Rotors

[gertcoetzee]

As you might have seen on my www.zu-dve.com log, I had a bit of a rotor scare today. Which makes for a good topic for training talk.

I was flying towards Aurora (approx 100ft ASL), which is settled against the southern slopes of the Piketberg (approx 4500ft ASL), and encountered a northerly of about 20mph (at 2000+ ft) almost all the way there. The last 10 or so miles at 800 to 1000ft ASL I did not encounter much of a headwind, and was quite relaxed until all of a sudden about 1.5 miles south of the airfield (and town)....

This Google track, with the mountain elevation shown in background, shows my altitude, and the vertical stripes are a second apart. Since I was flying in more or less a straight line, the spacing (seconds) is regular.
From this track, as well as the data I put on the log you can see that there was (middle of this picture) a sudden 2000ft/min gust, as well as some downward falls, whic scare the sh*t out of me.

My question is now....we all know not to fly on the downwind side of a mountain for fear of rotors. Is there perhaps a logical zone where it might be safer. I would have thought that slap bang against a mountain there would be a quiet "cushion" of air above which the rotors would occur?

Once I took of in from a quite FASH, and once I got above 1000ft I got clapped by the rotors of the mountains due to an easterly. Which is why I had this "cushion" theory, until today!



Shaken, but not stirred,
Yours
G

[Arnulf]

This Google track, with the mountain elevation shown in background

Ek sien niks. Net 'n wit blokkie met 'n rooi kruisie.

[Biggles]

No pic, but will attempt to shed light...

Airflow over mountains is a combination of laminar and turbulent flow. It behaves exactly like a wing in that respect. Slow wind over a smooth obstruction will produce a nice laminar flow. wind will go up and over and down the other side. So up on the windward and sink on the lee.

Slightly higher windspeed and this up down motion creates a wave pattern that will continue down wind for quite some distance.

Increase the wind speed and you will get a turbulent flow like a wing stalling... up down and all around, with a spin cycle.

If the obstruction is high enough a shadow will form that is untouched by the wind over head (often inversion layers will shield the air blow... FASH for example, but climb up into the tubulance above and you will know about it.

The other case will be where a rotating "whirl" of air will form in the lee. Essentaill detached from the airflow above but powered by it... like an eddy in a river. Close to the obstruction the air will be going up rapidly... further downwind it is going down rapidly. These eddies can detach from the lee of the mountain and go bouncing downwind (I kid you not)

All in all when wind is coming over a mountain it is very unpredictable... actually officially known as chaotic. (chaos theory). I did some work in flumes trying to predict heavy media behaviour in flows and just a fractional increase in velocity changes the flow characterists completely... and sometimes all by itself... and sometimes doesn't change when you increase the flow... it was all very frustrating

Do yourself a favour by lighting an incense stick placing it in a steady draught and then put obsticles in the way of the smoke...

[gertcoetzee]

Don't know why the red X (attachment download problem, again) - now posted the pic on www.zu-dve.com with a link.

Thanks for the explanation. And how to handle this?

[DieselFan]

I had a kak case of rotoring this weekend. Passed over mountain not too high and all was calm about 300m past the mountain (Magalies, Hartes) I was thrown around badly ground speed from 65-90 in 1-2 seconds. This continued even 2-3km away and till 200 feet agl. Left wing dropped and I was at full lock right to keep straight.

The windsock was stable showing 3-5mph wind no gust. Yet on downwind I was in a washing machine, landing was VERY smooth.

On the ground you would NEVER EVER think it was bad conditions. No wind. Perfect weather

Take off 30mins later and EXACT same 300agl was very turbulent all the way to 2500agl Airspeed was gusting 60-80. Although I doubt it was that much in gusts I think more a case of the trike twisting.

It was my pax's 2nd time and good thing he's a man of steel and was "fascinated" by the effect. This was worse than ANY midday flying thermals I've had - there's no comparison.

I did expect the rotoring on the leeward side but not like that. I mean it was hot air balloon weather!

My guess is I was flying "under" the wind over the mountain and encountered the splashing on the other side. I think the reason the ground was calm was that the wind was too strong and continued at around 200ft to the next koppied 10km away. Yet the southern ridge gave nothing away

[Nkwazi]

Is it not better to fly high over mountains to avoid the rotor??

[DieselFan]

Is it not better to fly high over mountains to avoid the rotor??

Yip, however once you start the descent on the otherside to land you would've encountered the turbs. This was at FAAP.

[Harlequin]

Diesel fan.

"On the ground you would NEVER EVER think it was bad conditions. No wind. Perfect weather "

I flew in the same area as gert but not as far north.
Questions, your turbulence, did you have any temperature changes in the immediate air?
Why I ask, the upper air Gert refers to was a constant smooth 20mph. Northerly.
The lower air, warmer, but Easterly.
The interface was turbulent as you would expect but it gave a warning of hot and cold then the thump.
The mechanisms of the two rotors are different but the effect on our tiny craft are the same.

I would like to hear more commentary on the "cushion" as Gert explained, I’ll do some research in the time being.
Fly safe.
Graham.

[DieselFan]

Diesel fan.

"On the ground you would NEVER EVER think it was bad conditions. No wind. Perfect weather "

I flew in the same area as gert but not as far north.
Questions, your turbulence, did you have any temperature changes in the immediate air?

Graham.

Actually, there was. Warmer air on top with colder on ground and the wind direction was different as I even took off downwind without thinking, even though I looked at the windsock I didn't register that it was opposite direction to what I had just had, long and short the two contradicted each other.

Wind was blowing north over mountain as shown with ground speed where as in the "valley" 2km further it was blowing south. Did think / register about that till now