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

[Ramapriya]

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

[G.R. Patterson III]

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.

[Peter Duniho]

"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

[C Kingsbury]

"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 here:

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.

[Will Robinson]

Take a demonstration flight at a flight school near you. An inexpensive way
to get a lot of your questions answered.

-0-

"Ramapriya" > wrote in message
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 :)
>

[Morgans]

"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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[Greg Butler]

> 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.

[Ben Jackson]

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/

[Judah]

(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...

[Ramapriya]

Er... actually I'm from India, where flying is affordable to a very
small section of the populace, and I've never been in that category.
Poverty hasn't helped keep my interest for flying down, although it
hasn't helped in making that actually happen :\

Also, I'm 37 and 64 inches tall, both prohibitive minuses to even
think of flying lessons, from what I've heard. One guy (a pilot) had
said I wouldn't be able to reach the rudder pedal with my short
stature, making me wonder if planes don't have adjustable seats like
cars do ;)

Ramapriya



"Will Robinson" > wrote in message news:<awPid.25$mL1.15@trnddc08>...
> Take a demonstration flight at a flight school near you. An inexpensive way
> to get a lot of your questions answered.

[Peter Duniho]

"Ramapriya" > wrote in message
om...
> Also, I'm 37 and 64 inches tall, both prohibitive minuses to even
> think of flying lessons, from what I've heard.

When I first read that sentence, I thought you were 101 inches tall. That
definitely would cause problems. :)

However, being 37 certainly has nothing to do with taking flying lessons (it
might keep you out of an airliner cockpit, that's about all), and your
height, while admittedly lower than average, is no shorter than many others
who pilot airplanes. Most airplanes do have adjustable seats, some other
airplanes have fixed position seats with adjustable rudder pedals, and not
all airplanes are created equal. Just as with cars, some are better suited
to smaller pilots, while others are better suited to larger pilots.

Even in a plane where you don't fit right off the bat, there are ways to
work around the issue. I've heard of at least one pilot wearing what amount
to platform shoes, for example.

Anyway, access to affordable training sounds like the biggest impediment,
and for all I know in India that's enough to prevent you from learning to
fly. But certainly nothing about your age or height would.

Pete

[Andrew Sarangan]

(Ramapriya) wrote in
om:

> Er... actually I'm from India, where flying is affordable to a very
> small section of the populace, and I've never been in that category.
> Poverty hasn't helped keep my interest for flying down, although it
> hasn't helped in making that actually happen :\
>
> Also, I'm 37 and 64 inches tall, both prohibitive minuses to even
> think of flying lessons, from what I've heard. One guy (a pilot) had
> said I wouldn't be able to reach the rudder pedal with my short
> stature, making me wonder if planes don't have adjustable seats like
> cars do ;)
>
> Ramapriya
>
>
>

I don't think 5'4" is a problem. I have taught students who were about that
height. You can pull the seats forward, but the rudder is at a fixed depth
behind the panel, and if you pull too far forward your face will be too
close to the panel. You may also face a problem with seeing over the panel.
For that you may need a seat cushion.

37 is not old. Some have started that late and even gone on to airline
jobs, but admittedly that is not very common. There are plenty of people
who took their first lesson past 30 and went on to earn all the ratings and
are very active in aviation. In fact, I would suspect that the majority of
GA pilots started in their 30's or later.


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[Blueskies]

"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...

[Blanche]

Ramapriya > wrote:
>Er... actually I'm from India, where flying is affordable to a very
>small section of the populace, and I've never been in that category.
>Poverty hasn't helped keep my interest for flying down, although it
>hasn't helped in making that actually happen :\
>
>Also, I'm 37 and 64 inches tall, both prohibitive minuses to even
>think of flying lessons, from what I've heard. One guy (a pilot) had
>said I wouldn't be able to reach the rudder pedal with my short
>stature, making me wonder if planes don't have adjustable seats like
>cars do ;)

64 inches --- so what? Lots of shorter people fly. I'm only 66 in
and fly with no problem -- pillows or telephone books!

37 years old -- so what? We're not talking flying for a major airline
in the US, just aviation knowledge.

You can probably finds books about aerodynamics and aviation in
a school library. Is there an airport nearby? Talk to people there
about working/helping.

[G.R. Patterson III]

Ramapriya wrote:
>
> Also, I'm 37 and 64 inches tall, both prohibitive minuses to even
> think of flying lessons, from what I've heard.

In this country, you could still manage to have a career of ten years or more as an
airline pilot, with a bit of luck. You are definitely not too old to learn to fly.
While there are a few aircraft in which you would have trouble reaching the rudder
pedals (mine is one), you would have no problems in many aircraft. They also make
extensions for the pedals for very short people, and these could be mounted in
aircraft like mine, which would let you fly them.

I have read that flying is very expensive in India, however, and there's no way
around that. Better move.

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

[Mike Beede]

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

[Mike Beede]

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

[Peter Duniho]

"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