One of the last modifications we made before we started flight testing two weeks ago, was to reduce the friction in the control system.
As we’ve seen in the past, small imbalances in lift can easily throw a blade sky high while driving the other one into the ground. When we actuate the controls, the lift on both blades of a given rotor should increase or decrease evenly. But if there are large amounts of static friction in the control line it could lead to one canard moving move than the other and throwing off the balance. So we set out to measure and reduce the static friction between the rotor hub and the canard at the tip.
There are three components between the hub and the canard that can cause friction:
- The control line running through the small sheaths in each rib.
- The pulley at the tip that turn the line towards the canard.
- The bushings that the canard rotates on.
We tested each system individually by putting hanging a 2.5 lbs mass from each end of the control line, and then adding mass to one side until the line started to slide. Improvements included replacing the control line with a teflon coated fishing line, re-aligning several components to eliminate line and pulley rubbing, and sanding down and greasing the axle for the canard bearings. The table below shows the order-of-magnitude reduction that we were able to achieve.
Last Friday we were able to test the improved controls in flight. Actuation of the controls caused no perceptible imbalance in blade lift. We’re now confident in our ability to control the helicopter safely, and ready for our AHS Sikorsky Prize attempt this Friday!