I have a futaba fm 72mhz 4EXA tx with memory for up to four models. how do I know which rx's I can use for this tx when adding a plane.

Thanks in advance</p>

Your radio has a crystal that sets it to a certain frequency. Look on the back of your radio, there should be a number... Something like 42 or 13. I forget the exact range but I think the numbers go from like 11 through 80, or something like that. Anyway, whatever number your radio is, when you buy a new receiver just buy the crystal that matches your radio. That way you can have multiple planes that work with your radio. You can change the crystal in a receiver to match your transmitter, but don't change the crystal in your Transmitter to match a receiver. That is a no no.

I have a plane that has an airtronics rx on the same channel but it only works with my airtronics tx, if i turn on my futaba tx and then turn  on my plane with the airtronics rx all the control surfaces just twitch. If I put a brand new crystal in this rx and tried it with the futaba first would it recognize that radio then?</p>

Changing crystals most likely wont help you.
Typically Futaba and Hitec Tx/Rx will play nice because theyre whats termed negative shift.
JR and Airtronics will usually play nice together because theyre positive shift.

Some Airtronics Rx's and others are capable of both negative and positive shift.
Unfortunately thou it sounds like your Airtronics Rx isnt compatible with your Futaba Tx.

If you give us the model of your Rx that would help.

Cheers.

No, Airtronics receivers are not compatible with Futaba transmitters.

Ken

Hitec sells the Supreme 8 is autoshift and will work both the Airtronics and Futaba

Berg sells receivers, full range receivers, that are pretty much compatible with everybody.

http://www.castlecreations.com/products/berg.html

CGr