We performed extensive experiments to study the impact of varying the payload length on the Packet Reception Rate of LoRaWAN packets with various spreading factors and transmission power levels. The experimental setup consists of motes connected to The Things Network back-end. Our results show that the payload length only slightly impacts the Packet Reception Rate, for the majority of the gateways. As a result, we conclude that besides the impact of the signal to noise ratio on the bit error rate, it seems that reception is conditioned on the initial packet detection. Our aim is to determine and experimentally model this probability with respect to the signal to noise ratio.