Aeronautics and astronautics – Aircraft control – Automatic
Reexamination Certificate
2002-09-16
2004-10-26
Barefoot, Galen (Department: 3644)
Aeronautics and astronautics
Aircraft control
Automatic
C416S001000, C416S027000
Reexamination Certificate
active
06808141
ABSTRACT:
TECHNICAL FIELD
The invention relates to a propeller driven aircraft, and more particularly to a propeller driven aircraft with a tractor propeller arrangement—“tractor” meaning a propeller mounted in front of the engine which pulls the aircraft forward as opposed to one behind the engine which pushes the aircraft.
BACKGROUND ART
It is a characteristic of propellers that they are more efficient producers of thrust at low forward airspeeds than jet engines but, as forward speed increases, the efficiency and thrust of propellers drop rapidly. This means that propeller driven aircrafts accelerate more rapidly at low speeds but have a lower maximum speed than a jet powered aircraft of similar power. Rapid acceleration at low speed permits, during the takeoff manoeuvre, a propeller driven aircraft to achieve its minimum flying speed in a relatively short distance and therefore it can be operated from short runways.
At low speeds the higher efficiency of propellers (over jet engines) also causes less fuel to be consumed for a given manoeuvre than a jet engine. Therefore, where high speed is not a requirement, propeller engined aircrafts are less costly to operate.
Where high speed is required and longer landing distances can be tolerated (e.g., in personnel transport and in war planes) jet powered aircrafts are more common. Training aircrafts used to teach potential pilots the skills necessary to control their future operational machines are traditionally relatively slow aircrafts, giving trainees, who are unfamiliar with the environment in the air, time to learn the techniques to control the machines safely. Hence the domination of slow and propeller powered aircrafts in the trainer role.
However, with the advent of faster operational aircrafts, it has become advantageous to teach trainee pilots to operate at high speed as quickly as possible in order to reduce the cost of training. Up to the present time it has been necessary to give trainee pilots experience in high speed trainer aircrafts before permitting them to fly in operational aircrafts. This high speed training has traditionally been executed in another type of aircraft which is normally powered by a jet engine with attendant high operational costs.
With a high powered propeller engined training aircraft it is now possible to achieve the speeds that would make training realistic, but there are inherent problems for safe aircraft handling when high power is applied at low speed to a propeller driven aircraft. If these handling problems can be solved to make the aircraft safe to fly by inexperienced pilots at low speed it is advantageous to have combined in one aircraft, the power to operate at high speed and also the performance and cost benefits of using a propeller at low speed.
The invention addressed in this patent application makes it possible to operate safely at low and high speed in an aircraft using a high powered engine driving a propeller.
The problems of operating at low speed with a high powered propeller engine are discussed below.
A rotating propeller
10
produces thrust by accelerating a mass of air rearwards, as illustrated in FIG.
1
. The higher the power of the engine the greater the mass, velocity and energy in this airflow. This airflow behind the propeller
10
is commonly called the propeller slipstream
11
. The slipstream does not flow aft exactly parallel to the propeller shaft, but rotates around the aircraft longitudinal axis in the direction that the propeller
10
is rotating, resulting in a helical flow
11
. On conventional aircraft, there is a fin
15
and rudder
14
combination mounted on the upper rear part of the fuselage. The purpose of this surface is to provide directional stability (like the feathers do at the back of an arrow). If the airflow over this surface is directly aft the stabilizing surface will function correctly and the aircraft
13
will fly straight. However, if the flow is not directly rearwards the vertical surface (which is behind the center of gravity of the aircraft
13
) will generate a side force and cause the aircraft
13
to yaw (rotate around the vertical axis). This motion must be counteracted by the pilot moving the rudder. In a low powered aircraft this tendency to yaw is low and can easily be controlled. However, as power increases, the yawing effect becomes stronger and at high power levels can exceed the control authority available by the rudder
14
, thus causing the pilot to loose control of the aircraft
13
. This effect was often demonstrated in the highly powered WWII fighter aircraft, where inexperienced pilots not infrequently went off the side of the runway during takeoff. The standard technique in such aircraft is to apply power slowly, only achieving full power after a safe speed has been achieved when the aircraft is safely airborne.
An additional undesirable motion is caused by the secondary effect of yaw, which is roll. A conventional, certified aircraft will roll in the same direction as the aircraft yaws, the rapidity of roll increasing with the amount of yaw. During the takeoff, roll on the runway the aircraft will tend to turn off to one side, requiring a pilot action on the rudder
14
to keep straight. This tendency to yaw is damped by the friction of the wheels on the ground. At aircraft rotation to lift off the ground, the directional stability provided by the wheels is removed and the aircraft
13
yaws further and at the same time will roll in response to the sideslip (yaw). Also during airborne low speed, high power manoeuvres such as recovering from a stall, the aircraft
13
will yaw and roll in response to an application of power where the requirement is only for a linear acceleration. The higher the power level, the greater the tendency for the aircraft
13
to yaw and roll and the greater the problem for the pilot to maintain acceptable control of the machine.
An aircraft engine causes a propeller to rotate. There is a reaction between the propeller and the airframe in which the engine is mounted that causes the airframe to try and rotate in the direction opposite to the rotation direction of the propeller. The aerodynamic resistance to rotation of the propeller increases as the power developed by the engine and therefore the thrust developed by the propeller increases, which increases the tendency of the aircraft to roll. As mentioned above, on the ground this rolling tendency is resisted by the wheels, but causes a greater load to be applied on one main wheel than the other. The higher loaded wheel has more rolling resistance and the resulting drag causes the aircraft to turn, which must be corrected by the pilot. At aircraft lift off the resistance to rolling provided by the wheels disappears and the aircraft is now free to roll and at the same time the drag from the higher loaded wheel disappears so the correction applied by the pilot to resist this yaw must be removed at the same time as a movement is made with the ailerons to resist the tendency to roll.
Certification regulations and good handling characteristics require a particular aircraft response under given conditions. Two requirements that can be negatively influenced by high power are lateral stability and rudder authority. Lateral stability, as mentioned above requires that when an aircraft yaws to the right it should also roll right and similarly left yaw causes left roll. Directional stability requires that when an aircraft is trimmed to fly straight (i.e., without yaw) and the rudder
14
is deflected to yaw the aircraft
13
and then released, the aircraft yaw should reduce and return to the trimmed condition.
a) Lateral Stability: The design of a wing (normally incorporating dihedral or sweepback) achieves lateral stability. When an aircraft yaws (or sideslips) the wing towards which the aircraft is side slipping develops more lift and causes the aircraft to roll in the correct direction. At low or moderate power the airflow over the wing is not significantly modified by the propeller slipstream/engine thrust and the lateral restoring effect remains ef
Bretscher Dietmar
Tyndall William
Barefoot Galen
Browdy and Neimark , P.L.L.C.
Pilatus Flugzeugwerke AG
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