It's very nice of you to "volunteer" your services to "help out" in this worthy endeavor, however, what these folks really need is someone who "knows" about concrete, not airplanes. Lots of things to be taken into consideration here, and not all have to do with the weight of the airplanes;
First, where is the "frost line" in the area you are going to pour the concrete ? Depending on many things, ( overall weather, climate, latitude, etc. ) the ground "freezes" down to a certain point in cold weather / winter; any concrete, post, or "whatever" above that point moves around almost like a "cowboy" on a "bull" at a rodeo ! Any excavation for "footings" MUST extend BELOW the frost line; Any concrete poured as a "footing" to support a "load" MUST rest on "something" which does not move, and trust me, the ground / soil does move, and it moves a lot ! All of which is known to local contractors who build things in that area.
As for the "concrete" you are thinking of using; what you're talking about is known as "sakcrete", ( or any one of a dozen or more "trade names"; it's O.K for some small projects, but even then, you MUST have some one with concrete experience, or you will end up with "airplanes all over the place"!
A short, 5 minute, free conversation with any local concrete contractor will "convince you" that I am attempting to give you "good advice" ! What you really need instead of "sakcrete" is "ready mix" ( as mixed and hauled out to the job site by a local "readymix" firm. Concrete is essentially a "mixture" of portland cement ( a powder ), sand, water, and aggregate ( gravel ); the problem is, they must all be in exactly the proper proportions, all MUST be clean, ( no soil of dirt ), and they must all be mixed properly. Anything less and you may as well leave the airplanes sitting on the "dirt".
I hate to be sounding so negative, but believe me, "I've been there, done that" and you don't want to get your "education" about what happens to improperly mixed and placed concrete via the "trial and error" method !
Obviously, there are "short cuts" and "ways around" just about any problem, if only one has some proper experience, or can at least talk with someone who does have the experience. What I am about to say is just "food for thought", and may possibly get you thinking in the right direction;
Let's say each airplane has three "contact points" with the ground; one for each main gear / wheel. and one for the nose gear; so you need three "stable" concrete pads to bear the weight; first, you need an "excavation' ( read: round hole in the soil ) down to "solid ground", bed rock, or whatever; ( here's an "idea" for a "shortcut" I mentioned ); the hole MUST go below frost line; that's a MUST; ( you'll need to find that out locally ) then, you are almost surely not going to have bed rock at that point, so here's the "short cut"..............take three (minimum) ( the more the better ) lengths of 1/2 inch ( 3/4 inch is better ) rebar, maybe 6 ft long, and DRIVE each into the soil at an angle ( think of the poles that hold up a "teepee" ( Indian tent ) the bottom of each rebar ending outward into a LARGE circle , and the top ends all coming together at or slightly below ground level; basically, you don't need any "forms" much below ground level, but you do need the "excavation" ( or hole ) to be as "round" as possible; after the "excavation" is complete. the rebar is driven in, then you need "something" above ground to "form" ( contain ) the wet concrete; all kinds of possibilities here, but probably the best, simpilest is "form tubes"; any large "home store" ( Home Depot, Lowes, etc ) sells cylindrical cardboard tubes for doing just what we are "attempting" to talk about. They come in various lengths; ( you only need maybe 4 or 5 inches above "grade" and about the same below grade, so one 4 ft tube can easily be sawed into four equal lengths.
You can use anything, but these tubes make things a lot easier; this "idea" just "popped" into my head..........I have never tried this, but I may, and probably will quite soon: find some old 10" X 20" truck tires; these things are FREE, and any tire shop will gladly help you load them on a trailer, just to get rid of them; one tire above each "hole" and the ready mix isn't going anyplace! Tires are STRONG ! Very strong; you could even cut them away from the concrete after it has set up for a few weeks.
All of this is just to get across the "concept"; concrete is INCREDIBLY STRONG in "compression"; that is, a heavy load, bearing straight down on it; but it MUST be supported by "something" which also has some strength, and believe me, soil does not, hence the rebar radiating outwards at an angle; ( makes it many times harder for the load to push it deeper into the soil.
I have bought, owned and sold a lot of rental property over the years, and I've been obliged to learn about how to do all kinds of things which had nothing to do with my "job"; I needed to build a concrete porch once, about 12' X 18' ; Because I already owned a large cement mixer, I decided I would "mix my own"; I had a large pile of "pea gravel", some sand, and a pile of regular 3/4 " gravel, and I bought 40 or 50 100lb bags or cement at a supply house; it seemed like a great plan ! But it didn't work! You still have to accurately measure all the "ingredients", then you have to get them "into" the mixer, then after only a few minutes of mixing, you still need to MOVE the wet concrete 50 feet, get it into the forms, work it, etc etc.
This would all have been "do-able" for a 4 or 6 man crew or experienced concrete guys, but I only had ONE guy available.......ME ! I ended up hiring two EXPERIENCED concrete guys; I had "constructed" a 20 ft "chute" to get the ready mix from the truck to the forms, but the truck driver laughed when he saw the "chute" I had built, explaining that all the concrete would end up in the front yard; wet concrete is HEAVY ! So the guys I hired ended up wheel-barrowing the concrete in 30 feet from the mixer truck. ( The 40 bags of cement ended up "setting up" from moisture ), and I used up all the sand and gravel for other things, but the resulting porch was still there the last time I looked ! ( I sold the property a few years later and made a "buck", and got an "education" in the process. )
I still have that big cement mixer, but it's in Ohio and I'm in Indiana; I'm going to be going over there in the next few weeks to cut some trees down for my "ex", so I'm finally gonna bring my mixer back over here and use it for "something" or else sell it and use the proceeds for something else. Much of what I have "learned" has been from the "trial and the error" method, ( which is usually not the best way to get your "education" ! ( better to let someone else make the trials, and live with the errors ! ) Good luck !
Stupidity: Doing the same thing over and over again and expecting a different result; Albert Einstein