![\"SquareCube\"](\"http:\/\/www.balsaworkbench.com\/wp-content\/uploads\/2017\/09\/SquareCube.jpg\")
<\/a><\/p>\nFor a\u00a0gliding\u00a0airplane, rather than mass and surface area, think of momentum and drag. \u00a0A smaller model has\u00a0less momentum per drag, while a\u00a0larger model has more momentum per drag. \u00a0The result you will see in flight performance is that when you cut the throttle 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. \u00a0By contrast, the small plane has\u00a0less momentum to carry it along, and a very large amount of drag compared to its momentum. \u00a0This results in the famously short and steep glide path of smaller models when the engine quits.<\/p>\n
This can be taken to extremes. \u00a01\/2A size planes have a really steep power-off glide slope because their speed is eaten up by drag. \u00a0The natural remedy is to keep some power on when landing. \u00a0But when you run out of fuel or battery charge\u00a0this isn’t an option. \u00a0The plane is coming down so you’d better head for the runway! \u00a0Some really tiny planes really drop like a rock. \u00a0If a small plane has inadequate momentum per drag, maybe you want to add extra weight so the object in motion will stay in motion. \u00a0This would increase the wing loading, and as stated point #1,\u00a0the smaller the wing the less lift it can make per square inch. \u00a0So increasing the loading will only make it come down even steeper. \u00a0A very small plane will either land steep and slow if light, or steep and fast if heavy. \u00a0In 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 and reduce drag.\u00a0 The Q-Tee is a great example of a very small plane that has a pretty respectable glide for its size, due to the shape of the airfoil.\u00a0 So if you like engineering, that’s your goal.<\/p>\n
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. \u00a0A 20 size plane is big enough to have a respectable wing carrying capacity, but it has\u00a0the advantage of short field performance when taking off and landing. \u00a0It doesn’t require a hugely long runway like a giant model, but\u00a0it doesn’t drop like a stone the way a 1\/2A plane does. \u00a0When you want to land a 20 size plane, you just cut the power and it lands very comfortably.<\/p>\n
<\/a>The Aerostar 40 has a slow and comfortably flat glide slope when landing.<\/p><\/div>\n
<\/a>The Aerostar 20 is the same plane but smaller. It has essentially the same flying characteristics, except the smaller wing needs more airspeed to create as much lift per square inch as the 40 size, 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.<\/p><\/div>\n
The third factor that I mentioned above, crash inertia, is self explanatory. \u00a0Due to the square-cube law,\u00a0the smaller your plane the less crash inertia it possesses in relation to the surface area striking the ground. \u00a0I can give you a recent example that illustrates the point perfectly. \u00a0My 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. \u00a0The plane didn’t even turn over. \u00a0It just bounced and then landed on its wheels. \u00a0A 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. \u00a0This plane actually did a cartwheel before coming to rest on its wheels. \u00a0The only damage was a small crack in the bulkhead at the trailing edge. \u00a0It didn’t even break the prop.<\/p>\n
Planes at the extremes of the size scale will demonstrate the crash inertia principle even better. \u00a0I’ve had 1\/2A planes that cartwheeled and flipped several times, and all they needed to get back into the air was more fuel. \u00a0A ten pound 120 size plane will get totally smithereened by even a relatively minor mishap.<\/p>\n
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. \u00a0Maybe the choice is already made for you: \u00a0big planes for old, tired eyes, or small planes for your small flying field. \u00a0Barring 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. \u00a0For my own part, I like all different sizes of planes. \u00a0I appreciate the smooth flying and flat gliding of the larger ones, but I’m always very careful to treat them right. \u00a0And 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. \u00a0It seems that 20 to 40 size planes represent a good compromise, so it’s no wonder that they have always been so popular.<\/p>\n","protected":false},"excerpt":{"rendered":"
Generally a single design will fly very similarly at different scales, but there are three major parameters that change\u00a0with the size of the plane: drag per mass, lift per wing area, and crash inertia. \u00a0I’ll address the first two factors … Continue reading