I'm using 1/72 aircraft with 28mm figures.
I know they're hopelessly underscale, and a better option would be 1/48 scale (which would be more consistent with my land vehicles which range from 1:55 to 1:43). But I've reached this decision after a lot of thought and for the following reasons.
1) When "flying" over the battlefield, the discrepancy in scale is less noticeable.
In fact it works well, because mounted on a flying stand 12-18" above the tabletop it gives a sense of false perspective. That plane's not small, it's just really far away! Real-world movie makers have been doing this for years, passing off footage of 1/4 scale models as the real thing without anyone batting an eyelid.
2) On the ground, is it a playing piece or a piece of terrain?
Most wargamers understand that the ground scale used by most rules is usually a lot smaller than the figure scale. So a vehicle model that's in scale with the figures, might be the size of a large house when compared to the ground scale. As a result, a lot of wargame terrain pieces are smaller than the true figure scale would suggest. Aircraft on the ground are more likely to be acting as terrain pieces than actually being in play the way figures and vehicles are, so it's not inappropriate for them to be scaled closer to the ground scale than the figure scale.
3) Aircraft are still pretty big
Even something tiny like the Kaydet I used in last year's Hillbilly game is the size of a small 1/43 van. If you're going to have anything larger on the table, it's going to start taking up an awful lot of ground space. A 1/72 Dakota, a must for any brushfire wargame, is going to have a 40cm wingspan. It's still pretty manageable at 60cm for 1/48, but anything larger than that is going to start becoming too big to be practical.
But the key thing is that pretty much any 1/72 aircraft on the table at first glance is going to look BIG compared to 28mm figures. It's only when you start looking closely at details like doorways and windows that you realise that the aircraft is underscale for the miniature.
4) 1/72 aircraft are cheaper and on the whole more widely available.
Because it's always been one of the most popular aircraft modelling scales, the range of 1/72 kits is much wider than their 1/48 cousins. And while you can find some quite expensive 1/72 kits if you're looking at rare or unusual aircraft subjects, on the whole they tend to be cheaper. You can get a 1/72 BAe Hawk for as little as £7, whereas in 1/48 you're looking at £18-20
Add it all up, and it makes a compelling argument for using under-scale 1/72 aircraft in 28mm games.
In case you hadn't figured out, I'm not only looking at this from the perspective of using models for on-table air support in large wargames, but also in the context of a future airport/airfield terrain layout. This latter was prompted by the discovery of this toy on Amazon.co.uk
Scale model purists may despair, but if you're looking for a practical wargaming terrain piece it's pretty hard to argue with. You don't want a perfectly accurate but fragile scale model that's going to break every time you take it to the gaming club, you want something that was designed to stand up to the uses and abuses of your typical 5 year old! I have two. (the toy planes, not 5 year olds)
The other toy plane I'm looking at is to recreate that classic hangar scene that crops up in pretty much every action and adventure TV show. You know the one where the rich bad guy is about to leave the country in his private jet, and they're boarding it in THAT EXACT SAME HANGAR IN EVERY SHOW when the good guys turn up to stop them? This is one case where 1/72 lets us down a little. For starters... try finding models of that sort of business jet in any scale. 1/72 supports historical and military aviation subjects pretty well, but there's precious little in the way of modern civilian aircraft in that scale. Secondly, if you'r reproducing this scene in a skirmish wargame, this is one instance where the figures will be closely interacting with the aircraft and the scale discrepancy will really stand out. It's one instance where I think a larger scale aircraft is more appropriate.
Pixar and Disney come to our rescue with their "Cars" movie tie-in toy range. I've heard several people recommending the Cars die-cast toys as being suitable for conversion to 28mm gaming. I've never seen the films, but the toy range includes a couple of aircraft, including "Siddeley the Spy Jet" which basically looks like it's modelled on a Gulfstream bizjet.
Finally, though I can't justify the purchase myself, I've just noticed that the other big plane toy in the Cars range, "Cabbie McHale the Transporter" is on Amazon.co.uk for just £20. He's a pretty good representation of a C119 "Flying Boxcar", and I've seen someone on Lead Adventure do a pretty good job of repainting him for use with 28mm figures.