\Data\cqe\RTModels is growing very large

For most customer the Cognos 8 runtime model directory which is only a temporary buffer pool grows very large (tons of gigabytes) after a while. Unfortunately there is no way to prevent the directory from growing. Usually this directory is designed to clean itself but depending on the usuage this cleanup does not work in most cases.

So, can this directory be cleared manually?

Yes, you can flush the directory, but the Cognos 8 Dispatcher and Content Manager Service has to be shut down at this time. Make sure that you delete only the contents (including subdirs), not the directory itself.