Introduction to Stability and Control Problems of High Speed Aircraft While airplane sp

145 Pages·2015·24.07 MB·English

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B, 1 - Introduction to Stability and Control
Problems of High Speed Aircraft
While airplane sp· eds were entirely below the subSonic critical,
the problems arising fran increased speeds were usually caused by the
propeller slipstream or by dyna.mj.c pressure effects such as large control
surface hinge moments, skin distortion, or aeroelastic wing twist. About
w
the time of World rtar II, compresaibili effects on the stability and
control of airplanes becall!e evident during high speed dives. 'lbese
appeared in the form of longitudinal .and directional trim changes, wing
drOpping, loss of control effectiveness, buffeting, large increases in
control forces, ~md changes in lift-curve slope and sta.bilitv. These
changes generally occurred ZJ.t about the same fach number within the high
subsonic range and the fact that they appeared in conjunction with a
rapid drag rise led to the popular notion of a "sonic barrier. tt
Successful atta.in'bnent of supersonic fiight with operationally useful
aircraft depended on developments which effectively eliminated the barrier
or .at least reduced the undesirable effects to tolerable ma.gni tudes. Qle
of these developnents was the turbojet engine which provided the required
high thrusts at h.i.gh speeds. Another development was -the introduction of
aerodynamic refinements such as sweptback and low aspect ratio p;Lanforms
for wings and tail surfaces, thin airfoil sections with no camber, and
higher fineness ratio fuselages. These developments, although alleviating
the problems formerly encountered in the traneonic region, introduced a
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wide variety of other problems covering the entire speed range of th~
aircraft.
The use of wings with sweptback leading edges, sometimes accompa.nj,ed
'tv considerable taper as in triangular or delta wings, introduces phenODiena
such ae strong spanwise air flow and separated vortez: flow· at the leading
edge. These in turn cause significant effects on the aerodynamic atabilitq
parameters, in particular, their variation with angle of attack. Low aspect
ratios with their consequent low lift curve slopes resulted 1n large angles
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of attack Vlhieh increase the severi of many a tab ili 'o/ problems. Body
aer~cs have became of increased signifieance because of the generally
increased sizes of fuselages compared to wings and also because of the high
angles of attack being encountered.
As a result of the changes in configuration, inevitable alterations
in the distribution of the mass of airplanes have occurred. 'lbe use of
thin lOW' aspee t ratio Wings and high fineness ratio fuselages results 1n
the mass of the airplane being distribut-ed primarily along the fuselage
ra. ther than along the wings. Thus the moments of inertia in pitch and
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yaw are much greater relative to the moment of inertia in roll than was
fonnerly the ease. In addition, increa.ses in wing loadings and operational
altitudes have cnused significant increases in the relative densiw .facto~.
'Ihese changes in mass distribution and relative density have resulted in
profound effect.s on the dynamic stabili1;ir maneuverablli ty, and spinning of
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'lirplanes.
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~~ '!he stabili t.Y and maneuverabili during landing and t..~ke-off o'! air-
planes have also deteriora tA:ld as a reeul t of the altered features required for
high speed flight.
1- 3
Historical Background
Ole of the first studies of aircraft stabili -cy was that of T..anchester CIJ
in the years before the first powered-airplane fl ight. Lanchester studied
the long period longitudinal motion., 1Yhich he named the phugoid, and also
considered lateral motions. Chly the most elementary of aerodynamics was
\
included. Somewhat later Bryan [P,] placed the theory of the stabili t.Y of
airplanes on a firmer basis with a more complete and rigorous stuey. He
considered both long and short period modes of longitudinal motion and the
three-degree-of-freedom lateral motion. The methods developed by Br,ran
are still the basis for most dynamic stability work today. During these
years practically no experimental aerodynamic information was available.
Some rudimentar,y mq;erimentB had been made by Lan..,.,ley on a whirling arm and
by the Wright brothers in a home-made wind tunnel. Laboratory experiments
in aerodynamics were performed in the F.ffel wind tunnel in Paris from about
1909 on, and in a whd tunnel at Gottingen, Germany, prior to 1914.
For the next two decades the work on stability and control consisted
mostly of addi tiona! theoretical work and improved experimental methods
for detennining the varicus aerocynamic stabil i tgr derivatives. The work
up to this time was well summarized by B. H. Jones [ $} • Further develop
ment included the effect:s of free controls [-f) ., the effects of aero
elasticity [5;'), and the effects of compressibility [7] • Initial
studies of the effects of compressibility considered only t.~e high subsonic
speed region. Later st dies extended the realm of investigation into the
8J •
supersonic region [
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In recent years a great amount of effort has gone into theoretical
studies of the configurations evolved from high speed flight requirements,
and into experimental determination of tile longitudinal and lateral
stability derivatives. Experi:lnental methods have been perfected for
determining not only the static derivatives but also the rota.r:y- and
(f!; "') •
acceleration derivatives In the field of dynamics no
fundamental changes in the method of study have taken place. Additional
methocls of solution of the equations of motion have been put to use,
such as the laplace transform ['!] , and electric analogue oanputing
equipment.
Maqy excellent treatises are available an the fundamentals of
aircraft stability and control. It is not the purpose of seetian
th~.s
of the present volume to furnish another such source or to supersede
previous studies. The primary purpose is rather to provide a review
of the special problems that arise as a consequence of the aircraft
configurations and flight conditions evolved in the quest for high flight
speeds, nnd to indicate insofar as possible the solutions that have been
obtained for these problems.
I -5"
HANDLING
ODALITIES·- Through a considerable portion of the histor,y of
flight, the subject of stabilit.y and control remained largely on a qualitative
basis, since desirable handling qualities of airplanes were not clearly defined.
Shortly before the beginning of orld a.r II, however, program aimed at the
specification of satisfactor,y handling qualities were instituted both in the
United States and Great Britian. The work consisted largely of fljght testing, in
"'''hich attempts were made to correlate pllots opinions of airplanes UlXier various
flight conditions with quantitative measurements of control positions, control
forces, and airplane response. Many pilots participated in the programs, and
airplanes of almost ever,y type were employed.
'fue programs conducted in the United States resulted in the issuance by
R. R. Gilruth of the National Advisory Con:mittee for :Aeronautics of a paper ( 10)
f.B~~in which the first comprehensive act of requirements ~ set forth.
This paper formed the basis for specifications adapted later by the military
services, although the original work has been e.xpazxied and modified at various
times as a result of continued testing and the discovery of new problems for the
more recent aircraft.
The specific requirements used in different countries and by different
services shaw same variations; ho ever, in general, the items treated essentially
the same. ith regard to dynamic motions, consideration is given to the phugoid
and short-peroid longitudinal modes, and to the rolling, spiral, and oscillatory
(short-period) lateral modes. Normally, only the short period modes are considered
to bear a definite relation to pilot's opinions. The specifications indicate
minimum damping rates, usually in terms of cycles or "ti!-ne to damp to a given
fraction of the initial amplituda following a disturbance.
Requirements for static-longit\Xlinal stability normally are applied to the
control position and force characteristics associated with speed variations from
4-6 -
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a trimmed straight flight condition and with maneuvering at essentially constant
speed. Stable control position variations (rearward stick movement with reduced
speed or with increased normal acceleration) are desired; however, it is usually
considered sufficient to require that stability be demonstrated only within cert in
specified ranges of critical flight corrlitions. It is also desired that the
control-force variations be stable in the sense that a pull force is associated
with decreased speed or increased normal acceleration from an initial trimmed
condition, ho ever, the requirements also restrict the magnitudes of the forces
within definite limits. Requirements for static lateral stability refer to
desirable control force and position variations associated with
a~ymmetrio
displacements of the airplane. The ability to trim an imposed as~,mmetric
condition and to provide a specified roll rate also are covered.
Additional requirements have been formulated with regard to such items as
stall warning and spin recovery. Appropriate trea~nt of some of the more
reoent difficulties, such as transonic trim change, pitch-up, and coupled motions
is in the formative stage of development.
The establishment of definite requirements has been of profound significance
lrl.th regard to the entire field of stability and control, since the application
of both wind-tunnel and analytical studies thereby have become firmly established.
~, r.t:,;.;reJ
The relationships of the requirements to more basic concepts have been ~taa by
William H. Phillips (B, J • 2)q(it).
2 - 1
B, 2 - Elementary Concepts
Before proceeding into a d tailed discussion of the effects on air-
crafts bilit:T and control of the configurations and flight conditions
th t h vo evolved .fron the requirements of high speed flight, 1 t is
beneficial to review some simplified concepts. 'Dlese concepts may be
more familiar to the reader and \Till serve to illuetrate some of the
e rliest o erved effects of high speed. flight. Oily steady motions of
the aircraft l7ill be considered at this point so that the mass and inertia
of the ire raft or of its component parts are of no consequence.
nte static stability and controllabilHu of an airplane are evident
to a pilot 'through the control otions and control forces he must supply
to change from one steady trimmed flight condition to another or to
produce certttin desirod m neuvers. We will consider first the longitudinal
am
or Gym!Iletrlc l"C~ie of flight. In the n:;e of any angular accelera tiona
or velocities the following simple expressions for the lift and pitching
mo ent coefficients can be written:
(2-1)
(2- 2)
'lhese equ:1t1ons represent an airpl· ne co.U'igura.tion nhich in the past
has been called conventional) 1hnt ia, one bnving a tail surface · th an
elevator for control, such th t C~ c n be assumed zero. For an all
moving 1:.411 or a tailless de 1gr. · ' : "n elovator control this ass tion
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is less v. lid but the refinement of adding tb tem lrill be rese~Ted
Cfe
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for a later llrticle. It is also aesumed. for this simplif'ied approach
that the various nerodynmc derivatives are linear functions o-f> oc 'Ltd
ie. 'i
nte QWllltit iC';l nd Cm. nre the lift and pitching moment
0 0
coefficients t zor·o a:; .. d cfe nnJ .wight be considered as representing
an effective camb r or unsymmetrical character of the irplane as a whole.
Solving equations (2-1) and (2-2) for tfe nssuming trLrr Jed flight
(cm=oJ ... ,.. CI'Noc
,d mald.n usc of the form ~Cm : '"'"'c -
t- we
JcL
eL~
obtain tho eQu:ttion:
--- c
11'1l*L ( CL - C: } - ("l' no (2-3)
C~ Lo CI7J~
e oe
In the absence of compressibili tg e!fec ts tL, md em_ .re constllnt and
0 0
tbe x· te of change of elevator deflection with lift coefficient will be
-- CmcL.
{2-4)
CmEe
S ce in trimmed level flight the lift coefficient is given by
(2-5)
then the variation of elevator deflection for trim with speed will be
dSe == Cm~ /_ 4 ~)
.__,7 (2-6)
dV c177~ ~f'~
oe
;~e ~ Cm and these
'lbus the stabili t,y criterion is proportion 1 to
Jc;_
two factors define the "static r inn in the absenct- co -Jressibili cy
effects. As the l~' ,,+ .r of fT"' oved reamnrd on a stable airplane
tho values or both ~~" ar.' (- ;~9 Jecl'!! so IUid tho center or gl'llvi t;y
2 - 3
for which they are zero is refeiTed to as the neutral point with eti.ck
fixed.
One of the first effects of compressibility observed as <>irplo.ne
fliqht sp~eds approached the tr.~msonic was a loss in effectiven~se, Cm ,
.Fe
of elevator controls. As indic;:~tfd in equation (2-6) this would cause an
inc rease · -dI V$'12 . even tho urh ~Cm were to remtu· n cons •w-.n t or even
~n ~
t.t 'JCL d.£ ~ Cm
decrease slightly. 'lhus although ooth ~ and would still indicate
C/V (1 CL
the same neutral point location they would not indicate the same variations
of stability with changing speed at other center of gravity positions.
Another of the early effects of comprcssibilit :-1 was a change in the value
of Cm (usuttlly in a negative di.rection) and as can be seen from
0
equation (2-3) this also ce.ttsee a variation of 8'e 1'rl.th speed. In this
rv
d.SQ d Cm
q,
case and ~ no longer indicate the same neutral point, and the
concept of neutral point loses some of its significance. Further comments
on this phonomena will be made in the later more detailed discussion of
compressibility effects.
A catastrophic erfect of the above mt'lntioned reductions in C.me "~'1d
0
negative changes in C~n0 wit1 increasing speed was occasionally encountered
in high speed dives of airplanes of the orld War TI era. As the speed
at w! ioh these aerodynamic changes took place was reached in a dive the
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airpl11ne would nose down or "tuck under." IT the pilot used the lonei tudinal
trim tab to retrim the airplane to the original ~.tti tude, then during the
pullout from the dive the changes occurred in the reverse direction as the
speed decreased caus ng a buildup or normal acceleration in excess of
c.
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