To demonstrate different aspects or viewpoints explaining the MAP drop in a steady/level turn look at the following simple simulated right turn
(Media player, 240Kb)
with corresponding controls:
Turn is initiating by changing the horizontal flight lateral cyclus of about -0.5 left to 0.25 right (0.75 change)
To maintain altitude aft cyclic is necessary, which goes from 3.7 degrees forward down to 2.5 degrees forward (1.2 degree change)
Speed reduces because of loading while maintaing collective, from 90 to 85 knots.
Altitude is almost maintained, except of a small climb due to simulator pilot errors (still learning to fly that thing...)
The corresponding pilot views
The Engineering data in the above figures show:
While level :
-> 9307 N MR trust,
-> 4.08 degrees rotor disk inflow angle
-> 2.75 degrees coning
-> 111 Main rotor SHP
While in the turn
-> 11200 N trust = higher
-> 2.39 degrees rotor disk inflow angle = lower see sketch
-> 3.32 degrees coning = higher consistent with higher loading
-> 106 Main rotor SHP = lower, so lower MAP
So indeed power needs are lower by approximately 5% in this case.
One way to explain the MAP decrease is to look is at the rotor disk inflow:
A lower inflow angle tends so to speak towards autorotation so reduction of SHP.
An other way to explain is look at the detailed rotor power needs. This consists of profile drag (the drag of the blade profile when it is rotating around the mast)
and the induced drag (the horizontal part of the lift due to the fact that net inflow is not horizontal, so the lift component is not strictly vertical as it were in an airplane fixed wing)
Drag before turn:
The drag plots show 3 quantities :
- in blue the profile drag, this is pretty much the same in both cases (slightly higher for more loaded disk in turn).
- in green the induced drag, the is lower when turning because the inflow comes more from below, so lift is more forward (or better less backward)
- in red the total drag, again lower in turn, so lower SHP.
Remark that profile drag is greater for the forward going blade the for the retreating blade, but that the induced drag is greater for the retreating blade.
Finally a very detail view consists at exploring all the angles of Attack
The meshed figure shows the mechanical angles of the rotor blades: this is the same in both cases (front view) corresponding to same collective and almost same lateral cyclic.
The solid blue figure show the effective angles of attack taking all airflow components into account : this also shows an increase corresponding with lower inflow and higher disk load.
Finally an examples of a detailed airflow plot, snap shot begin of turn right advancing blade fully to the right (270 degrees in my references, 0 starts centre front), at the 50% cord position. I indicated manually the major changes that occur in these flows when in level turn.
Air flow is influenced of course by helicopter speed and the blade rotation. But also by blade flap, induced air speed, and total rotor disk attitude changes:
So all these ways of looking at turns explain the drop in MAP.
Perhaps a final remark.
During rapid turns the effective rotor RPM equals the relative rotor RPM, which is kept steady at approx 404 rpm = 42 rad/sec = 2400 degrees/sec, plus the absolute yaw rate of the heli.
See the pilot view while in the turn (lower right corner): Yaw = -8, so in this turn there is a extra yaw angle of -8 degrees per second. This is a change of -8/2400 : -0.3 %. This has a cubic general influence on power (so -0.9%) The effects of this small change are just on the edge of the precision of this simulator (0.5-2%), so if precision were
to be increased, which is possible at the cost of more CPU time, it should be detectable, but in view of all the other much larger influences this may not be easy. It would also be outside the precision of onboard measuring instruments.