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Loopycrank

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  1. I used to think that the coaxial contra-rotating rotor setup was obviously the correct way to design a helicopter. No power goes to the tail rotor, no tail rotor pushing the helicopter sideways, tail can be shorter, main rotors are a bit more efficient, all good stuff, right? Turns out, like most things, they're a trade-off. One of the many things a helicopter's designers have to choose a balance between is hover performance versus forward flight performance. There are several design parameters that directly help in hover, but hurt in forward flight and vice versa. For instance, low disc loading is great for hover performance, but for a given helicopter weight means a lower Mu limit in forward flight. TANSTAAFL. Coaxials are the same. Most of their advantages show up in hovering flight, and their disadvantages show up in forward flight. Think about a helicopter travelling forwards. On one side of the rotor disc, the blades are flying into the wind, while on the other side, the blades are flying away from the relative wind. This causes lift asymmetry on the total rotor disc, and the faster the helicopter flies, the more this matters. Juan de la Cierva encountered the same problem in his autogyros and developed the mechanical solution that has been used in autogyros and helicopters ever since; a complex articulated rotor hub with flapping and lagging hinges that can allow the blade disc to tilt to compensate for this asymmetry. And before any smug Eurocopter shills start flapping their lips about their supposedly "flapless" or "rigid" rotor hubs, they're lying. Their rotor hubs have just as many degrees of freedom as any other design, they just use elastomeric-sprung flex elements rather than hydraulically damped articulated metal hinges. The only helicopters that have actually rigid rotors are the Sikorsky ABC aircraft like the XH-59 and Raider. Those superficially resemble Kamovs, but they're actually a whole 'nother can of worms because they have to re-think the fundamentals of how rotor-borne flight works from square 1. Again, these hinge systems go all the way back to autogyros, so any design that tosses it out is very much in unknown territory. So, why are flapping rotors a problem for Kamovs in forward flight? Well, think about the direct the rotor tip planes want to go. The rotors rotate in opposite directions, so the rotor discs also want to tils in opposite directions. Past a certain airspeed, or when performing certain maneuvers, the rotors actually smack into each other. That's not a theoretical concern either; Kamov has actually lost prototypes that way. One would think that preventing this would be a simple matter of spacing out the rotors far away enough from each other that they couldn't smack into each other at any realistic airspeed, but presumably stiffness limits of the rotor hub and blades preclude this. Second, the whirling rotor disc starts to create large amounts of drag as forward airspeed goes up. For a given disc loading, a coaxial helicopter has quite a bit more wetted disc area, so drag from the rotor system increases faster than a conventional tail-rotor helicopter as well.
  2. There are a lot of variables to consider. Sorry, that's a canned answer, but it's true here. So, this is an image of the raw plastic that gets fed into the injection molding machine, from some Chinese supplier I've never heard of before. Best quality, I'm sure. Each pellet is mostly nylon plastic, with either the glass or carbon fiber strands pre-mixed into each pellet. The pellets may or may not be pre-mixed with dye, and, maddeningly, this sometimes makes a difference. The pellets get mashed by a big screw into a heated tube that narrows along its length. Incidentally, this process also breaks up some of the fibers, so they're not as long going into the mold as they are when spec'd by the supplier. They might say that the fibers don't shorten but they're lying. Eventually, the combination of heat and pressure causes the plastic to turn into a liquid that gets shot into a mold that is, as you might imagine, the negative of whatever object it is you want to make. OK, so as far as carbon fiber vs. glass fiber goes, you need to specify the fiber length and the fiber fill percent. For gun magazine plastics I think they usually run 30-40% fiber fill. Glass is denser than carbon fiber, so the higher percentage of fiber, the greater the weight savings of switching to carbon. The glass fiber is also (much) more abrasive than the nylon, so the greater the fiber fill percentage, the greater the savings on mold life. Conversely, carbon is more expensive than glass, so the higher the percentage, the greater the raw material cost difference. For 40% glass vs 40% carbon fiber, the carbon fiber reinforced plastic is about 10% lighter. So, not a huge weight savings, but not trivial either. I can't find raw material cost differences, but from memory when I looked into a similar problem years ago, it was on the order of 50% higher for the carbon. This does need to be weighed against the greater times between overhauls of the mold with the less abrasive carbon fiber though, which not only translates to less downtime but also more profit. Mold refurbishment is very expensive. So, on the face of it, this isn't obviously a bad idea, nor is it clearly going to set the world on fire. The biggest problem I can foresee is that there is a lot less industry experience with carbon fiber reinforced nylon. Old salts who have been there and done that are worth a whole lot when it comes to de-bugging your molding and getting the designs dialed in. I've met a whole lot more industry professionals who knew their way around glass fiber than carbon fiber, and the glass fiber experts didn't exactly grow on trees either.
  3. Given that the cam blocks could break on steel framed hi powers if fed a steady diet of hot rounds, this seems most unwise.
  4. First we need to define what is meant by "carbon fiber." Because there's lot of kinds of things that can reasonably be called "carbon fiber." What this is very clearly not is a layered carbon fiber layup in an epoxy matrix, which is the normal kinds of carbon fiber composite. Look at the shape of the magazine and the swirly pattern of the fibers. This is very clearly a hexmag mold. Conceivably they've shot it full of carbon fiber reinforced nylon instead of the glass filled nylon they were using before, if they're not just BSing about it being carbon fiber. The material properties of carbon fiber reinforced nylon vs glass fiber reinforced nylon would depend very much on the percent weight fill of the fiber as well as the fiber length. But it's not night and day either way. From the manufacturer's perspective, carbon fiber fill chews on the mold a lot slower than glass fill does, since glass is harder than steel but carbon fiber isn't. This is offset, however, by higher base material cost. This isn't the first time this has been done either, the DD 32 round magazines were also carbon fiber filled nylon.
  5. Does anyone have pictures of the 105mm tandem HEAT round developed for the SK-105?
  6. Ah, you're right about the Type 89. It appears to be two outward swinging doors. The CV90 example suggests that it is not at all difficult to re-design for customer preference.
  7. The Germans did have the Bergepanther ARV, which the British seemed reasonably impressed by when they evaluated one. I'm sure that in the role of winching out stuck or disabled vehicles it was likely let down by the dodgy final drives, but it seems conceptually sound. Certainly more conceptually sound than yoking up three heavy half tracks to try and yank out a stuck panzer.
  8. Think about this carefully. On a vehicle with a door, there is a sudden transition from ground level to the height of the ground clearance of the vehicle plus floor thickness plus the height of the lip around the door. There will always be a lip, by the way; there has to be to ensure adequate sealing against water and NBC junk. The lip not only adds height to the distance the soldier needs to clear getting into the vehicle, it adds an inherent tripping hazard on the way out. The fact that a person can slip on a ramp does not in any way imply that doors are somehow easier to navigate, or have any advantage whatsoever. Forgive me for being cynical about the stated reasoning behind British military doctrine; there are multiple examples of cost-cutting features in their hardware being rationalized as being that way "for doctrinal reasons" as well as multiple examples of standards being revised to fit the performance of the hardware rather than the other way around. Some documents to this effect were posted on the Ed Francis Discord a while back regarding the purchase of the Challenger II vice the Leopard II, and there are multiple examples of jiggery pokery with reliability numbers in Steve Raw's book on the L85. Not including an ATGM on every hull of a vehicle that's ostensibly intended to face off against an army with a massive superiority in number of tanks is dubious. ATGMs have no recoil (at least not that the vehicle designers need to account for), are extremely lightweight, and give an IFV a fighting chance against tanks, especially in an ambush scenario. Not only that, but they are much better at destroying bunkers or other hardened targets than an autocannon. The Bradley goes one better and has thermal optics, which the Soviets largely lacked. A large-caliber gun is generally a better vehicle-killing weapon than an ATGM, but the cover of night will do a lot to even things out for the M2 against the godless hordes and their endless supply of tanks. Not including an ATGM system that can be fired from under armor is also dubious. A number of AFV design books from the period talk about how wargames and practical experience showed that mobility is not armor, but armor is definitely a kind of mobility. That is to say, light, nimble vehicles cannot avoid being hit well enough to forego having armor. However, in a large-scale war, the front lines are swept with enough artillery and small arms fire that not having adequate overhead protection to at least keep that out means that there are effectively large areas of the battlefield that a vehicle simply cannot traverse. The large percentage of infantry casualties caused by artillery in WWII would tend to bear this observation out. Given that, having to stick ones head out to operate an ATGM post stapled to the roof is a definite downside of the design, and that's before even considering NBC protection. Speaking of "spraying and preying" is an obvious strawman. The issue isn't that the clip-fed RARDEN lacks the ability to perform suppressive fire with its autocannon or maintain some arbitrary sustained rate of fire that looks good on paper and has no tactical value. The issue is that it's feeding from three-round clips, and it has a two man turret crew. Congratulations; you've managed to re-invent the same tactical problems that the T-34 had in 1941.
  9. Are you kidding? Warrior had so many deficiencies relative to the early models of Bradley that it's difficult to pick any single "obvious" problem. No stabilizer, no belt feed (belt-fed weapons were relatively new, having only been invented about ninety years prior), no thermals, no ATGMs on most vehicles, no ability to fire the ATGM from under armor; the lack of a ramp is interesting because while the other issues are just typically boneheaded MoD penny wise and pound foolish corner cutting, the fact that other British APCs are laid out the same way suggests it's a national eccentricity. Oh yes, how silly of me. The soldiers ingressing and egressing the vehicle will just rotate their local gravitational vector ninety degrees and then a door becomes just like a ramp. This is utter foolishness. I cannot believe that you're so triggered over the phrase "obviously correct" solution that you're arguing this nonsense. How is a power door better than a ramp? The argument that the motors are lighter is silly. The difference in weight and bulk in an electrical motor that can swing a door vs lifting a ramp is utterly trivial to the overall design of an IFV. The difference in complexity is trivial also; if the door actuator is what is going to make or break your IFV, maybe you should just buy IFVs and not try to make them. Which is what the British should have done with Warrior; aside from seating arrangements, the early Bradley enjoys a substantial list of advantages.
  10. Apparently British testing found that a lot of the noise inside an AFV comes from the running gear idler, of all things.
  11. There have been such studies, here is a summary of one of them: https://apps.dtic.mil/sti/pdfs/ADA490527.pdf Note that the vehicle in question has a ramp. This is wishy-washy thinking in the extreme. Yes, technical solutions to a given problem will come with an array of advantages and disadvantages. That does not mean that one solution cannot be obviously holistically better than another. Or do you have a different explanation for the lack of gasoline engines, interleaved road wheels, full-caliber AP, stereoscopic rangefinders, and Horstmann suspension on modern tanks?
  12. Ramps allow easier egress and ingress by far. This is one of those things where the fact that it's the overwhelmingly dominant design choice should be a clue. M113 has a ramp. CV-90 has a ramp. Type 89 has a ramp. Marder has a ramp. Puma has a ramp. Lynx has a ramp. K21 has a ramp. Redback has a ramp. Hell, even Kurganets has a ramp! It's not like there's a raging debate between two schools of thought on this. There is an obviously correct design solution that everyone chooses given the chance, and not choosing it is conspicuous. What's more, as I explained, the Warrior somehow manages to throw away the one, dubious advantage a door has by going with a power operated door that's just as slow, heavy and complicated as a ramp!
  13. Originally this was going to be a question about the warrior IFV and its unusual power door, which only occasionally smashes poor squadies' heads: This seems like a patently stupid way to design an IFV. It's all the weight and complexity of a power ramp, only not as good, and more dangerous. And then I realized it's not just Warrior, it's FV 432 and Spartan too! Why didn't the British put ramps on their APCs and IFVs?
  14. OK, so the article is a bit of a mess, but I think I get what they're getting at. Part of the reason that steel is used for everything is that carbon and iron are abundant on Earth's surface, and another reason is that an iron/carbon alloy can have several different microstructures with substantially different macroscopic properties. Often, it is possible to switch the microstructure of a part just with heat treatment. Martensite is nothing new. It's really old, actually. Think of a historical film about the Middle Ages. You know when the blacksmith thrusts the red-hot sword he's forging into water? That quenching process is to convert the steel of the sword from a ferritic microstructure into martensitic one. As the article notes, martensite only forms if the temperature transition occurs rapidly. Water is conductive enough that it sucks the heat out of the blade quickly, and the sword blade becomes the hard, wear-resistant martensite. Usually a purely martensitic structure is a bit too brittle for most uses, so the sword is usually tempered after quenching to create a tempered martensite microstructure, which is a little softer than pure martensite, but a whole lot tougher and less likely to shatter. There are some odd exceptions; traditional katanas have a pure martensite edge AIUI. Tank armor is usually also tempered martensite. But there is a problem. Usually a tank hull needs to be welded together. Welding is hot enough that it can disrupt the heat treatment of the armored plates. So, you may start with armor plates of the finest tempered martensite, but when you weld them, the entire rest of the tank hull effectively acts as a giant heat sink and sucks the heat out of the weld quickly enough that the weld material turns into the brittle pure martensite. So it may be necessary to pre-heat the area around the weld, tweak the weld filler rod metallurgy or re-heat-treat the weld and the area around it after welding. This is why armor is a bit weaker at the weld and in the area immediately surrounding the weld (the heat affected zone, HAZ). If you look at pictures of knocked out WWII German armor, you'll see that the armor plate frequently breaks at the welds. So, what I think is going on is that they're selective laser sintering little blobs of steel. The little blobs are welded together with the laser to build up the part. Each individual blob cools quickly enough that it forms martensitic microstructure. The fancy computer modelling allows them to optimize laser settings so that the printed part doesn't have a bunch of little micro-voids in it.
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