I just watched the new movie today; it's good. The dragonfly-like wings were an interesting choice. It seems like they'd be less efficient at lift than a jointed wing, but then I thought, maybe an unusual wing surface material could be used?
The problem is that when the wings go up, they're pushing air in the wrong direction and propelling the ornithopter down. But suppose the wing was covered in tiny little hinged flaps that close on a downstroke, but open on an upstroke? That way the upstroke would push the aircraft down much less than the downstroke lifts.. (These wouldn't even need to be controlled in any way, they could just be passive flaps. Though being able to lock them closed might be useful for certain acrobatic maneuvers.)
>A hummingbird (4 inch wingspan) flaps at 60 Hz though. It can hover too, just like an insect.
during hover hummingbird doesn't flap the wings up/down, instead it flaps it in horizontal plane. On each stroke, back and forward, the wing is set at an angle of attack and thus is kind of like propeller during each flapping stroke pushing air down :
Hummingbird weights 3 grams. An APC 4x4 prop at 3000 rpm would generate 4 grams thrust consuming 0.2 Watt. Such energy density would mean 6 KWt for human, ie. 60x our regular power. So no wonder :
"Hummingbirds are sugar addicts. Their metabolism is so incredibly fast that they need to refuel about every 10 minutes. Each day they consume 50 percent of their body weight just to maintain their normal weight. Hummingbirds burn from 6,600 to 12,000 calories per day. If a man had the metabolism of a hummingbird, he would have to eat almost 300 pounds of hamburger a day to keep from wasting away."
You just change the angle of the wing on the upstroke so it's slicing through the air. It's not actually a problem. :) Same thing real dragonflies and even birds do (though birds fold up their wings too).
I kind of assumed that's what actual dragonflies do (I mean, they have to get lift somehow), but for a very large wing that rotation must cause quite a bit of drag in itself. In the movie, the ornithopter wings are maybe 50 to 100 feet long and usually moving too fast to see. I would tend to assume that the aerodynamics would be different for something that huge than an insect, but I don't have any good intuition for what would be most efficient. Maybe a combination of wing rotation and self-opening flaps would be best? Or a wing that splits itself into multiple parallel wings on the upstroke and combines into one on the downstroke?
It's interesting that dragonfly wings are so long and thin, which I suppose minimizes the amount of drag as they rotate between the downstroke angle and the upstroke angle.
>> the aerodynamics would be different for something that huge
With such a large moving airfoil there are inefficiencies. Pretending that the wing is a solid object, very quickly the tips will be moving faster than the speed of sound. At such speeds void/shockwaves start to form, radically changing everything. The root of the wing is barely moving, the tips are supersonic, and somewhere along the middle is a transition zone that moves back and forth depending on speed. Good luck designing that airfoil. (This is what makes the TU-95 Bear so loud. Its propeller tips are supersonic while the rest of the aircraft isn't.)
There is a Russian saying about helicopters: You can place every problem in all aerodynamics on the tip of a rotor blade.
All these conversations are lacking a discussion about the atmosphere on Arrakis. We have only hints about the composition and density of the atmosphere. We can assume that the atmosphere isn't toxic and contains quantities of oxygen and carbon dioxide that make breathing both possible and comfortable. We can be sure it doesn't crush humans or explode their lungs.
We also don't know the gravity of the planet. It might be 1.2g or 0.85g.
So we don't know how much lift is generated per stroke, how much lift is required, or what the speed of sound actually is on the planet.
Ornithopter is a type of vehicle. It's unlikely any specific vehicle of the type would be generic enough to operate across the variety of planets that likely exist within the Imperium without modification to the design.
This particular branch of the discussion was focusing specifically on the feasibility of the 'dragonfly' ornithopters operating on Arrakis as represented in the new film.
The problem with the supersonic tips is identical for normal aircraft propellers.
For aircraft with fixed wings for generating lift, the propellers can be small, so supersonic speed is reached only at a higher rpm, but for a helicopter there is no difference from this point of view compared with an ornithopter.
The blades of a helicopter propeller must also be able to rotate along their longitudinal axis, like those of an ornithopter, but nonetheless they are more efficient than flapping wings, because there is no dead time and no energy lost for the reversing motions, and the fatigue resistance need not be as extreme as it would be required for flapping wings.
Possibly cost. But also suspensors only seem to give lift, or very small amounts of lateral thrust. All the pure suspensor vehicles we see move extremely slowly. It's possible the 'thopters use suspensors for primary lift, maybe 80% of vehicle weight, and the wings are just for manoeuvring lift and thrust.
The aversion to using force fields is more about their interaction with laser guns, when shot by one they cause an explosion equivalent to a small nuclear weapon. Not ideal to have wrapping a vehicle you may be flying in formation on an attack run.
In real insects there is no problem with the drag for wing rotation, because the rotation is not an active motion, but a passive motion caused by the flow of the air.
The insect wings have a rigid front edge, which is moved up and down, while the rest of the wing is flexible. When the front edge goes down, the rear part of the wing is pushed by the air upwards until it assumes a slanted position that pushes the air backwards. The reverse happens on the upwards beat.
So in most insects only the rigid front edge of the wing is moved actively while the rest of the wing assumes passively various shapes that are determined by the distribution of rigid veins and flexible membranes in the wing, which ensure that the shape of the wing is the right one for good efficiency.
The fatigue life seems to be the biggest problem next to power production. Superelastic alloys might survive though.
If the intuition is some kind of supersonic reverberation that lets the wings operate in a local low air density then it might not be that hard to find an envelope of lift in that regime.
However, momentum is a problem. The vibrations, and thus stresses, induced on the body of the thopter must be ludicrous. When the wings go up, the body must go down, and vice versa.
Actually this can be an explanation for the fact that in the movie the ornithopters have 8 wings instead of 4, like an insect.
On each wing position there are a pair of wings, one above the other. If when one goes up, the other goes down, and vice-versa, that eliminates the vertical oscillation of the body.
You might like this post also from today describes a simple science fair like way to demonstrate the effects of hinged flap surfaces. I called it a Venetian valve wing but whatever..
I thought about it a year ago but put it away due to no interest from my peers. I only brought it up to news.ycombinator recently because of the cut scenes from the new dune movie. You can try the experiment for yourself. It does in fact work. The only immediate benefit I see so far is that trust power does not get sheered at the edges with super sonic helicopter edges. Meaning the flaps, even if they are traveling at a whopping 100 meters per second are moving far less than would cause a super sonic shock wave like helicopter blades. An aircraft built around it would be more maneuverable, and have more lift power than similar sized aircraft. I can almost picture an ornithopter picking up a tank and transporting it to the battle field with the lift these things can develop. I'd animate it but there seems little point. This thing is going to show up soon and neither of us will get credit for the idea. But it does work..
The problem is that when the wings go up, they're pushing air in the wrong direction and propelling the ornithopter down. But suppose the wing was covered in tiny little hinged flaps that close on a downstroke, but open on an upstroke? That way the upstroke would push the aircraft down much less than the downstroke lifts.. (These wouldn't even need to be controlled in any way, they could just be passive flaps. Though being able to lock them closed might be useful for certain acrobatic maneuvers.)