One of the first successful test to receive UDP packets from X-Plane. X-Plane UDP Data Structure analysis.(May 2012)
It receives UDP packets from X-Plane and show it in the terminal in the raw view.

/*
X-Plane UDP Data Structure 

Test sketch that receives UDP packets from X-Plane in the raw view.
and prints to Serial terminal  data of the group #131 ("ground location") 
Check this group in the Data Input & Output Screen of the X-Plane for sending.

Packet size: 41
Xplane Data:
68-65-84-65-60-131-0-0-0-166-138-16-71-193-114-2-195-58-143-139-70-244-238-135-59-61-207-225-58-
0-192-121-196-0-192-121-196-0-192-121-196- 

May 2012
Vlad S
http://simvim.com
 */  

#include <SPI.h>        
#include <Ethernet.h>
#include <EthernetUdp.h>
    

byte mac[] = {  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
IPAddress ip(192, 168, 1, 6);    // local IP - address of my Arduino 
unsigned int localPort = 49001;      // local port to listen - default X-Plane port 
byte buf = 00;   // buffer for  UDP packet (BYTE, not char)

EthernetUDP Udp; // An EthernetUDP instance to send and receive packets over UDP

//-------------------------------------------------------------------------------

void setup() 
{
  Ethernet.begin(mac,ip);   // start the Ethernet 
  Udp.begin(localPort);     //..and UDP:

  Serial.begin(9600);       // init serial port
}
void loop() {
  int packetSize = Udp.parsePacket();   //  Checks for the presence of a UDP packet, and returns its size
  if(packetSize)                        //  UDP packet was received and its size defined
  {
    Serial.println();
    Serial.print("Packet size: ");
    Serial.println(packetSize);         // Packet Size in bytes

 // When Udp.read used without parameters, it returns next char (byte in this case) :
 
      Serial.println("Xplane Data:");  
        for (int i =0; i < packetSize; i++)
        {
          buf = Udp.read();
          Serial.print(buf);
          Serial.print("-");
        }
  }
  delay(10);
}