blueovalz Posted October 17, 2002 Share Posted October 17, 2002 If anybody on this forum is well versed with the Nissan Pathfinder 4wd trannies and/or real sharp about air flow over airfoils (specifically the current generation jet aircraft), I've got some questions I'd like to ask off site via E-mail. I know we have quite a varied group of experiences and intellect here, so I thought I'd throw my questions out here first. Quote Link to comment Share on other sites More sharing options...
Guest Mr. Big Business Posted October 17, 2002 Share Posted October 17, 2002 You plan and strapping a JATO rocket to your pathfinder? I doubt that it has the transmission or aerodynamics to withstand it! Henry Quote Link to comment Share on other sites More sharing options...
blueovalz Posted October 17, 2002 Author Share Posted October 17, 2002 Thanks for the laugh. 1) I think I'm getting shafted by the transmission repair guy. 2) On my way back from Seattle this week, I noticed a pressure (refractive) "wall" of some sort over the wing (visible as a thin shadow line along the entire length of the wing when the sun was lined up with it - diffracted light). This "wall" even distorted light rays coming from the other side when viewed on edge). I just wanted to discuss this with a good aerodynamicist being I've never read about this or seen this before. Quote Link to comment Share on other sites More sharing options...
zero Posted October 17, 2002 Share Posted October 17, 2002 i am not an expert or anything, but i have seen that on lemans cars coming off the rear wings. not sure what it means but just wanted to say ive seen it before. Quote Link to comment Share on other sites More sharing options...
Guest Tht1KSguy Posted October 17, 2002 Share Posted October 17, 2002 Terry, FWIW i'm not an aerodynamicist but my main interest is experimental aircraft & engine design. I'm pretty sure what you're seeing is the difference in density of the air flowing across the wing acting like a prism bending the light. You can really notice it in areas of high moisture such as Seattle. The pressure change can actually cause water to condense causing even more of the prism effect you're seeing. Too bad there's not an aircraft forum with as many great ppl as HybridZ. Quote Link to comment Share on other sites More sharing options...
clint78z Posted October 18, 2002 Share Posted October 18, 2002 Geeze you build a good wing Terry, it not only changes air flow but bends light. Warp engines ?? Quote Link to comment Share on other sites More sharing options...
aviatorx Posted October 18, 2002 Share Posted October 18, 2002 Clint is on the right track...... given sufficient air moisture (humidity) a pressure gradient will compress the ambient air further, thus 'squeezing out' visable water vapor in the form of a standing wave if the pressure gradient remains constant and is kept in place by any barrier (as in the rising edge of your wing), or as a vapor trail if the gradient remains constant and is unblocked as it seperates from your wing. The attendant water vapor would also act as a prism ..... Quote Link to comment Share on other sites More sharing options...
383 240z Posted October 19, 2002 Share Posted October 19, 2002 I'm pretty well versed in pathfinder trannys why do you think you are getting the shaft? Keith Quote Link to comment Share on other sites More sharing options...
blueovalz Posted October 19, 2002 Author Share Posted October 19, 2002 The tranny lost reverse only. All front drive gears are normal, and shift normal. The shop says I need a pump stator, planetary gear set, drum, etc. I could almost accept a new converter (why not just thoroughly flush the old one?), but these items above make me wonder, being everything about the tranny works great, except for reverse. And why would a rebuild kit come without the bands? I guess I've been away from automatic transmissions too long. Quote Link to comment Share on other sites More sharing options...
blueovalz Posted October 19, 2002 Author Share Posted October 19, 2002 A distant friend of mine (aerodynamicist) feels strongly that this phenomenon that I saw on the wings was indeed a shock wave. He said at the temperature and altitude we were at, and that the air speed over the top of the wing would be faster than the speed of the aircraft, thus it could be possible that this "vertical wall that refracted light" could have been a shock wave right at the transition of sonic speed. The shadow that this wave was leaving on the top of the wing very much resembled film of shock wave characteristics of supersonic air speed. Only this shock wave was perfectly vertical instead of sweeping rearward. Anyway, I consider the mystery solved, and even at 46 years of life, I still find nature's interaction with man's intellect to be most stimulating. Quote Link to comment Share on other sites More sharing options...
Michael Posted October 19, 2002 Share Posted October 19, 2002 Terry, Like your friend mentioned – what you saw was the footprint of a shock wave on the wing upper surface. I wish I got to this thread sooner; but at the present point, my post will just be review. In going over the front of the airfoil, the air accelerates to speeds significantly greater than the speed relative to the airplane itself. If the airplane is flying at, say 550 mph (well below Mach 1.0), there will probably be small regions over the wing upper surface where locally the flow is supersonic. Further aft on the wing, the pressure conditions are such that supersonic flow can no longer be sustained. Nature's solution is to introduce a shock wave, which stands like a "curtain" vertically over the wing. It abruptly terminates the region of supersonic flow, beyond which the flow is again subsonic. Almost all modern commercial jetliners cruise fast enough for there to be regions of supersonic flow over the wing. The speed at which this region gets to be so big that overall aerodynamic efficiency is reduced to unacceptable levels is the natural "speed limit" at which the airplane is operated. At somewhat higher speeds, the drag rises to a point where the engines' thrust is no longer sufficient to overcome the drag; that is the absolute speed limit of the airplane in steady level flight. Airlines will operate their planes at speeds somewhat below this absolute limit, in order to be below the point of diminishing return, to keep fuel costs down. For example, the Boeing 777 is rated at a maximum speed at cruise altitude of around 600 mph (probably more, I don't have the exact details), but you will never see a regular commercial flight faster than around 570 mph true air speed. The speed at which a region of supersonic flow first forms (called the critical Mach number) depends on many factors, such as the thickness of the wing and the lift coefficient at which it's flying. Jetliners are very efficient. They cruise at low lift coefficients and have thin wings. The root part of the wing is pretty thick, though - which is why I occasionally see the shock over the wing near the root portion, but not further outboard. In contrast, the upside-down wings of race cars sometimes operate at huge lift coefficients, so their critical Mach number is lower. Nevertheless, even an Indy car doing 240 mph is going too slow for shocks to appear, although more benign effects of air compressibility are still present at speeds in that range. Even with some very clever airplane design approaches, the drag penalty of trying to fly faster than the critical Mach number is so great, that it makes no commercial sense to do so. This is precisely why the speed of commercial jetliners has not changed much from 1969, when the Boeing 747 was introduced. They all cruise at around Mach 0.8 or so. It's really the critical Mach number, and not the magic Mach 1.0, that determines how fast conventional airplanes can fly. The Concorde flies much faster, of course, but it's so woefully inefficient that even at 10x the ticket price of a regular airplane, it still loses money. Boeing is now working on a new jetliner called the "Sonic Cruiser", designed to cruise at Mach 0.98. In my personal opinion, this airplane has no commercial future. Boeing is pursuing this design to keep their engineers busy. Anyway, the aforementioned footprint of the shock on the wing upper surface is due to a change in index of refraction in the air, which produces regions of contrasting illumination. More precisely, what you're seeing is the second derivative of the index of refraction (in the streamwise direction), which itself varies with the second derivative of the air density. Aerodynamicists call this the "shadowgraph effect". At speeds comparable to the speed of sound, significant changes in the thermodynamic variables of the air (density, pressure, temperature, etc.) are caused by changes in air velocity. Gradual changes are not visible. Violent changes, such as in a shock wave, are very visible, because the second derivate of density or temperature inside a shock is huge. Shocks are very thin, so their shadowgraphs are easy to see. Moist air has greater changes in index of refraction for the same change in temperature. This is why when the air is humid, the shadowgraph effect is stronger, and the shock more visible. By the way, the reason that this shock wave is vertical is that locally the flow is nearly parallel to the wing surface. In contrast, the bow shock of a supersonic airplane (under most conditions) is sloped with an angle equal to the arcsine of the inverse of the Mach number. It is caused by an entirely different mechanism. Quote Link to comment Share on other sites More sharing options...
383 240z Posted October 19, 2002 Share Posted October 19, 2002 Rebuild kits come in different levels (paper and rubber) this is just the gaskets and seals then you can get the more complete kits that come with clutches and steels the band always comes as it own part. as for the touque converter Would you tear down a motor with a broken ring and not replace the main bearings? This is the same as your torque conveter they can be flushed but any damage done by the failed planetary to the clutch surface (the lockup clutch not the apply clutches) will fail very soon I would never let one go with out a new TC (I also back my units with a good warranty) BTW I have had a TON of these units go down due to inadiquate cooler flow PLEASE add an auxilary cooler to your truck get a low psi drop style NOT the hayden tube and fin style sold at most auto parts stores B&M sells a very nice one that is availible from summit racing and BYPASS the stock cooler Keith Quote Link to comment Share on other sites More sharing options...
Doug71zt Posted October 19, 2002 Share Posted October 19, 2002 Regarding the aerodynamic q. That's a really great answer Michael! I'm an A&P guy who works on helicopters and I've noticed this on Boeing 767 a number of times. If you sit in row 18 or 19 (exit row) it is visible in some sunlight conditions. I always assumed it was a mach wave as it tends to move around as the airflow changes over the wing. Guess I spend too much time on the airplane and not enough at home with my family and Z(in that order, of course). Cheers, Doug Quote Link to comment Share on other sites More sharing options...
blueovalz Posted October 19, 2002 Author Share Posted October 19, 2002 Michael, thanks for the more detailed explanation, and it corresponds exactly to the characteristics witnessed from my seat on the plane (both a 757 and 737). I just could not make myself come to the conclusion that a shock wave existed, and still boast efficient thrust/drag numbers that current day airfoils have. The shadowgraph effect was right on in terms of what the refractive shadow looked like on the wing. It did indeed extend all the way out to the wing tip, but in a scalloped pattern (one slightly arcing pattern joining to another slightly arcing pattern down the entire length of the wing). Thanks so much for the explanation. As I knew when I posted this question, this site has a wealth of technical knowledge backing it up. Quote Link to comment Share on other sites More sharing options...
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