Try Cryptomator. You can’t encrypt a whole disk per se, but you can create an encrypted container within it.
I’m not sure whether you can create one on an disk direct from the iPad to a usb stick, but even if you can’t you can’t you can create one on the iPad and then move it in files and reconnect in the Cryptomator app.
It’s a cool app and it’s open source. It’s a bit behind on the iPad though, the integration is limited. Worth it though.
If you're trying to encrypt a flash drive to use on an iOS device, you should probably know that your security stops at the device. For example, if your 'world domination plans' include a video or image or other file it will be recoverable on the iOS device.
Currently iOS will allow you to read a flash drive, but you must open the files on the device itself which leaves a cached version of it in the background on the device itself.
If you'd like to leave no (or much less) trace on the iOS device, you need to not just encrypt the drive but open the files on a non-iOS native application.
The software is absolutely horrible, but it works 80% of the time for my needs. Go to San Disk (Western Digital) site and download PrivateAccess. It encrypts the files and has an app you can use on the iOS devices. Personally, I bought an iXpand flash drive that has USB-C and Lightning connections for this very purpose.