Finding Lift

[mgardner]

Hey you Sultans of Soaring...a question for ya'

Its thunderstorming here in central texas right now...so I'm on my way home, and notice big birds everywhere specking out...there are scattered storms around, but in between them are clear sky with huge pillars of puffy clouds...I'm wondering...would these conditions be good for thermals???

It gets gusty in the storms, but calms down dramatically after passage of the showers...and those birds just seem to be calling me....

I would think the air would be turbulent, and even if you did find lift it would probably be a wild ride...just wondering as I get into this new facet of a hobby I've been at for some time.

Thanks for any replies...

[Ollie]

First, you don't want to be flying too close to an advancing thunder storm. Sometimes that first bolt of lightning can be a real shock when it comes before it starts to rain. A transmitter antenna is an effective lightning rod.

There is another form of lift that is sometimes associated with thunderstorms. An advancing cold front can be a wedge of cold air that lifts the mass of warm air ahead of it. This can produce very strong lift over a wide area.

Moisture on the ground usually supresses conventional thermal activity.

[Daemon]

The conditions could range between good, and too good. Cloud suck can take a full scale glider and slurp it right up into a cloud at a couple thousand fpm or more. Imagine what it can do to your R/C glider. Still, with the birds there I'd also have been very tempted to fly it. They're generally no more fond of severe turbulence than your glider is, and also aren't likely to be around for impending lightning. They ain't stupid. They do this for a living. ;)
If they decide to book though, probably wise to come down.

BTW, just a little random nit. Lightning rods aren't actually designed to attract lightning (despite all popular cliches to the contrary), but to dissipate static charge as fast as possible, hopefully preventing a lightning strike. Static charges in the ground/structures/etc will rise to the highest point, and build up there. When an opposite charge in the cloud builds up enough energy to cause a lightning bolt, the cloud sends down many step leaders (a tiny spark that's trailblazing through the areas of lowest resistance through the air) and many of the high points on/above the ground will send up their own "positive streamers" toward the cloud which are the inverse of the step leaders. As soon as one from the ground, and one from the cloud touch.. BOOM. A bolt forms along the pathway, and the various visible branches of the lightning bolt are the step leaders being drained of their energy.

What's this have to do with lightning rods? Well, if you can rid the ground object of its static electricity fast enough, it'll never build up enough static to make a large positive streamer. To do that you place a rod, or rods on top of the object, each with a very sharp point. The static rises up the rod, and is pushed off the point into the atmosphere by the charge behind it. So while people see a lightning rod with a cable to the ground, and assume that it's there to attract lightning and conduct it to the ground, in fact the cable is meant to carry the building positive static charge from the ground, around the structure, and blast it off the point of the rods continuously, before it can build up enough potential charge to launch a big step leader toward the clouds above. This is why proper lightning rod installations put a whole bunch of rods across the roof of a building to spread out the charge and dissipate it quickly. Sometimes it just isn't enough though and the charge differential is just too great, and they get struck anyway. But in general, one building with properly installed lightning rods will get struck less than one without.

And yes, I lived in a house on top of a bare hill with a tin roof for a summer in Arkansas. We had 5 rods across the front edge of the roof which did their jobs well.

I have stood on a hill and brought my finger to within 1/4 inch of the TX antenna and had a near continuous stream of sparks jump to/from my finger. That's perhaps a little past time to get down. ;)

ian

[mgardner]

Interesting treatise on lightening rods...I learned something today...always wondered why they went to the trouble to put those sharp points on the rods...

So if I get the itch to go out and fly the HLG...what wind speed should I consider a max...just to go out and throw it some to make a loop and work on catching it. I was thinking 10 mph, but as I've read through some of the posts here it looks like guys are flying in higher winds..

For reference I'm flying an Eros 1.5 m...

Havent received my copy of the Old Buzzards Soaring Book yet...can you tell???

Later guys..

[Ollie]

The highest wind speed to fly in depends on the best penetration glide slope for the wind condition. In windy conditions you must fly faster than the best L/D to make headway over the ground with minimum altitude loss. In other words you must put the nose down and keep it down in a fast glide or even a shallow dive if the wind is blowing hard. To assess the conditions, just see how much you must put the nose down to make the most headway for the least altitude loss. If the best penetration glide slope is, say, 3 to 1 then the penetration glide slope angle is arctan 1/3 = 18 degrees. If you venture below 18 degrees you will not be able to get back to the field unless you encounter more lift. If you encounter sink wnen attempting a return along the 18 degree slope, the plane will be forced down short of the field. For this reason, it is best to allow a safety factor in selecting how far above the horizon to begin a safe return to the field. If you estimate the penetration glide slope to be 3 to 1 then I suggest doubling the angle to 36 degrees and not venturing below that angle to the horizon when down wind. If the penetration glide slope is 2 to1 then arctan 1/2 = 26 degrees and I would not venture below 52 degrees to the horizon when flying down wind. How much safety factor to allow depends on the strength of the down air and the pilots flying skill. If you can fly smoothly in a shallow dive without porpoising then a smaller safety factor is allowable. If you continually stall the model then you won't be able to make it back to the field in any sort of wind. On very strong thermal days the sink will also be very strong and a bigger safety factor is appropriate.

[mgardner]

Wow Ollie..I'm embarrassed to say it, but that went over my head...I'll have to re-read it a few times to try to internalize it...

Anyway, went out and tried to practice...wind was 10-15 mph, with some stronger gusts. Soaring birds were aggravating me, floating so high up on the clouds...I could manage it into the wind, no problem, but trying to make a downwind turn to bring it back around to catch was a near death experience...I was hand tossing, so didnt have enough altitude for the ship to gain enough airspeed...after a few attempts I called it a day.

This hand launch stuff is way different, and mui-challenging...

[Ollie]

mgardner,

OK, let's look at penetration glide slope another way. In no wind conditions the glide slope is defined by how far the plane will glide from a given altitude. Lets say the hands off glide slope is 20 to 1. That just means that the plane will glide a distance of 20 feet over the ground for every foot of altitude lost. If the glide slope is 20 to 1 and the plane takes one second to cover 20 feet over the ground, the ground speed is 20 feet per second and the sinking speed is one foot per second. The air speed is slightly greater than 20 feet per second because the right triangle with sides of one and 20 has a hypotenuse just a little longer than the 20 unit long side. ( arc tan 1/20 = 2.86 degrees )

Now let's add some wind. Let's say the wind is blowing at 10 feet per second. The ground speed flying into the wind is 20 feet per second minus the wind speed of 10 feet per second leaving a ground speed of 10 feet per second. Now the penetration glide slope is only 10 feet covered over the ground for each foot of altitude lost. ( arc tan 1/10 = 5.71 degrees)

If the wind is blowing at 19 feet per second, the progress over the ground is only one foot for every foot of altitude lost. ( arc tan 1/1 = 45 degrees ) Instead of flying at the hands off speed we put the nose down and the airspeed goes up to a little over 29 feet per second and the sinking speed to 2 feet per second. The progress over the ground becomes 10 feet for every 2 feet of altitude lost or five feet of progress for every foot of altitude lost. In this comparison the penetration is improved 5 fold even though the sinking speed is doubled.