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Originally Posted by MarkShot
BH Nuke and Bill,
You describe maneuvers with the sub basically behaving like a large aircraft; very interesting.
It makes me go back to all my online air combat days and start to wonder what AC maneuvering and concepts is analogous to subs:
(1) For example, energy (kinetic and potential) are often considered with the exchange being done through converting altitude. Of course, a sub is supposed to be more or less neutral and not a flying brick. However, BH Nuke has pointed out how blowing tanks can produce great speeds. So, is flooding or blowing ever used during evasive maneuvers to realize speeds in excess beyond propulsion driven speeds or acceleration?
(2) In flying, one is often concerned with making energy efficient turns as opposed to creating high wing loading situations. In evasive maneuvers does one try to not take too much speed off the boat by how one makes their radical maneuvers?
(3) Going back to radical maneuvers and evasion. A missile although a lot faster than an AC may for the same reason (high speed and momentum) be unable to maneuver as radically and turn as tight. (Of course, thrust vectoring and the lack of human G limitations do give a missile some advantages.) Thus, an AC can if maneuvered tightly at the right moment effectively dodge a missile in a turn (of course, there are proximity fuses for this). Is it correct to assume that none of this applies to subs and torps given that the relative speed difference is not that great? Or can a sub turn so radically as to create an overshoot situation with a fast moving torpedo?
(4) In flying, there are too things that happen to control surfaces that become very problem. First, if the angle of attack is too great, the surface could experience turbulence and effective cease to provide control. Second, if the speed is too great, the surface can experience compression and effectively become locked and/or have no affect on movement. Does any of this apply?
(5) Speaking of control surfaces, AC often have trim surfaces or trim applied directly to control surfaces to correct for various flight conditions. Do sub control surfaces have analogous features?
(6) As mentioned previously by BH Nuke, speed changes things. How significant do speed considerations impact the behavior of sub control surfaces? Is there a sweet spot of optimum control? Are there problems at the high and low ends of the speed range?
(7) With AC the fuselage itself also functions to provide lift and handling. Does the sub's hull provide a similar function?
Thanks. (feel free to not answer as appropriate)
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Quick response:
1) No.
2) Yes. The general rule in torpedo evasion is "speed before maneuvers."
3) Modern torpedos generally have much higher turn rates than their targets, so once they have acquired, don't count on out-turning them.
4) High-speed compression effects don't apply underwater because water is an incompressible fluid. Control surface 'stall' at high AoA: I never encountered it and it was never mentioned during training, so my supposition is that although theoretically possible, it is not a in reality not a problem. One control surface effect that does happen (and doesn't in aircraft) occurs at very low speeds: the force created by the planes isn't enough to cause a change in sub body angle, so the hull doesn't generate 'lift'. In practice, this means if you 'pull up' on the stern planes while at very slow speed, the generated force pushes the stern down more than the hull can create lift. Thus, instead of going up (as expected), the boat instead goes deeper.
5) Some sub classes have 'trim' devices on their planes and rudders, I've heard they are more often used for high-speed operations than for trim purposes. (At high speed, plane/rudder motions can cause violent responses if the operator isn't careful. By using the 'trim tabs' only when at high speed, this problem is mitigated).
6) I think I've already covered this, above.
7) Hydrodynamic 'lift' created by the hull, created when the boat takes an AoA or 'slip', is a major (if not THE major) contributor to submarine maneuvering. As diving officer, we were taught the mantra 'planes, angle, speed'. This means, first use the planes to control depth when the boat is 'heavy' or 'light', if this isn't enough then put an angle on the boat, and finally (as a last resort) increase speed. A good diving officer will keep the boat in trim so that only small plane motions are needed to control depth.
As an aside, when operating near the surface the hull can create significant lift even with no AoA (due to the Bernoulli effect). To the operator, it seems as if the boat is being 'sucked' up to the surface. This can make depth-keeping at periscope depth difficult (especially in boomers, which have a large, flat deck that exacerbates the 'suction' effect). One trick often used is to flood extra water into the aft trim tanks, to allow maintaining periscope depth in a 'stern down' condition to reduce the 'suction' effect.
(Surface ship drivers are familiar with the Bernoulli effect when conducting side-by-side underway replenishment: in this case the effect is to 'suck' the ships into each other.)