The Devon Air Ambulance Trust says it needs to raise an extra £300,000 a year to pay for a new helicopter.
The helicopter has carried out four to five missions a day this summer
It says it will improve the service to people needing to be airlifted to hospital
The trust says the new helicopter will allow it to reach the scenes of accidents more quickly and carry more passengers.
The new helicopter, which will cost £2.5m, is currently being built in France and Germany and should be ready by next August.
The trust has had a busy summer, with the air ambulance carrying out four to five missions a day.
It is funded entirely by local people, but the trust says it has got some reserves, which it will use if it does not get the extra funding it needs from the public.
The trust is asking people to take part in a coffee day on Wednesday, drinking a cup of coffee at about 1100 BST and donating £1 to the charity.