Cessna 172....Pitches for Vx and Vy
Sushidot
Just wanted to know generally what pitch on takeoff I should expect,
generally, for Vx and Vy, on the AI?
Thanks
SD
Robert M. Gary
The pitch won't be extreme. I don't teach students to climb based on
pitch because the pitch you choose is too dependant on current
conditions. Pitch for airspeed. Too many pilots don't make it off the
runway at high density altitude simply because they try to ask the
plane for a pitch it can't do.
-Robert, CFII
Barry
> pitch because the pitch you choose is too dependant on current
> conditions. Pitch for airspeed. Too many pilots don't make it off the
> runway at high density altitude simply because they try to ask the
> plane for a pitch it can't do.
Here's another opinion. I teach that the pitch should always be set to some
desired attitude (using position of the nose if flying visually, or the
attitude indicator if on instruments) and that the airspeed indicator should
serve only to confirm the desired performance. I find that pilots who set
pitch using the airspeed tend to have more trouble flying smoothly. This is
especially evident just after takeoff (pitch attitude oscillates as the pilot
chases the airspeed, which takes a few seconds to stabilize). Note also that
airspeed indicators can give faulty readings (water inside the pitot, for
example) and I think that this poses an additional risk for pilots who rely
heavily on airspeed for pitch control.
I'd like to hear from other instructors on this issue.
Robert M. Gary
I don't necessarily disagree with you. However, I teach at the foot of
the Sierra Nevada mountains. My number one concern with regard to
pitch is that students can handle a variety of density altitudes. I'm
willing to make things a bit harder for them in return for them
understanding that its IAS that makes the plane climb.
-Robert
Andrew Sarangan
That would greatly vary with density altitude. Even at a fixed
altitude, AI is not going to give you accurate pitch information for
Vx and Vy. What's wrong with using the ASI?
WingFlaps
I'm not an instructor but the idea of reaching Vx by reference to AI
sounds potentially dangerous to me. Surely, the AI is no proxy for AOA
and that is the most important factor at take off.
Cheers
WingFlaps
. I'm
> willing to make things a bit harder for them in return for them
> understanding that its IAS that makes the plane climb.
>
\Why is that so hard to undertsand/teach? Steady climb is only a
function of spare thrust (i.e. trust beyond that needed to overcome
drag)....
Cheers
Dudley Henriques
I don't think this distinction is the issue here; at least I'm not
reading this from the post you are answering.
One thing that might need a bit of "clearing up" in this thread is
whether the OP meant airspeed indicator or attitude indicator when he
wrote (AI). Normally, AI means attitude indicator, and ASI means
airspeed indicator when written out as acronyms. Since both instruments
have been mentioned as a pitch reference in this discussion, the
distinction should be made perfectly clear.
Getting back to the discussion;
I'm not an advocate of over concentration on instruments (considering
non IFR operations of course) during takeoff and rotation to a climb
attitude. This is a critical transition phase, especially involving high
density altitude takeoffs. This doesn't mean no concentration but rather
the normal procedure using visual cues combined with instrument
peripheral checks for verification. Visual cues are prime, instrument
cues verify as in any normal VFR takeoff.
I like pilots to KNOW the numbers for the aircraft/density altitude
equation that will be present on their intended takeoffs and fly those
numbers.
On high density altitude takeoffs, the pilot should be looking for the
predetermined indicated airspeed needed for Vx or Vy peripherally as
with any other takeoff. Rotation is made on that basis and the airplane
placed at a SAFE nose attitude as the desired climb airspeed is reached.
I've never found the chasing of airspeed due to lag to be an issue in
pilots taught in this manner.
Every instructor has their individual preferences for handling this
scenario, especially those teaching at high density altitude airports.
I don't fault instructors who concentrate heavily on the rotation to
climb attitude issue when involved with density altitude.
Actually, I've found in more than one instance that some instructors
teaching at sea level airports don't deal with the high density
operations enough.
--
Dudley Henriques
WingFlaps
My gut feeling is that high altitude is never going to be a problem if
you rotate at the correct speed and climb out of ground effect once Vx
is reached. This is not the same as setting pitch for climb, which
could lead to a departure stall -right?
Cheers
WingFlaps
>
> I like pilots to KNOW the numbers for the aircraft/density altitude
> equation that will be present on their intended takeoffs and fly those
> numbers.
> On high density altitude takeoffs, the pilot should be looking for the
> predetermined indicated airspeed needed for Vx or Vy peripherally as
> with any other takeoff.
The only numbers I can think of are are take off and landing distances
(maybe static RPM too). (ASI Vx and Vy don't change) but what other
numbers should I know?
Cheers
Dudley Henriques
ALWAYS, the most gradual transition allowed by the conditions is fine.
You work the airplane UP to it's climb speed, you never try to nail it
cold with the initial pitch attitude in rotation. Doing it by moving the
nose up with an initial attitude below desired, then adjusting it
accordingly should eliminate any tendency toward a departure stall.
--
Dudley Henriques
Dudley Henriques
Well, you want all the numbers for the takeoff that influence the
density altitude calculations for that specific takeoff at that exact
time and temperature.
Also, recheck your assumption that Vx and Vy IAS doesn't change with
altitude. There are in fact IAS changes in both these figures with
altitude increase.
IAS for Vx increases slightly with altitude and IAS for Vy decreases
with altitude. They will meet at the same point which should be the
absolute ceiling for the aircraft.
--
Dudley Henriques
WingFlaps
> WingFlaps wrote:
> > On Mar 31, 1:49 am, Dudley Henriques <dhenriq...@rcn.com> wrote:
>
> >> I like pilots to KNOW the numbers for the aircraft/density altitude
> >> equation that will be present on their intended takeoffs and fly those
> >> numbers.
> >> On high density altitude takeoffs, the pilot should be looking for the
> >> predetermined indicated airspeed needed for Vx or Vy peripherally as
> >> with any other takeoff.
>
> > The only numbers I can think of are are take off and landing distances
> > (maybe static RPM too). (ASI Vx and Vy don't change) but what other
> > numbers should I know?
>
> > Cheers
>
> Well, you want all the numbers for the takeoff that influence the
> density altitude calculations for that specific takeoff at that exact
> time and temperature.
> Also, recheck your assumption that Vx and Vy IAS doesn't change with
> altitude. There are in fact IAS changes in both these figures with
> altitude increase.
Interesting. Not listed in my POH, where can I find them? My
assumption about Vx and Vy was based on the idea that the Pitot
measures kinetic energy in the air stream and hence lift
availability... This not one of those funny compressibility things
that you jet jockies get to see is it?
Cheers
Dudley Henriques
Most of the older POH's show a Vx and Vy figured for GW at SL. Some
modern POH's show the difference adjusted for density altitude . The new
format is a great help in estimating an adjusted climb schedule.
What actually effects Vx and Vy are that they are a function of excess
thrust (power). The speed and angle of attack for Vy is dependent on
maximum excess thrust horsepower (excess power) and thus decreases with
altitude. Vx, our best angle of climb speed, is dependent on maximum
excess thrust (excess force). Vx increases with altitude
A Rule of Thumb for estimating the decrease in Vy with altitude is to
reduce indicated airspeed by 1% or 2 knots for each 1000’ increase in
altitude above sea level, excluding the first 1000’. For Vx, a good Rule
of Thumb is to increase indicated airspeed by ½% or 1/2 knot per 1000’
or, more simply, 1 knot per 2000’ increase in altitude.
There are good density altitude charts available that will help you
compute what you need to know to be safe on high altitude operations.
--
Dudley Henriques
WingFlaps
> or, more simply, 1 knot per 2000' increase in altitude.
>
> There are good density altitude charts available that will help you
> compute what you need to know to be safe on high altitude operations.
>
Cheers
Dudley Henriques
Anytime. My pleasure.
--
Dudley Henriques