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#41
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#42
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"Gary Drescher" wrote in message news:bnzLb.6520$8H.20195@attbi_s03...
"Dave S" wrote in message . net... Now... a question about realities.. The POH nazi's will say that the Word as written is good, praise be to the POH... if I base flight decisions and speeds on MY calculated numbers rather than the max weight sea level standard day numbers published in the almighty POH.. am I going to be asking for trouble here? It depends on what you mean by 'trouble'. The laws of physics prevail over the POH in determining whether your engine mount will break... Why do folks worry about engine mounts breaking? They are far stronger, in most cases, than the rest of the structure. For production airplanes, the legal standards for certification include a 9G strength for fuselage/cabin structure for crashworthiness, and I have seen other specs calling for the same 9Gs specifically on engine mounts. Dan |
#43
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"Dan Thomas" wrote in message
om... "Gary Drescher" wrote in message news:bnzLb.6520$8H.20195@attbi_s03... "Dave S" wrote in message . net... Now... a question about realities.. The POH nazi's will say that the Word as written is good, praise be to the POH... if I base flight decisions and speeds on MY calculated numbers rather than the max weight sea level standard day numbers published in the almighty POH.. am I going to be asking for trouble here? It depends on what you mean by 'trouble'. The laws of physics prevail over the POH in determining whether your engine mount will break... Why do folks worry about engine mounts breaking? They are far stronger, in most cases, than the rest of the structure. For production airplanes, the legal standards for certification include a 9G strength for fuselage/cabin structure for crashworthiness, and I have seen other specs calling for the same 9Gs specifically on engine mounts. Are those regulatory specs? In any case, it's just an example. The crucial point is that Va is a speed that limits the _acceleration_ that the control surfaces can impose before the plane stalls, whereas Vno is a speed that limits the _force_ that the wings can develop before the plane stalls. Therefore, staying below Vno is what keeps the wings attached and intact, whereas staying below Va is what keeps _other_ parts of the plane attached and intact (because the plane's acceleration determines the force exterted upon other structures). This distinction is key to understanding why Va is proportionate to the square root of weight, whereas Vno is independent of weight. (Whether or not the engine mounts are the weak link in the rest of the plane presumably varies from one aircraft to another.) --Gary Dan |
#44
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On Fri, 09 Jan 2004 16:50:14 GMT, "Tony Cox" wrote:
"Dave S" wrote in message . net... Gary... I was looking for actual formalas.. not wild ass guesses or rough approximations.. Computer spreadsheets use mathematical equations. Check your reactions. You were given the EXACT mathematical relationship, and a very close approximation to make in-flight calculations easier. I'm sure you didn't mean to sound ungrateful, but that is the way it came across! There are a few issues before you go off treating any equation you get here as gospel. 1) Va probably isn't what you think it is. See my other posts. 2) In the case that Va = Vs*sqrt(load-factor) (23.335 equality), don't *ever* be tempted to scale it up if you are over gross (Alaska, for example). Wings falling off may not be the limiting factor. 3) If you are under gross (and Va is 23.335 equality), the scaled Va is probably too conservative. If the 23.335 equality does not apply, then the adjusted Va may not be conservative enough.Without further specific analysis, you'll never be sure. This is something that may be used by others besides myself. This sounds foolhardy. You might want to ask yourself why manufacturers don't publish Va vs. weight. And if you go off telling people they can happily fly at Va without the wings falling off, you're setting yourself up to be sued. |
#45
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On Fri, 09 Jan 2004 18:43:43 GMT, "Gary Drescher"
wrote: "Roy Smith" wrote in message ... "Gary Drescher" wrote: (The calculations should be made with regard to CAS rather than IAS, but the difference is usually small.) Careful about that. CAS is usually very close to IAS near cruise, but at the low end of the scale, they may differ significantly. True. It depends a lot on the aircraft. Lately I've been flying Arrows, for which the discrepancy is tiny even near stall speed. But that's not always the case. Right! For a C-172 (most models/years) the error gets pretty large at Vs0 -- 50 KCAS = 33 KIAS. That difference is certainly NOT trivial. --Gary |
#46
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Wonderful.. Thankyou Blanche... I only have to tweak the name of the
variable A6 to plug this in.. This was exactly what I was lookin for. Dave Blanche wrote: For the type of aircraft your club will be flying, the formula in Kershner will be adequate. The formula in Excel is full_va*SQRT(A6/full_weight) where full_va printed weight in the POH (usually at gross weight) full_weight gross weight for aircraft (again, most recent W&B) A6 column with weight for calculation I fly a cherokee, so I have weights from 1800 (lightest load with fuel and me and gear) to 2400 (gross weight) in column A. And while you're calculating Va, the Glide speed can be done at the same time since it's also weight-based: full_glide*SQRT(A6/full_weight) have fun! |
#47
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"Tony Cox" wrote in message
. net... You'll find Va covered in the FAR's for part 23 certified aircraft he- http://www.access.gpo.gov/nara/cfr/c...4cfr23_00.html The relevant sections are 23.335 (where you see that Va can be *no less than* Vs*sqrt (load factor) -- which means that _it can be greater than this_, and 23.423 where its relationship to control surfaces is discussed. Short answer: Va is defined in terms of what the *control surfaces* can handle, not what the plane can handle. However, Va must be sufficiently high to satisfy 23.335, which means (oddly) that there is no regulatory requirement that stops you from flying at Va without exceeding the load factor. Well fancy that!. Your points are excellent, but I think the issue is over terminology. The "maneuvering speed" placarded in the cockpit is not Va. It is Vo. It is defined by 23.1507 and the placard is mandated by 23.1563. "Vs is a selected speed that is not *greater* than Vsvn". My *s. At or below Vo, the criteria that are generally (and erroneously) thought to apply below Va, do apply, i.e. the wing will stall before the positive limit maneuvering load factor is exceeded. Julian Scarfe |
#48
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"Dave S" wrote in message
.net... Wonderful.. Thankyou Blanche... I only have to tweak the name of the variable A6 to plug this in.. This was exactly what I was lookin for. Dave Dave, please forgive me for saying so, but if you found the statement "the speed is proportionate to the square root of gross weight" to be unhelpful, but Blanche's "full_va*SQRT(A6/full_weight)" is "exactly what you were looking for", then with all due respect, you do not understand the calculation well enough to base a life-or-death piloting decision on it. If you use the Excel expression without understanding how to derive it yourself or why it's correct, you're essentially choosing a speed to keep your plane intact by delegating the decision to someone on Usenet whom you don't even know. And since you were also uninterested in a very close approximation (within 2%) that lets you do the same calculation in your head, how are you going to check whether your implementation of the formula contains a typo or other problem that results in the wrong answer? I don't mean to be critical, but I implore you to be sure you understand exactly why and how some of the V-speeds (Vs, Vs1, Vx, Vy, Va, Vl/d) vary with weight, and why others (Vfe, Vle, Vlo, Vno, Vne) do not, and how the relation translates into a mathematical expression. (The reference I pointed to earlier contains a full explanation using nothing more advanced than high-school physics.) Fly safely, Gary Blanche wrote: For the type of aircraft your club will be flying, the formula in Kershner will be adequate. The formula in Excel is full_va*SQRT(A6/full_weight) where full_va printed weight in the POH (usually at gross weight) full_weight gross weight for aircraft (again, most recent W&B) A6 column with weight for calculation I fly a cherokee, so I have weights from 1800 (lightest load with fuel and me and gear) to 2400 (gross weight) in column A. And while you're calculating Va, the Glide speed can be done at the same time since it's also weight-based: full_glide*SQRT(A6/full_weight) have fun! |
#49
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I wrote in message ...
The "maneuvering speed" placarded in the cockpit is not Va. It is Vo. It is defined by 23.1507 and the placard is mandated by 23.1563. "Vs is a selected speed that is not *greater* than Vsvn". Just to clarify, Vsvn is the way that Vs*sqrt (n) pasted from the text. n is the "positive limit maneuvering load factor". Julian |
#50
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"Julian Scarfe" wrote in message ... The "maneuvering speed" placarded in the cockpit is not Va. It is Vo. It is defined by 23.1507 and the placard is mandated by 23.1563. "Vs is a selected speed that is not *greater* than Vsvn". My *s. At or below Vo, the criteria that are generally (and erroneously) thought to apply below Va, do apply, i.e. the wing will stall before the positive limit maneuvering load factor is exceeded. Indeed. Vo was discussed in the thread in rec.aviation.piloting that I referred to earlier. As I understand it, Vo is a more recent certification requirement. Not sure then it came in, but 23.1507 is dated 1993. My 1966 182 doesn't come with a Vo. Certainly, terminology caused a lot of confusion the last time this was discussed, and I expect it will again if the thread continues! -- Dr. Tony Cox Citrus Controls Inc. e-mail: http://CitrusControls.com/ |
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