Sperry Corp. said yesterday that it had a 28.7 percent drop in earnings in its fiscal year ended March 31 in spite of a 2.7 percent rise in sales and that its fourth-quarter profits were off 11 1/2 percent from a year ago.

Net income for the year was $221.8 million ($5.25 a share) on sales of $5.571 billion compared with $311.2 million ($7.63) on sales of $5.427 billion. Fourth-quarter profits were $81.8 million ($1.92) on sales of $1.573 billion compared with $92.4 million ($2.24) on sales of $1.546 billion.

Earnings for the year were increased $1.66 a share and earnings for the quarter were raised 54 cents a share by adoption of the new Financial Accounting Standard 52 on foreign currency translations.

Chairman J. Paul Lyet said that all the commercial-product divisions were hit hard by the recession, but defense orders were up 25 percent for the year and 26 percent for the final quarter.

Grumman Corp. reported yesterday that its net income for the first quarter rose to $7.57 million (55 cents a share) from $5.36 million (39 cents) a year earlier as revenues rose to $519.72 million from $419.05 million.

Aerospace operations led the company, up 19 percent to $67 million. Although Flxible bus sales were ahead of last year's initial quarter by $38 million, the division still suffered a $20.1 million loss, an increase of $10.8 million from the 1981 period.