Sub physics - experts needed

Dr.Sid · 12-03-07, 10:46 AM

I'm doing tests with bank angle. Now that really brings 'piloting' into it. Now I understand why they have 2 men driving the sub.

By my guess, there are 4 major forces effecting bank:

1) hydrostatic stability. Since center of buoyancy is higher then center of gravity, sub 'wants' to stand sail up even at zero speed.

2) hydrodynamic stability. Center of side-drag is similar to center of buoyancy for 688 .. near the centerline of the column-shaped body, or even higher because of the sail. Because of that, when water flows from the side of the sub, it will bank away from the water flow direction. This happens even in simple turn - in right turn sail banks to the right. Also if sub dives straight down, this helps keep the sail up. Sail is kind of flag so it turns to opposite of 'wind' direction.
If the sub surfaces straight up, this force would force the sail down (to 'flag' behind the main body). In this case hydrodynamic stability fights against hydrostatic stability.

3) screw rotation. Forces the body to rotate in opposite direction.

4) planes input. Used to fight all these effects.

Now amount of individual effects is quite mystery to me. I could compute at least hydrostatics quite well, but I would need to know where exactly point of gravity lies in 688. I don't know that. This too affects hydrodynamics. Screw effects ? No idea. At what vertical speed is the hydrodynamics stronger then hydrostatics ? No idea.
Planes effects .. is it possible to turn the sub upside down ? Can planes fight over hydrostatic stability ?

Anyway this really complicates all maneuvers. Simple right turn for example will bank the sub to the right which will make the banked rudder to pitch sub down. I know this really happens, but again, the amount is unknown.

Any ideas ?

OneShot · 12-03-07, 11:13 AM

I dont know if you have considered this till now, but maybe check out the aerodynamics and what happens with airplanes. For example the screw issue is quite similar if not the same with a helicopter.

While water as a fluid behaves differently in some aspects then air I think that some principles should be the same and thus it might be worth trying out some formulas for aerodynamics on your sub.

Dr.Sid · 12-03-07, 11:40 AM

Quote:

Originally Posted by OneShot

I dont know if you have considered this till now, but maybe check out the aerodynamics and what happens with airplanes. For example the screw issue is quite similar if not the same with a helicopter.

While water as a fluid behaves differently in some aspects then air I think that some principles should be the same and thus it might be worth trying out some formulas for aerodynamics on your sub.

I have all the formulas. But not the parameters. And not knowledge enough to compute them. Normally you just measure real aircraft to get the parameters. Even if you can't fly real plane, there are dozens of simulations available, so even virtual pilot can know what feels right and what not.

With subs it's different. Everything is secret. There are no simulation of this detail (I mean public).

Dr.Sid · 12-03-07, 12:36 PM

As for the bank effect .. after some tuning I came up with this subjective 'rules' (everything said concerns my sim, not reality):

1) Screw effect can be felt .. but it is not an issue except for flank speed, and even so you can ignore it, it will just bank you a bit.

2) Banking effect of the planes can almost do a roll but not really, at flank speed. However it is sufficient to counter all other banking effects except for high-speed corkscrew.

3) Corkscrew ? That's that funky think what sub does at high speed turns. Side flow banks it, banked rudder pushes sub down. At flank speed this can happen in seconds and you are past crush depth. I tuned it so that at 10 kts it is not really possible. Maximum rudder is possible. There are some minor banks but slow and easily to counteract with planes. At flank you better turn gently. It is possible to loose control. At least you are diving, turning, banking, not knowing what to do first. Engine stop, rudder center is first things to do. Correct pitch with planes, bank will correct itself. Emergency blow if needed.

4) Hydrostatic vs. hydrodynamic .. except for surfacing straight up with sub turned upside down there is no problem to keep sail up. At some extreme conditions the hydrodynamic effect can be felt anyway (and it's also responsible for the corkscrew).

Like this the sub is controllable and stable at most speeds, little wild at the flank. Anyway to keep bank zero requires constant attention.

Dr.Sid · 12-03-07, 02:43 PM

Here is the live simulation with these bank effects http://roger.questions.cz/other/sim2.zip
Check the readme for new controls.

XabbaRus · 12-03-07, 03:03 PM

One little bug. When using the arrow keys to turn the right arrow makes you turn left and the left arrow makes you turn right. I was at flank and hit full rudder and didn't see much roll.

Oh I can send Akula model if you want.

XabbaRus · 12-03-07, 03:10 PM

Here's a good simulator

http://www.jhuapl.edu/websub/

Some other helpful stuff

http://robotics.ee.uwa.edu.au/auv/su...l/physics.html

Dr.Sid · 12-03-07, 03:12 PM

Send whatever you have

Anyway for me the arrow keys works correctly. Left moves rudder toward negative values. Right toward positive.

To get some decent bank:
Press 0 for flank speed. Wait till speed hits 30. Ignore slight bank to the right due to screw reaction. Press ']' for full rudder right. It should perform something like barrel roll.
Now this isn't realistic from many points of view. You would loose the air from balast tanks when upside down. Screw and planes would not work above the water (I don't test it now).

Are you sure you have the sim2.zip version ?

12-03-07, 11:13 AM	#2
OneShot Grey Wolf Join Date: Mar 2005 Location: Germany Posts: 956 Downloads: 9 Uploads: 0	I dont know if you have considered this till now, but maybe check out the aerodynamics and what happens with airplanes. For example the screw issue is quite similar if not the same with a helicopter. While water as a fluid behaves differently in some aspects then air I think that some principles should be the same and thus it might be worth trying out some formulas for aerodynamics on your sub. __________________ All the things you always wanted to know about Dangerous Waters but never dared to ask!

12-03-07, 12:36 PM	#4
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	As for the bank effect .. after some tuning I came up with this subjective 'rules' (everything said concerns my sim, not reality): 1) Screw effect can be felt .. but it is not an issue except for flank speed, and even so you can ignore it, it will just bank you a bit. 2) Banking effect of the planes can almost do a roll but not really, at flank speed. However it is sufficient to counter all other banking effects except for high-speed corkscrew. 3) Corkscrew ? That's that funky think what sub does at high speed turns. Side flow banks it, banked rudder pushes sub down. At flank speed this can happen in seconds and you are past crush depth. I tuned it so that at 10 kts it is not really possible. Maximum rudder is possible. There are some minor banks but slow and easily to counteract with planes. At flank you better turn gently. It is possible to loose control. At least you are diving, turning, banking, not knowing what to do first. Engine stop, rudder center is first things to do. Correct pitch with planes, bank will correct itself. Emergency blow if needed. 4) Hydrostatic vs. hydrodynamic .. except for surfacing straight up with sub turned upside down there is no problem to keep sail up. At some extreme conditions the hydrodynamic effect can be felt anyway (and it's also responsible for the corkscrew). Like this the sub is controllable and stable at most speeds, little wild at the flank. Anyway to keep bank zero requires constant attention. Last edited by Dr.Sid; 12-03-07 at 02:43 PM.

12-03-07, 02:43 PM	#5
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	Here is the live simulation with these bank effects http://roger.questions.cz/other/sim2.zip Check the readme for new controls. Last edited by Dr.Sid; 12-03-07 at 04:17 PM.

12-03-07, 03:03 PM	#6
XabbaRus Navy Seal Join Date: Sep 2001 Posts: 5,330 Downloads: 5 Uploads: 0	One little bug. When using the arrow keys to turn the right arrow makes you turn left and the left arrow makes you turn right. I was at flank and hit full rudder and didn't see much roll. Oh I can send Akula model if you want. __________________

12-03-07, 03:10 PM	#7
XabbaRus Navy Seal Join Date: Sep 2001 Posts: 5,330 Downloads: 5 Uploads: 0	Here's a good simulator http://www.jhuapl.edu/websub/ Some other helpful stuff http://robotics.ee.uwa.edu.au/auv/su...l/physics.html __________________

12-03-07, 10:46 AM	#1
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	I'm doing tests with bank angle. Now that really brings 'piloting' into it. Now I understand why they have 2 men driving the sub. By my guess, there are 4 major forces effecting bank: 1) hydrostatic stability. Since center of buoyancy is higher then center of gravity, sub 'wants' to stand sail up even at zero speed. 2) hydrodynamic stability. Center of side-drag is similar to center of buoyancy for 688 .. near the centerline of the column-shaped body, or even higher because of the sail. Because of that, when water flows from the side of the sub, it will bank away from the water flow direction. This happens even in simple turn - in right turn sail banks to the right. Also if sub dives straight down, this helps keep the sail up. Sail is kind of flag so it turns to opposite of 'wind' direction. If the sub surfaces straight up, this force would force the sail down (to 'flag' behind the main body). In this case hydrodynamic stability fights against hydrostatic stability. 3) screw rotation. Forces the body to rotate in opposite direction. 4) planes input. Used to fight all these effects. Now amount of individual effects is quite mystery to me. I could compute at least hydrostatics quite well, but I would need to know where exactly point of gravity lies in 688. I don't know that. This too affects hydrodynamics. Screw effects ? No idea. At what vertical speed is the hydrodynamics stronger then hydrostatics ? No idea. Planes effects .. is it possible to turn the sub upside down ? Can planes fight over hydrostatic stability ? Anyway this really complicates all maneuvers. Simple right turn for example will bank the sub to the right which will make the banked rudder to pitch sub down. I know this really happens, but again, the amount is unknown. Any ideas ?

12-03-07, 03:12 PM	#8
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	Send whatever you have Anyway for me the arrow keys works correctly. Left moves rudder toward negative values. Right toward positive. To get some decent bank: Press 0 for flank speed. Wait till speed hits 30. Ignore slight bank to the right due to screw reaction. Press ']' for full rudder right. It should perform something like barrel roll. Now this isn't realistic from many points of view. You would loose the air from balast tanks when upside down. Screw and planes would not work above the water (I don't test it now). Are you sure you have the sim2.zip version ?