Fastest way home?

At 13:19 20 June 2013, wrote:
On Thursday, June 20, 2013 4:57:40 AM UTC-5, Jim White wrote:
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...

Herb, I think you are being unfair. Since your post I have done the calcs
in a spreadsheet (the raw math is too difficult for my little brain).

In the situation I pose BB can fly average better than me at all wing
loadings but here are some examples:

ASW27 50k to go 50k/m wing loading
BB flies at 70kts for 24 minutes avg 103.49kph
I fly a few knots faster striaght to the finish 103.36kph

ASW27 40k to go 50k/m wing loading
BB flies at 85kts for 15.6 minutes average 104.57
I fly a few knots faster 15.3 minutes average 104.47

ASW27 30k to go 50k/m wing loading
BB flies at 90kts for 13.48 minutes average 104.63
I fly at 100kts for 9.73 minutes average 104.36

In all case BB beats me but by less than 0.3% or about 1 point

I have never flown against BB so don't know whether he would beat me on the
day.

Steve Leonard makes the point in support of my suggestion that you do not
fly through still air so BB flying longer has the opportunity to gain more
energy allowing him to go further still. I will too which allows me to fly
faster still.

If you want to check the spreadsheet send me an email and I will forward it
to you to play with.

Jim

On Thursday, June 20, 2013 2:57:40 AM UTC-7, 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.

Okay Jim, it took me a little while to figure out what you were trying to do. Here's my take on your example.

We are assuming a MAT where there are some additional turn options. Steve is right that the distance increments are discrete rather than continuous as in a TAT, but let's assume that there are reasonable options available and you want to pick the optimal one.

The first issue I have with your example is that you are simply calculating for speed over a fixed final glide distance, but you have the option to extend your task distance under a MAT so flying slower and finishing really high hardly seems like the right alternative to test.

Allow me to demonstrate why there is an optimal answer that is equal to the McCready speed corresponding the the average speed achieved up to the final glide.

For your example (using an ASW-27 polar in this case) the McCready setting for 62 mph (100kph) is 4.2 kts. This corresponds to an 88 kt cruise speed. With your 5000' above finish height you can go 27.1 miles at 88 kts in 18.5 minutes, yielding a task speed of 66.2 mph.

If you fly slower at 78 kts, from 5000' above the finish you can go 32.4 miles in 24.9 min. for an average task speed of 65.8 mph. or about a half mile per hour slower, worth about 6 points.

If you fly faster at, say, 98 kts, you can fly 22.6 miles in 13.9 minutes for an average task speed of 66.1 mph.

At 68 kts you can go 37.1 miles and average 64.9 miles
At 108 kts you can go 19.1 miles at 66.0 mph.

The optimal curves are pretty flat and it is slightly less disadvantageous to fly x knots too fast than x knots too slow.

9B

On Thursday, June 20, 2013 9:51:55 PM UTC-7, wrote:

Steve - it is possible to make a better task speed by slowing down in this example. To recap, on a 4.3 kt McCready day when you are averaging 62 mph in cross country speed, you will achieve a better overall task speed by slowing down from any final glide speed in excess of 88 kts and speeding up from any glide speed less than 88 kts. This is simply because the polar is a quadratic curve. The summary table follows.

- At 68 kts you can fly 37.1 miles for a task speed of 64.9 mph.
- At 78 kts, you can go 32.4 miles for a task speed of 65.8 mph.
- At 88 kts, you can go 27.1 miles for a task speed of 66.2 mph.
- At 98 kts, you can fly 22.6 miles for a task speed of 66.1 mph.
- At 108 kts you can fly 19.1 miles for a task speed of 66.0 mph.

9B

In the UK we fly an AAT where you can turn anywhere you like in defined
areas. I am not sure which US task type this maps onto.

It is received wisdom that you should use surplus height to fly further
into the last area before turning but the spreadsheet turns up a surprise:
you don't gain much unless flying straight home fast brings you in under
time.

On reflection I would slightly modify my rule of thumb such:

If it is a strong day and you have surplus height at a time that allows you
to belt home without taking a time penalty, fly a final glide that takes
you a little further into the area before turning for the finish at the
strong MacCready setting. You would gain a few extra points.

If it is a weaker day (3kts) then use your surplus energy to fly straight
home as fast as possible consistent with getting there.

It is only worth going for the extra point by extending the distance if you
can still guarantee getting home. Flying a long final glide with a low Mc
is taking unecessary risk.

Jim

9B...I have sent you the SS by email.

On Friday, June 21, 2013 1:51:00 AM UTC-7, Jim White wrote:
In the UK we fly an AAT where you can turn anywhere you like in defined
areas. I am not sure which US task type this maps onto.

Jim,

AAT=TAT. The OP was flying a long MAT, which is a US task type where the pilots fly to an assigned sequence of turnpoints (each with a 1 sm turn cylinder). If you are still under time after flying all assigned turns you can add additional turnpoints of your choosing. For the purposes of this discussion MAT and TAT (or AAT) are roughly equivalent, the only difference being that under a MAT the distance increments you can add are not infinitely variable as in the TAT but rather come in discrete chunks. Generally there enough close in turnpoint options to use up whatever altitude you have and if you hit the finish cylinder first you even have an insurance policy of a provisional finish in the event you land out while adding distance (you mention this risk later).

You rule of thumb makes sense to me.

Ideally you set the Mc for final glide at the Mc that corresponds to the task speed that you have achieved. However there is a lower limit if you want to mange landout risk. I rarely set my final glide for less than Mc = 4. There are only a handful of points at stake from flying faster than optimal McCready and coming up short on a final glide is very costly should you get even small amount of unanticipated sink on a very flat final glide.

9B...I have sent you the SS by email.

Never got the SS

9B

A simpler summary.

First, don't think speed, think MacCready setting. The basic principle is, finish the flight at the MacCready setting that generates your average speed so far.

So, you adapt to lift and sink accordingly. If you have averaged Mc 3 so far, and you run in to 4 knots lift, take it and extend the flight some more. As long as you find Mc 4 thermals and cruise accordingly you are going to increase your average speed.

Yes, the tops of hills are flat. And this does not include the risks of landout. All that shades toward higher and faster.

John Cochrane BB

For us knuckle-draggers that can't quite wrap our brain cells around MacCready speeds in real time, I thought that the simple way to look at it was: If going over time is going to raise your average speed (i.e. a beautiful cloud street shows up at the end of a slow day) then extend as long as it is helping your average speed. Otherwise, finish just above min time on a normal (fading) day.

Without a handy chart of XC speed vs MC settings for my glider, all I got is my SN10's handy Average Speed display, which is what I use to decide whether I can make the number get bigger or smaller.

Ouch - math in the cockpit!

Kirk
66

On Friday, June 21, 2013 2:30:30 PM UTC-4, kirk.stant wrote:
For us knuckle-draggers that can't quite wrap our brain cells around MacCready speeds in real time, I thought that the simple way to look at it was: If going over time is going to raise your average speed (i.e. a beautiful cloud street shows up at the end of a slow day) then extend as long as it is helping your average speed. Otherwise, finish just above min time on a normal (fading) day. Without a handy chart of XC speed vs MC settings for my glider, all I got is my SN10's handy Average Speed display, which is what I use to decide whether I can make the number get bigger or smaller. Ouch - math in the cockpit! Kirk 66

Knuckle Draggers!
I (rep)resent that remark.
LOL
This is a good description of what I do, even if I don't beat BB all that much.
UH

OK, time to come clean. I don't do a lot of math in the cockpit either. This kind of exercise is useful as a general guide -- no, if you're over time it is not a good idea to fly off 14000' at VNE. And it's useful to adapt to unusual circumstances. But in practice flying in good air is more important than anything else, and I spend most of my energy looking out the window to figure out where the good air is and the gliders aren't.

John Cochrane

On Friday, June 21, 2013 11:30:30 AM UTC-7, kirk.stant wrote:
For us knuckle-draggers that can't quite wrap our brain cells around MacCready speeds in real time, I thought that the simple way to look at it was: If going over time is going to raise your average speed (i.e. a beautiful cloud street shows up at the end of a slow day) then extend as long as it is helping your average speed. Otherwise, finish just above min time on a normal (fading) day.

That's funny Kirk - the goal is to ensure that your knuckles don't drag on final glide!

The answer to your question is yes.

The way you manage it is with the McCready dial on your SN-10 and the turnpoint position (for a TAT), or turnpoint selection (for a MAT). You set the McCready to the setting that corresponds to the task speed achieved so far and adjust the final glide distance to have the arrival height you want. If you are in a cloud street, keep flying as long as you can maintain that final glide speed and are above your minimum arrival height (or think you can reach it without slowing down).

It does help to have a version of John's table to guide you, because in my experience I tend to cruise on course at a lot lower speed than the recommended McCready STF so I need to judge versus the optimal glide speed, not the one I've been flying to stay higher, follow lift bands, etc.

If you don't know how to make a table, send me three points off your LS-6 polar (dry), your max water ballast capacity and I'll email you one you can print out.

9B