TM 1-320
36-37 AIR CORPS
of attack beyond that giving the ma-ximum lift, the dynamic lift nat ..
urally decreases. At the same time the downward thrust on the eleva
tors is increased. The gain in the upward component of the propeller
thrust at reversing speed or below will not compensate for the loss
in lift just described and the airship will be under the action of a
greater resultant downward force than at the start.
f. The opposite effect to that described in c above occurs when an
airship is trimmed tail heavy and the elevator is depressed. In this
case the whole airsh ip will rise.
g. It might appear that reverse control would be a source of great
annoyance to the pilot. This is not the case when the phenomenon is
Ls
FIGURE 24.-Fl!gbt at constant altitude (airship statica lly heray, t rimm ed tail heavy,
·elevators neutral).
properly understood. I n fact, many maneuvers are executed by in
telligent use of reverse control, for example, heavy take-off. This is
described in paragraph 37.
37. Application of dynamic control to operation of air
ships.-a. The three major maneuvers in airship operation which
are assisted by dynami c control are-
(1) Flight at consta nt altitude.
(2) Take-off.
(3) Landing~
These operations are fully covered in TM 1-310 and are discussed but
briefly here to bring out the aerodynamic principles involved therein.
b. As soon as the take-off is completed and the obstacles in the imme
diate foreground cleared, the pilot climbs to the altitude at which he
desires to cruise. He then trims the airship so that with the contro ls
in neutral the algebraic sum of the vertical forces is zero. Since the
airship is almost never in static equilibrium, one of two situations will
prevail , static heaviness or ligh tness.
AIRSHIP AERODYNAMICS
TM 1-320
(1) Figure 24 shows the case in which the airship is statica:Jly·heavy
and trimmed nose light . In thi s case the equation of vertical forces
to give constant altitude flight with neutral controls becomes-
W =£ 0+ L e+ Lt+ L~
The pilot may be called upon to fly a heavy airship on account of vari
ous reasons such as--
(a) Collection of moisture if rain is encounte red.
(b) Leakage in envelope .
(c) Loss of super heat.
(d) Heavy tak e-off.
Most airships can car ry about 10 percent . of their gross lift dynamically
at the surfa ce of the earth . Since the dynamic lift varies as the air
·density, it decreases with nltitude. Table III shows results of some ex
periment s on an Italian M type airship at- full speed :
TABLE III. - Lijt of Ita lian M type at full speed
[In pounds]
Altitud e, 3,000 feet Altitude , 10,000 feet Altitude, 16,500 feet
Angle of I I <I) I I I I !XI 1:1 0 1:1 0 ~ 1:1 0 inclination in .... Q)Q) !').~ 1:1 .... Q)Q) '"'fl) .... Q)Q) .... fl)
~ ~ ~
:=l o. .... ~ .... 0.~-o radiants ..... ..... o. -~ ..... o. .._.a> ..... ;..::: ..... o. ._Cil .... oo ..... oo 0~ ..... - o- 0 - Q3 0~ 0 - 0 ..... 0 . <IS Q) . Q)
.5 s Q3 .... -+"> <!:0. .... -+"> .... 0. .... -+"> +>o. ....
0 - ~ 0 .... .... .... 0 - ..... ~ ·- ...... ·- ·- ·- ·- ·-E-< H H H E-. H H H E-. H H ..:I
0.03- - - -- - - - - 1, 224 330 60 834 I, 012 269 48 695 839 218 40 481 0.06 __ ___ ____ 1, 855 612 125 1, 118 1, 542 495 101 946 1, 287 400 82 805
0.09 _- ----- -- 2, 290 810 200 1, 2801, 914 657 163 1,095 1, 608 530 130 948 0.12 ___ __ ____ 2,497 913 290 1, 294'2, 101 742 235 1, 124 1, 778 599 189 99 0
(2) Figure 25 shows the case in which the airship is fl.yin~ statically
light at constant altitude with contro ls in neutral. In this case 'the
equation of the vertical forces becomes-
L0= W + L e + Lt + L8
(3) In the unusual case in which the airship is in perfect static
equilibrium, it will be necessary to trim the airship about 2° nose
heavy to overc6me the upturning moment of the propeller thrust. So
trimmed the airship will fly on an even keel at cruising speed. The
motorized observation ba1loon, having only one ball onet , cannot be
trimmed for an individua l flight. An approximate 2° nose heavy
Tl4 1-320
87 AIR CORPS
trim is given this type of airship during initial inflation by proper
adju stment of car suspension rigging .
c. It is customary to take off large semirigids and rigid s statically
light, but nonrigids are taken off as much as 6 or 7 percent heavy.
(1) The light take.-off may be made with the airship in any trim
from tail heavy to a few degrees nose heavy. In the latter case the
airship should be free-ballooned to a safe altitude before the motors
are opened. The light take-off presents little difficulty.
(2) For the take-off when the airship is in static equilibrium the
trim should be neut ral or a few degrees tail heavy, pref erably the lat
ter. In this case the car party of the maneuvering crew gives the ai r
ship a toss upward, the men on the nose of the car throw ing their end
up first, then the men to the rear throwing up their end. This gives
•
FIGURl'l 25.- Flight at constant alt itude (ai r sb ip statica lly Hght, tri mmed nose heavy,
elevators ne\ltral) .
t.he airship an initial angle of attack to the air stream. When clear
of the party the pilot opens his motc·rs and raises his elevators slightly.
The thru st of the prop ellers assisted by the slight force on the eleva
tors will further raise the nose of the airship and it . will climb
rapidly.
(3) For the heavy take-off the ajrship must be carefully trimm ed
tail heavy. The amount of the t rim varies with degree of heaviness,
type of airship, and wind velocity. If there is a good wind blowing it
gives the airsh ip an initial air speed to assist the a scent. Experience
has shown that an airship of the TO type with a trim of 9° tail heavy
will take off 700 pounds heavy in still air. For this degree of heavi
ness the car party should be augmented to at least 20 men and 4 men
should be assigned to lift on the tail surfa ce. At the proper signal
the c ar is thrown up, nose first as before. The elevators should be
depressed about 10° and as the pilot opens his motors he will find it
necessary to depress the elevators fully to keep the tail from striking
the ground. Action of the airship in rising is a pure case of reverse
control. The air from the s lipstream of the propellers strikes the
AIRSHIP AERODYNAMICS
TM 1-320
elevators and gives the tail a positive lift. At the same time the trim
of the airship will keep its nose elevated so that there will be the
familiar dynami c lift on the hull surface and the vertical component
of the propeller thrust to assist the ascent. In this cas:e-e -
Lu+Le+L t+ Ls> W
d. The most diffic•lit maneuver which confronts the pilot is the
landing. This operatio n may b~ divided into three parts, as · follows:
(1) Weigh-off.
(2) Approa ch.
(3) Arri val at the landing party.
e. Weigh- off is made at a safe altitude (1,000 feet for large airships,
250 to 500 feet for smaller ones). For this maneuver controls are
~~~W<tl9h oil horo
_,.• NJ • ......,.-JtOO
~------------- 2Mi------------------~
FIGURE 26.-Approa ch of an a.irs hip to a landing.
placed in neutral and air speed reduced to as low a speed as possible.
The airship will quickly assume an attitude determined by the trim,
which can be read from the inclinometer. At the same time the pilot
can notice wheth er the airship is rising or descending statically. It is
useless to bring the airship to an even keel to eliminate dynamic lift
caused by unavoidable residual speed incident to idling prop ellers, as
this would, in reality create a dynamic thr ust on the tail surfaces.
As a result of the knowledge of condition of the airship deri ved from
weigh-off and after due considera tion of existing meteorological condi
tions, the pilot is ready to make the approach.
f. The princip al object of the approa ch is to determine in advance
of the arrival at the party the behavior of the airship at landing speed.
Figure 26 gives a graphic al picture of the approach.
(1) From the altitude of weigh-off the airship is brought quickly
to the altitude of approach. This varies from 150 feet for a nonrigid
to 500 feet for a large rigid, depending in some measure on gustiness
of the atmosphere. During the descent the pilot arranges the trim
he estimates to be necessary to make the landing. On arrival at the
TM 1-320
87-38 AIR CORPS
approach altitude the speed of the airship is reduced to 15 miles per
hour plus the wind velocity. This is an excellent approach speed.
(2) The pilot now wishes to check behavior of the airship at this
speed. Controls are placed in neutral and if the trim is correct the
airship will maintain constant altitude in a manner described in b
above. If it does not, it is necessary to adjust further the trim to
effect that result. The principle is exactly the same whether the air.
ship is stati cally light or heavy. During remainde~ of the approach
·controls are useu to overcome gusts or changes in static conditions,
care being taken to observe principles of reverse control should the
speed fall below reversing speed or should the airship be placed in
dang er by loss of static lift.
g. When the airshi p arri ves within 50 to.200 yards of the landing
party it is brought to landing height. This depends on type of air
ship and length of handling guys used. Large nonrigids usually land
about 60 feet off the ground, rigids at a much higher altitude, while
the motorized observation balloon must be landed at an altitude of
25 feet or less. In this conn.ection it should be borne in mind that the
lower a statically h~avy landing can be made the smaller the drop
after aerodynamic control ceases. In that case,· also, care should be
t aken to level the airship by use of elevators as it falls into the hands
of the party, as otherwise the tail would be injured.
h. The landing described above is the usual type of landing. The
description is not at all complete since it omits nearly all the static
principles involved . The other types of landings, such as turn land
ings , will not be discussed, since the dynamic prin ciples involved
therein are simi lar to those alread y explained.
SECTION VI
AERODYNAMIC STRESS
Paragraph
Assumption as to condition of maximum stress ------ -------- ----- --- -- - 38
Transverse forces acting on airship flying at constant angle of pitch ______ 39
Transverse forces acting on airship in steady turn _____________________ 40
Forces caused by gusts------- -- ---- - - --- --- -- ----- ---------- --- ------- 41
Empirical formulas for maximum aer odyn amic bending moment on hull
and for forces on tail surfaces----- - ---- - - ----- ---- ----- - ------------- 42
Method of calculating shear and bending moment on hnll _________________ 4g
Conclusion____________________________ ________________________________ 44
38. Assumption as to condition of maximum stress.-a . For
airships designed prior to the World War the air speeds were quite
slow. The aerodynam ic forces acting on these airships were conse-
AIRSHIP AERODYNAMICS
TM 1-320
quently insufficient to give shear or bending moments large enough to
endanger an airship designed to care for the static loading. At pres
ent the speed of airships has been so materially increased that aero
dynamic forces, which vary as the square of the speed, must be con
sidered. While it is not the function of this manual to teach design
of airships, a general know ledge of results of these forces and moments
is sufficiently important to the pilot to warrant inclusion herein a
simplified discussion thereof.
b. As previously stated, the longitudinal aerodynamic forces are
usually not a source of danger to the airship. The single exception
to this statement occurs in the case of the pressure airship flying at
maximum or near ly maximum speed. At this time the nose pressure
· may attain such magnitude that it 'may very conceivably exceed the
pressure for which the airship was designed, in which event the nose
will cave in. Since it is the internal pressure of the gas which resists
such caving action, it should be the duty of the pilot to increase his
intern al pressure to the maximum allowable pressure when flying at
velocities approximating maximum design speed.
o. The most important aerodynamic stresses are those caused by
transverse forces. In order to design for such stresses, it becomes
necessary to make assumptions concerning conditions which give great
est transverse forces. It was early believed that the worst condition
occurred at the instant of si,multaneous application of full rudder and
elevator control. This assumption would appear reasonable in view
of the fact that momentaril y inertia of the airship will arrest any
tendency toward rotation, but as soon as an angular velocity is at
tained, rotation of the tail reduces the forces on the surfaces. How
ever, this argument omits one important consideration. It frequently
occurs that at the moment of application of the controls, the airship
may be under the acti~n of forces giving it yaw or pitch in a direction
opposite to that desired . In this case while initial yaw or pitch is
being overcome, the hull will be subjected to a twisting action caused
by two opposing moments. Theoretical treatment of stresses so caused
is quite complicated and many designers simply arbitrarily double
the forces which arise when full rudder and elevator are applied simu l
taneously.
d. During the design of the RS-1 airship various conditions of
stat ic loading, with a load factor of 4, were investigated. Stresses
found in the keel members under static loading conditions were com
bined with stresses found under the following conditions of aero
dynamic loading to determine maxi~um stress in any member: In
·arriving at !ow load factors applied to aerodynamic loading condi-
TM 1-320
88-89 AIR CORPS
tions, the effect of the envelope in relieving the keel by resisting a
portion of the shear and bending was neglected. It was found that
this was very conservative as subsequent tests on water-filled models
and full-scale tests on the RS-1 airship indicate that the keel resists
approximatel y 50 percent of total bending due to static loads. How
ever, in flight tests it was found that the keel resists only 10 percent
of the bending moment due to external air loads in pitch. ·In order
to l!>e conservative however in future designs of semirigid airships the
design should be based on the assumption that the proportion of the
total loads on the airship due to external air loads in pitch in flight
resisted by the keel is half that found in the case of static weights
and that a load factor of 2.0 be used.
(1) Horizontal flight at 55 miles per hour with a load factor of 4.0.
(2) Horizontal flight at 70 miles per hour with a load factor of 3.0.
(3) Pitch up or down at an angle of 3° 19' at 55 miles per hour
with a load factor of 3.0.
( 4) Yaw at 55 miles per hour with load factor of 3.0.
(5) Turning, 1,500 feet radius at 55 miles per hour with load factor
of 2.0.
(6) Mooring by the nose with pitch up, pitch down, and yaw of 4°
0', in a gale of 70 miles per hour with a load factor of 2.0.
e. From the foregoing discussion it is evident that the pilot should
be cognizant of maximum angles of pitch and yaw for which his air
craft was designed . Then when atmospheric conditions render it
impossible to keep the. airship within design limits he should reduce
his air speed to effect a reduction of the aerodynamic forces.
f. To simplify the discussion transverse forces will be considered
under three classes :
(1) Transv erse forces at fixed angle of pitch.
(2) Tran sverse forces in steady turn .
(3) Forc es caused by ·gusts.
39. Transverse forces acting on airship :flying at constant
angle of pitch. -a. 'When an airship is flying at a constant positive
angle of pitch it is acted on by the following dynamic transverse forces:
(1) Component normal to longitudinal axis of dynamic force on
tail surfaces.
(2) Component normal to longitudin al axis of dynamic force on
hull.
Since rotation is considered about the center of buoyancy it is neces
sary to divide the latter force into two parts. This is essential because
the normal force on the forebody is directed upward, whereas the
normal ·foroo on the afterbody is directed downward. It has been
'AIRSHIP AERODYNAMICS
TM 1-320
39=40
. shown by a member of the · National Advisory Committee for Aero
nautics that the algebraic sum of the forces on the fore and after
bodies is theoretically zero, which would indicate that the dynamic
lift on the hull was zero, and that the total lift obtained dynamic~lly
by the airship, exclusive of the vertical component of the propeller
thrust, was that furni s!1ed by the surfaces. This is not in strict agree
ment with the actual facts, since the down thrust on the afterbody is
-less than the theoretical down thrust. However, the fact remains that,
since the pitch remains constant, the sum of the moments about the
, center of buoyancy must equal zero.
· b. The turning moment of the aerodynamic forces on the hull
theoretically equals the formula :
(Vol)iv 2 (k2 -kl) sin 20
where O=angle of pitch.
k2 and k1 =constants correcting for additional masses of air carried
longitudinally and transversely . Values of k1 and k2 are given in
National Advisory Committee for Aeronautics Report No. 184.
o. If it is granted that the dynamic force on the tail equals the total
resultant static transverse force, its moment must equal the formula
given ~n b above. . Hence-
Fa= (Vol)~v2 (kz-k1 ) sin 20
where F = component of force on tail surface normal to longitu
dinal axis.
a= distance from center of buoyancy to center of pressure
of tail surface.
'The above formula will give an approximation of dynamic lift of the
airship. .
d. Practically, F need not be as large as indicated aboye due to the
discrepancy betwen actual and theoretical values of the down thrust
on the afterbody . The point of application of F is slightly forward
of the center of the area of the tail surfaces.
e. For method of calct1lation of shear bending moments due to
dynamic forces see paragraph 43.
40. Transverse forces acting on airship in steady turn.- The
theory in this case is quite similar to that described in paragraph 39.
· Assuming, as before, that the algebraic sum of the forces on the for&
.. :, and after portions of the airship hull equals zero, the other two forces
:>acting on the airship (the force on the fins and centrifugal force) must . · ... . ~