Low Altitude Troubles

But what are your thoughts about thermaling low above ridges or hills? Are the same rules apply? Many of us do it often and usually it is hard to tell your exact altitude above the ridge if you don't know the ridge elevation.

My thoughts are, er, "evolving." Due to a youth mis-spent flying hang gliders and some ridge time, I got used to turning really close to hills. Too many magazine articles rave about rock polishing. However, we have had a spate of crashes lately from pilots -- really good, famous pilots -- running in to hills. Peter Masak, Klaus Holinghaus are just two of the many.

Doing the 2012 safety review analysis of Brian Milner's flight -- available here

http://faculty.chicagobooth.edu/john...m#safety_rules

was another eye opener for me. Brian thermaled off the ridge, found 4 knots up at the 90 degree mark, still climbing at 180 degrees, well above ridge top height and decently away from the ridge -- now, tighten and finish the 360? He did, and at 270, the lift turned to sink, and tailwind, and he hit the top of the ridge. Remember, he was well above the ridge when he made the crucial decision to finish the turn. At least looking at the see you screen shots, it' something I have done many many times with equal or less clearance.

Frank Paynter's soaring cafe post about his crash at Moriarty is another eye opener. Standard ridge technique, and Frank is no rock-polisher, and all it takes is one big gust of wind.

For me, a trip to New Zealand was another eye opener. I expected, based on articles, to be told to get close to the mountains. No, in fact, my instructor G Dale was aghast at how close I was flying and how soon I wanted to thermal. Hmm.

Bottom line. Close to hills lie many of the same traps, and I think there are traps out west for us east-coast ridge pilots.

-Wind will be more turbulent close to the hill, especially if the ground features are more disruptive.

-More sun and unstable air makes it more turbulent still -- you're flying in a super adiabatic layer, made more so by lifting.

-There is not just lift and sink, there is headwind and tail wind. A gust of tail wind when you're pointing at the hill is no good.

-Ground illusions abound. In particular in the air, you think subconsciously that you're turning on a point, and forget how much radius a turn takes.

-If you follow common advice and add some airspeed, your turn radius gets larger still. Turn radius goes with airspeed squared. Adding water ballast really increases your turn radius, because you are flying 5-10 knots faster.. As an ex hang glider pilot, the huge turn radius close to the ground really surprised me on my first few mountain flights.

-By email, I was reminded of another illusion. When you really want to turn, now, you really want to see the nose sweeping across the horizon. Nothing like stomping on the rudder to get that accomplished.

And a bit of Russian Roulette. You can get away with things 99% of the time and, like me, convince yourself you're being safe. The 1% monster out there has gotten so many pilots.

In sum, I'm going to be putting a bit more distance between me and the mountain!

John Cochrane

Another idea that has been discussed regarding close ridge thermalling:

There can be both strong updrafts and downdrafts within one wingspan resulting in a force greater than the rolling forces of the ailerons. JJ wrote an article "Don't smack the mountain" about just that concept.

Mike

On Fri, 31 Jan 2014 11:06:55 -0700, "Dan Marotta"
wrote:
[snip]

Above a certain altitude when you make a coordinated turn with a given bank
angle the inside wingtip traces a circle in the opposite direction vs the
ground. In a low turn to final the wingtip goes the same way around versus
the ground leaving a subtle sensation that the turn lacks enough rudder.

[snip]

Pivotal altitude. Equal to TAS in knots, squared, over 11.3, or about
319 AGL at 60 knots.

rj

There can be both strong updrafts and downdrafts within one wingspan resulting in a force greater than the rolling forces of the ailerons. JJ wrote an article "Don't smack the mountain" about just that concept.



Mike

JJ's article can be read here. It is worth a look.

http://www.dg-flugzeugbau.de/index.php?id=mountain-e

On Saturday, February 1, 2014 8:42:15 PM UTC-5, Craig R. wrote:
There can be both strong updrafts and downdrafts within one wingspan resulting in a force greater than the rolling forces of the ailerons. JJ wrote an article "Don't smack the mountain" about just that concept.







Mike



JJ's article can be read here. It is worth a look.



http://www.dg-flugzeugbau.de/index.php?id=mountain-e

ThereisanotherexcewllentarticlewrittenbyHarryCombs regardingmountainridgethermalsandthepropensityfors tallsandunexplainedcollisionswithterrain:http://www-ee.stanford.edu/~hellman/soaring/Combs.pdf

It wasn't me that wrote the quote below, but thanks for the math. BTW,
there's absolutely no way I'd be thermalling at that altitude. I'll be
lining up on a landing spot (or a tree).


"Ralph Jones" wrote in message
...
On Fri, 31 Jan 2014 11:06:55 -0700, "Dan Marotta"
wrote:
[snip]

Above a certain altitude when you make a coordinated turn with a given
bank
angle the inside wingtip traces a circle in the opposite direction vs the
ground. In a low turn to final the wingtip goes the same way around
versus
the ground leaving a subtle sensation that the turn lacks enough rudder.

[snip]

Pivotal altitude. Equal to TAS in knots, squared, over 11.3, or about
319 AGL at 60 knots.

rj

On Saturday, February 1, 2014 8:42:15 PM UTC-5, Craig R. wrote:
JJ wrote an article "Don't smack the mountain" about just that concept.
http://www.dg-flugzeugbau.de/index.php?id=mountain-e

Dang. This is interesting. I do not fly in "mountains", but we have ragged and undulating relatively high ridges close-in to the east and west of my home airport and depending on the wind direction they throw rotor, turbulence and gusty lift in front of each other.

JJ says:
As I get within 300 feet of the rocks, I roll the ship into a 30 degree right bank and hold it there with a bit of top rudder. Why? Because, I'm already banked away from the mountain, so if a zephyr tries to get me, I'm ready to apply all three control inputs to fight it with right stick, right rudder and back stick to make it turn away from the rocks.

If I understand this, JJ is maintaining a constant turn to the right relative to the air mass movement (aka the wind) and thereby maintaining a flight path parallel to the slope, and since he is flying a high performance glider, he does not see much altitude loss due to the slight slip induced by the top rudder.

Having trained in low to medium performance gliders, I learned to crab (wings level) to fly parallel to a ridge. JJ seems to be saying that dropping the upwind wing is a better solution. I can see that this also makes it easier to observe, note and correct any residual drift towards the ridge (whereas the crab directs your attention somewhat away from the ridge).

JJ says:
It is estimated that our ailerons can only counteract a differential moment of 2,5 m/s. Let's say there's 6m/s under our right wing and only 1,5 m/s under our left wing. Reasonable figures? Sure, we see 6m/s all the time on the Whites and in the Sierras

I've rarely seen sustained 6m/s, but the gusty rotor/turbulence near the local ridge might easily produce a strong pulse. And if my reaction time is a little slow, then lesser differential lift will flip me. I've honed my reflex to keep the wings level when I hit rotor on tow, but I'm pretty sure that I don't react to every transient "light wing" when off tow. When flying in turbulence, the light wing will quickly alternate from side to side and a left-right PIO in turbulence is not helpful.

But, is flying below the ridge line an asymmetric special case? Should I counter every windward light wing with aileron and NOT likewise counter a light wing on the ridge side? (I'm not looking for flight instruction on the internet, I just want to hear some opinions.)

On Saturday, February 1, 2014 2:27:25 PM UTC-8, Ralph Jones wrote:

319 AGL at 60 knots.

I haven't done the specific math. At a 45-degree bank you are trying to solve for turn radius=altitude. It will be a different ratio at other bank angles. A shallower bank angle at the same airspeed will cross over at a higher altitude (cross-over is where wing stays on a single point on the ground). Turn radius is a function of bank angle and airspeed so presumably there are a range of altitudes where the visual cues go one way versus the other.

Of course the main point is not to get fooled into over-ruddering your turns at too slow an airspeed down low. Trying to thermal up against a steep mountain has a similar, even more disorienting effect as the "horizon" goes up and down in front of you around the circle. Add to this thermal boundary and ridge shadow shear and/or wind drift effects and you can end up being your own worst enemy.

9B

On Saturday, February 1, 2014 5:27:25 PM UTC-5, Ralph Jones wrote:
On Fri, 31 Jan 2014 11:06:55 -0700, "Dan Marotta" wrote: [snip] Above a certain altitude when you make a coordinated turn with a given bank angle the inside wingtip traces a circle in the opposite direction vs the ground. In a low turn to final the wingtip goes the same way around versus the ground leaving a subtle sensation that the turn lacks enough rudder. [snip] Pivotal altitude. Equal to TAS in knots, squared, over 11.3, or about 319 AGL at 60 knots. rj

I would be very interested in understanding:
First- what "pivotal altitude" is (definition).
Second- how was equation described developed.

UH

On Sunday, February 2, 2014 11:16:30 AM UTC-8, wrote:

A shallower bank angle at the same airspeed will cross over at a higher altitude

Oops - I meant you get lower altitude crossover at lower bank angles. At 60 knots and 45 degree bank the cross-over is 450'. At 60 knots and 30 degree bank the cross-over is 368 feet and at 60 knots and 15 degree bank it's 330 feet AGL. The absolute minimum for the shallowest possible bank angle is 318 feet AGL.

9B