RC transmitter and receiver for Raspberry Pi possible?
WiFi should be able to work well for your application.
ESP8266, ESP32  and their kin should be suitable. Operation with external antennas at 2.4 GHz can achieve ranges of many km - especially in ground to aerial operation. 



But, look at LORA ("Long Range") modules which should easily meet your needs.
LORA - Wikipedia. Ranges of "over 10km in rural areas" are cited (antennae unspecified) but there is every reason to expect that this range can be exceeded in ground to air operation with 'sensible' antennae. 

LORA utilises 'spread spectrum' techniques to achieve astounding-but-true real-world link budgets of typically 168 dB. eg -148 dBm RX levels at 100 mW tx power (!). 

ICs are available but modules offer vastly better cost-benefit.
There are numerous on web Raspberry Pi + LORA related pages. 

One example.  

A zillion examples

Digikey - LORA modules  - wow!!!  

Seeed Studio module from Digikey - $US7.91/1 in stock (!)

Datasheet for above - github

Raspberry Pi LORA-GPS module - $35 Digikey. Adds LORA to RasPi and can add GPS functionality with GPS input to RasPi serial port. When are you buying one ? :-).

Wiki for above

LORA alliance home page - more for reference. 



LORA SPECIFICATION

Lora Specification for above RasPi module:

168 dB maximum link budget. &lt;- !!!  (!!!!!)
+20 dBm - 100 mW constant RF output
+14 dBm high efficiency PA.
Programmable bit rate up to 300 kbps.
High sensitivity: down to -148 dBm.  &lt;- natural consequence of power and link budget
Bullet-proof front end: IIP3 = -12.5 dBm.
Excellent blocking immunity.
Low RX current of 10.3 mA, 200 nA register retention.
Fully integrated synthesizer with a resolution of 61 Hz.
FSK, GFSK, MSK, GMSK, LoRaTM and OOK modulation.
Built-in bit synchronizer for clock recovery.
Preamble detection.
127 dB Dynamic Range RSSI.
Automatic RF Sense and CAD with ultra-fast AFC.
Packet engine up to 256 bytes with CRC.
Built-in temperature sensor and low battery indicator.