Lessons from a Lucky Wind Dummy - Edith's 12/28

[krista]

Thanks to the work party today at the Mason's house.

I am writing the below details of the today's lucky wind dummy story since we are still learning the characteristics of the Edith's Gap site.

First off, the conditions were relatively on par with what the forecast had predicted (SW at 7mph), but with a higher wind speed during what appeared to be thermal cycles (8-12mph by the feel of it, at least to me). There were also cirrus clouds coming from the W (although SW was predicted by ADDS). Note: This is a SE site. The thermal cycles were coming straight in about two-thirds of the time, with a few bouncing around in the trees. Birds were soaring.

Decision calculus. The cross wind and the cirrus gave us pause, as we Dan, Paul, and I considered a jump to Daniels. Given the prediction of SW, we were considering calling it. But the straight in cycles and birds soaring gave some encouragement. I took up the role of wind dummy.

It was indeed easily launchable (can provide video thanks to Jim McClave), and I was mindful of the push coming out of the slot. That part wasn't too bad, although you will note from the video I was a bit too quick to wiggle myself into the harness after responding to minor wing tip flap. The ridge afforded me surprisingly good lift for a minute or so (can send flight track for Google Earth), but given the cross I wanted to try the bump to the left of launch. I must note, the moderately cross wind at the launch became much stronger as I moved away from the ridge. Whereas I was making headway up wind by the ridge, I was parked still by the bump. (I will also note that I was particularly light on my wing today given I did not have my usual 1.5L of water with me for ballast.) The little bump had some mild thermals, although I was not sufficiently aggressive in turning in them to utilize them well. The wind, however, was much stronger and I was not penetrating.

Despite what would have been more than sufficient altitude to reach the now GLORIOUSLY WIDE landing zone, my angle was not reassuring. I got on speed bar and it became even less reassuring. Given the garden, the house, and the other garden with power lines ahead of me, I thought it best to land in the field by the barn (which is particularly less glorious that the gloriously large LZ). I moved myself into the wind over the barn and prepared myself for a 360 over the field. I was quite convinced that I would get my first T1 rating of the non-tandem variety, hit the barn rotor and fall 15 ft from the air, and/or land with my wing falling into the trees (which I assessed as the best possible outcome).

The 360 started nicely, but hot damn did that downwind leg go quickly. I noticed that I was getting pushed down wind over the trees and resisted, pulling harder into the turn. Hard enough to pull a couple Gs, which also led to the depressurization (and deflation/collapse) of my outside wing. I came out of the turn and stabilized the glider over top of the small field. I wanted to be sure I didn't penetrate too much to hit the barn rotor, which I was certain would hit me (yet didn't--at least that low), but also wanted to stay in front of the trees so to not get my T1 rating. I got out of the harness and prepared to drop from the air, but put hands up to ensure I got the most speed possible. I touched down about 10-15ft in front of the tree line (no stepper to boot), and then walked my glider forward towards the barn and safely put it down away from the trees. Imagine my surprise.

I'm relatively certain that should I ever be given a similar set of circumstances, I would land in a tree and/or get injured. I was quite lucky, and I am extremely appreciative for (and apologetic to have used) any flying safety karma that I may have expended out of the build-up we made today.

So, key lessons from Edith's, which may be of use to the wider community, which also draw upon previous flights there as well.

1) The forecast was actually correct.
2) When 2 of 3 (or 2 of 4) indicators are good, that does not equal good. Flyable (and/or launchable) does not equal advisable.
3) The wind in the valley was parallel to the ridge. It seemed as though the thermals were pulling the valley wind up over the ridge in part, leading to a S dynamic along the ridge. But as soon as you moved away from the ridge, that effect went away and it became a SW (and strong) valley wind.
4) Whatever you calculate as the distance to get to the LZ, multiple by a factor of 1.5-2.0 depending upon the conditions. There are no good bail out areas (remember, this field itself is the "bail out"), so do get yourself stuck.
5) The bump to the left has consistently inconsistent thermals (i.e., quite hard to map).
6) The barn has a good thermal, but it's tight when you're too low. You need altitude. Same for the house.
7) The barn field is SMALL. See flight track and photo as evidence. Unlikely you can get below the tree line and glide to the grass there without either strong winds and/or very extreme maneuvers.
The Masons are awesome.
9) The really big LZ that is now cleared thanks to everyone's effort is exponentially better than it was previously, and I very much want to land there.
10) Recognize luck and when you have it and skill when you build it. I would have not come out of that landing unscathed if I was not horrendously lucky to start. But the two SIV clinics I had, particularly the one that focused on wing-overs saved my ass. Knowing the feel of my glider in the high energy turn saved me coming out of the 360 and positioning myself in the field. (Even though I wasn't anticipating how much energy went into doubling down on the turn when I was getting pushed over the trees, and the need for outside brake. This means I wasn't thinking enough since this is not yet muscle memory with my new glider.) That said, being lucky enough to NOT high-energize myself into a tree was all the work of the flying karma stored up over the past several un-flyable flying trips.
X) Wind dummies are an extremely useful tool, particularly when they don't require extraction.

Thanks again for everyone's hard work. Hope to see you all flying soon!

Krista

[Matthew]

Awesome write-up Krista!

Kudos to you!

Matthew

[sailin]

Krista that was a great analysis of the conditions and situation! Thank you for taking the time to write up such a thoughtful post. We will all definitely get something good out of that. Thank you!

Look forward to flying Ediths with you all!

Jon

[krryerson]

Krista,

Great launch and glad that you managed to pull of a safe landing.

[youtube]http://youtu.be/mmGCcpreuMg[/youtube]

Knut

[RichH]

Krista, great write up ..I'd be curious to see if any experienced PG's have any advice for Krista on her flight analysis?

[mingram]

Krista tried to work the spine to the left of launch that leads to the LZ and managed to find some lift there, but that meant she was already downwind of the LZ. After launching I would have continued to the right and tried to penetrate upwind with speed bar to set myself upwind of the LZ.

Going left after launching is not preferred because the ridge isn't as steep. People who have flown the site and sledded found that going right after launching made them more likely to catch a thermal.

I flew this site a month ago in a S cross that was strong enough to make me concerned with my ability to reach the LZ. I made sure that I was far enough upwind before leaving the ridge and then crabbed to the LZ to maintain my upwind position as much as possible. Other pilots new to the site that flew it the same day as me flew straight to the LZ after leaving the ridge and were able to make it.

[brianvh]

I'm not so familiar with the performance of paragliders: if you had enough altitude to decide to make a 360 over the barn, would that have been enough to get you to the square field past the house? PG make tighter turns, so maybe not I guess...

[krista]

My original thought was yes, but that assessment was quickly proven wrong when I used speed bar to push through the SW winds. It killed my angle and I was not confident that I had the altitude to get beyond all of the obstacles. I may have been able to get over the house, but not beyond the garden; I picked the barn field since it had less obstacles surrounding it and I was already there.

My glider has a 10:1 glide, but I was light on it that day, which undermines its performance particularly in strong winds on speed bar. Not knowing much about HG, I think there would have been a much better chance to push crosswind and upwind, and the added speed of the glider would have overcome some of the challenges as well. That said, so would have better piloting, particularly staying up wind right after launch.

[brianvh]

Given the newness of the site I wouldn't question the piloting skills. The more everyone hears about less than ideal flights the better - it's really good for HG pilots to be thinking that when the wind is SE (best conditions for landing) it's a really good idea not to get stuck to the left of launch. I did that, and didn't even have time for a 360 over the LZ after hitting sink on the way out. Let's keep reminding people!

You made a conservative decision at the end which saved the flight. I'd call that good piloting.

[Danny Brotto]

Hi Krista,

Thank you for posting your account of this flight. It really is enlightening to read (and reread) to help further understand the flight envelopes that various forms of soaring aircraft perform within.

You make reference to being light on the glider on this particular flight by virtue of not flying with your normal 1.5 liter of water. Forgive me if I am reading this incorrectly but that's less than 3 1/2 pounds (1.5 Kg) of additional mass. I would think that's hardly enough to meaningfully alter the flight profile. That mass is likely made up for 2 to 3 times over by winter flight gear. In all sincerity, I am curious as to why this amount of ballast is meaningful.

Regarding the glide ratio at least in sailplane and (for the most part) hang gliders, adding ballast does not alter max glide (L/D). With ballast, max glide is not improved but it is achieved at a higher airspeed (as is min sink). So in effect, glide over terrain especially into ahead wind is improved by additional of ballast. However absolute min sink and the ability to slow down and climb in turns is negatively affected. Also landing speeds are increased so most gliders have provisions to dump ballast as required when conditions lighten or prior to landing. I know that in hang gliding that ballast also alters the airfoil/twist of the glider perhaps altering other performance factors making for my "for the most part" caveat.

Does 1.5 Kg of pilot mass really alter the flight envelope of a paraglider?

Thanks again for initiating this discussion by posting you flight experience.

Danny Brotto

[laszlo]

Danny,

I have not done the math, so speak from experience. In the performance of my paraglider a difference of 1 kg in my total weight is fairly noticeable, especially when flying into strong wind. At an extreme, a difference of 5 kg makes me feel that I fly an entirely different glider, and need to learn its behavior before I feel safe on it. The global paragliding forum is full of topics pilots debating which weight is optimal to fly a particular glider, with differences of 2-3 kg at stake.

Laszlo

[Danny Brotto]

Hi Laszo,

Thanks for that input. It is very interesting. I suspect that increasing mass by a few Kg alters the airfoil enough to alter the flight dynamics? The 5 Kg that you mention, in my mind, is easily the difference between winter and summer clothing. Are you saying that your glider feels entirely different when you fly on a warm summer day vs. a brisk winter day?

Regarding polars and speed-to-fly, this is an interesting tutorial: http://www.5c1.net/Glider%20Performance%20Airspeeds.htm

It is based on sailplanes, albeit a low performance 1-26 but the thinking scales. The last segment discusses the points about ballast. Now the 1-26 and most sailplanes have a rigid airfoil so adding ballast essentially does not alter the airfoil. Hang gliders and I suspect paragliders will have their airfoils altered to some extent with ballast which it seems can create secondary effects. I recall watching a tandem tow with 2 very heavy people in the HG that was so washed out so as to barely get airborne.

I think this is a good discussion, thanks.

Danny Brotto

[RichH]

Krista, I would be cautious in accepting claimed glide ratios by manufacturers in my experience (owned over 10 different gliders from multiple manufacturers) I've seen claims of glide ratios grossly overstated. Also, I have to agree with Danny on the usefulness of the ballast you are using..It has its place but you usually see it in higher weight amounts to really show effectiveness. Again, great write up and thank you for being open to suggestions...Rich

[brianvh]

I would think the addition of a few pounds of ballast is not affecting the performance of the glider so much as the performance of the pilot. As an analogy, you really feel the difference when riding a bike with and without a backpack, most especially when you attempt to ride no hands, imitating the weight shift of gliders.

[Danny Brotto]

Hi Brian,

Yes, good point. I see the analogy; that Physics Instructor past in you coming out . But from what I gather the PG folks indicate a notable performance improvement(?) with just a few Kg of additional mass. I agree the "feel" would be different especially since the PG pilot is at the end of a pendulum but still unsure how it correlates to any meaningful alteration of the performance envelope?

Not to belabor this point, just interested from a technical standpoint.

Danny Brotto

[brianvh]

More weight to shift - I'm guessing better handling makes it feel like better performance (glide ratio) just because variations in lift are being worked more effectively. But it's a good point that handling and performance are not the same, and no doubt experienced pilots can tell the difference. In which case, I dunno.

[mingram]

Weight becomes more of an issue in windy conditions.

"Decide whether you'd rather sink out more often or be in a slightly higher risk of being blown back."
"Windy places needs extra weight, usually I fly 3 kilos over the limit. "
From Where do you fly in your weight range?

"...the ideal place in the weight range is more important with higher performance gliders and when competing or flying long XCs."
From Can bodybuilders paraglide? (drastic bodyweight changes)

I experienced this first hand in Dunlap, CA this summer for the Paragliding Nationals. Every flight requires flying upwind to get out into the valley and all the pilots there fly at the top or over the top of the recommended weight range. I found it very difficult to keep up with them and to penetrate into the wind even though with all my gear I'm over the middle of the weight range which is the manufacturer and pilot reviewed recommendation. I added 1 extra gallon of water ballast which helped a bit, but still didn't put me at the top of the weight range so I was really flying a wing that was too big for the conditions.

Flying on the east coast we typically fly in lighter winds or stable days where being lighter helps significantly. We typically don't risk it on the higher wind days unless we're comfortable that we're heavy enough. Speed bars aren't even that helpful for penetrating into the wind unless you're on a C or D glider. On a B wing you can penetrate a little, but sink a lot and most people are flying B wings.

[JohnE]

I tried to make a similar post as this one but do not see it. If it shows up and this is a double post, I apologize.

I just finished reading '50 Ways to Fly' by Bruce Goldsmith, et al. It is a collection of wisdom and experience, full of great info, and analysis for (mostly) paragliders. In it, the point is made that weight (including ballast) DOES NOT change the glide ratio of a PG. The airfoil is the airfoil. It is created by the shape of the fabric and the diverse line lengths (which do not vary with weight).

In it, they point out:
more weight = more speed and ability to penetrate
more speed = more air pressure in the wing
more pressure = more stability and resistance to collapses

However, more speed also means bad stuff happens faster and more violently when bad stuff happens. They recommend flying heavy (nearer the top of the range) when you anticipate more active air.

BTW My wing is rated from about 80kg to 105kg, so its range is 25kg. An increase of 1 or 2 kilos would move me up not even 10% in my weight range. I suspect to get the added benefits mentioned in the book, one would need to make a more impressive move up in the weight range. [If you know me, adding weight is not my problem...]

I know this thread has deviated from Krista's good save and great write-up, both of which I thank her for, as well as her attitude and willingness to help us learn, together.
John

[krista]

So on the weight issue. I cannot speak to other gliders but I do know from the research I did on my Blacklight that it performs particularly poorly when lightly loaded and in strong wind. I was 4kg lower than normal the day I flew (due to lack of ballast and other usual pieces of equipment, junk, etc.). For a glider with a range from 60-90kg, being at 79 versus 83kg did make a difference when upwind on speed bar (my google earth track shows this well). That said, if you see the launch video, you will also see the slow wing tip deflation and recovery, something that would be a snap dynamic ona heavily-loaded wing.

Generally, the lighter loading means that the glider is slower for everything, be it errors or recovery. The other part is the speed. Comp pilots, as Matt noted, tend to be heavy up on their wings because it increases the speed. That doesn't particularly help with glide performance per we, but it does help help cover more ground since it's quicker. It's the same logic (sort of) as applied to the speed bar and angle of attack. Just with an extra cheeseburger as justification.

[RichH]

I understand about your feelings about ballast..Having never flown Pg's I'm not sure about the effects of 3-4 Kgs on flight performance but I would just simply caution you as I would a Hg pilot that Im not sure the gains are all that you may perceive them to be and not to let that be your decision maker on whether to fly or not to fly on certain days. I've seen other pilots think they can fly in higher wind situations because they are caring ballast only to find out that in the end it really didnt give them the performance gain they were looking for..and as a result they came up short ..( D.B. knows the circumstance I'm referring to) Again it just a caution..certainly not saying not to fly with or with out ballast ..I've known plenty of pilots to do so.. I always chose to fly with out but then again I carry too much of my own internal belly ballast..RH

[Lauren Tjaden]

Krista,
Girls are SO smart! Great write up, and great perception in knowing the difference between luck and skill. We all make error judgements but intelligent pilots like you learn from them. Well done all around.
Lauren

[Danny Brotto]

There are some very good points being made here about ballast. I agree that adding ballast will increase the speed at which max LD and all other pertinent polar points is achieved. I also now understand that PG wing collapses and other upsets can be minimized at higher wing loadings. While I suspect that 10's of kilos would be noticeable, I'm still surprised by the effect of a few kilos. It seems that the comp pilots describe flying over the design limits (which frankly I'd be surprised is allowed in actual competitions) but that's consistent with highly ballasted discussion (vs. lightly ballasted.) I suspect the "feel" and control benefits could be different with a little ballast, but am still wondering about the physics behind the practical befits vis-à-vis glide over terrain.

I've flown my HG with up to 15 lbs of dumpable ballast in stronger conditions to help with handling and penetration. I always dumped ballast before landing to slow down a bit and as a safety precaution. My harness has 3, hand deployable ballast containers to accommodate sand. Regarding safety, in a blown landing the energy of decelerating ballast has to go somewhere and having it impinge onto the pilot is not a good idea. Same for a blown launch.

The situation that Rich Heigle refers to occurred in the early/mid 80's where a light pilot flying a Comet 130 would regular fly with up to 30 lbs (a meaningful amount) of ballast as lead shot around his waist with no ability to dump it. He blew a landing in a particularly turbulent day at High Rock and put himself into a hospital. It was felt that the ballast contributed to his injuries. Two things resulted from that incident. First, the LZ owner forbade the use of ballast while flying HR (which was not the root cause of the problem.) Secondly, 1/4 of the tree line to the south end of the field was cut down (turbulence was a root case contributor to the incident.) This made for a much more forgiving field on strong or turbulent days. HR LZ remains rather benign to this day thanks to that portion of the tree line being cut down 30 years ago.

In the competitive sailplane world, ballast is very important. We are talking hundreds of pounds of water. Gliders are flown at max gross (dumpable water ballast) all of the time even in light conditions (more on why later.) I crewed for a Belgium pilot at the worlds last year. I filled to max gross and the glider was weighed every day by officials to insure we were not even a pound over. Higher wing loading allows for faster speeds around the course. Even on weak days where flying with ballast throughout the task would be disadvantageous (slower climbs, wider circles) tasks are started with ballast which is quickly jettisoned upon leaving the start circle. A fully ballasted glider has lots more energy than an unballasted one. A ballasted glider leaving the start will dump ballast converting that extra energy into an initial altitude or speed advantage over the unballasted one. With everything else being equal, that slight initial advantage will mean the difference between a win or a loss.

Krista, you mentioned the ballast improvement "being at 79 versus 83kg did make a difference when upwind on speed bar (my google earth track shows this well)." I have to wonder how one can see this on a google earth track and also compared to what? Without a baseline far a side-by-side comparison in perfectly identical conditions, we really can't see the difference between 79 vs. 83 Kg. The effects of a few knots of headwind or a few knots of sink can be devastating when trying to reach a distant field and it completely messes up performance deductions. Without having the wind information, we really can't tell anything about the wing's performance from a terrain track. Please let's discuss this if I'm thinking about it incorrectly.

I am learning quite a bit via this discussion. I hope that we can continue to get practical data points to add to the collective knowledge base. Again, I am very keen to understand this connection of performance vs. small additional ballast in PG.

Be careful out there!

Danny Brotto

[brianvh]

I've been having a similar experience for years, Danny, trying to argue that the 10% difference in density between summer and winter will only lead to a 5% change in the speed of wind needed to ridge-soar, which is basically unnoticeable. Most pilots don't buy the math and insist in the winter they soar in 8 mph winds as opposed to 10 mph in the summer. I think either they mistake the wind speed because they don't hear the leaves rustle anymore, or the wind gradient comes closer to the ground because of lack of leaves, or as somebody suggested the glider airframe stiffens up in the cold.

I'm with you on the mathematics of ballast. Perhaps it's these other factors that don't come into the idealized flight equations that make a difference.

[Dan T]

Interesting discussion:

Brian, I'd like to see the math. I'm particularly interested in understanding the measurements and assumptions that yield the 10% winter/summer differential. In this part of the country the air tends to be dryer as well as colder in the winter. Are both of these factors taken into account in the 10% assumption? I confess I'm not sure I know exactly what it means to say that the air is 10% denser. Denser relative to what? Once I understand that, I hope to be able to understand the math behind your 50% translation ratio.

Thanks,
Dan

[brianvh]

dammit, I wrote a whole page, but didn't sign in and lost it. Here we go again, hopefully better:

The pressure in summer and winter is basically the same because we have the same mass of air above us. The temperature goes from ~300 K in the summer to ~270 K in the winter, or a 10% difference, leading to the same 10% difference in density.

When flying without acceleration, weight is balanced by lift. So weight = (form factor)*Area*density*V^2 Everything stays the same in winter versus summer (Ok, a few more pounds of clothing in winter) so we can rewrite this as V^2 = constant/density. As density goes up, airspeed goes down because more mass is being deflected so the wing can get the same lift from flying slower.

A 10% change is small enough to use small number approximations. The first one is 1/(1+x) ~= 1 - x. I already used this one implicitly when I said an 10% change in temperature leads to a 10% change in density through the ideal gas law.
The second one is (1+x)^e ~= 1 + e*x where e is any exponential. (Those unfamiliar with this approximation should try it: 1.1^2 = (1+0.1)^2 ~= 1+2*0.2 = 1.2 compared to 1.21.)

Now we apply these to flight:

V^2 = constant/density => V = sqrt(constant/density) = (constant/density)^1/2

V+dV = (constant/(density+10%))^1/2 ~= (constant/density - 10%)^1/2 ~= (constant/density)^1/2 - 10%/2 = (constant/density)^1/2 - 5%

or dV = 5% of V

{ I've been simplifying the math by writing things like density +10% instead of density + density*10% = density*(1+0.1) , hope it can still be followed. I can redo it more formally if requested. }

So 5% of a cruising speed of 20 mph is only 1 mph, which I don't think is really noticeable without instrumentation and careful quality control of measurements. PIlots swear up and down they notice a difference, but in most cases I think it's just because the trees are less noisy with no leaves, and when wearing more clothes the wind doesn't feel as strong on the skin. People have told me they've seen more than a 5% difference with wind tubes, but I don't trust the reliability. If anything, the lack of leaves would make the wind stronger near the ground. Other than the noise, I don't really know what may be going on when pilots insist they soar at significantly lower speeds in the winter, but it's not as simple as density differences. The math is pretty clear cut in that respect.