Why fly fast approaches?

"Peter Duniho" wrote in message ...
"Snowbird" wrote in message
m...
I'll be interested in what others say, but my take on this is:

As far as I know, all the V-speeds you're talking about are a direct
function of stall speed.

For landing, true. For takeoff, not true. Vx and Vy

'Scuse, Peter, but I said "all the V speeds you're talking about",
not "all V-speeds".

As best I can tell, the original poster was talking about
Vr, and landing speed.

I don't think he discussed being asked to climb out at a different
speed than Vx or Vy, but in any event I think a better argument
can be made for that practice in appropriate circumstances.

Now, if you want to argue for Vr being a function of excess thrust
and excess power -- go ahead.

Cheers,
Sydney

"Snowbird" wrote in message
om...
'Scuse, Peter, but I said "all the V speeds you're talking about",
not "all V-speeds".

As best I can tell, the original poster was talking about
Vr, and landing speed.

Whatever. I find Vx and Vy to be perfectly relevant in this thread, even
looking at only the first post. You want to be offended, go right
ahead...wouldn't be the first time.

"Peter Duniho" wrote in message ...
"Snowbird" wrote in message
om...
'Scuse, Peter, but I said "all the V speeds you're talking about",
not "all V-speeds".

As best I can tell, the original poster was talking about
Vr, and landing speed.

Whatever. I find Vx and Vy to be perfectly relevant in this thread, even
looking at only the first post. You want to be offended, go right
ahead...wouldn't be the first time.

Vx and Vy are certainly relevant to bring into the discussion, but
they weren't mentioned in the original poster's description, and I
referred to that. You were responding to my post, and saying "not true"
to me. I simply clarified what I'd meant.

That doesn't mean that Vx and Vy aren't relevant to introduce, as
additional factors which *are* dependent on factors other than stall
speed. It's a reasonable point -- provided it's not introduced in a
way which puts words into someone else's mouth or refutes a claim
they weren't making.

I await your explanation of Vr and what it depends upon, and I note
that one can fly behind an engine later shown to have one cylinder
completely flat, and not notice any particular changes to Vy, the
rate of climb obtained at that speed (under DA conditions not near
the edge of the envelope, admittedly), or cruise airspeed.

Cheers,
Sydney

"Snowbird" wrote in message
om...
I await your explanation of Vr and what it depends upon

I've never flown an airplane with a published Vr. Since you're the one
making claims about how it's determined, how about YOU explain Vr and what
it depends on.

As far as Vy and Vx not varying with engine power (your claim regarding "one
cylinder completely flat"), you may not have noticed the difference in the
cockpit, but that doesn't mean they didn't change.

Pete

(Snowbird) wrote
I await your explanation of Vr and what it depends upon, and I note
that one can fly behind an engine later shown to have one cylinder
completely flat, and not notice any particular changes to Vy, the
rate of climb obtained at that speed (under DA conditions not near
the edge of the envelope, admittedly), or cruise airspeed.

What you say is true, but only because what you refer to as a
completely flat cylinder is not in any way equivalent to a cylinder
not making power. A completely flat cylinder is what a mechanic will
call a cylinder that is not anywhere close to making compression in a
static test - meaning it only makes, say, 20/80 or even less.

I have in fact had this happen - sometime between compression checks,
which I tend to do at about 100 hour intervals, I had a cylinder go
from 76/80 to 20/80 due to a leak past the exhaust valve (it had 1400+
hours and 12+ years since major, and was not new even then, so I
wasn't terribly disappointed). There was no noticeable impact on
performance, but that's because it was still making nearly full power,
and would have continued to do so right up until the valve got stuck
or munched.

If you fly with a cylinder that isn't making power at all (for example
because you managed to foul both plugs - ask me how I know) but is not
otherwise interfering with the rest of the engine, there will be an
immediate and obvious change to the rate of climb and the cruise
airspeed - and that's when you have eight cylinders.

Vr is sort of a funny speed, and is indeed affected by power
available. Ideally, you want to rotate as soon as possible to reduce
wear on the tires. However, too early a rotation can back you into a
corner - as you lift the nose, the weight is transferred from wheels
to wings, and that can actually increase drag dramatically, especially
if you are taking off from a smooth paved runway. However, at lower
weights you have significantly more excess power available, and can
indeed rotate early - so Vr goes down with decreasing weight as well.
It will go up with increasing density altitude because excess power
available goes down, so you need to get closer to an optimum climb
speed before you add the aerodynamic drag.

A good way to look at it is this - you always rotate at less than Vy
and accelerate to Vy, but how much less depends on your ability to
accelerate - which depends on excess power.

All else being equal, reducing Vr by the same factor as the approach
and landing speeds is sensible, and indeed conservative because a
greater reduction is actually possible.

Michael

"Michael" wrote:
What you say is true, but only because what you
refer to as a completely flat cylinder is not in any
way equivalent to a cylinder not making power.
A completely flat cylinder is what a mechanic will
call a cylinder that is not anywhere close to making
compression in a static test - meaning it only
makes, say, 20/80 or even less.

I have in fact had this happen - ...There was no
noticeable impact on performance,

Yep, I've even had this happen on *two* cylinders on the same side of a
LYC O-360 and noticed nothing until a just-for-the-hell-of-it
compression test revealed the bad news. The engine was running nice and
smooth.
--
Dan
C172RG at BFM

Someone correct me if I'm wrong, but changes in engine performance
wouldn't affect Vx or Vy would they? It might affect the power settings
necessary for these speeds, or the plane effectiveness in holding them,
but my understanding is that Vx and Vy are determined by the
aerodynamics of the design of the plane and it's weight. Changes in an
engines performance with age shouldn't affect these speeds, should they?



Snowbird wrote:

"Peter Duniho" wrote in message ...

"Snowbird" wrote in message
.com...

'Scuse, Peter, but I said "all the V speeds you're talking about",
not "all V-speeds".


As best I can tell, the original poster was talking about
Vr, and landing speed.


Whatever. I find Vx and Vy to be perfectly relevant in this thread, even
looking at only the first post. You want to be offended, go right
ahead...wouldn't be the first time.


Vx and Vy are certainly relevant to bring into the discussion, but
they weren't mentioned in the original poster's description, and I
referred to that. You were responding to my post, and saying "not true"
to me. I simply clarified what I'd meant.

That doesn't mean that Vx and Vy aren't relevant to introduce, as
additional factors which *are* dependent on factors other than stall
speed. It's a reasonable point -- provided it's not introduced in a
way which puts words into someone else's mouth or refutes a claim
they weren't making.

I await your explanation of Vr and what it depends upon, and I note
that one can fly behind an engine later shown to have one cylinder
completely flat, and not notice any particular changes to Vy, the
rate of climb obtained at that speed (under DA conditions not near
the edge of the envelope, admittedly), or cruise airspeed.

Cheers,
Sydney

"Matt Young" wrote in message
news
Someone correct me if I'm wrong, but changes in engine performance
wouldn't affect Vx or Vy would they?

As I mentioned before, Vx and Vy are functions of excess thrust and power,
respectively. In particular, they are the airspeeds at which excess thrust
and power are at their maximum.

If thrust were constant, Vx would happen at minimum drag airspeed regardless
of engine power. But thrust isn't constant; it varies with airspeed, and
the airspeed at which the thrust in excess of drag is at its maximum depends
on how much engine power you have.

For the same reason that thrust isn't constant (propeller efficiency changes
with airspeed), power isn't constant either, and again, the airspeed at
which the power in excess of power required for level flight is at its
maximum depends on how much engine power you have.

You can demonstrate for yourself that Vx and Vy depend on engine power, by
considering the difference between relevant airspeeds when there's no engine
power. Best glide airspeed isn't the same as Vx, even though both airspeeds
provide the best performance of the aircraft in terms of altitude change
over distance. Likewise, minimum sink airspeed isn't the same as Vy, even
though both airspeeds provide the best performance of the aircraft in terms
of altitude change over time.

Generally speaking, the differences between all of these airspeeds isn't
huge, but it's important to understand that they aren't the same.

[...] Changes in an
engines performance with age shouldn't affect these speeds, should they?

Not significantly enough for the published speeds to be unusable. If the
engine performance has changed enough to affect Vx and Vy significantly,
there's something wrong with the engine. But it's false to say that they
aren't affected at all.

Pete