The ratings of four Aerospace and Defense stocks are down this week, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

Boeing (NYSE:BA) is on the decline this week, earning a D (“sell”) after receiving a C (“hold”) last week. Boeing is engaged in the design, development, manufacture, sale and support of commercial jetliners, military aircrafts, satellites, missile defense, human space flight, launch systems and services. Over the last month, the price of BA is down 3.4%. This is worse than the S&P 500’s 4.4% increase for the same period. For more information, get Portfolio Grader’s complete analysis of BA stock.

This week, Ducommun (NYSE:DCO) drops from a C to a D rating. Ducommun designs, engineers and manufactures aerostructure and electromechanical components and subassemblies, and provides engineering, technical and program management services mainly for the aerospace industry. The stock gets F’s in Cash Flow and Margin Growth. To get an in-depth look at DCO, get Portfolio Grader’s complete analysis of DCO stock.

National Penn Bancshares‘s (NYSE:NPK) rating falls this week to an F (“strong sell”), down from last week’s D (“sell”). National Presto Industries is a diversified manufacturing company. As of Sept. 17, 15.6% of outstanding National Penn Bancshares shares were held short. For a full analysis of NPK stock, visit Portfolio Grader.

AeroVironment (NASDAQ:AVAV) gets weaker ratings this week as last week’s D drops to an F. AeroVironment designs, tests, and manufactures diversified technological products, including small, unmanned, remote-control military aircraft and rapid battery-charging stations. The stock also gets an F in Earnings Growth. Over the last month, shares of the company are down 1.9%. For more information, get Portfolio Grader’s complete analysis of AVAV stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.