In my world, I want two primary things: Battleships to maintain their prominence much longer (Story takes place in roughly 1950s era tech), and as part of that, aircraft to initially develop much later.
You really need aircraft delayed (compared to our world). That alone saves the battleship.
Battleships didn't really stop being considered the mainstay of most fleets until WW2 was well underway and it became apparent that they didn't quite cut it.
A fifteen year delay in aircraft development is enough for this.
I'm mostly not worried about how fast aircraft would develop once they arrive, for the sake of this particular question. I want to focus on what factors would slow down the development of aircraft.
No WW1 would do it.
WW1 saw a major drive to develop the aircraft as an armed fighting arm. Until then it was considered little more than a way to do reconnaissance.
So WW1 could simply be avoided (or kept from developing into something huge).
No German state forming would have done that.
Or even something as simple as the failure of the assassination of Arch Duke Ferdinand.
If we put the story in 1950, I'd want aircraft that are maybe 15 years along the development cycle - So, late WWI. If aircraft develop at the same pace, this would put first flight at around 1935, a little over 30 years later than what happened in history.
I've identified a handful of factors that could slow and inconvenience aircraft development, but I'm not certain it is enough:
Higher surface gravity, about 30% higher. Combined with Lower atmospheric density, by about 15% lower.
An unlikely combination. More likely is that increasing gravity will increase atmospheric density.
These two mean larger wings and/or more powerful engines are required to get a plane of the same weight off the ground.
Yes, but it also means you have to deal with a world with a higher gravity and a denser atmosphere (not less dense, but more dense). You will have to develop engines that are more powerful anyway for all your surface transports and this means you can't avoid developing the engines.
Delay in easy Aluminium production. If I don't want other things to be affected, this would have to be minimal, and the effects on aircraft would similarly be minimal.
And we need Aluminum for what ?
Planes can be build from wood and canvas and they'll happily sink a battleship with a torpedo or a bomb (well, several bombs).
Lack of need. The world is largely on a single continent.
Single continent's don't need battleships.
If anything a single continent makes having long distance air transport even more desirable both economically and militarily. You want to be able to reach everything on the other side of that continent as fast and easily as possible.
Rail travel is common and easy. There ARE other continents, but they're significantly less populated, and ships are also relatively common. I'm considering shorter distances between continents and islands as well.
One huge continent makes seem unlikely. Rail travel would have to cross varying terrain and be developed in varying climatic conditions. Consider the difficulty of building the trans-American and trans-Russian railways.
Far from discouraging the development of air transport, I think it would encourage it, simply because business and government would want an alternative.
Military Disdain. A significant driver of early aircraft was their use by the military, which was resisted by many "Old-school" leaders. If these old-school leaders "Won" their arguments, the military money may not exist.
That happened here too, and didn't delay anything very much. One war proved they were useful and planes were instantly adopted by everyone.
A factor to their "Winning" could be:
Catastrophic accident. Something like an early test/demonstration happening with a large crowd, and ending up crashing into said crowd and causing significant injury and death. There have been more than one promising project canned because of a PR disaster.
The history of air flight, commercial and military, is littered with disasters. All that happened is that people pushed harder. In peacetime the first jet airliners had a lousy safety record due to a tragic fault. Result : hardly slowed the development of commercial jet aircraft at all. Crashes of commercial piston aircraft made very little impact in slowing the development of those.
Are these factors enough to delay aircraft development by three decades, or should I include some other factors?
Just prevent WW1. Job done.