Fastest way home?

Tony: you clipped off part of the formula but you did point me to the right section which is half the battle; so thanks.

I would summarize the rule like this: The scored time element for determining speed when undertime is 90% based on min time and 10% based on time on course. I didn't see anything about that rule changing for guys that are more than 15 minutes under time as Steve Leonard suggested.

The formula has a provision that further reduces the scored time element if the pilot has flown more than 85% of the maximum task distance. In that case the scale slides between 10% / 90% at and below the 85% threshold to 100% / 0% for the pilot that flies to the very back of every turn area. The effect then is that you are entirely scored based on time on course as long as you go to the very back of all the turn areas. This clearly is intended to keep a valid competition going in the case that the CD has called too short of a task for the prescribed minimum time.

By typing out this summary, maybe I'll be able to remember it for a couple weeks anyway.

JJ: Sorry, I suppose we hijacked your original question. Anyone able to respond to JJ's original question should please do so.

On Tuesday, June 18, 2013 9:38:45 PM UTC-5, Steve Koerner wrote:
Tony: you clipped off part of the formula but you did point me to the right section which is half the battle; so thanks.



I would summarize the rule like this: The scored time element for determining speed when undertime is 90% based on min time and 10% based on time on course. I didn't see anything about that rule changing for guys that are more than 15 minutes under time as Steve Leonard suggested.



The formula has a provision that further reduces the scored time element if the pilot has flown more than 85% of the maximum task distance. In that case the scale slides between 10% / 90% at and below the 85% threshold to 100% / 0% for the pilot that flies to the very back of every turn area. The effect then is that you are entirely scored based on time on course as long as you go to the very back of all the turn areas. This clearly is intended to keep a valid competition going in the case that the CD has called too short of a task for the prescribed minimum time.



By typing out this summary, maybe I'll be able to remember it for a couple weeks anyway.



JJ: Sorry, I suppose we hijacked your original question. Anyone able to respond to JJ's original question should please do so.

just in case it matters, my partial copy/paste job was from the 2013 regional sports class rules. I don't know if the Nationals and/or FAI classes use the same formula (they probably do)

Surprise, surprise, I've worked this one out. Lots of free time in the winter I guess...

To restate the question: you're finishing up a flight. You have excess altitude so you're going to be over time. You have a turn area ahead of you or choice of MAT turnpoints, so you can either fly fast but for less distance or short for more distance. What's the optimum?

Think of it as a classic final glide question. But instead of "how do I use up the altitude I have and just make the distance to the finish gate," now the question is "how do I use up the altitude I have and go around one more turn before going to the finish gate?"

It's obviously not either extreme. If you point the nose at the ground and fly VNE, you will add a few miles very fast, but it won't help your average speed much.

And clearly floating along at best LD, 53 knots, won't help your speed if you've been doing 80 all day. So, the optimum is in between. What is it?

Answer: First, figure out the MacCready setting that, in classic climb and glide mode, produces the average speed you have achieved so far. (Intersection of tangent line to the horizontal axis of the polar.) For example in a dry ASW27, in 3 knot thermals, climb and glide MacCready theory says you cruise at 80kts and achieve 54 mph average. So, if you have averaged 54 mph from start to now -- no matter how you did it -- the MacCready setting we're looking for is 3. You do carry around a little card of these numbers, right? (I keep pestering cleanav to put them in but they keep saying things like "John, nobody but you wants this stuff!")

Now, the final glide rule: use a MacCready setting on your final glide equal to this MacCready setting. If you achieved 54 mph from start to now, fly the final glide at MacCready 3, or about 80 knots in a dry ASW27. Set off at 80 knots, turn the glide computer to 3.0, and turn the final turn area when it says you will just make it home.

This is the simple rule to remember, and it's interesting. Classic final glide theory, where you only get to pick how high to climb in the last thermal, says equate the MacCready setting on the way home and the climb rate of the last thermal. Final glide theory when you can set the length of the final glide says to use the MacCready setting on the way home equal to what produced the average speed so far. You would now stay in the last thermal so long as it is stronger than the MacCready setting corresponding to your average speed so far, and then set out at that setting.

If you need a simpler rule, it would be "keep doin' what you've been doin' all along."

The actual formula has an extra term: The real formula is
MacCready setting on final glide = Setting that produces average speed so far +
+ current height / time on course so far.

So you do fly a little bit faster. For most tasks the last term is small. If you're at 6000 feet after a 3 hour task, it's 6000 / (3 x 60) == 33 fpm or 0.3 knots. So the real rule is to fly the final glide at MacCready 3.3, not MacCready 3.0. But we all push final glides a bit anyway, and this isn't a big difference. The accurate formula is more important to answer r.a..s. ers who will say "what if you're at 10,000 feet after a 5 minute task? Then, yes, H/T is huge and you point the nose at the ground and go VNE.

Quiz time: Tuesday June 25. Location: Hobbs NM.

John Cochrane

PS: Here is the table for an ASW 27. Ach is the achieved speed in classic climb and glide flight. stf is the classic speed to fly in knots.

|-----Dry ---||----Wet -----|
Mc stf Ach stf Ach
Kt Kt mph Kt mph
0 56 0 65 0
1 65 32 78 32
2 73 45 89 49
3 80 54 99 59
4 86 61 108 68
6 98 71 124 81
8 108 79 138 92


So, MacCready 3 is 80 knot cruise dry, 99 knot wet, and averages 54 mph dry and 59 mph wet.

If you're at Hobbs (say) ad you have achieved 68 mph so far, then you finish at Mc 4 and 108 knots. And if you've achieved 81 mph so far....

This is all just the very flat polar of modern gliders. Nobody flies that fast, which is a puzzle for another day

John Cochrane

On Wednesday, June 19, 2013 3:35:29 PM UTC-6, wrote:
PS: Here is the table for an ASW 27. Ach is the achieved speed in classic climb and glide flight. stf is the classic speed to fly in knots.



|-----Dry ---||----Wet -----|

Mc stf Ach stf Ach

Kt Kt mph Kt mph

0 56 0 65 0

1 65 32 78 32

2 73 45 89 49

3 80 54 99 59

4 86 61 108 68

6 98 71 124 81

8 108 79 138 92





So, MacCready 3 is 80 knot cruise dry, 99 knot wet, and averages 54 mph dry and 59 mph wet.



If you're at Hobbs (say) ad you have achieved 68 mph so far, then you finish at Mc 4 and 108 knots. And if you've achieved 81 mph so far....



This is all just the very flat polar of modern gliders. Nobody flies that fast, which is a puzzle for another day



John Cochrane

Very nice of you John.

Thank you.

Mike

quiz answer - Start of the 2013 15 Meter shoot out at the Hobbs corral. Great group of pilots. Same for the Open Class and regionals.

Here is hoping for good weather.

Rooting for the UT gun slingers

On Wednesday, June 19, 2013 2:28:57 PM UTC-7, wrote:

So you do fly a little bit faster. For most tasks the last term is small. If you're at 6000 feet after a 3 hour task, it's 6000 / (3 x 60) == 33 fpm or 0.3 knots. So the real rule is to fly the final glide at MacCready 3.3, not MacCready 3.0. But we all push final glides a bit anyway, and this isn't a big difference. The accurate formula is more important to answer r..a.s. ers who will say "what if you're at 10,000 feet after a 5 minute task? Then, yes, H/T is huge and you point the nose at the ground and go VNE.

John Cochrane

Thanks John - great grounding back the the original theory. I was going to answer JJ along similar lines until I realized that his calculation was correct. But his calculation told him to not do what he'd been doing, but rather to burn off the altitude by going home at a faster stf than he'd been flying all day. RASers might ask "why is that?".

The answer is that he'd been cruising too slow for the theory all day. I don't have the polar for JJ's awesome bat-plane, but just using your table for the -27, if JJ had been achieving 70 mph then his STF should be 98 mph, so perversely, speeding up the final glide is the right thing to do since he'd been cruising at 80 knots all day. By 90 knots is still less than 98 knots so any final glide less than 98 knots still increases average speed on task versus slower final glide speeds.

The nice thing about the final glide is it's the "equalizer" with respect to McCready theory since your achieved cross-country speed up to the decision point is an absolute determinant as to whether accelerating your final glide will be accretive or dilutive to your speed on task. I think the right procedure here is probably to look at your average speed on task, figure out what is the McCready setting that corresponds to that task speed, then set your glide computer to that McCready and extend or contract the task length (subject to MinTime) to arrive exactly at minimum finish altitude.

9B

In an AAT points are awarded for speed. Assuming you are always over time:

If you fly further at your average so far you will fly further but no
faster.

If you fly straight to the finish but end up high you have wasted time /
distance and therefore finish at less than potential speed.

So I conclude that flying further only helps if it is done at a
considerably higher speed than your average so far.

My calcs for a 3hr task are as follows:

Scenario is 5000ft left with 50k to go. First 160 mins at 100kph average.

Fly to the finish at 55kts = 100kph 30 mins 100kph average
Fly to finish at 80kts = 150kph 20 mins 105kph average

If you had sufficient height to fly the glide at 120kts = 225kph 13 mins
giving 109Kph average.

Using this height to fly as far as you can at 80kts would not improve this
average speed enough to warrant the additional risk.

For me I would just belt for the finish as flying further introduces risk
of losing time through bad air, or worse a landout etc.

On Thursday, June 20, 2013 4:57:40 AM UTC-5, Jim White wrote:
In an AAT points are awarded for speed. Assuming you are always over time:



If you fly further at your average so far you will fly further but no

faster.



If you fly straight to the finish but end up high you have wasted time /

distance and therefore finish at less than potential speed.



So I conclude that flying further only helps if it is done at a

considerably higher speed than your average so far.



My calcs for a 3hr task are as follows:



Scenario is 5000ft left with 50k to go. First 160 mins at 100kph average.



Fly to the finish at 55kts = 100kph 30 mins 100kph average

Fly to finish at 80kts = 150kph 20 mins 105kph average



If you had sufficient height to fly the glide at 120kts = 225kph 13 mins

giving 109Kph average.



Using this height to fly as far as you can at 80kts would not improve this

average speed enough to warrant the additional risk.



For me I would just belt for the finish as flying further introduces risk
of losing time through bad air, or worse a landout etc.

Jim,
A little harsh, but maybe that's why you don't win contests and BB does...

On Thursday, June 20, 2013 1:36:14 AM UTC-5, wrote:
Some snippage...The answer is that he'd been cruising too slow for the theory
all day.
9B

Ah, but theory assumes dead air between climbs. If you cruise in air that is going up, you can average considerably higher speeds than what MC theory says you should be able to achieve for the climb rate you get when you stop to circle.

JJ's question was on a MAT, not a TAT. Difference between them is that you can exactly tune distance on a TAT, where on a MAT, you are step functions to next available turnpoitn on your way home. Also, you will note that JJ said he was already above glide when he discovered he had a final glide decision to make. So, he didn't intentionally climb to that postion, he probably got there cruising under good clouds, making good higher than MC ground speed for the climbs he was taking. But, if it was one of the best days he has seen in a long time, I will agree with you 9B, that he might have had his MC set too low and been cruising slower than optimum for conditions.

As to how to handle that situation on an MAT, as long as your ground speed on the final glide is higher than your achieved speed to that point, you are going to have a very hard time convincing me that you can fly faster overall by flying slower for longer.

Steve Leonard
Long time scoresheet fodder, but sometimes not. Oh, and my best ever daily finish was on a MAT day during the US Open Nationals, 2006. I was just a touch under minimum time.