Normal airflow over wing

SS

Airflow at onset of stoll

Breakdown of airflow as wing is stalled

Ficure 1. The breakdown of airflow as a wing is stalled is shown, The stall warning
system provides a small vane at a point on the leading edge of the wing. Just prior
to a shift in airflow over the wing which heralds an impending stall, the small vane
igs pushed upward completing an electrical circuit.

Wing - Critical Stoll Angle,

< stalt Warning

Signal Angle

Stall Waming System

Control Column

Stall Waring

Corrective

stall Barrier

STALL BARRIER INSTALLATION

Ficure 2, The “Stall Barrier” is shown diagramatically. When the control column is
far enough aft to place the aircraft in danger of stalling, the stall warning system
ig activated and the servo motor in the Stall Barrier pulls the column forward. The
stall is thus averted.



warning is triggered by a high angle of attack,
the magnetic clutch is energized. The stick is
then pulled forward until the angle of attack
of the wing is restored to a normal flight range,
at which point the stick is released. A rheostat
was installed enabling the stick-pull force to be
varied between 5 and 90 pounds.

Thirty pilots of all degrees of flight proficiency,
ranging from low-time general aviation pilots to
highly experienced professional pilots having
thousands of flight hours, flew the T-34 with
the “Stall Barrier.”

Each flight followed a definite protocol. A
check pilot monitored the entire sequence. The
sequences consisted of the following:

1. Normal take-off.

2. Normal climb.

8. Cruise (4000-5000 feet).

4, Stalls (normal, climbing, descending, turn-

ing, accelerated, high speed, secondary).

a, power-off (differing gear and flap com-
binations).

b. power-on (differing gear and flap com-
binations).

5. Short Field take-offs, landings.

6. Normal landings.

The average flight lasted between one and
two hours.

III. Results.

Under all normal flight conditions, the subject
pilot was unaware that the device was present.
The need for stall landings has passed with the
advent of tricycle landing gear. It is unneces-
sary to stall such present day aircraft under any
but the most remote circumstances (possible ex-
ample: ditching at sea).

All subjects found that just prior to the onset
of a stall, the stick was pulled firmly forward,
preventing the stall. The timing in all cases was
precisely coordinated with the actuation of the
stall warning vane on the left wing.

It was determined that a stick pull force of
twenty pounds was near optimum. Under un-
usual circumstances, the pilot can elect to over-

ride this. force and keep the airplane in a stall.
Twenty pounds is firm enough, though, that it
cannot be disregarded readily by the pilot. In
all unopposed circumstances, twenty pounds
force will prevent a stall or, in a stalled attitude,
provide stall recovery.

It was also determined that it is desirable to
incorporate a two second time delay in the release
so that the angle of attack is moved away from
the edge of the retrigger position. This prevents
the development of any “porpoising” motion
about the pitch axis of the aircraft, such as might
occur if a panicked individual inadvertently
rocked the aircraft on the verge of a stall.

The effects of gusts were found negligible with
respect to the device. No tendency was found
near the ground for dangerous “nose down” atti-
tudes to occur. Apparently, the “ground cushion”
of air precludes this circumstance.

IV. Conclusions.

All subjects reported optimism over the ability
of the device to prevent accidents even though
many were initially skeptical.

The “Stall Barrier” as we call it is actually a
stall warning indicator that initiates corrective
action without the pilot having to sense the
warning, interpret it, and take action. Since al-
most all present day light aircraft have conven-
tional stall warning systems already installed, an
inexpensive servo unit is the only additional com-
ponent required for the “Stall Barrier.”

The findings of these studies with the “Stall
Barrier” provide, promise that it can prevent at
least 85% of all stall accidents in those aircraft
having the device installed. If all general avi-
ation aircraft had such a device, this forecasts the
prevention of over 200 fatal accidents each year.

REFERENCES
1, Civil Aeronautics Board Summary Reports.

2. Hawkes, Gienn R.: “Tactile Communication,” Civil
Aeromedical Research Institute Report 62-11, Federal
Aviation Agency, May 1962.




AM 66-31

THE “STALL BARRIER” AS A NEW PREVENTIVE IN
GENERAL AVIATION ACCIDENTS

C. W. Von Rosenberg
F. R. Keen
Stanley R. Mohler, M.D.

Approved by Released by
S Aha ?S.2 “A YT»)
J. R. Duzz, M.D. P. V. Smcer, M.D.
Cmer, Civ, AEROMEDICAL Feperat Am Surceon
InstrruTs

September 1966

FEDERAL AVIATION AGENCY
Office of Aviation Medicine



FOREWORD

Mr. Von Rosenberg was Chief, Flight Standards Technical Division,
Aeronautical Center, Federal Aviation Agency, Oklahoma City, Oklahoma.
He is presently Vice President, Lear Jet, Wichita. Mr. Keen is a research
pilot in the Flight Standards Technical Division. Dr. Mohler is Chief, Aero-
medical Applications Division, Office of Aviation Medicine, Federal Aviation
Agency, Washington, D.C. Presented at the April 17-21, 1966, Meeting of
the Aerospace Medical Association in Las Vegas, Nevada.

Qualified requestors may obtain Aviation Medical Reports from Defense Documentation
Center. The general public may purchase from Clearinghouse for Federal Scientific
and Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151


THE «STALL BARRIER” AS A NEW PREVENTIVE IN GENERAL
AVIATION ACCIDENTS

I, The Problem.

In 1964 the following accident experience oc-
curred in general aviation. There was a total of
5,185 general aviation accidents and, of this num-
ber, 508 were fatal accidents resulting in 972
fatalities There were 653 accidents that re-
sulted from stalls and these accounted for:

12% of all accidents (653 of 5,185)
26% of all fatal accidents (131 of 508)
25% of all fatalities (242 of 972)

This shows how much of a killer this type of
accident is. Actually, stall-spin accidents have
plagued aviation since its inception, and, as
shown above, continue to do so.

In the light of this, the Federal Aviation
Agency’s Flight Standards Technical Division at
the Aeronautical Center, Oklahoma City, was
asked to study the stall requirements of Part 23
(Airworthiness Standards) of the Federal Avi-
ation Regulations. This study was for the pur-
pose of determining if any revision of the regu-
lation may be indicated.

Early in the project, it became apparent that
the regulations were such that any substantial
increases in stringency would impose impractical
airworthiness requirements. In other words, it
was felt there was nothing that could reasonably
be done at present to the regulations.

It was concluded that in order to stop people
from killing themselves in stall-spin type acci-
dents, we would have to stop them from stalling
the airplane. With this thought in mind, we
began thinking of what has been done in this
direction and what else can be done. We re-
viewed the efforts of designers who used slotted
wings, restricted controls, stall warnings of vari-
ous types, etc. All of the methods had deficien-
cies or limitations. Stall warning has been looked
upon through the years as a means of helping
the pilot avert a stall.

Natural stall warnings have consisted of char-

acteristics such as change of sound as airflow
velocity changed, buffet, vibration, control pres-
sure changes, and others. Numerous methods
have been used to produce artificial warnings in
light aircraft such as horns, lights, and whistles.
In collaboration with the Civil Aeromedical Re-
search Institute, it was learned that visual and
aural sensory input channels are often overloaded
in flight, while the tactile sensory channel is not.?

It seemed, therefore, that a warning device such
as a “stick shaker” would possibly provide a solu-
tion. However, as the name implies this unit
requires the pilot to sense the warning, interpret
it, and take action. This system still has the de-
ficiency that a pilot who is not alert or is pre-
occupied with any one of a great many flight
situations is susceptible to being victimized by a
human failure: the failure to sense the stall warn-
ing and the impending stall.

As military aircraft were pushing into the
higher performance regions in the early 1950's,
stall difficulties were encountered which necessi-
tated the installation of powerful stick pushers
to prevent a stall from ever occurring. Refine-
ments of this device are in use today in some of
the higher performance military and civil air-
craft.

As we reviewed this work, it became apparent
that a similar device could be the means of sav-
ing many of the lives in general aviation that
are lost each year through inadvertent stalling.

IL, Method of Procedure.

In order to validate this approach to the prob-
lem, a small electric servo unit was installed in
the cockpit of the Beechcraft T-34B aircraft
operated by the Civil Aeromedical Research In-
stitute. A cable from the drum of the servo
unit was connected to the stick. The magnetic
clutch of the servo was wired into the airplane’s
conventional stall warning system (present in
almost all light aircraft) so that as the stall