Unexpected lift

[PatMc]

Due to family commitments & a short holiday I haven't had much opportunity lately to get any flying done.Today even though the wind was very light it was from the a good direction for my local slope - NNE. Knowing that my old Easyglider will stay up in the lightest breeze [plus it has an optional fan up front ] I nipped out for an hour this afternoon.

First flight was pleasant enough, chatting & answering questions from passing walkers, managed a few loops, stall turns & the odd roll to break the boredom of coasting up and down at between eye-level & about 50ft max. The model did complete some aeros well below slope level a few times but always managed to be clawed back up again without resorting to power.
Landed after 1/2 an hour for a 10 min break & chat with some people then re-launched.

This time the wind had died a little & a minute or two after launching it gradualy swung to about due E. I was flying from the N facing slope of a bay with E facing slopes at each end. Within three or four passes the model was below recoverable height so I powered up & climbed to about 300ft before cutting the motor & stooged around doing a few aeros on the glide as the model descended to eye level. Repeated the performance a second time but noticed that at the top of the climb the model seemed to maintain altitude until I started the aeros & it took longer to come down. After the third climb I coasted around flying over the E facing slopes & prolonged the glide at that altitude until I decided to land. Not until the model dropped below about 200ft did it sink at the sort of rate I'd consider normal.
The slopes are only about 60ft ASL & with such light wind the max altitude of the first flight was normal but during the second flight the lift at the higher altitude was steady & completely unexpected. I can only surmise that there was some sort of temperature inversion at work generating a steady updraft.
This second flight was for 30 minutes with three, roughly equal, motor run times for an overall total of 50 secs power on. The glide after the final motor run was about 20 minutes & could have been extended if I hadn't decided to land when I did.

Edited By PatMc on 08/05/2016 23:49:34

[Martin Whybrow]

Not sure about where you are, but we had a lot of thermal activity around here today (Cambs area), some fairly spectacular cbs built up this afternoon (in the distance, thankfully)

[PatMc]

This was on the coast of S. Northumberland, flying over the [cold] North Sea. There was no sign of any thermal activity.
We had sea fret all day Friday & Saturday most of this morning until about 11'ish.

[KiwiKid]

You might have been experiencing "slermal" conditions. These happen in low speed wind conditions when the slope lift picks up weak thermal air and gives it a boost up the ridge. The combination of the two produces stronger lift conditions than would normally be expected. I have seen this happen even in the middle of winter as thermal conditions occur due the difference in surrounding surface temperatures and this can occur at sea as cold currents get pushed to the surface. These slermal conditions won't happen in stronger winds as the wind breaks up the thermals.

Another thing that can happen when the wind is changing direction slightly is that the wind can hit the slope's "sweet spot". The amount of lift generated can change markedly depending on the slopes shape and can drop off rapidly (like 10% for every 5 degrees) in some  cases.

Edited By KiwiKid on 09/05/2016 01:49:18

[John Muir]

I remember a friend of mine coasting up and down along what he referred to as an 'edge'. He was flying a thermal glider on our seaside club field. He said that cool air blowing in gently from the sea could form a sort of wedge under the warmer air over the land driving the warmer air upward. Certainly his glider was able to stay up for a good while just cruising backwards and forwards along a specific track parallel to the coast. He's an ex full-size glider pilot so I believed him. Maybe you found some of that.

[J D 8]

Often see the local Buzzards taking advantage of such conditions on a cold but sunny winters day here on the coast.

They are circling so are in a thermal,when in slope lift only they remain in straight ahead flight.

[Ian Jones]

Martyn Kinder and myself were out on a slope the same afternoon. Broken clouds with periods of sunshine certainly gave us a combination of slope lift and thermals, together with assocaited sink, which is something to watch out for:

At last I have got the hang of left handed launches- Photo: Martyn Kinder.

[Martyn K]

A few more from the same flying session

I flew my Glass Slipper and the Bubble Dancer Thermal Soarer (which was decidedly hairy and a nightmare to land).....

I cant remember what Ian was flying but here are a couple of shots of Ian's foamies

 

and the Bubble Dancer - slightly over elevated and under ballasted for the conditions. So only one flight and a very nerve wracking landing

Ian took the photos of the Bubble Dancer, not sure who took the photos of the Glass Slipper as we were both snapping away at that and both flying the model (but not at the same time)

Absolutely great fun.

Martyn

Edited By Martyn K on 09/05/2016 10:15:16

Edited By Martyn K on 09/05/2016 10:17:52

[Ian Jones]

Pat, everyone of those models hit a mass of extra lift at times and I think that the weather warning for thunderstorms (which didn't materialise) perhaps explains a lot of it, cold air and warm air meeting each other. So rising air and sinking air came in quick succesion. This supports your suggestion of mixed tempretures at play. Apart from Martyn's Bubble Dancer which didn't want to come down no matter what, all the other models hit some sink at times too, resulting in a few hasty landings .

Edited By Ian Jones on 10/05/2016 01:19:48

[KiwiKid]

Another phenomenon I have noticed on some gentle slopes with light gliders like DLGs is that, sometimes when the wind speed increases, the lift actually reduces. My theory here is that the stronger wind is knocking the top off the lift so when the wind is at it's optimum speed for the slope, a sort of wave forms where incoming air rides up on the back of the ridge lift. This creates another feature with DLGs whereby you can fly them to a certain height in the ridge lift, but if you do a strong DLG launch the model will pass through the ridge lift up into the wave lift and sit there seeming to defy physics.

Here's a vid of one site I fly at where this happens a lot - my mate testing the boundaries of the lift with his Halfpipe - launch (and return) is at the tree line.

[PatMc]

Posted by John Muir on 09/05/2016 08:20:03:

I remember a friend of mine coasting up and down along what he referred to as an 'edge'. He was flying a thermal glider on our seaside club field. He said that cool air blowing in gently from the sea could form a sort of wedge under the warmer air over the land driving the warmer air upward. Certainly his glider was able to stay up for a good while just cruising backwards and forwards along a specific track parallel to the coast. He's an ex full-size glider pilot so I believed him. Maybe you found some of that.

That's more or less what I reckoned it to be, John.

It definitely was not thermic lift.

[PatMc]

Posted by Ian Jones on 10/05/2016 01:19:07:

Pat, everyone of those models hit a mass of extra lift at times and I think that the weather warning for thunderstorms (which didn't materialise) perhaps explains a lot of it, cold air and warm air meeting each other. So rising air and sinking air came in quick succesion. This supports your suggestion of mixed tempretures at play. Apart from Martyn's Bubble Dancer which didn't want to come down no matter what, all the other models hit some sink at times too, resulting in a few hasty landings .

Edited By Ian Jones on 10/05/2016 01:19:48

Ian, when the model was in the lift it cruised around without losing or gaining any height in a similar way to a model in light slope lift that's reached it's ceiling for the conditions. When I deliberately forced the Easyglider down to below the altitude I perceived the lift belt to be [aprox 200ft] it continued the descent at it's normal glide rate.

BTW I have experienced similar conditions before when flying vintage models at my club site which is a couple of miles up the road & 300 - 400 yards west of a beach. i.e. Light wind off sea, motor cut at about 200ft then gliding the model back and forth parallel to the coast.

[Ian Jones]

Ah, I get it now, I was thinking too vertically.

[brfc7]

Morning Pat, I've been wondering where you were hiding. I've been at the sluice about 10 times in last 5 weeks and no sign of you.

I was their for an hour on Monday 4pm and again Tuesday 4.30 both times Chris C was their. I will be nipping down probably ever day this week as the wind is between N-NE until at least Sunday. Might see you their.

Baz

[Simon Chaddock]

If you have the advantage of a local pair of Buzzards they will demonstrate that thermal lift exists even where there are no other visible signs and even with their guidance they are so much better at using it than me!

[Steve Houghton 1]

Posted by John Muir on 09/05/2016 08:20:03:

I remember a friend of mine coasting up and down along what he referred to as an 'edge'. He was flying a thermal glider on our seaside club field. He said that cool air blowing in gently from the sea could form a sort of wedge under the warmer air over the land driving the warmer air upward. Certainly his glider was able to stay up for a good while just cruising backwards and forwards along a specific track parallel to the coast. He's an ex full-size glider pilot so I believed him. Maybe you found some of that.

Yes, this is what is called a convergence and what I believe a lot of model fliers mistakingly think are thermals on coastal sites. This convergence usually takes place between November and April, with April being a particularly strong month for it. And coincidentally, this is when most model fliers report sea thermals.

And you are correct in saying that it is when the warmer air over the water, (remembering that sea temperatures around the UK are around 10°C at that time of year), meets with the colder air of the land, (particularly if its frosty), and that warmer air then rises, making the model flier believe he is in a thermal.

[PatMc]

Steve, I think you have it the wrong way round, it's the cold air coming off the sea that's causing the warmer air over the land to rise.

Currently sea temp is well below the ground temp, certainly in this area & we haven't had any ground frost near the coast since about late January - early February.

When I was flying on Sunday afternoon, we'd had sea "fret" & generaly heavy overcast skies for a solid 2½ days whilst most other parts of the UK apparently enjoyed glorious sunshine.
This pattern isn't unusual on the S. Northumberland coast when there's a N to NE wind. Quite often in spring & early summer we'll have a bank of mist on the coast whilst a mile or so inland is basking in sunshine. OTOH in winter we don't get as much frost or snow etc as inland areas.

Simon, most of the birds around my local coastal slopes are waders, divers or gulls, no buzzards.

[Speedster]

At the beach yesterday

Not much slope needed to keep the Arthobby Odyssey 2,7 in the air

Cheers Soren

[FilmBuff]

Posted by Speedster on 16/10/2016 11:02:35:

At the beach yesterday

Not much slope needed to keep the Arthobby Odyssey 2,7 in the air

Cheers Soren

That is amazing Soren - and great control to keep it all going. What was the wind speed?

[Speedster]

That is amazing Soren - and great control to keep it all going. What was the wind speed?

Windspeed was 20-24 knots

Regards

Soren

[Steve Hargreaves - Moderator]

Is that an example of the same phenomena that the Albatross uses to stay aloft I wonder? What we more commonly call Dynamic Soaring......

In Pats original post I wonder if "Wave Lift" might be responsible....this is where an obstruction (a hill or such like) causes "standing waves" in the air, the peaks of which can often be determined by the appearance of lenticular clouds spaced at regular intervals in the lee of the obstacle...

[Martin Harris - Moderator]

I don't think that's very likely - wave classically appears when there are two layers of stable air - low (such as a temperature inversion) and high and the air deflected over the mountain "bounces" between these layers. It results in uncannily smooth lift - the glider seems to be much quieter than normal and everything feels lighter. Because of the need for that stable lower layer, it's not usually contacted at low level.

What might be happening if the hill is on the edge of a plateau, is some form of convergence lift. At the end of a long warm day, the air in contact with the higher ground begins to cool. It becomes heavier and sinks, spilling over the edge of the hill - this is called a katabatic wind. If it opposes the general wind direction, then the two air masses meet and there would be an up-flow at that point. The change in wind direction noted certainly fits with this hypothesis.

I remember seeing a hot air balloon doing circuits at Dunstable many years ago - impossible? No - the wind was blowing towards the ridge, the balloon was flown a few hundred feet above the hill height until close to it where it was allowed to descend into the katabatic flow, travelled back a few hundred yards at low level, climbed back into the general wind flow, flew back to the hill and repeated several times.

[PatMc]

Posted by Steve Hargreaves - Moderator on 16/10/2016 22:10:52:

In Pats original post I wonder if "Wave Lift" might be responsible....this is where an obstruction (a hill or such like) causes "standing waves" in the air, the peaks of which can often be determined by the appearance of lenticular clouds spaced at regular intervals in the lee of the obstacle...

The nearest hills from which the wind was coming that day would have been in the snow capped Danish Alps about 400 miles away.