Ribeyrolle Automatic Carbine- The Ribeyrolles 1918 chambered the experimental 8x35 mm round, used straight blow-back, was fed from a 25 round magazine and had an effective range of 400 m. This was a very early attempt at creating a submachine gun. Which is a weapon designed to fire pistol rounds at a very high rate.
However since the weapon's design came very late in world war one, it did not see much service.
Coaxial Helicopter- This configuration is a feature of helicopters produced by the Russian Kamov. It allowed for the helicopter to have an inreased payload for the same engine power. This also made the helicopter much quieter making it much more suitable for recon missions. Coaxial Helicopters like the Ka-25 'Hormone' were used quite heavily in the naval front. Where space was readily avaliable. The Coaxial design made it effective for the Russians to carry them on their ships acting as early carriers. Due to the fact that the tail rotor no longer existed it would be able to take off with little danger to the crew of the ship.
Radio Crystal Oscillator
Quartz crystal oscillators were developed for high-stability frequency references during the 1920s and 1930s. By 1926 quartz crystals were used to control the frequency of radio broadcasting stations and were popular with amateur radio operators. This invention replaced the escapement and pendulum (as the timing reference), relying instead on the natural vibrations occurring in the quartz crystal as the oscillator. The introduction of the radio oscillator made it possible extend the range of the communications between allied countries.
Cyclist Infantry and Motorcycles
As the war was coming to and end, both sides began getting more and more desperate. A need for a fast moving vehicle that could deal damage yet was cost efficient was becoming a necessity. Both sides could not handle mass tank production, and so they turned their eyes to Motorcycles. They would often mount machine guns onto the motorcycles, a downfall of this was that back in 1918 many countries did not have many roads. So to be able to mobilize an army of motorcycles was very difficult. This is why it was not used until the late years of the war. Simply because it could not be very helpful unless they had the roads for the Motorcycles to travel on.