Characteristics of a T-tail/ deep stalls

Hello all,
Have you ever wondered about when you get this when flying the b717?


I shall explain the science behind the T-Tail and the deep stall
(Btw if you are doing a PPL i think you need to know how to recover this)
A deep stall (or super-stall ) is a dangerous type of stall that affects certain designs, notably jet aircraft with a t-tail configuration and rear-mounted engines. In these designs, the turbulent wake of a stalled main wing, nacelle-pylon wakes and the wake from the fuselage “blanket” the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall. Aircraft with rear-mounted nacelles may also exhibit a loss of thrust. T-tail propeller aircraft are generally resistant to deep stalls, because the prop wash increases airflow over the wing root.

(Source: Wikipedia)

IMG_2897
(Source:Wikipedia)

How to fix it
The optimal sequence of control actions first uses the rudder to yaw the horizontal tailplane out of the aircraft’s own wake to regain elevator effectiveness, and then uses the elevators to pitch the nose down and recover from the stall.
And now you know
(Source: google)

Dutch roll
A dutch roll is a type of aircraft motion consisting of an out-of-phase combination of “tail-wagging” (yaw) and rocking from side to side (roll). This yaw-roll coupling is one of the basic flight dynamic modes (others include phugoid, short period, and spiral divergence. This motion is normally well damped in most light aircraft, though some aircraft with well-damped Dutch roll modes can experience a degradation in damping as airspeed decreases and altitude increases. Dutch roll stability can be artificially increased by the installation of a yaw damper. Wings placed well above the center of gravity, sweepback (swept wings and dihedral) wings tend to increase the roll restoring force, and therefore increase the Dutch roll tendencies; this is why high-winged aircraft often are slightly anhedral, and transport-category swept-wing aircraft are equipped with yaw dampers.
(Source:wikipedia)

IMG_2901

(Source:Wikipedia)



note what the auto pilot is doing in this photo

How to fix it
To manually negate a Dutch roll, the pilot can use rudder inputs to supplement the yaw stability and negate the alternating oscillations , thus returning the aircraft to straight and level flight more quickly than if it were left to self-stabilize. Most modern aircraft will not require any intervention for Dutch roll, since the oscillations tend to die out on their own. However, if a plane is susceptible to this phenomenon, a integrated yaw damper can be used to counteract it.

(Source:google)
This is why on T/O and landing the 717 gives you a stall warning

Or its because of old physics and i just explained a deep stall and a Dutch roll

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Hmm
Pretty interesting things! I would like to see more of this on the forum.
I have a question though. Unfortunately I didn’t study deep stall recovery for T-tail aircraft, but my aerodynamics knowledge confuse me with this topic.
Don’t we get sideslip when applying rudder pedals? I mean sideslip prevents laminar airflow recovery and this is opposite of what we need in this situation. Could you clarify that please?

By the way, we also get side g-overload when yawing, which increases the stall speed even more 🤔

If you have the same overall yaw input as the rotational speed of the aircraft when approaching a stall/ Dutch roll, it should recover the aircraft

Great! Now I know how to fly the 717 or the 727 on xp11.

Thanks for the information

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i did not know this till Now now i know how to be extra careful on the b717

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Who would’ve thought… Thats terrifying, lol.

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