From: HAHLLUND@finabo.abo.fi (Hans Ahllund Inf)
Subject: Best wing-loading ?(Was: Speed to Fly.....)
Organization: Abo Akademi University, Finland
Date: Thu, 10 Sep 1992 14:41:06 GMT
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In <BuC4q4.BMM@usenet.ucs.indiana.edu> petri@venus.iucf.indiana.edu writes:

> " well yes that is all very good, but you know that the correct speed to fly
> is only about 5% of the total equation. The other 95% is choosing the correct
> path over the ground!"

Always remember this when you read the following....
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Okay ! 
Here`s something more to chew on that I have been trying to figure out lately.
Especially for those who are short of programming tasks.

*How do you figure out exactly what your optimum wing-loading would *
*be for a given set of meterological conditions ?                   *

I have given it some thought , trying to create a simple rule of thumb.
My own theory (may it be good or not) ,is valid for only one type of
aircraft, the one that I mostly use, (Centrair 101A Pegase). It`s a 
standard class ship roughly equal to LS-4, that can hold 120 litres
of water making the maximum wing-loading about 44 kg/m2, and minimum
loading about 33 kg/m2.

Here goes !!!
This is based on the McCready theory of best speed between thermals.
Thermals can be either large( bank angle <30 deg.) or they can be 
tight (bank angle >40 and <50 deg.).
The average lift that you get is always in a fully loaded glider.
Here is what you do to always have optimum wing loading loading.
Large thermals :
If average lift is below 1 m/s (maximum loading) ,you let that much
water out that you start getting 1m/s of lift. 
(the differece in climb for a empty versus loaded glider is according
 to my calculations ; heavy 1 m/s  , empty 1.2 - 1-3 m/s )
Tight thermals :
IF average lift is above 1.5 m/s you should have full wing-loading.
With average lift under 1.1 m/s, fly dry.
Average lift 1-1.5 m/s is the critical part. Rule is this .
 Reduce water by 30 litres for every .1 m/s that lift is below 1.5 m/s.
 ( difference in climb in a tight bank (empty/heavy ship) is 0.4m/s)

The differences in average speed if you have a completely wrongly loaded
(too light or too heavy) would be 7 % maximum. This sounds a bit small
to me. 

What is not taken into conscideration here is what patterns of lift you
are able to use. Are the thermals wide apart, so that you will be lower
than you want to be if flying an empty ship. You don`t lose that much
altitude in glides with a heavier ship. 
Or if there are lots of thermals closly , or maybe cloud-streets.
In that case you would like to have a lighter ship so you can gain
a lot of height in straight flight at small speed.
The variables are infinite.

How about wind then?  You would logically think that a strong wind demands
for lots of water, but I found out with a couple of calculations that with
a given wind through the whole task , the average speed proportionally stays 
the same with with maximum and minimum wing loading .That is, no increase
in average speed if you fly with a heavier ship than my theory suggests in
a stronger wind.
A different situation would be if you fly heavy in headwind and light in
downwind.(you could calculate how heavy and how light ?)

Then another thing. Should you increase inter-thermaling speed when you fly
against a headwind, and decrease speed in downwind, when using McCready, as 
you do in final-glides ?
If this is the case then my previous wind calculation is trash and the 
conclusion would be that a heavier ship is better in strong wind.
But then there is the aspect that thermals are more irregular in strong
wind, favouring a lighter ship. Also in long glides against the wind you
tend to get very low, not so low with a heavier ship.

You may think that all this is just bullshit, you either fly in good
weather or in bad. In good water , bad no water, but if you start
timing your average lift every time, (including entering,centering and
leaving a thermal), you will find out that most of the flying, even in
fairly strong conditions is done with lift between 1-1.5 m/s, and this
is the critical section of choosing the right wing-loading.           

This theory of mine has not yet been tested in practice, because our
flying season is already finished for this year.
Anyway, the gain in flying at exactly the best wing loading (especially
with a ship that only holds 120 litres) is so small according to my
theory that it makes almost no difference, taking also in account that
you have to fly with a ship that acts a bit unfamiliar to you.
But in contest there is a lot of psychology going on, so by knowing
that you use the ship`s abilities to the fullest may give you a 
psychological edge over other contestants.

Standard class ships are ,what I think , more sensitive with choosing
the optimum wing-loading. Gliders with flaps always are better in climb
when they are heavy.

Think about this and email me your opinions on the subject.
Maybe I am totally lost in my theory, I don`t know.

                                             Hans Ahllund
                                             glider&tow-pilot
                                             Comp.sci student
                                             Turku Finland.
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