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#11
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G-forces are directly related to weight. Since the size of the
control surface is directly related to the forces exerted on it, control surfaces are dependent on weight. Sorry, you lost me. The forces exerted on the control surfaces are going to depend on airspeed and angle of deflection. The size of control surfaces is irrelevant in this discussion, since they are fixed at design time. |
#12
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"G.R. Patterson III" wrote in message
... Seems to me that you have listed most of the effects correctly. One thing you should consider, however, is the fact that the balance envelope for most (if not all) planes gets narrower at the top. A true generalization as far as I know, but I'm sure there are a number of exceptions and in many cases, the shape of the W&B envelope has as much to do with what test parameters the manufacturer chose to look at, as it does any real structural or aerodynamic issues. The main thing is to make sure one is paying attention to the W&B envelope. When flying overweight (with FAA approval, of course) one can make an educated guess by extrapolating the existing graph, but the bottom line is you don't really know what the shape of the W&B envelope is over gross, unless the manufacturer has been kind enough to publish it (and they usually aren't). In other words, the more weight you put in an aircraft, the closer to the center of lift that weight has to be. Not really. In some aircraft, the envelope is sloped on the aft portion too as weight goes up. For rearward CG configurations, additional weight needs to be put farther from the center of lift, not closer. All you can say without seeing the actual W&B envelope is that usually you have a narrower range at higher weights. You can't say which direction that range trends, and even that generalization has exceptions. [...] At some point, all of the weight will have to be in the front seat. Even if the previous statement were true, not all airplanes have their center of lift aligned with the front seat. I have read of cross-Atlantic ferry flights in which the aircraft was loaded to weigh about 1.6 times the normal MGW. In one account, a Bonanza loaded that way took over 6,000' to get airborne. How much runway did Voyager take? I'll bet it was a LOT. Pete |
#13
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"Greg Esres" wrote in message
... Since the forces on these control surfaces will not vary with weight, you certainly can't scale it up. Huh? You have to scale Va with weight. Even within legal configurations, a specific Va is valid only at a specific weight, with lower weights resulting in lower Va and higher weights resulting in higher Va. Just because you went outside the design/certification envelope, that doesn't change the nature of Va. Pete |
#14
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"Greg Esres" wrote in message
... BUT, there are 2 things (at least) which contribute to the setting of Va in the first place. Neither one of the things you mentioned is given in Part 23 as a requirement for Va. Part 23 uses the speed solely to provide the design requirements of the elevators, ailerons, and rudder. Well, right conclusion even if you don't agree with my method. Va might be used in the design of the control surfaces, but I was eluding to how Va is established in the first place. And certainly one of the limitation is ensuring that you can't exceed 3.5G at Va by yanking the yoke back. Isn't that what Va is all about? Since the forces on these control surfaces will not vary with weight, you certainly can't scale it up. Now you've lost me. If that were the case, Va would be the same at any aircraft weight, which it certainly isn't. -- Dr. Tony Cox Citrus Controls Inc. e-mail: http://CitrusControls.com/ |
#16
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a specific Va is valid only at a specific weight, with
Show me a Part 23 requirement that says so. Todd Pattist has lectured on this a couple of times, and he's right. |
#17
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Isn't that what Va is all about?
Conventional wisdom says so, but there is no requirement in Part 23 that says this must be true. Part 23 only uses this speed in its requirements for control surfaces. In my view, the most correct definition of Va will be it's the speed above which you cannot make full or abrupt control movements, due to control surface integrity. New airplanes are supposed to come with a new Vo speed, which DOES require that the airplane stall before exceeding the load factor. Here's a copy from a draft copy of an AC 23.something that I found. The AC was intended to make this clear to test pilots, but I don't think the draft was ever finished: ------------snip----------------- VA should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits nor should it be interpreted as a gust penetration speed. Only if VA = Vs sqrt(n) , will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For maneuvers where VAVS n , the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor. Amendment 23-45 added the operating maneuvering speed, VO in § 23.1507. VO is established not greater than VS sqrt(n) , and is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits. ------------snip----------------- Va would be the same at any aircraft weight, which it certainly isn't. It is in some airplanes. My Piper arrow doesn't scale it with weight. Moreover, Part 23 says that Va is *only* defined at max gross. Some manufacturers do publish Va's at lower weight, but that appears to be at their option. As written, it doesn't match Part 23 definition. |
#18
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Pete: Let me elaborate on my terse response (and see my response to Tony). I agree that maneuvering speed, as defined in the aerodynamics books, must be scaled with weight. However, Va, which is called DESIGN maneuvering speed by the FAA, doesn't really match the definition of plain ole "maneuvering speed". They really should have called it something else, IMO. However, it appears that most manufacturers are shooting for a maneuvering speed, even though the regulations don't require it. If, however, they chose to make the speed higher for some reason, it won't protect you from overstressing the airplane, and neither will the speed when you scale it for weight. GIGO. ;-) Still, all this is of only academic interest. The one thing that IS known is that the control surfaces must be protected at VA, and that won't scale UP from published Va. Agreed? |
#19
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"Greg Esres" wrote in message
... a specific Va is valid only at a specific weight, with Show me a Part 23 requirement that says so. Part 23 isn't what makes an airplane fly. Aerodynamics are. And those aerodynamics clearly show that at a given weight, a slower airspeed is required in order to limit acceleration to a given number. Oddly enough, many aircraft manuals bear this out, providing lower Va speeds for lower weights. Todd Pattist has lectured on this a couple of times, and he's right. I seriously doubt Todd has told you that Va remains the same regardless of aircraft weight. You obviously misunderstood him. Pete |
#20
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"Greg Esres" wrote in message
... Still, all this is of only academic interest. The one thing that IS known is that the control surfaces must be protected at VA, and that won't scale UP from published Va. Agreed? No. Just because Part 23 doesn't stipulate that at a lower weight, a lower airspeed must be used to ensure not overstressing the airplane in turbulence, that does not mean that the maximum speed at which you can fly and be assured of not overstressing the airplane does not go down as weight is reduced. Put another way: the minimum airspeed at which a given load factor can be achieved before stalling the aircraft is positively correlated with weight (i.e. it goes down as weight goes down, and goes up as weight goes up). This is *known*. The fact that it's not stated in Part 23 does not make it any less known. Even your control surface tangent isn't really relevant to this particular thread since you are intentionally limiting your comments to a single weight. Again, just because Part 23 only requires a number to be defined at a specific weight, that does not automatically mean that the number doesn't exist at a different weight, nor does it necessarily mean that number is the same at a different weight. The definition of Va in Part 23 is clear. It has nothing to do with control surfaces and everything to do with stall speed and load factor. Just because Va is only used again within Part 23 for some other use, that does not change the nature of the calculation. It is commonly understood that, even though by definition Va exists only for a specific weight, that for the purposes of flying, one needs to adjust the "operational Va" according to weight if one expects to remain within the certificated load limits. Pete |
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