Originally Posted by
Ascend Charlie
The advantage is not having to rely on getting any lift from the retreating side of the disks.
Sure, in a one-disk system, the advancing blade can generate a lot of lift, but it has to throw most of it away, because it can only provide the dismal amount of lift that the struggling, half-stalled retreating blade provides. Otherwise, the sides are not balanced. The advancing side, at top speed, could provide around 7 times more lift than it is allowed to generate. But use a coaxial system, and there is an advancing blade on both sides, forget the retreating blade, and now because all that lift can be used, the diameter can be smaller. Simples. Tccchhhk.
To get most of that advancing blade benefit, though, simply being a coaxial rotor system isn’t enough. You have to be able to carry large moments across the hub and hold what would normally be an unusual rotor trim on each rotor so the center of lift on each rotor is offset from the rotor shaft. The Kamov style coaxial rotor systems don’t do this, so they don’t get an advancing blade benefit. Only relatively rigid rotor systems can. Unfortunately, the ability to carry large 1P moments in a rotor also means you carry large 2P, 3P, 4P, 5P, etc. moments as well. This is where the real problems begin. While 1P moments are very large (and generally commanded), 2P moments are also quite large (and unhelpful). 3/4/5P harmonic blade loads are smaller (still unhelpful), but also significant. In a flapping rotor hub, you have some design freedom to select the number of blades and blade tuning to choose how to minimize the impact of these harmonic loads, and with enough blades, rotors can fly quite smoothly. In a rigid coaxial rotor, the same problem exists, but there are more design constraints. If the rotors have more than 3 blades, the large 2P loads will generally cancel out at the hub, hence the 4 bladed designs on the current ABC aircraft. A 4 bladed rotor will interact with 3/4/5P harmonic blade loads. Those harmonic blade loads WILL end up as vibratory hub loads that must be mitigated somehow. Studying a helicopter dynamics textbook will show that. It’s not just a matter of “poor blade tuning” these aircraft struggle with, it’s the intersection of concept and physics.
Maybe Sikorsky finally gets a really excellent anti-vibration control system working for all flight conditions. Maybe they lose both FVL contracts. We’ll see.