Steve, here's what I know, please correct me if i am wrong. A straight shaft with a balanced coupler will always induce less vibration than a flex shaft. Especially an unsupported flex. Your setup is unsupported from the pump body to the impeller. If there were opportunity for wobble, it would be with the flex shaft, not a straight shaft. So the flex cable would be a more likely cause of increased tolerances (stator to impeller, impeller to tunnel wall, etc) than the straight shaft. A bushing is not suitable support for the end of a flex cable. To remedy this you would need to install a straight 'prop' shaft on the end of the flex cable to reduce the friction, wobble and wear. You stated the relationship between RPM and thrust is linear, but a few sentences later you say "power to thrust relationship is very non-linear". Really? Power and thrust are one and the same in this scenario. We have done extensive testing ourselves and found the NQD jet can only pump so much water. You can apply more power, but it reaches a 'point of diminishing returns' and this happens at around 25KRPM's, or about 3000kv on a 3S system and 4500kv on a 2S (3000kv x 11.1 x 80% efficiency). What this means is it will go just so fast, and if pushed beyond that range, it will make more noise, but go no faster (but may cause the impeller to spin on the shaft!). To go faster you need a larger pump, not a larger 4800kv motor on 3S! That will only induce wear and make more noise (giving the impression of more speed) without actually going any faster. The diminishing returns effect can be observed on some Youtube videos. You'll notice the jump from a (quieter) throttle setting to WOT (wide open throttle) with little increase in traveling speed. That lower throttle was peak efficiency, any more is just wasting power and reducing runtime. But notice that most people run from slow to wide open (jumping past the mid range speeds entirely) making it difficult to observe this phenomena, but it occurs nonetheless.
To sum up:
1. A flex shaft cannot 'eliminate vibration'. It can only induce and mask vibration. If you have vibration, get a decent coupler. Don't try to drill out a smaller coupler to a larger size, it will almost never work and will only make things worse. If a flex appears to reduce vibration, then you have an alignment problem, and a flex will only mask the real problem, not fix it. And it will be robbing power while matching the two misaligned parts.
2. There is nothing inherently wrong with the bore tolerance in the NQD drive. Just keep the shaft bore lubed with a good marine waterproof grease and keep the coupler from rubbing the seal (spinning it and slinging grease out). Develop a schedule for re-greasing, re-apply more frequently if you have a high RPM system.
3. I don't need to try something to know it's not a good solution. Wisdom comes with making the mistakes and I have made my share, but I don't believe I'll be making this one. To each his own. I just don't like misinformation. To some people, the high pitched whine represents more speed, and there is your customer base. But some people will see past the rhetoric, and I am only too happy to help clear things up. G'dday, mate.
