If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Troubling story and some questions
Hi Gang
Last week Minden had some excellent wave soaring. Off tow at 7.7k msl and within minutes up to 18k. I decided to go up north skirting Reno Intl. I had the transponder on and was monitoring Reno Approach. My goal was to try and fly as fast as possible maintaining an altitude very close to 18k by speeding up in lift and slowing down in light lift or sink. This worked well for the first 35 miles where my IAS (indicated air speed) ranged between 50 knots and 110 knots flying never less than 17k. Then I got into some real lift and pointed the nose down and noticed my Becker transponder registering 18.2k which is, of course, a no no. I then glanced at the IAS. It read 138 knots which is 165 knots TAS. (Every 1000 feet of altitude above sea level results in an error of 1.5% in IAS.) So what to do? If I pulled the spoilers at that speed, the shock might destroy them or the glider. So I gently pulled the stick back and translated speed into altitude going above 19k. At about 70 knots I pulled the spoilers and got myself down to below 18k. I wonder if ATC caught that? Why was this so bad? Well the VNE at sea level for the SparrowHawk is 123 knots and it has been demonstrated that at 171 knots the wings come off. This really gave me cause for concern. How quickly one can get into trouble by not paying attention. In the future I will fly slower and use the spoilers to compensate for excessive lift so as to maintain altitude. This story raises some questions about VNE at various altitudes which should be of interest to all of us glider pilots. I Googled combinations of words such as "flutter altitude", "VNE altitude" and "aircraft breakup altitude" to try and come up with information on whether the flutter/breakup characteristics of an aircraft are less at altitude than sea level at the same TAS. Intuitively it would seem so but intuition may not work here. I found nothing useful. I know that the World's ultimate high altitude motor glider the U2, which was designed in the 50s, had much study done on it with regard to its operating speed window of about 20mph (stall to breakup)at 80k feet msl. There should now be declassified documents on those studies which might answer my questions. I would appreciate any pointers anyone. If I find anything useful I will summarize it on RAS. Flying is often unforgiving of errors and I will definitely be more vigilant after this wake up call. Dave |
#2
|
|||
|
|||
Troubling story and some questions
|
#4
|
|||
|
|||
Troubling story and some questions
Dave,
You had another option open to you other than pulling up to slow down to extend brakes and therefore bumping higher into "no-no" airspace. You could have maintained your airspeed but turned left or right to get into weaker lift or light sink. Once you had lost enough altitude then eased back over into the stronger lift regions. Larry "01" USA Ventus 2bx " wrote in message : Hi Gang Last week Minden had some excellent wave soaring. Off tow at 7.7k msl and within minutes up to 18k. I decided to go up north skirting Reno Intl. I had the transponder on and was monitoring Reno Approach. My goal was to try and fly as fast as possible maintaining an altitude very close to 18k by speeding up in lift and slowing down in light lift or sink. This worked well for the first 35 miles where my IAS (indicated air speed) ranged between 50 knots and 110 knots flying never less than 17k. Then I got into some real lift and pointed the nose down and noticed my Becker transponder registering 18.2k which is, of course, a no no. I then glanced at the IAS. It read 138 knots which is 165 knots TAS. (Every 1000 feet of altitude above sea level results in an error of 1.5% in IAS.) So what to do? If I pulled the spoilers at that speed, the shock might destroy them or the glider. So I gently pulled the stick back and translated speed into altitude going above 19k. At about 70 knots I pulled the spoilers and got myself down to below 18k. I wonder if ATC caught that? Why was this so bad? Well the VNE at sea level for the SparrowHawk is 123 knots and it has been demonstrated that at 171 knots the wings come off. This really gave me cause for concern. How quickly one can get into trouble by not paying attention. In the future I will fly slower and use the spoilers to compensate for excessive lift so as to maintain altitude. This story raises some questions about VNE at various altitudes which should be of interest to all of us glider pilots. I Googled combinations of words such as "flutter altitude", "VNE altitude" and "aircraft breakup altitude" to try and come up with information on whether the flutter/breakup characteristics of an aircraft are less at altitude than sea level at the same TAS. Intuitively it would seem so but intuition may not work here. I found nothing useful. I know that the World's ultimate high altitude motor glider the U2, which was designed in the 50s, had much study done on it with regard to its operating speed window of about 20mph (stall to breakup)at 80k feet msl. There should now be declassified documents on those studies which might answer my questions. I would appreciate any pointers anyone. If I find anything useful I will summarize it on RAS. Flying is often unforgiving of errors and I will definitely be more vigilant after this wake up call. Dave |
#5
|
|||
|
|||
Troubling story and some questions
On Dec 31, 6:10*am, "Byron Covey" wrote:
"Chris Rollings" wrote in message ... Flutter speed (the normal cause of the break-up you are concerned about) varies with altitude but not so much as does the IAS. Are you certain? *Isn't aero flutter a function of true airspeed? My understanding has always been that the reason Vne drops with altitude is that flutter is a TAS phenomenon. 9B |
#6
|
|||
|
|||
Troubling story and some questions
On Dec 31, 8:26*am, wrote:
On Dec 31, 6:10*am, "Byron Covey" wrote: "Chris Rollings" wrote in message ... Flutter speed (the normal cause of the break-up you are concerned about) varies with altitude but not so much as does the IAS. Are you certain? *Isn't aero flutter a function of true airspeed? My understanding has always been that the reason Vne drops with altitude is that flutter is a TAS phenomenon. 9B True sort of. I think what Chris is referring to is that it apparently is not a simple direct relationship. Hard core wave pilots and some aerodynamic experts can tell you why. It's a fairly esoteric area. Each machine has different flutter characteristics, loading varies... It really gets weird above 40k but even at 18k you can have a bad day pushing the limits. Matt Michael |
#7
|
|||
|
|||
Troubling story and some questions
Been there done that and glad you got home OK. High performance gliders are
hard to dive out of lift - they just go faster and keep climbing. I'd suggest to all that, before you get into a situation like this, find out how your spoilers react at airspeeds near Vne. Work up to it in 5kt increments starting at 80 knots or so by cracking the spoilers open and trying to hold them open just a crack. You won't need much spoiler since the drag produced by them increases with the cube of airspeed. Even if you need more, most spoiler systems are fairly forgiving once cracked open so you can smoothly add more drag as needed. Bill Daniels wrote in message ... Hi Gang Last week Minden had some excellent wave soaring. Off tow at 7.7k msl and within minutes up to 18k. I decided to go up north skirting Reno Intl. I had the transponder on and was monitoring Reno Approach. My goal was to try and fly as fast as possible maintaining an altitude very close to 18k by speeding up in lift and slowing down in light lift or sink. This worked well for the first 35 miles where my IAS (indicated air speed) ranged between 50 knots and 110 knots flying never less than 17k. Then I got into some real lift and pointed the nose down and noticed my Becker transponder registering 18.2k which is, of course, a no no. I then glanced at the IAS. It read 138 knots which is 165 knots TAS. (Every 1000 feet of altitude above sea level results in an error of 1.5% in IAS.) So what to do? If I pulled the spoilers at that speed, the shock might destroy them or the glider. So I gently pulled the stick back and translated speed into altitude going above 19k. At about 70 knots I pulled the spoilers and got myself down to below 18k. I wonder if ATC caught that? Why was this so bad? Well the VNE at sea level for the SparrowHawk is 123 knots and it has been demonstrated that at 171 knots the wings come off. This really gave me cause for concern. How quickly one can get into trouble by not paying attention. In the future I will fly slower and use the spoilers to compensate for excessive lift so as to maintain altitude. This story raises some questions about VNE at various altitudes which should be of interest to all of us glider pilots. I Googled combinations of words such as "flutter altitude", "VNE altitude" and "aircraft breakup altitude" to try and come up with information on whether the flutter/breakup characteristics of an aircraft are less at altitude than sea level at the same TAS. Intuitively it would seem so but intuition may not work here. I found nothing useful. I know that the World's ultimate high altitude motor glider the U2, which was designed in the 50s, had much study done on it with regard to its operating speed window of about 20mph (stall to breakup)at 80k feet msl. There should now be declassified documents on those studies which might answer my questions. I would appreciate any pointers anyone. If I find anything useful I will summarize it on RAS. Flying is often unforgiving of errors and I will definitely be more vigilant after this wake up call. Dave |
#8
|
|||
|
|||
Troubling story and some questions
|
#9
|
|||
|
|||
Troubling story and some questions
|
#10
|
|||
|
|||
Troubling story and some questions
wrote:
Then I got into some real lift and pointed the nose down and noticed my Becker transponder registering 18.2k which is, of course, a no no. Not really - our 18,000' limit is measured with the altimeter, typically set by adjusting it to field elevation (msl) while on the ground. Your transponder was reporting the altitude measured by the encoder, which is set to 29.92 to measure "pressure altitude". These two measurements can vary by a thousand feet or more at 18,000', depending on the weather. As some of the other posters have pointed out, determining flutter speeds is a tricky business. "Fundamentals of Sailplane Design" discusses altitude effects on pages 58-60, indicating the "constant TAS" limit is generally very conservative, but it's best to stick to the flight manual or contact the manufacturer for guidance. The flight manual for my ASH 26 E uses a combination of constant IAS and constant TAS for Vne: it's constant IAS up to 10,000', and essentially constant TAS above that. That TAS value is equal to Vne (IAS) at 10,000'. I recommend the "Fundamentals..." book be on every glider pilot's bookshelf, and that the pilot read it through at least once. It's a great resource, and a better place to start than wading through a bunch of hits by Google. -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly * "Transponders in Sailplanes" http://tinyurl.com/y739x4 * "A Guide to Self-launching Sailplane Operation" at www.motorglider.org |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
More Troubling Planetary News!!! | Michael Baldwin, Bruce[_2_] | Products | 1 | August 24th 07 07:10 AM |
More Troubling Planetary News | Michael Baldwin, Bruce | Products | 3 | January 24th 07 03:40 AM |
More Troubling Planetary News | Michael Baldwin, Bruce | Products | 2 | November 20th 06 03:15 AM |
More Troubling Planetary News | Michael Baldwin, Bruce | Products | 10 | November 17th 06 02:57 AM |
Erosion of U.S. Industrial Base Is Troubling | The Enlightenment | Military Aviation | 1 | July 29th 03 06:57 AM |