The Bethesda-based Martin Marietta Corp. reported a more than 30 percent drop in second-quarter profits over the same period a year ago, blaming a depressed market for its aluminum and cement products.

Company officials reported, however, that the firm's aerospace operations, which accounted for 58 percent of its sales in 1981, continued to show record growth in both sales and profits. The firm's aerospace business, much of which is defense-related, has actually been strong enough to help improve the company's position since the first quarter, officials said.

Net income for Martin Marietta in the second quarter was down from $61.1 million ($1.63 a share) $40.2 million ($1.13) this year. Sales for the same periods were up, however, rising from $880.2 million last year to $909.7 this year.

Results for the first half parallel the second-quarter results. For the first six months, the firm's profits were $56.0 million ($1.57 per share) compared with $100.8 million ($2.69) for the same period in 1981. Sales were up modestly to $1.7 billion from $1.6 billion.

A Martin Marietta spokesman said forecasts for the firm's aluminum business, which made up 19 percent of the company's 1981 sales, were particularly bleak. "The corporation now expects aluminum to be unprofitable for the rest of the year," he said.

Atlantic Research Corp. of Alexandria, maker of small rockets and gas generators, reported a 41 percent increase in net income for the second quarter.

Profits increased to $1.1 million (49 cents a share) compared with $802,000 (35 cents) last year. Sales were also up, going from $24.1 million in last year's second quarter to $29.2 million this year.

For the half, net income was $2.1 million (93 cents) compared with $1.5 million (67 cents). Sales were up from $45.8 million to $54.9 million.

Fairchild Industries Inc., the Germantown, Md.-based aircraft manufacturer, said a weak market for the airplanes it makes contributed to its 59 percent drop in second-quarter earnings compared to the same period a year ago.

Earnings dropped to $11.9 million (64 cents a share) versus $28.9 million ($1.56) for the same period last year. Sales also dropped, from $359.9 million last year to $303.3 million this year.

A sharp decline in sales of commercial turboprop aircraft and aerospace fasteners had a major impact on second-quarter results, company spokesmen reported yesterday.

Earnings in the second quarter included a pre-tax gain of $5 million paid by Continental Telecom Inc. to Fairchild in exchange for Fairchild's waiver of preferential rights to certain future profits of American Satellite Corp., which the two companies jointly own.

In the half, earnings were $21.2 million ($1.14) compared with 1981 net earnings of $46.3 million ($2.50). Sales declined to $565.8 million from $672.1 million.

A recent decision by the Air Force to award Fairchild the contract to build its next training aircraft will not contribute to company sales and earnings until the mid- to late 1980s, officials reported.

Reynolds Metals Co., the Richmond-based producer of aluminum and aluminum products, reported a 44 percent drop in second-quarter earnings from last year.

The company, the second-largest aluminum producer in the United States, has been suffering, like Martin Marietta, from the depression in the aluminum industry and yesterday blamed its poor results on low demand and deteriorating prices.

Reynold's net income for the second quarter was $25.5 million ($1.29 a share) compared with $45.3 million ($2.33) last year. Sales dropped from $950.2 million last year to $805.4 million this year.

The company's second-quarter figures do not entirely reflect its operating results. Net income was bolstered by an extraordinary gain of $4.6 million (24 cents) from the exchange of 431,623 shares of new common stock for $14.3 million in debt.

Net income also included foreign currency translation and exchange gains of about $15.6 million (80 cents). Moreover, the company's last-in, first-out (LIFO) determination of inventory contributed about $16.5 million to net income compared with $500,000 in the second quarter of 1981.

Company officials reported that its aluminum shipments declined from 315,500 tons in the second quarter of last year to 266,000 tons in the second three months of this year.

Net income for the first half of 1982 was $39.1 million ($1.97) compared with $78.3 million ($4.02) in 1981. Net sales for the first half of 1982 were $1.5 billion compared with $1.8 billion last year.

Ethyl Corp., maker of gasoline additives and other specialty chemicals, yesterday reported record earnings of $27.9 million ($1.39 a share) for the second quarter of this year, up from $24.5 million ($1.24) last year.

Net sales for the second quarter fell to $417.6 million from $453.4 million last year.

The Richmond-based firm recently acquired First Colony Life Insurance and earnings from that company were included with Ethyl Corp.'s beginning May 14. First Colony contributed about $800,000 in net income for the second quarter after deducting financing costs related to its purchase.

Ethyl officials also said second-quarter earnings include $1.3 million from the sale of its oil and gas interests in the Dutch North Sea.

For the half, Ethyl's earnings dropped to $45 million ($2.25) from $45.8 million ($2.30) last year.