In a steampunk world with airships and airplanes, but which lacks radio, how would different aircraft communicate, and how would they communicate with the ground? Would a form of semaphore, using flags controlled from within the aircraft be effective? What other methods would be possible? As a subpoint of that, what combination of message speed, and message complexity would be optimal? Would multiple systems, one for in-combat, quick messages and one for long battle plans be plausible? How could one effectively keep enemies from spying on messages?
You have a good few options, I think the realistic answer is a bit of everything.
I'm sure that a lack of communication between flying planes could be a great plot device.
Maybe if two planes fly in a really tight formation, they can use cans & string.
Use a Morse-type system where one can send messages by covering/uncovering a lamp or flashing sunlight In the other pilot's face with a mirror in some pattern. The lamp could be built into the plane with a lever in the cockpit to open/close the shutter, although this would be rather heavy. Probably a good idea to have a spring to make the shutter "closed by default".
Have semaphore flags affixed to the outside of the plane, controllable with levers from the cockpit. Very heavy. Probably more practical on airships than on planes. May require the receiving pilot to use binoculars.
Of course, none of these work outside line-of-sight, so:
(d) Smoke signals
e.g., the ones used by the war party chasing the war rig in Mad Max: Fury Road
Small, long-range aircraft. Give the pilot a piece of paper, hope he isn't shot down en-route.
Airships: Semaphore system for communicating Intel with each other, the ground, and to pilots in good visibility; heliograph in poor visibility.
Pilots: heliograph (mirror or lamp) to talk back to the airships and to each other;
Couriers: for no-line-of-sight comms; they dock with a blimp and hand over the piece of paper or use one of the other systems to commune directly to pilots.
But it's your novel; you can do whatever you want.
If you write this book please tell me
Here are three more options:
(d) Pigeons for one-way communication back to a ground base.
(e) Railway-style mail-hooks. Railways solved the problem 150 years ago of how to grab and drop written records without stopping the train. It's a close shave while flying, but barnstormers in biplanes can do it too. Not something to try with a Zeppelin.
(f) Fulton/Skyhook lifting system for grabbing dispatches from the ground or lower-altitude craft. This is the coolest. Airplanes can do it easiest. Airships have a harder time simply because their fly-around time due to a misjudged wind is so much longer.
Wire catching baskets on the airships will be popular, as will ground targets for dropped mailbags....and floating mailbags at sea.
Most airships communicated with their ground crews using good old megaphones.
It is important to remember that in the age of sail and into the early age of steam warships, captains of warships had a certain level of independence. They didn't need to be told every little maneuver or who to engage. There would also be a series of set battle plans (sweep left, engage at will, etc) that could be activated by a simple set of semaphore flags, a specific color sequence of flares, etc. Redundancy is key, signals in battle must be clear, concise, and unlikely to be mistaken for something else. The battle maneuvers are painstakingly rehearsed beforehand and designed to account for the chaos of battle, loss of the admirals flagship, broken communications, etc.
The concern with airship is how frequently they make landfall since captains can't just row across to another ship like they might in a naval fleet. If airships are perpetually aloft, then they will need to exchange more complex messages in a more secure manner, including the transfer of personnel and sensitive equipment. If they land every night or dock at a tower or something then face to face meetings can replace most communications.
So for airships, there are numerous ways to ensure commo.
- Semaphore flags run out on a long line along the airship.
- Large colored panels of fabric stretched out across the inflated bladder. These could even be raised and lowered (or function like those changing billboards) to display real time information.
- Signal flares, colored smoke, rockets, even colored powder bags dispersed into the wind, or dropped into the water or land below.
- Flashing lights using a form of morse code.
- Trained birds, homing in on specific markers on ships, could pass messages.
- Balloons released with messages scooped up by planes equipped with wingtip hooks, then dropped into a large basket/funnel on another ship.
- Loud air horns blasting audible code sequences or direct speech from one ship to another.
- Rappelling from a higher ship to a lower one, possibly even using a glider to carry people or messages. The glider could land in a hanging net below another airship or into suspended nets on top.
- Potentially even firing aerodynamic message containers from ship to ship, aiming for a catch funnel or net.
- Pilots/passengers in planes could even use hand signals to each other to coordinate actions.
Ultrahigh balloons or blimps could serve as line-of-sight extenders, relaying flashing codes or colors to the entire fleet. Obviously protecting codebooks would be paramount. Long messages with sensitive information would have to be encrypted in case they are intercepted or miss their target.
Morse or other code using mirrors or lamps is going to be the best for complex communication, but for simple messages there are other options.
Dipping the wings or wagging the tail can be used for simple signals to other nearby aircraft. This would be important when trying to get a small group of planes to swing left or right or to dive.
Setting off a flare or smoke canister works for longer range messages or communicating with more recipients.
For Zeppelins, it would most likely be Semaphore OR Morse code lamps, but these are line of sight solutions. These were used by Navies up to even WWII and most real world military Zeppelins were developed by Naval components and given names fitting Ocean Vessels.
Most planes used some radio system as the Wireless Telegraph was invented before a military use of air planes. Most planes would have a co-pilot or crew member who's job was to relay messages from the flight deck with the pilot as the officer in charge of the whole plane (for a good demonstration of this, watch the film "Midway" which features the famous battle. Given that it was the first battle to rely on pure carrier tactics rather than battleship theory, it demonstrates the plane to carrier communications quite well. There are some segments that use actual recorded voices from real pilots in real combat in the real battle). Without radio, your airplane communications would be limited to line of sight and given the 10-15 minute limited operation of the planes, it would be better to issue orders to the flight crews and then let them execute them. There are no messaging systems that can overcome the speed and line of sight factors in the short time they have to remain operational.
It's your story, but in this case, one of the earliest uses of military aviation is lost without the radio (recon) so you might want to redesign your fleet to be more Battleship theory than Carrier theory.
If we are assuming Zeppelins, airships or blimps, then we can assume analogous signalling as used by surface ships. Signal flags and lights would be very easy to adapt, and have the great advantage that airships can be used as adjuncts to naval task forces since they would easily be able to communicate with each other.
For fixed wing aircraft, this was actually an issue during WWI. Aircraft were generally too small to carry the bulky radio sets of the era, and of course vacuum tubes are not very keen on aerobatic manoeuvres, especially if things start crashing into them. Early aircraft also lacked compact reliable alternators or generators needed to power radios, so much more space and lifting power would have to be devoted to batteries if you were to carry a radio.
Pilots instead operated by following plans developed and briefed on the ground, and inside a flight formation, wingmen generally watched their flight leaders and copied whatever manoeuvres they were doing, right up until a dogfight broke out.
Pilots could communicate to each other through a system of rocking or "waggling" the wings to attract attention, then using hand signals to indicate things like the location of the enemy, which direction to go and so on.
Ground troops could signal aircraft using coloured panels and markers to indicate their locations, preventing friendly fire attacks. Pilots could also be briefed on the location of enemy ground targets relative to known friendly locations, so looking for particular panel markers on the ground would indicate the point where they take a bearing and distance as they run in to the attack.
And finally, although this seems like a cheat, airfields were often identified by painting the name of the airfield in large letters on the roofs of hangers. Given air navigation often consisted of following railway lines or canals , having the names of airfields painted on the roofs made locating the start and finish points much easier and more reliable.