Too fast vs too slow

Say overall speed is not an issue (you are not racing) and the only objective is to achieve best glide to make it home with a marginal final glide. Also lets assume you fail to maintain the correct speed and deviate +/- 10 knots. Which will have less penalty, flying 10 knots too fast or too slow than best glide? so for example, if best glide is 60 knots, which speed will have better chance to get you home, 70 knots or 50 knots?
Assuming stable air with no wind, but will be interested to hear opinions for other conditions.

Ramy

If you look at the lines on the bottom of this graph:
http://soaringcafe.wpengine.netdna-c...io-Polars1.jpg

You will see the glide ratio plotted versus the speed. For every speed this diagram gives you the glide ratio for that speed.

If you then look at the optimal glide point, that is the point where graphs reach their lowest point (the Y-axis is on the right for these graphs), you see that on both sides of this peak the line
has roughly the same slope. This leads me to conclude that it does not really matter, for this particular aircraft at least.

W dniu środa, 8 maja 2013 10:02:44 UTC+2 użytkownik Roel Baardman napisał:
If you look at the lines on the bottom of this graph: http://soaringcafe.wpengine.netdna-c...io-Polars1.jpg You will see the glide ratio plotted versus the speed. For every speed this diagram gives you the glide ratio for that speed. If you then look at the optimal glide point, that is the point where graphs reach their lowest point (the Y-axis is on the right for these graphs), you see that on both sides of this peak the line has roughly the same slope. This leads me to conclude that it does not really matter, for this particular aircraft at least.

Bad example - compare this polar with real Concordia performance from igc file ;-)

Your opinion would have been correct if the polar was a straight line, but it isn't...

On Tuesday, May 7, 2013 11:58:34 PM UTC-7, Ramy wrote:
Say overall speed is not an issue (you are not racing) and the only objective is to achieve best glide to make it home with a marginal final glide. Also lets assume you fail to maintain the correct speed and deviate +/- 10 knots. Which will have less penalty, flying 10 knots too fast or too slow than best glide? so for example, if best glide is 60 knots, which speed will have better chance to get you home, 70 knots or 50 knots?

Assuming stable air with no wind, but will be interested to hear opinions for other conditions.



Ramy

Folks, it is a trick question. The proper answer is follow Ramy and do whatever he is doing.

Darryl

At least at 70 you'll get to wherever you are going faster

On May 8, 1:58*am, Ramy wrote:
Say overall speed is not an issue (you are not racing) and the only objective is to achieve best glide to make it home with a marginal final glide. Also lets assume you fail to maintain the correct speed and deviate +/- 10 knots. Which will have less penalty, flying 10 knots too fast or too slow than best glide? so for example, if best glide is 60 knots, which speed will have better chance to get you home, 70 knots or 50 knots?
Assuming stable air with no wind, but will be interested to hear opinions for other conditions.

Ramy

You can easily answer the theoretical question visually. Open your
glider manual to the polar page. Draw a line from the origin to the
tangent of the polar. This is the best glide line; the intersection of
straight line and polar gives your best glide speed and sink rate at
that speed. The slope of the line is your glide ratio. Now draw lines
from the origin to a point on the polar 10 knots slower and 10 knots
faster. Those lines represent directly your glide angle, which ever
one is flatter is the better glide angle.

Now to reality... The chance that you are doing this in exactly still
air is pretty small. And if it's perfectly still air, anyone can
actually hit their best glide speed, typically more like 55 dry, no?
So, if you're struggling to make it home, you're likely going through
rising and sinking air. Now, if there were any rising air around,
you'd stop and thermal in it. So that means you're managing to neatly
follow the sink.

So now ask the question, in gentle sink, are you better off at "best
glide" + or - 10 knots? You'll see that the 50 knot (really 45 knot)
answer is a disaster. Unless you have a really good speed to fly vario
set up, knowing that you're in 1 knot sink and what to do about that
will escape you.

So, my 2 cents: when in doubt, fly a little faster, because if you're
not in lift you're in sink, and getting out of sink is important.
Equivalently, fly the Mc 1 speed not the Mc 0 speed if there is any
motion to the air

John Cochrane

Your opinion would have been correct if the polar was a straight line, but it isn't...

I was trying to say that, since the shape of the curve on both sides of the theoretical optimal point is symmetric, it does not really matter. Optimal +10kts gives roughly the same L/D as Optimal
-10kts.


Furthermore, I think John Cochrane hits the nail on the head here (again).

Roel

At 12:29 08 May 2013, Roel Baardman wrote:
Your opinion would have been correct if the polar was a straight line,
but it isn't...

I was trying to say that, since the shape of the curve on both sides of
the
theoretical optimal point is symmetric, it does not really matter.
Optimal
+10kts gives roughly the same L/D as Optimal
-10kts.


Furthermore, I think John Cochrane hits the nail on the head here
(again).

Roel


But the polar curve is not symmetric about the point of intersection with
the tangent that passes through the origin, ie the maximum glide - we're
not talking about min sink here.

Infact, I bet you will achieve a better glide ratio if you fly 10 knots
slower than best glide than 10 knots faster (in still air).

If you're really in still air, just fly at each speed (LD +/- 10 kts) for a minute or so and observe what your glide computer says. Most have a "Instantaneous L/D" display. See which LD is better for your unique situation.

Rob Dunning

On Wednesday, May 8, 2013 2:58:34 AM UTC-4, Ramy wrote:
Say overall speed is not an issue (you are not racing) and the only objective is to achieve best glide to make it home with a marginal final glide. Also lets assume you fail to maintain the correct speed and deviate +/- 10 knots. Which will have less penalty, flying 10 knots too fast or too slow than best glide? so for example, if best glide is 60 knots, which speed will have better chance to get you home, 70 knots or 50 knots?

Assuming stable air with no wind, but will be interested to hear opinions for other conditions.



Ramy

we're not talking about min sink here.

This sums up my error quite nicely. Thanks for correcting me!