Aviation Forums

Cecil Chapman · Guest

Nope, not finished with the Commercial yet,,, planes and weather are not
cooperating (our fbo REALLY needs another complex plane ((we only have one,
and it keeps showing up in maintenance))).

So, as most of you know,, I'm studying for my CFI written since it really
doesn't matter if I take it 'early'. I've been studying for a good few
months now and am trying to do my best on the written. I think I will also
take the BGI (since I have my Instructor's ground certificate,,,, I
figure,,, why not?).

Anyway, there is one concept that is being presented in one of my (King)
courses that I am not 'getting',,, looking for help here:

So according to my King study material, the best rate of climb speed
DECREASES (!???) approximately 8/10's of a knot for each one-thousand feet
of altitude gained. So the best rate of climb at a higher altitude, say
10,000 feet is LOWER than it is at sea level..... WHY??????

The latter REALLY confuses me - For example: We fly indicated airspeed at
altitude even though our true airspeed at altitude is higher/faster and our
ground speed (in no wind) will be higher, since the plane is having to
travel faster to get the same amount of molecular stuff over the wing and
prop.... So,,,,, why should the best rate of climb speed be LOWER at
altitude,,, shouldn't it be a HIGHER value; after all, at altitude the air
is 'thinner', the prop and wing need more of the molecular stuff to keep
aloft... So why this thing with the best rate of climb speed...?????

Help>????

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Good Flights!

Cecil
PP-ASEL-IA
Student - CP-ASEL

Check out my personal flying adventures from my first flight to the
checkride AND the continuing adventures beyond!
Complete with pictures and text at: www.bayareapilot.com

"I fly because it releases my mind from the tyranny of petty things."
- Antoine de Saint-Exupery -

"We who fly, do so for the love of flying. We are alive in the air with
this miracle that lies in our hands and beneath our feet"
- Cecil Day Lewis -

Marc J. Zeitlin · Guest

Cecil Chapman

Jim Burns · Guest

Think about this question from the point of view of your own flying
experience.

You know that rate of climb is dependent upon excess HP. What happens to HP
when density altitude goes up? HP goes down, right?

So you take off on a standard day, lots of excess HP and your Arrow climbs
at Vy like a home sick angel right? Say, 1000fpm even. You keep climbing
and climbing, feeding in more MP to maintain that excess HP so you can keep
climbing. At some point you have full throttle, max MP, and then MP starts
to go down as you continue to climb. What happened? Density altitude
increased to the point that your engine simply can't put out it's maximum
power with so little air. Notice your climb rate? You're not climbing at
1000 fpm anymore either, even though you maintained your climb airspeed at
Vy for sea level. As you keep holding that original Vy speed, your rate of
climb as indicated by your VSI decreases.

You level off and let your airspeed build then center calls and wants you up
another 1000 feet. You've already got max MP for that altitude, so all you
can do is pitch up. You have to replace the vertical lift lost with the
lost HP with something else, angle of attack. Remember, we want best RATE,
which is most feet up for a given time.

Watch your airspeed, compare it to your VSI. At this higher altitude that
slight pitch change sure does kill that airspeed doesn't it? And your VSI
isn't very astounding either, but you notice that as you slow down, the VSI
readings get better, even increasing as you slow past your sea level Vy
speed.

This is opposite of best angle of climb because there we want most feet up
for a given distance, so to hit our best angle at higher density altitudes,
we need to reduce drag, lower the angle of attack and fly at a higher
indicated airspeed to keep us from falling towards the back side of the
power curve. Trying to maintain best angle, we loose excess thrust as we
climb, therefore we have to replace it with something, and that is more
airflow over the wing (more forward speed). So a higher indicated airspeed
is required to maintain best angle of climb as density altitude increases.

Watch your airspeed, compare it to your VSI. At this higher altitude that
slight pitch change sure does kill that airspeed doesn't it? And your VSI
isn't very astounding either, but you notice that as you slow down, the VSI
readings get better, even increasing as you slow past your sea level Vy
speed.

Both best angle and best rate are related to the airplanes absolute ceiling.
Theoretically, if you draw a line from the sea level best angle speed
upwards to the max indicated level flight airspeed (as fast as you can go)
at the absolute ceiling (as high as you can go) (Vx increases with altitude)
you can draw a line downward from there to the sea level best rate speed,
showing that Vy decreases with altitude. You end up with a triangle, in
otherwards, Vx and Vy converge at the absolute ceiling where you simply
can't climb no matter what you do and there is only enough thrust and only
enough horsepower for straight and level flight at your given weight.

Another analogy. Ever run, bike, or drive in the mountains? Down in the
foot hills you find yourself able to go 50mph up a 3% slope with your foot
right to the floor. As you get further and further into the mountains and
higher and higher altitudes, you still have your foot to the floor, it's
still a 3% slope, but your "best rate of climb" is now suddenly 30mph and
decreasing the higher you go.

Jim

Cecil Chapman · Guest

Thank you for the links, though I probably should 'come-out' and tell you
I'm a 'Cro-Magnon pilot' ,,, that is, I need it relatively simple (cave
drawings might help <grin>). I looked at some of the graphs and became
'ascared' LOL!

I'm not all that sharp, but I DO appreciate you thinking the best about me
in the latter regard! <GRIN> LOL!

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--
=-----
Good Flights!

Cecil
PP-ASEL-IA
Student - CP-ASEL

Check out my personal flying adventures from my first flight to the
checkride AND the continuing adventures beyond!
Complete with pictures and text at: www.bayareapilot.com

"I fly because it releases my mind from the tyranny of petty things."
- Antoine de Saint-Exupery -

"We who fly, do so for the love of flying. We are alive in the air with
this miracle that lies in our hands and beneath our feet"
- Cecil Day Lewis -

Wade · Guest

Marc J. Zeitlin wrote:
<snip> Also, see:

Cecil Chapman · Guest

Hilton · Guest

Wade wrote:

Wade · Guest

Hilton wrote:

Hilton · Guest

Wade wrote:

Highflyer · Guest

"Wade" <wade (AT) foo (DOT) com> wrote

Bob Chilcoat · Guest

Nice explanation, Todd. It's actually intuitive if you think about power as
force times speed, but that's not all that easy to get to. I've always
found it fascinating that a jet or rocket engine delivering a fixed thrust
generates more power the faster it goes. Now that's really NOT intuitive,
at least to me, even though I understand the physics and believe it.

--
Bob (Chief Pilot, White Knuckle Airways)

"T o d d P a t t i s t" <tpattist (AT) dontspamme (DOT) snet.net> wrote