Originally Posted by chornedsnorkack
YouŽd expect the spiral mode to be the worst, because it is the one with negative stability, and therefore the mode that will fly the craft into ground...
In fact, no. Negative spiral stability and weakly damped Phugoids are both quite flyable - they add to workload, of course, but a simple autopilot can easily accomodate both of these. This is because both have relatively long periods (usually) and so it's relatively simple for a human or an artificial system to compensate.
The Dutch Roll and SPPO are much more serious impacts on handling qualities because they are higher frequency modes, at or about the natural human response frequencies. Therefore they get excited by normal human behaviour, and are hard to damp manually sometimes. Most 'Pilot Induced Oscillations' are interactions between the human and one of the higher frequency modes.
Because the frequencies are higher, it's relatively harder to design compensation systems, too.
Originally Posted by chornedsnorkack
Hm... wouldnŽt there be important economics points about stability?
Not really. Stability and Control - usually the two are considered together - have generally minor impacts on the operating economics of an aircraft. They can affect the design, and if bad enough make the design uncertifiable (and hence by default uneconomic) but assuming you get the aircraft certified there are only a few areas where the S&C affects the operations, and those are really more 'control' than 'stability' issues. Examples might be:
1. Crosswind landing and takeoff capability may restrict operations if control power is poor.
2. Minimum control speeds may impact operations at lighter weights
3. Poor handling in (usually high altitude or high speed) areas of the flight envelope may restrict operational use, especially for dispatch with failures.
But, compared to the huge impact of Performance (the 'other' aerodynamic discipline) these are generally minor effects on the economics of operating the aircraft.
Originally Posted by chornedsnorkack
Pitch stability defines the CoG range - move CoG too far to the rear, and the tailplane AoA equals that of main wing, so that the pitch stability vanishes; move CoG too far forward and the tailplane has inverted stall and the plane falls nose over.
How do you add CoG range to a plane? I suspect it would require enlarging the tailplane - with economic penalty in form of trim drag.
Ah, have to correct you there. Pitch stability doesn't define the cg range, or certainly not alone. There are a number of factors which define the cg range, some are related to stability, some to controllability and some to other factors, and the way they are usually designed (in the traditional sense) is with a graph relating the various different limits to tailplane area. Not sure if it has an official name, but let's call it a tailplane sizing diagram.
On it one plots cg along the horizontal axis, and tailplane area on the vertical. Once you have a general layout and an idea of the aerodynamic characteristics you can start to express various design constraints as a function of tail area verus cg.
So, for example, with no tailplane, I may find that my most aft allowable cg while reatining adequate stability is 20% mac. If the tail is 100 sq ft I calculate I can go to 30% mac cg.
I also calculate that for trimming at stall speed with the landing flap, with no tailplane I cannot be further forward than 25% mac, but with 100 sq ft I can be as far forward as 10% mac.
Having worked out these - and many more - constraints (which will include considerations of things like a minimum weight on the nosegear statically to ensure the plane doesn't tip over when parked!) I end up with a whole bunch of lines, some of which constrain aft cg, some forward.
I now decide how much cg range I NEED for the aircraft - which will be based on predictions of fuel and cargo variations - and that defines how large my tail must be.
For the case above, assume I decide I need 10% cg range (not very much).
At 0 sq ft my aft limit is 20%, my forward limit 25% - obviously, this plane needs a tail! (-5% cg range)
At 100 sq ft my aft limit is 30%, my forward limit is 10% - 20% cg range, more than I need. (20% cg range)
By simple interpolation I can work out that to meet those requirements and a 10% cg range, I need a 60 sq ft tail (16% to 26% will be the limits).
So, yes, adding cg range usually means a bigger tail. But it's not a trim drag problem - it's a profile drag problem, and a weight issue. The trim lift on the tail stays the same for the same conditions, whatever size the tail is.
Also, the items you mention for the forward and aft limits are not really accurate.
For the forward limit, you're close (no-one designs for tail stall; we design for a margin under tail stall for forward trim, and that may not be the limiting forward cg factor. Often its a loads issue at forward cg).
For the aft limit, Im afraid that tail AoA and wing AoA being the same has absolutely no effect on stability. Moving the cg aft does indeed lead to instability, but what matters primarily are the relative slopes of pitching moment and lift with angle of attack (for the wing) and the tailplane lift curve slope and its area and position. Stability is all about small changes and the relative change in forces and moments - in essence, gradients, or DERIVATIVES - and much less to do with values of parameters themselves.