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

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?

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.

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?


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?

Cheers,

Ramapriya

Ramapriya wrote:

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.

The aircraft will accelerate most rapidly by keeping the wings nearly level and
keeping the nose or tail wheel off the ground, however, the plane will still
accelerate if the nose is held higher. Consequently, the pilot can raise the nose to
takeoff attitude as soon as the plane is traveling fast enough for the elevators to
work. When the plane is moving fast enough, it will lift off the ground. *If* the
pilot has held the nose at the correct attitude, it will continue to climb.

There are two reasons why this is not usually done, even with light aircraft. The
first is that the aircraft will accelerate better with the nose fairly low. The plane
will use less runway if the nose is keep down until the normal "rotation" point (the
speed at which the nose should be raised). The second is a phenomena called "ground
effect". When very close to the ground (within about 1 wingspan) an aircraft will
climb at a lower speed than it will higher up. A careless pilot who gets the nose too
high may climb well for a couple of wingspans and then find that the plane isn't
going fast enough to keep climbing. In extreme cases, planes have been known to
descend back to the runway again. This is, at best, embarrassing.

Taking off this way is used by some pilots for taking of an aircraft which has a
tailwheel when the winds are strong and blowing from one side (as "crosswind"). This
keeps the tailwheel on the runway, which helps keep the plane straight until it
leaves the ground. I personally don't like doing this.

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?

Pilots usually descend by using only the throttle. Putting flaps down, however,
changes the attitude of the nose even without further pilot input. You will also
probably have to raise the nose to slow down. This can be delayed until the last
minute, but it still must be done before touchdown.

For example, I will cruise at about 120 mph. I will adjust the attitude of the nose
(this is called "trimming" the plane) until the wings are level and set the throttle
at 2600 rpm and stay that way until I want to come down. If I slow the engine down to
2400 rpm, the plane will descend at about 500 feet per minute. The wings will still
be pretty level, and my speed will still be 120 mph. This is not like a car, where
slowing the engine slows the car.

Now, let's say there's a runway in front of me and I decide to fly straight in and
land. I can descend just by slowing the engine down, but I can't touch down in my
plane at 120 mph. Lowering flaps will slow me down and raise the nose a bit, but it
also increases lift, so I have to slow the engine down even more to descend, and it
won't slow me down enough or raise the nose enough. I know from experience that I
will have to raise the nose to slow the plane enough to land safely. If I want a
smooth landing, I will either raise the nose even more in the last few feet or add a
bit of power to slow the descent (or both).

Now. In theory, a pilot could land some aircraft without touching the trim, but the
plane would touch down much faster than it is designed to do. This would abuse and
possibly blow the tires. It would also require much more runway than usual. With some
aircraft, it might also result in the nosewheel touching down first, with possibly
disastrous consequences.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

"G.R. Patterson III" wrote

Snip

The second is a phenomena called "ground
effect". When very close to the ground (within about 1 wingspan) an
aircraft will
climb at a lower speed than it will higher up. A careless pilot who gets
the nose too
high may climb well for a couple of wingspans and then find that the plane
isn't
going fast enough to keep climbing. In extreme cases, planes have been
known to
descend back to the runway again. This is, at best, embarrassing.
Snip

George Patterson


Anybody have a clue what would be happening, if an Airliner took off,
settled back to the runway, and took off again? Mis-calculated rotation
speed? Seems hard to understand, to me.

Recently happened to a niece of mine, but not a knowledgeable flyer, so no
more details to be had.

Any confessions out there? ;-)
--
Jim in NC


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Anybody have a clue what would be happening, if an Airliner took off,
settled back to the runway, and took off again? Mis-calculated rotation
speed? Seems hard to understand, to me.

Recently happened to a niece of mine, but not a knowledgeable flyer, so no
more details to be had.

Any confessions out there? ;-)
--
Jim in NC

I have had one wing get picked up by strong cw, the wing lifted off the
ground then set back down. That would have to be a strong cw for that to
happen to an airliner.

In article , Morgans wrote:

Anybody have a clue what would be happening, if an Airliner took off,
settled back to the runway, and took off again? Mis-calculated rotation
speed? Seems hard to understand, to me.

I think you are asking how this could happen. One way I can think of is if the
wind died partway down the runway. Another would be some sort of boo boo
as you speculated. Rotated too soon and decided to lower the nose to pick
up speed, for instance.

I've taken off twice in one run down the field before, but I think it was always
when I was trying to get off of a wet field or some other high-friction condition.
Not in an airliner, of course....

Mike Beede

In article ,
G.R. Patterson III wrote:
The aircraft will accelerate most rapidly by keeping the wings nearly level and
keeping the nose or tail wheel off the ground, however, the plane will still
accelerate if the nose is held higher.

There are some jets that will rotate into a high drag configuration and
never gain enough speed to fly. I remember reading about an accident where
an inexperienced pilot (maybe a new owner of ex-Soviet equipment?) ran off
the end of the runway like that.

--
Ben Jackson

http://www.ben.com/

"Ben Jackson" wrote in message news:45Vid.468979$mD.64699@attbi_s02...
In article ,
G.R. Patterson III wrote:
The aircraft will accelerate most rapidly by keeping the wings nearly level and
keeping the nose or tail wheel off the ground, however, the plane will still
accelerate if the nose is held higher.

There are some jets that will rotate into a high drag configuration and
never gain enough speed to fly. I remember reading about an accident where
an inexperienced pilot (maybe a new owner of ex-Soviet equipment?) ran off
the end of the runway like that.

--
Ben Jackson

http://www.ben.com/

F-86, Sacramento, CA, quite a while ago...

"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

In article , Peter Duniho wrote:

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.

At least ones with constant-speed props. It seems to me that you get
much better thrust at low speed with a CS prop. My understanding,
which may be defective, is that at low speed much of the fixed-pitch
prop is stalled. I'd like to see a plot of airspeed vs. thrust for this, but
don't have any idea where to look for one. Any suggestions?

Mike Beede

"Mike Beede" wrote in message
...
At least ones with constant-speed props. It seems to me that you get
much better thrust at low speed with a CS prop. My understanding,
which may be defective, is that at low speed much of the fixed-pitch
prop is stalled.

My answer was intentionally oversimplifying the issue. It is true that prop
efficiency and total thrust generated depends not only on engine power
(which itself depends on RPM, which may be limited with a fixed prop
installation), but airspeed as well.

But over the course of an entire takeoff run, assuming thrust remains
constant as a first approximation is perfectly reasonable, especially for
the purposes of the question asked.

Pete