I have done instrument design for various aircraft. These include the ER2 and the especially prepared DC-7 which was used by NASA Glenn Research Center for zero gravity simulations.  

Because of the special applications and conditions of use for these aircraft, crew safety, structural integrity, "fail safe" structures, and well considered handling of pressure vessels, power supplies, cryogens (both experiments had liquid gasses aboard) were of unusual concern.

The ALIAS instrument, still flying, is a tunable diode laser spectrometry that flies in the under-wing pod of an de-militarized U2 aircraft.  Low cost, ease of installation and aircraft safety were of central importance in this installation.  For details of the ALIAS instrument try: http://laserweb.jpl.nasa.gov/alias.htm

The superfluid helium slosh experiment investigated the damping properties of superfluid helium.  Superfluid helium is a "quantum fluid" having "no" viscosity.  Since superfluid helium is important as a cryogen in various planned spacecraft including SIRTF and Gravity Probe B, there was serious concern about the properties of this material in motion as a potential disturbing factor on the pointing accuracy or microgravity stability of the spacecrafts on which it is intended to be deployed.  The aircraft platform was a relatively low cost vehicle for investigating the zero-gravity properties of this interesting material..

The DC-7 aircraft, consumes a full load of fuel in about an hour, flying repeated parabolic trajectories.
The Glenn Center currently uses a specially prepared Boeing 707 (KC-135) for this purpose.

ER 2 (U2)
ER2 (de-militarized U2) High Altitude Aircraft

DC 7 
DC7 converted for low gravity simulation 

for  descriptions of  the instruments click below:

-Airborne Laser Spectrometer
-Superfluid He Slosh Experiment