(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