Gress Aero Bi-Rotor VTOL Aircraft; Unlike Anything Else!!
Posted on August 7, 2009 by aaronredbaron
One of the beauties of model aircraft is the potential to try new and exciting concepts. Where as it might not be practical to build and design your own flying machine which you ride in, it is totally within reason for most people to build their own ideas and make them fly by radio control. Since the Wright brother’s first experiments, aeronautical concepts have been tested with miniature models of the full sized counterpart. Gary Gress is not using any new technology, but his method of control for a twin rotor VTOL aircraft is unlike anything on the planet. Although it may look like the control arrangement is similar to the Marine’s new V-22 Osprey, its actually markedly different and far simpler. Gary would like to someday build a Bi-Rotor VTOL he could actually fly inside, but for now he’ll settle for promoting his concept through functional models. I called Gary and picked his brain recently, and he was kind enough to share some of his ideas.
At one point I even incorporated fly-barred rotors (similar to Art-Tech’s current K22 model, and all the top rotors of simple coaxials), where I did get stability but no control (I believe the K22 is pretty much the same). Any further in this direction and I would have added swashplates, but stopped because I realized I would be just reinventing the helicopter. That wouldn’t have been any fun because we all know it’s possible to do (aka the V-22 Osprey). And two heli heads means a lot of complicated mechanics.
So, returning to conventional airplane propellers, what was missing was a very strong (aircraft) pitch-control moment – something in the order of what a helicopter has. Simply tilting them forward or back – for thrust vectoring – often gave me the opposite result; if the props and motors were heavy in relation the aircraft, tilting them forward would cause the rest of the plane to pitch backwards! Adverse inertial effects were far more powerful and immediate than any moments arising from thrust vectoring.
To summarize the state of affairs at this point:
1) effective pitch control needed something more powerful – and immediate – than the thrust vectoring obtained either by control vanes or tilting the props longitudinally.
2) adverse inertial effects (due to tilting the props) had to be avoided or minimized.
It was here that I wondered whether the fact there were two counter rotating props could be used to advantage. Also, perhaps in envy of helicopters and their exceptional stability and control, I began to think the only solution was somehow gyroscopic. Then it dawned on me: try to tilt a gyroscope one way and it will want to tilt in a direction 90 degrees from the intended direction. That is, push on gimballed propellers sideways (both outwards or both inwards) and they will tilt forward or back all on their own. Now I had thrust vectoring without the adverse inertial reactions.
That was the initial foray into gyroscopics, and with it I finally had some promising results. However, with a lot of tinkering, the concept was improved further by making use of a converse property of gyroscopes: actually tilt the propellers sideways (and not just apply a moment to them) and they will generate powerful gyroscopic pitching moments. If the pivots are constrained so the props can only tilt sideways, then these moments are directly transferred to the aircraft. You now have gyroscopic pitch control, with no thrust vectoring.
However, it turned out that – even though the gyroscopic moments are immediate and very strong – you still need thrust vectoring; otherwise the whole aircraft has to pitch for it to travel forward. Also, gyroscopic moments are only temporary and arise only during the tilting. You still need thrust vectoring to provide a sustained moment. So, in the present configuration, the propeller pivot axes are oriented so that the props tilt in a direction 45 degrees from directly forward (ie., half way between longitudinal and lateral). This gives the best of both worlds, (pitch control moments from both gyrscopics and thrust vectoring), and very helicopter-like qualities.
