TheGreatBoo

As already stated I would scale it up by 26% i.e. multiply all measurements by 1.26.

BMatthews

Well, that's an odd situation since you'd expect a twin design to use two drive systems in the first place.

OK, at it's most basic level a twin setup is like having two singles flying in very tight formation. So a model of a given size and weight that flies to a given performance level with ONE drive system (EDF and motor in this case) will fly about the same if you double the wing AREA (not span) and can make it so that the weight is also doubled. If you simply take the smaller model and blow it up directly this means that the twin version would be 1.42 times the wingspan to get 2 times the wing area.

Note that in many cases with hollow internal construction that you'll need to increase the number of wing ribs and spar sizes. You can't just enlarge the plans on a photocopier and expect the structure to not be too sparse. Foam or foam and sheeting or foam and epoxy-glass are different since those will scale for the structure decently.

Watch the weight. 1.42 times the size for span also means that you have 1.42 cubed or almost 3 times the internal volume. So it's easy to end up overly heavy with the larger model and climb rate and slower flying can suffer.

TheGreatBoo

Basic math;
1D scaling (linear) scales to the power of 1
2D scaling (area) scales to the power of 2 (of linear scaling)
3D scaling (volume) scales to the power of 3 (of linear scaling)

When talking about scale we almost always mean linear scale, i.e. a 1/4 scale aeroplane is 1/4 the the size (linear) of the "real" plane. And a 26% increase of a model plane means we mean increasing wing span from fro example 1m to 1.26m.

I you intend to keep the internal structure and only do a (linear) increase of scale, a 26% up-scaling will increase the volume of each construction element by 100% (which will increase weight by 100% if the same materials are used). The good thing is that you also increase thrust by 100% by adding a second engine, the bad is that the wing area will only increase by 59%(since area only scales to the power of 2)... So a pure upscaling (nothing else changed) will increase wing loading. To some degree the increased wing loading will be offset by improved Reynolds numbers and, hopefully, to some degree by improved construction techniques and lighter equipment (i.e. not increased by 100% in weight).

If you scale it up by 42% you will have to seriously reduce weight compared to the original model or you will have a truly anemic plane. If I scaled a plane up by 42% I would prefer a 186% increase in power, i.e. add a third engine

doxilia

My Feedback: (3)

Interesting exchange.

I just wanted to point out that Watts are a unit of power, specifically SI units. While on this side of the pond we like to use horsepower as a unit of power, it is a very old unit and not SI which means "conversion factors" are required if one were to use this unit within a mechanical equation which otherwise is using SI units for its other variables or constants.

English units have fascinating origins and names but are rather impractical in the context of modern physics and mathematics. Many think of Watts as a unit of power related to electrics but this is incorrect. Power is power. There is nothing about an internal combustion engine (gas or glow, piston or turbine) that doesn't allow it's power output to be expressed in Watts.

If an airplane is going to be powered by a horse or two, even they produce a certain amount of power in Watts (~745 Joules/second a horse if we want to be even more SI specific).

Curiously Mr. James Watt was Scottish...

David

griesel

It goes by weight. Twice the scale, with everything else equal (it seldom is ) the weight would go up 8 times which would require 8 times the
horsepower. This will get you in the right ballpark.