Thinking outside the box

Guys, I've got a question. It is not a trick question.
Assume that you have a tailess flying wing with a span anout 36 feet and an aspect ratio about 17. Gross weight is about 330 pounds. There are no ailerons or flaps, no rudder, no elevators. The wing is a visco-elastic membrane supported by an articulated leading edge spar, but without a trailing edge tendon. The wing is tension battened, not compression battened, and you can control camber by slightly increasing or decreasing local surface area of spots about the size of your hand, thereby allowing the pressure jump between the upper and lower wing surface to locally modulate the shape of the camberline. The wing is not bat-like.

The question is, how do you provide substantial yaw command authority (on the loose order of 90 ft-lbs) without simultaneously creating pitch or rolling moments. Again, this is not a trick question.

To yaw to the right, decrease camber on outboard end of left wing (to decrease drag) while simultaneously increasing camber on outboard end of right wing (to increase drag).

You're going to have to increase lift slightly on the right wing while slightly decreasing lift on the left wing during the right-hand yaw-turn.

Thanks.
Goal is not to turn, but to correct a yaw upset. Your first paragraph is correct, but won't generate the 90 ft-lbs of yaw. Need more. Airspeed BTW, is on the order of 35-70 mph.

Second paragraph, you don't have to do that if you don't wish to. If you want to keep the wings level while still yawing, you just increase the camber of the inboard wing enough to offset the lift lost due to the outboard decrease in camber. I should have mentioned that these critters don't usually use opposing movements between left and right wing. In other words, a roll for example, isn't initiated by simultaneously increasing lift near one wing tip while decreasing the other by the same amount. Instead, one wing at a time. Same holds for yaw (in this particular scenario, at least). Pitch is implemented by moving the wings fore and aft. Either by sweep, or by translating the entire wing fore and aft.

My head hurts.

When you got to tailess, I was trying to figure out who shot off the tail, and how you were still flying!:???:

I need more education;-)

Jim,

What kind of dinosaur are you building?

Ursa Major said:

Jim,

What kind of dinosaur are you building?

Click to expand...

!!??:lol:

Mike, not a dino - a pterosaur. And not building one; I did that over ten years ago. Actually, for the last two decades, I've worked on the biomechanics and flight mechanics of the real thing, the largest species known.

Candidly, they do actually have a very powerful aerodynamic tail complex consisting of the hindlimbs, the biological tail, and a membrane that connects the two. They also have a ten foot long, rather stiff neck and an eight foot long head out in front of the wings. However, I have a close friend for whom I have a great deal of respect, and he is convinced they had no tail membrane and flew with the legs retracted up under their body. I feel that I owe it to him to make a serious effort to find a way (if they had one) to provide adequate yaw command authority to overcome the eighteen feet of head and neck stuck out front without making use of the tail structure. In twenty years, I have not been able to do so. Neither could my friend Paul MacCready. In the half scale replica that he built back in the 80's, he had to artificially shorten the neck and head and also 'break' the neck with a non-existent bend to move the head far enough aft to allow his replica to fly without a tail.

So, I'm obligated to give that scenario a fair look. When using the tail complex, the animals could generate a 90 ft-lb yawing moment without difficulty and without substantially altering pitch and roll moments. This was more than enough command authority to serve their needs.

But to make my friend's scenario work (if it did), I would need to find a way to equal that moment using wings alone. Can't do it using the head and neck as a rudder -- the yaw forces they create are quite substantial, but too non-linear and erratic to serve as a rudder. In twenty years, I have not been able to make the tailless scenario work. Neither could Paul. But that doesn't imply that there was no way to make it work. Just means no one has found one. And, if someone does, it doesn't imply that the animals did it that way either. So, I'm doing the Fair Witness thing, attempting to look outside the box. I could write thousands of pages on how this stuff all interacted in life, but for purposes of this conversation, I'd rather just look at the big picture.

Here's a photo of me out in my front yard holding a left humerus cast of the big guy. I'm holding the shoulder with my right hand and the elbow with my left. The doohicky sticking out from the shoulder is the delto-pectoral crest. It attached a few of the muscles associated with flapping flight.

If you none of you have seen the Peregrine Falcon defending their territory it's quite amazing. Pelicans seem to be unable to reposition their neck once unsettled from it's normal flight position. This may be why the big bird couldn't use it's head for yaw control.



A quick search found this cheesy clip, but the cameraperson holds steady most of the way through. I'm sure most will find it interesting.

Could your beasts have used their legs and feet for yaw control?

Is this what you are asking about? It seems that just turning the head should be enough to control yaw.

[video]http://video.dailymail.co.uk/video/bc/rtmp_uds/1418450360/2015/09/07/1418450360_4468383406001_4468338198001.mp4[/video]

Hi, Skywagon. No, the still image is of a Tapejara imperator, and the video looks like a Pteranodon longiceps. Both short-neck pterosaurs. The long-necks couldn't afford large head crests, so had only quite tiny crests.

Here's an image of a small long-neck, Quetzalcoatlus species (15.75 foot span) seen from above in maximim sideways deflection. Vertical mobility is similarly restricted. The head goes on the end of the neck nearest the ladder. The grid in the background is 6 inch squares. The big guy's neck was over twice this length. They also couldn't achieve the pelican like neck bend as seen from the side. The short horizontal structure at the top of the image is the spine, from the base of the neck back to the front of the hip. Head length was about 70-80% of neck length. For this small morph, head length was about 43 inches, neck length about 58 inches, and distance from shoulder joint to hip joint, about 14 inches. To give a visual sense of scale for the head of the large morph, sit the base of the skull on the floor, snout pointing straight up. The tip of the snout will touch the ceiling. The neck was about 25% longer than that.

Anyway, if you disallow use of the aerodynamic tail structure (which I myself, don't), then how do you create enough yaw moment with wings alone to overcome inadvertant head and neck displacements?

As an aside for additional visualization, in those pterosaur fossils with wing membranes preserved, in no case is the trailing edge of the wing membrane at the elbow located more than 45% of the length of the humerus behind the elbow.

Birds can descend vertically using vortex lift, as demonstrated by Wittold Kaspar, with his Kasparwing ultralight, and several demonstration aircraft he built. My observations of birds at the airport have been concentrating on how they do roll control in this mode, but it suddenly occurred to me that a tip could be bent to produce yaw control using this method.

Or, you could go to New Guinea and observe one in flight.....:???:

Hi, fobjob.
What you say is true, though due to the substantial difference between the way bird and pterosaur wings articulate, and the aeroelastic number limitations on pterosaur wings that don't apply to birds, pterosaurs can't configure their tips to create that outboard drag vortex.

The antique critter is a model of Pteranodon longiceps (at that time, still called Pteranodon ingens). Note the fake bat fingers in the wing membrane. Cute though.

I tend to disagree that limited mobility in the neck would preclude the use of the head in yaw control, indeed it would be an advantage in preventing over-control. An experiment would be in order, I think. How many degrees of deflection are likely? The picture you posted would imply that there is adequate deflection. The sharp leading edge and membrane trailing edge is ideal for vortex lift, and I would be astonished to think that a pterosaur did not use it, in at least part of it's flight regime......it is still largely unknown how birds affect roll and yaw control while in vortex mode. Limiting our discussion to non-vortex mode, and eliminating (perhaps prematurely) movements of the head, then we are pretty much left with differential CG and CL shifts to accomplish the yaw, though that's where my idea engine grinds to a stop, at least for now. More coffee!

The neck limit does serve the purpose of limiting divergent deflection to an amount that be compensated for by the wings and hindlimbs. The head serves as an anti-servo. As the neck goes left, it goes right. Forces generated however, are quite non-linear and erratic.

We have done quite a bit of experimenting. By coincidence, Paul and I both budgeted a little over $550,000 for our two seperate projects (which were done about twenty years apart). Our results each confirmed what I said above.

Note though, that my focus in this thread is to accomodate my friend -- attempt to find a way to overcome by wings alone, the yaw forces created by a neck offset to the maximum limit.

Don't know diddly about bio mechanics, etc. But how about increasing dihedral? If you flew with the wings at an increased angle it seems that it would also increase stability. Then maneuvering would just be an adjustment of each wing. Think about a V tail Bonanza.

Web

The head acting as an anti-servo to prevent excessive yaw is an indication that the critter could and would use the head as a yaw control. So, why would a wings-only method of yaw mitigation even be necessary? From the falcon video, it looked like the pelican lost control of his ability to correct his head position once the falcon bent his neck enough to get the wind to keep it there. Is that kind of situation what you are describing? The pelican crashed, so maybe the answer is "crash"...

Well, from my perspective, wings only correction is not required. From my friend's perspective, it is. I'm doing him the courtesy of looking at his scenario as fairly as I can. My ability to be fair to him is somewhat limited, more limited than I would like.

It is possible to recover from the divergent yaw scenario, but there is a good bit of altitude loss. You have to be careful not to let the airspeed build up to the point that the aeroelastic number of one wing falls below the bistable limit.

As an aside, these animals were not stable in flight. Their ancestors were stable.

I agree with something Paul said about the long-necked pterosaurs (azdharchidae). "It's like having the feathers on the wrong end of the arrow". We both found it most effective to detect the relative wind with repect to the head, and just keep the head pointed into it.

Another aside. You can sometimes power yourself out of a bad situation by increasing the flapping amplitude on one side.

These are interesting little beasties.

JimC said:

Another aside. You can sometimes power yourself out of a bad situation by increasing the flapping amplitude on one side.

Click to expand...

Well, you just said it...flapping one wing would provide asymmetric thrust, inducing yaw....if you flapped the other wing at the same time, and were able to induce vortex lift in the vertical direction, (instead of thrust)you could compensate for adverse lift, and have the desired yaw left over for your benefit.

You (the critters) can safely create vortex lift when flapping, but typically not during soaring (except in special cases like lee shear soaring). The reason you usually shoot for turbulent attached flow when soaring or gliding, is that vortex lift risks transiently lowering the aeroelastic number below the flutter limit through a local reduction in membrane camber. That makes for a bad day.

However, to make my friend happy, I have been looking for a possible non flapping solution.

Another aside (I'm full of them). It is yaw transients that create the problem. Within limits, steady-state yaw isn't an issue.
A 10 degree steady state yaw only creates about a 1.5% decrease in gliding efficiency.

JimC said:

However, to make my friend happy, I have been looking for a possible non flapping solution.

Click to expand...

Attitude control rockets? We're at the bottom of my list, I think....

JimC said:

vortex lift risks transiently lowering the aeroelastic number below the flutter limit through a local reduction in membrane camber. That makes for a bad day.

Click to expand...

OK, the trailing membrane is tightened or loosened by forward/backward wing position, I'm guessing. I'm also guessing that membrane tension being too tight induces flutter????

"OK, the trailing membrane is tightened or loosened by forward/backward wing position, I'm guessing"

A lot more complex than that. Chordwise tension tends to be an order of magnitude less than spanwise tension. Spanwise tension outboard of the wrist is quite discontinuous with spanwise tension inboard of the wrist due to the different actinofibril layout (tension battening). Actinofibrils are each about 10 cm long, and intercalated. The amount of intercalation is variable, adjustible both by aerodynamic loading and by direct internal manipulation of the interconnected membrane lattice as well as by pteroid positioning, wrist positioning and rotation, elbow positioning, shoulder positioning, etc. The skeletal spar can also be used to adjust tension, but the effect inboard of the wrist is rather independent of that outboard and tension isn't always controlled by the skeletal spar. Aeroelastic number decreases as lift coefficient decreases (as speed increases), so that pterosaurs have an upper speed limit that birds don't have. Increasing speed will shift maximum camber aft until the the wing camber first becomes bistable (can invert), then as the aeroelastic number continues to fall and falls below the flutter limit, flutter and trailing edge flagging are initiated, usually beginning near the elbow. I can send you the equations that define the aeroelastc number if you wish. There are a couple of different ways to define it, but consequences are similar for each. Easiest way to inhibit it (other than just watching your speed), is to use the actinofibril lattice to locally increase or decrease the surface area of the wing in patches about the size of your hand to use the local pressure jump to modulate the shape of the camberline as desired. At least, that's how the animals do it. Current state of materials technology isn't quite good enough for us to emulate it adequately. In another ten years maybe. These things react nothing like hangglider wings, bat wings, or boat sails. The actinofibril function is the most remarkable feature in the wing, followed by the pteroid function.

"I'm also guessing that membrane tension being too tight induces flutter????"

Again, more complex than that, but yes, if you pull the camberline too tight, or let it shift too far aft, the wing will either invert or flutter. The animals have about a dozen seperate mechanisms for modulating it. That said, if they retract the wing too much, the wing will go slack and everything will go to crap on them. The animals can stop it. The models that use tension battened membrane wings usually can't. They tend to enter a steep, nose down, divergent spiral and go splat. Paul and I have both had a number of models go rapidly from being 'Critter One' to being 'Litter One'.

Most models that you see flying sucessfully cheat and either use compression battened wings, or semi-rigid wings (Paul's 'cheat'). No model to date implements all of the mechanisms the the animals used to control the membrane.

Also, the materials are viscoelastic rather than elastic. That is, under uniform loading and stress, they slowly creep, changing shape. You have to move the wing in order for them to reset. Normal atmospheric turbulence is enough to do that. And stress and strain responses are different when increasing loads on the wing vs decreasing the loads.

So the model program told you that materials and autopilot technology wasn't up to the challenge, yet....I can see why, now. If there was a sheet of muscle fibers in the membrane, it would have a whole new level of control ability... There is a lot of info out on the web, and I've had some fun grinding through it. Makes me wish I hadn't given up on my second smoke tunnel project. I built my first R/C equipment in 1960 from vacuum tubes, and every time I get back into it, the learning curve is hell, the technology improvement in the last ten years is really impressive. I think the autopilot tech is ready for you now...

Keep in mind when browsing internet content about this stuff, that there are currently only about five or so people in the world who have a good handle on the interaction between pterosaur biomechanics and flight mechanics, and they don't agree on all details. Even have major differences on some of it. There are some other folks who are very, very good at avian flapping flight, but don't know much about the differences in avian and pterosaurian skeletal articulations and the very different muscle arrangements they used to power flapping flight. Consequently, they tend to over 'avianize' and try to force the pterosaur wings to move in ways that are impossible for them - trying to make them birdlike when they aren't.

By that, I don't mean that pterosaur wings are inferior to bird wings. Maximum steady state avian lift coefficients are correlated with aspect ratio, and frigate birds are about the best they can do, at 1.65. Pterosaurs can achieve steady state about 2.2. Pterosaurs were also far more powerful flappers than birds, but could do so for only short durations. In no-lift conditions when part time soaring is not an option, birds are more versatile. When soaring is an option, pterosaurs are more versatile. Pterosaurs also had less fat stores, so were more prone to starvation when long duration conditions were not suitable for flight (which is why the Chicxulub impact killed off the pterosaurs, but not the birds).

As an unrelated point of interest, the largest individual bird known to have flown by means of continuous flapping flight was a male Whooper swan named Stonker (called JAP in the research papers). He weighed as much as 44 pounds at the beginning of long flights and less than half that when he landed. Stonker died during a trip from Iceland to Scotland sometime around 2002-2003 (memory fails me). He was well up in age for a swan.

A lot of folks don't seem to be aware that Darwin was quite religious, and a deacon in his church.
A good man.

I haven't said anything here about evolution, nor do I intend to. Wrong venue for that.

Another aside - pterosaurs were warm blooded, but flew with an exothermic flight style. They had a fur like body covering. No feathers. A reasonable visual approximation of their actinofibrils would be hair from a horse's mane or tail, cut into four inch lengths, laid parallel and intercalated, with a diamond shape lattice connecting and powering them. There are of course, strong survival advantages in unstable flight (as long as you aren't human)

Still another aside. A lot of pterosaur illustrations show roach in the trailing edge, particularly near the wingtips. Pterosaurs can't support trailing edge roach.

fobjob,
Yeah, but the materials technology ain't made it yet. Getting close, though. I haven't built a pterosaur model in over ten years. Could probably build one now for $30,000 vs the half million back then.

Last model froze a lot of the various articulations. Different articulation choices in different flights. All that could be done more realistically now.

Jim,
You seem to have very intimate knowledge of the structure of a pterosaur. Did you or someone else find a complete fossil/skeleton/remains and do you have any photos which you can share?

That video of the pelicans seems to answer a lot of the aerodynamic questions. Do you suppose that a pelican is an improved descendant of the pterosaur?

It boggles my mind that someone would expend a half a million samolians on a model in an attempt to find out what made an extinct creature tick. I hope that you do eventually get your answer and are able to share it with us. Perhaps it is something too simple for our analytical minds to see?