obi 0 #1 June 8, 2004 After the discussions on how to avoid object strike and how to set up your canopy (http://www.dropzone.com/cgi-bin/forum/gforum.cgi?post=1098618;sb=post_latest_reply;so=ASC;forum_view=forum_view_collapsed;;page=unread#unread linking to http://www.blincmagazine.com/forum/dcboard.php?az=show_topic&forum=12&topic_id=1167&mesg_id=1167&listing_type=search), I thought I'll ask Adam to share some of his knowledge with us to help people to set up their gear right. I was specifically asking for the DBS. Here is his answer, enjoy --- The bottom line (IMHO) is that you avoid object strike. I feel this is best achieved using rear riser input because it is a very radical and effective control input. Having said that it is not the best method for all situations. If you are opening with room to spare, use your toggles. They are how the canopy was designed to change directions and will give better results. However using them takes more time, can cause you to turn in a wide arc rather than pivot on your "y" axis and can be dropped, which is another problem entirely. Don't believe that because you can grab your toggles more easily that they are the best method to avoid an object strike. The so-called "big-grab" toggles (no particular brand insinuated) have been around for decades and are also know for there ability to snag things and prematurely fire. What's your plan if you lose one? I make the decision prior to exit as to what method I will use and that is what I do. It is site specific. If object strike is immanent the approach I have successfully used is both r. risers down to cause a stall and cease all forward momentum, then reduce input on one riser to force a pivot. Now my heading is improved and I initiate stall recovery. It's not pretty, but it's an emergency procedure. The entire DBS discussion is also as old as BASE jumping. A ram-air canopy cannot consistently open well with zero forward air speed. Most ram airs have a stall speed in the range of 5-10 mph. asking it to fly slower than that especially at deployment will not yield good results. Finding the balance between ideal and too much forward speed requires trail and error testing. Winds, altitude deployment speed, wing loading, momentum, reefing, canopy design, canopy manufacture and canopy condition all play a part. The goal is to have enough forward speed as to 1) have sufficient internal pressure to respond to input and 2) to have a defined horizontal direction of flight -a prerequisite to redefining ones' flight direction. At deployment you are converting a lot of energy moving vertically toward the ground onto a more horizontal vector. This is achieved by letting the canopy move forward. Trying to contain this energy and airflow on one axis is like balancing on a beach ball. Converting this energy (momentum) is also part of the reason why a steady state stall occurs at a different part of the control stroke than a deployment stall. In a steady state stall, one has forward momentum gradually reduced until flow separates from the wing. In a deployment stall, flow never exists and is in the process of being established as the canopy pressurizes. Once lower surface pressurization has occurred , if flow cannot attach to the upper surface the canopy will stall. Making a deployment brake setting deeper has a variety of effects on a canopy. It pulls more of the tail down which creates a more concave profile to the lower surface. This speeds lower surface inflation by using more of the tail's surface area to decelerate the system. This factor has another effect in that it can compromise heading reliability. When your canopy deploys, an instantaneous high-pressure area forms underneath it. This must vent somewhere. By design it will go rearward and exit from under the canopy at the center as a byproduct of how canopies are typically rigged. If however, this "mono-rail effect" is reduced by pulling more tail down, the stabilizing effect will be reduced. We have even seen this air vent to the front causing the canopy to rock backward. Additionally, weight bias under the canopy is altered with deployment brake settings. This can hinder secondary inflation both as a result of pressure and forward speed. Initial cell pressurization of a ram air is accomplished not by moving forward and "ramming" air into the cells but rather by lifting the upper surface away from the lower surface. This lifting is archived by the pressure gradient between lower and upper surfaces. A little forward speed will maximize this gradient and hence improve pressurization speed. When you add a slider to the mix the rules change again. However, with respect to your question about why a deployment brakes setting appropriate for slider-down may be too deep when slider up; the simple answer is that the control lines are forced to take a much longer path when routed through a slider. This results in an effective brake setting that is deeper during the initial part of deployment and thus can induce a stall. In all this DBS stuff the dynamics are not complex but they are all interdependant and hold true only to a point. Often, changing variable "A" will produce a given result only to a point then the effect reverses. It's best to use the manufacturer's recommendation as a starting point and then trail and error testing to further refine your ideal setting. It comes down to this: An ideal DBS is defined as minimal forward speed - not zero forward speed. Deployment variables (speed, altitude etc) will effect the deployment so it's better to err to the shallow side. A brake setting that is marginally too shallow is much better than flirting with a deployment stall. --- Now, this topic already has been dicussed a gazillion times in the past, so everyone who wants to know more should first search the archives of blinc. A BIG thanks to Adam for taking the time again to explain it so well. Share this post Link to post Share on other sites
TomAiello 25 #2 June 9, 2004 QuoteA ram-air canopy cannot consistently open well with zero forward air speed. Most ram airs have a stall speed in the range of 5-10 mph. asking it to fly slower than that especially at deployment will not yield good results. Finding the balance between ideal and too much forward speed requires trail and error testing. Winds, altitude deployment speed, wing loading, momentum, reefing, canopy design, canopy manufacture and canopy condition all play a part. The goal is to have enough forward speed as to 1) have sufficient internal pressure to respond to input and 2) to have a defined horizontal direction of flight -a prerequisite to redefining ones' flight direction. Does anyone have any thoughts on if this might be different with secondary inlets? My experience has generally been that a canopy with secondary inlets can be turned even without forward speed. Essentially, because the inlets maintain internal pressure during a stall, you can collapse part of one side to turn in that direction without overall forward speed on the canopy. Thoughts, anyone?-- Tom Aiello [email protected] SnakeRiverBASE.com Share this post Link to post Share on other sites
