The 'air sound' /ɛr/ is an r-controlled vowel. Technically this sound is two distinct sounds (vowel sound+'r sound' /r/). It is presented here under the name 'air sound' to distinguish the fact that the vowel portion of the sound is different from the 'long a' /eɪ/ (although the sound's spellings are very similar to those for the 'long a' /eɪ/).
The 'air sound' /ɛr/ begins with the tongue rounded slightly upward in the middle of the mouth. The sides of the tongue may lightly touch the bottom teeth during the formation of the beginning of this sound.
To transition to the /r/ portion of the sound, the body of the tongue moves upward and forward. The mid-section of the tongue raises so the sides of the tongue touch the mid-side teeth. The air travels over the body of the tongue to create the /r/ portion of the sound. (NOTE: The secondary method of producing the /r/ may be used to produce second portion of the 'air sound' /ɛr/ instead of this technique.)