Aeronautics and astronautics – Aircraft – heavier-than-air – Helicopter or auto-rotating wing sustained – i.e. – gyroplanes
Reexamination Certificate
1999-09-28
2001-11-06
Poon, Peter M. (Department: 3643)
Aeronautics and astronautics
Aircraft, heavier-than-air
Helicopter or auto-rotating wing sustained, i.e., gyroplanes
Reexamination Certificate
active
06311924
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a device and to a process for reducing the vibration generated on the structure of a rotary-wing aircraft, particularly a helicopter.
It is known that the main source of vibration in a helicopter is the main rotor thereof which provides lift and forward drive. Such vibration constitutes a significant problem which needs to be addressed, because this vibration causes:
cyclic stresses throughout the helicopter, leading to phenomena of fatigue and therefore having a direct influence on safety; and
vibration in the cabin, which is, of course, highly prejudicial to the comfort of pilots and passengers.
A well-adjusted and fault-free helicopter causes the fuselage to vibrate at frequencies which are a multiple of b&OHgr;, b being the number of blades and &OHgr; the rotational frequency of the main rotor.
The occurrence of frequencies other than harmonies of &OHgr; indicates faults. Likewise, an increase in the intensity of said harmonics may also betray the emergence of faults of various types.
By way of example, it may be noted that, in the case of vibrations:
the frequency 1&OHgr; represents the emergence of an imbalance in the wing structure, the source of which, may, in particular, be detachment at the leading edge of the blades, defective mast bearings, or a problem with the stiffness of the drag dampers; and
the frequency 4&OHgr; is the natural frequency of vibration of a helicopter with a four-bladed rotor. This frequency must not change for a given flight configuration. Its degradation is generally caused by a problem in the connection between the main transmission gearbox and the fuselage.
DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 4,937,758 discloses a process and a device for reducing the vibration generated by the main rotor of a helicopter. This known document recommends that the following operations be performed:
a) measuring the values of measurement parameters that represent the vibration of said structure;
b) determining, on the basis of these measured values and of a predetermined influence matrix, the adjustment values of adjustment parameters capable of reducing and of minimizing said vibration; and
c) adjusting said adjustment parameters to said adjustment values.
This known device and this known process, which allow a certain number of different vibrations to be minimized, do, however, have a major drawback: they do not allow vibrations of the structure of frequency other than 1&OHgr; and 2&OHgr; to be reduced.
It is known that in the case of a helicopter comprising a four-bladed rotor, the vibrations in 3&OHgr; combine with the natural vibrations in 4&OHgr; in such a way as to give rise to a vibration which is felt as an unpleasant knocking at 1&OHgr; and which is highly prejudicial, particularly to the comfort of crew and passengers.
SUMMARY OF THE INVENTION
The object of the present invention is to overcome these drawbacks. The invention relates to a device for reducing the vibrations in all the harmonics, and particularly the vibrations in 3&OHgr;, generated on the structure of a rotary-wing aircraft, particularly a helicopter, at a rotational speed &OHgr;, and to thus increase the comfort of passengers and crew.
To this end, according to the invention, said device comprises adjusting means for adjusting the second mode of flapping of each of the blades of the main rotor which provides said rotary-wing aircraft with lift and forward drive.
Thus, by virtue of the invention, the natural frequency of each blade, which corresponds to the second mode of flapping of the blade, can be adjusted and therefore moved away from the 2&OHgr; and 3&OHgr; harmonics of the rotational speed, between which this frequency is defined, so as, in particular, to reduce the vibrations in 3&OHgr;.
What happens is that the main cause of said 3&OHgr; vibrations in a stationary frame of reference associated with the structure, is an excitation resulting essentially from the second mode of flapping of the blades, the frequency of which is close to a frequency of the structure which frequency is, itself, of the order of 3&OHgr;. This main cause is therefore eliminated by moving the frequency of said second mode of flapping far enough away from said frequency.
Furthermore, said natural frequency needs to be adjusted to move it away from the 3&OHgr; harmonic and, in particular, by way of example, to make it below:
2.6&OHgr;, in the case of a three- or five-blade aircraft; and
2.7&OHgr;, in the case of a four-blade aircraft.
Advantageously, said adjusting means comprise adjusting masses, preferably metal washers of varying thickness and density, arranged along the span of each blade and/or arranged at various points along the chord.
Furthermore, in a preferred embodiment, said device additionally comprises:
a number of sensors, for example accelerometers, capable of measuring the values of measurement parameters that represent the vibration of said structure;
additional adjusting means capable, as a function of the adjusting values, of adjusting adjustment parameters capable of reducing the vibrations of said structure; and
a calculation unit, preferably a personal computer, capable, on the basis of the values measured by said sensors, of calculating the adjusting values of said adjusting means and of said additional adjusting means to allow the vibrations to be minimized, the adjusting values thus calculated being applied to said adjusting means and to said additional adjusting means.
It is thus possible to reduce and to minimize most of the various vibrations generated by said main rotor.
Furthermore, advantageously, said additional adjusting means comprise at least one of the following elements:
auxiliary masses arranged in the cuffs of the blades;
pitch rods; and
balance tabs.
The present invention also relates to a process for reducing the vibration generated by a main rotor on the structure of a rotary-wing aircraft, and more particularly, to a process such as the one disclosed in the aforementioned U.S. Pat. No. 4,937,758, and according to which:
a) the values of measurement parameters that represent the vibration of said structure are measured;
b) on the basis of these measured values, the adjustment values of adjustment parameters capable of reducing and of minimizing said vibration, are determined; and
c) said adjustment parameters are adjusted to said adjustment values.
According to the invention, said process is noteworthy in that wherein at least the second mode of flapping of each of the blades of a main rotor providing said rotary-wing aircraft with lift and forward drive is adjusted by way of an adjustment parameter, which, in particular, allows vibration in 3&OHgr; to be reduced.
Furthermore, advantageously, in the aforementioned step b) said adjustment values are determined using an influence matrix:
which is formed on a reduced-vibration aircraft, from a number of different adjustments and measured values of said measurement parameters each or which is the result of these different adjustments; and/or
which is formed taking account of the following assumptions:
all the blades of said main rotor have the same behavior;
said aircraft represents a solid body; and
the relationship between the adjustment values and the measured values is a linear relationship.
Furthermore, advantageously, in order to determine all of the adjustment values Pn+1 for a particular flight n+1, a matrix is minimized, said matrix &Ggr;n+1 representing said vibrations and being obtained from the expression:
&Ggr;
n+
1
=M
(
Pn
+1
−Pn
)+&Ggr;
n
in which:
M is the influence matrix;
Pn is the set of adjustment values from the previous flight n; and
&Ggr;n is a matrix containing all of the measurement values of said flight n which represent the vibrations and which are the result of the adjustment values Pn.
It will be recalled that the aforementioned U.S. Pat. No. 4,937,758 does not, unlike the invention (with masses arranged along the span and/or along the chord) teach how to adjust the second mode of
Eurocopter
Jakel Kevin
Poon Peter M.
Stevens Davis Miller & Mosher LLP
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