How to sending data string with nrf24l01

#include <SPI.h> // Call SPI library so you can communicate with the nRF24L01+
#include <nRF24L01.h> // nRF2401 libarary found at https://github.com/tmrh20/RF24/
#include <RF24.h> // nRF2401 libarary found at https://github.com/tmrh20/RF24/
#include <Adafruit_MPU6050.h>
#include <Adafruit_Sensor.h>
#include <Wire.h>
const int pinCE = 9; // Pin is used to set nRF24 to standby (0) or active mode (1)
const int pinCSN = 10; // Pin is used to tell nRF24 whether the SPI communication is a command or message to send out

RF24 radio(pinCE, pinCSN); // Create your nRF24 object or wireless SPI connection

#define NODE 1

const uint64_t wAddress = {0x7878787878LL, 0xB3B4B5B6F1LL, 0xB3B4B5B6CDLL};
const uint64_t PTXpipe = wAddress[ NODE ]; // Pulls the address from the above array for this node's pipe
Adafruit_MPU6050 mpu;
Adafruit_Sensor *mpu_temp, *mpu_accel, *mpu_gyro;

String start= "aaaa";

String trame;

float data1, data2,data3, data4,data5, data6;
//--------------------------------- Partie de configuration -----------------------------------
void setup()
{
Serial.begin(115200); // start serial to communicate process
radio.begin();
radio.setChannel(108);
// radio.setPayloadSize(2);
radio.setPALevel(RF24_PA_LOW);
radio.setDataRate(RF24_1MBPS);
radio.setAutoAck(true);
radio.setRetries(2,15);
radio.setCRCLength(RF24_CRC_16);
radio.openReadingPipe(0,PTXpipe);
radio.stopListening();

while (!Serial)
delay(10);

Serial.println("Adafruit MPU6050 test!");

if (!mpu.begin()) {
Serial.println("Failed to find MPU6050 chip");
while (1) {
delay(10);
}
}

Serial.println("MPU6050 Found!");

// set accelerometer range to +-8G
mpu.setAccelerometerRange(MPU6050_RANGE_8_G);

// set gyro range to +- 500 deg/s
mpu.setGyroRange(MPU6050_RANGE_500_DEG);

// set filter bandwidth to 21 Hz
mpu.setFilterBandwidth(MPU6050_BAND_21_HZ);

mpu_temp = mpu.getTemperatureSensor();
mpu_temp->printSensorDetails();

mpu_accel = mpu.getAccelerometerSensor();
mpu_accel->printSensorDetails();

mpu_gyro = mpu.getGyroSensor();
mpu_gyro->printSensorDetails();

}
//----------------------------------- Programme Principal ---------------------------------------
void loop() {
// /* Get a new normalized sensor event */
sensors_event_t accel;
sensors_event_t gyro;
sensors_event_t temp;
mpu_temp->getEvent(&temp);
mpu_accel->getEvent(&accel);
mpu_gyro->getEvent(&gyro);
data1=accel.acceleration.x;
data2=accel.acceleration.y;
data3=accel.acceleration.z;
data4=gyro.gyro.x;
data5=gyro.gyro.y;
data6=gyro.gyro.z;

trame=start+" "+data1+" "+data2+" "+data3+" "+data4+" "+data5+" "+data6;
radio.openWritingPipe(PTXpipe); // open writing or transmit pipe
if (radio.write( &trame, sizeof(trame ))) {
Serial.println("done");
}
else { Serial.println("false"); }

delay(500);
}

This will only send the descriptor of the string, not its content. You could try

if (radio.write( trame.c_str(), trame.length())) {

I would not send text, but a struct of binary data.