. Modifying Flight by Darwinian Evolution or Unintelligent Design? 
Slowrotor,
Some more un-thought-out thoughts about compressed air assist for an aircraft.
The engine will be bigger than normal so that it can provide good STOL, therefore during cruise some of the excess power could be used to 'top up' the tank.
The wall of the compressed air storage cylinder could also serve as the fuselage aft of the pilot's seat. This will maximize it's size and minimize the additional weight.
The pressure of the air flowing out the wings to the tips will not be exceptionally high, therefore the modifications to the wings should be minimal.
The cylinder will extend to near the tail of the plane, therefore the tail thruster would consist of only a valve, an expansion chamber and a discharge, which might be located under the horizontal stabilizer and in front of the elevator (& flaps?).
________________________________
Put the fuel in the same tank and then ignite the vapor at the discharge locations.
________________________________
Carbon filament wound cylinders are very very strong and very very light.
An example
If the cylinder can hold sufficient air to provide 20 - 30 seconds of assistance to the conventional thrust and control, I'll race you to the patent office.
Dave
Edited to add the following.
The Slepcev Storch ~ stall speed 22 mph
The e.Volution Car ~ 124 miles before being refueled with compressed air.