Pitch trim & stability

From Velocity Aircraft Wiki
Jump to: navigation, search

Pitch Trim and Pitch Stability

Here's a pretty good discussion of Velocity pitch Stability by Larry Coen from the Velocity builder's email Reflector discussion list.

As the subject suggests, pitch trim and pitch stability are not the same thing in spite of the fact they do interact. All certified aircraft have some form of pitch trim and must exhibit pitch stability. If we look at the history of the Velocity it started as what we now call the "Standard" with a 180 HP Lycoming. The horsepower on that same airframe has increased over time but the trim system remained the same. Is it any wonder that a faster plane starts to run out of down trim? There have been other major modifications like the "173" or "Long Wing", the "XL Series and the RG option. These are factory options and don't begin to address the creativity of the builders. The spring trim system that we use is probably the simplest design of any trim system. It pushes or pulls on the elevator control with a spring loaded force so we don't have to.

There are two basic areas that builders are struggling with. First is having more than enough up trim and not enough down trim. A change in the spring torque tube attach to give more down trim and less up trim is likely the simplest solution. A more difficult problem is lack of trim range. This is the situation if you have enough up trim, there isn't enough down trim and visa versa. The increased flight envelope from minimum to maximum speed is likely the culprit here. A longer linear actuator or a stiffer spring could help. I don't like the idea of reducing the incidence angle of the canard unless it was set wrong in the first place. The unintended consequences are reduced pitch stability, increased stall speed and rotate speed.

Now let's talk about pitch stability. In a conventional aircraft, the main wing pushes up and the horizontal stabilizer pushes down. When you reduce power the plane slows causing less up on the wing and less down on the tail and the nose pitches down. Speed up. nose up, slow down, nose down. That's pitch stability, not pitch trim. I used a conventional aircraft as an example because it's simple to see how it works. Not so clear with a canard. The wing and the canard both make positive lift both of which increase with an increase in speed. No pitch up thus no inherent pitch stability as in a conventional aircraft. This is overcome by setting the canard angle of attack about six degrees greater than the main wing. The greater angle of attack causes the lift of the canard to change more rapidly than the main wing giving us pitch stability. The sparrow strainer was not added to somehow set pitch trim speed but to increase pitch stability. The normal configuration has it supplying a downward aerodynamic force on the back edge of the elevator proportional to speed. Increasing speed causes the nose to pitch up and the reduction of force when you slow allows the trim spring to raise the elevator allowing pitch down. Inverting the sparrow strainer will give you more down trim but will actually give you less pitch stability than no sparrow strainer at all.

If you got this far, you're a brave soul.

Larry Coen N136LC