4 engined scale model, 55" span advice needed


I am about to embark on a 4 engined scale model using foamboard and depron, wing span is approx 55" with a tapered wing. So questions to those of you with more experience in foam models than me what should i be aiming for ready to fly weight wise, yes i know as light as possible but what at a guess would i need to keep it under for any chance of successfully flying it. At that weight can anyone recommend motors to run on a 3s lipo pack with prop diameter 5 to 6" that is the maximum on clearance to the fuselage. All replies with constructive advice appreciated, i want to build this model at this size and if successful i will double the size for my next build.


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I'm going to assume you've got around 400in^2 of wing area and that you're shooting for a wing cube loading of 7. That would put weight at around 900g. Here's a preliminary look at an electronics mass budget:
2250 3s 170g
4x1806 motors + ESCs 120g
Servos and RX 60 g
Wiring 100g
Which totals to 450g, leaving another 450 for the plane itself. Foam is ~110g/600in^2 sheet. The wing needs a top and bottom layer and a spar - say, maybe 2.5x the total wing area in total, or 1+2/3 sheets or 183g. Let's assume the fuselage and tail use the same amount of foam as the wing, bringing the foam weight total to 366g. Add in a factor of 20% for glue and you're basically on the 450g remaining.

I think if I actually built a plane with these specifications it would end up closer to 1000g due to extras like landing gear and all the little things like control rods and firewalls.

In any case if we take the 2280kv variant of the 1806s swinging 6x4 props, they make 460g of thrust on 125W. That'll put thrust-to-weight close to 2, which is quite excessive - 6x3s or smaller may be a better choice for efficiency. In any case there's plenty of margin for excess weight or smaller motors.
I think that bigger is easier to build and easier to fly and if you're going to use 4 working motors then you should go bigger because 4 motors are extremely overkill on a 55" model unless they're very small. A 4 motor setup is a lot of work too, spending it on a little 55" seems like a waste.. especially because there isn't any kind of special mixing you could utilize at that size in a way that would be visible. You want to go bigger anyway I say go for it right away. If you really want to do the 55" maybe consider having a couple of static or free spinning propellers/engine nacelles
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Many thanks for the detailed replies although the battery weight will obviously push it up a fair bit from your calculations Pieliker. Build has now been started, 56" span hand launch so no u/c, may look at the two motor option with two wind milling but am limited on prop size so the motors have to be reasonably small.