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#1
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Ok... all you closet aeronautical engineers... I'm asking for someone to
help do my work for me.. with regards to Va.. I have an Excel Spreadsheet application that does W&B and plots it on a graph... The form also lists certain speeds that are "static": Vx/Vy, Vne, etc.. I would like to modify this form to list Va dependent on the given calculated gross weight, and perhaps even doctor it up to do density altitude computations.. If anyone HAS or KNOWS (or has the formulas)how to do this in Excel, please feel free to pass it on.. I'm sure I will figure out or find what I need sooner or later, but I'm not wanting to reinvent the wheel if I dont have to.. this is for myself and some flying club members (and for anyone else who happens to see it on here).. not a school project or work assignment of any kind. Dave PP-ASEL Doug wrote: Kershner's "The Advanced Pilot's Flight Manual" has the following definition for Va. Va - The maneuvering speed. This is the maxiumu speed at a particular weight at which the controls may be fully deflected without overstressing the airplane. Now, Va is commonly taught as turbulent air penetration speed. But nowhere in the definition does it say that Va will protect the airframe from damage due to turbulence. Does slowing down even slower than Va protect the airframe from even more severe turbulence? Or is Va the best speed for turbulence penetration? Or is Va just used as a turbulence air penetration speed becauase of tradition or some other non-technically correct reason. |
#2
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Dave S wrote:
I have an Excel Spreadsheet application that does W&B and plots it on a graph... The form also lists certain speeds that are "static": Vx/Vy, Vne, etc.. I would like to modify this form to list Va dependent on the given calculated gross weight Easy. Va as published is for max gross weight, and goes down with the square root of weight. So: MGW = Max Gross Weight W = Weight of the aircraft at a given moment Va = Maneuvering speed as published in the POH Va,w = Maneuvering speed for a given weight Va,w = Va * sqrt (W / MGW) Stall speeds (Vs0 and Vs1) both follow the same formula, and so does your final approach speed, which is usually calculated as 1.3 * Vs0. So, if you really want to do landings right, you should calculate your weight at the end of the flight (taking into account fuel burn), calculate a Vs0 based on that, and multiply by 1.3 to get your proper final approach speed (keeping in mind that the multiplication needs to be done in CAS, not IAS). It turns out that for the majority of light airplanes, the difference between max gross and a reasonable minimum landing weight (pilot and minimum fuel) is a small enough percentage of max gross that stall speed only varies a few knots between the upper and lower limits. As a result, most people don't bother with this (nor is it often taught in a private pilot course), and they never have a problem. On a bigger plane where half the takeoff weight can be fuel, it's a much more significant issue and these calculations are done for every takeoff and landing. If you were really paranoid, you could calculate Vfinal and Va for three loadings: pilot and minimum fuel, max gross, and halfway in between, then keep these on your cheat sheet. In flight, just take a WAG which of those you are closest to and use the appropriate number. |
#3
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Thanks Roy...
And it didnt take me long to find more than I ever wanted to know about this, too.. Stuff that brought me back flashbacks of my physics and calculus classes.. Your formula breaks it down a little more simply than what I did stumble across.. http://142.26.194.131/ and http://142.26.194.131/aerodynamics1/Lift/index.htm I will be puttering around with this and will see what I can come up with, and if its useful. 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? This originally was to come up with weight specific Va for the crib sheet.. but I see (or was reminded of the basics) that Vs is weight dependent too... even if the difference is negligible in the small spam cans with only 300 pounds of fuel. Dave Roy Smith wrote: Dave S wrote: I have an Excel Spreadsheet application that does W&B and plots it on a graph... The form also lists certain speeds that are "static": Vx/Vy, Vne, etc.. I would like to modify this form to list Va dependent on the given calculated gross weight Easy. Va as published is for max gross weight, and goes down with the square root of weight. So: MGW = Max Gross Weight W = Weight of the aircraft at a given moment Va = Maneuvering speed as published in the POH Va,w = Maneuvering speed for a given weight Va,w = Va * sqrt (W / MGW) Stall speeds (Vs0 and Vs1) both follow the same formula, and so does your final approach speed, which is usually calculated as 1.3 * Vs0. So, if you really want to do landings right, you should calculate your weight at the end of the flight (taking into account fuel burn), calculate a Vs0 based on that, and multiply by 1.3 to get your proper final approach speed (keeping in mind that the multiplication needs to be done in CAS, not IAS). It turns out that for the majority of light airplanes, the difference between max gross and a reasonable minimum landing weight (pilot and minimum fuel) is a small enough percentage of max gross that stall speed only varies a few knots between the upper and lower limits. As a result, most people don't bother with this (nor is it often taught in a private pilot course), and they never have a problem. On a bigger plane where half the takeoff weight can be fuel, it's a much more significant issue and these calculations are done for every takeoff and landing. If you were really paranoid, you could calculate Vfinal and Va for three loadings: pilot and minimum fuel, max gross, and halfway in between, then keep these on your cheat sheet. In flight, just take a WAG which of those you are closest to and use the appropriate number. |
#4
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"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, whether your climb angle will clear an obstacle, whether you can stop before the end of the runway, whether you can glide to a landing spot, etc. And those things are what the V speeds are all about. In fact, though, I don't think there's any contradiction between the physics and the way the POH speeds are supposed to be interpreted. But the question is a good illustration of why understanding the basic physics helps understand how to use the POH numbers safely. --Gary |
#5
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I guess what Im getting at is.. if the POH and checklist says one thing,
and a homebrew Vref, Vx, Vy, etc. doesnt match "the book" exactly (but is scientifically correct).. which would prevail if something went wrong and my decisionmaking was analyzed after the fact by G-men, insurers, usenet readers, etc.. I get the feeling (without having done any of the math yet) that this truly is an academic exercise in the typical 4 seat or less light spamcan anyways, something akin to a few knots here or there... Gary Drescher wrote: "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, whether your climb angle will clear an obstacle, whether you can stop before the end of the runway, whether you can glide to a landing spot, etc. And those things are what the V speeds are all about. In fact, though, I don't think there's any contradiction between the physics and the way the POH speeds are supposed to be interpreted. But the question is a good illustration of why understanding the basic physics helps understand how to use the POH numbers safely. --Gary |
#6
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"Dave S" wrote in message
. net... I guess what Im getting at is.. if the POH and checklist says one thing, and a homebrew Vref, Vx, Vy, etc. doesnt match "the book" exactly (but is scientifically correct).. which would prevail if something went wrong and my decisionmaking was analyzed after the fact by G-men, insurers, usenet readers, etc.. I don't think there's actually a disparity here between what the POH says and what physics says. Va, Vx etc. are defined at max gross weight, and are _intended_ to be scaled down for other weights. But suppose there _is_ a disparity. If you're executing a high-performance takeoff from an obstructed short field, would you rather use a speed that gives you the best climb angle, or one that produces a shallower angle, but gives you an excuse for the crash investigators? (That's not to say that your question about the legal consequences isn't still of interest, though.) I get the feeling (without having done any of the math yet) that this truly is an academic exercise in the typical 4 seat or less light spamcan anyways, something akin to a few knots here or there... Well, there's not much math to do--if you're 30% below gross (quite possible in a typical 4-seater), then Va, Vx etc. get reduced by about 15%--not a trivial difference. --Gary Gary Drescher wrote: "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, whether your climb angle will clear an obstacle, whether you can stop before the end of the runway, whether you can glide to a landing spot, etc. And those things are what the V speeds are all about. In fact, though, I don't think there's any contradiction between the physics and the way the POH speeds are supposed to be interpreted. But the question is a good illustration of why understanding the basic physics helps understand how to use the POH numbers safely. --Gary |
#7
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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 |
#8
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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 |
#9
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"Gary Drescher" wrote in message news:wKGLb.9047$8H.23200@attbi_s03...
"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? Yes, they are. being a Canadian, I can quote the CARs but the FARs are a different matter. I'l see what they have to say. In any case, when have you ever heard of an engine departing an airplane in turbulence or during violent maneuvering? Our Citabria has a G-meter in it, and we have seen some pretty big numbers when students get clumsy on landing. Landing forces don't affect wings much, since they're still generating lift and the landing forces on the structure tend to be negative, and if the engine mount was a 5G structure like the rest of the airplane it would have fallen off long ago. A missing 300 pounds or so during a hard landing would be disastrous: CG way back near the trailing edge, an airplane suddenly much lighter, and airspeed still sufficient to flip the whole works over into a crash and burn scenario, all for the lack of another pound or so of tubing. The only times I have heard of engine mounts failing on light airplanes is when a prop throws part of a blade, or maybe the whole blade on a constant-speed prop. The imbalance is more than enough to rip the engine off the airplane. Blades will fail when propeller nicks are left untreated and cracks develop. The prop is the most highly stressed bit of metal on the whole airplane, and THAT'S what pilots should be concerned about, not engine mounts. Dan |
#10
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In article ,
Dave S wrote: 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? I'm not sure what it is that you're asking here. The POH gives you experimentally derived performance numbers under stated conditions. There are standard formulas to extrapolate those numbers to other conditions of temperature, altitude, etc. A typical POH will contains tables or graphs showing these extrapolations for a number of various combinations. |
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