Using an ultrasonic system is not that hard!
In fact it's not hard at all!
If you've gotten past the "blinky LED" stage you should have no problem.
Schematics and built examples here
You don't HAVE to use 40kHz, a 32.768Khz watch crystal will work.
There is a good reason cars and some motorcycles use an ultrasonic system to measure -
- air flow rate
- temperature
- humidity
Because it is EASY!!
Having A CRO to setup helps, but you can get by without one.
And befor the usual suspects start telling you the receiver bit is hard, it isn't, my initial prototype took all of an hour to get together.
With an aircraft you can not only sense airspeed but a stall condition as well.
Now I have a bit of time I'll put together some notes and a video on how to do it.