Torpedo angle of attack

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marcelo_malara
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Torpedo angle of attack

Post by marcelo_malara » Tue Oct 20, 2020 3:15 am

Hi guys. WWII torpedoes had negative buoyancy, even with the fuel, water and air consumed. As they don´t have forward fins like a sub, they can not maintain depth generating lift from the fins, lift from the after fins alone would create a nose down momentum. I think that a nose up (positive angle of attack) attitude would produce two things:

-some lift from the torpedo body itself
-a vertical component of the propeller thrust

Anyone knows what is the angle of attack of a torpedo traveling thru the water? I estimated about 20 degrees would be needed if the component of the thrust alone is used, but seems pretty much.

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Re: Torpedo angle of attack

Post by OpanaPointer » Tue Oct 20, 2020 4:20 am

I do know that the special torpedoes used by the IJN at Pearl Harbor were modified so as to hit the water closer to horizontal.

Wouldn't gyroscopic stabilization keep the torpedos running level?

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Re: Torpedo angle of attack

Post by Thorsten Wahl » Tue Oct 20, 2020 8:46 am

Depth rudders
Meine Herren, es kann ein siebenjähriger, es kann ein dreißigjähriger Krieg werden – und wehe dem, der zuerst die Lunte in das Pulverfaß schleudert!

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Re: Torpedo angle of attack

Post by OpanaPointer » Tue Oct 20, 2020 12:13 pm

Thorsten Wahl wrote:
Tue Oct 20, 2020 8:46 am
Depth rudders
AKA horizontal tail fins. :ok:

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Re: Torpedo angle of attack

Post by marcelo_malara » Tue Oct 20, 2020 2:38 pm

Yes guys, the control would ultimately fall to the depth rudders, but how? The rudders would push the tail down, so the nose will be pushed up, but that attitude would be maintained for all the travel, as the torpedo would constantly accelerate to the bottom because of its heaviness. The question is, what angle is necessary for depth keeping?

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Re: Torpedo angle of attack

Post by OpanaPointer » Tue Oct 20, 2020 5:40 pm

marcelo_malara wrote:
Tue Oct 20, 2020 2:38 pm
Yes guys, the control would ultimately fall to the depth rudders, but how? The rudders would push the tail down, so the nose will be pushed up, but that attitude would be maintained for all the travel, as the torpedo would constantly accelerate to the bottom because of its heaviness. The question is, what angle is necessary for depth keeping?
The horizontal tail fins would be driven by the gyroscope to keep it level. The gyro would be trying to keep the torp at the pre-set running depth. It would tweak the fins as needed to get that depth. That's the point of interest, not any incidental and transient angle of attack.

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Re: Torpedo angle of attack

Post by wadinga » Tue Oct 20, 2020 5:47 pm

Fellow Contributors<
Wouldn't gyroscopic stabilization keep the torpedos running level?
The main function of the gyro was to keep the torpedo on course. Depth was controlled by hydrostatic pressure ie weight of seawater on a flexible disc balanced against a spring adjusted to give desired depth and working a linkage to the depth rudders. There is an instructional WWII RN film with plummy accents on You Tube explaining how it all works.

On this navweaps page http://navweaps.com/Weapons/WTBR_WWII.php are given the negative buoyancy values for various weapons, which are considerably lower than the all-up weight in air. Because the negative buoyancy is small, water is dense and the torpedo speed is high, the estimated angle of attack of 20 degrees is way, way too much. Consequently at operating speeds the horizontal rudders do not have to deflect far to produce sufficient lift through a very small angle of attack to overcome this small negative buoyancy value. If the weapon gets too shallow the hydrostatic pressure adjustment backs off the rudders a bit and the weapon regains preferred depth.

Because torpedoes are complex , expensive machines, practice ones were given positive buoyancy so they could be recovered, re-fuelled and serviced to use again.

All the best

wadinga
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Re: Torpedo angle of attack

Post by marcelo_malara » Tue Oct 20, 2020 7:01 pm

Hi Wadinga
wadinga wrote:
Tue Oct 20, 2020 5:47 pm
Because the negative buoyancy is small, water is dense and the torpedo speed is high
Small or not, the negative buoyancy is present, and must be cancelled, if not the torpedo would accelerate vertically to the bottom, albeit almost in a horizontal position.

wadinga wrote:
Tue Oct 20, 2020 5:47 pm
the horizontal rudders do not have to deflect far to produce sufficient lift through a very small angle of attack to overcome this small negative buoyancy value
The horizontal rudders in fact would not generate lift to the the surface, but to the bottom, pushing the tail down and the nose up, so the vertical component of the propeller thrust (and may be the body lift) would cancel the negative buoyancy.

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Re: Torpedo angle of attack

Post by Steve Crandell » Tue Oct 20, 2020 11:36 pm

Do practice torpedoes have positive buoyancy, or does something happen at the end of the run to change buoyancy from negative to positive? It seems like running one at positive buoyancy would not be a proper test of it's combat performance.

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Re: Torpedo angle of attack

Post by wadinga » Wed Oct 21, 2020 1:18 am

Fellow Contributors,

Can you international guys see the film on You Tube?
AIRCRAFT TORPEDO ROYAL AIR FORCE INSTRUCTIONAL FILM MARK XII TORPEDO 75624
YouTube · 24,000+ views · 19/08/2015 · by PeriscopeFilm

It explains everything. :cool: It's especially cool because air dropped torpedoes have to initially maintain attitude in a less dense fluid, air, and then in a completely different one 800 times more dense.

For instance:
The horizontal rudders in fact would not generate lift to the the surface
They don't have to. When their rear edge is raised they push the stern of the torpedo down and
a nose up (positive angle of attack) attitude would produce two things:

-some lift from the torpedo body itself
-a vertical component of the propeller thrust


They are rudders and thus crudely divert water flow creating side thrust disrupting the hull's smooth passage through the water. They are just like a ship's rudder that pushes the stern to port to make the ship turn to starboard. They are not aerofoils/hydrofoils creating lift through differential curvature like an airplane's wing.

The 178/9 ish degree angle of the axis of the body of the torpedo to the thrust direction of the diverted prop wash gives the few hundred pounds of positive buoyancy necessary to balance the intrinsic negative buoyancy built into the design. High speed in an off-axis direction of travel in a dense medium (water) generates lift. With no heavy warhead a practice torpedo operates the other way. When either runs out of fuel, its speed drops away, the lift or anti-lift provided by the body travelling through the water at a small angle to its axis disappears, and the torpedo sinks to the depths or floats to the surface. Then a surface vessel can hook onto the ring in the practice torpedo's nose and crane it aboard, whereas the armed weapon drops away to the seabed either exploding on impact, detonating on a time fuse or exceeding its crush depth.

Photos of WWII practice torps waiting to be picked up show them floating vertically, nose up, with no external floatation. These days with AUV capabilities the torpedo probably backs itself into the tube again once the exercise is over. :lol:

I am sure Steve knows a lot more about the influence of fore and aft planes on submarines (very different to torpedoes) and the interaction of vessel speed and blowing tanks on the attitude of the boat when it breaks surface than he has let on so far...... What about those spectacular crash surfaces when the boat breaks surface at 40 degrees? Is the crew all lying in a heap at the back? Bet the cooks are complaining. :D

All the best

wadinga
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Steve Crandell
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Re: Torpedo angle of attack

Post by Steve Crandell » Wed Oct 21, 2020 2:09 am

I think submarine stern planes work just the same as those on torpedoes. I've stood watches (a lot of them actually) on stern planes and "fairwater" planes so I know how it all works in practice, but not the technical hydrodynamics of it.

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Re: Torpedo angle of attack

Post by paul.mercer » Wed Oct 21, 2020 9:25 am

Gentlemen,
In a post on the attack on Tarranto, I asked this question:
"It is widely assumed that the Japanese attack or Pearl Harbour was motivated by the RN attack on the Italian port of Tarranto. After watching a documentary on the Pearl Harbour attack it showed the tail of the Japanese torpedoes being encased in some sort of wooden box which broke away on impact with the water and stopped the torpedo diving too deep"
On looking at some old film of the US attack on the Japanese carriers, I'm sure i saw a torpedo with a 'box' like structure around the propeller area being dropped by a US aircraft. Was this fact or am I imagining it?

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Re: Torpedo angle of attack

Post by wadinga » Wed Oct 21, 2020 10:25 am

Fellow Contributors,

Indeed this thread is converging somewhat on that re Tarranto(sic). And slightly with "Bouncing Bombs".
Was this fact or am I imagining it?
Yes and no respectively.

From that other place:
In daylight such slow moving targets would likely be annihilated even before dropping their weapons since volume and accuracy of defensive fire had developed rapidly in just a few years. Faster, high performance aircraft needed to drop at higher speeds and for shallow harbours, techniques to minimise the dangers of high speed water impact and deep diving were required. The box tails and structures developed for the Pearl Harbor attack were necessary because the weapons were dropped from greater altitudes and faster speeds in daylight.
The Americans/ Italians/Germans also all developed "box tails" to allow drops at higher speeds and altitudes to give the launch aircraft a better chance of survival against enhanced enemy AA. This was true no matter how deep the water. The Italians produced a parachute-equipped programmed track torpedo to be dropped from safer higher altitudes. The FIDO A/S acoustic torpedo developed in the US could be dropped
Maximum drop altitude: 200 to 300 ft (60 m to 90 m)
Maximum aircraft launch speed: 120 knots (220 km/h).
Post war the Australian Ikara weapon used a rocket to carry the homing A/S torpedo to the drop point and a parachute to slow the fall until it hit the water.

The revolutionary and enormous increase in submarine underwater speeds produced post war by improved hydro-dynamics and nuclear power means their control surfaces are much more significant in "flying" the vessel as they can travel at wartime torpedo speeds. WW II submarines rarely made even double figure underwater speeds and quickly exhausted their batteries doing so. Their hydroplanes acted more as auxiliary assistance in trimming or tuning what the ballast tanks were making the sub do.

All the best

wadinga
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Re: Torpedo angle of attack

Post by Mostlyharmless » Wed Oct 21, 2020 1:33 pm

There was a trend to attacking with torpedoes at higher speed and higher altitude by both the IJN and the USN. According to http://www.navweaps.com/Weapons/WTUS_WW ... 29_Mark_13 the USN Mk 13 went from "The early models were further handicapped by the need to drop them low and slow - typically 50 feet (15 m) and 110 knotswhich made the torpedo planes carrying them vulnerable to attack" to "On one occasion in early 1945, six torpedoes were dropped from altitudes between 5,000 and 7,000 feet (1,500 to 2,100 m). Five out of the six were observed to make their runs hot, straight and normal." There is a very good site commemorating Eugene Slover with training videos such as https://eugeneleeslover.com/VIDEOS/Aeri ... ttack.html to give an idea of 1944-5 USN methods. Similarly, the IJN's Type 91 (1931) Mod 3 Strong had "These modifications allowed a 350 knot launching speed" according to http://www.navweaps.com/Weapons/WTJAP_W ... d_7_Strong. British torpedoes could also be dropped at higher speeds by 1944.
The German's went on a different route over 1944-5, possibly because they were thinking of using aircraft such as the Arado 234. This was the Bombentorpedo
https://en.wikipedia.org/wiki/List_of_W ... of_Germany. These were flat nosed bombs designed to be dropped short of a ship and would travel through the water like a Japanese Type 91 shell.

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°

Post by marcelo_malara » Thu Oct 22, 2020 2:37 am

wadinga wrote:
Wed Oct 21, 2020 1:18 am

They are rudders and thus crudely divert water flow creating side thrust disrupting the hull's smooth passage through the water. They are just like a ship's rudder that pushes the stern to port to make the ship turn to starboard. They are not aerofoils/hydrofoils creating lift through differential curvature like an airplane's wing.
I want to clarify this. In the correct circumstances any surface exposed to a flow will generate lift. An airfoil flat on a side and curved on the other will generate lift because of the velocity differential between sides. A symmetrical airfoil (both sides equally curved) will also generate lift, in this case an angle of attack is needed, ie there needs to be an angle between the middle line of the foil and the flow. Even a flat surface will generate lift if exposed to a flow with a positive angle of attack, the difference between a curved foil and a flat one, is that in the curved one you can increase the angle of attack, which augments the lift, till about 20°, where the foil would stall, whereas a flat airfoil will stall at a much lower angle.

Ship´s rudders are like airfoils, even have NACA profiles, till about 20° of rudder angle they generate sidelift. The difference with an aircraft wing is that beyond stall the aircraft wing will not generate enough lift to maintain the aircaft in the air, whereas the rudder will still generate a sideforce.

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