Choosing the right dimensions for an underwater glider

Question

I'm looking to potentially build an underwater glider, a type of submarine that's slow but can operate on extremely low power draw. However, in order for it to work effectively I've found several sources hinting that the dimensions of the components, especially the wings, are critical to its success.

However, I've found very sparse information about what these dimensions should be! I'm happy to do a bit of trial and error if it comes down to it, but to save some work does anyone have any information on what the critical dimensions should be?

Answer 1

There is no right answer.

However, I think I can clarify what you've read so far. You need a buoyancy engine to supply the thrust, wings to direct that thrust, and some ability to change your pitch angle when you change the buoyancy. So, you'll need to balance the strength of your engine with the drag created by your wings. At the same time, your wings need to be big enough to convert vertical forces of buoyancy into horizontal motion. (Generally, the shallower depths you work with, the less power your buoyancy engine will require.)

Existing Gliders

This is a complex problem, and the existing solutions to it vary wildly.

Seaglider

The Seaglider AUV (built at the University of Washington, now owned by iRobot) is shaped to achieve laminar flow. The antenna attaches at the back, allowing the vehicle to communicate while pointing nose-down at the surface. It has no external moving parts; all control is done by shifting weight internally.
_{(source: washington.edu)}

Spray

The Spray Glider (built at Scripps Institute of Oceanography, now owned by Bluefin) is a more cylindrical shape. The antenna is located in one of the wings, and the vehicle lays on its side on the surface to communicate.

_{(source: auvac.org)}

Slocum Gliders

Slocum gliders go for a cylindrical shape that's more suited to the pressure housing. They have an electric powered version, but the real magic is in their more modern "thermal glider" -- they take advantage of the fact that pressure increases with depth but temperature decreases with depth to make an engine that requires no (or very little) electrical input. Like Seaglider, the antenna is in the tail.
_{(source: rutgers.edu)}

Liberdade XRAY

The Liberdade XRAY is shaped like a flying wing and (to my knowledge) is actually the fastest autonomous underwater glider ever made. It's more for naval use than scientific use -- it tracks submarines.

Approaching the problem (from scratch)

If you can't find a hydrodynamics expert to help (which would be the best option), I would suggest starting with your buoyancy engine and working outward.

1. Figure out how much buoyancy your engine can create
2. Figure out how much power your engine requires
3. Decide how large of a battery you'll need -- this will be a significant proportion of total weight
4. Explore your ability to trim your vehicle (likely by moving the battery fore and aft) and calculate the pitch angles you'll be able to produce
5. Repeat steps 1-4 as necessary
6. Make a prototype shape, make its buoyancy half of what the engine can produce (minus a little for safety). Then drag it to the bottom of a pool and see what happens.
7. Repeat steps 1-6 as necessary

Answer 2

This is a classic design problem. It starts with want you want something to do - a design specification. Hence there is no one right answer. I have designed a number of both aerial and underwater drones, they vary greatly in design because each was to meet a specific problem. The approach I took for a sea glider to map an offshore reef and record the biodiversity was;

1. Figure out what you want the glider to do - speed, endurance, depth, activity, sensors, communication, reliability,emergency systems etc. seperate into must have and nice to have.
2. Figure out what is needed to do the sensor part in 1 above - sensors, cameras, etc
3. Estimate the weight of that payload and it’s energy budget
4. Guess your size of glider payload bay to fit the payload. Roughly estimate this at 20% of total volume.
5. Decide on the glider structure and given the operating parameters in 1 above calculate the thickness of the pressure vessel. Estimate the overall structural weight. Add this to the total weight.
6. Estimate the weight of the structure and payload to be 50 to 60% of the total weight.
7. Figure out how much buoyancy your engine needs to create - allow for changes in bouyancy with depth, temperature and salinity. Check this roughly fits in your size estimates in 4 and 5 above.
8. Figure out how much power your engines require to operate the bouyancy engine, pitch and roll control elements.
9. Decide how large a battery you'll need - allowing for 20% reserve capacity and allowing for decreased battery efficiency at low temperatures, and then decide if you need to have power harvesting to meet the aims at 1 above. Add the battery weight and any power harvesting systems weight (mine was solar on the wings which meant increased surface time but extended duration).
10. Decide on the control method and Comms set up. Add the weight to the total.
11. Given the design speed over ground calculate the wing parameters required - this will largely be a function of the aerofoil shape, the angle of attack, wetted area, (together giving the drag) and the speed through water.
12. Iterate the parameters to create an efficient trim for your vehicle (wing placement, moving the battery fore and aft and the approach I took by shifting hydraulic fluid as part of the buoyancy engine) and calculate the pitch angles you'll be able to produce at different settings - and therefore the speed through water (lot of calculations here). You don’t have to be too pedantic here - you are simply looking to take out the obviously inefficient designs. The point up to here is to optimise the drag at design speed for your wetted surface to maximise performance (and hence minimise the power budget).
13. Repeat steps 1-11 as necessary until something looks like it might work.
14. Make a prototype shape, make its buoyancy roughly half of what the engine can produce. Then as noted above drag it to the bottom of a pool and see what happens. Repeat this with the trim and bouyancy settings at both extremes of the operating envelope. Weird things happen at the operating extremes - which is where design survival occurs (or not).
15. Now build the prototype and test it (sink it) - I have yet to have something work perfectly first time. Consider if you want real time data during testing or can live with data post test. Consider how you recover it when it sinks. In the pool it’s easy - in 30m of sea water less so and at 200m it’s really hard(when that through hull sensor leaks).
16. Repeat steps 1-15 as necessary to get a reliable working glider.