*General Notes*. The chassis or landing gear carries the weight of the
aeroplane when resting on or running over the ground, and is subjected
to very heavy shocks, especially when landing. It is provided with
pneumatic tired wheels, an elastic shock absorbing device, and the
structural members that connect the axle with the fuselage. In some
forms of landing gear, the wheels are supplemented by long horizontal
skids which serve to support the machine after the elastic shock
absorbers are fully extended or when the wheels collapse. The skids also
protect the aeroplane in cases where the wheels run into a ditch and
also prevent the machine from nosing over in a bad landing. Since the
skids and their structural members cause a high resistance they are now
seldom used except on the larger and slower machines. In running over
the ground, or in making a hard landing, part of the shock is taken up
by the deflection of the tires and part by the deflection of the shock
absorber. The greater the movement of the tires and absorber, the less
will be the stress in the frame.
In the majority of cases, the shock absorbers consist of rubber bands or
cords, these being wound over the axle and under a stationary part of
the chassis members. Since rubber is capable of absorbing and
dissipating a greater amount of energy per pound of weight than steel,
it is the most commonly used material. Rubber causes much less rebound
or "kick" than steel springs. The principal objection to rubber is its
rotting under the influence of sunlight, or when in contact with
lubricating oil. The movement of the axle tube is generally constrained
by a slotted guide or by a short radius rod.
The design of a suitable chassis is quite a complicated problem, for the
stresses are severe, and yet the weight and resistance must be kept at a
minimum. In running over rough or soft ground for the "Get off," the
shocks and vibration must be absorbed without excessive stress in the
framework, and without disturbing the balance or poise of the machine.
There must be little tendency toward nosing over, and the machine must
be balanced about the tread so that side gusts have little tendency in
throwing the machine out of its path. It must be simple and easily
repaired, and the wheels must be large enough to roll easily over
moderately rough ground.
[Illustration: Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane.
Courtesy "Flight."]
*Types of Chassis*. The simplest and most extensively used landing gear
is the "Vee" type shown by Fig. 1, and is equally applicable to
monoplanes, biplanes or triplanes. Primarily, the Vee chassis consists
of two wheels, an axle, a rubber shock absorber, and two sets of Vee
form struts. The chassis shown by Fig. 1-a is that of the
Hansa-Brandenburg and is typical of biplane chassis. The winding of the
rubber cord and the arrangement of the chassis struts are clearly shown.
The two struts are connected at the bottom by a metal fitting, and the
rubber is wound over the axle and under this fitting. No guiding device
is used for the axle, the machine being freely suspended by the chord.
The struts are made as nearly streamline form as possible.
[Illustration: Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg
Biplane.]
Fig. 2 is a front view of a typical Vee chassis, and Fig. 3 is side view
of the same device, the same reference letters being used in each view.
The vertical struts C run from the fuselage at F to the connecting axle
guide plate G. The wheels W-W are connected with the steel tube axle A,
and the struts are braced against side thrust by the cross-tube D and
the stay wire braces B-B. In Fig. 3 the metal fitting G is provided with
the guiding slot S for the axle A. The elastic rubber cord absorber
passes over the axle and is fastened to the plate G by the studs I. Fig.
4 is a side view of the chassis of the Lawson trainer, which like many
other primary training machines, uses a front pilot wheel to guard
against nosing over. The rear two wheels (W) are elastically supported
between the Vee struts C and F, while the front wheel X is attached to
the fuselage by the vertical strut E, and to the rear wheel frame by the
tube G. It will be noted that the front wheel is smaller than the rear
main wheels, as this wheel carries but little load. The tail skid T is
hinged to the fuselage and is provided with elastic cord at the upper
end so that the shock is reduced when the tail strikes the ground. Fig.
5 shown directly above the Lawson trainer, is the complete assembly of
the Hansa-Brandenburg already described. The tail skid of the
Hansa-Brandenburg is indicated by T.
[Illustration: Figs. 2-3. Typical "V" Chassis With Axle Guide.]
The metal shod ash skid stick is hinged to the lower face of the
fuselage, and at the upper end is attached to a stationary fuselage
member through four turns of elastic cord. When the skid strikes an
obstacle the rubber gives and allows the tail to move in relation to the
ground. By this arrangement the greater part of the device is enclosed
within the fuselage and, hence, produces little head resistance.
[Illustration: Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5
(Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail
Skid (t).]
Fig. 7 is the skid chassis of the Farman biplane which shows clearly the
arrangement of the skids and the shock absorbing suspension. A metal
bridge is attached to the axle, and a series of short rubber bands are
used in connecting the axle bridge, and the bridge on the skid. A
triangular tubular radius rod is attached to the axle and hinged to the
skid. This restrains the travel of the axle in a fore and aft direction.
Another form of skid shock absorber is given by Fig. 8, in which the
rubber rings pass over a spool on the axle. The guiding links or radius
rods on the inside of the skids regulate the axle travel. In general,
the use of a radius rod is not desirable as it transmits a percentage of
the shock to the machine.
[Illustration: Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another
Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or
Lever.]
[Illustration: Fig. 9. Chassis Details of the Nieuport Monoplane. This
Has a Central Skid and Uses an Automobile Type Steel Spring Instead of
Rubber Cord. Fig. 10 Is a Detail of the Nieuport Spring. (At Right.)]
Fig. 9 is an older form of Nieuport monoplane chassis, a steel cross
spring being used in place of the usual rubber bands. This is simple,
but comparatively heavy, and is subject to frequent spring breakage. To
guard against spring failure, a long ash skid is placed under the axle.
The spring system is connected with the body by three sets of oval steel
struts. An old type of Curtiss chassis is given by Fig. 11. This has
been widely used by amateurs and exhibition flyers, but requires a
fairly smooth landing ground as there are no shock absorbers. The only
shock absorption is that due to the deflection of the tires. The extreme
forward position of the front wheel effectually prevents any tendency
toward nosing over when landing.
[Illustration: Fig. 11. An Old Type of Curtiss Exhibition Chassis With
Three Wheels.]
[Illustration: Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13.
(Right). Rubber Cord on Axle.]
A Standard H-3 shock absorbing system is given by Fig. 12. This has a
bracket or hanger attached to the axle over which the elastic cord is
wrapped. The cord is wrapped in continuous turns between the axle hanger
and the bottom of the Vee support members. As shown, the upper
streamlined bar is the axle, while the lower is the cross bar brace
which serves to hold the lower ends of the U’s. I am indebted to "Aerial
Age" for this cut. In order to guide the axle in a straight line in its
up and down movement, two radius links are attached between the axle and
the front vertical strut. One decided advantage of the "Standard
construction" is that the cords are wound without crossing the strands,
thus reducing cutting and wear between the cord turns. Fig. 13 is a
variation of Fig. 12, the cord being wound directly around spools on the
axle and the lower stationary cross tube. The axle is guided by a slot
in the guide plate at the right, while end motion is controlled by a
radius link. Fig. 14 is the double wheel arrangement of a large "Twin"
bombing plane. Two wheels are placed directly under each of the motor
units so that a portion of the load is communicated to the chassis by
tubes. Diagonal tubes transmit the body load to the chassis.
[Illustration: Fig. 14. Chassis for Twin Motored Biplane of Bombing
Type.]
*Folding Chassis*. Owing to the great relative resistance of the chassis
it has been suggested by many designers to provide a folding frame which
will automatically fold up into the body after the machine has left the
ground. This would be a decided advantage but the gear is complicated
and probably not altogether reliable.
*Height of Chassis*. The height of the chassis is made as small as
possible with a sufficient clearance for the propeller tips. It is usual
to have the tips of the propeller blades clear the ground by from 10 to
12 inches when the aeroplane is standing with the body in a horizontal
position. Any smaller clearance is almost certain to result in broken
blades when landing at a sharp angle or when running through high grass.
If the chassis is excessively high the resistance will be high and the
machine is also likely to be top heavy.
[Illustration: Figs. 15-16. Methods of Calculating Wheel Position on Two
Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.]
*Location of Wheels*. The exact location of the wheels, in a fore and
aft direction, is of the greatest importance. If they are too far ahead
of the center of gravity, too much weight will be placed on the tail
skid and excessive running will be required to get the tail off the
ground. If the wheels are too far back, the machine will be likely to
nose over when landing or running over the ground. In any case, the
wheels must be well ahead of the center of gravity so that the weight
will resist a forward overturning moment. In the majority of orthogonal
biplanes, in which the leading edges of the upper and lower wings are on
the same vertical line, the center of the wheel is from 3 to 6 inches
back of the leading edges. In staggered biplanes the wheel center is
from 6 inches to one foot in front of the lower leading edge. This
difference is caused by the fact that the center of gravity is nearer
the leading edge of a staggered wing than with the Orthogonal type, and
hence the wheels must be further forward.
Fig 15 (upper diagram) shows the conditions when the machine is running
over the ground with the body horizontal. The vertical line a-a passing
through the center of gravity C G is a distance N from the center of the
wheel. The weight acting down has a tendency to pull the tail down, this
moment being equal to the weight of the machine multiplied by the
distance N, or W x N. The elevator flap M exerts a lifting force Ky
which acts through the lever arm L, and opposes the moment due to the
weight. The force K must be equal to K = WN/L. The distance I is the
distance of the wheel center line from the entering edge of the wing.
The weight on the tail skid S when the machine is resting on the ground
will be equal to S = WN/M, and this may range anywhere from 40 to 200
pounds, according to the size of the aeroplane.
Fig. 16 illustrates a principle of wheel location advanced by Capt.
Byron Q. Jones, and published in "Aviation and Aeronautical Engineers,"
Nov. 16, 1916. The body is shown in a horizontal position with the
propeller axis X-X horizontal. The center of gravity is at G on X-X, the
weight acting down as at P with the line prolonged meeting the ground
line at B. A line E-E is a line drawn tangent to the wheels and the tail
skid at D, the angle of this line with the ground determining the
maximum angle of incidence. E-E is the ground line when the machine is
at rest. For the best conditions, Capt. Jones finds that the line
connecting the point of tangency C, and the center of gravity at G,
should make an angle of 13 degrees and 10 minutes with the vertical GB
dropped through the center of gravity. With the line GA drawn
perpendicular to the resting line E-E, the angle BGA should be 10
degrees as nearly as possible. This is for a two-wheel Vee chassis, but
with a third front wheel as with the training of type the angle CGB can
be made less. Capt. Jones has found that with the wheels in the above
location there will be no tendency to nose over even with very poor
landings, and this method has been applied to the training machines at
the San Diego Signal Corps aviation school. If the angle BGA is greater
than 10 degrees it is difficult to "taxi" the machine on the ground,
this tending to make the machine spin or turn into the wind. Capt. Jones
claims that a two-wheel chassis arranged according to these rules is
superior to the three-wheel type for training purposes since the
tendency toward spinning is less.
The location of the tail skid S should be such that the elevator and
rudder surfaces are well off the ground with the skid fully deflected,
and yet the skids must be low enough to permit of the maximum angle of
incidence or an angle of EXX = 10 degrees. To a certain extent, the
maximum angle of incidence determines the chassis height. If the angle
EXX is made greater than the greatest angle of incidence, the wings can
be used as air brakes in bringing the machine to a quick stop after
landing.
The track, or the distance between the centers of the wheels measured
along the axle, must be about 1/7 or 0.15 of the span of the lower wing.
This makes the track vary from 5 to 7 feet on the usual types, and as
high as 15 feet on the large bombing planes. The track must be great
enough to prevent overturning when making a landing on soft ground or
with a cross wind. If the track is excessive, there will be a heavy
spinning moment in cases where one wheel strikes a depression or soft
spot in the ground.
*Shock Absorbers*. The axle movement allowed by the elastic shock
absorbers and guiding appliances averages from 5 to 6 inches. The
greater the movement, the less will be the stresses induced by a given
drop, but in practice the movement is generally limited by
considerations of chassis height and propeller clearance. It can be
proved that a movement of 5 inches will produce a maximum stress equal
to 8.6 times the weight of the machine under conditions of a one-foot
drop, while with an absorber movement of 6 inches the stress is reduced
to 7.5 times the weight. This calculation takes the tire deflection into
consideration. With the absorber movement limited to one inch, the
stress may be as high as 35 times the weight of the machine.
F=W (2 + 2.77/x) where W = weight of machine in pounds, F = the stress
produced by the fall, and x = the absorber movement in inches.
*Landing Gear Wheels*. The wheels are generally of the tangent laced
wire spoke type, and are enclosed with discs to reduce the resistance.
They must have very wide hubs to resist the heavy end stresses caused by
landing sidewise. The length of the hub should be at least twice the
diameter of the tire and a greater width, say three times the tire
diameter, is preferable. The narrow hubs used on motorcycle wheels are
not safe against side blows, although they may be capable of
withstanding the vertical load. The wheels are rated according to the
outside diameter over the tire, and by the diameter of the tire casing.
A 26" x 4" wheel signifies that the outside diameter is 26 inches with a
casing diameter of 4 inches.
The 26 x 4 tires are used on the majority of training machines of the
two-wheel type, while a 20 x 4 wheel is used for the front wheel of the
three-wheel trainer. Two larger sizes, 30 x 4 and 34 x 4, have also been
used to some extent, particularly on the Ackerman spring wheels.