HlGHT 4nternational. 16 April 1964
AIR
COMMERCE
V, V, AND ALL THAT
Part 5 of CCJ's "V" Classifications
Research for Part 4 of this discussion has disclosed a lack of certain
essential data, and the author accordingly feels it desirable to
postpone the article to a later issue. Part 5 is therefore brought
forward, and appears below. Parts 1, 2, and 3 appeared on January
16 and 30 and March 5 respectively.
FEW years ago the dictionary definition of an aeroplane as
being a "flying machine" was accurate enough for most
purposes. Now it would be closer to the truth to describe
it as "a machine which only just flies." You will see what I mean
when we have looked at the next four operational speeds, Vr (the
rotation speed), Vmu (the minimum unstick speed), Vioi (the lift-off
speed) and Vsmin (the minimum take-off safety speed).
In processing these we will take them somewhat out of their
ascending order of speed and deal with V2min first; this is because
V2miH is a derivative of the primary factors Vs and Vmca and is the
next governing speed after Vx; both Vr and Vior are conditioned by
V2min, while Vmu stands more or less on its own, except that it
provides a minimum of Viof-
A
(5) Minimum Take-off Safety Speed (V2min) In the almost forgotten days when an aeroplane was indeed a flying machine and
not a machine which only just flies, V2mm was unheard of; the
simple speed V2 followed directly on V! and was the speed at which
the aeroplane flew off the ground or was gently eased off; it
depended only on the weight of the aircraft and on the altitude and
temperature of the air. V2 (the initial climb-out speed) and its
half-brother V2min (the minimum take-off safety speed) still do
depend on these parameters but they are now applied not on the
runway but at 35ft above it. The theory here is that, with modern
accelerations, if V2 were applied to a runway position (as used to be
the case), a certain amount of "unavoidable acceleration" would
provide an "excess" speed at the end of '"the take-off distance
available"—i.e., at the 35ft height point; therefore the concept of
V2min is introduced and is applied at or before the 35ft height point;
the old V2 would have found a place somewhere between the rotation
speed (Vr) and the lift-off speed (Viot); however, the concept of V2
on the ground is now obsolete and a new V2, introduced under the
name of the "initial climb-out speed," is applied after the 35ft
height point. In this number, however, ignore V2 and concentrate
on the range of operational speeds between Vt and V2min.
Fixing a neat definition for V2min involves a little difficulty,
mainly because of variations in the stalling speed parameter. The
ICAO PAMC visualizes a true 1-g stall (Vsig); this being a conservative figure, the V2min factor of the PAMC is low: 1.15 V sig for
jets and 1.10 V sig for props. The FAA's SR 422b visualizes a
minimum speed stall (Vs or Vms), which is unconservative and
therefore carries a rather higher V2min factor, so as to achieve
approximately the same performance margin. BCARs are close
to SR 422b in their concept of the stall, except that Vs (or Vms)
must not be lower than 96 per cent VSiB Both of these codes
therefore put V2min at 1.20 Vs (or Vms) for jets and 1.15 Vs (or
Vms) for props. A definition which attempted to cover these
variations would not be simple; and since the trend is towards the
concept of the 1-g stall (Vsig),* as defined by the 5th Session of the
ICAO Airworthiness Committee, I propose to keep fairly closely
to the ICAO concept. (PAMC 6.2.2, Doc. 8283, page 29; SR 422b
4T114;BCARD2 3.1.)
^ Incidentally, perhaps I should remind the reader here of the
"commonsense" ground rules which I declared in the first number
of the series as a means of achieving simplification. One of these
rules was that we would deal only in terms of a four-engined aircraft.
It mast be borne in mind that this is often erroneously contracted to Vs
—see "Flight International" January 30, 1964, page 163.
Slightly different parameters for V2min do in fact apply to twos and
threes. Another of the common-sense rules was that we would
assume throughout the "expected" configuration. At V2min this is
gear up but flaps at take-off. It would be as well to bear in mind
these taken-for-granted aspects of the "definitions," since they can
be important.
Definition:—
(5) The Minimum Take-off Safety Speed (Vamin) is a speed
attained at the 35ft height point, with one engine inoperative,
such that it is equal to 1.15 times the stalling speed (Vsig) or
1.10 times the minimum control speed in the air (Vmca),
whichever is the greater. (PAMC 6.2.2 Doc. 8283, page 29.)
(6) Minimum Unstick Speed (Vmu) The next independently
determined parameter to consider is "the minimum unstick speed,"
Vmu. 1 have taken Vmu immediately after V2min because the two
have fixed values for any given set of conditions and together they
govern Vr (the rotation speed) and Vior (the lift-off speed).
I sometimes think that Vmu might be better referred to as Vsn—
"a scrape-off speed very close to the stall"—because that is what
this certification manoeuvre calls for.t At one time the manoeuvre
had to be demonstrated as a follow-up from the Vt demonstration
—with one engine inoperative—but that was found to be rather
too hazardous; and now Vmu, unlike V2min is, for certification
purposes, not an engine-out speed but an all-engines-operating-speed
(though SR 422b still contains reference to an engine-out Vmu).
However, it still tends to be close to the stall and carries with
it a rock-bottom standard of airborne performance, safeguarded
only by the requirement that at Vmu the aeroplane must exhibit
at least the performance ability to overcome its weight (i.e., it must
show a minimum climb capability or be able to sustain, without
sink, the application of stick force).
Definition:—
(6) The Minimum Unstick Speed (Vmu) is the all-enginesoperating speed at which it can be demonstrated that the
aircraft can be made to leave the ground and, maintaining a
positive climb, continue the take-off without displaying any
hazardous characteristics. (PAMC 6.2.4. Doc. 8283, page 29;
SR 422b 4T114(d); BCAR D2-8 3.2.3.)
(7) Rotation speed (Vr) and (8) The Lift-off Speed (VioO Having
defined Vmu and V2min, the remaining transitional speeds (i.e., those
applying when the aircraft "only just flies"), Vr (the rotation speed)
and Vior (the lift-off speed) fall into place.
In numerical terms only one of these (namely, Vr) really needs a
definition, since the lift-off speed follows directly from the rotation
speed and the pilot has no control of it once he has executed the rotation manoeuvre. If he strays from the recommended procedures by
early or late rotation, or by under- or over-rotation, then the lift-off
speed or the point of lift-off will in all cases be adversely affected.
There is some protection against selecting a hypercritical Vr and
two safeguards are built in against variations in pilot technique
leading to a dangerous take-off. Firstly, a rotation 5kt early must
not diminish the speed at the 35ft height point (i.e., the V2miu speed)
by more than 5kt (PAMC 6.2.5.3.); secondly, if the aircraft is
rotated so as to lift-off at 93 per cent Viot and be at 35ft at 93 per
cent of V2min, the take-off distance (to the 35ft point) must not be
longer than when the correct Vr speed is used (PAMC 6.2.5.4).
Finally, SR 422b makes a distinction between the all-engines and
t Also the mnenomic would be consistent with those introduced for the
Low Buffet Speed (Vslh) and the High Buffet Speed(K6ftB)—see "Flight
International" 30/1/64, page 164).