Doug Riley
Platinum Member
- Joined
- Jan 11, 2004
- Messages
- 6,968
You can put your flex point anywhere south of the rotor, but of course the design will vary with the location. An important goal is to keep the moving part of the rotor-mast assembly as light as possible. IOW, don't hang a diesel-powered prerotator off your torque bar.
To be clear, the flex device is not a "dampener" to any great extent. A dampener (such an auto shock absorber) extracts energy from the system via friction and dumps it overboard as heat. Barry mounts don't do much of this; they'd melt if they did. They are basically springs, not shock absorbers. The manufacturers call them isolators, which is accurate.
Gyro builders have taken stabs at building this type of device. "Slider" heads (a la Chuck Beaty and Ernie Boyette) use springs up top to allow motion, usually just along one specific axis. Fore-aft sliders are most common, but I believe there are lateral sliders out there, too. The RAF rubber bushing accomplishes somewhat the same thing.
Designing spring suspensions for machinery is a bit more complex than it may look. The engineering science of such designs is extremely well-developed, however. There's no need for guess-n-gosh here.
For example, Barry mounts are made and sold in a wide range of spring rates and resonant frequencies. How you deploy them will determine the loads they "feel" and therefore the selection of mounts and configuration. The masses on each side of the spring assembly also play into the design; e.g. the more massive the rotor/mast assembly, the lower its natural frequency.
Engineers familiar with designing such things as car suspensions should be able to come up with a reasonable first shot at a good design. Not being remotely qualified to do such work myself, though, I'd be inclined to copy Robinson. Again for myself, I wouldn't try mid-mast magic bushings or top-o-the-mast variants. Being naturally lazy and cowardly, re-inventing the wheel just doesn't appeal to me.
BTW, notice that part of the classic Bell-Robinson mast design is an allowance for VERTICAL motion. Many of those who drive 2-bladed big-iron gyros talk about "cabin hop." Two-blade rotors do, in fact, experience a 2/rev variation in their total thrust. It's just the nature of the beast. It should be possible to isolate some of this from the airframe with a proper flex mount.
To be clear, the flex device is not a "dampener" to any great extent. A dampener (such an auto shock absorber) extracts energy from the system via friction and dumps it overboard as heat. Barry mounts don't do much of this; they'd melt if they did. They are basically springs, not shock absorbers. The manufacturers call them isolators, which is accurate.
Gyro builders have taken stabs at building this type of device. "Slider" heads (a la Chuck Beaty and Ernie Boyette) use springs up top to allow motion, usually just along one specific axis. Fore-aft sliders are most common, but I believe there are lateral sliders out there, too. The RAF rubber bushing accomplishes somewhat the same thing.
Designing spring suspensions for machinery is a bit more complex than it may look. The engineering science of such designs is extremely well-developed, however. There's no need for guess-n-gosh here.
For example, Barry mounts are made and sold in a wide range of spring rates and resonant frequencies. How you deploy them will determine the loads they "feel" and therefore the selection of mounts and configuration. The masses on each side of the spring assembly also play into the design; e.g. the more massive the rotor/mast assembly, the lower its natural frequency.
Engineers familiar with designing such things as car suspensions should be able to come up with a reasonable first shot at a good design. Not being remotely qualified to do such work myself, though, I'd be inclined to copy Robinson. Again for myself, I wouldn't try mid-mast magic bushings or top-o-the-mast variants. Being naturally lazy and cowardly, re-inventing the wheel just doesn't appeal to me.
BTW, notice that part of the classic Bell-Robinson mast design is an allowance for VERTICAL motion. Many of those who drive 2-bladed big-iron gyros talk about "cabin hop." Two-blade rotors do, in fact, experience a 2/rev variation in their total thrust. It's just the nature of the beast. It should be possible to isolate some of this from the airframe with a proper flex mount.