Over the years a large amount of Flight Logistics' work has been air to air. From vintage bi-planes to four engined airliners, all have one thing in common it is complex and expensive work.
For fast jet work, the standard a periscope mounted through the floor of a specially modified business jet. Above left is a Learjet (with the lens head showing in front of the rear wheels).

The periscope and camera are free to pan through 360 degrees and either by tilting mirrors or prisms, simultaneously tilt the image up and down.
With air to air filming, pre-planning is everything. When we said that it was expensive, filming with an airliner can cost over £200 ($300) per minute. This is not the time to say "let me have a think about it".
What the Director requires and how it is achieved is completely different for all involved. Each element has to understand what is required of them. The ideal is to treat each flight like an aerial ballet with a strict order of shots and procedures to maximise the light, location and time.
Flight Logistics has spent a great deal of time in developing methods to ensure efficiency and to easily "preview" what is desirable or possible.
The circling manoeuvre above is beautifully simple and allows the two formation Pilots to settle down. 

This shot guarantees the ripple of sun sparkel line across the subject aircraft.  .

Flight above a solid cloud layer provides soft uplighting reflection and if close enough an impression of speed.
Pictured below is a Hasselblad stills camera that we modified to allow us to photograph 6cm x 6cm stills (centre), through a 35mm format lens ,without vignetting.

Pictured (right) is a horizontal 35mm Show scan plate camera.
Clearly not every shoot warrants specialy modifried aircraft and complex camera systems. We  have frequently used a turboprop buisiness aircraft which has particularly large windows,and an unobstructed rear three quarter view.
The picture (above right) was taken with a 28mm stills lens, 1 ft from the single layer plexiglass window.
For the BBC series "Flying Soldiers", an Army Lynx helicopter was used to shoot the largest formation of helicopters and fixed wing ever assembled in the UK.

Even from within the camera helicopter it was possible to hear the formation!
Whatever the requirement, it is essential to use the appropriate camera aircraft and plenty of time to plan, this is not an area of filming where you can make it up as you go along.
Another option is to use a Partenavia type aircraft. This light twin engined aircraft is unusual in that it has a high wing and thus an unobstructed downward view. It also has a very good range compared to a helicopter.
Shown below is the Aerospatial Corvette with which we worked extensively on the "Bird of Prey" video.