Well now Ah'm just totally corn-fused. Maybe Ah'll never understand these helicopter thingees. See, Ah'm jus' a simple ol' country-boy pilot, but Ah takes some eck-sepp-see-ohn to the following statement.
Nick sez:
But, helicopters generally have strong dihedral due to the way the rotor behaves when we impose a new wind direction on it. If you can, picture the rotor having been trimmed with the cyclic forward at high speed, so that the disk is level and the nose is down to trim at that speed. Then simply rotate that helicopter a bit to the right, like a right yaw would do. If you look at the rotor from the wind's perspective, where the air meets the disk as the new forward, you can see that the cyclic is now trimmed to the right relative to the airflow, so it has a small roll input. this makes the aircraft roll in the direction of yaw, so-called positive dihedral.
An airplane wing has a hard and very well defined leading edge over which the air flows. Helicopters have no such device. Once a helicopter rotor is in forward flight it doesn't have any clue as to where "straight ahead" is. Nor does it care. You simply *cannot* fly a rotor "out of trim" like you can a fixed wing. You can fly the
fuselage out of trim, but this has absolutely no effect on the wind that the rotor is seeing.
In my aeroplane, if I'm scooting along in cruise and I push on the right rudder, the plane will most assuredly bank immediately and turn to the right. In my helicopter, if I do the same thing the fuselage will merely slew off to the right, the trim ball will slide out to the left, but the helicopter's track across the ground will remain fairly unchanged (banking the rotor is the only way to change direction). In fact, if I persist in pushing the right pedal, the helicopter will perhaps take on a bank to the
left, which is how the ship normally behaves when the trim ball is displaced thataway, which is also the opposite of my airplane.
Maybe larger helos with articulated rotor systems behave differently than the small ships I fly. But I have demonstrated this time and time again to the non-believers. They freak out when I do this, fearful as they are (and legitimately so) of mast-bumping as we reduce the hub clearance in the 206. Perhaps they remember all those Cobra accidents that occurred until we learned the importance of diligent attention to yaw-trim in high-speed helicopters with teetering rotors.
If helicopters did indeed have "positive dihedral effect," then pushing on a pedal would cause a bank in that direction. But this does not happen.
What this tells me is that so-called "dihedral effect" in small helicopters is either non-existant or opposite to fixed-wing. Nick evidently agrees with me. In clarifying a previous post where he was talking about the behavior of the "shark fin" attached to the fuselage roof of the Bell 212, he writes:
Phoinix, I wasn't clear enough. It counteracts the roll left with right pedal, which is unstable, and not allowed for IFR flight.
Roll left with right pedal? Yeah, that's exactly what I've been saying all along. We do not need to be test pilots to witness this. Merely go up in cruise, and perfectly trim the aircraft. Now slightly press on a pedal (your choice- but not too much!), just enough to get the aircraft out of trim. DO NOT MOVE THE CYCLIC. Come back and tell me which way the helicopter banks.