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26 Posts |
 Posted - 07/31/2010 : 14:11:40
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I thought it might be worth posting something on the principals of human powered air craft. (I would rather be editing a dedicated wiki, but my post on the subject didn’t exactly takeoff.)
First and foremost are the immutable laws of physics.... specifically the law of conservation of energy.
The law says that energy can be neither created nor destroyed. As such the energy in an isolated system will stay the same over time. For HPA this results in a balancing act between Potential Energy (PE) which is a product of height and weight; Kinetic Energy (KE) which is a product of mass and velocity2 ; and Human Power in watts (HP) which ranges from 1200W for very short periods, to a sustained 300watts for a good athlete. So if a HPA glides it loses high, which results in more speed, PE converts to KE.
Add to this drag, which is a force acting on the craft in opposition to it motion. A craft will experiences drag, which converts energy in to turbulence in the air flowing over the craft and is lost in the wake. This drag force must equalled by energy inputs to the system in order to maintain a constant energy state for both KE and PE. So the power input of the pilot (HP) must equal the drag to maintain speed and height. Clear as mud!
Now the really interesting part about HPA is the apparent contradiction of the law of conservation of energy.
If a pilot and craft together way 100kg they will need a lift force of 981N to remain aloft. Yet with a power output of only 300w they would only be able to generate a force of 300N. The paradox is therefore that an aerofoil can produce a greater lift force than is imputed to overcome its drag. So it is possible to build a craft with the needed 981N of lift for a 300N of trust from the pilot. The air that is part of the system somehow is foiled in to giving up energy. And though the total system remains balance the craft somehow ends up with a greater share of the energy.
For me the ultimate question is which form or type of locomotion is best from first principals alone?
I have made some criticism of HP-Helicopters in the past, for having the mass of the pilot isolated from the KE of the craft. As such they will struggle to travel anywhere above walking pace. I still struggle to see their attraction beyond a student design excurse.
So this leaves HP- fixed wing aircraft and HP- ornithopters.
If we look at drag and efficiencies of locomotion i believe it can be seen that HPO’s are the superior design in theory, though clearly not in terms of ease of design, manufacture or flight.
The efficiency of locomotion of a propeller is critically affected by two main factors; the smaller the diameter, the faster it must spin to achieve the desired thrust. Yet the faster it moves the air the lower its efficiency becomes as turbulent losses increase. So a larger propeller would be more efficient at moving the air to generate thrust, yet it would in its self create a large structure that experiences more skin drag. This situation means that there will only ever be negative compromise in terms of propeller size.
With a HPO the drag from the propeller is completely eliminated. The motion of the wings will be, by comparison with a propeller, very slow. This slow speed large air mass motion will be more efficient. On these two points the HPO should have lower drag, and be more efficient than a conventional HPA.
Talos do we agree?
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Talos
United Kingdom
32 Posts |
Posted - 08/01/2010 : 07:41:14
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Yes Countrymike. I agree with most of your analysis apart from these points:
Clear as mud! .... Allow me to say the same thing differently. Newton’s first law of motion states:- ‘A body will continue in a state of rest or uniform motion in a straight line unless it is affected by external forces’.
This means an aircraft will continue its flight in a straight line if there are no external forces acting on it. Unfortunately, an aircraft experiences two external forces:- Gravity and Drag. Gravity is counteracted by the lift produced by the aerodynamic force of the wings, which in turn is generated by their speed through the air. Once the aircraft has powered up to the flying speed no further power is required (Newton’s 1st law), where it not for the drag. Satellites don’t require any power either, because they don’t experience drag out in space, but an aircraft does experience drag in the atmosphere, which tries to slow it, so the engine must produce power to equal this drag (assuming 100% efficiency) thereby preventing the speed from slowing.... Crystal clear!
Now, about your paradox. We have just seen that the engine must produce enough (and no more) power to counter the drag, and in so doing the speed is maintained. This means the lift too is maintained. In other words, the engine does not produce any power to provide lift, as this is already accomplished by the aircraft’s speed. The paradox simply does not exist.
I haven’t given much thought to HP Helicopters but I suspect you are right in your assessment.
I agree wholeheartedly that HP ornithopters will be better than HP fixed wing aircraft, for the reasons you give but also from a ‘mechanical engineering’ point of view. Any propeller must rotate, but we humans don’t produce power by rotation. Our efforts are push or pull, which means a crank and crankshaft is required to convert our push/pull efforts to rotary form. And this is further modified with gearing and power transmission through chain drives or pulleys and belt drives. All of these absorb a lot of our energy before it arrives at the business end!
Power from push/pull efforts are ideally suited to flapping wings using leverage principles for power transmission with almost 100% efficiency. And for any human powered vehicle this is critical.
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26 Posts |
Posted - 08/01/2010 : 10:28:43
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Talos,
Perhaps i was using the term “paradox” a little bit casually. But you have to admit a device that can produce a force greater than the input force would, to someone first trying to understand the dynamics of flight, appear to contravene Newton’s laws.
And thank you for expressing more eloquently what i could not. Make sense isn’t my strongest suit.
I would however council caution with how you portray the efficiency of rotary motions. It is a basic fact that a rotary motion is generally more efficient than a reciprocating one. The forces to decelerate and reaccelerate a object in reciprocating motion are in most situation lost as inefficiencies.
Equally important in considering a linkage system is the deflection of pivot points and associated losses in strain energy. These could, in a poorly designed structure, be as affected as a crank setup.
Perhaps it would be worth explaining the balance of forces in an ornithopter. The basic explanation of the phases of the stroke might help more people see the potentials of a HPO. I’m sure i understand it, but it will take some time to find a form of words that works.
What are the chances of you changing your mind on collaborating on a dedicated wiki? You don’t need to give away all your secrets, just the basics, so people can learn the principals without the enormous investment in research time otherwise nessasery.
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Talos
United Kingdom
32 Posts |
Posted - 08/02/2010 : 03:33:54
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Countrymike, I must disagree with your assertion that a rotary motion is generally more efficient than a reciprocating one. This is simply not true. As a ‘materials’ engineer you must surely know that every material in a machine is designed to work within ‘elastic’ limits (recall Hooke’s law). A material which is allowed to exceed these limits will deform or break, rendering the machine useless.
To jog your memory and for the benefit of readers unfamiliar with elastic limits, a concomitant phenomenon of operating within them is that of ‘strain’, i.e., the stretching or compression of the materials used in a machine. This strain is caused by absorption of energy, and because the materials are ‘elastic’ they will (by definition) sooner or later spring back to their original shape and in so doing, release the energy that is contained in the strain (extreme examples are an Archer’s Bow, a bungee catapult or athletes pole in the pole vault...etc). In other words, elastic materials store energy as ‘strain energy’ which is given back later; the energy absorbed is not wasted, apart from a very small amount due to tiny atomic/molecular movements (friction) within the material (almost immeasurable). And because they absorb and give back energy they are true reciprocating devices.
Countrymike, you seem to have missed my point, which is; that our push/pull actions are reciprocating actions to start with, and as such they can be applied directly to the flapping mechanisms within the ornithopter structure. They are not the result of external acceleration and deceleration forces.
Also, a lever does not absorb or waste any energy; again, apart from a very small amount due to tiny atomic/molecular movements within the lever. Energy wastage due to external friction is totally eliminated because a lever can pivot over a (non-yielding) rigid fulcrum, i.e., the lever doesn’t slide.
An ornithopter that uses appropriately designed elastic materials and leverage, will operate with almost 100% efficiency in terms of applied energy being delivered to the business end.
On the other-hand, rotating devices will experience friction around hubs, axles, bearings, chain links...etc, all of which waste energy. In other words, rotating devices can never be as energy efficient as true reciprocating ones.
Sam Whittingham has pedalled his speedbike at over 80 mph. I wonder what speed he could get if it flew using flapping (reciprocating) wings driven by more efficient reciprocating inputs?
For further insights into my ‘secrets’ as you put it, interested readers are invited to visit my website: www.talosperdix.co.uk, or Google: Elements of Ornithopter Design. |
Edited by - Talos on 12/07/2010 03:28:03 |
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26 Posts |
Posted - 08/02/2010 : 10:54:17
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An interesting light page you have their Talos.
I shall assume then that you intend to use a spring in some form or other to apply a force at the end of the stroke. Such a contrivance would indeed help to achieve a high degree of mechanical efficiency. In general however a reciprocating motion (like a piston in an engine) will require a force to decelerate then re accelerate. Where as a rotation will continue until opposed....
The mechanical efficiency of a fixed gear bike in optimal conditions is 99% or possibly over. Further the power output of the rider increases as the cadence increases, (to about 140 rmp or something). This rate is critical and NO other motion has produced a greater amount of power from a human.
My personal leanings are not to use a crank and to use a linkage with a direct linkage to the leg motion. This is not in the belief that it might be more powerful, but that it might afford some degree of control.
My understanding was that the hour record on a bike was set at an output of about 400watts. A generally fit athlete might maintain 300watts. With this in mind it might be reasonable to assume a lower output for the less efficient locomotion you are proposing; 200watts perhaps?
Still i’m glad to see you have afford 20% for the mechanical efficiency. This does seem sensible.
I guess 160watts to sustain flight is starting to seem a little optimistic. If a good hand glider has a sink rate of 0.8m/s then a HPO would need to make a marked departure to work at 0.16m/s.
I’m not sure if you have come across a post by a chap called flyboy51 on another forum? Claims to have achieved flight with a HPO back in the 70s. any way the page seems to be down at the moment, but it was on http://www.ornithopterresearchgroup.com/
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Talos
United Kingdom
32 Posts |
Posted - 08/03/2010 : 13:01:48
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Countrymike: Nine points.
Are you really trying to say that a decelerating force and an accelerating force in some way wastes energy in a true reciprocating device?
I know that one test (the best I’ve seen) on bicycle gears had a claimed (doubted by other equally capable researchers) efficiency of 98.7% with the power provided by an electric motor. Had the power been provided by a human then cranking arms and pedals would have been necessary. A cranking arm is simply a lever, but the pedals would have had to rotate around a shaft on the end of the arm. This would have introduced some power loss at source due to pedal friction, which didn’t come into the laboratory test calculations. If we allow 0.5% loss for the pedal the overall efficiency would drop to 0.987 x 0.995 ~ 98.2%. But this is only half the story. To convert the rotary motion to a flapping wing would require a cranking arm and rotary attachment points on each of the wing spars (delivery pedal) at the business end with further losses. Assuming the same loss for each, the overall efficiency then becomes 0.987 x 0.995 x 0.995 x 0.995 ] = 97.2%. If the pedal and cranking arm losses were higher at say 1.0% each, the overall efficiency would be 0.987 x 0.99 x 0.99 x 0.99 ~ 95.8%. The overall efficiency for a gear driven ornithopter is therefore also dependant on these peripheral components, which can have a large influence.
As regards the cadence, different speeds of sprocket can only change the flap frequency and will not change the amplitude for the flap input movement, due to the final output cranking arm being of a fixed length. A reciprocating device does not suffer from this handicap. It can accommodate any frequency and produce any degree of flap amplitude, from zero in the glide to near 180 vertical degrees of flap at full power, i.e., around 300watts or 400watts in level flight to 1400watts or more during take-off, depending on the L/D ratio, required flying speed, and pilot fitness. And a true reciprocating device’s efficiency is higher than 99% at any cadence.
I don’t know how you can say that my ‘locomotion’ is less efficient since you don’t know what my ‘locomotion’ is?
How did you arrive at 200watts?
Where does that sink rate of 0.16m come from?
My website’s calculation is intended only as an illustration, (for obvious reasons) and you should not take it literally.
In Elements of Ornithopter Design, methods are shown where designers can determine the overall efficiency for their own machine so that they can ensure that their personal energy levels will exceed the aircraft’s energy requirement.
As regards flyboy51’s claim...the silence is deafening.
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Edited by - Talos on 08/07/2010 00:56:58 |
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26 Posts |
Posted - 08/07/2010 : 09:29:32
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I still haven’t managed to join the forum, but it has started working again. http://www.ornithopterresearchgroup.com
Flyboy51; “30+ years ago I was competing against Paul McCready for the Kremer prize. His Gossamer Condor obviously beat me to the finish line. Where he followed fairly conventional methods, I took a more novel approach; something I'm sure you all would be interested in. I built, and flew, a foot-launched, human-powered ornithopter. This was pre-internet. Since I lost out on the prize, I never sought publicity and hung up the project to work on other endeavors.”
And yes i am suggesting that a basic reciprocating motion would lose power in arresting and reaccelerating its mass, unless useful work can be derived from it.
And 200watts was an estimate based on a less than perfect athlete, with a less than perfect locomotion. 0.16m/s sink rate is gained from a 20% mechanical efficiency in converting to PE.
Much of my understand of the efficiency and power outputs are based on what i have read in Human Power in the archives on this sight. I’m sorry if I’ve miss understood, or miss represented, or miss remembered anything that I’ve read. I’m just too lazy to recheck it all.
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Talos
United Kingdom
32 Posts |
Posted - 08/08/2010 : 06:02:32
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Countrymike; Sir Isaac Newton (1642-1727) and Johann Kepler (1571-1630) would disagree with you, as do I.
Energy (and therefore power) is lost only by frictional resistance. Remove the friction and no power is lost by acceleration or deceleration.
All heavenly bodies that orbit the Sun do so in elliptical orbits (almost circular, but definitely rotary motion) and accelerate as they move towards their nearest point to the Sun (perihelion), only to decelerate as they swing past on their outward journey to their farthest point (aphelion). In space there is no frictional resistance, hence the accelerations and decelerations merely cancel each other, and the body loses no energy or power.
In a reciprocating device, whether it does useful work or not, it is frictional resistance that wastes power; not the acceleration or deceleration.
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Edited by - Talos on 08/08/2010 10:06:10 |
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26 Posts |
Posted - 08/10/2010 : 12:33:39
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| So what is your spring for then? |
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Talos
United Kingdom
32 Posts |
Posted - 08/11/2010 : 06:47:25
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Countrymike; I’m sorry, but I never said I had a spring! I did say however, that all parts of a machine used materials, that were ‘elastic’.
In this regard, I see elasticity (or flexibility) as essential in a machine such as an ornithopter. This is because a bird’s feathers are highly flexible, especially at their tips. A bird’s bones are also flexible, compared to ours. This is because they are generally longer and thinner than ours; compare the length of their wings with that of our arms. You might have come across a thin and very flexible, almost needle like bone, when you have eaten chicken; and some people have unknowingly broken these bones in their mouth and actually choked on them. The wishbone is another example of a flexible bone.
Then there are the tendons, which stretch as they pull, absorbing strain energy in the process.
All this stretching and flexing absorbs energy which can be recovered when needed to supplement muscular effort.
In other words, surplus energy can be stored in several places within a bird’s skeletal and physiological make-up.
When I think ‘ornithopters’ my thinking isn’t hard and fast; it’s ‘flexible or elastic’. |
Edited by - Talos on 08/11/2010 06:53:27 |
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15 Posts |
Posted - 08/26/2010 : 17:33:31
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Some articles that maybe of interest;
some are a couple of years old.
At: http://news.sciencemag.org/sciencenow/engineering/?p=1
Floppy Wings = Efficient Flight
Computer simulations reveal why desert
locusts are such economical fliers
Lord of the Wings
Dragonflies get a boost from
out-of-sync flapping
A Swift Understanding of Flight
Birds could inform better aircraft design
Taking a Page From the Book of Flight
Artificial evolution helps researchers design
craft that may someday fly like birds
Harry \../ |
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Posted - 11/26/2010 : 13:16:08
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I am sorry if i resurrect very old threads, but i see a lot of ideas that would need some further clarification.
Countrymike, in your first post you seem to confuse power with force. Having 300W doesn't mean one can only generate 300N of force. one can generate 1 million N of force with 1W, but at a very slow speed. Power is Joules/time = force*velocity.
For an HPA calculating the drag is rather simple: for example, take the speed, 10m/s (meaning 36km/h, a fast HPA), the pilot supplies 200W, thus, if we assume a 100% efficient propeller/transmission the pilot effectively fights 20N of drag.
The air never gives any energy. Again, dont confuse the units. needing 981N of lift doesnt mean any energy is required. A blimp can stay alof without dissipating any energy, as it's weight is matched by buoyancy.
The weight of the airplane is supported by lift, which is generated by air moving over it's wings. The air generates drag, which, if the pilot applies no power, would slow the airplane down. The pilot only fights the drag of the airplane, he never supports the weight of the airplane directly, even for a helicopter.
Talos, i agree with you assessment that humans are more efficient pulling - pushing than rotating shafts really fast, and thus our HPO has a rowing motion for the pilot, with a very simple pulley system.
However, I do believe you are mistaking in saying that reciprocating systems do not experience any power losses, because they do (just as rotating systems do, if the rotation velocity changes). Lets look at what power is: change of energy over time. Take a horizontal piston's cycle:
time 1) position A, stationary. Energy = 0
time 2) between A and B, travelling at max speed. Now the Energy is v^2*m
time 3) Position B, stationary. Energy = 0
As we can see, the overall change of energy in the system is 0. However, between time 1 and time 2, the energy changed, in a finite amount of time. This means that the average power needed is v^2*m/(t2 - t1). Power is needed to decelerate the cylinder back to 0 in (t3 - t2). The power to decelerate the cylinder is negative, meaning it could be used to do work somewhere else in the system, but nonetheless, it must be dissipated somewhere. Unless the system is designed right, the pilot will need to supply both the accelerating and decelerating power requirements.
same applies to rotating systems where the angular velocity changes in time. A rotating wheel at constant velocity is equivalent to a piston traveling at constant velocity, of course nothing changes and no power is needed. However, as soon as acceleration is present, power is required.
cheers
Victor |
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Talos
United Kingdom
32 Posts |
Posted - 11/27/2010 : 03:08:05
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Victor. All dictionary definitions of the word ‘reciprocate’ use descriptions such as receive mutually, interchange, return...etc.
I am not talking about a horizontal piston per se. I’m talking about devices that deliver Simple harmonic motion (SHM). A pendulum swings with SHM and is a reciprocating device. It accelerates and decelerates repeatedly and would go on for ever if it didn’t experience friction and air resistance. None of its energy is lost (not even for an instant) due to accelerations and decelerations. Accelerations and decelerations are part and parcel of the energy store within the reciprocating device itself.
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