President Mei
　　Tsinghua Unviersity
　　Kunming , Yunnan
　　China

　　Dear President Mei ,

　　I understand that the Tsinghua University has considered the continuation of fellowships for graduate students studying in this country .

　　I am glad to endorse , very heartily , the application of Mr . H. S. Tsien. Mr . Tsien passed his doctor ’s examination last month with good success , however , he is still working on his thesis. The delay is not his fault , because , at my suggestion , he has been , in the meantime ,occupied with important research problems . The result of one of　his　researches has been published in The Journal of the Aeronautical Sciences as a joint publication with me. His second paper on“the theory of projectiles ”is finished , and will be printed very soon in The Journal of Applied Mechanics of the American Society of Mechanical Engineers . This paper found the interest of the Ordnance Department of the U. S. Army , as an　important contribution to the science of theoretical ballistics . A third joint paper with F. Malina on “pooblems of the rocket propulsion ”is finished in manuscript form , and will be published in a short time. So, you see that Mr . Tsien ’s activities are very promising . I especially believe that if his fellowship is continued for one more year , he will be quite an expert in the theory of compressible fluids , in high speed , and ballistic problems . It seems to me that the last field especially , is of very great importance for the future of your country . I hope that you will find it possible to continue Mr .Tsien ’s fellowship for the next academic year.

　　I met Dr. Wattendorf a few weeks ago. Fortunately , he has made great progress as far as health is concerned , and now , for the time being , he is starting with home work . I understand thathe plans the publication of one or two papers on the wind tunnel work carried out in China under your and Dr. Ku’ｓsupervision .

　　Please give my regards to Dr. Ku and other members of your Faculty .

　　Very sincerely yours ,

　　Th. von Karman

　　钱学森同时亦附信请求延长学习一年。

　　钱学森致清华办公处函

　　(1938206207 美国加州—昆明)

　　径启者, 学生于民国二十四年出国习航空工程,第一年在麻省理工大学, 第二年及第三年在加省理工大学。然学问非易事, 学生现在始觉对独立研究有相当把握。今年二月间, 曾与冯．卡门教授联名在美国航空学会年会发表论文一篇(已在该会会刊发表) , 题为《可压缩流体中之界流层》(Boundary Layer in Compressible Fluids ) , 现在待发表者又有论文一篇题为《非金属弹偏斜时所受之空气阻力》( Supersonic Flow over an Inclined Body of Revolution )。然学生以为, 如能在冯．卡门教授门下再有一年之陶冶, 则学生之学问能力必能达完善之境, 将来归国效力必多。冯．卡门教授亦以为在现在情形之下, 此亦上策。故学生乃敢呈请再延长公费一年, 至民国二十八年七月为止。此致

　　国立清华大学校长办公处

　　1939 年6 月当钱学森在冯．卡门指导下, 完成“高速气动力学问题研究”等3 篇论文, 获得航空、数学博士学位后,7 月清华第4 次聘委会即作出决议,聘请钱学森为航空研究所副教授, 月薪280 元。

　　1940 年2 月钱学森在美国航空学会年会上宣读了“薄壳稳定性”和“高速气流突变的测定”2 篇论文。他对薄壳研究中有极大难度、而且具有很大实用价值的科研课题, 作出了创造性、开拓性的解释和回答, 受到与会者的高度评价。他们的合作第一次发现了在可压缩的气流中, 机翼在亚音速飞行时的压强和速度之间的定量关系, 被称为“卡门- 钱”公式, 从而准确地表达了这种定量关系, 且为实验所证明。

　　1938—1945 年钱与冯．卡门合作发表了多篇重要论文, 使他成为国际空气动力学重量级人物。清华在这期间, 通过各种渠道聘请钱回校任教授, 其间周培源于1943 年冬赴加州理工学院工作, 曾亲自找钱谈话。他认为作为一个海外学子重任在肩, 应该以身许国, 把学到的科学知识和取得的科研成果献给祖国,以改变国家的面貌。但由于多种原因未能成行。