"Ramapriya" wrote in message
om...
I'm encouraged by your non-disparaging response to my first posting
here yesterday. I have a few more Qs that will look utterly idiotic to
you guys -- but remember that I'm not a pilot

You need to go take a flying lesson. Anyone with this much interest in
airplanes ought to be thinking about being a pilot.

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?

First, thrust during takeoff is relatively constant, from the beginning of
the takeoff roll, to actually leaving the runway. This is more true for
jets, but is reasonably close to the truth even for propeller-driven
airplanes.

As far as "guaranteeing" a takeoff, sure...given a particular airplane,
engine power, runway characteristics, air temperature and density, etc. you
can calculate the distance required to takeoff. Compare that to the runway
length itself, and that will tell you whether you can take off.

Of course, mechanical failure, sudden change in wind, that sort of thing can
screw up the calculations. But theoretically, yes...it's easy to calculate
whether an airplane can take off or not.

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.

Most airplanes will "fly itself off" even without lifting the nose early.
Climb rate is a result of excess thrust, beyond that required to counteract
drag. For any given configuration of the airplane, there is a particular
airspeed that the airplane will "want" to fly (this can be adjusted by the
pilot using "elevator trim"). Once reaching this speed, the nose will pitch
up on its own, and any additional thrust not required to maintain that speed
will be used to climb.

Lifting the nose early may slow down the takeoff by increasing drag, but as
long as there's enough power (which would be true most of the time, provided
the nose isn't raised *too* much), the airplane will still eventually
accelerate to the given climb speed and take off.

Raising the nose during the takeoff roll is a common practice when using
unpaved runways, to help protect the nosewheel and even the propeller.
Usually the nose isn't really raised, so much as the weight is lifted from
the nosewheel. But it's basically the same idea.

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?

Sure. Just as a climb is a result of excess thrust, a descent is a result
of insufficient thrust. If engine power is reduced below that required to
maintain the trimmed airspeed in level flight, the airplane will descend,
taking energy from gravity to make up for the difference.

In fact, there has been at least one accident I'm aware of in which the
pilot became incapacitated (from carbon monoxide poisoning), ran out of gas
and the airplane simply glided to a landing in the middle of a soy bean
field. Minor damage to the airplane, and if I recall correctly the pilot
did eventually recover from the CO poisoning (he wasn't injured in the
landing).

As far as it being "a lot longer to do it this way", that's not actually
true. Well, it is compared to not reducing power, but it's not compared to
normal descent practices. The *primary* way airplanes descend is by control
of engine power, because without a power reduction, an airplane will usually
wind up faster than is safe during the descent. Generally, we'll reduce the
power enough to keep the airspeed as high as possible while still being
safe, but for many airplanes (and especially when the air is not perfectly
smooth) that airspeed is not much higher than the basic cruise airspeed.

Hope that helps.

Pete