From: David M. <da...@me...> - 2001-09-26 14:00:07
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(Since this has to do with environmental rather than flight modelling, I've copied it to the devel list, and all follow-ups should go there.) ea...@un... writes: > *Much* harder is likely to be creating interesting air movements to > fly in that reasonably represent the ridge lift, thermals, wave > lift and whatever other lift may occur in real life. I've been thinking about this -- these effects matter for powered flight as well, since a mountain wave can literally tear a C172 to pieces under the right circumstances. The goal for all of this is that the calculations have to be fast and simple, using only data that is easily available. 1. Thermals ----------- I expect that the materials file is going to become much more sophisticated soon -- it is a fairly inexpensive operation to find the material type directly under the plane, so we can use that for some simple calculations. I've already mentioned adding roughness and buoyancy fields to feed information to the gear code; we could also add a field with a heat absorption/reflection for each material type to feed information to the environmental code. It's far from totally realistic, of course, but you should be able to fake at least some thermals using the following inputs: 1. heat absorption/reflection factor for the current material. 2. temperature 3. altitude above ground 4. angle of sun 5. time of day (or angle of sun, say, one hour ago) 6. percentage cloud cover Even a simple formula could give some kind of thermal effect over the right kinds of materials by adjusting the up velocity for the wind. Running the formula every second or two would give fairly good responsiveness (we might want to interpolate to smooth changes) without hitting the framerate. 2. Waves -------- Modelling these properly would require far more computing power than FlightGear uses for everything else combined, but we might be able to fake things a bit. Here's the input: 1. wind direction and velocity (x/y only) 2. slope and elevation of triangle directly below the plane. 3. slope and elevation of triangles under points, say, 500m downwind and 500m upwind (the WGS84 calculations will be slightly expensive, but not too bad). 4. plane's altitude above ground. Again, this should be enough to fake things and give some kind of a wave over ridges and mountains -- with the slopes and elevations of the three triangles, we can tell in a general way how the ground is rising and falling relative to the wind and how high the plane is above the ground; we can use that information in a simple formula (with some randomness thrown in) to decide how to adjust the up velocity for the wind. Real pilots at tradeshows would be quite impressed if the plane suddenly jolted upwards crossing a mountain ridge. All the best, David -- David Megginson da...@me... |
From: D L. <ea...@un...> - 2001-09-26 14:47:26
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David Megginson writes: > > 1. Thermals > ----------- > > I expect that the materials file is going to become much more > sophisticated soon -- it is a fairly inexpensive operation to find the > material type directly under the plane, so we can use that for some > simple calculations. > > I've already mentioned adding roughness and buoyancy fields to feed > information to the gear code; we could also add a field with a heat > absorption/reflection for each material type to feed information to > the environmental code. It's far from totally realistic, of course, > but you should be able to fake at least some thermals using the > following inputs: > > 1. heat absorption/reflection factor for the current material. > 2. temperature > 3. altitude above ground > 4. angle of sun > 5. time of day (or angle of sun, say, one hour ago) > 6. percentage cloud cover > > Even a simple formula could give some kind of thermal effect over the > right kinds of materials by adjusting the up velocity for the wind. > Running the formula every second or two would give fairly good > responsiveness (we might want to interpolate to smooth changes) > without hitting the framerate. > The problem is that for hang/paraglider cross-country flying you want the thermals to drift, and also possibly visual indication from cumulus cloud. Certainly in this country a lot of XC is flown by circling at cloudbase in the drifting thermal and then finding another when it expires. I would envisage that over different textures a distribution of thermals would be generated according to the factors that you've listed above. This would have to be random in the sense that thermals need to be in different places on different runs of the sim, but non-random in that they need to drift with the wind, and the pilot needs to be able to attempt to refind one if he looses it. I believe that there are also thermals that are pretty much generated in the same place on similar days ('house' thermals), but I don't think our scenery is detailed enough at present to start thinking about that. Disclaimer - I've never flown in a thermal. Cheers - Dave -- David Luff Engines Research Group University of Nottingham 0115-9513814 dav...@no... |
From: <js...@ha...> - 2001-09-27 17:15:06
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> Yikes. I don't disbelieve the story, exactly, but some of this is > awfully hard to buy as the straight truth. I mean, certainly _some_ That was my feeling, too, but I wasn't brave enough to say it on the list ;-) I'll second it, though! :-) Jon |
From: <Wol...@t-...> - 2001-09-27 20:27:40
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On LinuxTag, one of the top 3 questions was for thermals, soaring planes etc. Many Germans soar. The answer I gave might still stand: The problem is *NOT* saying where the thermal is, how strong, which form etc, since a trivial model for this is enough for maybe 80-90 % of people. It would be enough for now to have in the *.ind file a line very roughly like THERMAL xcenter ycenter radius strength The problem is to make the FDM react to this. In Germany there is a hanggliding/paragliding simulator that was in one version called Thermix. I think it changed name now, but thats not the point. This simulator was AFAIK only sold via the DHV, the German hanggliding association. So, it was mainly sold to real life HG/PG pilots. The physics was very bad, it didn't even have kinetic energy!! What it did have and why people bought it is that a ) it had very realistic scenery. The author once told me that quite a lot of the selling price goes into the license fee for the map data. b) all the house thermals were where they are in real life, i.e. thermals that are fixed at one point! This was enough to make real life pilots who flew or wanted to fly the flying sites of the program buy the program. Of course later on, it would be nice when thermals lead to clouds, when thermals can depend on sun, wind, surface material, material change, birds, shadow of clouds, other shadows, underground material, valley systems, waves, humidity of air and ground, fronts etc etc. Even though it is easy to explain some physical concepts behind thermals, thermal generation is *very* difficult to explain in the concrete cases. I could go on for hours :-). Doing it correct is probably 1000 times more complicated than doing the simple thing, which is enough for many pilots, so lets do the simple thing first. BTW, I am not sure you can do it really correct. What is the status of the weather DB, can it already tell the FDM things like the three components of the wind? What does the FDM do? >The problem is that for hang/paraglider cross-country flying you >want the thermals to drift, and also possibly visual indication from >cumulus cloud. Certainly in this country a lot of XC is flown by >circling at cloudbase in the drifting thermal and then finding another >when it expires. First of all, how many HG/PG pilots do cross country? Secondly, the drifting is normally not so important. A more typical flight is to enter a thermal, go up until (almost) cloudbase, look for the next cloud in the direction you are going, glide to it (loosing height), iterate. >I believe that there are also thermals that are pretty much >generated in the same place on similar days ('house' thermals), Yes. >but I don't think our scenery is detailed enough at present to start >thinking about that. People would simply set them manually. AFAIK, often even the very best pilots would not be able to reliably predict them. There are many effects affecting thermal generation. For exmaple, one of the most important effects is "tearing edges". To generate a thermal, you need two things: warm air and the warm air has to leave the ground. This does not happen automatically. For example, you would say that in the gravity field of the earth, water falls down, right? Now think about a horizontal glass pane that is wet on the underside. The water will take seom time until at one point a drop forms at finally falls. Even then, the drop will not take all the water with it. This is because the water clings to the glass. Also, all of the glass looks the same, so the drop doesnt know where to form. The drop forming breaks the simetry. Similarly, the hot air bubble clings to the earth. It will raise for example when it gets really, really hot (compared to the surroundings). Another typical thing that we would need if we want to simulate thermal generation at all are "tearing edges" (Abrisskanten in German). It means, there is a wind and the bubble moves across the earth. Then there is something that breaks the simetry, for example earth of a totally different temperature (snow vs non-snow) or height (hedges, a row of tress etc). The bubble can also very easily be burst. BTW, good pilots in the alps fly above snow - non-snow boundaries, just to get these thermals that rise there and were heated somewhere else. Like I said, this is a major effect to thermal generation. There are several others (for example, a ripe, dry corn field is one of the best thermal generators although it is not dark) and many, many other things affecting this. IMHO a sound way to progress would be a) Write into files (maybe *.ind) where thermals are and make the weather DB and FDM work with this information. Importance: Very High, Difficulty: ? b) Make an acoustic vario Importance: Very High (at least for real pilots), Difficulty: ? c) Make the thermal form more complex and bend it out of the wind Importance: Medium - High. Difficulty: Fairly low - Medium d) Show thermals via cumulus clouds Importance: High. Difficulty: Fairly low - very Heigh (can be done differently) e) make ridge uplift and waves Importance: Very high. Difficulty: Don't know. The guy from Thermix has done this and said it isn't that difficult. f) Simulate valley systems: Can be done while doing d), maybe, but probably increasing the difficulty g) Simulate thermal generation Importance: Medium. Difficulty: Either very heigh or impossible. Obviously importance is "inside of the soaring issue". Off course importance and diffculty are (sometimes rough) guesses. Bye bye, Wolfram. |
From: David M. <da...@me...> - 2001-09-27 21:00:14
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Wol...@t-... writes: > The problem is to make the FDM react to this. That is no problem at all -- you can set the wind for all three directions, and have been able to for quite a few months. Try setting it interactively through the HTTP interface. All the best, David -- David Megginson da...@me... |
From: BERNDT, J. S. (J. (JSC-E. (LM) <jon...@js...> - 2001-09-27 21:36:32
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> From: Wol...@t-... [mailto:Wol...@t-...] > IMHO a sound way to progress would be > a) Write into files (maybe *.ind) where thermals are and make the > weather DB and FDM work with this information. Interesting discussion, here. As I see, David has already mentioned the ability to change the wind vectors in JSBSim (don't know about LaRCSim). This has the effect of changing the relative wind seen from the aircraft. This is really meant for winds that do not change rapidly, or the effects of which are seen equally by all parts of the aircraft at any point in time. This means that there are no rotational effects modeled (yet). It is also beyond the scope of the FDM to model wind *dynamics* such as downbursts, terrain-induced flows, etc. What I mean by this is that something on the FlightGear side (the weather model?) will have to calculate the wind vector at any point in time and space. Many of the desired wind effects can probably be modeled as potential flow math models (if memory serves me correctly). There is room here for some nice modeling work. Ultimately, the wind speeds in NED frame will be passed to the FDM which will convert them to body frame. Turbulence is another matter because the effects on the aircraft are directional, rotational, and the dynamics are also dependent on the aircraft geometry. This *will* be done by the FDM. It will, however, be up to the driving program (FlightGear) to provide a *level* of turbulence expected (perhaps 1 - 10). Jon |
From: <Wol...@t-...> - 2001-09-27 21:53:34
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JON wrote: >This is really meant for winds that do not change rapidly, What do you mean by rapid? Is one second rapid? Is it possible to make JSBSim work better for rapid changes? >or the effects of which are seen equally by all parts of >the aircraft at any point in time. This means that there are no >rotational effects modeled (yet). I understand. However, this is quite a biggie for thermalling. When you hit a thermal, statistics says you will seldomly hit it in the centre. Lets say you hit the thermal on the left side, that means the updraft will be stronger on the right side of you. So, when you enter a thermal and the right wing goes up, you turn right (straight away or a bit delayed). Lets say you once hit 30 meters to the left of the centre and once 30 meters to the right and you keep on flying straight. Obviously, the up / down movement will be the same in both cases, so you can not decide which side you are on from the vario alone. It is possible to thermal without rolling, but harder and more unrealistic. Also, when going into the thermal (even when hitting the centre), there are rotational forces around the nick axis. > Jon Bye bye, Wolfram. |
From: Jim W. <ji...@ke...> - 2001-09-26 18:33:04
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David Megginson <da...@me...> said: <snip> > > 1. Thermals > ----------- > > I expect that the materials file is going to become much more > sophisticated soon -- it is a fairly inexpensive operation to find the > material type directly under the plane, so we can use that for some > simple calculations. > </snip> On the subject of the materials...it occurred to me that above (or below) certain lattitudes (inexact) there's a change in surface color. Same with above certain alititudes. For example above 42-3 degrees or so the forests become a more darker evergreen color...dark firs instead of pines or redwoods and rocks/barren land (with the except of volcanic locals in the south) tend to be darker. Same with higher altitudes. Above 65 degrees the forests start to disappear, but the grass land isn't necessarily so lush and bright green, exept maybe on rainy days when the lichen turns green. Also coastlines along the ocean are pretty much never green...anywhere...again at about 42-3 degrees they change from mostly sand or sandy colored rock to dark rock (with a few exceptions). What I'm getting at is perhaps some general way of varying texture colors within a given material type according to broad variables. Or maybe the data is already there and we just need more textures? It noticed that the evergreen.rgb is lighter than the dec_evergreen.rgb and then there's also a mixedforest.rgb, but I really don't know much about how the data works and what texture is used where and why. Just know that Maine and Alaska look kinda tropical (relatively speaking). Also it might be nice to have a variant to resgrid.rgb for more urban/industrial areas. Maybe I can help out on this as well (still working on the gui stuff that Norm suggested)...as an occasional diversion--like everyone I like messing with gimp and think I'm pretty good at it :-) But I don't know enough about the data, how the data is used in the scenery or what could be done. This make any sense? Any ideas or suggestions? Best, Jim |
From: David M. <da...@me...> - 2001-09-26 19:14:29
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Jim Wilson writes: > What I'm getting at is perhaps some general way of varying texture > colors within a given material type according to broad variables. > Or maybe the data is already there and we just need more textures? > It noticed that the evergreen.rgb is lighter than the > dec_evergreen.rgb and then there's also a mixedforest.rgb, but I > really don't know much about how the data works and what texture is > used where and why. Just know that Maine and Alaska look kinda > tropical (relatively speaking). Also it might be nice to have a > variant to resgrid.rgb for more urban/industrial areas. I've already done some experimentation with using rock on steep slopes and putting snow on high mountain tops. So far, the results have been less than satisfactory: http://www.megginson.com/flightsim/mountains.jpg Still, it can be done. To improve things, I'll have to divide up triangles into smaller ones and include transitions of some sort. I did the changes on scenery-generation time, but things like seasonal snow and vegetation cover changes really need to be done at run time. We cannot get all of this right without a lot more data -- for example, we'd need a way to know the soil and bedrock types for the whole world. We can make some useful guesses, though, based on the data we do have. All the best, David -- David Megginson da...@me... |
From: Ralph J. <ral...@ho...> - 2001-09-27 00:25:48
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I might add a few comments here: David Megginson wrote: [snip] > I've been thinking about this -- these effects matter for powered > flight as well, since a mountain wave can literally tear a C172 to > pieces under the right circumstances. > That might be overstating things a bit. We regularly fly sailplanes on tow into the Pikes Peak wave, and this almost always involves transiting the rotor region. It can be an interesting ride, but light aircraft are too low in wing loading and airspeed to be in much structural danger. [snip] > 1. heat absorption/reflection factor for the current material. > 2. temperature > 3. altitude above ground > 4. angle of sun > 5. time of day (or angle of sun, say, one hour ago) > 6. percentage cloud cover > Ground slope is quite critical. East-facing slopes commonly develop strong, cumulus-topped thermals in the morning, then go dead as the cloud cover overdevelops, just about the time good lift develops in the flatlands. [snip] > Modelling these properly would require far more computing power than > FlightGear uses for everything else combined, but we might be able to > fake things a bit. Here's the input: > > 1. wind direction and velocity (x/y only) > 2. slope and elevation of triangle directly below the plane. > 3. slope and elevation of triangles under points, say, 500m downwind > and 500m upwind (the WGS84 calculations will be slightly expensive, > but not too bad). > 4. plane's altitude above ground. > > Again, this should be enough to fake things and give some kind of a > wave over ridges and mountains -- with the slopes and elevations of > the three triangles, we can tell in a general way how the ground is > rising and falling relative to the wind and how high the plane is > above the ground; we can use that information in a simple formula > (with some randomness thrown in) to decide how to adjust the up > velocity for the wind. > I think it would be a mistake to tie things to the aircraft. There is very little that happens in the lift picture over time constants of less than five minutes. It might be more productive to generate a map of lift vs. x,y position at zero altitude, perhaps 10 km across with 50m granularity, and update it at a five-minutes-per-cycle rate. In each frame, you could use the altitude to index into a table of wind vector vs. altitude, then use the resulting wind vector to offset the lift map from the aircraft position. > > Real pilots at tradeshows would be quite impressed if the plane > suddenly jolted upwards crossing a mountain ridge. > More so if it jolted downwards. Ridge effect produces smooth lift on the upwind side and turbulent sink on the downwind side; lee wave lift is so smooth as to be eerie. As for the rotor -- well, that goes in all directions! rj |
From: David M. <da...@me...> - 2001-09-27 10:44:57
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Ralph Jones writes: > > I've been thinking about this -- these effects matter for powered > > flight as well, since a mountain wave can literally tear a C172 > > to pieces under the right circumstances. > > That might be overstating things a bit. We regularly fly sailplanes > on tow into the Pikes Peak wave, and this almost always involves > transiting the rotor region. It can be an interesting ride, but > light aircraft are too low in wing loading and airspeed to be in > much structural danger. The July 2001 issue of FLYING has a piece about that topic in its regular "I learned about flying from that" column. The author was flying a Cardinal RG straight and level 3500' above the highest crests near Mena, Arkansas when he hit a mountain wave (surface winds were up to 20kt) -- the plane shot up suddenly at over 1,500fpm, both doors blew open, the rear window smashed, his shoulder harness ripped off its anchor (and his head smashed the interior, causing a lot of bleeding), and many of the instruments failed, including the tachometer and the gear up/gear down indicator (he had to get a visual confirmation of gear down from the ground before landing, holding onto his seat with one hand to keep from falling out the open door). He found out later that the battery box also smashed and that the battery was holding on only by its cables. In this case, the plane's main structure, control surfaces, and engine all stayed intact (hence the fact that he's able to write the column), but it could have been worse. These were only small mountains and the surface winds weren't that bad -- in his case, there were not even any lens-shaped clouds to warn him about the wave. All the best, David -- David Megginson da...@me... |
From: Andy R. <an...@ne...> - 2001-09-27 17:05:28
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David Megginson wrote: > The July 2001 issue of FLYING has a piece about that topic in its > regular "I learned about flying from that" column. The author was > flying a Cardinal RG straight and level 3500' above the highest crests > near Mena, Arkansas when he hit a mountain wave (surface winds were up > to 20kt) -- the plane shot up suddenly at over 1,500fpm, both doors > blew open, the rear window smashed, his shoulder harness ripped off > its anchor (and his head smashed the interior, causing a lot of > bleeding), and many of the instruments failed, including the > tachometer and the gear up/gear down indicator (he had to get a visual > confirmation of gear down from the ground before landing, holding onto > his seat with one hand to keep from falling out the open door). He > found out later that the battery box also smashed and that the battery > was holding on only by its cables. Yikes. I don't disbelieve the story, exactly, but some of this is awfully hard to buy as the straight truth. I mean, certainly _some_ of it could be due to aerodynamic forces -- windows and doors (gear doors, too, which would explain the indicator failure) can blow out from excessive pressure differentials. But the tach? That's a big, stiff cable attached to the engine. No way is it going to come lose from the 6-7 G's of acceleration that a Cessna wing can produce*. Likewise, the battery box? At maximum acceleration, the battery is going to "weigh" a few hundred pounds at most. That's nowhere near enough to smash the housing -- you can verify this by stacking batteries on top of one if you like. Maybe the battery was loose, and "dropped" onto the box? That might have done it, but a loose battery would have been an awfully serious maintenance problem all by itself. Especially goofy was the notion that the seatbelt pulled out. Now, the seat belt is just a loop that goes around an object. If it pulled out, it was because the strapped-in object pulled on it. Now, that object was the pilot! The tensile strength of seatbelt anchors is, what, a few dozen times higher than bone? His shoulder should have been pulled apart, and all he got was a bump on the head. OK, now maybe the seatbelt was already broken (or he wasn't wearing it), and the tach cable was loose, and the battery was loose in its box. That certainly COULD happen, and I'm not saying it didn't. I am saying that aerodynamic effects alone don't explain this very well at all. And, this being Arkansas and all, how is he sure this was a mountain wave and not, say, a tornado? Pulling a window out requires a BIG pressure differential, and I just can't see that from a mountain wave -- there's nothing to "hold" the pressure in place as the localized rotor of a tornado can. I don't mean to pick on this author in particular, especially not having read the column. But I _have_ seen a lot of pretty loopy physics and weather "advice" dispensed via the general aviation press. Some very good pilots can have some pretty whacked ideas about how their planes work. Not all of them do, of course, and our friends at the NTSB** generally have their heads on just fine. I'd be curious to see what they said about this incident. Andy * Really, that's all the acceleration you can cause on an airplane. Getting any more requires extra airspeed from somewhere. At 3x stall speed (~170 KIAS in this case), you can still only pull 9G's at best AoA. and if you pull much harder than that the wings fall off and you DEFINITELY can't get any more acceleration. :) ** IMHO, probably the single most consistently competent agency our government has to offer. Chuck NASA and hand it over to these guys. :) -- Andrew J. Ross NextBus Information Systems Senior Software Engineer Emeryville, CA an...@ne... http://www.nextbus.com "Men go crazy in conflagrations. They only get better one by one." - Sting (misquoted) |
From: Alex P. <ale...@ie...> - 2001-09-29 07:51:50
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> David Megginson wrote: > > The July 2001 issue of FLYING has a piece about that topic in its > > regular "I learned about flying from that" column. The author was > > flying a Cardinal RG straight and level 3500' above the highest crests > > near Mena, Arkansas when he hit a mountain wave (surface winds were up > > to 20kt) -- the plane shot up suddenly at over 1,500fpm, both doors > > blew open, the rear window smashed, his shoulder harness ripped off > > its anchor (and his head smashed the interior, causing a lot of > > bleeding), and many of the instruments failed, including the > > tachometer and the gear up/gear down indicator (he had to get a visual > > confirmation of gear down from the ground before landing, holding onto > > his seat with one hand to keep from falling out the open door). He > > found out later that the battery box also smashed and that the battery > > was holding on only by its cables. Andy Ross wrote: > Yikes. I don't disbelieve the story, exactly, but some of this is > awfully hard to buy as the straight truth. Well, given the variability of maintenance on GA aircraft ... 1. The older cessnas have doors that pop open on a hard landing. This was only fixed for acft with the big silver locking handle. 2. Tachs are a torque based instrument, so it will continue to work even when there is nothing actually holding the cable in place, as long as nothing trying to pull the cable out of the instrument. 3. Battery box mounts are prone to corrosion due to the acids, and the boxes themselves tend to crack and become brittle. There are documented cases of an aircraft having the batter _fall_ out of the bottom of the cowling during eg an accelerated stall (intentional) from a steep turn. 4. The seat belt is a thin piece of metal and a bolt; the former tends to get bent (weakening it) from being kicked around a lot and the latter is often loose due to wear and vibration and therefore has a large longitudinal torque when force is applied. The plate can wear a slot in the side of the bolt, which acts as a crack initiator under load. 5. It doesn't take much force on the human to apply a huge tensile force to the shoulder belt because the path between the endpoints is only slightly deflected by the human body. The force amplification can easily be a factor of five or so. 6. The window popping out is the only one I cannot comment on. |
From: David M. <da...@me...> - 2001-09-29 12:54:37
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Alex Perry writes: > 6. The window popping out is the only one I cannot comment on. I think that it was smashed by objects in the plane flying out. All the best, David -- David Megginson da...@me... |
From: Jon S. B. <js...@ha...> - 2001-09-29 14:58:22
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> 1. The older cessnas have doors that pop open on a hard landing. > This was only fixed for acft with the big silver locking handle. Doors popping open doesn't surprise me. This happened to me once just prior to when I crossed over the threshhold. It didn't really bother me. I noticed in the origonal post they didn't mention anything about needing to clean the seat upholstery ... ;-) |
From: Alex P. <ale...@ie...> - 2001-09-29 07:55:24
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> the plane shot up suddenly at over 1,500fpm As I've mentioned before, the Julian region down here routinely has lift in excess of 2000fpm with a completely smooth ride up (or down). The turbulence can be "extreme" (a technical term) if you go at the wrong time. |