A few notes on weight are in order.

First of all, it's important to distinguish between weight and inertia
(yaw inertia in particular).  Inertia depends on where the weight is
added.  Let's look at weight-related issues in order of priority.


1.  Yaw inertia.  This is the #1 killer of handling quality of a
rudder/elevator glider.  Every effort should be made to keep the tips
and tail light. This is the percentagewise breakdown of yaw inertia
on the Allegro-Lite:

   44    mid  panels
   24    tip panels
   10    center panel
    7    rudder
    6    stab
    4    boom + rods
    2    RX battery
    1    fuselage pod + radio
    1    18g nose weight
  ---
  100%

Each gram (weight of $1 bill) added at the wingtip or tail will
increase yaw inertia by about 1%.  Roll rate will decrease by 1%, and
Dutch roll damping will worsen.  It doesn't take many grams out there
to really clobber the airplane's handling.

Adding 1 gram to the mid panels or tailboom will increase yaw inertia
by about 0.3%.  Not quite as severe but still a concern.

Adding weight to the fuselage or nose has no significant effect on yaw
inertia.  It will actually improve things a bit, since the
inertia/mass ratio (which is what really counts) will get smaller.

A larger vertical tail can overcome a larger yaw inertia, but it has
its own drawbacks from the added tail weight and drag.  Larger rudder
sideforces also cause larger sideways translation in the "wrong" 
direction before the bank kicks in, making spot landings more difficult.
It's best to nip the problem at the source by keeping yaw inerta
as small as possible.


2.  Total weight (turning).  The most severe drawback of added weight
is an increased minimum turn radius.  Or equivalently, an increased
bank angle for a given turn radius.  Increasing weight by 1% also
increases the minimum turn radius by 1%.  This is a major concern when 
scraping for lift at low altitude.  HLG flying suffers the most from
excess total weight.


3.  Total weight (electric version climb rate).  Adding 1% to the
total weight reduces climb rate by just over 1%.  The electric A-Lite
has a good but not exceptional climb rate, so keeping the total weight
down is important.


4.  Total weight (sink rate).  Adding weight increases sink rate, 
butimproves L/D via larger Reynolds number.  As a rule of thumb,
increasing the weight by 5% increases the sink rate by 2% and
increases the max L/D by 0.5%.  The sink rate increase from excess
weight is only a concern on weak days.


5.  Pitch inertia.  This is dominated by the weight of the tail group.
This is a relatively minor concern on the A-Lite, since the pitch
damping is already exceptionally good.  Even doubling the pitch
inertia would not worsen the handling all that much.  An overweight
tail is more of a problem via the additional required noseweight and
increased total weight (items 2,3,4).


Bottom lines:  

* Put your weight saving efforts into the tips and tail.  

* HLG and electric fliers should be also very concerned with total
weight.


- Mark
October 18, 2000