A few observations, if I may ...
(a) I like Mago's idea the best ....... unless you are a sailplane pilot, of course ...
The CARs to which he refers predated the FARs and the CAA (actually the CAB - originally the Civil Aeronautics Authority (CAA) dating back to the '30s - which subsequently split into two organisations, one of which, the Civil Aeronautics Board ..) was the predecessor to the FAA. Some of the old yarns about how the original rules were dreamed up make for wonderful bar tales ...
(b) the downgoing blade idea, while appealing and of considerable use in pilot education and understanding, is only a part of the problem/answer ... the airflow patterns downstream of the prop discs are influenced considerably by the prop rotation and their interaction with the aft keel and empennage surfaces is not as simple as might be thought .. djpil, if he sees this thread, might be enticed to talk on the matter at some length ..
(c) further to mcdhu's observations, and totally off topic of course, some, if not all, of the 125s have a Va limited by directional control considerations for the same reason ...
(d) following on Genghis' thoughts, in the normal certification sense, we are principally concerned about the failure of whichever engine results in the aircraft's exhibiting the most critical handling characteristics. If there be no immediate systems consequences (hydraulics etc) which might affect the controls, then a jet is not usually going to have much of a problem when compared to a prop aircraft.
One handling factor to which mcdhu, Mutt, and Genghis are alluding is wind. Crosswind will provide a handling concern, regardless of certification considerations. I gather that some airlines, and I seem to recall CX's being mentioned in this regard in other threads, take the view that the upwind engine is, for directional control reasons, the "critical" engine during the takeoff. This concern is valid and is addressed in the various test pilot assessments of the aircraft throughout the flight test program.
More importantly, but largely ignored, there is one area of operations where wind is a critically important factor which is ignored by the design standards.
For a FAR 25 twin, the minimum V2 (often referred to as minV2 or V2min) is limited by either 1.2 Vs or 1.1 Vmca and the minV1, similarly, is limited by Vmcg. The FAA (and I presume the JAA, likewise) makes the assessment of Vmcg for nil wind conditions (AC 25-7 refers), while the BCARs used 7 knots crosswind (I had a reference long ago .. but can't recall it these days).
The problem arises when one considers that the AFM-scheduled speeds, which in the case of V1 can be based on this Vmcg-limited case, raises no concern with the crosswind. The "actual" Vmcg, with a crosswind, increases substantially above the certification value for the same reasons as discussed above. On one narrow-bodied twin jet, for which I have manufacturer's test data, this increase is half of the crosswind. On another thread it was reported that, for a particular 4-engined jet, the increase is more than the value of the crosswind. Of far greater importance is the observation that Vmcg handling problems are characterised by a very rapid onset, regardless of the wind. On one aircraft test program with which I was involved and where we looked at this in detail, you are looking at 5 knots or so making the difference between a comparatively "no-sweat" operation and quite significant centreline excursions ... another knot or two and it is a case of onto the grass ....
The problem now, is quite clear. If the takeoff be predicated on a low weight minimum speed schedule and the crosswind is significant, then the pilot WILL lose control of the aircraft and likely exit the runway edge - unless the runway is very wide and the V1/Vr split is small. Even if the pilot manages to drag the aircraft off the ground, in the handling confusion, there is a very real risk of the bank angle's being permitted to be driven by yaw-roll coupling .. which, itself, increases the Vmca and sets the pilot up for a Vmca departure. This is, as I have observed on other threads, an interesting sim exercise and, for the crews, very informative ...
Operational circumstances permitting, ths problem can be avoided on takeoffs where the RTOW is considerably higher than the actual TOW by using an appropriately higher weight speed schedule. The TOW used, naturally, still has to be not greater than the RTOW permitted value.
Upnorth ... "Active control", as a term is used in a variety of applications so it really depends on the context in which you saw the term used. Ignoring non-aviation areas, such as the common computer lexicon use, in the aerospace arena you are possibly looking at the use of computer controlled feedback systems which provide primary control inputs for gust load alleviation or flutter control. Alternatively, I notice that BAe is using the term to describe some of their work in the pilot control interface on the JSF program.
As a polite request it would be better if the topic header were more appropriate to the subject matter .. I only discovered this one after Mutt steered me to it by email .. you don't think that it might be useful if you changed the topic header to a more appropriate one ... ?
Last edited by john_tullamarine; 23rd May 2002 at 01:45.