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后掠翼为什么翼尖先失速？改善后掠翼飞机翼尖失速特性的措施有哪些？

答案

参考答案：

后掠翼翼尖先失速的主要原因在于：

①翼根效应和翼尖效应，使机翼上表面翼根部位压力大于翼尖部位压力，压力差促使气流展向流动，使附面层在翼尖部位变厚，容易产生气流分离。

②翼尖效应使翼尖部位上表面吸力峰增强，逆压梯度增加，容易气流分离。改善措施主要是为了阻止气流在机翼上表面的展向流动，有：

①翼上表面翼刀

②前缘翼刀

③前缘翼下翼刀

④前缘锯齿

⑤涡流发生器

问答题 简答题

轮背内侧距离最大限度是怎样规定的？
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单项选择题

It’s all annual back-to-school routine. One morning you wave goodbye, and that (1) evening you’re burning the late-night oil in sympathy. In the race to improve educational standards, (2) are throwing the books at kids. (3) elementary school students are complaining of homework (4) What’s a well-meaning parent to do.As hard as (5) may he, sit back and chill, experts advise. Though you’ve got to get them to do it, (6) helping too much, or even examining (7) too carefully, you may keep them (8) doing it by themselves. "I wouldn’t advise a parent to check every (9) assignment," says psychologist John Rosemond, author of Ending the Tough Homework. "There’s a (10) of appreciation for trial and error. Let your children (11) the grade they deserve. " Many experts believe parents should gently look over the work of younger children and ask them to rethink their (12) . But "you don’t want them to feel it has to be (13) ," she says.

That’s not to say parents should (14) homework-first, they should monitor how much homework their kids (15) . Thirty minutes a day in the early elementary years and an hour in (16) four, five, and six is standard, says Rosemond. For junior-high students it should be " (17) more than an hour and a half," and two for high school students. If your child (18) has more homework than this, you may want to check (19) other parents and then talk to the teacher about (20) assignments.

9（）

A.single

B.piece

C.page

D.other
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