SR- 71/ Blackbird lore

On Tue, 29 Jul 2003 01:19:30 GMT, "Jack G"
wrote:

I am trying to make sure I understand this concept so please humor me.

Clarification: Glide Ratio is the distance traveled forward for each
equivalent distance traveled vertically. Is Glide Angle the same thing but
measured in degrees?

If two aerodynamically identical gliders (but one being heavier) are
launched in a straight line from the same vertical distance with the same
horizontal force, they will both arrive at the same horizontal distance from
the launch point, but the heavier one will take less time to get there.

This will be true only if the lift generated can sustain a glide ratio of
0:1


This is where I get confused:

Does the lift required to sustain a glide ratio 0:1 increase as the weight
of the glider increases?

Your help IS appreciated!

Jack


Glide angle, as I use the term, is the angle of the glide relative to
the horizon.

Yes, the lift force required to sustain an aircraft in un-accelerated
flight is proportional to the gross weight of the aircraft. For
instance, if 300 lbs of water ballast is added to a glider then the
lift required to maintain a given, un-accelerated glide ratio, or
glide angle, is increased as required to offset the additional 300
lbs.

Since the glider's wing size and shape are not altered simply by
the additional water ballast, and the wing now must produce additional
lift to maintain the additional 300 lbs, the glider must fly faster in
order to produce the additional lift force required.

In a way perhaps, this is kind of good news and bad news depending
on what the pilot wants. With the additional weight, the glider MUST
fly faster at a given glide ratio, on the other hand, the glider thus
CAN fly faster at a given glide ratio. Remember, the glide ratio
itself is not altered (significantly) by the added weight.

A glider pilot will want a glider that CAN / MUST fly faster at a
given glide ratio if he/she wants to cover distance quickly,
such as in a competition. However, the additional weight of the water
ballast will require stronger thermals for the glider to gain or
sustain altitude. If the thermals are weaker than the pilot had hoped
for he/she can dump the additional water ballast to lighten the wing
loading so as to make use of the weaker thermals -- but thus giving up
the ability to fly faster.

All this can confuse me too, and I often have to think it through
again.

One other note; with the added weight of the water ballast the glider
CAN / MUST fly faster at a given glide ratio, which is good for
covering distance quickly but the faster speed will put the glider on
the ground faster if no thermals or other lift sources are found.

Have I managed to make any sense?