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#21
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#22
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"Nathan Young" wrote in message ... Jay's original comment of the absurdity of vacuum systems is dead on. But the solution isn't to replace with electric gyros, the solution is to replace with solid-state ones. Very few piston general aviation airplanes have dual alternators and you could not even begin to consider a non-vacuum-system airplane without both dual alternators and dual electrical buses. Even dual electrical buses can fail and that is why business jets often have an emergency battery-powered gyro which completely self-contained; these backup gyros can easily cost over $7,000. At those prices, it seems a lot more practical or at least economical in a GA airplane to simply have a conventional vacuum AI backed up by a conventional electric AI. -------------------- Richard Kaplan, CFII www.flyimc.com |
#23
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Why spin a gyro at all? Laser ring gyros are the way to go. No moving parts. Sandy Mustard Jay Masino wrote: Jay Honeck wrote: This whole topic drives me crazy, since the idea of a "vacuum system" is ridiculous in the first place. I'm stuck with one, and have replaced both of my vacuum instruments in the last six months. Why? Because the danged electric replacements are absurdly over-priced, and the back-up battery that would make an all-electric system prudent (and legal) is even worse. Personally, I think the fact that we're flying around behind vacuum instruments in the 21st century is patently absurd. I suspect it's a lot easier to get a gyro to spin at 10 or 15,000 RPM (or whatever) using vanes and vaccum, than it is to design a reliable electric motor to spin the gyro at that speed. It can obviously be done, but I suspect the parts neccessary to do it might be a little more exotic and expensive than vaccum gyro parts. Add to that the immense liability that a manufacturer of such devices are taking on, you start to see why all of these devices are expensive (even the vaccum devices, really). Turn coordinators spin a gyro with an electric motor, but I bet the fact that horizons have to pivot in two directons (roll and pitch) makes the internal design way more complex. --- Jay |
#24
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David Megginson wrote
Afterwards, I read the Air Safety Foundation report that everyone cites on the danger of vacuum pump failures: it turns out that they did not find a *single* fatal accident from 1983 to 1997 involving a vacuum-pump failure for a fixed-gear plane flying IFR -- losing control partial panel seems to be a retractable thing. As a gross generalization, that makes sense to me. IMO the really critical parameters are drag coefficient and roll stability. Airplanes which are roll stable and draggy (Cherokees, C-172's, and their ilk) are easy to fly partial panel and it takes a long time for a nose low unusual attitude to develop to the point where the airplane will redline. Airplanes that are clean and not terribly roll stable (Bonanzas, Mooneys) are much more demanding partial panel, and will go to redline in a heartbeat once you let a nose-low unusual attitude develop. I suspect, though, that the new crop of high-speed low-drag fixed gear singles from Lancair and Cirrus are likely to behave more like the Mooneys and Bonanzas, whereas retracts like the Arrow and Cutlass are probably not significantly more likely to have loss of control problems when partial panel than their fixed gear cousins. Personally, I consider backup vacuum/gyros to be a low priority for something like a Cherokee. I suspect that the same money spent on regular recurrent training would have a significantly higher payoff in safety. I would put the backup vacuum/gyro for something like a Cherokee lower on the list than some sort of weather avoidance capability (spherics, datalink, etc.) and lower than a good handheld GPS with fresh batteries. Once you have those things, and you're doing regular recurrent training, then sure - get athe backup. I'm sure it must have some marginal safety advantage. Michael |
#25
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Richard Kaplan wrote:
Very few piston general aviation airplanes have dual alternators and you could not even begin to consider a non-vacuum-system airplane without both dual alternators and dual electrical buses. Even dual electrical buses can fail and that is why business jets often have an emergency battery-powered gyro which completely self-contained; these backup gyros can easily cost over $7,000. It can be a fraction of that if you don't mind yet another portable device cluttering your cockpit: http://www.icarusinstruments.com/microEFIS.html People have also reported some success under the hood flying a plane using the display on the Garmin 196. Of course, until people are forced to used it in actual IMC, we won't know how well it really works in an emergency. At those prices, it seems a lot more practical or at least economical in a GA airplane to simply have a conventional vacuum AI backed up by a conventional electric AI. That's just it -- even my cheap little Warrior has a lot of options for keeping the wings level in IMC: Vacuum-powered AI and HI Electic-powered TC Battery-powered GPS Magnetic compass Of course, these become less and less useful as you go down the list (I wouldn't be much on my chances with just the magnetic compass), but in real life, but how much redundancy do you need before you've overdesigned the system? As I mentioned earlier, I have not yet managed to find a single example of a fatal accident caused by a vacuum-pump failure in a fixed-gear plane flying IFR. There must be one or two, but it does not appear to be a significant risk. All the best, David |
#26
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Michael wrote:
Personally, I consider backup vacuum/gyros to be a low priority for something like a Cherokee. I suspect that the same money spent on regular recurrent training would have a significantly higher payoff in safety. I would put the backup vacuum/gyro for something like a Cherokee lower on the list than some sort of weather avoidance capability (spherics, datalink, etc.) and lower than a good handheld GPS with fresh batteries. Funny, you just listed my major purchases over the past six months: I bought a Garmin 196 in December, and just ferried my plane back from Montreal this afternoon with a (used but factory updated) WX-900 Stormscope installed by an experienced shop. It was the perfect afternoon for it: solid IMC above 1,200 ft AGL, with a small risk of occasional embedded TCU and CB (normally, I cancelled flights under those conditions). All the best, David |
#27
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On Mon, 10 May 2004 16:15:48 GMT, "Richard Kaplan"
wrote: snip Very few piston general aviation airplanes have dual alternators and you could not even begin to consider a non-vacuum-system airplane without both dual alternators and dual electrical buses. Even dual electrical buses can fail and that is why business jets often have an emergency battery-powered gyro which completely self-contained; these backup gyros can easily cost over $7,000. At those prices, it seems a lot more practical or at least economical in a GA airplane to simply have a conventional vacuum AI backed up by a conventional electric AI. I would be interested in the maintenance requirements for a couple of the new "electric" aircraft. Another thread was asking about the batteries keeping the engine turning on the Liberty XL2. My totally uninformed guess is that on the Liberty, when the batteries quit, the engine does as well. This was indeed the case with the certified Porsche-powered Mooney. Having maintained a few turbine aircraft & "business jets", periodic maintenance and inspection/cap check of the various batteries and specific DC generator life limits (one specific example has three primary DC buses, six batteries, and three generators installed) are an important factor in maintaining the "normal" and "emergency" systems. I would like to think that this same matter (periodic cap check and alternator life limts) is specifically addressed in the new GA designs. BTW, I've also thought that it curious that cap checks on the main ship's battery (which is the sole source of emergency electricity) on "classic" GA designs seems to be a non-issue. Regards; TC |
#28
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Airplanes that are clean
Aye and not terribly roll stable (Bonanzas, Mooneys) are much more demanding partial panel, My 65 C Mooney is incredibly roll stable and extremely easy to fly partial panel; so much so that some view it as a weakness (I dunno, maybe in a flat scissors?). I fly partial panel approaches IMC for practice. Later M20's are more pitch stable as well. |
#29
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Jay Honeck wrote: It's amazing when spending something approaching $1500 for two lousy vacuum instruments makes me a "cheapskate"... If you manufactured a clock to that level of precision, it would cost more than one of those items. And wouldn't have to be certified. Don't believe me, check out the prices on Swiss watches these days. I repeat. Quitcherbitchin. George Patterson If you don't tell lies, you never have to remember what you said. |
#30
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"David Megginson" wrote in message e.rogers.com... It can be a fraction of that if you don't mind yet another portable device cluttering your cockpit: If we are talking about why airplanes have vacuum systems then we need to keep the discussion to certified equipment -- other options may work but if they are not certified then the FAA will require a vacuum system for legal IFR. system? As I mentioned earlier, I have not yet managed to find a single example of a fatal accident caused by a vacuum-pump failure in a fixed-gear plane flying IFR. There must be one or two, but it does not appear to be a significant risk. I think part of this may be related to how the accidents are classified. For example, there are a number of examples each year of IFR airplanes which simply suffer inflight break-up for unknown reasons; these could well be due to vacuum pump failure. -------------------- Richard Kaplan, CFII www.flyimc.com |
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