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Found 119 results

  1. Wait ,.. just a second Bro, I've had many charcoal lighter fluid fires under my beloved Weber charcoal grills' grill . The grilling surface is very thin guaged steel. Should I expect a catastrophic failure next time I cook up some wings? Blues, Cliff 2muchTruth
  2. Not sure that you got much of an answer on the actual question there! Funny enough I was looking at this the other day (when I was supposed to be working!) There are two types of drag primarily acting on a parachutist and their equipment: Parasitic Drag and (lift) Induced Drag. Parasitic Drag is simply caused by the effects of an object moving through a fluid (in this case air) and being resisted by it. This can only be reduced (as far as I know) by changing the object size or surface type (more shiny = more speed!) Induced drag is caused at the wing tips of your canopy by the forces which give it aerodynamic lift. Essentially the 'faster' air moving underneath your wing spills around the end and creates a vortex which 'sucks' your canopy back a little = drag. That's how I understand it anyway. Elliptical wings suffer less from this and it is also the reason for airliners to have winglets at the end of their wings, which disrupts the creation of the vortex. Perhaps this is why the Nitron has those little winglets too? HTH Frank. *************** Not one shred of evidence supports the theory that life is serious - look at the platypus.
  3. That is an interesting brain-teaser. Yes, the loss of groundspeed is a lesser precent of the overall airspeed when you are flying faster. But the issue is not groundspeed at all when we are talking about turbulence. It is: 1) How long are you in the bad air (downdraft, updraft or choas) And: 2) Is the variance of the fluid going to effect the flight path? Number one is clear. The faster you are flying, the sooner you will pop out of the nasty stuff into clean air. The second issue, partially predicated on the first, is a more complex question that requires all of the relevant details. The example you presented is a helicopter, which is a very different set of dynamics. Not to say that helicopters do not suffer in turbulence, but the effects will be somewhat different, and I do not pretend to be an expert in that field. Actually, I do not pretend to know everthing about parachutes either. I just seem to be saddled with this responsibility of the "go-to" guy for this kind of question. So I will try... The flight path of a parachute is based on 1) Lift 2) Drag 3) Flying Environment The first two partner up to give us our efficiency, calculated by L/D resulting in glide ratio. The flying environment is a variable that includes vertical air movement (+ and -) as well as chaos which reduces the efficiency of the airfoil. If the flow of air over the wing is disrupted and becomes less "laminar", the drag will increase and the lift will decrease. Increasing the airspeed will increase both forces, Lift and Drag. This does not necessarily result in a linear correlation between the variables as they are altered by the conditions, but it stands to reason that the higher the airspeed, the less the percentage of the whole the loss of both figures will represent. If airplanes did not weigh as much, we might choose to increase the airspeed in bad air due to this assumption, but we do not. We slow down. This is beacuse of the limited ability of the airframe to handle the stress. If you increase the airspeed, you increase the forces on the aircraft as the loading on the wings varies positive and negative. This is why there is "maneuvering speed" on your airspeed indicator. If you fly too fast in turbulent air, your wings may fall off. That sucks... Parachutes are quite different. The mass of the suspended load is far less than an airplane, and parachutes are designed to handle quite a bit of stress for opening shock. This means that there is pretty much no chance of you flying so fast that the positive "g's" actually bust your parachute. Good thing. Not to mention, negative "g's" are not an issue at all, as all that does is release the line tension and collapse the parachute. Not good for descent rate, but you parachute will fare well from the encounter. This all suggestes that the first issue is the most important. Flying fast will keep you from lingering in the oatmeal air, and will super-pressurize the parachute to handle the hits. As far as we are concerned, the faster the better, as long as the maneuvers utilzed to create the airspeed maintain significant line tension to keep the skeleton of your system intact. OK, that was the most verbose answer I could have given. Sorry about that. On cold winter days, a little mental masterbation can be a good thing. Anybody got a towel? ;) Instructional Videos:www.AdventureWisdom.com Keynote Speaking:www.TranscendingFEAR.com Canopies and Courses:www.BIGAIRSPORTZ.com
  4. Thank you Scotty,...that means alot coming from you.....and I appreciate all your tips on my flying during the past few months. Guys and Gals, I have not spent any time on this forum in the past other than to check events and who is coming. I'm also only a two finger typer, a 1000 and beyond total jumper who began in 66 but stopped logging jumps after I became a jumpmaster for Jacque Istel's Parachutes Incorporated( Lakewood/Orange, Ma.) in 1973 or so ( C-8909,..never bothered to get my D),.. and a 240 or so wingsuit flight jumper who was lucky enough to have Jeff Nebelkopf as my instuctor and paid coach ( he is "the man" for me on flying the Tony suits ) However,..I received help from every one I have met since Z flock 3.5 , each listening to my ideas and problems and each offering their insights and assistence to me. The most recent learning experience came from jumping and then staying as a roomate and hiking buddy with a Frenchman named Stephann (Zun) in Puerto Rico after flying behind him in our flocks there. AS for landing a wingsuit after intentionally departing a flying aircraft/helicopter and then landing it with no chute used and no landing devices or wheels/skids etc,..and on doing it onto totally unprepared hard solid and "level" grass covered ground;.... I wanted to be the first at something in my life,...I wanted to live through it,... to highlight our Z flockers event , and to give some idea of what is now possible...... I "did" what I did,...nothing less and nothing more. Others ( not me again) will do amazing things that are much more dramatic and well thought out( I hope) and do it when the technology and the person COMBINED ARE "right" to do so and to have at least a 85% chance to BE SUCCESSFUL... Anything less will just perpetuate or even degrade the our current image with the public as something done by mostly nuts with a death wish.... Scotty Burns was going to do my video and stills but his camera lost power, so I have given my permission to the Gray brothers (who jump at my old DZ home in Orange Va) to edit the video, add slow-motion,... add music, and add arrows pointing to the most critical moments. I will not rush them to do this properly and I hope all the other spectators who may have captured video or stills "honor" my then verbal request that they wait to show them or post them until the Gray Bortheres have posted theirs exclusively on Dropzone .com.... ( This is ABOUT and for about "us" ,...as jumpers,...and not about geting cash from millions of wuffos on U tube! ( but that's not a bad idea Chris and Scott---if you want to do it ....but if you do , I hope you will decide to remember to share the proceeds with the man in pain or put them toward Orange's hanger fund or toward some kid's charity. After you bear witness to what I did ,..if anyone of you are willing to risk a smashed face, a maybe broken but definitely now crooked and dried-blood filled nose,...the two days or more of chest/belly/back aches and (for some reason) the slight pain behind my ear and around my brain stem,...well then, please be my guest and go "do" what I did, to show all your friends how easy it is! Or better yet,...do the deed from even higher and higher and see how things turn out for "you" Now,... let me dispel the fall-out theories,...I did not fall out,..and I was not pushed. I departed the skid as directly out into the then relative wind as I could do,... being careful to not "push off" and cause the chopper control problems,... Nigel, our helicopter pilot and David , our crewchief ,...had both warned and briefed me prior to our liftoff on that potential danger to the craft as well as to the spectators on the ground around the landing area if the helicopter lost control and crashed. Yes,..I was indeed somewhat close to the ground ( but above the one meter suggested in one of the forum replies) Nigel requested at least 12 ft. above the ground to give him the ability to avoid catching a rotor tip.... When I departed the skid I was hoping to be using the helicopter's reverse rotor wash deflection from the ground as well as the "gound effect" ( atmospheric compression under my body near the surface) to establish a cushion to the extent available. I did not get my arms out as far as I would have liked and had envisoned before contacting the ground so I lost much of the benefit these two aerodynamic properties could have offered me. A more powerful tubine engined helicopter could have allowed me to "push off" better and directly away from the skid without endangering the helicopter or people on the ground and allowed me to further benefit from the aforementioned aerodynamic effects , but this "was", as someone has already suggested, a somewhat "spur of the moment " decision for me because we had low scruddy clouds...( BUT actually with dry cold air at ground level which further assisted my lift and with what "my instruments" detected as negative ionization in and around the landing field and right at the max high tide slack time adjusted for the distance inland to Z hills from each coast.) Alignment was perpendicular to the Northern Magnetic flux lines of the earth for reasons that will become apparent when Jeff actually does a full altitude to earth landing in a modified Tony suit that I hope will incorporate several of my ideas from my years of jumping and my 25 years experience participating in the design and sales of Citation, Israel Aircraft Company, Beechcraft, Gulfstream IV and Hawker 800/800XP Jet Hawker. ( as well as what I saw and liked done in aerodynamic design by Sabreliner, and by my strongest competition at Dassualt and at Bombardier. Remember NEW is not necessarily BETTER!... ( as I proved to my son using a Flexible Flyer sled vs his Rocket/composite sled on the hills up north after a snowstorm) We did "not" clear the landing area of rocks and indeed I have numerous mirror image bruises from where I believe those rocks likely were when I touched down...I did jump a Tony suit,..but the super mach 1 that I jump has a special wing design that I requested based on what I learned from working 7 years with Cessna Citation, and being the sales liason to the President's Advisory Board there interacting with the Citation's senior designers and manufacturing Senior management. (It's all about fluid dynamics,...lift, mass, gravity, power and drag as well as combining those optimally in the SIMPLEST POSSIBLE WAY. I spent a full day last October watching and filming the giant Condors in the Colca Canyon of Peru....and how they use their wings and body to ride the thermals and swoop down to land on small rocky cliffs---( magnificent 9 to 12 ft wingspans) With more planning, and with other resources,....I "could have" eliminated many factors that limited and then caused me harm in this jump,.. but I was not thinking in a fully logical manner when I made the decision to "go for it". (-- and there was no random or compulsary drug or Alcohol testing after the deed, which might have explained to me why the hell I did this?!)...( yes, I was beyond 8 hours and at 12 or 13 from my last Corona) Let's just say I am a somewhat spontaneous person who depends on guidance and thanfulness in advance to see me through tough or dangerous situations.( but not without having "some" thoughts and "dreams' ( "visions"?)and research, study and sort-of preparing before-hand ... as to what I might do)---I just never KNOW exactly "when".BUT I always keep my promises to myself, and this was one of those things I had promised myself to try( to see how it felt and/or to move onto and into my next grand adventure on the other side). Hell,...why do we jump? I have already offered some ideas to Jeff in the past that I see he has included in his wingsuit designs along with ideas from others and from HIS VISIONS and results of his almost constant test flight mentality. I also want to support Jeff with some of the aerodynamic modification ideas I now have that could bring the day of "typical advanced wingsuiters" landing without a main chute and or possibly with just a small safety chute or a droque chute or two deployed prior to or after touchtown to slow the speed and provide stability....AS USUAL, KISS is the best way to look at whatever man wants to do in nature. Gotta go, want to find Jeff and start talking to him about my ideas...... Regards to all, "Stoney" Life is what happens while we are making other plans.
  5. 10am update hes now keeping down fluid if he can keep food down tonight maybe he can come home :-) on the up :-) ``````````````````````````````````` " Cant keep a good woman down " Angels have wings, but devils can fly !
  6. To make a long story short, you don't need an A&P ticket unless you're taking apart anything considered a complex assembly, flight control, major structural components, etc. Aircraft owners and pilots can legally change their own oil, clean sparkplugs, replace hoses and filters as long as they've been educated in the manufacturer's approved procedures. A&Ps end up doing most of that stuff simply because pilots are afraid of dirty hands, and even more afraid of breaking something they can't fix. From FAR 43 appendix A: (c) Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations: (1) Removal, installation, and repair of landing gear tires. (2) Replacing elastic shock absorber cords on landing gear. (3) Servicing landing gear shock struts by adding oil, air, or both. (4) Servicing landing gear wheel bearings, such as cleaning and greasing. (5) Replacing defective safety wiring or cotter keys. (6) Lubrication not requiring disassembly other than removal of nonstructural items such as cover plates, cowlings, and fairings. (7) Making simple fabric patches not requiring rib stitching or the removal of structural parts or control surfaces. In the case of balloons, the making of small fabric repairs to envelopes (as defined in, and in accordance with, the balloon manufacturers' instructions) not requiring load tape repair or replacement. (8) Replenishing hydraulic fluid in the hydraulic reservoir. (9) Refinishing decorative coating of fuselage, balloon baskets, wings tail group surfaces (excluding balanced control surfaces), fairings, cowlings, landing gear, cabin, or cockpit interior when removal or disassembly of any primary structure or operating system is not required. (10) Applying preservative or protective material to components where no disassembly of any primary structure or operating system is involved and where such coating is not prohibited or is not contrary to good practices. (11) Repairing upholstery and decorative furnishings of the cabin, cockpit, or balloon basket interior when the repairing does not require disassembly of any primary structure or operating system or interfere with an operating system or affect the primary structure of the aircraft. (12) Making small simple repairs to fairings, nonstructural cover plates, cowlings, and small patches and reinforcements not changing the contour so as to interfere with proper air flow. (13) Replacing side windows where that work does not interfere with the structure or any operating system such as controls, electrical equipment, etc. (14) Replacing safety belts. (15) Replacing seats or seat parts with replacement parts approved for the aircraft, not involving disassembly of any primary structure or operating system. (16) Trouble shooting and repairing broken circuits in landing light wiring circuits. (17) Replacing bulbs, reflectors, and lenses of position and landing lights. (18) Replacing wheels and skis where no weight and balance computation is involved. (19) Replacing any cowling not requiring removal of the propeller or disconnection of flight controls. (20) Replacing or cleaning spark plugs and setting of spark plug gap clearance. (21) Replacing any hose connection except hydraulic connections. (22) Replacing prefabricated fuel lines. (23) Cleaning or replacing fuel and oil strainers or filter elements. (24) Replacing and servicing batteries. (25) Cleaning of balloon burner pilot and main nozzles in accordance with the balloon manufacturer's instructions. (26) Replacement or adjustment of nonstructural standard fasteners incidental to operations. (27) The interchange of balloon baskets and burners on envelopes when the basket or burner is designated as interchangeable in the balloon type certificate data and the baskets and burners are specifically designed for quick removal and installation. (28) The installations of anti-misfueling devices to reduce the diameter of fuel tank filler openings provided the specific device has been made a part of the aircraft type certificate data by the aircraft manufacturer, the aircraft manufacturer has provided FAA-approved instructions for installation of the specific device, and installation does not involve the disassembly of the existing tank filler opening. (29) Removing, checking, and replacing magnetic chip detectors. (30) The inspection and maintenance tasks prescribed and specifically identified as preventive maintenance in a primary category aircraft type certificate or supplemental type certificate holder's approved special inspection and preventive maintenance program when accomplished on a primary category aircraft provided: (i) They are performed by the holder of at least a private pilot certificate issued under part 61 who is the registered owner (including co-owners) of the affected aircraft and who holds a certificate of competency for the affected aircraft (1) issued by a school approved under Sec. 147.21(e) of this chapter; (2) issued by the holder of the production certificate for that primary category aircraft that has a special training program approved under Sec. 21.24 of this subchapter; or (3) issued by another entity that has a course approved by the Administrator; and (ii) The inspections and maintenance tasks are performed in accordance with instructions contained by the special inspection and preventive maintenance program approved as part of the aircraft's type design or supplemental type design. Matt
  7. Irrespective of how long you've been jumping, piloting today's high-speed canopies is not for the faint of heart. With thousands of landings on old zero performance canopies such as round 1.1s, PCs, Piglets, and Strato Stars, many of our founders are frankly fearful of fast canopies. Moreover, as canopy development continues in the present direction toward ever faster, smaller models, skydivers new and old need to be continuously educated on landing technique. As one who recently returned to skydiving after a lengthy layoff (13 years) I knew I needed to get better acquainted with today's high-speed wings. They were obviously different from what I had been used to. Faster ... make that "swoopier" ... and although they looked to me to be more fun, there were too many people getting hurt under them. Wanting to avoid that, I set out to discover what I needed to know that I didn't about piloting these new canopies. To provide some perspective, before learning these tips I'd rather have had to shoot down-wind accuracy on a round than land a small Z-Po 9-cell on a hot still day. Surprised? Remember that a landing in 110 F temperatures, say at Perris' 1,450 MSL, is like landing at 5-6,000 feet. One of the first persons I got turned onto was John LeBlanc, design engineer for Performance Designs. He explained that my old-fashioned notions about the handling characteristics of ram air wings have little relationship to designs now on the market. New high performance Z-pos are lighter and more durable, but they also demand much more attention to landing. Because what you don't know can hurt you, John tried to explain why I couldn't land a zero porosity canopy the same way as my old ram air. Here is my understanding of how to land today's canopies. While some of these ideas, tips, and techniques are from John LeBlanc please remember that they are all filtered by an old time skydiver: all mistakes are my own. This is advice from a canopy expert interpreted by a relic: Stepping up to the ground?On a nice sunny day, John and I watched some landings at the DZ. He used his hands and feet to show me how, having picked my landing area, it should be done. 'You simply level out,' he told me. 'Convert your forward and down approach into forward speed. Eliminate any down for now, but stay inches off the ground.' Inches? with a high forward speed? That seems scary; why not feet? Says John: "The idea that neophytes should be several feet off the ground is okay for flying super big student gear, but it's not what the goal is, and is definitely not okay on the smaller stuff! Several feet up feels worse and worse as you go smaller, whether you're a neophyte or a self-acclaimed expert. As a result , we (Performance Designs) consider it unwise to go smaller (in canopy size) until you can consistently level out with feet at ground level under your existing canopy. Going smaller won't make it any easier, but rather it becomes more difficult!" John compares good landing technique to getting off an escalator. "The down escalator is like the ideal descending approach, level off and landing." Escalators do offer a good canopy landing analogy. Both modes of transport demand coordinated, mindful movements at journey's end. Try visualizing a landing approach as John describes how your landing will resemble stepping off a descending escalator: "Now, think of an escalator. When it levels out, your feet are just below ground level by an inch or two. You can gently transfer your weight from the step (the harness) to the ground because you are at that level. The forward speed is no problem, because you're at ground level. You're stepping up onto the ground, rather than down to it." "If the escalator dumped you off even as little as a foot high, the first few steps would be tough! This is because your forward speed is still there, as it will be on any no-wind landing. (If you level out too high) you crunch down with a (higher) rate of descent. This is why leveling off several feet up for neophytes is not a good idea. They have to come down sooner or later, and when they do, it will be with a (greater) rate of descent at the time of contact. With a little canopy, that will be a bad landing because it will hurt!" I mull this over. There you are, storming across the turn, just above the ground. While you still have forward speed, your feet just brush the weed tops. As your speed decreases, you provide a tad more flare so as to maintain your feel of the grass. Then, just step up. Step up? Wait a minute, I protest. Easy enough for you to say that stepping out of my harness should be as simple as getting off an escalator, but if it's so easy why do so many people crash and burn? Obviously this analogy only applies to a smooth, known landing surface. Life and landing, I remind John, are both dangerous. Yes, he agrees, "You are wise in emphasizing that brushing the feet applies to a known, smooth landing surface." and adds: but notes that "the altitude of your body should be the same, even if you're lifting your feet to stay clear of a poor surface until touch down time." More importantly, John continues, "As wing loadings go over 1.1 lbs per sq. ft., this technique is a required for acceptable no-wind landings." In my words, if you have a 150 square foot canopy and weigh 170 pounds, and your suspended weight per square foot of canopy exceeds a ratio of 1:1, then you gotta swoop the ground to avoid eating it. Then, a good landing will allow you to cautiously stand up out of your harness, starting from where the parachute is holding you up to where your feet are supporting you. The major tactile feedback is that your weight is transferred from your leg straps to your shoe soles. Low wind landings and high speed dirtWhat about the special challenges of no-wind landing conditions? "You will still stand up out of your harness," says John, "but you'll do so at a fast walk to a run, depending on canopy and wing loading. " The more wing load, the faster you'll have to run. We watch several more landings in which many of the canopy pilots flare too high or too early. One thing hasn't changed about landing, I tell John. Landing softly requires precise timing. How do you time your flare? He explains that if you flare too high, you'll land from higher up with an increased rate of descent, "if its done too soon, it results in a big gain in altitude, which means that you are too high (to land softly) again! " When you flare too high and then crash in on a little canopy, you'll likely get in a few front loops. Of course, if you flare too low or not at all, no matter what size canopy you're under you'll eat much dirt and still do several front loops. People will laugh. Late flares are not considered good form; they tend to dirty your jumpsuit and gear. It's a good idea to avoid them, so you'll eat less dirt less often. As John adds that a good way to learn how, "is to figure it out on a bigger, more docile canopy. (less dirt eaten.)" A backyard swing modelRemember the fun you had as a kid swinging in a backyard swing? You could go real high or not. You could even try to jump out at the top of the arc or lower. Or, if you had a littler kid in your lap, you'd just let the swing slow down to nearly stop and then just step out of the seat onto your feet. You do it so smoothly that there is no fear and no pain. It is satisfying. The little kid is happy but not scared. "The swing can be moving slow or fast, but if you get off at the right time, it's easy in either case." [to step out of the seat and onto your feet. No sweat, no fear. Like on a slow-moving swing, it's easier to time your touchdown under a bigger canopy], "The slow swing (big canopy) is easier to time, and the steps are slow. " A fast moving backyard swing is something like a landing small fast canopy notes John, "The fast swing, (like a small canopy), is harder to time and the steps are quicker. But (even fast steps) don't hurt if you step (up) onto the ground at the bottom, when the rate of descent is exactly zero." Just imagine you are stepping out of that swing seat and onto your feet. If done smoothly it's fun, even satisfying. You've just had a good landing and you know it. But "Now try goofing on the timing ... get off on the upswing; things get real rough if you're moving fast! That is just like finishing your flare too high." The transition from sitting in the harness to standing on the ground is subtle enough for another analogy. Thinking for a moment, he used a child's walking chair: and said something like 'I'm talking about those contraptions they put toddlers into. It's a seat with four wheels, and the kid's feet just barely touch the ground. They can paddle around and get into all sorts of trouble. Or they can quit paddling and just stand up. The difference is so smooth that they hardly notice whether it is their feet or the seat holding them up. That's what a good landing is like.' Yes, John said, "The walking chair analogy. Nice." Putting this to practice, I find the idea of a two-stage flare is helpful for transitioning to the horizontal. First, flare with only about 6-12 inches of toggle. This converts the ground-rush into a swoop just above the ground. At the end of the swoop, when the canopy won't stay up any longer, depress (bury) the toggles for the second part of the flare. After thinking about it, John added these remarks: "OK, I like the idea of a two-stage flare except for the part about burying the toggles at the end. This will make many canopies stall, and others might just quickly mush onto the ground. If your feet are at ground level, then this doesn't make much difference. You step up onto the ground just the same. But, if you haven't realized that you're a little higher than the ideal, you'll get a rude awakening when you bury the toggles. [Burying the toggles then] you drop down onto the ground with a thud." He also strongly endorses flare-practice, before finial approach, while still high up, "I like... practicing the flare a lot. I do it on EVERY jump. its fun!" Practicing upstairs helps because you can hear and feel what your canopy is doing without the distraction of high speed dirt coming at you. Keep "hands-on" canopy controlCanopy control inputs should be smooth and fluid, not abrupt and mechanical. Whatever landing you do make, says John, "you must still keep your hands controlling the canopy, even if you have the urge to swing your hands as you step (or run). If you are unknowingly moving the toggles, the canopy will do some unwanted maneuvers! People also use their hands for balance cause some pretty wild gyrations, too!" However, "If you continue to fly the parachute properly when you are taking your first steps, [then] the parachute will continue to help support you during those initial steps. Again, this technique is not critical on a big canopy, but becomes more and more important as the wing loading increases." So, remember, parachute canopies only do what you tell them to do. They are so responsive that heretofore unnoticed hand movements will give you yaw and cause you to veer off. In other words, they're responsive enough for perfect landings every time. Or they can turn a twitch into a turn. Keep your hands in sight so you always know what they're doing. Smaller is not always smarterWhile today's new smaller Z-Po parachute canopies are faster, most do appear to have wider safety margins than did the squares of yesterday. However, I'm convinced that going to a smaller canopy shouldn't be an automatic goal. For some of us, consistently painless landings require lower wing loadings via bigger canopies. As PD's John LeBlanc puts it: "Square foot for square foot, today's canopies are generally more forgiving than those squares from years ago. But as you downsize from one size modern ram air to a smaller canopy of the same type, you give up some of that forgiveness. "So, make sure you've really got things well under control before you even consider going smaller. On the larger canopy, little technique problems will not affect the softness of the landing noticeably, but the same poor technique will cause problems on the next size smaller canopy." Pat Works, SCS-1 Legal Disclaimer: Serous injury or death can result from applying written techniques to a high speed sport. Although the quotes are from John LeBlanc, Neither he nor Performance Designs endorse, condone, apporve, or reccomend anything herein. Parachutes are dangerous: you could kill yourself using 'em. Copyright 1994 by Pat Works RWu Parachuting Publications 1656 Beechwood Ave. Fullerton CA. 92635 (714) 990-0369 FAX 529-4769
  8. IMO, best feature is the one that causes most of the 'turning' of the fluid flow, so I went with shape. "birds can climb without flapping their wings, but we're not quite there yet" from SoulFlyers 2. "The evil of the world is made possible by nothing but the sanction you give it. " -John Galt from Atlas Shrugged, 1957
  9. I don't know why everything exists. Science has not yet explained it so it would be arrogant of me to pretend I could give an explanation. For a moment though, consider this - 10 or 15 years ago (an age in the world of fluid dynamics) a study was made on the flight of bumblebees. The findings were that based on all our current knowledge bumblebees were too heavy for their short, weak wings to enable them to fly (Cue lots of tabloid headlines "scientists declare bumblebee impossible"). We've now found that bumblebees use vortices in previously unexpored ways to create their lift. Anyway - during the time we had no idea how a bumblebee flew was it ever suggested that God picked bumblebees up and carried them where they needed to go? Of course it wasn't, it's an absurd suggestion which no-one would have been satisfied with. So why does it suddenly become a valid proposition on the larger questions when on smaller issues it is unacceptable? One last thought (A Dawkins paraphrase) granted we cannot absolutely prove or disprove God through science, this does not for a second mean that the chances are 50/50. Do you want to have an ideagasm?
  10. from part 43: Sec. 43.3 Persons authorized to perform maintenance, preventive maintenance, rebuilding, and alterations. (a) Except as provided in this section and Sec. 43.17, no person may maintain, rebuild, alter, or perform preventive maintenance on an aircraft, airframe, aircraft engine, propeller, appliance, or component part to which this part applies. Those items, the performance of which is a major alteration, a major repair, or preventive maintenance, are listed in appendix A. (b) The holder of a mechanic certificate may perform maintenance, preventive maintenance, and alterations as provided in Part 65 of this chapter. (c) The holder of a repairman certificate may perform maintenance and preventive maintenance as provided in Part 65 of this chapter. (d) A person working under the supervision of a holder of a mechanic or repairman certificate may perform the maintenance, preventive maintenance, and alterations that his supervisor is authorized to perform, if the supervisor personally observes the work being done to the extent necessary to ensure that it is being done properly and if the supervisor is readily available, in person, for consultation. However, this paragraph does not authorize the performance of any inspection required by Part 91 or Part 125 of this chapter or any inspection performed after a major repair or alteration. (e) The holder of a repair station certificate may perform maintenance, preventive maintenance, and alterations as provided in Part 145 of this chapter. (f) The holder of an air carrier operating certificate or an operating certificate issued under Part 121 or 135, may perform maintenance, preventive maintenance, and alterations as provided in Part 121 or 135. (g) The holder of a pilot certificate issued under Part 61 may perform preventive maintenance on any aircraft owned or operated by that pilot which is not used under Part 121, 129, or 135. .... (c) Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations: (1) Removal, installation, and repair of landing gear tires. (2) Replacing elastic shock absorber cords on landing gear. (3) Servicing landing gear shock struts by adding oil, air, or both. (4) Servicing landing gear wheel bearings, such as cleaning and greasing. (5) Replacing defective safety wiring or cotter keys. (6) Lubrication not requiring disassembly other than removal of nonstructural items such as cover plates, cowlings, and fairings. (7) Making simple fabric patches not requiring rib stitching or the removal of structural parts or control surfaces. In the case of balloons, the making of small fabric repairs to envelopes (as defined in, and in accordance with, the balloon manufacturers' instructions) not requiring load tape repair or replacement. (8) Replenishing hydraulic fluid in the hydraulic reservoir. (9) Refinishing decorative coating of fuselage, balloon baskets, wings tail group surfaces (excluding balanced control surfaces), fairings, cowlings, landing gear, cabin, or cockpit interior when removal or disassembly of any primary structure or operating system is not required. (10) Applying preservative or protective material to components where no disassembly of any primary structure or operating system is involved and where such coating is not prohibited or is not contrary to good practices. (11) Repairing upholstery and decorative furnishings of the cabin, cockpit, or balloon basket interior when the repairing does not require disassembly of any primary structure or operating system or interfere with an operating system or affect the primary structure of the aircraft. (12) Making small simple repairs to fairings, nonstructural cover plates, cowlings, and small patches and reinforcements not changing the contour so as to interfere with proper air flow. (13) Replacing side windows where that work does not interfere with the structure or any operating system such as controls, electrical equipment, etc. (14) Replacing safety belts. (15) Replacing seats or seat parts with replacement parts approved for the aircraft, not involving disassembly of any primary structure or operating system. (16) Trouble shooting and repairing broken circuits in landing light wiring circuits. (17) Replacing bulbs, reflectors, and lenses of position and landing lights. (18) Replacing wheels and skis where no weight and balance computation is involved. (19) Replacing any cowling not requiring removal of the propeller or disconnection of flight controls. (20) Replacing or cleaning spark plugs and setting of spark plug gap clearance. (21) Replacing any hose connection except hydraulic connections. (22) Replacing prefabricated fuel lines. (23) Cleaning or replacing fuel and oil strainers or filter elements. (24) Replacing and servicing batteries. (25) Cleaning of balloon burner pilot and main nozzles in accordance with the balloon manufacturer's instructions. (26) Replacement or adjustment of nonstructural standard fasteners incidental to operations. (27) The interchange of balloon baskets and burners on envelopes when the basket or burner is designated as interchangeable in the balloon type certificate data and the baskets and burners are specifically designed for quick removal and installation. (28) The installations of anti-misfueling devices to reduce the diameter of fuel tank filler openings provided the specific device has been made a part of the aircraft type certificate data by the aircraft manufacturer, the aircraft manufacturer has provided FAA-approved instructions for installation of the specific device, and installation does not involve the disassembly of the existing tank filler opening. (29) Removing, checking, and replacing magnetic chip detectors. (30) The inspection and maintenance tasks prescribed and specifically identified as preventive maintenance in a primary category aircraft type certificate or supplemental type certificate holder's approved special inspection and preventive maintenance program when accomplished on a primary category aircraft provided: (i) They are performed by the holder of at least a private pilot certificate issued under part 61 who is the registered owner (including co-owners) of the affected aircraft and who holds a certificate of competency for the affected aircraft (1) issued by a school approved under Sec. 147.21(e) of this chapter; (2) issued by the holder of the production certificate for that primary category aircraft that has a special training program approved under Sec. 21.24 of this subchapter; or (3) issued by another entity that has a course approved by the Administrator; and (ii) The inspections and maintenance tasks are performed in accordance with instructions contained by the special inspection and preventive maintenance program approved as part of the aircraft's type design or supplemental type design. (31) Removing and replacing self-contained, front instrument panel-mounted navigation and communication devices that employ tray-mounted connectors that connect the unit when the unit is installed into the instrument panel, (excluding automatic flight control systems, transponders, and microwave frequency distance measuring equipment (DME)). The approved unit must be designed to be readily and repeatedly removed and replaced, and pertinent instructions must be provided. Prior to the unit's intended use, and operational check must be performed in accordance with the applicable sections of part 91 of this chapter. (32) Updating self-contained, front instrument panel-mounted Air Traffic Control (ATC) navigational software data bases (excluding those of automatic flight control systems, transponders, and microwave frequency distance measuring equipment (DME)) provided no disassembly of the unit is required and pertinent instructions are provided. Prior to the unit's intended use, an operational check must be performed in accordance with applicable sections of part 91 of this chapter. ... The only sure way to survive a canopy collision is not to have one.
  11. Maybe a cheeky egg? I do know that I am not menstrual fluid, cuz that's just eeeeuuugh. (red wings are icky!!!)
  12. Oh yeah, neat aerodynamics. Although the aerodynamics are different, they remind me strongly of modern high wing loading parachutes: When one has a crappy glide ratio, diving for extra speed gives one the energy to create an effective flare and plane out long enough to gently ease down to a landing. So the issue still remains, how to describe the type of aerodynamics that are "pre-stall" ... in terms of conventional aircraft wings, not lifting bodies or vortex flow low aspect ratio deltas or whatever. The best I could come up with was 'attached flow aerodynamics'. Can you think of a good way to describe it to those not trained in aerodynamics, other than just saying 'like a normal wing when it is flying'? Why do the fluid dynamics have to be defined before we can call it "lift". It is lift - end of story. I bet this generated vortex lift in spades. ... The only sure way to survive a canopy collision is not to have one.
  13. admin

    Flying for Food

    I majored in Business Management with a minor in Spanish and dreamed of working for an environmentally and culturally sensitive corporation like Coke. My life was on track, and thinking back, I imagine I'd have been quite successful. But I fell for a girl who lured me off course. I was wrapped around her finger and didn't know at the time that she would entirely derail my dreams and change the course of my life. She made me an addict, and today I'm selling the same stuff that became my addiction, and I'm perpetually unemployed between jobs. She took me skydiving and now I fly camera for a living. It's a job I hadn't imagined, much less considered, but I like it! In this article, I'll try to give you an idea of what goes into bidding, planning, and filming for a production, and some of the things that can happen along the way. Bidding:The first call from production is normally from a researcher who's never jumped. He or she explains the aerial sequence, or some version of it, and then asks what I'd charge to do the job. Most of the time I can tell that what I'm being told is probably far from what's actually needed. But the researcher doesn't need an earful from me about how little he or she knows about the sport. Her job is to get numbers as quickly as possible, and my job is to give her something that she can take to her boss, some rough numbers, ideas, and a few of my own questions to pass along. All things going well, her boss will call. The basic bid for a skydiving project includes the location, aircraft, pilots, and skydivers, simple enough if the project requires a couple of jumpers in freefall over the local DZ. Most of the time, however, projects call for something more. A skydiving sequence for a commercial or movie might call for two hidden rig jumpers grappling for a briefcase over downtown Los Angeles. Suddenly the project requires riggers, custom equipment, ground crew, ground transportation, landowner permission for some parking lot or rooftop, FAA paperwork and permits, police to keep the landing area clear, ground to air communication, sometimes green screen and studio facilities, and especially in the USA, endless and often impossible insurance issues. When the producers call me with a story to tell. They need me to see the logistics behind the story and to bid accordingly. Sometimes I'll deliver a bid in an hour, sometimes it will take days to complete. The most valuable tool at this point is my rolodex. SAG:When I bid a SAG job, my terms include that I will be treated as a performer, just like the rest of the skydivers on the project. When a producer balks, my argument is simple. I ask them when they last hired someone to perform a stunt off-camera. Camera people sometimes hang off cliffs or race along in cars following the action, but they don't actually perform the stunt, nor do they need the kind of training that goes into flying a camera. Picture this: A stunt driver flips a car over a river. The shot requires a chase vehicle with mounted cameras to follow behind and make the same jump. Imagine telling the second stunt driver he's not on-camera, so he doesn't get a SAG contract. There are plenty more arguments like this, but they don't always work. A producer might stick to the off-camera argument and that's that, we reach an impasse. As much as I want the work, I don't bend on this issue. Sometimes I'll get a call from a producer who just wants to know if I'd consider working without a SAG contract, and I'll know that they've reached the same impasse with one of my competitors. That's the only time I'll give in...just kidding! Skydiving isn't a stunt:After my rant about SAG, now I say skydiving isn't a stunt, but bear with me. Skydiving is a sport, much like surfing or skiing and skydivers are practicing it on all different levels and making thousands of jumps to better their skills. There are skydiving stunts, no doubt, just like surfing stunts, but skydiving itself isn't a stunt. In productions, when there are surfing stunts, producers seek out surfers. When there's skydiving in a production, they call in stuntmen. Bummer for those who train! So I've been campaigning pretty hard to promote trained flyers. I was advised early in my career to keep things "in the family" by some of the stuntmen I met along the way, but I saw things differently and ultimately "broke the code" as it were, to bring in trained jumpers who I knew from the competitive circuit. Some say I "shot myself in the foot", and its true to some degree. Why would a Hollywood stuntman recommend a camera flyer who works with outsiders? So I miss out on some work, but overall my reel is strong, mostly because the performers I film are terrific flyers. Communication:Storyboards, often called "boards", are a series of drawings, much like the frames of a comic book, that help us to visualize what the writer or director is hoping to see onscreen. After all, we're making a moving picture. I know more about skydiving stunts than my clients, but they know more about storytelling than me. Often, a non-jumper will have some gem of an idea that I'd never considered. So I try to listen carefully before projecting too much of myself into the creative process, and my reel is better for it. When I'm clear about what the director wants, I'll plan skydives that show each storyboard frame in a way that will transition smoothly from one shot to the next, and some skydives just to film "plates". Skydiving plates are aerial scenes without skydivers that are combined with action that's filmed in front of a "green screen". The Green Screen:If an actor is featured in the aerial sequence, we'll film him in front of a green screen where we try to match the light and wind as close as possible to the real action. When we do it right, I can claim the shot as my own. When we do it wrong (picture Elvis surfing), I'll blame production for screwing it up. The green screen is both my friend and enemy. As a friend, it puts Lucie Lu into the action, and gives the impression that I'd filmed impossible things. As the enemy, it entirely replaces me. Insurers and lawyers, curse them all, sometimes drive the simplest of aerial stunts into the studio to be faked. And some directors feel that they can make skydiving look real, or better than real, in studio, with fans wires and green screens. Fortunately, the fake stuff just doesn't look all that good, so I'm not out of a job yet. The Helmet:I jump a modified 35mm motion picture camera with a video assist camera that lets the director see what I've filmed after each jump, and stills. My helmet weighs about 20 pounds. On some jobs, depending on the cameras I carry, the helmet can weigh up to 30 pounds. It's like jumping a bowling ball strapped to the top of my head. People ask how I handle the weight, what sort of workouts I do to build my neck and shoulders. The answer is, not all that much. I'm relatively fit and have done a few shrugs with dumbbells, and I swim to stay loose, but nothing extraordinary. There are a lot of camera flyers who are far stronger than me, but suffer neck and back problems from jumping even light cameras. Maybe it's genetics. My father is 73 and has never seriously hurt his back. Considering some of the disgusting deployments I've endured over the years, I can only guess that I have him to thank for my longevity in this business. Now I've been to the chiropractor a couple of times and its like "OK Joe, take a deeeep breath….. gooood, now let it out slowly and just relaaaax…. I'll just take a running start from the other side of the office and slam into your spine, and then twist your head around so that you can take a good look at your own ass." So I swim. It seems to keep me straight. Before production:At least a few days before a job, someone from the team will entirely take over coordinating. I'll have some input, but my attention is best focused on what the director wants and how to get it. I think over the boards, configure my helmet, choreograph and dirt dive with the team, set exposures, check my gear, and so-on. I don't multi-task well enough to worry about other details at the same time and I'm not concerned with being credited as a coordinator. Ultimately, I'm in this business to fly camera. Production Day:A lot of things have come together to get this far. Depending on the size and complexity of the project, anywhere from ten to fifty people or more will converge on set. It's refreshing to finally get out there to film, and by far, production is my favorite part of the job. It's also time to step up and deliver on a variety of levels. The pressure to perform reminds me of what if felt like to compete. Competition experience is probably my biggest asset as a camera flyer. Training to compete taught me a lot about flying the camera aggressively, and competing taught me about teamwork and about staying loose under pressure. The Team:I rely heavily on the team around me. And my work is only as good as the team's work, Cliché, perhaps, but true. Flying with Rob Harris in skysurf competitions, I had my part on the team, and even part of the score. Mostly, though, my work supported Rob's performance, and I found myself cheering for him in flight. When Rob had a good round, I had a good round. When he won, I won. Whenever possible, I fly with skydivers who've trained to compete on some level, and who perform well under pressure. On the ride to altitude, we're generally pretty quite, breathing and visualizing, eyes closed sometimes, rehearsing the dive. And it works. In the air, we're more fluid and relaxed, communicating but not over thinking, just doing what we'd planned. A rigger who can keep a cool head under pressure is also a great asset on any production. On hidden rig projects, for example, almost everything is riding on the rigger's work. Hidden rigs look spectacular when they work, but the slightest flaw can cause the rig to be exposed or even to blow the wardrobe clear apart. So I find myself cheering for the riggers as much as for the performers in front of me. And when it all comes together, we're golden. Harder than it looks:"Easy" shots on the boards can be deceptively hard because storyboards invariably call for moves we haven't practiced or even tried. I've seen talented skydivers entirely caught off guard by some simple change in a move that causes them to flail around like a first jump student. Try back layouts while "drinking" a soda, or "talking" on a cell phone, or maybe straddling another jumper without tumbling, or flying feet to earth while swinging a baseball bat. The list goes on and often includes hiding rigs or parts of rigs, or grappling while holding props, all the sort of things one rarely trains to do. On most jobs, we don't have the chance to practice new things before filming, so we'll just take our best guess and go for it. The challenge for me is to predict fall rates and how much slide there will be. I can react as things move and chase the action around the sky, but my shot is a lot better when I'm able to fly proactively. Things I don't like: Assumptions: "Well, I assumed you'd have a black rig here for me, so I didn't bother to bring one", or, "I thought they were taking that frame off the boards. That's why I didn't bring the prop", or "Nobody called me about it, so I just assumed you wanted the cables to be built like this". Screwing up: More than once, I jumped with the lens cap still on, jumped without a load of film, forgot my battery, forgot to turn the camera on, left my leg straps loose, forgot to latch on the helmet, set the focus for under a foot, exposed the same film twice, got onto the wrong plane, pulled too low, brain-locked in freefall, forgot to clip on my wings, walked to the plane without a rig, exited over the wrong place….. There but by the grace of god go I. Grandstanding: The last thing I want to hear on set is some skydiver making a big announcement for everyone to hear, like "You might get some idiot to do it, but you'll never tape over that handle as long as I'm wearing the rig", or "If you want to see a helicopter crash, sure, hook that line to the skid". Of course we have to set and enforce our own parameters for safety, but it isn't us against them. With a bit of diplomacy, we can explain our limits without embarrassing our clients. Jerk Producers: It's rare, but it happens. Producer: "The director needs you to pass close to the cliff like on your reel." Me: "If the winds calm, great, but right now they're blowing at 20+ miles per hour." Producer: "Look, nothing personal, pal, but I've been in this business longer than you think. I hope you understand I didn't come all the way out here to pay for excuses." Me: "Ok, let me talk with the guys" A few minutes later…."OK, we figured it out. We'll fly 100 meters from the wall. If the wind calms, we'll fly closer. And, we'll hang onto the exposed film until all payments for this job are received, nothing personal." I suppose there are reasons to become jaded over the years in this business, but it isn't worth the negative energy. Most productions are great, and some are epic! And most producers are good people, and good at what they do. Things I don't like (part 2): Crash landing in an airplane Hitting power lines in a helicopter Landing on thin ice Seeing another jumper get hurt Landing in a yard with guard dogs Being lost at night under canopy Opening too far offshore Hard openings Extra heavy cameras Whining (oops) What I like: Flying camera to tell a story Visiting cool dropzones Meeting great people Traveling Having family time between jobs Planning unusual projects Working through the physics of how objects fly Getting the shot Working with talented skydivers Dropping big things like cars Bragging about my cool job Being called a "Bald Headed Action Figure" by my kids Back in college, If someone said I'd fly camera for a living one day, I wouldn't have even known what they meant. And when I think back on the series of events that led me here, so much fell into place in ways I could never recreate. It makes me wonder where I'll be 20 years from now. I should thank the girl who lured me away from suits and polished shoes, maybe bring her something nice when I go home tonight. ~ Joe Joe's Photo Galleries on Dropzone.com Skydive.com - Joe's new project Skydive.tv - Joe's web site
  14. Inspired by Jaap's post some time ago (http://www.dropzone.com/cgi-bin/forum/gforum.cgi?post=1577452;search_string=kayaking;#1577452) referring to some guy's fluid dynamics solver, I decided to replicate that work and see what it had to say about the kind of stuff I like to look at. Anyway, as a result I've built a virtual wind tunnel. If you own a Mac running OS X, email me and I can fire it off your way. If you own anything else and would like to take a crack at compiling it (requires OpenGL, LibTiff, and Lapack), email me and I'll send you the source. ANYWAY... I've attached a series of simulations (each is a frame grab from an animation, taken when things had settled out) that came up in a discussion Jaap and I had last night. The first is a simple circle in the wind tunnel. I've hand-drawn a flow line (and pointed out some vortices) to point out the presence of a Karman street (see http://www.galleryoffluidmechanics.com/vortex/karman.htm for a good explanation and links to a bunch of excellent animations). When something (like a pilot chute) is dragged through the air, it develops a low-pressure region behind it. This region is unstable, and collapses into a Karman street, in which the object sluffs off vortices first from one side, then the other. Here's the important thing for BASE... It's not the "spilling" of air that causes a PC to oscillate or a round to pendulum. It's the presence of this Karman street, which places alternating sides of the round (or a location on the PC which circles around the perimeter) at low pressure. The spilling of air is a result, and is directly responsible for the higher descent rate on a penduluming round. The next two attachments give the same simulation for a semicircle and an arc in the tunnel. The Karman street is still there. I half expected these to turn out about the same, since each should generate a dead space in front of itself, but there do seem to be important differences. I'm working on quantifying the force on the object (which I think is the really significant difference here) -- I'll get back to you all with that. Finally, we have a "vented" arc. Notice that letting a little air through the apex of the arc results in the Karman street being pushed back, so that it no longer affects the object directly. It also has an effect on the dead space inside of the arc, of course, and I assume the pull force will drop a bit accordingly... I've done some simulations on buildings, wings, cliffs... All with results a lot like one might expect. Let me know if you're interested in seeing any of this stuff, or if you've got any requests. What fun!
  15. Floats18

    FINALS WEEK

    5 EXAMS! Who wants to take my Fluid Dynamics exam tomorrow??? Who else is going through hell this week? --- and give them wings so they may fly free forever DiverDriver in Training
  16. I should take that to my Fluid Dynamics professor tomorrow. Wonder what he'd make of it ME 309 RULES!!! Expecially the part when Prof. Werely says, "Since the averages on the exams were both less than 68%, most of you will fail this class." --- and give them wings so they may fly free forever DiverDriver in Training
  17. I am confused. My understanding is essentially that the principle is a simle expression relating pressure of a fluid to its velocity, and roughly explains certain aspects of wings, etc. Is there a reference you can point me to confirm your thoughts or mine? My background is not in air flow theory but in atomic theory. Looks like a death sandwich without the bread - Steve Deadman Morrell, BASE 174
  18. Yes, it's true that wings needn't be curved, but Bernoulli's theory still explains why a balsa wood airplane flies. The weight is in place in such a way that the aircraft trims itself for best glide. If you take a flat piece of balsa and place it horizontally in a wind tunnel, absolutely no lift will be produced. Once you make an AOA w.r.t. the relative wind in the tunnel, then the pressure on top of the "wing" decreases and lift is produced. I was merely using a standard GA wing because it best explains why a wing with an AOA equal to zero (steady and level flight) can continue to stay aloft in such a configuration. Look at an aircraft tail (which has no camber), without deflection of the rudder there is no yawing occuring. As soon as you deflect the rudder, the airflow "inside" the yaw is slower and hence of higher pressure, which pushes the tail out. Kutta-Joukowski and Bernoulli fundamentally explain lift in the same way, one approaches it from a circulation standpoint, one from a dynamic pressure standpoint. What we are talking about here (with Bernoulli's theory) is basically venturi effect, yes. Now simple Newtonian mechanics themselves do not explain lift. They are the basis for continuum mechanics and hence can be used to apply conservation of energy and prove Bernoulli's theory, but they alone do not explain lift, i.e. you can't sit there and just say "F=ma explains why an airplane flies." In that both theories are derived from continuum mechanics, i.e. conservation of energy. They don't explain lift the same way If it sounded like I was saying it directly alters the L/D, that is not what I meant. Altering the weight alters the speed a glider flies out, as there is no other way to alter the speed of a glider (as gravity is constant). This means that changing the payload changes the speed which changes the glide slope. You idealy want to ballast a glider for best glide. Bernoulli's theory in my above response explains how lift is produced. No, it isn't the entire explanation of how an aircraft flies. But go ask an aerospace academic the equation for lift and they'll give you L = q*S*Cl, where q is dynamic pressure, S is planform area, and Cl is coefficient of lift. Even a barn door, inverted wing, or hanging-body-under-parachute will have a coefficient of lift, so this equation will give the lift force due to any object through any fluid. I got a strong urge to fly, but I got no where to fly to. -PF
  19. Any altitude within the atmosphere. If there is enough fluid (air) there to get some kind of flow over the wings, the possibility of shock waves exist. This altitude would be way up there, but I don't know where the standard atmosphere stops and space starts. Shock waves occur when your velocity passes the local speed of sound, ie, when you pass Mach 1. The speed of sound varies with temperature. The higher you go, the lower the temperature is, and the lower the speed of sound is. Sonic booms can be heard if a shock is present. A sonic boom is caused by a shock wave. A shock wave causes the local pressure, temperature and density to increase. By local I mean those 3 properties will increase at the shock wave. If I recall correctly, I remember one of my profs saying the sonic boom is caused by the pressure change. In other words, we hear a sonic boom as a result of the pressure change. *Looking it up now in my stack of aerospace books*