shiv wrote:JayS wrote:As IR said, Static instability has nothing to do with 9G capability.
Please. I need to understand this if possible in lay terms and this is related to my own games with model aircraft for many years.
A statically unstable aircraft will not fly, but if it is forced to fly it will perform the most unbelievable and uncontrolled "manoeuvres" before crashing. A stable aircraft cannot be made to perform those mad manoeuvres. So a statically unstable aircraft CAN be controlled by computer by forcing it to fly stably by keeping control surfaces oriented in a particular degree and direction. This would actually add to drag because the statically unstable plane, left to itself without those pesky control surfaces forcing it to fly stably, "wants" to perform mad manoeuvres and crash.
But when hi-G "mad" manoeuvres are required - the fly-by-wire system allows those movements to occur by reducing control over the control surfaces allowing the plane to depart from its stable flight regime to the unstable manoeuvre in a controlled fashion, while preventing complete loss of control.
To that extent it seems to me that Hi-G manoeuvres and static instability are inextricably linked. Instability is not needed for Hi-G manoeuvres, but it helps
I will write a longer post later, but a quick comment - when we say statically unstable aircraft, that means when it is flying in designed configuration at designed point with all the control surfaces in neutral positions. Of coarse this is unsustainable position and the aircraft will not be able to maintain it on its own (means no changes in control surfaces).
But when an aircraft is flying in say cruise flight all the forces are balanced, pass through CG and the aircraft is neutrally stable. You achieve this situation by deflecting control surfaces from their neutral position appropriately. For example, if you take typical civil airliner types simple configuration, the wing with positive camber aerofoil is always statically unstable on its own (ignore effects of fuselage) a tail is placed behind to make the whole aircraft statically stable by providing small force at longer lever arm. Now in this config it means a down force by the horizontal tail i.e. negative lift. Generally the HTail uses a symmetric aerofoil - no camber. So to get this negative force in design config without deflecting elevators or the HTail itself (if its all moving), the tail is mounted at a negative angle, just enough so that it will make the level flight sustainable without elevator deflection. When you want to pitch up you need unbalanced force which is created by elevator deflection. You deflect elevator, pitch up the plane and put the elevator back to neutral position (not entirely correct but let say so for simplicity).
In short static stability changes with changing configuration of the control surfaces. And as I said, static instability has no implication on whether an aircraft can do 7G or 8G or 9G. It only tells you how easily you can touch the limit. 9G limit appear, first and foremost due to the human endurance and second by the structural limits of the aerostructure, thirdly the max AoA/max C_l and fourthly by excess Thrust. Static stability margin just tells you how fast it can go from 1G to 9G.
Regarding trim drag, you can actually have less drag in cruise flight for unstable aircraft (this depends on config - for ex canard vs tail and so on). For example in X29, they wanted least supersonic trim drag, means the static stability margin must be almost zero or negative even. But since there is quarter-chord distance shift in Lift centre from subsonic to supersonic flight (so while LCA is statically unstable while subsonic its quite stable in supersonic flight), this design limit meant that X-29 end up having 35% static instability margin at subsonic flight..!!
FunFact - It is estimated that the Wright brother's 1st aircraft had a whopping -20% static stability margin.!!
Point 2 - Why nose down Sir, that would put the AoA to negative no??