Crossing layers of air?

[chongas]

This is a question for the DSexperts .
When crossing from the still air mass into the moving air mass at the top of the ridge line, at what angle to the wind would say produces the most energy to the sailplane? And is the angle different when passing back through at the bottom of the hill?

See the attached diagram for discussion.

Consequently, I am a engineering student working on my senior project (the study of the aerodynamics of dynamic soaring) and trying to gather information on the subject. I am also a super newbi to the sport and fly on the front side of the hill, combat, unless I get knocked to the backside, heehee.

Thanks
Cullen

[chongas]

attaching file

[Daemon]

I can't make sense of your diagram.
I never cross any boundary layer like that when DSing. We cross the boundary layer parallel to the wind direction, to minimize drag and the effects of turbulence, then pull either above it, or below it.

The wind is blowing straight back over the top of the ridge perpendicular to the face. eg.
http://www.houseofthud.com/rc/ds1.gif

Note that at the transition point.. the glider is pointed straight into the wind.

And a side view..
http://www.houseofthud.com/rc/ds2.gif

Note the green transition point, and also note the rotor lines coming back up the backside. The best DS is when you fly the glider back to the rotor and gain some energy there, so that the full circuit is like two areas of wind blowing toward each other with a calm space in between.

Ok.. now the side view shows that the loop is tilt down over the back, thus you hit the wind coming over the top at an angle..
Here's a closeup..
http://www.houseofthud.com/rc/ds3.gif

If you're asking what that angle should be, it depends 100% on the conditions and the shape of the hill. The goal is to keep the entire loop almost parallel to the shear boundary and just pop up above it on the front, and below it on the back, so that when you pull up on the frontside, the wings are perpendicular to the wind. BUT. The shear layer is very turbulent so you have to rise higher, and dip deeper sometimes which increases the angle of the loop. Also when thermals come through they raise the entire shear boundary higher, and post thermal they can push it down flat.. even eliminate the calm area on the back side for a while thus making it un-DSable.

Also, if you just dip below the shear you may be able to DS, but you'll get more speed if you bounce off the rotor, which often means digging much deeper down the back side flying parallel to the slope.

Ok, so starting with that.. What was your question again?

ian

[chongas]

You answered my question. Based on your top view drawing it looks like when the glider passes from the still to moving air, the glider is pretty much headed directly into the wind. Once in the moving air, the gilder continues its turn (180 degrees) heading down wind and then crosses back into the still air (which will actually be a head wind with respect to the glider). Is this correct?

Another question: After entering into the moving air from still air, about how far around the 180 degree turn would you say the glider is accelerating?

Can you elaborate more on using the rotor. I am probably wrong, but it looks like once re-entering into the still air if you continue further down the hill the glider can cross into another mass of moving air (the rotor). Is this correct?

Thanks for your help
Cullen

[bjaffee]

Chongos, sounds like you got the idea.

Where and when the glider accelerates, or at least where you can see it accelerate, probably depends more on the conditions and the hill then something else. Sometime the glider will just smoothly accelerate, and other time you will get a noticable "ping" when you cross the boundry and you will see/feel (by the control feel) the glider's energy go up quite quickly.

[Daemon]

Ya you got it.
The glider heads as straight into the wind when heading for the front as it can, then you pull back and convert as much of your newfound airspeed (you gain airspeed passing from the still air to the moving air), into ground speed. When you pass into the still air you convert your increased ground speed into airspeed, then make a turn trying to lose as little as possible (that's the glider design's job actually), and repeat.

And yes, if you aim for the right spot and dive deep enough you'll find the rotor air coming back toward the top of the hill. If you go from the still into the rotor and then pull back, you gain even *more* airspeed/groundspeed before passing back through the still air headed for the frontside, so you pick up energy at both ends of the ellipsoidal loop. Practically speaking, none of the air is really smooth, or still, or blowing in any totally predictable direction, so finding the ever changing DS groove is the skill you have to develop to extract the most energy from the hill.

Lastly, the thing you've got to remember with regard to "headwinds" is that to a glider *everything* is a headwind (except in very extreme turbulence). Only the angle of attack changes slightly as you pass from one moving airmass to another. As you round a turn in the still air, the glider sees say, an 80mph headwind at say.. a 3 degree angle of attack. As it passes into the moving air over the top, it may jump up to a 100mph headwind at 1 degree angle of attack.. etc..

Today I was DSing this ridge.
http://www.houseofthud.com/rc/zion-pan4-2.jpg
in very variable and mild winds. I'm pretty sure a crunchie would have wound up pretty good, but my Bluto just couldn't accelerate beyond a certain speed. It reaches a point where the drag from the turns costs as much airspeed as the gain on the front and it just finds a speed beyond which it won't go. And I never did find the rotor. When I saw the turkey vultures cruising along at eye level, 50 yards back behind the slope, I realized why.

ian

[Ade]

Hi,

I find there are a few things to look out for, Crossing the boundary as close to the "source" (where the flow breaks away from the hill) as possible means crossing it where it is crisper and less turbulent gives more power.

The other thing is making the bottom turn as close as you can to the boundary means you are fighting gravity less round the bottom turn. This means you are putting less strain on the model and less drag because you need less force to get round the turn.

The last thing is the size of the circle. Fly as small as you can. 2 things govern this. The model and the boundary. The model will start to feel draggy when you get too small or it will fall apart :-) The boundary is site/conditions dependant. Some days the seperation will be so wide that you have to fly quite large in order to cross fully from the moving air to the dead air.

Ade