspension mechanism, or anything able to twist the wing using joints (our wing is solid, it twists only due to carefully engineered structure) would simply be too heavy for the available thrust.
More efficient wings are possible, and, due to our wing not breaking even in crashes, it was obviously over engineered (heavy). cutting down the weight of the wing might allow a joint system to be used close to the tips of the wing and twist the wing much more aggressively. For an HPO we still dont see any possibility to incorporate a landing hear.
We did not build a fully successful ornithopter. It can only fly in dead calm conditions (like other HPOs), when the air density is quite high and needs to be towed off the ground. It is however the first HPO to achieve sustained flight.
I do not understand the lack of respect and hostility of some of the forum members towards this attempt. The Ornithopter community, and especially the HPO community is fairly small and the challenge is great, yet when someone achieves an important milestone (or claims they achieve it), they are asked why they havent achieved all the other important milestones as well.
I would love to see other HPO flying, but so far there are none achieving anything close. http://www.dcgeorge.com./ProjectFalcon.html is interesting, but, unless the builder will obtain a very light structure, it will remain a project on paper.
Victor Ragusila |
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10 Posts |
Posted - 11/22/2010 : 09:55:21
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my appologies, forgot to list the website for the engine powered machine:
http://ornithopter.net/index_e.html
the human powered machine is at http://hpo.ornithopter.net/.
both were built at University of Toronto Institute for Aerospace Studies. A number of students worked for both, and the supervising professor was the same for both projects.
cheers
Victor Ragusila |
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26 Posts |
Posted - 11/24/2010 : 01:52:56
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Talos:
Interestingly the Mesh that is used to for mechanical or fluid computational analysis is identical in its construction to that used in electromagnetic analysis. There is an open source software “mesh” and “solver” available, but the mesh is generic and is frequently used in academia for all types of analysis.
Victor:
My apologies if myself and talos have been less than supportive, it is fair to say that we don’t always see eye to eye. The reason for this i clearly going to be different for the two of us.
For myself i am delighted that progress has been made in the field of HPOs. However i think i would have a nervous breakdown if i were involved in your project.
1; i can’t agree that the form of your HPO is optimal or efficient or even embodying the current understanding of HPAircraft.
2; to my knowledge you are the fourth such HPO to fly. I will clearly have to do some homework and give names and dates, but it surprises me that your clamming this 100 year late!
Despite of these things; congratulations in your achievement and good luck in your future endeavours.
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26 Posts |
Posted - 11/24/2010 : 04:29:58
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1925: Wasserkupe Dr. Martin Brustman “ flew his bat-winged bicycle about 66 feet (20 m) the longest man powered flight in history to that date.”
Source: Gossamer Odyssey.Morton grosser. P9
Admittedly a “bat-winged” bicycle could be a fixed wing, with only wheeled traction and a gliding, or a air screw. However Dr Brustman did colabirate on HPO which flew in 1929, though it was un able to sustain flight.
1929: Dr Alexander Lippisch the primary designer that Dr. Brustman collaborated with did manage to convince his reluctant pilot to put some real effort in and flow the HPO for 300 yards. An unverified and unifical record me thinks.
Source: http://www.humanpoweredflying.propdesigner.co.uk/
And finaly... the least well verified of my sources, but its got some good pictures...
Flyboy51 cleams to have achived true HPO flight in 1976.
If he’s telling the truth......... then it is the real achivment in HPO.
http://www.ornithopterresearchgroup.com/view_topic.php?id=150&forum_id=8&page=2
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10 Posts |
Posted - 11/24/2010 : 07:06:57
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We would actually be 6th, after Daedalus and Icarus, but the sources for their human powered achievement are even less reliable :P
I do agree that different designers, especially of HPOs, would consider our airplane not optimal or efficient, as it is probably not.. it is just the most efficient we could build according to our calculations, which i do hope at some point the two main designers publish.
I would be interested to know why you mean by the "current understanding of HPAircrafts" however. We went and visited the team being the Daedalus (the modern one), and our airplane is as close in construction and aerodynamics to the Daedalus as we could achieve. It is slightly heavier due to a much stronger wing spar, and the fuselage is arguably a bit less aerodynamic, but the performance is actually quite close..with a propeller it would have been a much more performer HPA. It anything, we thought we inspired too much fro the "current understanding of HPAs", so i am curious about your comment.
cheers
Victor always-looking-for-constructive-criticism Ragusila |
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26 Posts |
Posted - 11/24/2010 : 09:33:22
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As with so many things it is the Japanese that have taken the lead. They have an annual competition for ammeters constructors of HPA, as i understand it. Its fairly easy to find youtube footage of their efforts. As designs have been refined, it has been common place for these craft to fly “strait out the box” and test flights of 20min rather than 20 sec.
More specifically, however, i’m referring to wing design. The tendency has been away from the thick – high lift foils of the 70’s, for thinner and thinner designs. In essence these designs have abandoned conventional foil theory, and perhaps function on a more Newtonian principal. Regardless, thinner sharper wings cause less drag, meaning higher speed, and so more lift.
As such the tendency or direction in HPA design is away from massive slow craft barely able to support their own weight; instead they are sharper, cleaner, and faster. Thin “accelerating” foils would be my primary observation.
I would invite you to investigate the evidence to support conventional foil theory, the early computational models, wind tunnel results, and actual flight performance. Big holes everywhere!
On the flip side, Computational Fluid Dynamics (CFD) is now very accurate in giving results, but it does not explain theory or allow formulas to be manipulated to give definitive answers. As such it is a tool for the artisan; observation and informed understanding take president over theory or mathematical ability.
I have long intended to post some more involved thoughts on the nature of HPO’s. but i’m dyslexic, and tend to ramble somewhat and don’t actually like my work to be read.... regardless i shall edit what i have and post it soon.
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Talos
United Kingdom
32 Posts |
Posted - 11/25/2010 : 03:32:53
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Victor
I am happy to say, having seen more ‘evidences’ of the flight, that your team are to be congratulated in sustaining the height and speed for 20 seconds or so, after the aircraft became airborne from the tow. But, gee, it was a close call.
It is interesting to see that the flight data/record had to be critically analysed to find a (record) starting point where the height and speed were less than a (record) finishing height and speed. Nevertheless, you found them.
You have pushed the frontier of HPO flight into that of reality. Well done. I am now more convinced than ever that true HPO flight from the ground up will soon be achieved.
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10 Posts |
Posted - 11/25/2010 : 10:04:22
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The flight was indeed borderline...one can look at more data and see that when the air density was high, we flew better, and when it was low, we couldn't sustain flight. We could probably find data to tell us what breakfast the pilot had as well...
Countrymike, I think there are two types of HPAs out there: the large span, slow ones such as the Albatros, Daedalus and our own HPO, which are designed to fly slow and thus generate very little drag. This means they can achieve flight with very little thrust, and are mainly used for endurance or distance attempts (or in our case, attempt to fly with very little thrust...)
Airplanes such as the Bionic bat and Muscleair 1 are indeed using thinner airfoils and less wingspan, but they are built for speed competitions. Overall, the efficiency is lower ( in terms of km/W) but it can achieve higher speeds. These were developed because most of the remaining prizes at the time were for speed competitions.
The last 2 remaining Kremer prizes are even more difficult:
1) 26 mile in under 1 hour would test the upper speed limit for endurance planes and the endurance limit for speed planes. I am not aware of anyone building a plane for this prize.
2) flying in normal weather conditions would be very difficult indeed on the structure of the airplane. The Daedalus broke in flight 20m from the beach when struck by a very moderate gust of wind.
I dont think the thinner airfoil, smaller planes are an improvement on designs such as Daedalus, but are merely build for different goals and flight speeds.
I am curious what you mean by "Newtonian principal". The airfoil theory (which is actually comprised of several theories) is not wrong, it just makes a number of assumptions, which, in the case of flapping wings, generally do not apply. The main designer of our plane had to write his own code for CFD, starting from basic equations, to determine the thrust of the wing. Determining the lift was actually based on normal airfoil theory. Can you point me to some of the holes you saw in this theory?
on the same topic, Bernoulli's equations are also not "wrong" they are just simplifications of the real flow, and in most conditions they do apply quite precisely. It is true that a lot of wrong lift explanations are based on Bernoulli's equations, but that doesnt mean that the equations are wrong, just wrongly applied.
CFD is generally based on simplifications of the Navier-Stokes equations, one of them being Bernoulli's equations (if the flow is steady and inviscid and taken over a streamline). and it is unfortunately no silver bullet, being wrong more often than it is right.
cheers
Victor |
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26 Posts |
Posted - 11/26/2010 : 02:16:30
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Firstly “accelerating” foils are a type I believe to have been championed by the write brothers. It is said to be Newtonian as it differs in its function from more conventional foils.
Specifically the camber on a conventional foil acts to increase the speed of the air travelling over the top surface. This means the pressure above the foil is lowered (on Bernoulli’s principal) and lift is created.
An accelerating foil is idealised as having no camber. The leading edge is sharp, and the profile curves down ward. It is the Mass of air Accelerated downward that produces the lift Force. F=MA hence Newtonian. As the leading edge is so thin the effective plan form area is idealised as zero. This helps minims form drag, though not skin drag.
The argument goes that a conventional foil must put energy in to the air passing over the foil in order to speed it up (i’m trying not to use accelerating) in order to create the low pressure region. Of course energy input to the air will be less than energy recovered. Which is a fundamental problem in a low powered craft such as HPA. However on faster powered flight, the tradeoffs are very different.
Secondly, CFD is a hit and miss affair, but it is improving. It is a situation where the rate of refinement is decreascesing as it near the final form, yet its accuracy and usefulness is increasing massively. If you look at F1 cars and the correlation between race performance of new untested aero-parts , and the success of the teams and their differnt CFD, is pronounced. (and if you don’t follow F1, red bull won with the lowest powered engine of the top teams) unfortunately it will probably be a few years before the developments come down to the general public (or academia).
Thirdly, i am in agreement that there are two distinct forms of HPA, and that speed and size has been a tradoff with efficiency. My contention is that the conventional foil theory is sufficiently miss applied or worse, that improvements are possible.
If you read this 1974 paper from a leading aero dynamist you find a high lift foil comically recommended for HPA. Either showing a monumental lack of understanding or that somehow the reality fails to fit the theory. The same theory used in your calculations? http://pdf.aiaa.org/jaPreview/JA/1975/PVJAPRE59830.pdf
I haven’t managed to find the article i was looking for on the error between foil theory – Computational stream lining and wind tuned results, but i beg you to research it. It is a huge error (30% ish in places) and one that if it were presented now, in modern engineering and scientific community, would be laughed out the building.
Its a pleasure to talk these things out, even more so for the fact that your in the position of experience that you have.
Mike
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Talos
United Kingdom
32 Posts |
Posted - 11/27/2010 : 08:40:58
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Countrymike, I thought we had moved on from Bernoulli.
When still air comes in contact with the surface of a moving foil it is dragged along in the same direction as the foil due to its viscosity (stickiness). Form drag is similar, in that the air is dragged along with the aircraft in the same manner, and I repeat, in the same direction as the foil. This is why we call it drag.
The air above the foil is dragged less than the air below because it is rendered less viscous due to the angle of attack. Conversely, the air below is dragged along more than the air above, but this time because it is rendered more viscous due to the angle of attack. The upshot is this: the air below the foil is dragged along faster than the air above.
This appears in a wind tunnel where the top streamlines travel faster than the lower streamlines along the length of the foil’s chord. At the trailing edge the top and bottom streamlines rejoin and their velocities should quickly become the same. If they don’t become the same quickly then a turbulent wake is created as the different air velocities mix with each other.
With an ideal foil all the air from above and below leaves at the same (drag) velocity. The air over the foil is dragged along slower than the air below. It follows that the air in proximity to the upper surface of the foil experiences localised decompression and the air in proximity to the under surface of the foil experiences localised compression; hence the foil experiences lift. It’s got nothing to do with Bernoulli.
Addendum: see my post on page 2.
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Edited by - Talos on 01/10/2011 07:47:13 |
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26 Posts |
Posted - 11/28/2010 : 02:25:23
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A slight correction first, a Accelerating foil is idealised as having no thickness. It does have camber – brain fart......
Right. Form drag is Newtonian in its construct. That is because it has thickness it must move a mass of air around it as it progresses. This mass of air needs accelerating and decelerating to achieve this, so requiring energy, and hence creating drag. This is generally approximated by the Plan Form Area normal to flow. It is then fudged with a constant for the apparent stream lining features of the shape. An old school, but usfull concept.
Of course my very first point in this forum was that technology has sufficiently advanced as to mean an improvement is our potential to design efficient craft. It never ceases to amaze me how many different and contradicting theory’s there are out there. What is perhaps most telling is that nobody has succeeded in shoehorning all the different theory in to a single approach. I assume that this is what is happening in the finer detail of CFD.
The truth is that all competing ideas are simplification and convenient miss representation. They were born of the absolute need to find some reasonable means of progressing in a situation where there may never be a convenient little formula that makes all ends meet.
It is chaos – quite literally.
The idealised form of a conventional foil is described in the paper i posted the link to above. It is a high-lift Liebeck airfoil. It doesn’t work! Hench my scepticism.
Last year I took some time to test the apparent performance of different foil types. Test is very simple, make different foils of the same basic size and mount them next to each other on a spar, with a central spindle. It looks something like a model glider with no tail fins. The test is simple, the foil with the better performance will create more lift. When held loosely it will start to rotate in a stiff wind. Secondly you can throw the whole thing like a glider; the foil with the highest lift will again rise, but also the foil with the higher drag will act to create a rotation force. What i observed form this simple test was that the accelerating foils had the lowest drag and the most lift. Simple, perhaps two simple.
May i ask what foil profile other people are advocating? I assume that the snowbird uses the same as where employed in the Daedalus team? Or was it modified for the change in usage? |
Edited by - Talos on 10/02/2012 15:53:11 |
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26 Posts |
Posted - 11/28/2010 : 03:06:49
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the only resonable account of an accelerating foil that i can find at the moment is here http://www.freepatentsonline.com/y2010/0150714.html
its long wided and you need to start reading from BACKGROUND OF THE INVENTION, but it dose make the point that i was trying to...
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10 Posts |
Posted - 11/28/2010 : 10:00:46
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| I am not the aero-designer for the Snowbird, but i will post your question to the person who designed it. The airfoil shape i know was a compromise between the Daedalus team, which provided good lift efficiency at the speeds we want, and the foil that would give best thrust. There are no 2 identical airfoil sections on the whole wing, the shape changes from the root to the tip. I will hopefully provide you with more answers when i talk with the aero designer. |
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Posted - 03/13/2011 : 20:20:30
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| agree with you that the key to unravelling the secrets of human ornithopter flight centres on compressibility (air), dynamic stress (in the structure of the aircraft), and kinetic energy; but also on the understanding and replication of the mechanics for flapping wings. |
Edited by - Talos on 10/14/2011 05:46:43 |
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Talos
United Kingdom
32 Posts |
Posted - 04/11/2011 : 07:40:53
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It won't be long now.
Take a look at this site: www.festo.com/cms/en_corp/11369.htm and find 'smartbird videos'. Watch the videos and in the tab 'making of' you will eventually see an ornithopter take off from the ground without assistance. |
Edited by - Talos on 04/11/2011 07:50:13 |
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