Originally Posted by Osprey Driver
BTW, the 40KIAS VRS limit doesn't exist because VRS starts there--it starts there because pitot-static instruments don't give accurate indications below 40KIAS, so that's why most r/w manuals say to avoid high RODs below that airspeed.
I could not recall such a restriction in any Sikorsky Manual, but since I retired a few years ago, I called a couple of still active pilots at SA this morning. Starting with the 53E and continuing thru the UH-60, SH-60, S-76 and S-92, my two friends reported that the manuals haven't changed on that issue, i.e., there are no such proscriptions. Perhaps others can post concerning what is in, for instance, CH-47 and AH-64 manuals, if anything, on this subject.
Let me attempt to explain an aerodynamic reality that occurs in slow, relatively steep descents that I believe several in the forum have incorrectly believed to be the edge or onset of VRS (there are other members whose reports are clearly VRS).
In descending flight at speeds in the 15-30 kt range and at rates of descent in the 700-1500+ fpm range, the advancing tip penetrates the tip vortex from the preceding blade (one can immediately see how the flight conditions for this effect are affected by weight, altitude, temperature), and the instantaneous angle of attack change on that tip results in a lift, drag and pitching moment change that effectively "rings" the main rotor at N/rev and can produce, if air conditions are stable enough to stay in this situation, very high vibration levels at N/rev frequency. One has to consciously try varying the ROD and airspeed to find this effect (in day to day flying we usually we fly right on thru it), but it is there if one goes looking, or if the combinations are just right. SA has always used this manoeuvre (at least since before I started doing engineering flying in 1969), called a "rough approach" as a flight loads survey manuever, as the associated vibrations may impact engine mount loads, or perhaps electrical, hydraulic, or other hardware that is mounted on the transmission deck (they are typically far higher than the N/rev vibrations associated with VRS as discussed below). The blade stress guages clearly reflect what is happening here. HOWEVER, it is key to this discussion to realize that when in this condition the vehicle is rock solid in pitch, roll, yaw ROD and airspeed.
Finding the right combination for a "rough approach" as described above is a piece of cake compared to getting VRS data. Since you are looking for 2500 fpm (ballpark) and a stable situation, one starts at 6-8000 ft. The vehicle has to be coming down
exactly vertical relative to the airmass, so on the most recent vehicle we did this on, the S-92, we thought that since we had a super new, state of the art low airspeed system onboard it would be easier than usual, but it wasn't to be, and thus we were forced to use the sides of clouds etc. Once VRS is found, the effect upon the vehicle is dramatic, with rather sharp excursions in the pitch and roll axes, some smaller excursions in yaw due to the power changing (at fixed collective) as a result of non-steady inflow, and increased N/rev vibrations. If not ready for the ride, one can understand the reference offered in Tcabot's posting:
"Fact is one of the investigating board members took a 53 into VRS as part of the investigation and stated he would never do that again. You can read that for how he meant it."
In the case of fully developed helicopter VRS, the vehicle is hardly rock solid.
Two very different rotary wing aerodynamic situations.
Misconstruing a "Rough Approach" situation with the profile necessary for a VRS event can create unneeded restrictions. I mention that because some friends have heard unconfirmed "buzz" about possible regulatory restrictions (civil) on steep approach profiles.
Thanks,
John Dixson