Why big planes and small planes fly differently

Generally a single design will fly very similarly at different scales, but there are three major parameters that change with the size of the plane: drag per mass, lift per wing area, and crash inertia.  I’ll address the first two factors first, and discuss the way they affect flight performance.

Due to reasons that are just a bit beyond the scope of this article, if you have two models of the same plane at different scales, the larger one will be capable of lifting more weight per square inch of wing area.  Thinking like an RC pilot, a 20 size trainer will fly pretty well at 16 oz per square foot, but the same trainer in 60 size will fly just as well and give you that same light, airy feeling at 25 oz per square foot.  This is a scale factor of wings that you can’t do much about.  So if you’re a heavy handed builder or you want to carry extra equipment, the plane has to be big enough to handle extra wing loading.

The other factor, drag per mass, is an even greater reason why big planes and small planes seem so different.  To gain an understanding of drag per mass, imagine a single cube 1 inch long.  The volume is one cubic inch, and the surface area of course is 6 square inches.  Now stack eight of them together to make a bigger cube with sides 2 inches long.  Although the mass increases by a factor of 8 the outside surface area increases by only a factor of 4.   When an object is scaled up, the mass increases at a greater rate than the surface area.

For an airplane, mass per surface area translates to momentum per drag.  The smaller model has less momentum per drag, while the larger model has more momentum per drag.  The result you will see in flight performance is that when you cut the engine to land the plane, the big plane will have a longer, flatter glide slope because it has a lot of momentum due to its greater weight, but the drag per weight is less because of the mathematical relationship between the square and the cube.  By contrast, the small plane has less momentum to carry it along, and a very large amount of drag compared to its momentum.  This results in the famously short and steep glide path of smaller models.

This can be taken to extremes.  1/2A size planes have a really steep power-off glide slope because their speed is eaten up by drag.  The natural remedy is to keep some power on when landing.  But when you run out of fuel or battery charge this isn’t an option.  The plane is coming down so you’d better head for the runway!  Some really tiny planes really drop like a rock.  If a small plane has inadequate momentum per drag, maybe you want to add extra weight so the object in motion will stay in motion.  This would increase the wing loading, and as stated point #1, the smaller the wing the less lift it can make per square inch.  So increasing the loading will only make it come down even steeper.  A very small plane will either land steep and slow if light, or steep and fast if heavy.  In any case, a small plane comes down steeper, and the only way to change the glide slope angle is to use a more efficient airfoil to reduce drag.  So if you like engineering, that’s your goal.

Somewhere in between the steep descent of the tiny plane and the slow, flat glide of the giant plane is a sweet spot, which I would say is around 20 size.  A 20 size plane is big enough to have a respectable wing carrying capacity, but it has the advantage of short field performance when taking off and landing.  It doesn’t require a hugely long runway like a giant model, but it doesn’t drop like a stone the way a 1/2A plane does.  When you want to land a 20 size plane, you just cut the power and it lands very comfortably.

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The Aerostar 40 has a slow and comfortably flat glide slope when landing.

The Aerostar 20 is the same plane but smaller. It has essentially the same flying characteristics, except the smaller wing can't create the same lift per square inch so everything happens a bit quicker, and it has a steeper, faster glide when landing. It's still slow and friendly, but it just feels a little bit more zippy.

The Aerostar 20 is the same plane but smaller. It has essentially the same flying characteristics, except the smaller wing can’t create the same lift per square inch so everything happens a bit quicker, and it has a steeper, faster glide when landing. It’s still slow and friendly, but it just feels a little bit more zippy.

The third factor that I mentioned above, crash inertia, is self explanatory.  Due to the function of the square vs the function of the cube, the smaller your plane the less crash inertia it possesses in relation to the surface area striking the ground.  I can give you a recent example that illustrates the point perfectly.  My 40 size low winger had a wing tip strike while landing, and it broke the rear bolt blocks out of the fuselage where the wing was attached.  The plane didn’t even turn over.  It just bounced and then landed on its wheels.  A month later I was flying a friend’s 20 size low winger and it got caught in a cross wind when landing, and it dropped the wing tip.  This plane actually did a cartwheel before coming to rest on its wheels.  The only damage was a small crack in the bulkhead at the trailing edge.  It didn’t even break the prop.

Planes at the extremes of the size scale will demonstrate the crash inertia principle even better.  I’ve had 1/2A planes that cartwheeled and flipped several times, and all they needed to get back into the air was more fuel.  A ten pound 120 size plane will get totally smithereened by even a relatively minor mishap.

I hope I have helped you to feel at least a little bit more informed about why you would choose one size plane over another.  Maybe you have old, tired eyes, or maybe you have a very small flying field, and your choice will be made for you.  Barring that, try some airplanes at different sizes and keep these factors in mind, and you’ll start to see the differences in flight performance and why they happen.  For my own part, I like all different sizes of planes.  I appreciate the smooth flying and flat gliding of the larger ones, but I’m always very careful to treat them right.  And I still love 049 and 10 size planes, especially the ones that are engineered correctly to make them fly well, and I especially love how they just bounce when I don’t land them properly.  It seems that 20 to 40 size planes represent a good compromise, so it’s no wonder that they have always been so popular.