JCTherik Posted January 29, 2023 Posted January 29, 2023 Situation: ~80kn full flaps down, wings auto throttle to maintain altitude gear up speed brake and DLC irrelevant trim full nose up, stick full nose up stick lateral centre rudder centre As it is now, when pulling more and more AOA, way above 15 units, the plane eventually gets itself into a bit of a roll oscilation. But if one resists the urge to add lateral stick in the transition, increases AOA further and just rides it out, on average it flies perfectly straight. Even when disturbing the flight with momentary asymetric thrust, the cat quickly returns to the wobbly, but very straight flight when returning to symetric thrust. Lateral stick or rudder doesn't have any effect in this AOA, which is unsurprising, since the stabs are at full deflection anyway and there isn't really any air hitting the spoilers or rudders. Still, I disabled all SAS channels just to be sure, and still, stable cat. I'm not saying this is wrong, but it just seems suspiciously easy to fly high alpha pass with no electronic help, well, practically even hands off. I even ejected and the plane continues to do high alpha pass, with a bit of a roll wobble. Why doesn't it depart? What's causing it to return into nose-away-from-ground position even after putting it into 90 degree bank via asymetric thrust? Is this working as intended? Attaching trackfile. tomcat-aoa-pass.trk
Spurts Posted January 29, 2023 Posted January 29, 2023 Because the Tomcat doesn't stall. Once above 30 AoA the Normal Force (aerodynamic force acting 90 degrees from center of pressure) stays stable, so even at 45 AoA you would still have 71% of max lift, and the Tomcat makes a ton of lift. It is stable because you are at the limit of the tails, they can't pull the nose any higher, so as long as you have attitude and thrust you will never fall. Go to higher altitudes where you do not have enough thrust and you will fall, and likely spin, then likely die 5 minutes later.
JCTherik Posted January 30, 2023 Author Posted January 30, 2023 50 minutes ago, Spurts said: Because the Tomcat doesn't stall. Once above 30 AoA the Normal Force (aerodynamic force acting 90 degrees from center of pressure) stays stable, so even at 45 AoA you would still have 71% of max lift, and the Tomcat makes a ton of lift. It is stable because you are at the limit of the tails, they can't pull the nose any higher, so as long as you have attitude and thrust you will never fall. Go to higher altitudes where you do not have enough thrust and you will fall, and likely spin, then likely die 5 minutes later. I get why it doesn't fall down, my question is, why doesn't it yaw or roll out of control? In a regular coordinated flight, even if the airplane is properly trimmed, if you let go of the controls for long enough, a little bit of roll will creep in, which would slowly make your bank creep up until you spiral into the ground. But the tomcat in this regime does the opposite - it self-corrects any roll/yaw inputs, and in fact very strongly. It takes about 5-10 seconds for it to go from 90 degree bank back to more or less straight flight, with zero input from the pilot.
Spurts Posted January 30, 2023 Posted January 30, 2023 Because it is stable in roll, it has a high mounted wing with sweep (even when forward) so when sideslip begins with no other input the sweep makes the "down" wing make more lift and a zone of high pressure builds under the wing-body joint to push that side back up. I have done the same thing in a Cessna 172, a power on stall with full throttle, full aft yoke, and no rudder or aileron input. It snapped to the left, corrected wings level and pulled up level. In the case of the Cessna 172 with no sweep, the wings have dihedral to increase roll stability. 2
Rinz1er Posted January 30, 2023 Posted January 30, 2023 19 hours ago, Spurts said: Because it is stable in roll, it has a high mounted wing with sweep (even when forward) so when sideslip begins with no other input the sweep makes the "down" wing make more lift and a zone of high pressure builds under the wing-body joint to push that side back up. I have done the same thing in a Cessna 172, a power on stall with full throttle, full aft yoke, and no rudder or aileron input. It snapped to the left, corrected wings level and pulled up level. In the case of the Cessna 172 with no sweep, the wings have dihedral to increase roll stability. Great response btw!
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