Speed triangle on ridge

I've got a FAI triangle set up where 2 of the legs are on ridges and the 3rd leg will be a down wind leg. Will it be fastest to climb in a thermal to just high enough to fly back to the ridge at min. sink, or would it be faster to thermal higher and fly back faster? Would I set the MacCready setting to the climb I'm achieving in the thermal or what?

Boggs

On Sunday, November 17, 2013 10:57:25 AM UTC-5, Waveguru wrote:
I've got a FAI triangle set up where 2 of the legs are on ridges and the 3rd leg will be a down wind leg. Will it be fastest to climb in a thermal to just high enough to fly back to the ridge at min. sink, or would it be faster to thermal higher and fly back faster? Would I set the MacCready setting to the climb I'm achieving in the thermal or what?



Boggs

That's probably not as easy as it sounds. Classic McCready theory would say to climb at whatever rate you get at or near the downwind turnpiont, then fly that setting back to the ridge. Not knowing the geometry/geography of the task, I'm assuming this means you would have to approach the ridge from the lee side and cross back over it to get to the windward side to close the triangle? So, you'll be dealing with potentially strong sink and the risk of smacking into the back side of a very hard object, so you'll probably need to need to start by adding significant safety margins to clear the ridge and account for sink.

All turnpoints are on the windward side of a ridge. I would probably mark a thermal coming off the ridge near the start, start the task and go back to the thermal and get high enough to make the second turn. ( in a perfect world ) These are bluffs with flat tops so not a lot of sink downwind of the face.

I'm not totally clear on the geometry, but if you are trying to get to a ridge it is perfectly classic final glide MacCready theory. Set a ridgetop point as your final glide point, then fly to it as if final glide. It is much faster to climb higher in a strong thermal and bomb to the ridge than it is to stop soon and go to the ridge slow. For example, if your best glide is 53 kts, and you're in a 53 kt wind but have 10 kt thermals, going higher and faster will get you there, flying 53 kts will not. Same downwind.

John Cochrane

You would need a modified MacReady calculation that takes account of the
tail-wind as, unlike the next thermal, the ridge isn't drifting with the
wind (or at least you hope it isn't, there's been some pretty extreme
weather lately). Likelt result would be somewhere between normal MacReady
setting for the rate of climb and best glide speed.

At 15:57 17 November 2013, Waveguru wrote:
I've got a FAI triangle set up where 2 of the legs are on ridges and the
3r=
d leg will be a down wind leg. Will it be fastest to climb in a thermal
to=
just high enough to fly back to the ridge at min. sink, or would it be
fas=
ter to thermal higher and fly back faster? Would I set the MacCready
setti=
ng to the climb I'm achieving in the thermal or what?

Boggs

Surprised there aren't more replies to this. The fastest way through the
air (and hence over the ground) is to fly MacCready cruise speed for the
actual climb rate. Assuming you can achieve a faster speed on the ridge
than the thermal phase of the flight, then climb until at the correct
MacCready speed you can reach the ridge at or above the top. (Halfway up if
you are feeling brave, but with a downwind middle leg, a little safety
height won't make much difference.

At 15:57 17 November 2013, Waveguru wrote:
I've got a FAI triangle set up where 2 of the legs are on ridges and the
3rd leg will be a down wind leg. Will it be fastest to climb in a thermal
to just high enough to fly back to the ridge at min. sink, or would it be
faster to thermal higher and fly back faster? Would I set the MacCready
setting to the climb I'm achieving in the thermal or what?

Boggs

No and yes.



At 15:57 17 November 2013, Waveguru wrote:
I've got a FAI triangle set up where 2 of the legs are on ridges and the
3r=
d leg will be a down wind leg. Will it be fastest to climb in a thermal
to=
just high enough to fly back to the ridge at min. sink, or would it be
fas=
ter to thermal higher and fly back faster? Would I set the MacCready
setti=
ng to the climb I'm achieving in the thermal or what?

Boggs

And the 1% rule applies so on a 100K triangle, I can finish 1K lower than I start, right?

Boggs