Pravda, a "Russian semi-propaganda newspaper," recently reported that Russia [is] infuriated with Chinese export copies of Su-27 jet fighters (h/t commenter at Marginal Revolution). The Sukhoi Su-27 fighter, and its numerous variants, is Russia's most advanced jet fighter in service with the Su-27SM being the most advanced. Back in 1991, Russia sold China 24 Su-27s for approximately $1 billion. A second delivery of 24 Su-27s was made in 1996, and the total deal is worth about $1.7 billion. In the same year Russians and Chinese agreed to a $2.2 billion licensing agreement allowing the Chinese to produce up to 200 Su-27s, to be called the J-11. The license rights did not include export rights to foreign countries. Russia already had a healthy Su-27 export business with customers in Vietnam, India, and Indonesia. The Chinese license agreement, and sales of the Su-27 is an important revenue source for Russia's military which is using the profit to finance the development of their next generation jet fighter, the Su-37. (Almost all of this data was sourced from FAS's pages. FAS was founded by scientists who worked on the Manhattan Project and were wracked with guilt following the atomic bombing of Hiroshima and Nagasaki. Their mission is combating nuclear proliferation and the small arms trade, and world peace in general)
Now, the Russians are accusing the Chinese of manufacturing Su-27s with an intent to export the jets in violation of the licensing agreements. Pravda reports that "Moscow promised to launch legal proceedings to protect its intellectual property." This will probably have to be a legal proceeding based on violation of international law or breach of contract in selling to third parties. It is highly unlikely that there are any patent or trade secret claims. A patent claim would require that the Chinese misappropriated patented technology. Sophisticated military hardware often avoids patents because that requires disclosure. Also, competition for military contracts operates differently than the free market making patents less purposeful. A trade secret claim is unlikely because the Chinese already have the trade secret and would not be divulging it by selling the jets to a third-party. This would just give the third-party the opportunity to reverse engineer the jets. This also makes it less likely that the jets would be patented in Russia.
Selling jets involves more than just the delivery of a plane. There are also the training costs, maintenance costs, and upgrade costs. Russia might be worrying less about its IP, as its sales record demonstrates that it is willing to sell to anybody who can afford their jets, and more about the income from all of the services that go along with selling jets.
Russia upset by China's imitation fighter
China copies obsolete Russian fighter