If you working with a wrapped app with phonegap you can use the preference orientation: Phonegap documentation
You have to modify the config.xml before building your app with phonegap.
orientation Device orientation; possible values are default, landscape, or portrait. Please note that default means both landscape and portrait are enabled. If you want to use each platform's default settings (usually portrait only), remove this tag from your config.xml file.
< preference name="orientation" value="landscape" />