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mdiehl -> RE: An article I found. Not for JFB's. (6/29/2006 7:42:50 PM)
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Ursa. You should write bad comedy. Then you could write for a sitcom and have it canceled after the premiere.
Chez -
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Again, nobody said it kept the torp level in the air. I said they reduced the angle at water entry.
All of which is to say that you claim that the attachments reduce the angle in the air as well. In effect it keeps it more level.
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Given the fact that they are designed to break off at water impact, I don't see how any additional drag is created.
That is your claim. It is not something that you have substantiated. It is clear that they break off some time during or after entering the water. Well, at least part of them break off.
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But let's say for moment that you are correct and they did create additional drag. This would cause the torpedo to angle downwards after water entry... the exact opposite of what is needed.
It would have that effect unless of course their more important function was to reduce the speed of the torpedo when it entered the water. Reducing the speed reduces the momentum and KE, and prevents it from plunging as deeply. If the depth control settings kick in as they should, and if the wooden attachments are otherwise "attitude neutral" the depth control settings will bring the torp to operating depth faster than they would if the wooden fins were not attached.
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Your statements are contradictory. First you say that they were intended to create additional drag then you say that the additional drag would be detrimental and cause the torpedo to pitch downward. Which is it?
They're only contradictory of one is so uninformed as to imagine that angle of entry is the *only* consideration. As I have noted several times in several ways (and as Mandrake has observed) there are other considerations.
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These fins (the wooden anti-roll and tail fins) were designed to minimize the drag in the air and simply act as a stabilizing mechanism.
No kidding. ALL FINS STABILIZE WEAPONS IN THE AIR. Even the unguided dumb HE bombs dropped by B17s had fins. If you put a fin on the back end of the weapon the weapon is pitched downward.
Ironically, were there even a little bit of angle on the fins, they'd be far more effective in the water than in the air. Because water is a much denser medium. If you attached said fins such that a little bit of angular stress on their attachment would shear them off, then even a slight pitch on the fins would cause the torp to pitch up and the angular stress would then shear the fins off.
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If nothing more, just the additional weight from the fins at the rear of the torpedo will change the center of gravity and produce a flatter entry angle.
For your claim to be correct, you would have to be able to assert that the added weight of a couple of small plywood fins on the back of a (1.5-2 ton?) torpedo substantially shifted the center of mass well aft of the normal mass center of the torpedo. If you have documentation of this claim I'd like to see it. Said shift of center of mass would have to be sufficient to overcome the designed center of the torpedo as well as the additional drag (which would force the torp downward) caused by the plywood fins of themselves.
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As the anti-roll stabilizers were pitchable and had wing-shaped wooden fins attached while in the air, seems reasonable to me that they would exert some type aerodynamic force on the rear half of the torpedo.
No. They're antiroll devices. That means they control ROLL not PITCH. All they can do is increase drag a bit (they have to, because drag is what provides the force to make the torp roll). Increasing drag aft would pitch the torp downward a hair. Note please, before you start splitting imaginary hares again (beep beep), that I said *control* pitch not *affect* pitch. The antiroll fins cause a problem for the pitch that they do not solve of themselves.
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That is true during a normal run. But for a shallow water run, the torpedo needs a little help. The decreased angle at water entry provides that by keeping the torpedo from plunging as deep as it would otherwise.
You have not substantiated that the stabilizers aft reduced the angle at water entry. Moreover, reducing the speed of the torpedo at impact will reduce the plunge depth of the torpedo, giving the usual control mechanisms more time to correct the torpedo's depth when dropped in shallow water.
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Again, who said anything about keeping the nose of the torpedo up?
If you want to assert that the wooden attachments reduced the angle of entry at the water, you must assert that the pitch of the torpedo is affected. That can be phrased in many ways. You push the stern down or you push the nose up, relative to the "normal" (sans wooden attachments) attitude of the torpedo that enters the water. The effect is to keep the nose of the torpedo's nose up.
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It does seem to me like the wooden fins on the anti-roll stabilizers are acting as canards and would have some influence of the flight of the torpedo.
They'd influence the roll. Possibly in the air. Definitely in the water because water being denser would impart more force interacting with the control surfaces. Preventing the torp from rolling much in the water would likewise allow the gyros to bring the torp to operating depth faster, I suspect. I wonder if the antiroll devices broke off too?
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The main point to remember here is that the Japanese were not trying to stop the torpedo's assumption of a down angle once launched; they were simply trying to limit it to less than the standard water entry angle of 17 to 20 degrees when deployed under normal circumstances.
OK. But why insist that they accomplished this in the, uh, 1-2 seconds the torp was in the air with such tiny surfaces, when they could do it that much more effectively when the torp enters the water?
You can get all the effects you want much better if you don't assume that everyone else is wrong and instead assume that the attachments had their main effect (however briefly) after they were underwater. They'd still break off with a not-at-all sophisticated bit of engineering. (Shear pins on the things would have to be sufficiently robust that a low density medium like air would not affect them, but force imparted by the water pushing on these things to alter the pitch of the torpedo underwater could shear the pins.)
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Those are your words. Again, no one said anyone was trying to keep it level in the air.
Take a level. Place it on a wooden board inclined at 45 degrees. Now change the inclination of the board to 15 degrees. Is the board (a) more level, (b) less level, or (c) just as level, as it was at 45 degrees? If you imagine that the board is a torpedo and the downward end of the torpedo represents its nose as it falls through the air, changing the pitch of the "torpedo" by pushing down on its back end pushes the nose up. That is what "pitch" is all about. You're keeping it more level.
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They were simply trying to keep it from assuming the normal down angle. Doesn't really take a lot to do that.
Oh really? How much directional force is imparted by those stabilizing wooden fins aft? Remember, you're trying to alter the pitch of an object that weighs... what... 1500 Kg? And you have to have sufficient force to overcome the drag that these fins add to the back of the torp.
If you are correct ,those Kates probably really disliked lugging torpedoes around in this way, because to compensate for the highly effective planes on the back of the torpedo that you posit substantially lifting the nose of the torp, the Kates would have to add a lot of downward trim. Both the torp's fins' effect and the planes added trim would substantially increase drag. Their fuel consumption would have been an ugly thing.
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I will admit that the plane of the wooden tail frame is flat, that it has no chord, does not provide lift in the way an airfoil would and that its primary purpose is air stabilization. But certainly not for additional buoyancy as you had earlier claimed.
I'm just going by what everyone else I've read (they don't go into a heck of a lot of technical detail there) said. I will agree that their main effect probably was not flotation. For the same reason that I can't see them doing alot to the pitch of the airborn torp. There's not enough stuff there. Maybe it was just energy lost from the torp as it plunged into the water, through drag or as Cap'n said, through cavitation.
The thing is, there would, one way or the other, have been a slight increase in buoyancy. It could not be otherwise if you attach a device to the thing and the material from which it is constructed -- plywood -- has a specific gravity of around .70-.85. I can see why the previous authors on this have subsumed it all as "flotation." Wood being the sort of thing that floats (well, most woods).
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However, reviewing the engineering plans contained in the Nav Tech Intell document (figures 5 and 6) shows that the horizontal fin is hinged and contains what they call an “angle plate.” While it doesn’t say so, there does appear to be a way to adjust the angle of it. This may be so as to produce a downward movement of the rear of the torpedo. Regardless, I can’t prove the purpose of the hinge and the angle plate so I’ll just say it’s not germane to the discussion at hand.
That is actually very helpful. It would seem to me to put the nail in the coffin of the "flotation" argument. I'd bet a dollar to a doughnut that they stayed on a bit after the torp entered the water.
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But we do have “wing-shaped” fins added to the anti-roll stabilizers. These are aerodynamically shaped. It would be rational to assume that they function to influence the flight of the torpedo in the air and quite probably provide a downward force on the rear of the torpedo.
I'd put my money on that hinge plate rather than on the antiroll stabilizers if for only this reason. For them to work to pitch the nose up (or the aft down), they'd interfere with the antiroll property. They'd both be, uh, "pushing" or imparting force in the same direction; where if you wanted them to be effective as antiroll devices, they'd have to push in opposite directions.
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Second, we have the added weight of the fins themselves which moves the center of gravity towards the rear of the torpedo. This alone will slow the rate at which the nose of the torpedo drops.
How much can they weigh, though, relative to the mass of the torp and how do they compensate for their own added drag?
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It sounds to me as though every modification was done in an attempt to influence the torp’s flight through the air until water impact.
We can't rule that out I supposed but I suspect that their main effect occurred during the first few seconds after the torpedo entered the water.
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As the NTM documents prove, the fins broke away upon water entry…they had to… otherwise the control surfaces would be locked in place by the wooden fins. So they are incapable of providing any drag whatsoever in the water.
You know, I think you're just ebing way too literal. The fins broke away on water entry seems to mean to you "instantaneously without in any way affecting the torp in the water." I don't think that the NTM document necessarily means to say that which you think it means to say. As I said, if they just "hung on there" a second in the water they'd be waaaay more effective at pitching the torp up than they would while dropping through the air. And this could be easily controlled by simply having the right kind of shear pins holding them on.
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The Combined Fleet source you quoted only mentions that the purpose of the fins were to keep the torpedo from diving as deeply as it normally would. Hardly a definitive source or description.
OK fair enough. But look, it's not like the guys at Combinedfleet.com are a bunch of allied fanbois, or have a record of getting things manifestly WRONG. They seem pretty informed to me. And Parshall and Tulley warrant alot of respect in my book for getting published a peer-reviewed book on Midway.
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Likewise, the Osprey article. The author states that the fins were for added buoyancy, a fact that is neither supported nor even mentioned by any other source I have seen, including the NTMs and the one other one you listed. Indeed, if added buoyancy were the purpose, it would seem logical to place them at the front of the torpedo to lift the nose. Certainly not at the rear where the added buoyancy would actually force the nose down thereby increasing the running depth.
Yeah. But if you wanted to change the pitch of the torp in the air you'd likewise put something on the front. Like a canard. That's one reason why guided A2A missiles have canards (well, fins forward), not rudders.
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I, unlike you, do not claim to have got it right. What I do claim is that the authors of the NTM I cited are far more knowledgeable about the subject than either you or I. Even you dismissed many of Osprey author’s claims as suspect in your first posting on the subject. Are you now saying that isn’t so?
I'm saying that when I reject someone's argument I require a good reason for it. I've seen lots of publications that claim that the device on the torps prevented the torp from plunging as deeply when they entered the water. Some like the Osprey guy invoke flotation. Some imply some sort of submerged effect on pitch. None stipulate that the wooden thingies broke off instantaneously on hitting the water. None stipulate that they were intended to affect the torp's pitch or angle of entry in the water. And the devices, to the best that one can tell by looking at them, if they had any effect on pitch at all, would have been far more effective underwater even if they'd stayed attached for merely a second, than they would in the air.
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So against these experts, who do you line up?
Physics. I do wonder what the expertise was of the good lt, and whether or not the translation is rendered correctly, and I strongly suspect that you are reading far more into the NTM document than it actually states.
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And as it seems you haven’t bothered to read the NTM article, it seems a bit presumptuous of you to dismiss it out of hand in favor of one article written by someone you initially regarded as presenting suspect data and with no stated experience in aerial torpedoes.
I'm not dismissing it out of hand. I'm saying that the document does not say that which you have inferred. I think you've read it too literally, in your desire to one up the Osprey guy, or win a round vs me, or just to seem like you know whereof you speak. I think you've attached yourself, like a limpet, to a very narrow view of these things as aerodynamic devices, when they are I suspect hyrdodynamic devices.
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