Flying Coffins is a Great War theme Trail of Cthulhu scenario in which the players are knights of the air, battling the enemy and the Mythos in flying machines made of canvas, wood, and not much else. The pilots have to work out how to defeat the forces of the Mythos without getting shot down by more conventional forces. There's a review here. Naturally, since the scenario involves some wargaming elements, people have questions about the rules, and one that's come up has significant implications for gameplay. Namely, just how fast is that Byakhee?
It's not a trivial question. The speed of the creature factors into the air combat mechanics. If the players can get an advantage by flying a faster aircraft, they've got a better chance of beating the scenario. However it's not a question I feel comfortable answering, for a number of reasons.
The first is also the most obvious. I have no idea how fast they'd fly. I can guesstimate "pretty damn fast," but that doesn't really provide a miles per hour figure. It's not as though there are many references I can use for comparison. I think Lovecraft used them only once, and while there may be other fictional references out there, I'd be surprised if any of them went into the nitty-gritty of Byakhee flight mechanics.There may or may not be one or two real-world beasts of similar size and capability, but even if someone has statted those out, I'm not sure how they'd compare to a supernatural entity from beyond the stars.
The second is less obvious, and has to do with emperors and the lack of clothes thereof.
People tend to take rules very literally. After all, that's kinda what they're for. They provide structure, and without them the game can flounder in a mire of doubt, misinterpretations, and what-ifs. That doesn't mean you can't have a good time without rules, but it does mean that if you want to attempt anything like a real-world simulation, you need some rules that everyone's going to have to obey.
That said, if I provide a miles per hour figure, people are going to take that figure seriously. They're going to assume that this really is how fast Byakhee fly, because I said so, and I must be right because I wrote the scenario. It could mean that, in future scenarios, Byakhee always fly at X m.p.h. and therefore, although they compared favourably to Great War era aircraft, they don't do well in comparison to Spitfires, and have a lot of trouble keeping up with commercial airliners. That may or may not suit those scenarios, which will have plot demands of their own that may not have anything to do with air combat.
People have a lot of difficulty telling the emperor that he has no clothes; it's why that relevation seldom sees the light of day. Either they spend a lot of time wrangling over trivialities, trying to work out what was "really meant" by a particularly odd rule, or they insist that there has to have been some reason why a rule was written in such-and-such a way, and so it has to be rigidly adhered to. I used to play a lot of wargames, particularly Napoleonics. I remember those old rules arguments, but I don't remember them being all that much fun.
In Call of Cthulhu, the best example of this problem is the reading time given in the grimoire statistics. Each book, from the Necronomicon to the King InYellow, has a time, expressed in hours, it takes to read and completely understand. Once you understand the contents, you get the benefits, which could mean increased Mythos knowledge or a chance to learn spells. There's a tangible benefit to completing a book, but there really isn't much of an in-game timekeeping mechanic, nor is there any means of adjusting for character statistics. A dummy with EDU 12 and INT 6 will take just as long to read the Necronomicon as a genius from Harvard, and unless someone's keeping track of days spent in-game, it's easy to lose track of where your character was in his reading schedule.
Yet what few people seem to grok, is that the time listed is a completely arbitrary figure, dreamed up by some writer, who maybe spent all of thirty seconds thinking time working out that number. Perhaps they based it on some real-world measurement. They might have sat down, read War and Peace, timed how long it took, and said to themselves, "right! that's the time it takes to read the Unaussprechlichen Kulten!" Alternatively, they plucked a number from the air and said, "yeah, that's about right. What will I have for lunch?"
Which do you think is more likely? [Hint: my money's on option two.]
For me, if I had to say in-game how fast a Byakhee flies, I'd say it's at least as speedy as the best technology man can put up against it. So whatever warbird happens to find itself sharing airspace with a necrotic horror is no faster than the Byakhee, and might be slower. Since this is a horror game, as a general rule the opposition shouldn't be easily outclassed by technology-reliant mankind. It's a concept you see in movies all the time. If those flamethrowers in Alien had worked really well, it would have been a short movie, and the Alien wouldn't have seemed nearly as menacing. Ergo, the flamethrowers can't work well, or at least not well enough to provide the crew of the Nostromo with an obvious advantage.
That sounds good enough for the scenario, without getting too bogged down in the mechanics of the thing. Ultimately, in a roleplaying system, the rules exist to suppliment the narrative. They don't exist to replace the narrative; it's the difference between a wargame and a storytelling game.
So, how fast is that Byakhee? Just as fast as it needs to be!