The concept of flapping flight will eventually succeed!

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Dan Thomas

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If you look at a landing duck the wings are cupped for a high camber airfoil, the webbed feet are spread out as airbrakes, tail also spread for max area. And they are at a high angle of attack as noted above. Usually flap a bit to break the descent just prior to touch down.


Bird style flapping efficiency may require bird style highly variable wing geometry.
That duck's wings have also swung forward, putting the center of lift far ahead of the center of gravity, mandating the tail's spread to avoid tumbling backward.

To duplicate that in a flapping-wing airplane will require computer-control of every aspect of wing and tail movement. Things happen too fast with moving CL for a human to control that. So now you're getting to the level of modern fighter jets with their highly unstable configurations to enable radical maneuverability. That won't be cheap or easy.
 

DennisK

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This. it wont matter what state of the art or futuristic motor/engine/actuator, power source, materials etc. you invent but without the knowledge and understanding of the mechanisms responsible for lift and thrust generation then flapping flight will not succeed.
There is a decent amount of information available to understand the overall aerodynamics for birds (I haven't done much reading on bats). John Videler's book, Pennycuick's "Modelling the Flying Bird", various articles on jeb.biologists.org, Al Bowers' lectures on Prandtl twist. This video has some good visualization of the spanwise location of the wingtip vortices (which are not at the wingtips)

This video shows the covert feathers' ability to flip up and shed turbulence to delay stall, and the "venetian blinds" effect of primary feathers allowing some air to flow through the wing during upstroke (jump to 2:30)

Videler's book talks some about leading edge vortices being used to generate extra lift, particularly in swifts.

Thrust is mostly from the hand wing, because it is farthest from the center of rotation for the flapping motion so it gets the most added airspeed. And the Prandtl twist (which is good to have anyway to minimize structural weight and induced drag and generate proverse yaw when turning) allows the hand to be tilted so its lift vector points partially forward, while the arm wing remains at positive angle of attack to generate plenty of upward lift.

If you look at our three prime examples (birds, bats and pterosaurs), the essential elements appear to be:
1. 3-axis shoulder with high torque to weight ratio
2. Wing skeleton with 3 segments, where the elbow does not extend to fully straight
3. Ability to quickly fold and extend the wing

Start with that and you'll probably get something usable. The hand segment of the skeleton appears to be the place to experiment with different designs, and may have additional degrees of freedom to provide fine control. But nothing major, because any actuators out on the wing add inertia which wastes energy during stroke reversal.
 

jedi

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To duplicate that in a flapping-wing airplane will require computer-control of every aspect of wing and tail movement.
I do not know of any scientific requirement for computer control of the wing movement.

modern fighter jets with their highly unstable configurations
The designer can create a stable airplane or an unstable airplane. The same can be done with an ornithopter.

That duck's wings have also swung forward, putting the center of lift far ahead of the center of gravity, mandating the tail's spread to avoid tumbling backward.
There are many who would say this is an unstable situation, but is it? Does it need to be unstable or can it be designed to be stable. There is a project in works to demonstrate how this configuration can be stable.


2. Wing skeleton with 3 segments, where the elbow does not extend to fully straight
I am still trying to understand the use and benefit of the bent elbow. I suspect it is to control aeroelastic twist as a function of aerodynamic torsion. Any other ideas?

Perhaps it is details such as this that allows an aeroelastic property to negate the need for computers and servos to control the wing shape and dynamics.
 
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Dan Thomas

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There are many who would say this is an unstable situation, but is it? Does it need to be unstable or can it be designed to be stable. There is a project in works to demonstrate how this configuration can be stable.
Any winged aircraft, in which the CG is behind the CL, will be unstable. If airspeed falls, the nose rises and airspeed falls further, eventually stalling the airplane and often entering an unrecoverable spin. GA airplanes must legally be designed to be stable to avoid this, and the POH of every airplane will have weight and balance information, and CG limits, to keep it that way.
 

DennisK

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I am still trying to understand the use and benefit of the bent elbow. I suspect it is to control aeroelastic twist as a function of aerodynamic torsion. Any other ideas?
It allows you to have control of the twist axis without much energy consumption. If the elbow were straight, the center of lift would be far behind the skeleton, putting a large static torque on it which the muscles would have to support. With the elbow bent, the humerus points toward the center of lift, so the static torque is only on the forearm and hand, which are supported by ligaments (no energy consumption or fatigue).

Alternatively you could add more wing structure forward from the skeleton to shift the center of lift, but that would probably get in the way when folding the wing. Plus the torque you can generate with a bent arm is much greater than you can by twisting a straight arm, so you have more control authority too. Think arm wrestling :) That part is not really relevant for robots, though. At least not when using a gear or belt reducer mechanism for humerus twist.
 
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OrVNstabilize

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flapping wings..."is a concept about which we know frighteningly little" ---Dr. Strange

the literature describing the mechanisms of lift and thrust production over the last 50 years is ...scant to say the least... most only pertaining to insects but almost nothing about larger birds.. it's not as easy as just observing, identifying and describing certain mechanisms or phenomena, there's a reason why our best attempts at replicating it looks so crude and ham-fisted.
 
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Aesquire

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To the Premise, I say... For values of Succeed. ;)

I'll point out that we humans walked for a Really Long Time until Honda and Boston Dynamics built working walking machines that aren't toy level spider type walkers. Bipedal and DYNAMICALLY stabilized walking is far more complex than thought just a very few decades ago. If you look at the progress from the early Honda 'bot that first climbed a stair, to the acrobatic version of today, it's impressive.

So, sure. Flapping wing flight has promise.

But I'll point out that Walking Vehicles are coming soon to military, recreational, and Forestry purposes, and will be delivered to the battlefield, track, and woods, in/on Wheeled vehicles and trailers just like tracked vehicles are, and for the same reasons. Off road optimized travel modes may be/are inefficient and even destructive to existing roads.

In Aviation, the infrastructure ( big flat runways ) isn't a problem for ornithopters, like it is for rocket/turbojet VTOL ( F-35 ) that will tear up "soft" surfaces. ( like, "not fire resistant armor" ) & I'll ignore regulatory issues with many ideas, so, sure...

For niche, specialty, purposes, flapping wings Will eventually succeed. See Heinlein's "The Menace From Earth".

But it's going to be a long time before the theoretical advantages are fully realized, it ever.

Neat stuff! The micro-flapping of tiny actuators as seen in DLP projection video systems micro-mirror DMD chips, applied to a wing surface, may someday equal or exceed the control of flow of bird feathers.

Re: Duck landings.

My SOP for approach path control in my hang glider ( one of my old ones ironically named Duck ) is to fly at higher than max L/D speed and use my body/legs as drag devices by changing position from prone or supine to upright. Final flare greatly resembles the image above, albeit with much less grace and style.
 

jedi

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Any winged aircraft, in which the CG is behind the CL, will be unstable. If airspeed falls, the nose rises and airspeed falls further, eventually stalling the airplane and often entering an unrecoverable spin. GA airplanes must legally be designed to be stable to avoid this, and the POH of every airplane will have weight and balance information, and CG limits, to keep it that way.
You are repeating text book aerodynamics akin to the Bernoulli lift theory. It is a simplified partial truth that does not contain a complete understanding of the processes at work.

The statement you make assumes the CL (Center of Lift) is the CL for the wing and the CG (Center of Gravity) is the CG for the aircraft. The complete example includes a stabilizing tail at the rear of the aircraft. The tail also has a lift component that may be positive or negative.

It is because of examples like that given that many pilots to believe the tail needs to have a downward lift force for the aircraft to be stable. That is not true. An aircraft can have a lifting tail and still have pitch stability.

If your statement “Any winged aircraft, in which the CG is behind the CL, will be unstable.” considers the CL as that of the total aircraft, the aircraft would pitch up with the CG behind the CL and it would pitch down with the CG ahead of the CL. Only if the CL and CG are aligned vertically would the aircraft continue in steady flight. Even this more detailed explanation assumes some unspecified simplifying assumptions such as the center of thrust and the center of drag are aligned to eliminate additional aerodynamic pitching moments.

If the CG is fixed by the aircraft loading (and the thrust drag moment remains zero) how does the pilot control the pitch? The elevator controls the pitch by moving the CL forward or aft or keeping the CL aligned with the CG for steady flight. This has nothing to do with stability.

Stability has to do with how the CL of the entire aircraft changes with a change in the angle of attack (AOA). Lock the elevator in a fixed position. If the CL moves aft with increasing AOA the aircraft is stable in pitch. This is how the designer configures a “flying wing aircraft”. If the designer uses a wing that has the center or pressure (another name for the CL so as to not be confused with the lift coefficient) move forward with increasing AOA a horizontal tail lifting surface is included to cause the total aircraft CL to translate rearward with increasing AOA. This gives the aircraft “stick fixed pitch stability”. Unlock the elevator and let it float free in the airstream. If the CL still migrates aft with increasing AOA the aircraft has stick free pitch stability. Now the pilot is able to reposition the elevator to control the pitch of the aircraft.

So what? We now know that the designer can make a wing that is pitch stable or an aircraft configuration that is pitch stable. We know that the tail can assist the wing lift by both lifting in the same direction or it can counter the wing lift. We know that IF we want pitch stability the CL must move rearward relative to the CG with increasing AOA. This can be done by aerodynamic design or by changing the CL or changing the CG or by computer, human or other means to control of an aerodynamic surface.

What is the point of all this? The duck is simply using his/her ability to control wing sweep, incidence and dihedral in addition to tail (elevator) position to improve landing performance by controlling not only the CL but also the center of thrust and drag.

The design of a flapping wing aircraft may use the additional variables to optimize the design and operation of the aircraft.
 
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Dan Thomas

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So what? We now know that the designer can make a wing that is pitch stable or an aircraft configuration that is pitch stable. We know that the tail can assist the wing lift by both lifting in the same direction or it can counter the wing lift. We know that IF we want pitch stability the CL must move rearward relative to the CG with increasing AOA. This can be done by aerodynamic design or by changing the CL or changing the CG or by computer, human or other means to control of an aerodynamic surface.
Where do you get this stuff? The CL moves forward with increasing AoA, not aft. It's due to the boundary layer separation happening on the aft top surface of the wing, and the max low-pressure area movement. It's a destabilizing force.

1641834381077.png

Please post a link showing that a lifting tail is a stable configuration.
 

DennisK

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I'll point out that we humans walked for a Really Long Time until Honda and Boston Dynamics built working walking machines that aren't toy level spider type walkers. Bipedal and DYNAMICALLY stabilized walking is far more complex than thought just a very few decades ago. If you look at the progress from the early Honda 'bot that first climbed a stair, to the acrobatic version of today, it's impressive.
Mostly it was a matter of power to weight ratio. Those old robots used slow and rigid worm gear reducers, which allow the robot to stand up without constant expenditure of energy, but also force it to remain in a statically stable position at all times, even when only one foot is on the ground. That approach was obviously never going to give life-like performance, as it is with all the flapping attempts which ignore the three essentials I listed in my previous post.

Watch through James Bruton's robot dog development on youtube. He started out with ballscrew actuators, which are technically backdriveable but not without a fair amount of force due to the high effective reduction ratio and cogging torque of the motors. Predictably, the robot couldn't balance worth a darn. But as soon as he made even a crude backdriveable leg, it was able to balance effortlessly. After that it was just a matter of getting the power to weight ratio high enough that it could move without running up against its torque limit all the time and failing to complete the intended motions.

Probably the most successful ornithotper I've heard of is Adalbert Schmid's, which used fixed wings for lift and a small pair of flapping wings for thrust. That is analogous to using legs to propel a wheeled car like Fred Flintstone :) It works, but you gain no functional advantage over any other kind of thruster. Most other ornithopters flap the primary wings, but still only in a fixed cycle, giving no functional advantage. The 3-axis shoulder and folding wing is where the magic lies (and I should amend that the shoulder also must be backdriveable). But as with the robot dog, until you get the power to weight ratio high enough, you'll constantly run up against your limit and sink back to earth, if you can maintain control at all.

Bipedal walking certainly is more difficult than quadrupedal, but nonetheless if you build the machine correctly, the control programming goes from essentially impossible to relatively easy. Acrobatics are obviously more difficult, and the same will be true for bird-bots, but my bet is that basic flying will be easy. After all, birds figure it out between the time they jump out of the nest and the time they hit the ground.

And if you copy nature's designs as closely as possible, then the most difficult work of analyzing performance and optimizing and balancing all the modes of flight is already finished.
 

henryk

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The duck is simply using his/her ability to control wing sweep, incidence and dihedral in addition to tail (elevator) position to improve landing performance by controlling not only the CL but also the center of thrust and drag.
=KASPERwing can do too...

-KASPERwing have STATIC stability and controlibility !
 

jedi

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The CL moves forward with increasing AoA,
This is the problem with PP ground school classes. Sometimes they teach just enough to be misleading or even dangerous.

I get it from aero engineering classes. The CL [Edit: of the typical airfoil] moves forward with increasing AoA. This destabilizing moment about the wing 1/4 cord point is generally countered with the stabilizing moment of the horizontal stabilizer (either a tail or canard).
 
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Dan Thomas

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This is the problem with PP ground school classes. sometimes they teach just enough to be misleading or even dangerous.

I get it from aero engineering classes. The CL [Edit: of the typical airfoil] moves forward with increasing AoA. This destabilizing moment about the wing 1/4 cord point is generally countered with the stabilizing moment of the horizontal stabilizer (either a tail or canard).
So if I'm right, what is misleading or dangerous about it?

The center of pressure moves forward as AoA increases due to accelerated airflow over the leading edge and the decay of the boundary layer on the aft upper surface. Once the AoA reaches stall angle and all the airflow separates, the CP moves back suddenly since the lift is now all coming from dynamic pressure on the underside. This aft shift is one of the factors that causes the nose to drop in the stall.
1641869292475.png


I still need that link from you showing that a lifting tail (not a canard) can be made to be a stable configuration.
 

jedi

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I still need that link from you showing that a lifting tail (not a canard) can be made to be a stable configuration.
Glad you ask.

This is just one reference. There are many good aero design books. Study stability and control. I hope this helps. Happy to explain further if you would like to talk. Otherwise this is too much back and forth typing for me so if you want print check the internet or a book. Diagrams work best. Easy to understand once you get the overall picture.

A quick google search gave the following.

Lift
Depending on the aircraft design and flight regime, its tailplane may create positive lift or negative lift (downforce). It is sometimes assumed that on a stable aircraft this will always be a net down force, but this is untrue.[2]

2. Burns, BRA (23 February 1985), "Canards: Design with Care", Flight International, pp. 19–21, It is a misconception that tailed aeroplanes always carry tailplane downloads. They usually do, with flaps down and at forward c.g. positions, but with flaps up at the c.g. aft, tail loads at high lift are frequently positive (up), although the tail's maximum lifting capability is rarely approached..p.19p.20p.21
 
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qchen98

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Do birds suffer inflight structure failure as well?

Imagine a seagull doing a highspeed dive. Theoretically, the aerodynamic load could break its wing if the seagull exercises poor judgment when pulling out of the dive- but why is it so rare to see any cases of birds doing controlled flight into terrain?
 

jedi

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So if I'm right, what is misleading or dangerous about it?

This is the plane that Mark died in. If memory serves me right, on take off, it pitched up violently,stalled , then flipped over into a noes-dive. There was no way Mark could survive. RIP.
View attachment 67145
Mark Stull was a firm believer in the tail download and ignored the advice of others. This design had tail download that was excessive. The aicraft could not be flown as there was not enough nose down elevator control. The result was a stall crash on the first takeoff. This issue was not discovered in taxi testing because the main gear was placed too far aft.

A poor understanding of pitch stability and pitch control was not the only problem but it was a contributing factor.
 
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BBerson

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The statement you make assumes the CL (Center of Lift) is the CL for the wing and the CG (Center of Gravity) is the CG for the aircraft. The complete example includes a stabilizing tail at the rear of the aircraft. The tail also has a lift component that may be positive or negative.
I think Dan may have missed the point above from Jedi about the ”complete aircraft”. The tail area cancels the nose pitching moment of the wing. The tail can be lifting if it is large enough, such as a tandem wing configuration. For speed stability, the tail or rear wing needs to be large enough to operate at a lower angle of attack than the front surface (lower lift per square foot).
 

jedi

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The center of pressure moves forward as AoA increases due to accelerated airflow over the leading edge and the decay of the boundary layer on the aft upper surface.
Sorry. My post was lost. I may try again later but for now I just want to agree that the above bold print is one of the factors.

That explanation does not recognize the many other details at play that determine overall stability and control that allow the duck pictured above to ace his landing relative to the typical Cessna student enrolled in the PP course and trying to land with that silly wheel hanging off his nose.

Once you get that picture firmly in mind, picture adding giant bazookas for booties on the duck's feet, each with a teeny tiny wheel hanging off the front.

Is it any wonder that the typical sea plane has such PP (P*s Po*r) performance.

:) :) :) :) :) :) :) :)
 
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