(Ramapriya) wrote in
om:

Hi all,

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

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?

Successful takeoff is never guaranteed... But specifically to your dictum,
if your throttle setting is not high enough to create adequate lift, you
may get 100% of the selected thrust and never get off the ground.

More seriously, though, since takeoff distance is affected by other factors
besides thrust, such as wind speed and direction, angle of attack of the
control surfaces, runway terrain and condition, trim position, etc. I can't
imagine that your dictum, which by the way I never heard before, is set in
stone.

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. To a non-pilot like me, it's intriguing how this can
be possible. I know that plane manufacturers prescribe takeoff flap
settings, which means that there's gotta be some predetermined angle
of the wing with reference to the horizontal that'll give the aircraft
an optimal kind of lift at some speed enough to make it afloat and
keep it afloat. How then would increasing this wing angle, which is
what would happen by an early nose-lift, help? If at all, I feel it'll
get the craft airborne without enough speed to sustain itself,
whereupon it should start descending before too long... I could be
completely wrong in the way I'm thinking here but would love to hear
how this principle works.

Your intrigue comes, IMHO, from a lack of understanding of the power curve
and of the relationship between pitch and power how they are controled.
This is even more pronounced in your third question.

For the answer to this question, though, look to the Elevator Trim tab,
which attaches to the elevator and essentially stabilizes the elevator at a
certain attitude, which produces a specifc speed based on the power
setting. Basically, the Elevator Trim tab is like a cruise control. The
plane will constantly seek out the set speed - if the plane is flying more
slowly than that speed, the nose will start to fall and the plane will go
faster. If the plane is flying faster than that speed, the nose will start
to pitch up and the plane will start to slow down. If left on its own, the
plane will sort of bobble up and down until it finally levels off at the
set speed...

On the ground, part of the pre-takeoff checklist includes setting the trim
tab to the takeoff position, which is marked on the trim control, and
generally sets the trim tab to about climbout speed at full power. If the
plane were in the air already flying at that trim setting and full power,
the plane would be climbing in a climb attitude at climbout speed. As the
plane speeds up on the runway, then, and the airspeed increases past
climbout speed, the nose of the plane will pitch up to slow the plane back
down to climbout speed. This will essentially start to lift the plane off
the groud all by itself. In fact, no pulling back on the yoke is necessary
to take the plane off the ground if the trim tab is set properly. It will
do it all by itself if you can keep the plane on the runway.

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?

A plane can cut power completely, pitch the nose up, descend in a stall all
the way to the ground, and do this at a very aggressive rate. In fact, a
plane can have full power, pitch way up, and be descending in a stall.
Without the proper angle of attack on the wing, the wing doesn't produce
enough lift to carry the weight of the plane, and the plane falls. Even
with thrust,though, the airspeed relative to the angle of attack of the
wing is what really impacts lift.

You may be stifling laughs by now at these but I hope to get better in
the days to come through such Qs... not wrong to hope, is it?

If you want to get better, I have a better idea - just go to your local
airport Fixed Based Operator and make an appointment for a discovery
flight. If you're not sure where to start to look, try www.beapilot.com .
They will give you a certificate for a $50 intro flight, and help you find
the nearest flight school...

It's great to talk about it in a newsgroup, it doesn't compare to the
first-hand experience...