The experience gained in over four years of STOL operations with the augmentor wing research aircraft and the background of operation of other STOL powered-lift aircraft indicate that the use and percent of lift achieved by powered lift have significant effects on the operational characteristics of STOL aircraft and, therefore, on the performance that can be achieved. A brief description of the augmentor wing including the means by which it achieves its powered lift is presented. Specific problem areas relating to the control of longitudinal flight path are discussed as well as the consequences these might have on the design and operation of this class of aircraft. Particular emphasis is given to the approach and landing phase of flight where the consequences of the powered lift are most pronounced, and the resultant characteristics most different from those of conventional aircraft

Innis, R. C.

Quigley, H. C.

English

NASA Technical Reports Server

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DESCRIPTION OF AIRCRAFT
Tho augmenter wln_ wa_ deHlgnod an a low-cost, low-speed rt.t_earch vehicle
that could be urged to Investigate the opt:rational charactorlt_tlc_ of a
powered-lift Jet STOL aircraft in the environment of thr terminal area
including takeoff, transition, approach, and landing. The a_rcraft, shown
in figures I, 2, and 3, was modified from a deliavilland C 8A Paffalo whlch
was donated by the USAF. The G. E., T-64 turboprop L,ngine24 were replaced by
R. R. 3pey turbofans, The wing area was reduced by removing about 2 m ,
from each wing riband fixed, full-span slats were ,installed -n the leading
edge. The landing gear was fixed in the down position and modified to
accommodate a higher gross weight. The spring tab controlled elevator
system was changed to a hydraulic powered unit, and the c,mventlonal d_uble
slotted flaps were replaced with an entirely new augmented Jet flap system.
This flap, illustrated in figure 4, consists of two nearl_ parallel surfaces
wlth a continuous double slot nozzle located between them which acts as an
ejector pump with air drawn in from both the upper and lower surface of the
wing.
Air for the flap nozzles is provided by the fan section of the Spey
compressors. The lower nozzle is supplied by air from the engine o_ _he same
side of the airplane, while air for the upper nozzle is cross dueled from the
opposite engine. This arrangement reduces the as)munetry which would occur
should an engine fail during takeoff, approach, or landing. The purpose of
all this is to augment the thrust from the ejector nozzle and also to induce
airflow over the surface of the wing which increases its llft. The aft
portion of the lower surface of the flap is hinged so that it can be closed
thereby choking the au6_nentor and spoiling the lift. The outboard chokes
are used for lateral control while the inboard chokes are modulated for direct
llft control. Additional lateral control is obtained from drooped ailerons
provided with BLC and from spoilers located in front of the ailerons.
The hot gases from the Spey engines are exhausted through Pegasus-type
swiveling nozzles which are located on both sides of each engine nacelle.
They can be positioned from nearly straight aft to slightly forward of the
vertical and are controlled by levers located adjacent to the overhead
throttles in the cockpit. During Lhe approach where the nozzles are deflected
nearly normal to the flight path, they contribute about 1800 newtons
(8000 Ib) of direct llft to the airplane. However, this is only a small part
of the powered llft that is achieved by the augmenter wing as shown in
figure 5. This bar graph compares the airspeed that corresponds to a given
• angle of attack with varying amounts of thrust. The center bar represents
our nominal approach conditions, 65 knots at 4* angle of attack, utilizing
about 2/3 of the available thrust. If there were no thrust, the airspeed
corresponding to this angle of attack would increase to i00 knots. The
thrust from the swiveling nozzles would account for only about 7 knots of
this difference. Applying maximum thrust, which might occur during a
wave off, would decrease the airspeed by about i0 knots.
32
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Following the initial documentation ,_nd proof-of_conc¢;pt flight tenting,
the aircraft wan oqulppod with STOLAND, Thin in an experimental digital
avionlc_ eystom which, throu_;h It_ computer, _onhe'.;-_and nerves, can drive
any or all of the prtmary and necondary controln, Thin nllow_ u_ to Indopon_
don_ly vary tht: llft, drng and ntabiiity characto,ristlcn of the augmenter wing
so a_ to ropre_ont the resp,mz_o charactoritltics of a wide. rang_ of aircraft
of thi_ cla_s. Subsequent fl_$:ht testing ha_ emphal_lzed the e',am:nation of
STOL handling qualttiv_ over a_ broad a ronge of thene chnractert_tics as tfi
pract ical.
Most of these flight tests were conducted at a Naval Au_tliary Landing
Facility called Crows Landing, located in the San Joaquin Valley of California.
The approaches were conducted on a 7-1/2 ° glide slope with guidance provided
by an experimental microwave landing system called MODILS. Some of these
approaches were hooded to simulate instrument meteorological conditions. The
landings were made to a 518 m " 30 m (1700 × 100 ft) STOL strip marked out on
one of the main runways.
DISCUSSION OF RESULTS
The environment in which the airplane has operated, in terms of wind and
turbulence, is indicated in figure 6. The points represent the maximum wind
velocities and direction relative to the landing runway. The lines extending
from the points indicate the gust factor. The grid resolves them into their
headwind and crosswind components. These are tower reported winds which do not
accurately depict the conditions at the touchdoxcn zone but are at least
representative. Approaches with headwlnds of 30 to 40 knots and I0- to
15-knot gusts were negotiated without great difficulty although they did take
a considerable length of time and were sometimes subject to large flight path
excursions. Landings with crosswind components in excess of 20 knots were
relatively easy even though the decrab maneuver of some 20 ° required full
rudder. The most critical condition in terms of both safety and performance
was approach and landing with a tallwind component. The higher descent rates
tax the capabilities of both the aircraft and the pilot_and landing distance
increases dramatically. This is illustrated in figure 7 which depicts the
results of some landing performance tests. These landings were performed
on two back-to-back flights, the first of which was made with a light tail
: wind which steadily increased to about i0 knots as the flight progressed.
The second set of landings was made into the wind. It is apparent that as the
wind velocity approaches i0 knots, landing with the wind rather than Int( it
_ effectJ.vely doubles the stopping distance.
It was recognized early in the design of the aug_entor wing that stabil-
ity augmentation would be required to achieve satisfactory handling qualities.
: This is typical of those aircr_ft which operate at h_gh llft coefficients and
low dynamic pressure. The initial flight tests were made with a ]ateral-
directional SAS which provided positive spiral stability, increased roll and
yaw damping, and improved turn coordination. Later in the program, more
advanced augmentation schemes were examined. Attitude command and rare-
33
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command attitude hold w,2rc ,_v,lu_,led in both the pitch and roll axis. With
attitude command, the .hlllty to zeturn the alrcraft to wings level and trimmed
pitch attitude was ;q)pJecJated by tilepilots; however, the sustained control
forces and defhctloJl=_ ,equ]r,,l wl,_-nmancuverlng in either pitch -Jr roll were
objectionable. W:Ith those cot_Ilguratlons which _equlred pitching the aircraft
for flight path control, the film button which was used to change the refer-
ence pitch attltud_ b_,came a IZlmaly but somewhat awkward controller. Because
of this the pi.lot:ss,.,ttl(,,.1o_ int¢. command attitude hold as the basic SCAS
configuration. It sl,o_iI,Jbe pointed out that acceptable STOL approaches and
landings were perfor[_,_dwitl_m:t any SAS or SCAS in light to moderate turbu- "_'"
fence, but only trader v [.';uaJ fl.[ght conditions.
Having arrived ;_t-an acceptable stability and control augmentation
scheme, we proceeded to _.,×am_nethose characteristics which are peculiar to
powered llft. These, of coulse, occur primarily in the longitudinal axis as
shown in figure 8. I;ith co_v,:mi:Iont_lhi,craft, the thrust exerts a force
along this axis whicb In qt,.;_dF flight balances the drag force. Changes in
thrust produce a l.mr'_ t u,.l_,._ 1 at(..,:, [eration.
The concept _[ p,,',.'er_,,Jlift implies that the lift produced by the wing
is dependent up_,:_tl,.....T,o,1,t _,I thrust applied. In order to achieve a low
approach speed and ,,a[.,taina :__teepdescent angle_ the thrust must be also
deflected or turned so a_" t,,properly balance the longitudinal and normal
forces. In this ,:.,:ala_;l_:,,tl,e thrust vector includes the contribution of both
the cold air fro,, tl,e fJap ,,ozzle and the hot gases from the swiveling nozzles.
Changes in thrust cow ii:._,re_ethe lift _.hich produces more change in normal
force than longitud_n,_l a,:,:-<],,.ati:.,n;,,adin some cases may even cause the
aircraft to decelerate, _:llenthou,st Js increased. This provides the pi_ot
with a powe_ ful me.ms by whicl, he can change flight path angle but leaves him
somewhat at a loss as tu how te ma_mge airspeed control. In the case of the
augmenter wing, the :_t:.[::elh!g_,,z'_:[eswhich divert the hot gases from the
Spey engines provide an eff,:_'t_veInt'ausof changing airspeed. As the pilots
gained expe_.ience aud fa,_ilia,.!tywith the airplane, they learned to use the
nozzles in conjunction %,it:,!Ira lhrott!es to adequately control the flight path
(and airspeed), lh>wcver, la [l.e p_'_,+nce_of turbulence and wind shears, the
pilot workload became quite higl, _md there was sometimes confusion as to
which set of levers to move first. In an IFR environment, glide slope
tracking was poor, th_!.r,..fc,re the pHots concluded that the use of three
different control].et._ I,_, t],_H:!_a_.,c,:,:ntof the longitudinal task was too
much to cope with ['o,: +'_:,,lvd_,_. ,In_r;ttfon.
Leaving the n->:z,,a !_:._:,,!.t :,ome predetermined value requires the pilot
• to control airspee,l by ,I_,,,}<_i,!,.l,itch attitude. If the effective thrust
turning exceeds about _!_I",n,1_,er:_,:coupling can occur between thrust and
longitudinal acc_.)t,.,l{,_ _lt-_I_w;ll r,_,mpoundthe control problem. As thrust
iS increased at c,,_ :t:_ ._. :i,:,_,, :_i_q_ucd dec;,\'s, which puts the
aircraft furth,'r ,,,' _i._,. _'., i: i,i, ,_" I_hc lll_u:;t required curve, As this
occurs, th,_ Ii[g}_l i''!,' i,:,l,_:< .l[,aini..hesand tlm pilot is forced to add
"" still more power, 'if,,:l,_,,I,l_.l, ;'; illu;_t_ated in fivure 9 which is a time
hfstory of an al,l,_,,ah witl_ :,,¢ml _;uratiun which has substantial adverse
thrust-alrspeed cou!,l I,_,.,
34
ORIGINAI_PAGE L_
0_' POOR QUALITY
00000001-TSD01
Glide slope intercept is from above and is initiated by a change in
pitch. Tracking is accomplished with thrust while attitude is held relatively
constant. At about 80 sec the aircraft starts to descend below _he glide
slope which prompts the pilot to add thrust. Airspeed decays though, and the
aircraft descends still lower until the thrust is at the maximum allowable
and the airspeed is well below the desired value.
The obvious soluKion to this problem_to everyone but the pilo_ is to lower
the nose to gain airspeed. However, to be effective, this requires a falrly
large change in attitude -- at least 51 and the initial response of the
airplane is to descend even steeper. Furthermore, the recovery time to regain
airspeed is such that the approach had best be abandoned.
One question which the pilot must address with a powered-lift aircraft
is how much power can be used in the approach. Assuming he has the option
of changing the inclination of the effective thrust vector by either flap
or nozzle deflectlon or by some other means, he can increase the amount of
thrust used and thereby reduce the approach speed while maintaining the low
effective llft to drag required for the steep flight path angle. In other
words, the approach speed depends upon the amount of thrust used; but the
margins in terms of fllghtpath capability depend on the excess thrust '
available.
Our pilots felt that they would llke to have the capabillty of achieving
level flight without requiring a change in configuration. Assuming that this
performance is available under standard conditions, the pilet must also
concern himself with what adjustments must be made to accommodate temperatures
above standard and higher altitudes. Figure i0 presents a chart which was used
for this purpose with the augmentor wing. It allows the pilot to determine
what rpm is requir, d to achieve the thrust that would be realized on a standard
day. For example, our approach speed was predicated on a nominal 93 percent rpm
for standard day conditions. For a day on which the temperature was I0 degrees
above standard, 94.5 percent rpm would be required, and if in addition, the
fleld elevation was i000 m (3280 ft), about 97 percent would be needed. Under
these conditions, there would be insufficient thrust remaining to allow adequate
flight path corrections. In this case our pilots would select a lesser nozzle
deflection and accept a higher approach speed with its reduced thrust
requirement.
SUMMARY OF OPERATING PROBLEMS AND CONSEQUENCES
Perhaps the greatest asset of a STOL airplane in terms of safety is its
low closure rate to the intended touchdown point. It allows the pilot time
in which he can observe, react, and make corrections. Powered llft is an
attractive means of achieving this performance while still malntaln_ng the
high speed cruise and efficiency of a Jet airplane. There are, however,
_ certain operating problems which are inherent to the concept. Some of these
35
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are listed in table I along with the implications they might have on either
, the design or operation of the aircraft. They are divided into two categories,
i the first of which includes those problems which are brought about by
"_ operating at low dynamic pressures and high lift coefficients. Our experience
has shown that the low aerodynamic stability and damping associated with this
condition will require some form of augmentation in order to provide satlsfac-
_-_i tory handling qualities. The effect of w_nd naturally becomes more pronounced
as its velocity becomes greater relative to the approach speed• More direc-
tional control is required to accommodate the higher sideslip or crab angles
associated with a given crosswind component. In addition, turbulence or
: gustiness will probably dictate a requirement for increased flight path --'"
control. Runways whose length is determined by no-wlnd stopping distance are
_ comfortable to land on in a head wind but suddenly become too short with a
::_ light tailwlnd component.
The second category includes operating problems which are the direct
;_ result of powered llft The first three of these are the subject of discussioni •
:. in reference I. I would llke to comment on them from the viewpoint of a pilot.
:, The first two items should actually go together, since the adverse effects of
__ speed variations are due in part to the poor ability to control airspeed.
_" Because of the operation on the backside of the _nrust required curve, these
•:_ aircraft will probably experience greater flight path excursions when encoun-
terlng wind shears. Airspeed management through the use of an additional
_: controller to be operated by the pilot seems impractical, so some form of
_t
_; automatic speed stabilization may be required. Adverse coupling can, of
_: course, be minimized by design, but if the full performance benefits of the
o_, powered llft system are to be realized, some form of control _ugmentatlon
!_ may be required. The final item is a fact of life which must be accounted for
in the day-to-day operation of this type of alrcraft. The use of flat-rated
engines will allevlate the situation because takeoff thrust will be available
_: under all conditions up to the rating limits. However, charts will still have
il to be used to determine proper thrust settings, and operation outside these
J_ limits will sometimes require a configuration change if adequate safety
!i margins are to be preserved.
_
_; REFERENCE
_r
i: I. Franklin, James A.; Smith, Donald W.; Watson, De Lamar M.; Warner,
_!= David N., Jr.; lnnis, Robert C.; and llardy, Gordon H.: Flight
Evaluation of Advanced Flight Control Systems and Cockpit Displays for
_; Powered-Lift STOL Aircraft. A_rcraft Safety and Operating Problems,
:' NASA SP-416, 1970. (Paper no. 4 of this compilation.)
•_
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TABLE I.- SUMMARY
Operating problems Consequences
Due to low speed
Reduced stability and damping SAS or SCAS required
Effects of wind and turbulence Increased control required
Field length more sensitiv_ to wind
Due to powered lift
Poor ability to control airspeed More sensitive to wind shear
Adverse effects of speed variation May require speed stabilization
Possible adverse coupling between Can be minim/zed by powered-lift
thrust and airspeed system design
May require SCAS
_ncreased effect of temperature Landing performance must be
&ud altitude on landing performance computed like takeoff performance
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STANDARD DAY
GROSS WEIGHT, 18,700kg
55 "_"
OFF APP MAX
THRUST
Figure 5.- Effect of thrust on approach airspeed constant angle of attack (40).
0 m l
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x 30-
'°ro(ous,
_ COMPONENT 0E ot i , i_
x -10 0 0
-20 I I 1 I
-30 -20 -10 0 10 20
CROSSWIND COMPONENT, knots
Figure 6.- Haxlmum wlnd conditions encountered in flight tests.
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_, TOUCHDOWN ZONE NO WiND
METERS
ii 100 200 300 400 500
:- 0--.. I I I t I I I t I
: i' 400 800 1200 1600
_ FEET
DISTANCE FROM THRESHOLD
Figure 7.- Effect of wind on landlng distance.
i.I' W W
CONVENTIONAL
_' POWEREDLIFT
_: Figure 8.- Effect of thrust varlatlon.
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._.,+ _ Figure 10.- Effect of temperature and altitude on approach thrust.
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Operational experience with a powered-lift STOL aircraft

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