Forum: aviation.stackexchange.com
Tags: <aerodynamics><aircraft-performance><commercial-aviation><flight-controls>
Title: Is it possible to stall a plane so badly that the nose refuses to go down due to lack of airspeed?
Question: So basically, a stall from a high AoA to the point that the entire plane just falls with the rear end pointing straight down?   Because if wind is no longer passing over the wings, then this includes the elevator, and the elevator can’t deflect the wind with no forward speed?

Answers:
sents: text: Yes it's called a Deep Stall , and is mostly a problem with T Tail aircraft, especially jets with Supercritical Airfoil wings (like the CRJ Regional Jet line).
text: Such wings stall from the leading edge and the stall's flow separation spreads rapidly and completely across the entire wing all at once, so there is very little residual nose down pitching moment.
text: Plus the T tail ends up in a spot where it's in the wake of the flow from the wing, and gets blanked out, and loses its ability to pitch the nose over with positive lift since it's sitting there in the turbulent wake of the wing.
text: So the plane just mushes downward in an unrecoverable, stabilized, well, mush, and will pancake into the ground like that.
text: A CRJ200 test aircraft was lost in development testing when it got into a deep stall and IIRC, the stall/spin recovery parachute in the tail failed to deploy (or it didn't have one at the time; I forget which).
text: Such airplanes require, in addition to the usual stick shaker, a stick pusher to force the nose over, by shoving the control column for you if you do nothing while the shaker is going off, before the natural stall can occur, since the natural stall can be unrecoverable.
text: Generally if an airplane has a stick pusher system it means it has a deep stall mode.
answer_details: author: aviation.stackexchange.com/users/30504/John K
score: 40
sents: text: What you describe is a tailslide, as another answer has noted -- but there is a condition in which the wing is stalled and the normal recovery method (apply down elevator and wait for the nose to drop and airspeed to build) can't be used.
text: It's called a "deep stall" and is only a problem with certain layouts of flying surfaces.
text: One of the best known is a T-tail in which the spacing between wing and horizontal tail is such that it's possible for the turbulent wash from the stalled wing to completely blanket the stabilizer and elevators, making it impossible for those surfaces to overcome the drag pushing the nose up.
text: This can make a deep stall unrecoverable in certain T-tail aircraft (the F-104 was infamous for this, and it also affects some sailplanes).
text: Worthy of note is that deep stalls (commonly called a "falling leaf") were a common maneuver in the days of fabric covered biplanes; many of these had enough elevator authority to hold the stall, and enough rudder to keep the stall "straight ahead" rather than letting it turn into a spin.
text: They were used as an alternative to a slip, to dump altitude without letting airspeed increase excessively.
text: Most monoplane designs (that I'm familiar with) don't have enough pitch authority to hold a deep stall (part of making aircraft spin resistant is making them stall gently), so the maneuver has fallen out of general familiarity.
text: The difference between a controlled deep stall and an unrecoverable one is loss of pitch authority due to blanketing of the horizontal tail.
answer_details: author: aviation.stackexchange.com/users/31630/Zeiss Ikon
score: 7
sents: text: Yes.
text: Moving CG farther and farther back will eventually cause the plane to be directionally stable falling backwards.
text: Abusing rearward CG limits is contributory to this condition.
text: Secondly, poor design of horizontal stabilizer, particularly, lacking sufficient "weathervaning" area, will cause an aircraft to be more susceptible to the unrecoverable "deep stall".
text: With models, this is tested by holding the plane horizontal to the ground and releasing it with no forward motion.
text: The relative wind, being 90 degrees to the wing and tail, means they are both stalled, but the pitch torque from the DRAG on the horizontal stabilizer, the rear fuselage and the trailing edge of the wing should flip the nose down and unstall the plane.
text: Higher aspect wings and/or
text: a shorter fuselage requires greater tail area for the same pitch torque, AND, a larger weight to surface area ratio (bigger plane) also requires a greater tail/wing area ratio.
text: Placement of the all important horizontal stabilizer can also affect its performance.
text: If it is in the "shadow" of the wing, as with T tails, a very high angle of attack can limit its ability to create pitch down torque.
text: Downwash from the wing can also affect a "low" Hstab.
text: Lengthening the fuselage is a remedy for not only wing airflow effects, but also increases the pitch torquing lever arm of the aircraft, enabling the same size Hstab to be more effective.
text: Thrust angling is also a key factor to assisting in pitch down torque.
text: Many aircraft have their thrust line angled down a few degrees, which helps control pitch up tendency as the plane accelerates.
text: Choice of sound and proven design is important, as well as keeping CG within limits.
answer_details: author: aviation.stackexchange.com/users/32094/Robert DiGiovanni
score: 3
sents: text: No, it is not possible that the nose will “refuse to go down” with “the rear end pointed straight down”.
text: If the aircraft is going down tail first, then there IS airflow over the wings.
text: It may be briefly in the wrong direction, but the center of gravity and placement of the wings will soon have it pointed in the correct direction.
text: With the nose pointed down again, the wings may still be stalled, but there will be airflow over the wings, and the stall will be recoverable with the correct control inputs.
answer_details: author: aviation.stackexchange.com/users/12589/Mike Sowsun
score: 1
sents: text: If a conventionally laid out aircraft is flown into a tail slide, it's unlikely that it could maintain that attitude for long.
text: The tail has low mass relative to the rest of the aircraft and a substantial moment between the tail surfaces and aircraft center of mass.
text: As negative airspeed (falling down tail first) builds, aerodynamic forces on the tail will cause it to flip around.
text: Angular momentum will continue the rotation until there is enough opposing aerodynamic force to stop it.
text: While a deep stall may be unrecoverable, a tail slide should be recoverable, given a sturdy airframe, enough altitude and the correct control inputs properly timed.
text: Hypothetically:
text: Given an aircraft of conventional layout, if it was released at zero airspeed and level attitude, the greater surface area of the tail as well as the greater surface area of the wing aft of its center of gravity will create a force that will rotate it nose down.
text: What distinguishes this sort of scenario from a deep stall is that the condition is unstable and robustly forcing the airframe towards a normally-oriented airflow.
answer_details: author: aviation.stackexchange.com/users/3218/Anthony X
score: 1

Summary of the answers:
Yes it's called a  Deep Stall , and is mostly a problem with T Tail aircraft, especially jets with Supercritical Airfoil wings (like the CRJ Regional Jet line).  Moving CG farther and farther back will eventually cause the plane to be directionally stable falling backwards.   Secondly, poor design of horizontal stabilizer, particularly, lacking sufficient "weathervaning" area, will cause an aircraft to be more susceptible to the unrecoverable "deep stall".  
While some would argue that this kind of stalling is impossible or at least highly unlikely to happen, and when the aircraft is descending tail-first, there is airflow over the wings, it is mostly claimed that this kind of stalling is possible and is called a "deep stall" or a "tailside" and can manifest due to many factors, namely poor design of the stabilizer or moving the CG too far back. It is very hard to recover from this.