Can a parasail support its tow vehicle like a parachute sailer?

[DaveC426913]

TL;DR Summary

This is for a story and I want to be sure the aerodynamics are plausible. It involves a parasail (briefly) carrying its tow vehicle.

If this is on the wrong forum, apologies. This is for a science fiction story, but presumably the physics won't change. It is specifically the aeronautics behavior I'm interested in.

Executive summary: could a sufficiently-large parasailing vehicle be designed to lift its tow vehicle out of the water and carry it for a (short) time?

• Payload: Lightweight frame with huge glider-like wings. Call it maybe a couple hundred pounds with pilot.
• Tow-vehicle: Rather than a surface boat, it's a submarine drone (gets better purchase on the water, has more freedom of movement). Is as large and powerful as necessary to tow payload, but not so heavy as to negate any chance of glider carrying it.
• Connected by cable of variable length, can be reeled in or eased as desired

What I'm envisioning is
- tow vehicle torpedoing just under the surface, towing the glider behind it many dozens of metres in the air (or a hundred?)
- pilot reels in cable while pitching up, so that the glider pulls forward until it is almost vertically on top on the tow vehicle (you can do this with a kite)
- pilot commands tow vehicle to breach - porpoising out of the water
- tow vehicle dangles from cable while glider carries it (on a slow descent) until tow vehicle touchs down again

This is effectively what windsurfers do:

The difference in my setup is that the airborne payload is much more than just a parachute - it's a winged frame with pilot. This raises a key question of weight ratios between tow vehicle and pilot vehicle. The tow vehicle must be powerful enough to control the pilot vehicle with ease but not so heavy as to not be carryable by the pilot vehicle.

Is it too much to hope for some amount of forward motion and some glide-ratio?


Oh. Also, full disclosure: the tow vehicle is a critter - like a giant manta ray. That shouldn't change the physics, but it does shed some clarity on my diagram:

 

[Baluncore]

It was tried a while back, but the idea never took off.
https://en.wikipedia.org/wiki/Focke-Achgelis_Fa_330

 

[DaveC426913]

It was tried a while back, but the idea never took off.
https://en.wikipedia.org/wiki/Focke-Achgelis_Fa_330

Weeeeell, I'm pretty sure that little thing wasn't going to lift the submarine out of the water...

 

[Rive]

Gliders do 'winch launch'
Sometimes by car
Sometimes it goes wrong...
So, I think it's possible. Not easy, but should be possible. That kind of lift do exists.

By doing it with a special tow-ship could be less trouble, I think - the ship can be kept in the water (by adjustable, submerged wings, for example), and then released, if not even launched (by setting negative pull of the wing into positive lift): no need for much mass, like for a car.

 

[DaveC426913]

and then released, if not even launched (by setting negative pull of the wing into positive lift): no need for much mass, like for a car.

OK, but not released. They stay attached and the tow vehicle is lifted into the sky to dangle on a gliding descent. (Oh wait, You knew that. It's hard to tell from your response if you were misunderstanding, but I think you got it.)

What configuration (weight ratio, angle of attack/stall) is required for the air vehicle to be able to carry the tow vehicle and still move forward on its gliding descent?

As you point out, the advantage of the submersible is that it can provide pulling horsepower much more than by its weight alone by angling downward into the water. That way, it doesn't actually have to weigh much to service as a tow vehicle. And when it breaches, it doesn't have a huge mass to lift.

 

[Rive]

OK, but not released.

What I meant there is not the cable, but the downward pull (of the tow) got released...
I'm not really familiar with the required kind of engineering, but a ship (tow) can have absurdly good power to mass ratio, so I definitely see hope for this.

 

[DaveC426913]

What I meant there is not the cable, but the downward pull (of the tow) got released...
I'm not really familiar with the required kind of engineering, but a ship (tow) can have absurdly good power to mass ratio, so I definitely see hope for this.

I'm particularly interested in what happens once they'e dangling. Can the glider carry the mass - given the contstraints - and maintain a glide path? Plausibly, of course.

 

[Klystron]

Your manta-ray-like creature could alter its shape, as well as angle of attack and attitude, to maximize lift.

Several actual fish species speed along just under the surface, breach, then take to the air and glide for many body lengths before reentering the water and continue swimming. So-called 'flying fish' use powerful caudal fins to swim fast and launch, spreading ventral fins to extend and control glide.

I have witnessed flying fish in the Gulf of Siam take air then 'skip' on surface tension for several hops to escape a school of hammerhead sharks. The fish appeared to use their tails, still in the ocean, to increase flight time. The gulf contained warm (80 F.) salty water and very strong currents.

IMS in the penultimate scenes of "Moby Dick", Melville caused his speedy giant sperm whale to laterally breach, take air and ram the Pequod amidships causing the whaling ship to roll and crush beneath the cetacean's enormous bulk.

https://en.wikipedia.org/wiki/Cetacean_surfacing_behaviour

Terrestrial rays, skates and cartilaginous sharks alter their body shapes under certain conditions. Your tow critter, once airborne, could flatten or otherwise alter its shape to maximize your literary goals.

 

[cjl]

Sure, this should be totally possible. Obviously, once the tow vehicle leaves the water, you've got a glider, so the parasail will move slightly in front of the dangling weight and will be losing altitude until the vehicle reaches water again, but with a reasonably long cord, you could certainly glide for a little while (the glide ratio won't be amazing though).

 

[jrmichler]

TL;DR Summary: This is for a story and I want to be sure the aerodynamics are plausible. It involves a parasail (briefly) carrying its tow vehicle.

Is it too much to hope for some amount of forward motion and some glide-ratio?

Completely plausible, but you need a very powerful winch to get very far. The parasail must be designed for the weight of the pilot, the cable, the winch, and the weight of the tow critter. Parasails have glide ratios that can exceed 10:1. A parasail with cable and tow critter will be less, say about 5:1 if the tow critter behaves while in the air. The rate of descent is calculated from the airspeed and glide ratio. The scenario will be roughly as follows:

1) Tow critter tows until parasail reaches maximum altitude.
2) Tow critter breaches while winch pulls in.
3) The parasail has an airspeed and glide ratio, therefore a rate of descent.
4) The tow critter stays in the air as long as the winch pulls cable in faster than the parasail rate of descent.
5) Sploosh!

 

[DaveC426913]

Completely plausible, but you need a very powerful winch to get very far.

Yeah. Given that this is on another planet (i.e. well in the future ), I can get away with light, powerful technology.

(Oh shoot. Did I forget to mention that it must be completely mechanical? No electrics. I guess my characters will be grinding away on a self-tailing winch... But that's not germane to this discussion.)

The parasail must be designed for the weight of the pilot, the cable, the winch, and the weight of the tow critter. Parasails have glide ratios that can exceed 10:1. A parasail with cable and tow critter will be less, say about 5:1 if the tow critter behaves while in the air. The rate of descent is calculated from the airspeed and glide ratio.

Yeah, this isn't meant to last long. It's a stunt, really.

4) The tow critter stays in the air as long as the winch pulls cable in faster than the parasail rate of descent.

Derp. I hadn't even thought about extending the hang-time by reeling in one cable. Duh.

So is my intuition right? That the first step is to lift the air vehicle up-and-forward so it's almost on top of the tow beast. Otherwise, when it breaches it will just fall backward. Also, I'd want to reduce the 'penduluming' as much as possible.

I envision the cable is not let out to maximum length; the shorter the better as far as the stunt goes, although that might make the execution a fair bit more challenging.

 

[Baluncore]

Derp. I hadn't even thought about extending the hang-time by reeling in one cable. Duh.

Replace the rigid towline with an elastic bungee, and all things become possible. Then energy can then be stored in the towline, that will keep the towline under tension.
Likewise, gyro kites fly higher than normal kites, with the wire towline closer to vertical. They have energy stored in the rotor, that can become lift when needed, to keep the towline under tension.

There is a maximum energy storage per length for a towline. Towlines snap when they are not long enough to store all the energy being used to control the movement of the attached objects, and the movement of the line, at the speed of sound in the line.

I'm particularly interested in what happens once they'e dangling.

Does the tow vehicle leap out of the flat water surface, perpendicular to the tension in the line, or does the tow fly out of a steep wave face? With the wind, or against the wind?

 

[DaveC426913]

Does the tow vehicle leap out of the flat water surface, perpendicular to the tension in the line, or does the tow fly out of a steep wave face? With the wind, or against the wind?

Well that would be the trick to get the effect desired.

My current expectation is this
- the glider is nearly on top of the tow beast, both traveling horizontal
- the tow beast aggresively turns up and breaches, slackening the cable, which the glider must reel in and/or gain altitude
- there is, ideally mimimum rocking

I have not considered conditions of water or wind, and therefore have simply been assuming no conditions. Although, I can certainly see one or both being requirements if it helps the stunt, and incidentally makes for a more interesting story.

 

[mfb]

Doing this with headwind will help a lot.

I love the diagram, but I'm skeptical about the glide ratio you can achieve. Animals have a poor power to weight ratio and they swim slowly compared to the velocity you want for gliders. This isn't a big deal if you have a human with a giant sail that's driven by the wind, but here we want the animal to power everything.

 

[Filip Larsen]

A small practical matter (like it is for real world glider design) would be to ensure the cable is attached so any positive or negative pitching moments from the cable force remains controllable in all phases of flight. Usually this involves getting the attachment point close to the center of mass and use elevator (or trim) to cancel out any remaining moment.

A complicating matter could here be where to put the winching mechanism. I assume it has to be internal to the glider and the last mechanical contact point with the cable then "act" as the attachment point. In real world gliders the attachment points are on the belly for ground winch launches and in the nose for tug plane launches, and both has a pilot operated release mechanism. In your case I assume any (emergency) release mechanism has to be either releasing the cable full to let it freely spool away from the glider or some sort of guillotine cable cutter.

By the way, real world cables also include safety links that will break at a known load, but these usually sit on the cable near the attachment ring. If your design has to include both a winch and a safety link it is again not obvious where to put it. Putting it near the load (your tug vehicle) is one option but leaves the full length of the cable still attached to the glider. Perhaps the safety link can mechanically be combined with an emergency cutter so the cutter automatically engages when the cable load exceeds some limit, but its difficult to see how using purely passive mechanism (i.e. no strain-gauge measuring device and electronically triggered cutters).

Perhaps the easiest solution is to simply deem such issues to be technical irrelevant for your story

 

[Filip Larsen]

Doing this with headwind will help a lot.

Doing winch launches in slow (old, wooden) gliders in strong headwind (with significant wind gradient) really does make you feel like you are sitting on a kite, not really moving much forward relative to ground but just slowly climbing up. As a winch operator in this condition it means reeling in as for normal launch at first but then fairly quickly having to slow cable reeling to a (relative) crawl and then watch the glider hang above you for what feels like a long time. I have heard old stories about conditions where the winch operator let the reeling stop and even run in reverse but I'm not entirely sure if those stories are accurate

 

[DaveC426913]

A small practical matter (like it is for real world glider design) would be to ensure the cable is attached so any positive or negative pitching moments from the cable force remains controllable in all phases of flight. Usually this involves getting the attachment point close to the center of mass and use elevator (or trim) to cancel out any remaining moment.

Yes, the cable(s) enter the glider belly near the CoM through a funnel which is mounted on a gimbal.

A complicating matter could here be where to put the winching mechanism. I assume it has to be internal to the glider and the last mechanical contact point with the cable then "act" as the attachment point. In real world gliders the attachment points are on the belly for ground winch launches and in the nose for tug plane launches,

Winche(s) are mounted in the center of the cockpit on top of the pedestal which is gimbaled. Note: not like a glider cockpit: like a sailboat cockpit. You sit on a bench around the pedestal and operate the various winches (which run bundled control cables).

and both has a pilot operated release mechanism. In your case I assume any (emergency) release mechanism has to be either releasing the cable full to let it freely spool away from the glider or some sort of guillotine cable cutter.

By the way, real world cables also include safety links that will break at a known load, but these usually sit on the cable near the attachment ring. If your design has to include both a winch and a safety link it is again not obvious where to put it. Putting it near the load (your tug vehicle) is one option but leaves the full length of the cable still attached to the glider. Perhaps the safety link can mechanically be combined with an emergency cutter so the cutter automatically engages when the cable load exceeds some limit, but its difficult to see how using purely passive mechanism (i.e. no strain-gauge measuring device and electronically triggered cutters).

Perhaps the easiest solution is to simply deem such issues to be technical irrelevant for your story

It has a harness release mechanism (don't want the critter to 'sound' and have to go along for the ride!), but I hadn't thought of an emergency cable release. That seems critical, now that you mention it. Don't want your dangling cable catching on a reef or boat....

The guillotine idea sounds good. That scenario doesn't come up in this story, but it sounds perfect for the sequel.