SR- 71/ Blackbird lore

On 28 Jul 2003 20:43:34 GMT, (B2431) wrote:

A U-2 constructed of lead would have the same glide ratio as one
constructed of balsa wood. It would glide faster, but just as far.
vince norris

Say what?
Dave

Um, I think there's be a slight difference in wing loading which just might
have a minor affect on glide ratio. Translation: the lead U-2 would glide like
a bowling ball.

Dan, U. S. Air Force, retired

What a wonderful question. Glider pilots often add water ballast, 200
- 300 lbs I think, to their gliders to increase airspeed at a GIVEN
glide angle, thus improving the glider's penetration through the air
at that glide angle. The extra weight does not alter the glide angle
(except for very small improvements due to higher Reynolds numbers at
the higher airspeed). The glider's descent rate is increased by the
extra weight, but the airspeed is also equivalently increased so the
glide angle remains (pretty nearly) constant, glide angle being a
factor of only lift and drag, weight not even being in the
calculation.

As for a lead U-2 vs a balsa wood U-2, the lead U-2 would certainly
glide as long as its wing loading was such that its wing could supply
sufficient lift to sustain equilibrium in steady flight. The "catch"
here is probably the inclination to picture the lead U-2 and the balsa
U-2 as the same size. In such a case, if the lead U-2 were the same
size as the gliding balsa U-2 I expect the lead one would do as you
so delightfully describe, and glide "like a bowling ball".

Jim wrote:

On 28 Jul 2003 20:43:34 GMT, (B2431) wrote:

A U-2 constructed of lead would have the same glide ratio as one
constructed of balsa wood. It would glide faster, but just as far.
vince norris

Say what?
Dave

Um, I think there's be a slight difference in wing loading which just might
have a minor affect on glide ratio. Translation: the lead U-2 would glide like
a bowling ball.

Dan, U. S. Air Force, retired

What a wonderful question. Glider pilots often add water ballast, 200
- 300 lbs I think, to their gliders to increase airspeed at a GIVEN
glide angle, thus improving the glider's penetration through the air
at that glide angle. The extra weight does not alter the glide angle
(except for very small improvements due to higher Reynolds numbers at
the higher airspeed). The glider's descent rate is increased by the
extra weight, but the airspeed is also equivalently increased so the
glide angle remains (pretty nearly) constant, glide angle being a
factor of only lift and drag, weight not even being in the
calculation.

As for a lead U-2 vs a balsa wood U-2, the lead U-2 would certainly
glide as long as its wing loading was such that its wing could supply
sufficient lift to sustain equilibrium in steady flight. The "catch"
here is probably the inclination to picture the lead U-2 and the balsa
U-2 as the same size. In such a case, if the lead U-2 were the same
size as the gliding balsa U-2 I expect the lead one would do as you
so delightfully describe, and glide "like a bowling ball".

But why would you go changing things?...if you're gonna do that
then you could have bigger wings, you could fit leading edge
slats, you could do lots to give one or the other the advantage.

I think that the only way to do these comparisons is to have big
ballast tanks fitted over the CG then play with weight.

Perhaps I'm getting old and slow but I'm having trouble seeing
how a glider (or an aircraft) can glide ...better?...
(farther/faster) when it's heavier.

I really don't follow your description about adding ballast
(above) at all...?

--

-Gord.

Curious that no one has mentioned aspect ratio in the discussion. I guess I
should have included that in my original "lead-covered rock" post.



Ed
"Those who have long enjoyed such privileges as we enjoy forget in time that
men have died to win them."

Franklin D. Roosevelt, Address for Bill of Rights Day
15 Dec 1941

(Delete text after dot com for e-mail reply.)

(snip)

I realize this has nothing much to do with this discussion of glide ratio
but you've just dug up a memory/question.
A couple instances back around Iraq #I when I was out running in
the Palo Alto Baylands when I would pretty much stop and watch in
awe as a U-2R2 would take off from Ames Moffett about three
miles away. The aircraft would fly a perfect straight line in pretty
much an up-westerly direction until I would lose the speck in a
perfectly clear sky directly overhead. Yes, I did the 360 deg.
twist to verify the directly overhead part. Always wondered
where it may be heading.
JK (recalling the great old sights and sounds living near Moffett)

Jim wrote in message . ..
On 28 Jul 2003 20:43:34 GMT, (B2431) wrote:

A U-2 constructed of lead would have the same glide ratio as one
constructed of balsa wood. It would glide faster, but just as far.
vince norris

Say what?
Dave


Within limits, weight has no effect on glide ratio (or Lift/Drag,
L/D). As has been mentioned, the speed that the plane achieves that
glide ratio goes up, by a function of the increase of the wing
loading. My glider (an LS6-b, a racing 15 meter glider) carries over
300 lbs of water for strong days - which increases my wingloading from
about 7.5 psf to over 10 psf. Wet, my average cross country speeds go
up about 10 mph (this is when lift is strong and consistent). My sink
rate goes up a bit, but in strong thermals (average acheived rate of
climb of 400fpm or more) the glide speed increase is worth it.

And of course, when the lift gets weak, we dump the water and creep in
- at up to 40/1, in my case!. Using a conservative 30/1 L/D, you can
go a LONG way from 17,999ft!

There is also a small theoretical increase in performance due to the
increases Reynolds number at the higher speed, but this is perhaps .5
to 1 L/D point.

Basic aerodynamics, guys!

Kirk
Phantoms Phorever, but real planes don't need no stinkin engines!