Originally Posted by
ShyTorque
LoneWolf50,
The fenestron has a geared, direct drive from the main gearbox.
I am aware of that.
If the main rotor rpm is gone, then so proportionally is the possibility of thrust from the fenestron.
I am aware of that as well, but MR and TR stall are not the same thing, nor brought about by the same airflow problems. There were certain cross wind and GW conditions in some helicpters I flew that, our safety center assured us, would lead to loss of TR effectiveness due to angle of attack changes in the tail while the Main is cooking merrily along at 100%.
I am not sure you understood what I posted.
Please go back and consider what I asked about, with the initial point being possibly stalled main rotor blades (decelerating, yes, but still turning for x seconds as they head toward about zero, which is what we are led to believe at this point. )
For that period, however brief and certainly transient, MR stalled but finestron/anti torque not yet stalled. One produces no thrust, the other "some value" of thrust about the vertical axis, as designed.
The blade pitch of the fenestron (or any other tail rotor type) is controlled via the yaw pedals.
I understand that as well. I spent some years flying helicopters. It is because I understand that feature that I am asking this question, since I had never considered a MRH / MRB stall beyond the old "retreating blade stall" at high speeds and high gross weights/DA's.
The finestron (or a tail rotor on othe models) provides thrust that, due to the lever arm of the tail boom, works to rotate the helicopter about the vertical axis. When we yaw in a hover, our rotation rate about that axis is based on the differential of the torque from the MR trying to rotate us one way, and the finestron acting in rotation in the other direction. (Idealized vectors for simplicity of explanation.) We both know this, it's the basics of what makes helicopters such fun to fly.
MR blades provide us torque (that which rotates the body of the aircraft) as a by product of lift production. Equal and opposite reactions, Newton, etc.
If the main rotor blades are stalled ... NO lift and so I infer no torque reaction to the body (edit: though certainly some precession due to mass and inertia) ... but the finestron is still turning (be it with a lot of pitch or a little pitch, control position dependent) then I'd expect there to a value of thrust, however modest, whenever that set of blades is spinning until they too either
1. stall
or
2. the lack of V gets down to a low enough value that V^2 isnt big enough to matter.
So back to my point, which you did not answer, nor address.
There is a transition point between NR = ~ 100% and, as we are led to believe, NR = ~ 0 at (or even a bit before) time of impact.
It is that transition period that I am concentrating on, specifically in the case that henra mentioned about main rotor blades having a stall point -- so no lift, no torque.
This threshold does not necessarily coincide with a condition that would stall the finestron blades: a differently shaped and oriented set of rotating airfoils.
The period I am asking about may have only lasted a few seconds as NR decayed. I am puzzling over a few seconds of thrust from back there, shafts still turning albeit slowly, where the usual torque/antitorque counter is cast aside by a stalled condition in the main blades. X seconds of anti torque, nominal value, acting agianst no torque. Resultant dynamic action? Rotation about vertical axis ... maybe.
After an initial impulse lasting x seconds or x fractions of seconds, momentum carries that rotation forward. As the major rotating body (MR) slows towards zero, thrust in whatever direction the finestron can act (even to counter the modest impulse noted above) is rendered moot by a continued reduction in its rotation speed too an amount lowered, by a couple orders of magnitude or so, until it too reaches nil when the MR stops.
Now do you see what I am trying to sort out?