Depending on the plane you use you will have some kind of Frame rate hit.
To minimize this edit the aircraft.cfg
Lets use Premier Aircraft Designs DHC-5 as an example
Lets add another entry to the list by coping the [fltsim.0] section
Change the Title name to add AI in the front
Point the panel to a folder that does not exists or has only a bmp of solid black in it. This way when your sim generates the aircraft it is not loading all of the gauges in the panel
Do the same with the Sound folder
Now because we did not make any changes to the air file this plane will show up in our list of aircraft to fly. Add AI_ to the Variation so you will know this is your AI version of the plane.
Textures: copy your texture folder and rename it AI_XXX. XXX is not porn it defines the variable that is the original name of the folder. Use
DXTBmp by Martin Wright to change all of the Textures to DX3. There are some other utilities that will change all of the files in a folder but I don't use them. Someone else can point you to them.
This is still the full model with in some cases a lot of polys to draw and will still cause a frame rate hit. Their really is no way around this one unless the developer created an AI version. Like the AI version of the A-26.
I used a variation of this method with the SectionF8 Sabre when it came out. In this instance I copied the whole aircraft folder and edited the aircraft.cfg to change the title then just deleted the panel and sound folders. I then put all twelve aircraft at one airport (after I added Parking). I set them all to take off at the same time. (Sweet) The frame rate hit I knew was going to be Big but I wanted to see how my system would handle it. Frame rates with all of the aircraft on the ground were 7. Once they all started moving to take off it crashed the sim (CTD). I knew it would be bad but still I laughed when it crashed. I cut the planes down to three.
Good luck
[fltsim.
X]
title=
AI_Buffalo_Canadian Forces S & R
sim=PAD_Dhc5
model=
panel=
AI_Panel
sound=
AI_Sound
texture=
AI_CAF
kb_checklists=DHC-5_check
kb_reference=DHC-5_ref
atc_id=115452
atc_airline=Air Force
atc_flight_number=SR452
atc_parking_types=gate,ramp
atc_parking_codes=ramp
ui_manufacturer=de Havilland
ui_type=DHC-5 BUFFALO
ui_variation=
AI_Canada Search & Rescue
ui_typerole="Twin Engine TurboProp"
ui_createdby="Premier Aircraft Design"
description=
Modified for AI de Havilland Canada DHC-5\nTwin Turboprop STOL transport\nVisit the Premier Aircraft Design Web Site\nwww.premaircraft.com\nAll Files Copyright 2008
atc_heavy=0
editable=1