I researched this place online, asked questions here and finally got there while in Beijing. Museum is relatively easy to find, get off Xitucheng station (line 2) and walk up the road for about 5-8 min. Enter the main gate to the university (passport essential otherwise forget it) and then walk a couple of hundred meters towards the museum building. English sign is on the door, big sign in Mandarin on the building.
Entry is not so simple though. If you translate their website you will notice that entry to foreigners is only on Saturdays and Tuesdays between 9am and 12pm and online booking is essential. That's fine, but I am yet to find a link to online booking via their website. Maybe Google translate is not good enough but even scanning the page in Mandarin and trying to find a link turned out to be fruitless.
I asked a friend of mine (Chinese) to book for me. Not sure how she did it, but she did it and it was booked for Tuesday. Then she called them to make sure I can get in. Apparently, they do not take booking on passport number, but they gave her a booking reference number and I was just supposed to show that. Easy right?
So I showed up at the ticket office (a hole in the wall) where a very seriously looking woman in some kind of a uniform and no language skills whatsoever shoved a sign in front of my eyes saying "foreigners Sat and Tue, 9am-12pm, booking online only". This was in English. I kept nodding and kept saying that I have booking and showing her the booking reference number. She looked puzzled, looked at me, looked at the number, looked at the computer screen, kept mumbling something. Looked like the booking is not there or she is going to be difficult. After a few minutes of this, she pointed to my passport (had it in my hand, learned at this stage that without ID you cannot enter anything in Beijing). She examined it thoroughly including validity of the visa and finally gave me a ticket to enter.
IT'S FREE OF CHARGE.
Once inside, I was the only Westerner. There was heaps of mothers with little kids and older families though, so the place seems to be popular. In a corner there was some kind of a lecture given in English to a bunch of students.
The collection is very cool, consists of gems such as P-61 Black Widow and Chinese used aircraft, Mig-9 and La-11 being highlights for me. Overall, not big, but definitely worth it if you are an aviation buff. Plaques have a short English description, otherwise there is no English in there. Would be good to read about the Chinese space exploration program, from photos it looked like there is plenty of good old communist propaganda there, but my Google translate was not coping well and it was taking too long.
Go and visit, do not give up. Woman in the ticket office just wants to be important, but probably has no authority whatsoever. Just in case, the entire place if full of students and it's a technical university, so you can ask somebody for help. I am sure they would speak English.