flying at dusk...why the lift?

alexsally · Oct 08, 2012, 02:22 PM

For my last video I flew the radian a couple of times at sundown..So why did I get good lift and a long flight. It had been sunny ish in the day but the sun was going down and temp dropping. I thought lift and thermals were stronger midday? dumb question no1

dumb question 2

I did several flight and the one with the gopro on seemed longer and more bouyant than with the camera off???

mmm sorry if it's really obvious.
alex

Svede · Oct 08, 2012, 11:01 PM

Evening air is often described as being "buoyant", not sure if I've ever come across an actual scientific explanation to the phenomenon. You could look in the soaring or dlg section.

wetstuff · Oct 09, 2012, 12:03 PM

I would bet it was increased 'density'. As a kitesurfer, in mid-summer I wish for the Fall when the air is cooler, more dense ..and more powerful for a given wind speed. I would guess in your area, it cools quickly when the Sun gets low in the sky.

Svede is probably correct; the Glider guys would know. Cheers.

Jim

alexsally · Oct 09, 2012, 01:33 PM

Thanks guys. I got a lot to learn on this glider stuff :-)
Maybe I should hang about the glider forum :-)
Alex

CenTexFlyer · Oct 09, 2012, 01:54 PM

You pretty much described what the situation was that helps create lift.
It was a sunny day, so the ground heated up pretty good and holds heat for a while.
Sun went down and the ambient air became cooler and denser.
The ground still being warm heats the air near the surface and causes a thermal.
The more difference there is in temperature (ground vs. air) the more the effect is.

Swanlander · Oct 10, 2012, 07:13 AM

I have experienced a similar phenomenon; when evening flying at our flat field site in settled weather we would often encounter a 15 to 20 minute period of sustained lift.

This was, we understand, generated by the change in local wind as the evening coastal breeze picks up, pushing cold (heavy) air inland and lifting the warm (lighter) air above it as it moves. A bit like a moving slope.

The camera weight issue might be associated with this. If the airspeed is optimised for minimum sink (no ballast) you drift downwind. Pushing forward on the elevator to maintain station will reduce the efficiency. If the camera is fitted, assuming no dramatic increase in Cd, you may be flying in the max range band. (longest glide) Which is why sailplanes carry water ballast for upwind legs of cross country flights.

Not sure if this is 100% correct, but as you say, loads to learn; surf the web, explore the soaring knowledge base and put me right.

Oct 08, 2012, 02:22 PM	#1
alexsally Registered User Joined Dec 2011 64 Posts	flying at dusk...why the lift? For my last video I flew the radian a couple of times at sundown..So why did I get good lift and a long flight. It had been sunny ish in the day but the sun was going down and temp dropping. I thought lift and thermals were stronger midday? dumb question no1 dumb question 2 I did several flight and the one with the gopro on seemed longer and more bouyant than with the camera off??? mmm sorry if it's really obvious. alex

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Oct 08, 2012, 11:01 PM	#2
Svede Registered User United States, CA, Lemoore Joined Nov 2008 386 Posts	Evening air is often described as being "buoyant", not sure if I've ever come across an actual scientific explanation to the phenomenon. You could look in the soaring or dlg section.

Oct 09, 2012, 12:03 PM	#3
wetstuff Registered User United States, MD, Salisbury Joined Jul 2011 50 Posts	I would bet it was increased 'density'. As a kitesurfer, in mid-summer I wish for the Fall when the air is cooler, more dense ..and more powerful for a given wind speed. I would guess in your area, it cools quickly when the Sun gets low in the sky. Svede is probably correct; the Glider guys would know. Cheers. Jim

Oct 09, 2012, 01:33 PM	#4
alexsally Registered User Joined Dec 2011 64 Posts	Thanks guys. I got a lot to learn on this glider stuff :-) Maybe I should hang about the glider forum :-) Alex

Oct 09, 2012, 01:54 PM	#5
CenTexFlyer Registered User Near Austin Joined Dec 2001 4,893 Posts	You pretty much described what the situation was that helps create lift. It was a sunny day, so the ground heated up pretty good and holds heat for a while. Sun went down and the ambient air became cooler and denser. The ground still being warm heats the air near the surface and causes a thermal. The more difference there is in temperature (ground vs. air) the more the effect is.

Oct 10, 2012, 07:13 AM	#6
Swanlander I fly with my little eye Lancashire, England Joined Jul 2005 1,783 Posts	I have experienced a similar phenomenon; when evening flying at our flat field site in settled weather we would often encounter a 15 to 20 minute period of sustained lift. This was, we understand, generated by the change in local wind as the evening coastal breeze picks up, pushing cold (heavy) air inland and lifting the warm (lighter) air above it as it moves. A bit like a moving slope. The camera weight issue might be associated with this. If the airspeed is optimised for minimum sink (no ballast) you drift downwind. Pushing forward on the elevator to maintain station will reduce the efficiency. If the camera is fitted, assuming no dramatic increase in Cd, you may be flying in the max range band. (longest glide) Which is why sailplanes carry water ballast for upwind legs of cross country flights. Not sure if this is 100% correct, but as you say, loads to learn; surf the web, explore the soaring knowledge base and put me right.