I wanted to try and make this a little more understandable so I had a look at the Hellcat 3-view and I did some very rough calculations to use as an example, i think if anyone is interested in what difference different airfoils do in relation to each other that deserves more than just an opinion.

I measured the tip and inner section at the fold break of the wing as suggested so in my example I have just taken in to account the outer part of the wing, obviously if you want to foam-cut the complete wing half in one go then the airfoil or washout progression will be all the way from root to tip and the numbers will be a little different - anyway this is just a rough example to provide some understanding or background for my previous stated opinion.

First of all what happens to the stall point (what Angle Of Attack the airfoil will stall) when the wing chord with a given airfoil gets smaller towards the tip is that the stall capacity decreases, with the 2415 airfoil on the Hellcat, the tip will stall about ~2* before the inner fold/break section does. That is a problem because it means that when the wing is at it's limits of how much AOA it can take the tip will stall first, and it will likely occur at the least wanted time of flight - close to the ground during a takeoff or at low altitude in the final last 90* turn towards the landing strip.
(((the sound of 6 months dedicated work reduced to fit in a relatively small plastic bag)))

So the solution to this as we all know you find in the Hellcat design itself, what did they do to make this airplane safe and aerodynamic effective; most importantly they lowered the AOA of the tip chord so that it matches the stall characteristics of the root perfectly. This does not just benefit the safety from tip stall on the airplane but it also increases it's aerodynamic efficiency at slow speed or in a hard turn making it fly cleaner and better.

If we wanted to make this wing without washout would it be possible? The answer is yes it would, but would it be better? I think at this point you have to ask yourself exactly why you would want to make it without the washout, what is the motive for doing this differently than the professors and scientists did when they designed this airplane?

Is it to get better performance? Is it to make it less complicated to build, or is it to make it more aerobatic capable like inverted flight? Note that this is originally designed as a fighter plane that should be capable of high speeds and all sorts of extreme maneuvers - not a Sunday flyer, however I doubt you would ever see it flying inverted for long periods of time or making rolls at a perfectly straight line.

If you want to use a symmetric airfoil at the tip like NACA 00xx we can do that, if you have 2415 at the root and tip - the tip chord will need about 2* washout to match the characteristics of the inner section. If you have 0015 (symmetric with the same thickness) at the tip you need about 4* washout, with NACA 0016 to 0018 about 2* washout (the same as with 2415 but it starts to get much thicker than the root), if you have NACA 0020 at the tip then you don't need the washout anymore. Now you have to ask yourself; will it be better to use a 20% thick airfoil at the tip chord than a 15% thick airfoil at the tip chord? The answer from an aerodynamic point of view is no, "but" - it is possible.

If we extend that example to use the same 24 series airfoil at the tip instead of the symmetric with 2415 at the inner section you need about 1 to 1,5* washout with 2416 at the tip and with 2417 you might get away with no washout at all. But from an aerodynamic point of view you want the tip airfoil to be as thin as possible compared to the root so we are heading in the wrong direction considering that, the reason for this is that at same time as the stall capacity decreases when the wing chord gets smaller - the drag increases rapidly.

So the contradictory elements that makes up the compromise is that you ideally don't want the tip airfoil to be any thicker than absolutely necessary because it's already rapidly increasing it's drag as it gets smaller and with a thicker airfoil you kinda multiply that factor. If you make the tip airfoil thinner which would solve the drag issue you have another problem to solve because of the decreasing stall capacity as the wing chord gets smaller and if you make the airfoil thinner you kinda multiply that factor.

My conclusion is back at if you want the scale model airplane to fly as good as possible in a variety of situations (just like the original) then using a proven airfoil (whatever that is) at the root and tip with about 2* washout is a very good compromise, if you need the wing to be without washout for whatever reason that may be it is possible - but it might result in a less effective and less pretty flying model airplane.

However, as a summary of my investigation of this; if the starting point is that the root airfoil or inner section is NACA 2415, the closest alternatives as I see it would be to use 2415 at the tip with 2* washout or 2417 at the tip with no washout, or something in between (that is with no safety margin). I just think this keeps the compromise within reasonable limits - whatever that is.

Finally after a huge pretentious looking post like this I just wants to say that I'm just an ordinary bloke like everybody else and I might be wrong just like everybody else, I don't want you to trust me on this - I want you to trust yourself.