SR- 71/ Blackbird lore

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.)

"Keith Willshaw" wrote in message
...

"Jack G" wrote in message
...
Only in a vacuum would both "glide" at the same speed and go the same
distance - I think.
Jack



In this case you are probably right but there is some truth in the
assertion.

No, my idiot.

(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!

(Walt BJ) wrote:

Glide distance - if you take the same airplane, it will glide farther
if it's heavier.

I have a lot of trouble with that statement Walt...perhaps I'm
slow but I cannot see that at all.

I mean normal load ranges from empty to MTOGW, not
'balsa' and 'lead'. The reason is that there is more potential energy
(energy of height) in the heavier-loaded one. Yes, the heavier bird
does have to glide faster to maintain best L/D. This is depicted in
the engines-out glide charts in the performance manuals of jet
airliners. I gave my manuals away when I retired but trust me, it's
there.
Walt BJ

Yes Walt, I can see that the heavier a/c would have more energy
(potential energy?) but wouldn't it lose that advantage by the
fact that It's heavier and therefore require more energy to carry
that extra weight?...just sounds like 'getting something for
nothing' otherwise. IOW, if an a/c will 'glide farther' when it's
heavy then why wouldn't it 'fly further' when it's heavy under
power?. And it sure won't.

I'm not trying to be argumentative, I just can't grasp it...
--

-Gord.

Thinking of the question from a different angle - the wing needs a range
of speeds to give good performance (lift versus drag) and the weight
dictates how much lift, and therefore speed, will be needed. An
extremely light load will need less speed to create enough lift, but
below a certain speed, the wing's drag vs lift will get in the way, and
though it is going very slowly, both forward and down, the glide ratio
may not be all that good. As weight increases, the speed necessary
increases as well, and there is a best combination of speed vs lift vs
drag for best glide ratio. Excessive weight will demand yet more speed,
but drag increases as well, so the glide ratio suffers. Our theoretical
balsa versus lead U2 might, ironically, have not dissimilar glide
ratios, while the real thing at an intermediate weight, have the better ratio.

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?

Ron Parsons wrote:


Back in the last century when as students we used to have to do
simulated flameout approaches to fields down near the Rio Grande with
Cessna T-37's, it was quickly discovered that if you dived at a field
that you were too close to, you would end up overshooting but if you
pulled the nose up almost to a stall, you could mush down at a great
angle.

But that doesn't make sense Ron...you certainly would end up at
the field with too much airspeed but why would you 'overshoot'
the field?...hell you could 'pointer straight down' and you
certainly wouldn't 'overshoot the field' (although you'd be going
like a scalded cat - and may need to 'overshoot' for that reason)
--

-Gord.

In article ,
"Gord Beaman" ) wrote:

Ron Parsons wrote:


Back in the last century when as students we used to have to do
simulated flameout approaches to fields down near the Rio Grande with
Cessna T-37's, it was quickly discovered that if you dived at a field
that you were too close to, you would end up overshooting but if you
pulled the nose up almost to a stall, you could mush down at a great
angle.

But that doesn't make sense Ron...you certainly would end up at
the field with too much airspeed but why would you 'overshoot'
the field?...hell you could 'pointer straight down' and you
certainly wouldn't 'overshoot the field' (although you'd be going
like a scalded cat - and may need to 'overshoot' for that reason)

Never been to the Rio Grande Valley I presume. All those nice flat
fields have palm trees as a fence row. Granted you could make a vertical
hole, but the point is to get over the near palms, get on the ground and
stop before you arrive at the far palms.

This was in the days when Primary was conducted by civilian instructors,
most of whom expected you to brush tall grass with your wheels on such
approaches and then go around.

My instructor at the time was a bit more than a civilian. He taught us
in the T-34 and T-37 and was still finishing up a few stragglers in the
T-28. He was also in the Texas ANG where he was an instructor in the
T-33, flew the F-86D and was qualifying in the F-102. He used to say,
"Just don't ask me any numbers. "The only time he ever touched the stick
with me was on the last day when he took the bird for a few minutes on
the way home. One of the smoothest I've ever ridden with.

--
Ron