What were some problems with the planes in World War 1?

In general, the wood and fabric airplanes were very delicate. The biplanes were strengthened by use of wire braces and if the airplane had a lot of wire braces, then this could cause drag. Many of the aircraft were very delicate resulting in failures during high g-load combat maneuvers. Generally, the engines were heavy and not very powerful. Some aircraft designs flew on the ragged edge of stalling and falling out of the air.

Specific examples

The Fokker Eindecker E-I was copied from a French mono-plane but with a welded metal frame structure covered with fabric that used the warp-wing control surfaces. The first ones to enter service at a flying school resulted in 3 crashes. The desginers went back and improved it with a 100-hp engine, larger landing struts and a triangle under-fin and a reduction in the wing area. The E-II and the later E-III proved to be a success in combat and a scourge of the air.

Structural failure of wings

The Fokker Dr 1 was a tri-wing fighter that proved to be a very successful airplane. However the first ones out of the factory had a flaw with the glue and the skin would peel away from the top wing.

Also, the Fokker Triplane was very short and had a small engine. The 3 wings gave it exceptional climb and fast turning. The British were surprised to see it perform a flat turn without banking in just a few seconds. Due to the drag of the 3 wings, its top speed was barely more than 100mph. And when it came in for a landing, as the aircraft flared to land, the airflow over the wings would kill the effect of the tail controls, so the pilot was committed to land.

The Albatros D-III & D-V and the Nieuports were biplanes but the bottom wing was much smaller. During dives and high G turns the bottom wing would fail. Usually this was fatal, but some pilots survived. The solution for the Albatros D-V was to add a small brace to the outboard strut to stabilize the bottom wing from flutter.

Poor Performance

The Short 184 seaplane was a torpedo bomber built early in the war and continued into service throughout the war. It was large and ungainly and under-powered. One pilot reported in 1915: "Unhappily, the torpedo-loaded Short seaplane could only be made to get off the water and fly under IDEAL conditions. A calm sea with a slight breeze was essential and the engine had to be running PERFECTLY. Further, the weight of the torpedo so restricted the amount of petrol which could be carried that a flight of much more than 3/4 hour was not possible. So it came about that while a number of torpedo attacks from the air were attempted, only 3 were successfully concluded."

The R.E. 8 was another bi-plane observer/bomber built by the Royal Aircraft Factory. After entering service on the front, the aircraft experienced several accidents due to it poor flying characteristics. The Wing command had to issue reports to the pilots to warn them about its problems. Their report stated: "The chief thing to remember is that the machine gives little indication of losing its speed until it suddenly shows an uncontrollable tendency to dive which cannot be corrected in time if you are near the grount."..."With the engine pulling the machine will not stall at 50 mph but it not advisable to get the speed as low as this."....and my favorite....."One more point as regards loosing speed. Observers must be cautioned that when an aeroplane is gliding down from work over the lines they must not stand up in order to look over the pilot's shoulder for the fun of the thing, as the EXTRA HEAD RESISTANCE caused may lead to the aeroplane falling below its critical gliding speed, and so bring about an accident." The pilots name it the "Harry Tate".

High Performance Aircraft

The Sopwith Camel was the most successful fighter aircraft to be produced in large quantity. By the end of 1917, 1325 Camels had been built. It was designed to be agile and quick. But due to these characteristics, the Camel was difficult to land and (I can't find referene to exact figures) as many aircraft were destroyed at the hands of the pilots as from enemy shooting them down.

American Aircraft

The US began producing the British DeHaviland D.H.4 under license and installed the American "Liberty" V-12 engine. The aircraft was pressed into production without inputs from designers or pilots. A 67-gallon fuel tank separated the observer/gunner from the pilot and thus preventing them from communicating. The undercarriage had the wheels too far aft which made it easy to tip over. Oh, it was not uncommon for the Liberty engine to catch fire in combat. Thus it received the name the "Flying Coffin". Actually, this aircraft was not much worse than most at the time and problems were overcome. The US produced 4,436 aircraft at a price of $5,500 each. This was the first US-built airplane to enter combat over the front lines with the US Marines.

References

Profile Publications: "The Fokker Monoplanes"... "Fokker Dr.1" .... "The Short 184"..."The R.E.8"..."The American D.H.4"