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PRACTICAL DESIGN SHORTCOMINGS OF THE SEAVIEW DESIGN
As I mentioned in part-8, the SEAVIEW revealed nasty
inherent design flaws (the wrongly stated manta-fin rolling problem aside) that evidence themselves when the model operates beneath the surface of the water.
The two big 'V' arranged Cadillac fins
overhanging the stern work to over stabilize the boat about the yaw axis while submerged … even the three big rudders, two of them benefiting from the high velocity flow from the propulsion nozzles, were
not enough to adequately overcome the stabilizers 'weather cocking' effect underwater.
Turning radius above the surface (with the fins sticking in the air) is good. Turning radius submerged (with
the fins in the water flow) is awful!
Additionally, those huge 'V' arranged Cadillac fins contribute to another stability problem: In a submerged high-rate turn they produce a torsional moment
that rolls the boat into the turn – that torque, when coupled with that of the sails torsional force, is enough to roll the boat so far over that rudder deflection begins to contributes a 'down pitch'
component; as the boat rolls nearly on its side the rudders act to pull the stern up and the boat heads to the bottom, out of control.
As pointed out above, the manta-fins induce a stabilizing
force about the roll axis, negating a good portion of the 'V' shaped Cadillac fin/sail inboard torsion/rolling force. Without the manta-fins the SEAVIEW would be an impractical, almost impossible to
control r/c submarine.
Nearly all modern American combat attack submarines employ a set of downward canted (anhedral) stabilizers at the stern (situated between the horizontal surfaces and lower
rudder). Their primary function is to serve as foundations from which either evasion devices or towed cables are launched or streamed clear of the propeller/pump-jet disc. The secondary purpose of the
stern mounted anhedral stabilizers is to generate a torsional force (created as the boat's angle of attack about the yaw axis increases) to counter the boats tendency to roll inboard in a turn. On a
'real' submarine this unwanted inboard rolling moment originates solely with the sail and a big reason that today's submarine sail structures are kept as short and low of area as possible. Sail
structures are either well faired in to the hull (as practiced by the Russian 'Ruben' design bureau) or are so shaped as to limit the structures ability to produce lateral 'lift' at a high yaw angle of
attack (American LOS ANGELELS class).
With the SEAVIEW we are cursed with three surfaces that produce a unified torsional moment in a turn: the large sail and two upward angled fins as the stern.
The rolling, and reduced turn rate experienced by the SEAVIEW underwater was observed and noted. Like any other type submarine I drive, I first work out the maximum underwater speed I can attain
and still maintain depth control once the rudder is put hard over. Same test and observations as I work the horizontal control surfaced to maintain or change depth. The objective during these sea-trial
activities is to determine the submarines 'performance envelop'. I determine the edges of that envelop and try not to exceed them during normal vehicle operation. Sea-trials are more than working out the
mechanical bugs and trimming the boat, it is also that initial period of operation where you, the Driver, learn what you can and cannot do with the vehicle above and below the surface.
Sometimes,
sea-trials are not a happy time between the operator and his new ride. For example:
On the maiden dive of the SEAVIEW model, after a few circles and figure-of-eight turns on the surface to check
out the running gear, I commanded an 'all-stop' and waited for the big model to coast to a stop. I then commanded a vent of the ballast tank and took the SEAVIEW to submerged trim. With only an inch of
the sail projecting above the water (the boat is trimmed a tad light in submerged trim) I was ready to run the model submerged for the first time … little did I know! I slowly advanced the throttle and
noted a slight pitch-down of the model, so I threw both transmitter sticks full over to command full rise on the sailplanes and full rise on the stern planes. No change! Even with a low throttle setting
the SEAVIEW, both sets of horizontal planes on 'rise', continued to pitch down to a dangerously high angle. It was headed to the bottom! Only full astern and ballast blow commands arrested the dive and
got the boat back to the surface, in a flurry of foaming water and swirling thick gas vapor – I just managed to keep the SEAVIEW's bow out of the lake mud. An embarrassing performance to say the least.
What went wrong?!
After a reflective pause, I repeated the maneuver. Same thing! Again and again, each time I tried it, the boat dove to the bottom at a severe down angle. Obviously there was
something intrinsically wrong with the design – it was not a problem of static trim or improper control surface response to commands, I checked all that. No matter how much 'up' I cranked into the stern
planes and sailplanes, the SEAVIEW model, once the ballast tank was full and the throttle advanced and the boat got some way on, it would pitch down.
Yikes!
Back at the shop and a good
hard look at the SEAVIEW bow in profile. OK, I see … how could I miss that! It became obvious then what was going on: In profile the bow of the SEAVIEW is in the shape of a wedge which assured that water
passing over it has to produce a downward force at the bow. The faster the boat goes, the more pronounced this pitch-down moment. As I demonstrated at the lake, the sailplanes and stern planes did not
have the authority to overcome the designs inherent tendency to pitch down when advancing submerged.
The fix was to install permanent vanes within each propulsion nozzle, their job to direct the
exhausted water upward, countering the pitching moment at the bow. In water tests verified that the fixed vanes countered the bow induced pitching problem throughout the SEAVIEW's speed regime, net angle
change as a consequence of submerged speed was zero. Mission accomplished! But, keep in mind that the two pitching forces (shape of the hull forward, the fixed vanes in the nozzles aft) are directed
down; the net force on the vehicle is a downward one. However, this downward force acting on the submerged submarine is of low magnitude and is easily countered by operating the boat at a slight up-angle
or simply by cranking in a bit of 'rise' on the sailplanes.
After installation of the fixed vanes in the nozzles depth control of the SEAVIEW became no more difficult than driving a 'traditional'
type r/c submarine.
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The SEAVIEW, during sea-trials, with sailplanes set slightly to dive, sinks beneath the surface as its ballast tank fills with
water. As is my practice, I build 'soft' type ballast systems: the tank is open at the bottom and fills only after a single vent valve atop the tank is opened, permitting the gas within the tank to vent
out, letting water in. The boat is trimmed with fixed lead weight and buoyant foam so that a full ballast tank takes on just the right amount of weight to get the submarine to a nearly perfect neutrally
buoyant condition when submerged.
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