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Aircraft Weight

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This is a good point to talk about aircraft weight. Many will have you believe that light is always best. Unfortunately, flight is not always so simple. In the old days engines were anemic. The only way to compensate was to take weight out of the plane. Today’s engines are decent power wise, so we need to look at weight issues from a different perspective. There are two facets of aircraft weight. The first is the ratio of power to total weight, and the second is wing loading.

For a 3D only airplane, wing loading is king! 3D requires the minimum wing loading possible. Power is important, so long as you do not sacrifice wing loading to get it. That is why people are never satisfied when they add a huge motor to a tiny airframe and expect it to 3D. The power is there, but they kill the wing loading with all that engine weight. In building a 3D plane, weight savings becomes a religion. Use no extra glue, no unnecessary bracing, and the lightest possible equipment.

Pattern is another story entirely. Let’s start by pointing out that the goal for a pattern plane is for it to fly the same speed throughout the pattern. This is not because speed is judged, but rather because we need constant speed to ensure consistent performance. Changes in airspeed are always accompanied by changes in response characteristics, and that means you have to compensate your flying to match an ever changing speed… that is bad. Level, up-line, and down-line speeds should all match - so your inputs will always give the same results. Consistent speed ensures consistent performance.

In calm winds a light plane is great. It is slower on the down-lines, so you fly level and up-lines slower to match. let’s look what happens when the wind picks up, though. First, with a slower plane, you have to use larger angles when correcting for wind…a little more challenging. Then your penetration into the wind is worse…meaning your downwind legs are a lot faster than your upwind legs. Sure, you can push the power up and fly faster, but the problem is your down-lines will still be slow - and we already talked about changing speed being a bad thing. Finally, gusts buffet a light plane more, making your lines bounce all over. Pushing the power up will not fix this problem.

A heavier plane will fly faster on the vertical down-lines, so you will fly your other lines faster to match. Higher speed reduces the angles needed for wind correction, and penetration also improves. Gusts have less affect on a heavier plane. These are all good things. Naturally, there is a limit…otherwise we’d all be flying bricks around.

The upper limit for weight is the speed on the down-lines. As the plane gets heavier, the down-lines get faster. You reach a point where the plane no longer hits terminal velocity and just keeps accelerating. As you remember, changing speed is still a bad thing. What’s the weight where this line is crossed? I don’t know. Sure, you were hoping for a great answer, but it depends on a lot of things unique to your airplane. An Ultimate biplane can have a lot more weight than a sleek Extra, since it has a more drag to balance it.

The purpose of this whole discussion is to give you a feel for the simple truths of weight planning. Those are:

1. For 3D, light weight is everything. Period.

2. For pattern, a little extra weight can be a good thing. Don’t sweat it if your plane is a little on the heavy side…just be reasonable.

 

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