I was thinking on hanging some aircraft over my layout what scale of aircraft would work . My layout in ho scale with 1/64 scale vehicles . I also was going to have one parked on the ground . Thanks in advance
1/64 scale vehicles are somewhat large for HO scale which is 1/87.
Aircraft in the AIR over the layout might be in a scale slightly smaller than the layout scale on the principle of “forced perspective”. The airplanes are supposed to be far away high in the sky. They might be 1/96th (1/8th inch to the foot scale), 1/100 or even 1/120, and would look right in that context.
An airplane parked on the ground near an HO scale building would look best if the airplane was also HO scale. I believe there are a few of these around.
I think some aircraft models come in 1/76 scale which is larger than HO-- but closer than the 1/64 vehicles you say you are using.
This is what I would do- but whatever works for you, works for you. So happy railroading and happy airplaning. (The sky’s no limit!)
Back when I built model aircraft, 1/32, 1/48, and 1/72 were really common aircraft scales. 1/144 and 1/200 were somewhat common. 1/100 was rather rare. Model train companies have produced a few aircraft in 1/87.
I use matchbox vehicles because I have collection of them so I use the layout to display them .On the layout 1/87 fire Equipment , 18 wheelers and Construction 
Equipment.
I have a really expensive Herpa 1/87 Cessna, & also (not sure the manu.) Bell Huey In 1/87, I got both from Walthers. I also got a 1/96 Vickers Viscount Passenger plane Model Kit from Squadron.com. It is not a real nice model, but can be made nice if some effort is taken into it. I treat Aircraft like cars, & boats, I usually try to go ‘equal &/or slightly smaller’ in scale gauge to get the right look, many models have exaggerated features & the ‘equal to & less than’ scale has been working for me quite well. I know at one time Walthers has some WW2 aircraft in HO, but I have not seen them lately.
Is there a question in there somewhere?
It’s your layout. If it looks good to you, or it’s what you want, go for it.
If you want my opinion, now, that’s another matter. I think a 1/64 (approx 25% bigger than HO scale) would be too big, especially for something that, as a previous poster mentioned, is in the air at a distance.
A friend of mine has some 1/100 aircraft flying over his, and they don’t look horribly out of scale. OTOH, my son uses a 1/64 helicopter model with his 1/72 scale toy soldiers, and it is noticeably out of scale sitting next to them.
Your layout, your rules.
Next time you’re in a store, Matchbox does a line of aircraft as well. Liek their cars, the planes are scaled more to fit in the card than to any scale, but a lot of the helicopters and a few smaller planes are close enough that if you put a person in the right spot, the viewer couldn’t argue the point too hard. You might knock the shine on them down though.
The human eye tends to oversize things viewed in a closer distance. A to scale HO aircraft hung over your layout may appear way too big - better to scale it down to, say, 1/100.
1/64 scale is far too big for an HO scale layout. That goes for vehicles on the ground or “flying objects”.
Promotex in Canada is a distributor for Herpa; they have some 1/87 private planes, mostly Cessna
http://www.promotex.ca/provisioner/catalogue/index.php?ProductLine=26&CategoryID=30
They have lots of other planes, mostly passenger jets (but including a B-2 stealth bomber and an SR-71), in 1:160 (N scale), 1:200, 1:400, 1:500 and 1:1000 scales. Maybe some of these could be used as a plane passing up high.
I made a model of a p-47 thunderbolt that I thought looked good on my layout. Revell makes it and the scale is 1:72. It’s not a large model (about 5 inchs) but being of a larger scale I thought it looked better in the foreground.
My son has a model of an Apache attack helicopter which looked pretty good, scale wise, for HO. I checked the box, and it’s a 1/82 model, pretty close to HO. If you can find some of these, that would work.
Right, but the Matchbox stuff is scaled at “4 inches long by 4 inches wide max” scale, so the real kicker is what the model is, and where it’s going on the layout.
Also, Ateharn made plane models for flatcar loads, and I think they did them in BB kits, though I could be mistaken.
Walthers sells or sold an HO C47/DC3 airplane. Time period WWII to ?. Some may still be flying.
Did you consider going online and searching, 1/87 Scale Airplanes?
Ray
We use HO scale wherever we can and smaller in the air is acceptable, up to a point. And like other’s have said, it’s your layout. In my opinion S Scale(1/64) can look a little toyish in an HO scale model scene. There aren’t a lot of HO scale aircraft however.
Thanks for info
I also what to add a photo of part of 
my layout