Submarines and dynamic stability… rocket science?

[KevinMC]

For a long time now I've read with great interest any posts I’ve come across which deal with stability, turning authority, and effectiveness of stern planes/bowplanes/sailplanes. This curiosity stretches all the way back to when I first started into my adventure with model subs when I saw a video clip which promoted a particular models’ forward CG as a contributing factor to its excellent stability and turning performance. As a model airplane flyer of many years, and a model rocket enthusiast, the second part of that statement immediately struck me as being incorrect: Stability is achieved by a forward CG, but turning performance should be achieved by moving the CG aft to a point near (but just ahead of) the center of dynamic forces. Airplanes, rockets and submarines: they should all be subject to the same principles, shouldn’t they? In not being able to find many discussions specifically on this topic I wonder how many modelers appreciate the interrelationship of CG, dynamic stability and turning/diving performance.

Admittedly when I built my OSCAR II I took a WAG on where to place the CG. Folks that have seen it run at the Sub Regatta have commented that it turns “respectably well” for a model of its size and limited rudder area, but I remain curious if further improvements in turning/diving performance can be had by additional tweaking of the CG. To answer my curiosity I’d planned on shifting everything aft an inch or three before hitting the pond this spring, but before I start tearing anything out I thought I’d ask: To those of you who have designed your own boats how do you choose where to place the center of gravity? Have you ever investigated gains in controllability afforded by shifting the CG of a boat aft? Is there a formula/process to follow that suggests where the CG of a given design ought to be? (I’ve read a reference to “counting squares” but no one’s fessed up to the details of this method…) If you're willing to tell I'm listening!

[skip asay]

OK, I’ll take a stab

OK, I’ll take a stab at this. But first, a little background.

I’ve been listening to Dave Merriman talking about CG and it’s proper location for years. But, I’ve never really paid much attention. After all, my boats were working just fine. I’ll admit that Marlin didn’t turn as well as Albacore but I wrote that off as the difference in length as well as hull form coupled with those vertical “sails” in the form of rudder supports. Then I built a 3 1/2” wtc to be used in both boat. I mention the diameter because that size doesn’t leave a whole lot of room on the sides for foam but I had plenty of room ahead of the wtc. So, I moved the fixed 2 or 3 pounds of lead ahead to compensate for the extra foam. I ended up doing the same thing with Albacore. But, even though they both performed reasonably well, they didn’t perform as well as they had with the original wtc. The bow planes, which used to work exceptionally well, didn’t do hardly anything anymore. Turning circle was somewhat larger, as well. After much head scratching, I remembered Dave’s treatises on CG and moved the fixed lead back about 4” which put it just about where it was originally and VOILE! I had my original performance back again. Perfect depth keeping with just the bow planes (APC-4 stands alone on the stern planes) and the tight turning radius came back. My short term plan is to move the fixed lead (CG) back even further until the boat becomes terminally unstable and then move it a tad forward.

I’ve noticed a post on subpirates which quotes the “Laws of Similitude” ( http://en.wikipedia.org/wiki/Similitude_(model) ) but these do not really apply here. The reason for that is that these “laws”, while entirely valid, are used by naval architects when designing a full size boat (or submarine) where determining how the full size version will perform/handle is paramount. In the model submarine community, we’re doing the exact opposite - we’re stuck with the design now let’s make it work.

So Kevin, the short answer is try moving the CG back some. If my experience is anything to go by, you should see an improvement.

Hope this helps.

Skip Asay

[skip asay]

My bad. The correct url

My bad. The correct url is

http://en.wikipedia.org/wiki/Similitude_%28model%29

Sorry about that.

Skip Asay

[KevinMC]

Thanks Skip. It's good to

Thanks Skip. It's good to have confirmation that I'm not "out to lunch".

Can you tell me (even if only roughly) what % of overall length the CG is at on your Albacore pre- and post-shift?

[hakkikt]

I don't understand how moving

I don't understand how moving the CG further back can have a positive effect on *both* the bow planes and the rudder. Whenever you move the CG, the distance of the one from the CG gets larger and the distance of the other from the CG gets smaller, since they are on different sides of the CG.

-Harald

[mylo]

Kevin,

ANYTHING that has to

Kevin,

ANYTHING that has to do with improving an r/c sub's performance and stability interests me.

I was under the impression that having the CG in the exact ....centre..... was the way to go, particularly for a U-Boat with it's tower generally located in this area. To discover now that putting the CG more aft is better, makes me happy that I was smart enough to design my bulk heads with this kind of flexibility in mind. Shifting CG at this point in my project is not going to be a problem. The 'problem' is where to shift it to.

So, how does a sub that has an aft CG maintain zero bubble, both surfaced and submerged ? ....seems to me she'd go arse down. Is this compensated by having more buoyancy aft as well with foam so in effect, you add weight and foam aft to cancel out any buoyancy issues ?

Is there any way you can post some simple drawings to clarify what you're talking about ? I'm RIGHT at that phase in my build of considering weight/buoyancy/CG/CofBuoy ....and all things similar. Must.....do.....properly.

Thanks much,

Mylo

[skip asay]

Kevin -

Now that I’ve had

Kevin -

Now that I’ve had a chance to actually measure it, I moved the weight back 2” (not the 4” I originally stated) which is 5% of the overall length of the boat. I hope to be able to move it another inch or 2 this coming weekend.


Hakkikt -

The single biggest difference achieved by moving the weight back that 2” was it made the bow planes more effective. They now moved the bow of the boat up and down without requiring full travel of the planes themselves. That said, I noticed a slight improvement in turning circle which I will attribute to the same thing.....it let the bow move left and right more easily. As I mentioned, I’ll be trying moving the weight another inch or 2 back this weekend and see what happens.


Mylo -
You answered your own question. Yes, foam is added to compensate for the weight. Keeping a zero bubble when submerged is paramount.

Skip Asay

[JWLaRue]

Skip,

Is another way of stating

Skip,

Is another way of stating this that the farther forward the weight (c.g.) the more stable the boat (i.e. resistant to turning) and the closer the weight is to the center, the less stable and therefore easier to turn?

I seem to remember someone (Merriman?) using the analogy of an arrow...all the weight is up forward making it very stable in flight.

Seems like this may be similar to the aircraft designs where the more agile aircraft are actually inherently unstable flight platforms. (an extreme example)

-Jeff

[hakkikt]

I seem to remember that

I seem to remember that elsewhere on this forum (was it about the turning radius of Ohios?) it has been said that a boat gains manoeuverability if the CG is moved *forward* because the distance CG-rudder becomes longer and the momentum arm of the rudder with it. Makes sense, in my book.

Based on that, the forward diveplanes should become more effective if you move the CG backwards because the distance between them and the CG becomes longer.

However, the rudder should become less effective! I dont understand how both can happen at the same time.

[aeroengineer1]

If you are to go

If you are to go and read the replies on this same thread on SP, then you will get a more in depth answer to this question, but needless to say there is a point of diminishing returns in moving the CG back. I do not know where it is, and it is model specific. Though, moving the cg back, as well as not having a set of 5lb batteries at opposite ends of the hull will also help.

Adam

[feet wet]

Several years past I discussed

Several years past I discussed this with David Merriman. He recommended placing the battery, a SLA, as far forward in my 1/48th 688 as possible. In doing so I would cause the boat to steer around the battery, so to speak, and greatly improve my turn radius. I placed my 7 amp SLA in the evry bow. At seven feet it turns on a dime, except for snap rolls at speed.
On my Jimmy Carter, which is bigger, I distributed the SLA's thru out he length of the boat(contrary to David's advise) and guess what? It doesn't turn. Some of this is caused by the pump-jet hanging off the stern like a big sea anchor, but I'm rearranging my batteries and as soon as the ice melts(18 inches to go) I will report on her ability to ...turn?
Jonathan

[skip asay]

Jeff -

That about sums it

Jeff -

That about sums it up. Your analogy to aircraft brings to mind the “X” fighter (I believe that’s what it’s called) with the forward swept wings . It CAN’T fly without the computer. No “seat of the pants”.

Hakkikt -
I’ll be the first to admit that this whole CG placement thing is new to me. I’ve been very lucky with all of my boats but with his permission, I’ll quote David Merriman, the guy who taught me about CG and its importance:

“The greater the vehicles control surface distance from the center of rotation (moment arms), the greater the force they exert about the axis point."

“With the c.g. too far forward the boat is too dynamically stable; the boat is difficult to steer about the yaw and pitch axis; the pitch and yaw rates are low. However, if you position the boats c.g. too far aft the boat becomes either dynamically astable (presenting no righting forces regardless of angle presented to the fluid stream) or unstable (the vehicle reorients and becomes stable at an undesired orientation to the fluid stream), making the boat a total Bitch to drive. Good drivers strive for a boat that is astable -- the boat has good pitch and yaw rates, but requires constant attention by the Driver!”

The important thing here is that we have to establish a compromise between having your boat stable enough but not TOO stable. And another thing. What works well for YOU is all that counts.

And concerning the thread on subpirates Dave has this to say:

“What we have here are three engineer types working very hard to be the smartest guy in the room. Now that they've hosed the hall down with three flavors of testosterone, please go in there and explain just what the **** they meant to the rest of us drooling, knuckle-dragging, screaming morons. Single syllable words would be appreciated.”

I mention this because, while it’s nice to be able to talk that way, very few “regular folk” can understand just what the heck they’re saying. Come on guys, we’re trying to make our boats work a little better, not design another lunar lander.

Skip Asay

[aeroengineer1]

Skip,

I am not too sure

Skip,

I am not too sure how you do it, but you seem to try to stir the water wherever you go. We have an engineer type who asked a question on an engineering level, and hence the engineering level answer was given. I will let you know that I have absolutely nothing to prove, it is not in my nature, and even David knows this about me. While I respect your past achievements and things that you have given to the hobby, I challenge you to either keep innovating and quit trying to tell people that you have done great things in the past; we know that you have done great things, I have a few of those great things in my subs.

I have done some pretty great things in the past too, but unfortunately it only gets food on the table for a little while after it is done, then I have to do something else to bring in more food. This is not intended as an insult, but as a solid shot across your bow to please be a little politer and to get out and keep contributing to the community in a positive way.

Adam

[skip asay]

Adam -

Don't shoot the messenger.

Skip

Adam -

Don't shoot the messenger.

Skip Asay

[Guest]

This has been one of

This has been one of the more interesting threads on here in recent times, lets not spoil it by throwing mud around. Lighten up a little please, the humour in those words added a little levity to the thread.

Despite all the mathematics, there still seems to be a large 'fudge factor', which means empirical methods will still need to be used at some point- especially if your boat is far from an ideal shape.