Just to be clear here, downloading videos is against YouTube's TOS.
5. General restrictions on use
5.1 YouTube hereby grants you permission to access and use the Service, subject to the following express conditions, and you agree that your failure to adhere to any of these conditions shall constitute a breach of these Terms on your part:
you agree not to access Content through any technology or means other than the video playback pages of the Website itself, the YouTube Player, or such other means as YouTube may explicitly designate for this purpose;
you agree not to access Content for any reason other than your personal, non-commercial use solely as intended through and permitted by the normal functionality of the Service, and solely for Streaming. "Streaming" means a contemporaneous digital transmission of the material by YouTube via the Internet to a user operated Internet enabled device in such a manner that the data is intended for real-time viewing and not intended to be downloaded (either permanently or temporarily), copied, stored, or redistributed by the user.