I'm not a native English (or German) speaker, but this is how I understand:
Aircraft is the general term for "flying machines". It can be anything from air balloon to helicopter or airplane. Airplane is a fixed-wing, power-driven aircraft.
aircraft - das Luftfahrzeug (die Luft - air, das Fahrzeug - vehicle, craft) airplane - das Flugzeug
When a translation you think is correct is not accepted, you should use the ‘report’ button to bring it to the attention of the staff. If you have doubts, you can first ask in the comments whether the translation is indeed correct; but if you're sure of your translation, commenting isn't going to be very useful.
But in English, we use the exact same word for Luftfahrzeuge and flugzeuge - aircraft. An airplane is an aircraft, as is a helicopter or an airship or a hot air balloon or an autogyro, or even a model aircraft. It's an aircraft no matter whether it carries passengers or not. If it flies and it's human-made, it's an aircraft - and it applies whether it's singular or plural. So aircraft has to be a correct translation.
But that is not what ‘Flugzeug’ means. A hot air balloon is not a ‘Flugzeug’, neither is a zeppelin, nor (in spite of what Duden seems to be saying) is a helicopter a ‘Flugzeug’ in most German speakers' minds (in fact the German Wikipedia seems to be pretty clear that Flugzeuge are fixed-wing aircraft).
In English, there's a word we can use to describe cats, dogs, lions and monkeys, but if someone asked you to translate ‘Katze’, ‘Hund’, ‘Löwe’ or ‘Affe’, ‘animal’ would not be the best word.
‘Aircraft’ is ‘Luftfahrzeug’; ‘plane’ is ‘Flugzeug’.
"flythings" (or something like "flyequipment") would be the literal translation of the individual parts of "Flugzeuge", but obviously, that's not accepted since it's not an English word. There often isn't much point in translating each part of a compound word separately, other than to maybe deduce its meaning or to admire the logic with which German words are constructed, but either way you'll end up needing to translate the word as a whole to actually find the English equivalent.
In another question, "car" was rejected as a translation for "Fahrzeug" because it was too specific. Here "planes" and "airplanes" are accepted. Is "plane" not a bit too specific? Helicopters are never called planes. Nor are lighter-than-air craft. Are there other words for these in German? Am I being too pedantic in thinking that Duo shouldn't accept "planes"?
There is an important difference to be made: there is no specific term for plane in German other than ‘Flugzeug’, which does technically mean ‘aircraft’ (and this should definitely be an accepted translation) but is commonly used to refer to planes specifically. In fact, means of aerial transport more in general can be termed ‘Luftfahrzeuge’, which would never be understood as a plane specifically.
‘Fahrzeug’ and ‘Auto’ instead map pretty well to the English words ‘vehicle’ and ‘car’. Accepting ‘car’ as a translation for ‘Fahrzeug’ is, in my opinion, the same as accepting ‘dog’ for ‘Haustier’ (‘pet’) or ‘parakeet’ for ‘Papagei’ (‘parrot’). Yes, both are very prominent and common subsets of those classes, but why be specific when the original German sentence is clearly and purposefully general?
How does one know when to use f or v for the f sound... Viele, Flugzeug
Just memorisation, I'm afraid; I don't think there are any rules.
Sometimes you even have homophones, e.g. fiel "fell" / viel "much".
And the fact that "full - fill" in English is voll - füllen in German makes me think that the distinction might have been arbitrary at some point (with the letters being used interchangeably) but that the spelling crystallised at some point when spelling started to get standardised and now we end up with whatever happened to get standardised.
Please always quote your entire sentence when you have a question.
If Duolingo marked your sentence wrong, then it probably was wrong.
Nobody can see what you wrote, so references to "my answer" or "the translation" or "this" are not helpful.
Take a screenshot, please, that shows the question and exercise type, your answer, and any error message there may be; then share it with us by uploading the screenshot to a website somewhere (e.g. imgur) and telling us the URL.
I wrote "We have no aeroplanes" which was marked incorrect.
viele Flugzeuge means "many aeroplanes", not "no aeroplanes".
Duo's 'correct' answer was "we have no planes" or no airplanes.
Do you have a screenshot of Duolingo suggesting "no planes" or "no airplanes" as a correct translation for this sentence?
I can't see either of those incorrect versions in the back end.
I believe airplanes is the American term.
That's right. And Duolingo uses American English to teach German, so the default translations will always be AE.
BE variants are usually accepted as well if you type them in, as long as the remainder of the sentence is correct.
I wrote "we have several planes" and it marked it wrong. Why do I need to use "many" instead? Is there a significant grammatical difference?
Not a grammatical difference (they are used identically from the point of view of grammar), but a semantic difference (they have different meanings).
"several" is less than "many".
"several" and "many" mean different things.
If you have several planes but not very many, then "several" will be preferable as it's more accurate.
But if you have lots and lots of planes, then "several" will not be accurate nor preferable.
Here, Duo has viele Flugzeuge (many planes) and not just einige Flugzeuge (several planes).
Thanks mizinamo (rewriting this since it didn't appear to register the first time). I did think about it for a second and yes, you're correct. It just sounded inelegant to my ears to say "many" when referring to a quantity of planes. Colloquially in English I probably would use "a lot of." So my use of "several " was more an aesthetic choice, but I guess incorrect, nonetheless. I noticed this query has come up in the past. Thanks for your patience and follow-through
Unfortunately, "aircraft" is one of those words that many non-English speakers don't understand. For some reason, many people don't understand that it can be both singular and plural. It should be a perfectly acceptable answer, yet it isn't, and hasn't been for at least four months. Come on Duolingo moderators - surely it can't take this long to fix what is obviously an error.