This work attempts to reduce the hub vibratory loads of a lift-offset rotor using IBC (individual blade pitch control) in high-speed forward flight. As a lift-offset rotor for the present study, the rigid coaxial rotor of a XH-59A compound helicopter is considered and CAMRAD II is used to predict the hub vibration and rotor performance. Using the IBC with a single harmonic input at 200 knots, the vibration index of the XH-59A rotor is minimized by about 62% when the 3/rev actuation frequency is applied with the IBC amplitude of 1° and control phase angle of 270° (
Lift-offset helicopters using ABC™ (Advancing Blade Concept, [
Lift-offset helicopters using ABC™.
XH-59A Technology Demonstrator [
X2 Technology Demonstrator [
S-97 Raider [
Although lift-offset compound helicopters have showed excellent high-speed flight performance, they have a serious vibration problem during high-speed flights. In flight tests of the XH-59A compound helicopter, significant 3/rev cockpit vibration was observed because of the use of a rigid coaxial rotor and the absence of a vibration control system [
There have been numerous experimental and numerical works on active rotor controls such as HHC (higher harmonics pitch control, [
There are limited works on vibration analyses of the lift-offset rotor using rotorcraft comprehensive analyses [
Therefore, this paper is aimed at reducing the vibration of the lift-offset rotor using IBC in high-speed flights. As the lift-offset rotor, the XH-59A rotor is considered and CAMRAD II is used to analyze the vibration and performance of the XH-59A rotor using IBC. When 3/rev hub vibratory loads are minimized using the IBC with a single harmonic input, a decrease in the rotor effective lift-to-drag ratio is investigated. In addition, it is shown that the vibration reduction and performance improvement of the lift-offset rotor can be simultaneously obtained when 2/rev and 3/rev actuations are applied in combination for the IBC using multiple harmonic inputs. This study for the XH-59A lift-offset rotor using IBC does not correlate the analysis results of vibration reduction and performance improvement with the measured data, since there is not the test data for vibration and performance of a lift-offset rotor using IBC. Furthermore, the present work is the first attempt for the study of the lift-offset rotor using IBC. However, it is believed that this work will show reasonable prediction results because the present analysis model without IBC has already been validated well in the authors’ previous work [
The XH-59A lift-offset rotor is used as an analysis model using IBC in this work. The XH-59A helicopter using ABC™ was initially developed as a pure helicopter configuration without auxiliary propulsions in 1964 [
General properties of XH-59A lift-offset compound helicopter.
Hub type | Hingeless |
---|---|
Radius (ft) | 18 |
Number of rotors | 2 |
Number of blades | 3 |
Total solidity, |
0.127 |
Tip speed (ft/sec), | |
Helicopter mode | 650 |
Compound helicopter mode | 450 |
Maximum speed (knots) | |
Helicopter mode | 160 |
Compound helicopter mode | 240 |
Horizontal tail | |
Area (ft2) | 60 |
Span (ft) | 15.50 |
Tail length (ft) | 20.30 |
Vertical tail | |
Area (ft2) | 30 |
Span (ft) | 12 |
Tail length (ft) | 20.30 |
Fuselage | |
Length (ft) | 40.5 |
Width (ft) | 6.08 |
Height (ft) | 6.08 |
Rotor separation (ft) | 2.5 |
Power plants | |
Lift | PT6T-3 turboshaft engine |
Thrust | J60-P-3A turbojet engine |
Definition of the crossover angle.
Crossover angle = 0°
Crossover angle = 90°
This work uses CAMRAD II [
The CAMRAD II model for the XH-59A rotor using IBC in the present study is based on the model constructed in the authors’ previous work [
CAMRAD II model for the XH-59A rotor.
For a single harmonic input,
For multiple harmonic inputs,
Since the actual airfoil data for the XH-59A rotor are not available, the airfoils similar to the actual airfoil characteristics of the XH-59A rotor are used as given in Figure
Blade thickness and airfoil distribution.
Actual airfoils
Present airfoils
The trim analyses are conducted using the six primary rotor controls of the upper and lower rotors. The pitch angle of the XH-59A compound helicopter is fixed at 0° since it provides the best performance for the lift-offset rotor [
For the propulsive trim, which will be used in Sections
The 3/rev hub vibratory loads of the XH-59A rotor are calculated using the following:
The vibration index (
The rotor power (
The rotor effective lift-to-drag ratio (
The modeling and analysis techniques of CAMRAD II using the prescribed wake model are validated for the XH-59A rotor without IBC in this section. For the analyses, the XH-59A rotor in compound helicopter mode with auxiliary propulsions (gross weight of 13000 lb) is considered and the lift-offset of 0.25 is used. Figure
Validation of the rotor effective lift-to-drag ratio.
Validation of 3/rev hub pitch moment.
In this section, the 3/rev hub vibratory loads and rotor performance are investigated when the IBC with a single harmonic input (equation (
Figure
Vibration index in terms of control phase angle for IBC using a single harmonic input.
Maximum reduction in vibration index for various IBC conditions using a single harmonic input.
Changes in the 3/rev hub load components in terms of the control phase angle are investigated in Figure
3/rev hub vibratory load components in terms of control phase angle of IBC using a single harmonic input.
Axial force
Normal force
Pitch moment
Figure
IBC input conditions using a single harmonic input for maximum reduction in 3/rev hub vibratory load components.
As shown in Figures
The variations of the rotor effective lift-to-drag ratio are given in Figure
Rotor effective lift-to-drag ratio in terms of control phase angle of IBC using a single harmonic input.
Maximum improvement in rotor effective lift-to-drag ratio for various IBC conditions using a single harmonic input.
As shown in the previous section, the IBC using a single harmonic input reduces significantly the vibration index of the XH-59A rotor at a flight speed of 200 knots; however, the rotor effective lift-to-drag ratio is reduced when the IBC condition to minimize the rotor vibration is used. Therefore, a new input scenario of IBC is required to reduce significantly the hub vibratory loads while maintaining or increasing the rotor performance of a lift-offset rotor in high-speed flights. In this section, the IBC using multiple harmonic inputs is proposed to reduce the vibration index and increase the rotor effective lift-to-drag ratio (or at least maintain the baseline value) of the XH-59A rotor at 200 knots, simultaneously. The different actuation frequencies (
Two actuation frequencies of 2/rev and 3/rev, two IBC amplitudes of 1 and 2°, and control phase angles from 0 to 360° with an increment of 30° are combined for the IBC with multiple harmonic inputs. As previously discussed in Section
Figure
Vibration index in terms of control phase angle (
Figure
3/rev hub vibratory load components in terms of control phase angle (
Axial force
Normal force
Pitch moment
IBC input conditions using multiple harmonic inputs for maximum reduction in 3/rev hub vibratory load components.
Figure
Rotor effective lift-to-drag ratio in terms of control phase angle (
When two present prediction result sets using control phase angles (
Changes in vibration and rotor performance using IBC with multiple harmonic inputs.
Control phase angle ( |
Percent change | |
---|---|---|
Vibration index | Rotor effective lift-to-drag ratio | |
190° | −37.53 | 1.64 |
195° | −43.71 | 1.33 |
200° | −50.00 | 1.01 |
In this work, the vibration and performance of the XH-59A lift-offset rotor using IBC were investigated by the rotorcraft comprehensive analysis code, CAMRAD II. At a flight speed of 200 knots, the vibration index was minimized by about 62% from the baseline value but the rotor effective lift-to-drag ratio was reduced by about 3.43% to the baseline result when the actuation frequency of 3/rev, amplitude of 1°, and control phase of 270° (
Actuation amplitude of IBC (deg.)
Drag force (lb)
3/rev hub force (lb)
Lift force (lb)
Rotor effective lift-to-drag ratio
3/rev hub moment (lb·ft)
Hub rolling moment (lb·ft)
Actuation frequency of IBC (/rev)
Number of blades of each rotor
Per revolution (/rev)
Rotor power (hp)
Coaxial rotor power (hp)
Induced power (hp)
Profile power (hp)
Parasite power (hp)
Radial position of the rotor (ft)
Radius of the rotor (ft)
Thrust (lb)
Flight speed (ft/sec)
Hover tip velocity of the rotor (ft/sec)
Weight of the aircraft (lb)
Wind axis drag force of the rotor (lb)
IBC equivalent blade pitch (deg.)
Solidity of the rotor
Control phase angle of IBC (deg.)
Azimuth angle (deg.).
The data used to support the findings of this study are included within the article.
This paper was presented at the 44th European Rotorcraft Forum, Delft, the Netherlands, September 18-21, 2018.
The authors declare that there is no conflict of interest regarding the publication of this paper.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2016R1C1B1007199). This work was supported by the research fund of the Korea Aerospace Research Institute. This work was conducted at the High-Speed Compound Unmanned Rotorcraft (HCUR) research laboratory with the support of the Agency for Defense Development (ADD).