The linked samara decelerator

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The linked samara decelerator

The principle of the linked samara is simple: latch a load to a long rope fixed to a tough little glider. The glider flies in a circle above the load and brakes its fall like a parachute would. The glider is much littler than a parachute, yet by circling above the load it covers a virtual ring-shaped surface equivalent to the surface of a parachute. Flying at high speed, forced into a circle by the rope, the glider exerts a huge lift force.

Formerly I called this device "rotating parachute". I was advised by Yuri Mosseev that this term is already widely used for another device: a conventional tissue parachute with large cutouts that make it rotate slowly while falling. So I choose another name. Images of a rotating parachute shape can be viewed through links on this page: http://www.mtu-net.ru/mosseev/pl/paralab.htm

Maybe the linked samara system has some advantages on a parachute:

A big problem with parachutes arises once the load is on the ground and there is a lot of wind. The parachute will be pushed by the wind and will pull the load on the ground. The linked samara avoids this: once the load lands the glider can do nothing else but land too or smash to the ground. It can by no way be pushed by the wind.
The little surface of the glider, the fact it can be transparent, the high speed of the glider and the single rope, allows for stealth capabilities.
The lift capability can be tuned by changing the rope length. This changes the diameter of the virtual lift surface. Also the flight attitude of the glider can be changed, using ailerons and tail. This allows for example to adapt to the decreasing speed of a Mars lander.
The glider and the rope can be made to withstand higher or lower temperatures than a tissue parachute and can be stored for a longer time under difficult conditions like moist.
It makes the Rogalo delta wing even more interesting for spacecraft recovery because a rather little delta wing can be used as rotating glider. A part of the external structure of the spacecraft can be released and serve as glider, saving more weight than a conventional parachute and perhaps being even more reliable.
It is a simple landing or safety system for planes. The load the plane carries just has to be dropable and latched to a long thin wire. The plane body itself then becomes the glider of the linked samara. No need for a parachute, the plane structure itself is the parachute.
The glider can be easy to manufacture and low-cost. A few wooden pieces of good quality, correctly latched together, will do the job. Mac Gyver can assemble one in a few minutes to escape a mad plane.
The system can make sideways movements. Indeed if the glider has ailerons it can make the virtual lift surface to rotate sideways like a helicopter propeller does. This could be interesting for a Mars lander, to cope with sideways wind, avoid dangerous zone or aim desired zones. (This can be done with a parachute too, using a parafoil or releasing some parachute ropes on one side of the parachutes just before landing.) A silly way to compensate for sideways wind is to make use of the horizontal rotation of the load inherent to this system. The load is released just when its speed vector is aligned with the wind speed vector yet in the opposite direction.
Like a parafoil, the linked samara can be powered and allow to rise in the air and fly:
- By using a motor and a propeller on the load, just like a powered parafoil. This needs some flight control ailerons on the glider, to orient it like the blades of an autogyre.
- By using a motor and a propeller on the glider itself, making a powered plane of it. A sophisticated version of this can be used for the take-off of a conventional plane: the plane takes off easily without its load yet is latched to it by the rope. It turns around the load and rises in the air, till the load is pulled into the air too. The rope is hailed by either the load or the plane till the load joins with the plane and the plane goes into a straight flight. This is quite complicated yet it can be the only way to launch on the spot a heavy plane with almost no infrastructure and making not too much noise.
- By pulling and releasing the rope alternatively, while changing the glider incidence.
Pulling on a long rope, with an electric motor using a one-shot high current battery, can be useful to make the landing smooth. (This is true for conventional parachutes too and could be more interesting than JATO rockets.) A very long and thin rope is used. When the surface is near, the electric motor is switched on. The load will experience a huge pull force while the distance between the load and the glider shortens. Till it stands still a few meters above the ground. To compensate for the rotation inherent to this device the glider can make a steep turn and bring the load to complete immobility above the ground.
Devices working on the principle of the linked samara can be interesting. For example a kite that rises in the air and go into rotation around an oblique virtual line (without touching the ground) or sweeps horizontally. It can be used as a huge virtual sail for boats, high in the air where the wind speed is higher or the wind direction is different. Or produce energy for example by pulling back and forth on the rope.

And surely some disadvantages:

The load is brought into rotation too. This is no problem for furniture but it makes the use of such a parachute difficult for humans. I bet some sportsmen will try it out but I doubt it could become of common use. A very long rope is a solution to decrease the angle. Aerodynamic brakes along the rope can help. One interesting possibility is to use a glider a little bigger than a man and jump with the body latched to the glider. This allows to fly at high speed a long while and make acrobatics. To land, just release the glider and let the rope roll out. A way to recycle your old longboard...
Several conventional parachutes can be dropped at the some time with almost no risk they hamper each other. Yet two linked samaras that meet in the air will twist their wires and fall to the ground.
A conventional parachute is far easier to fold and put into a small volume. A bendable glider can be conceived, or even a tissue glider resembling a little parafoil, yet this will be difficult. For some applications like droppings from the rear of a carrier plane, the glider can be placed above the rectangular load and can have the same surface as the load. Only the vertical tail surface will have to fold in order to reduce the height or in order to be able to stack several flat loads each latched to its glider (the rope can be placed inside a box or be lousily fixed above the load).
The opening of a conventional parachute can be very quick. This is not the case of the linked samara because it takes a while before the glider goes to full rotation speed.

The linked samara must be compared to a samara tree seed with its propeller blade. Yet with some advantages:

The surface of the glider wing is more effective than the surface of the samara seed blade because it is located further from the load and sweeps over a much wider virtual surface.
By using a bearing on the rope the load can be allowed not to rotate.
Latch the load to the rope is easier than to a blade.

Here are the rules to follow to build a linked samara. I found them out while trying to make it work, by making bad prototypes and trying to understand how to enhance them. Most trials were awful. It took me three months to get results because my first approach was nearly the opposite of the right solution. These rules are a first base, for sure they will have to be changed further to get best performances. I do not give mathematical formula to calculate sizes because I don't know them. I suppose the formula for conventional planes and the laws of mechanics will do the job.

The glider must be of conventional shape. Roughly, it needs to be able to fly by itself. Yet it does not have to be a very stable glider since it is guided by the rope. The weight at the front and the vertical tail surface at the rear are both very important. Yet the weight does not need to be the optimal weight for a normal straight flight: it can be a little too heavy or a little too lightweight. Better a little too heavy than a little too lightweight, tough.
The rope must be latched to the belly of the glider at the place of the center of gravity. Better a little to the rear of the center of gravity (probably because this forces the wing to a higher incidence).
A second little rope must be attached to the middle of one wing and to the main rope a further away down from the glider. This second piece of rope must pull down the glider wing towards an angle of about 30° with the horizon when the main rope falls vertically. This is necessary to force the glider to turn while flying above the load. Once the rotation is launched the little rope is of no more use because the angle between the glider wing and the rope will be less than 60°. The little rope will hang lousily. (Except when a very long main rope is used for stability and the glider turns by itself instead of being forced by the main rope and the load.) (The problem that arises when the glider is latched at the middle of each wing and the main rope is short is that once the rotation is launched and the glider is forced strongly outwards, its outer wing is forced lower than the inner. The glider then plunges its nose towards the ground and exerts few lift. The braking of the fall is weak and the whole could perhaps even accelerate to a high fall speed.)
The rope must be as thin as possible. The side surface of the rope brakes the glider. Best is to use a steel cable. For little prototypes I use 0.07 millimeter diameter nylon fishing wire.

An important menace for some applications is that the rope could wrap around the glider body and blockade it into a dead position. The only possible solution is to make the glider be a convex shape. Either by giving it the shape of a flying wing, like a bird, or by using thin cables between the wing ends and the nose and tail, and between the vertical tail and the nose.

This is a photograph of my best prototype. It is miniaturized that much because it had to go into full rotation while being dropped from just 6 meters height.

balsa glider

In order to make tests, you can buy a little balsa or plastic glider in a toy or model store for a few $ and latch two thin nylon wires to it.

Best way to make tests without having to climb to some height to drop the device and go back down each time is to latch the rope to a rod and run with it. You can also use a long rod and turn it around you. Once the rotation goes on you will feel the brake force, like if a parachute was latched to the rod.

Just like a plane, a linked samara can seem to work yet have a poor yield. If it rotates slowly and has an absolute trajectory resembling the path of a corkscrew, it will have few brake capabilities. The rotation must be fast in order to create a complete ring-shaped virtual brake surface. The glider should be more rotating than falling. To achieve this some tuning will be necessary.

Maybe a ladder of gliders would be fun:

ladder

Many variants are possible, using several gliders. Gliders can be linked through several ropes and rotate in concentric cones... They can be latched to the extremities of a large load and tune the attitude of the load... they can be latched together by horizontal wires and become closer to the petals of a rotating parachute... All ways to have the wires mangle and crash the whole. Yet an interesting possibility is to have the glider turn neither around a vertical axis nor an oblique axis but straight around a horizontal axis, flying below and above the load each revolution. This creates a virtual balloon around the load. For example a long thin wing can be latched along a rocket booster. To brake the fall it will be released and turn around the horizontal booster. A tissue wing in a spiral around the load would be charming... I once hoped that the reciprocal rotation of the load would allow to decrease the impact speed but this seems difficult to tune.

Maybe a linked meta-samara is possible: the glider would turn in a circle but this circle would turn in bigger circle, producing a cycloid. Maybe a simpler approach would be to have the glider flow in a spiral; with a radius continuously increasing and decreasing, in order to sweep over a broader surface.

ATAIR Aerospace seems to have achieved an interesting design: http://www.atair.com/parachutes/heli-chute/

A link: http://home.att.net/~dannysoar2/Whirlygig.htm

I whish to thank Yuri Mosseev and Didier Bizzarri for their advice and data.

Eric Brasseur - May 12 2000 till December 1 2010