Wing position and roll stability

wing position and roll stability


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18 replies

  1. Would a moderate to high degree of negative stability improve roll performance (and by extension transient performance) then? It would be hard to control such an aircraft, but if it worked, it would be quite impressive.

    • Yes, higher negative stability means quicker roll response. However, too high degree of negative stability has multiple distinctive negative effects, and ability of FCS to compensate for these is rather limited.

    • I do remember reading that too high a negative stability makes it really hard on the pilot to fly the aircraft. What other negative impacts are there? Perhaps moderate negative might be the best compromise?

      Are you referring to static or dynamic stability here, or both?

      Finally, I would guess a CAS aircraft would also want a low negative stability as well?

      • I’m talking about roll and directional stability.

      • Regarding the effects of directional stability, insufficient directional stability is dangerous because it means high probability of departure and uncontrollable spin. Excessive directional stability on the other hand causes Dutch roll, where aircraft rolls around the line of flight (basically, moves as if it will depart to one side, automatically returns towards the stable flight, overcompensates, returns towards the center, rinse and repeat). This is obviously not dangerous, but it causes extreme drag and thus reduced range, and worsened flight characteristics in general.

      • Yeah I think that there needs to be some compromises there.

        The problem with the Dutch roll is that it could be very fatal in a dog fight. I suppose so too would an uncontrollable spin.

        It seems that moderate-high negative stability may be the best compromise?

    • You need lotta cpu power and very sofisticated software to fly an high negative stability fighter: this the case of typhhon,; that’s why (not considering advanced materials) russian cant built one: typhoon has by far the higher STR in supersonic of every existing plane : STR is equivalent to G6 at mach 1,6; thats why typhoon, raphale and f22 gonna all fight in very similar way in air vs air : supersonic, possibly supercruising, at high G’s, lock,fire,turn…Meteor and very good radar complement each other with with high STR in supersonic…No more wvr…everyone gets killed in wvr…

      • F15 aerodinamic seems less optimized then sukhoy’s on in subsonic but much less punishing in supersonic: am I right Picard?

        • Yes, you’re right. Also, it seems that Flanker series has made some compromises for ground attack: podded, widely separated engines are excellent for redundancy in face of ground fire, but are terrible for transient maneuverability in dogfight (due to high roll inertia); gun is of a relatively large calibre, with heavy shell and large HE charge but low muzzle velocity and low rate of fire – again, better for ground attack than air-to-air combat.

      • WVR will still be impossible to avoid in any larger engagements vs a competent enemy, especially with limited ID range of visual sensors (cca 40 km for PIRATE and OSF IR channel, 45 km for OSF video channel), but you are correct that supercruise and supersonic maneuverability are a large advantage, at least in initial engagement.

      • This.

        It seems to have been an example of a “multirole” compromise. If one of the engines is taken out in a dogfight, the aircraft is a sitting duck and will yaw badly.

        I suppose the greatest advantage the Su-27 series has is that it can outlast the enemy through it’s size and fuel fraction (top models are 0.42).

        You cannot avoid WVR – that’s the problem. You may as well build your force around the assumption of WVR. Many of the world’s air forces seem to have done the exact opposite. The F-35 is a good example of this.

      • There are a few ground attack variants of the Su-27. The Su-34 comes to mind right away.

  2. Before I state my main point, I want to express this idle thought.
    Even though it is not part of aviation nomenclature, I often use the term ‘delaminated’ when describing the state of air on a wing at stall. The air stops flowing smoothly over the wing, and the boundary layer is more or less removed from the wing’s surface, so I’ve come to call airflow at stall “delaminated.”

    As I understand, canards create vorticies which help in keeping the boundary layer attached to the wing; however, I am curious as to how wing sweep and aspect ratio affect the vorticies.

    • Higher sweepback means that vortices form at lower angle of attack. This increases drag but also allows higher maximum AoA (and thus lift). Hence usage of highly-swept LERX with moderately-swept wing, as on F-16, Rafale, F-22 and Gripen NG, to combine high L/D ratio and good takeoff/landing characteristics of moderately swept wing with strong vortex flows and high stall AoA of highly swept wing.

      It is a bit more complex when canards are included, as canards also affect lift-drag ratio. Generally, highly swept canards are optimal for maximum lift while low or no sweep canards are best for L/D ratio. 45* canard (as used on Gripen and my FLX proposal) is a 50-50 compromise between the two, and IIRC Rafale’s canard has a sweepback about the same as its main wing (48*).

  3. Typhoon design resemble more and high negative stability then a moderate one; designing an high negative stability is more complex in developing software control that translates pilots orders or in high stress materials know how ? Tornado resemble an high stability design, and that it is quite obvious as its main aim was originally a low level penetrator/bomber; what it looks awful is mig31 design : how can it fly supersonic effecitevely beeing a no delta no negative stability design? May be i can fly supersonic only in a straight line with an enormous consumption of fuel; same for other sukhoys; pretty sure a mig 31 flying supersonic can be detected trough eurocanards IRST from 150km at least.

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