This Friday, February 8th, we will be making another attempt at the AHS Sikorsky Prize. Following our flight tests two weeks ago, we have made modifications to the truss in order to keep the rotors more level, and to prevent the bracing lines from going slack under the higher drive loads seen during a harder sprint.
Adjusting the Shear Lines
Throughout flight testing we’ve made many modifications to the shear lines on the truss, mainly lashing them tighter to achieve more tension. Adding uneven tension to lines, however, has caused the truss to twist out of true. The first step in attaining a level truss (and thereby a level rotor) was to re-adjust the tension of these shear lines. Assembling the truss structure upside-down we were able to adjust the tension in the lines to get the rotor axles level.
Adjusting the Length of the Bracing Lines
The next step was to attach the bracing lines (the long lines that connect and stiffen the truss arms), and adjust them so that the rotor axles remained level. This we did with the truss assembled right-side up.
Setting the Tension of the Bottom Lines
At higher drive loads, the lines that connect the bottoms of the rotor axles can go slack, as the rotor axle is bend inwards. When the lines go slack, the torsional stiffness of the truss arms is reduced to about 1/3 of its original value and an imbalance in blade forces can have a devastating effect. In order to be able to apply higher sprint forces we increased the tension on the bottom lines, so that they only begin to go slack when the pilot is reaching peak power. We did this by simulating a drive load by winching together opposing rotor axles and determining at what load the lines go slack. This was also an opportunity to proof load the structure to 1200 Watts with a 70 lbs drive load as shown in the pictures below.
Yes, of course we are very nervous when we see the axle with this magnitude of deflection (it’s normally straight). Proof loading is always very nerve-racking event, but we would much rather see it fail here than see it fail 3 metres up in the air. These tests also made us realize that the axle is almost certainly rubbing against the inside of the drive tube (which slides on top), potentially with a very significant force. We’ll be adding a lubricated teflon coating to the contact point, which we estimate may save an additional 10 Watts of flight power.
We feel that we’ve done everything we can to prepare ourselves for the next prize attempt. Once again, I think we are very, very close.