Three more newbie Qs, if you don't mind :)

"Ramapriya" wrote in message
om...

1. Is there a way of mathematically justifying the dictum that a
successful takeoff is guaranteed if you develop 70% of the desired
thrust in half the runway length? And is this dictum kind of set in
stone or are there riders?

I've never heard it but I don't fly jets. I doubt it can be mathematically
justified. Too many variables.

2. I've heard that you can let an aircraft fly itself off, so to
speak, by lifting the nose early in the takeoff roll to the desired
takeoff attitude.

You're barking up the right tree, at least partially. The Angle of Attack is
the critical element, though for most purposes you can use airspeed instead.
There are three numbers that apply he

Vx: Best angle-of-climb speed: This is the speed at which you gain the most
altitude per unit of distance traveled over the ground. If there's a tall
tree at the end of the runway you need to clear, this is what you want to
use.

Vy: Best rate-of-climb speed: The speed at which you gain the most altitude
per minute. This is both faster and more efficient than Vx, but because it's
shallower you may hit the tree at the end of the runway if you climb at this
speed.

Vr: Rotation speed: This is the speed at which you want to lift the nose off
the runway. If you rotate too soon, you will create drag and actually
lengthen the takeoff roll. If you rotate too late, you lenghten takeoff roll
unnecessarily. This number becomes pretty important on big jets, not as much
on small planes unless you're trying to dig out of a short runway.

There's also the phenomenon of "ground effect." When a wing is within one
wingspan of the ground, it will generate more lift. The result is that you
can get a plane to lift off the ground and fly in ground effect at lower
airspeed than it will fly at. If you try to climb out of ground effect
without sufficient airspeed, the most likely result is that you'll bounce
back down onto the runway. If you wait too long to abort you'll meet that
tree at the end of the runway. However, you can use this to your advantage
in some situations. For instance, if you're taking off from a grass field
(which tends to slow you down more than asphalt) you take off and pop up
into ground effect and then stay there (not climbing) until you accelerate
to Vx and then climb. Because there is less drag up in the air you will take
off in less distance.

Regarding the core question of the airplane "flying itself off" I don't
think this would work in most larger planes, but it would definitely happen
in most light airplanes.

3. Is it possible for a cruising aircraft (say at 35000 feet) to
descend and land without the pilot having to pitch the nose downward
even once? I mean, is it possible to lose altitude by just a
combination of the throttle and flaps? I know it might take a lot
longer to do it this way but is it a theoretical possibility?

This is a pretty complicated question actually. There have been several
incidents where aircraft lost all primary controls and landed with some
success. United 232 (?) is one of the most famous, but the DHL plane shot by
a missile over Baghdad last year also suffered total hydraulic failure and
managed to land on its gear under control. UA232 might have also, but for a
puff of wind at the very end.

A full description of the challenges this situation poses would have to
start by teaching you basic aeronautics. If you are interested in that, I
suggest you read "Stick and Rudder" by Langewiesche, and is written for the
non-pilot. Suffice it here to say that it is a deadly serious challenge,
among the worst situations you can find yourself in short of plummeting
straight towards the ground. But as these crews demonstrated, it is not
hopeless.

-cwk.