The danger of assumption

"Jay Honeck" wrote:
Here I had assumed that she was holding it down too long before rotation --
but it had now become apparent that she (and I) were in fact rotating
prematurely. Wind conditions were calm, temperatures were in the 80s,
humidity was very high, and a ground fog was developing as we landed.
Otherwise, everything was done according to Hoyle, with 2 notches of flaps
set for take-off.

Usually the plane just "flies itself off" the runway in this
configuration -- but not that night. Conditions of flight were fairly
unusual, for us -- the back seat was empty, no wind, high humidity, fairly
light on fuel -- so I suppose it was just pilot error.

Is there a reason why you *shouldn't* rotate at the published rotation
speed? ... more precisely, is there a reason why continuing the takeoff
roll beyond the published rotation speed before lifting off is not safe
(assuming you have enough runway ahead of you)?

In the C152, rotating at the published rotation speed nearly always
produced nice, smooth takeoffs. I recently bought a Shinn/Varga (low
wing, tandem, stick, trike), and in early flights, rotating at the
published rotation speed often produced a couple of seconds of "can't
decide if I'm really ready to take off and start climbing" hesitancy on
the part of the airplane (and yes, everything checked out during run-up).

We then tried letting it continue the takeoff roll beyond published
rotation speed, not really "rotating", but pulling back just enough to
take the weight off the nosewheel and letting it lift off in its own
time. Presto, all the takeoffs have been nice and smooth ever since, and
once it lifts off, there is NO hesitation to begin the climb. How far
beyond published rotation speed it lifts off, using this technique,
varies depending on temp, humidity and weight.

Other than runway length concerns, is there a reason why this technique
would not be preferable to abruptly rotating at the published rotation
speed?

Other than runway length concerns, is there a reason why this technique
would not be preferable to abruptly rotating at the published rotation
speed?

Particularly with low wing airplanes, this can produce a wheelbarrowing
effect where your mains get light, and even lift off, but you are holding
the nosewheel on the ground. Not good for the nose gear and any crosswind
gust could produce some rather interesting and dangerous effects.

Jim

In article ,
"Jim Burns" wrote:

Other than runway length concerns, is there a reason why this technique
would not be preferable to abruptly rotating at the published rotation
speed?

Particularly with low wing airplanes, this can produce a wheelbarrowing
effect where your mains get light, and even lift off, but you are holding
the nosewheel on the ground. Not good for the nose gear and any crosswind
gust could produce some rather interesting and dangerous effects.

With two notches of flaps and two adults in the front seats, your center
of lift was way aft and your center of gravity was way forward. Think
about it.

Jim Burns wrote:
Other than runway length concerns, is there a reason why this technique
would not be preferable to abruptly rotating at the published rotation
speed?

Particularly with low wing airplanes, this can produce a wheelbarrowing
effect where your mains get light, and even lift off, but you are holding
the nosewheel on the ground. Not good for the nose gear and any crosswind
gust could produce some rather interesting and dangerous effects.


I think you missread what he wrote: not rotating is a far cry from holding it
down. When I accelerate in a Cherokee, I hold the yoke neutral until I'm ready
to fly, then rotate and fly off immediately. In a Cessna, I ease the yoke back
once I'm at or beyond stall speed and let it fly off when it's ready.... rolling
on the mains only until it is.

I would agree holding it down is a poor practice.



--
Mortimer Schnerd, RN

VE

What indicated airspeed do you normally rotate at? (no real answer, just
curious) Take off with the twin requires us to hold it onto the runway
until 80mph, we definately do not want to rotate prematurely in case of an
engine failure.

We usually start to rotate (and that's not really what we're doing -- more
like applying mild back pressure) around 70 mph -- but that's not a hard and
fast figure. It all depends on "feel", in my experience -- which, in turn,
is based on weight, wind, temperature, etc.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

"Jay Honeck" wrote

We usually start to rotate (and that's not really what we're doing -- more
like applying mild back pressure) around 70 mph -- but that's not a hard
and
fast figure. It all depends on "feel", in my experience -- which, in
turn,
is based on weight, wind, temperature, etc.

Ground fog forms because the ground is cooling the air near it (due to
radiational cooling) faster than the air further from it. You already knew
that, though.

MY guess what happen, is that after you started to rotate, the warmer air
above started getting pushed down into the cooler air, warmed it up, (or you
got into the warmer air) gave you less lift than the cool ground air, then
you started not lifting so good. Just a guess, though.
--
Jim in NC

It might be worth considering the possibility that something about the
rigging has changed. An extra-careful preflight or even a trip to the
shop may be in order. If someone has backed a truck into it while it
was parked, the damage might not be immediately obvious.

Everything checked out normally before flight.

I've had dragging brakes produce interesting effects when breaking
ground.

Low tire pressure changes things pretty dramatically in our plane.
Take-off performance suffers a surprising amount, for one thing.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

My first thought was that perhaps you have less power than you usually do,
causing you to hang at the rotation speed a little longer than usual. Did
you rotate by the same point as you normally do? Other things: Mag check
normal? Normal RPM on takeoff run?

Yep, everything checked out during preflight and run-up. The JPI EDM-700
engine analyzer showed 6 good bars (meaning all six cylinders were firing
normally) manifold pressure was 26-27, and RPM was nailed right at 2650 or
so at full throttle.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

MY guess what happen, is that after you started to rotate, the warmer air
above started getting pushed down into the cooler air, warmed it up, (or
you
got into the warmer air) gave you less lift than the cool ground air, then
you started not lifting so good. Just a guess, though.

Man, that would require a layer just a few feet thick -- is that possible?

Actually, I know it is, as I've seen it at altitude. I suppose it can form
close to the ground, too, in a calm wind.

Strange how I can study weather in college, I can observe it carefully for
years, and I can spend so much time in the air -- and yet not understand it
very well at all.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jim Burns wrote:
Particularly with low wing airplanes, this can produce a wheelbarrowing
effect where your mains get light, and even lift off, but you are holding
the nosewheel on the ground. Not good for the nose gear and any crosswind
gust could produce some rather interesting and dangerous effects.

"Mortimer Schnerd, RN" wrote:
I think you missread what he wrote: not rotating
is a far cry from holding it down. When I accelerate
in a Cherokee, I hold the yoke neutral until I'm
ready to fly, then rotate and fly off immediately.
In a Cessna, I ease the yoke back once I'm at or
beyond stall speed and let it fly off when it's ready....
rolling on the mains only until it is.

He did misread what *she* wrote. I was not referring to "holding it
down", I meant pulling back *just enough* to get the weight off the
nosewheel (not enough to pull the nosewheel up), letting the nosewheel
and the airplane lift off the runway when it's ready, as you said,
rolling on the mains until it does. Is there a reason (other than runway
length) NOT to do this vs. abruptly rotating it off the runway at the
published rotation speed? The airplane performs better (no second or two
of hesitation before beginning the climb) with the former than with the
latter technique ... yet some pull the airplane up when they see the ASI
reach the published rotation speed even if the airplane doesn't act
ready to begin climbing. What say you?