When learning to fly a radio-controlled aircraft, you will likely crash quite a bit, which can damage your model. They're generally easy to repair, but what if you didn't need to?
I suggest adding a simple airbag to be activated immediately before crashing. It would be just a simple lightweight plastic bag packed into a compartment at a strategic location on the airframe, connected to, say, a 12-gram CO2 cartridge, with a solenoid valve in between. Many model aircraft are easily capable of carrying such a mass. When a crash is imminent, it inflates, cushioning both the aircraft and whatever it collides with.
On an airplane with a pusher propeller, the setup is easy: just put it in the nose. With a tractor configuration, it would be more difficult and would have to be up to the designer/builder of the plane. Maybe the motor could retract, or the airbag could be reinforced against propeller damage, or it could expand around the propeller. Helicopters and multicopters are more difficult still—perhaps detach the rotor(s) immediately prior to airbag deployment? And the airbag would need to cover all sides of a helicopter/multicopter, because they're far more likely than airplanes to crash in random orientations and directions.
Deployment could be manual via a button or switch on the transmitter, and/or triggered by proximity sensors on the model, to detect uncontrolled (i.e. not a deliberate landing) approach to obstacles or terrain.
Could also act as a flotation device in case of a crash into water, if your aircraft isn't buoyant enough on its own, triggered by water contact.-- notexactly, Feb 10 2016 External airbags for car, against rear-end collisions Bumper AirbagsPrior art [notexactly, Feb 10 2016] External airbags for fighting vehicle, against HEAT warheads airbag active armorPrior art [notexactly, Feb 10 2016] External airbags for car, against wild animals Deer puncherPrior art [notexactly, Feb 10 2016] External airbag for human body, that of an extreme sporter Personal airbag vestPrior art [notexactly, Feb 10 2016] External airbag for human body, that of an elderly person Air Bag SuitPrior art [notexactly, Feb 10 2016] External airbags for car, against side collisions Exterior airbagsPrior art [notexactly, Feb 10 2016] External airbag for human body, that of [Alterother] Motorcycle airbagPrior art [notexactly, Feb 10 2016] External airbag for human head, activated by technological precognition Peril Sensitive AirbagPrior art [notexactly, Feb 10 2016] External airbags for human body, drunk AirBagz(tm) for ClothingPrior art [notexactly, Feb 10 2016] External airbag for bus wing mirrors Soft wing mirrorsPrior art [notexactly, Feb 10 2016] External airbag for human body, waist-mounted Airbag Fanny PackPrior art [notexactly, Feb 10 2016] External airbag for collapsed bridge, providing both cushioning and flotation Bridge AirbagsPrior art [notexactly, Feb 10 2016] External airbag for mobile phone, Amazon version https://www.google....tents/US20110194230Prior art [notexactly, Feb 10 2016] External airbag for mobile phone, Honda version https://www.youtube...watch?v=t_l_8r7NgJ4Prior art [notexactly, Feb 10 2016] External airbags for car, providing flotation Car flotation airbag systemPrior art [notexactly, Feb 10 2016] External airbag for laptop screen, against fists Computer AirbagsPrior art [notexactly, Feb 10 2016] External airbags for human body, riding a motorcycle Bikers GlovePrior art [notexactly, Feb 10 2016] External airbag for human crotch, against dog paws Dogproof trousers with airbagPrior art [notexactly, Feb 10 2016] External airbag for staircase, to protect fallers StairbagsPrior art [notexactly, Feb 10 2016] External airbags for Mars lander, providing landing cushioning https://en.wikipedi...descent_and_landingPrior art [notexactly, Feb 10 2016] External airbags for Mars rovers, providing landing cushioning https://en.wikipedi...ation_Rover#AirbagsPrior art [notexactly, Feb 10 2016] External airbag for child car seat Exploding Infant SeatPrior art [notexactly, Feb 10 2016] External airbags for human body, that of a car passenger https://web.archive.../idea/Body_20airbagPrior art [notexactly, Feb 10 2016] External airbags for car, providing flotation Submerged ProtectionPrior art [notexactly, Feb 10 2016] External airbag for car, to protect wild animals Bunny BagPrior art [notexactly, Feb 10 2016] External airbag for mobile phone of automobile driver, to accomplish something I can't understand Car phone airbagPrior art [notexactly, Feb 10 2016] External airbag (and other devices) for avalanche victim safer thrill sportsPrior art [notexactly, Feb 10 2016] External airbags for boat, providing flotation SailboatAirbagsPrior art [notexactly, Feb 10 2016] External airbag for human body, driving a car without airbags Whoopee-SuitPrior art [notexactly, Feb 10 2016] External airbag for computer monitor, against coffee Coffee fountain airbagPrior art [notexactly, Feb 10 2016] External airbags for broken-up airplane pieces (see [yikes!!!]'s anno) Commercial airliner parachute systemPrior art (closest yet!) [notexactly, Feb 10 2016] External airbag for small car, to launch it into the air to avoid crashing (see [doctorremulac3]'s second anno) Crash Survivable Design For Micro CarsPrior art [notexactly, Feb 10 2016] External airbag-equipped driverless cars, to cushion other vehicles' collisions (see [Vernon]'s anno) Driverless Car Buffer VehiclesPrior art [notexactly, Feb 10 2016] External airbag-equipped remotely driven cars, to cushion main vehicle's collisions EV BumperSafetyCarPrior art [notexactly, Feb 10 2016] External airbags for intact (but not model) airplane External AirbagsPrior art (even closer!) [notexactly, Feb 10 2016] External airbag for ballistic bird's-eye-view camera, to cushion landing, triggered by acoustic rangefinder Eyes in the sky for the regular guyPrior art [notexactly, Feb 10 2016] External airbags for mobile phone, to provide cushioning and act as a footbag Hacky Sack Cell PhonePrior art [notexactly, Feb 10 2016] External airbag for laptop computer Laptop AirbagPrior art [notexactly, Feb 10 2016] External airbags for human body, falling from aircraft Layman's ParachutePrior art [notexactly, Feb 10 2016] External airbag for human body, that of an elderly person https://wayback.arc...idea/PersonalAirbagPrior art [notexactly, Feb 10 2016] External airbag for mobile phone and human body distracted from surroundings thereby Safety CellphonePrior art [notexactly, Feb 10 2016] External airbags for human head, part of the hat that caused the collision in the first place Side_20Scanning_20(...d_20Detection_20HatPrior art [notexactly, Feb 10 2016] External airbags for car Standardized bumper carsPrior art [notexactly, Feb 10 2016] External airbag for car, to save pedestrians Switl external airbagPosterior art (my own) [notexactly, Mar 20 2020] random, halfbakery