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From: gopal@sun701.UUCP (Gopal Ramachandran)
Newsgroups: rec.aviation
Subject: Re: Airliner stalls
Message-ID: <1743@cirrusl.UUCP>
Date: Tue, 22 May 90 19:51:27 PDT
References: <1030@cica.cica.indiana.edu>
Sender: news@cirrusl.UUCP
Reply-To: cirrusl!sundial!gopal@sun701 (Gopal Ramachandran)
Organization: Cirrus Logic Inc.
Lines: 174

In article <1030@cica.cica.indiana.edu> greg@cica.indiana.edu (Gregory TRAVIS) writes:
>Not sure if this should go in sci.aeronautics, but...
>
>Can anyone comment on the stall characteristics of modern airliners?
>I do know that a sweptback wing is trouble in a stall, as the outboard
>portion tends to stall first, trashing any roll control and lateral
>stability.
>
>Now, a 727 has a "T" tail, so the elevator may be blanketed by the
>wings at high AOA/Stall.  However, I recall that the BAC-111, an airliner
>similar to the 727 in configuration, had to be modified early in
>its career as one was lost from elevator blanketing during stall testing.
>
>Is a 707/727/747/DC-9/DC-10/etc. unrecoverable in a nice and/or deep stall?
>What types of spins does an airliner tend to get into?

You bring up some interesting questions. I did a little research into
this and here's some of the info I've been able to come up with:

BACKGROUND:

Modern jet transports have stall characteristics quite different from
propeller driven transports for several reasons:

- lack of propwash-induced lift
	Nothing startling here. Power-off and power-on stalls are
about the same except for the effect of the slight vertical thrust
component helping to reduce effective weight. Also lack of the speed
stability contributed by props (read on).
	
- use of sweptback high-speed wings
	Sweepback is used to raise the critical Mach No. or Mcrit
(when flow over some part of the a/c first reaches the local speed of
sound). Of course, cruise Mach is generally higher than Mcrit, but
must be below limiting Mach no. or Mmo (when transonic effects can
cause real stability and/or controllability and/or structural problems)
  
Sweepback, along with dihedral, increases static lateral stability.

 Sweepback (with tapered wings) causes flow separation to occur at the
wingtips first (spanwise flow and higher loading due to taper) and
this means that the center of lift moves forward, causing pitchup.
Pitchup is destabilizing, as it implies moving further toward higher
angle of attack (alpha). This can happen at extreme alpha, even though
designers  use fences, inboard stall strips, different airfoil
sections, and wing twist in order to reduce this tendency at moderate
alpha.  The danger is in going beyond the initial moderately high
alpha (stable nose-down recovery point) with further pulling back of the yoke.

- need for high-lift devices at low speeds
	Sweptback wings are not efficient at low speeds, so complex
and expensive devices (slats, multiply-slotted flaps, vortex
generators) are used to increase lift (Cl) at low speeds and keep the
flow attached at high alpha. Unfortunately, this also means that when
the flow does separate, it can let go pretty abruptly, and any slight
asymmetry or yaw could cause a sudden roll. This says that getting low
and slow in a jet with everything hanging out could be very dangerous
to your health.

- fuselage lift
	At high alpha, the fuselage continues to develop lift and has
a significant effect because it can be so long, and wide, and have so
much area in front of the wing. This fuselage lift is destabilizing as
the fuselage will generate lift even after the wings get tired.

- speed instability at low indicated speeds (near best L/D)
 All jet transports have this to some extent, though some are a lot
worse than others (the Concorde, for example). The ones which have a
tough time holding speed are the ones that really need to be flown
with auto-throttles at low speed. There are a couple of reasons for
this. One is that they dont have the helpful stability contributed by
propellers. Piston engines at a constant MAP and constant RPM are
producing constant BHP, so as speed goes up, the thrust goes down, and
conversely, so there is a natural stabilizing factor.  Turboprops are
also flown at constant RPM and constant torque, so shaft BHP is constant.
	Jets are flown at constant THRUST, which doesnt have any
natural stabilizing effect.
	The other reason is the shape of the Lift/Drag curve for jet
transports compared to the curve for propeller transports. Jets are
sometimes operated on the back side of the L/D curve (where the speeds
are lower) and this in itself is destabilizing (increase in speed may
lead to decrease in drag and vice-versa). In many jets, 1.3Vso is less
than the speed for minimum drag, leading to speed instability.

- transonic (Mach) effects
	At high altitudes, stall buffet can be mistaken for Mach
buffet (and has contributed to some accidents) and in coffin corner,
these speeds converge.

- large changes in CG and center of lift positions
	By their very nature, large jet transports have huge ranges of
CG (long, lots of variations in cargo/people loading configurations)
so they invariably have all-moving tails (stabilators) to accommodate
for this. In addition, the center of lift can move a lot because of
the effect of all the high-lift devices, the Mach effects, fuel
burnoff.

- reduced aerodynamic damping at high altitudes
	In the thin air at high altitudes, the lateral and vertical
empennage areas and the wings have much reduced damping effect in yaw,
pitch, and roll, allowing excursions about the axes to have larger
amplitudes.

OK, now to address the main question.

AIRLINER STALLS:

All the above factors should make it clear that jet transports can be
rather unforgiving at slow speeds. The worst case situation is with
transports that can be "deep stalled", the rear-engine, T-tailed
configurations.

	Rear engine transports generally also have T-tails in order to
get the stabilizer into clean air. The weight of the engines biases
the CG back and this means that the wing must be moved back to reposition
the center of lift aft of the CG where it normally belongs (in order
to let the nose drop when the tail downforce goes away at high alpha as the
separated wing wake hits the tail, causing natural aerodynamic buffet) .

In the case of the 727 and similar birds (Learjets, for example), the
 aft wing location and the high tail interact unfavorably. At high
alpha, the T-tail is awash in turbulent wake from the stalled wing and
moving the surface has no effect. Meantime, the airplane has pitched up because
of the other reasons mentioned. Of course, the high drag and
consequent high descent rate just increases alpha and the bird is
quite happy to stay in this stable pitch attitude until it strikes
the ground.

Aircraft capable of a "deep stall" may get into it inadvertently. High
pitching inertia (especially with aft-mounted engines) can cause the
the airplane to pitch up a further 6-8 degrees (when rotating up) even
 after the pilot applies forward yoke. That may be enough to enter a
deep stall. This may be even more likely when the airplane has
acquired pitching momentum following an accelerated stall entry.

Once the a/c is in a deep stall, the occupants are in deep s**t.
As the a/c starts descending rapidly, the alpha becomes so high that
the air cant "turn the corner" at the engine inlets, and all engines
will flame out. The good news is that the view will be good all the
way down.

Jets with low stabilators, and wing-pod mounted engines do not deep
stall (as far as I've been able to find out) but of course are still
capable of normal stalls (unless alpha is limited by computer
control) and are subject to all the above-described problems
instability, lack of natural aerodynamic warning, etc

So they wont deep stall, but they're still capable of raising your
blood-pressure at high alpha, and still require lots of altitude for
recovery. Consequently, most jet transports have stall warning
devices, and even alpha-reducing devices (stick-pushers).

Now spinning a jet transport is a whole different thing altogether.
No-one does it on purpose, not the factory test pilots, not the FAA
certification pilots, not the line pilots. Let's assume you're not
thinking about spinning a 727. Assuming you get enough yaw after the
stall to get a spin going (why not pull the thrust lever back on one
side, and push it forward on the other?) the airplane will probably
spin very nicely. The rotational momentum will probably be high enough
that there wont be enough rudder to stop the spin. Ane even if you do
get it stopped, your engines flamed out long ago (axial-flow engines
like to get their air straight down the inlet, and even centrifugal-flow
engines probably wont tolerate the sideslip angles generated in a
full-blown spin) and you lost too much altitude in the spin and
subsequent pull-out (watch out for accelerated secondary stalls and
for ripping the wings off, remember, we're talking only some 2.5g
positive for a load limit, not 6 or more) for a restart.

Most of these factors are true with tactical jet aircraft, and are
even worse with some of them. Think about air combat maneuvering in
early, non-FBW airplanes like F4s, F-106s, etc The pilots really
earned their (meager) pay. No wonder they wear ejection seats.

Gopal