Aircraft Geometry and Configuration

PANAMs parametric semi-empirical noise source models need geometric data to calculate noise emission. Data of wing geometry, horizontal tailplane, vertical tailplane and the landing gear is necessary. Furthermore, fuselage diameter and the number of engines are required as an input. Dedicated variable names can be found along with a short description in table C.1.

Values of the reference aircraft are also provided. PrADO writes all data in classified databases as depicted in figure B.2. Table C.1 includes the database number in which the required parameter can be found. Furthermore, a few parameters from former data input files have been passed but not used on a later basis for noise prediction in PANAM (e.g.

FNKBESF in table C.1). Other parameters used for checking e.g. the existence of flaps or the number of vertical tail planes became inessential since all checks are now handled by IOPANAM. Variable names of those parameters are crossed in table C.1 to indicate that they are not passed anymore in the current version. Additional vectors out of the database are needed for calculating parameters that are not included in the database in the requested format. For example, the slat length is not to be found in the database as an absolute value.

Therefore, the plan view vector of the front wing box is used for computing the length.

Vectors are read by applying a specific coding. These vectors of geometric information are essential for calculation only and are therefore also not passed as an input for PANAM.

Table 4.1 Aircraft configuration requirements (status in July 2008) A/C component Max. No. in PrADO Max. No. for PANAM

airfoil wing 2 1

horizontal tailplane 4 1

vertical tailplane 4 1

gear strut > 5 3

fuselage 2 1

identical engines 10 10

Before parameter values are read out of the database the basic aircraft configuration is surveyed. Numbers of major aircraft components must not exceed a specific value as shown in table 4.1, although greater quantities would be possible to consider in PrADO. Thus, only conventional aircraft configurations can be taken into account. A further restriction is defined by allowing only a single vertical tail plane and identical engines. The latter is not necessarily a restriction since common aircraft hold identical engines regardless of engine quantity. This limitation in aircraft configuration is due to PANAMs ability in computing only a specific

number of such aircraft components. Since PANAM would not be able to handle such issues, IOPANAM was programmed to give an error message if one of the maximum quantities, as stated in table 4.1, is exceeded by the user in PrADO.

The number of engines is taken into consideration as stated in equation 4.1. Engine noise source models compute noise emission of a single engine. Sound pressure levels are calculated for relevant one-third octave band frequencies. A doubling of sound pressure levels, as is the case with two engines, is therefore independently done for each octave band before determining the overall sound pressure. As stated in equation 3.7, this summation equals a doubling of the mean square sound pressure. Two engines result in a logarithmic increase of 3.01 dB, three engines in 4.77 dB and four engines in 6.02 dB respectively. With equation 4.1 in mind, only identical engines are to be considered.

( ) ( )

Aircraft wing parameters are among others: wing loading for take-off and landing, wing span, wing sweep, dihedral angle, length of flaps and slats, etc. For horizontal- and vertical tailplanes span, trailing edge sweep, dihedral angle and mean aerodynamic chord are required data. PrADO allows for different types of leading edge flaps: Krueger flap, leading edge flap and slats. As long as no flap (leading- or trailing edge) is extended, the airfoil wing is considered to be in a clean configuration. Parameter of flap position is provided by the trajectory file. With this input set for a clean configuration, PANAM calculates airframe noise for a cruise configuration with no influence of high lift devices. The noise source model for leading edge flaps is based on a slat. Although input parameters would be the same in quantity and definition for Krueger or leading edge flaps, only slats are considered in IOPANAM. In all other cases, the program writes an error message and aborts. This is because the so far implemented noise source model needs further research if applied on other

types than slats²⁰. The geometric slat model is described by a spanwise length and an averaged depth as in figure 4.2. The η-coordinate in PrADO is defined perpendicularly to the aircraft’s longitudinal axis for describing a percentage spanwise extension of various elements on the wing such as flaps, slats, wing tanks, ailerons, etc. Noise source models need spoiler length along the η-coordinate as an input and not the actual length of the spoiler. However, leading edge sweep φLE out of airfoil wing parameters, set as slat trailing edge sweep, is additionally considered in noise source modelling. Therefore, leading edge wing sweep is also important for the predication of slat noise. Slat length is computed out of a geometrical vector reserved for the front wing box. The sum of the percentage length of different segments is multiplied by the half wing span to obtain the spanwise length in units of meters. The projected area Aproj of each element (top view) is additionally added up to obtain an averaged slat depth. Thus, the sum of the projected area divided by the actual slat length (length in η-coordinate divided by the cosine of the leading edge wing sweep) gives the actual averaged slat chord cS (equation 4.2). Finally, slat length is doubled to obtain the length on the right and left wing of the aircraft. This is necessary because the implemented noise source model accounts for only one piece of slat and assumes that therefore a slat, with a total length over the whole wing span, swept according to the leading edge and with an averaged slat chord.

( ) ( )

The same principle is applied upon trailing edge flaps. Out of possible PrADO types i.e. split flap, plain flap, Fowler flap, single-, double-, or, triple slotted flap and single slotted flaps with upper surface blowing; only fowler flaps are to be considered. Flap geometry is taken from the aft wing box geometry vector. The same principle as in the preceding paragraph applies computing flap length and an averaged flap chord. A further simplification is made by averaging the trailing edge over a straight line, which is swept backwards according to an averaged trailing edge sweep (figure 4.2). A kink would normally subdivide the trailing edge

20 e.g.: the noise source model, developed to predict noise originated from a slat, is based on an acoustic model that considers the gap between the slat and the aircraft wing. In the case of a Krueger flap, this gap would no longer exist and the aeroacoustic physics behind lose validity.

flaps into two pieces with separate tailing edge sweeps. The same principle is applied on spoilers whereas the spoiler length of upper- and lower wing spoilers is accumulated.

0.130 0.319 0.358 0.945

ç/ -ç/

-x/m

Slat coordinates

Flap coordinates

longitudionalaxis/fusealge c^S1

c^S2

0.092 0.374 0.776

Spoiler

averaged straight-lined trailing edge

ö_LE

Figure 4.2 Calculation of averaged slat, spoiler and flap length

For the prediction of landing gear noise, gear leg length, tire diameter and the number of axles are required data out of PrADO. The noise source model in PANAM can be applied to multi-wheel main landing gears commonly referred to as bogie gears. The number of axles applied on one gear leg is a control parameter for the wheel arrangement. The implemented noise prediction model was validated on single- and double bogies (Pott-Pollenske 2006, p. 9). A further applicability for e.g. triple bogies might be possible. The landing gear arrangement must be tricycle due to the character of the implemented model in PANAM. Therefore, only three gear struts (with any number of axles and wheels) are available as a maximum as shown in table 4.1. The nose gear out of all gear struts is found by a parameter set for nose gear steering. The length of a gear strut is the sum of fully compressed gear strut length and maximum shock-absorber travel. Left- and right main gear are considered as identical.

Therefore, data of the second main landing gear has not to be passed.