I'm going to quote wikipedia here as it sums up the controversy, and I ask the question because my own research is inconclusive.
Slave trading
Drake accompanied his second cousin Sir John Hawkins in making the third English slave-trading expeditions, making fortunes through the abduction and transportation of West African people, and then exchanging them for high-value goods.[28] The first Englishman recorded to have taken slaves from Africa was John Lok, a London trader who, in 1555, brought to England five slaves from Guinea.[29] A second London trader taking slaves at that time was William Towerson whose fleet sailed into Plymouth following his 1556 voyage to Africa and from Plymouth on his 1557 voyage. Despite the exploits of Lok and Towerson, John Hawkins of Plymouth is widely acknowledged to be an early pioneer of the English slave trade. While Hawkins made only three such trips, ultimately the English were to dominate the trade.[30]
Around 1563 Drake first sailed west to the Spanish Main, on a ship owned and commanded by John Hawkins, with a cargo of people forcibly removed from the coast of West Africa. The Englishmen sold their African captives into slavery in Spanish plantations. These activities undermine the tendency to view Drake as simply an untarnished English hero. Although slavery was legal throughout the world at the time, its expansion by Hawkins (and Drake) is now widely seen as a great blot upon their records. In general, the kidnapping and forced transportation of people was considered to be a criminal offence under English law at the time, although legal protection did not extend to slaves, non-Protestants or criminals. Hawkins' own account of his actions (in which Drake took part) cites two sources for their victims. One was military attacks on African towns and villages (with the assistance of rival African warlords), the other was attacking Portuguese slave ships[citation needed].
One can assume he may have bought and sold slave just by his presence on the ship (and this seems to be the assertion here, unsupported). Or by the very nature of Sea Travel at the time he may have been on he ship simply because it was going where he wanted to go.
Are there any contemporary sources that support the assertion that Sir Francis Drake was involved in the slave trade in any way?