I come from a game called Besiege that is very similar to this one. Some key differences that I found were that the community is a lot stronger in besiege than in simple planes. Here are some of my suggestions to help this game out. I just a week into this game though so my information might not be correct.

1. Make uploaded planes easier to be played.
   Besiege uses the Steam Workshop to share creations. I find that it is way easier to use then this website. Firstly, users can hover over a creations thumbnail to see the description and more information and the subscribe button (to download the creation) is huge and easy to click. Creations are downloaded very fast and are saved in a sub-menu called subscribed creations so players don't need to constantly re-download or save downloaded machines.

2. Encourage people to view, upvote and download planes
   Another thing that the Steam Workshop does well is the sorting system. The most subscribed creations are at the top like simple planes, however there are many different categories that can be chosen easily (unlike the current tag system in simple planes). Views are also a large part in the creations success making it easier for smaller creations to be seen and downloaded. I don't know how up voting could be encouraged but my idea would be to have an upvoting counter on a users home page sort of like a bookmark for all of their favourite creations.

3. Structure the forum
   Make many sections such as: General, Creations, Bug reports, Modding, Off topic, etc. This will help users advertise their creations without spamming, make mods easier to find and generally make the forum easier to use. This will probably be the best step towards making the community stronger

Boy that was long. Hope this helps!