Looking for 90mm CW/CCW EDF pair

[Tom_B]

Well, I am looking for a 90mm CW/CCW EDF pair. Any advice?
Tom_B

[Alpha]

Tom_B Hi Tom, welcome! CW/CCW 90mm impellers are increasingly rare these days, perhaps due in part to the fact that there aren't many twin 90mm jets with nacelles spaced far enough apart to warrant needing CW/CCW rotation. What airframe are you installing these in, and how many total watts are you targeting?

For good 90mm CW/CCWs, JP's set comes to mind. They have 6s, 8s, and I believe 10s setups as well, all available with CW/CCW impellers.

[Tom_B]

Thanks Alpha,
I just had a look at the JP's and indeed they have that stuff... mightily expensive though. Right now I am still debating 90 mm vs 70 mm (I can get some less nice 70 mm CW/CCW EDF pairs).
I want to do my own "safe", or et least safer quadcopter and I don't think I can use all the thrusters rotating in the same direction.
My other question is, what is a practical difference between number of blades on the EDF impeller? I think I might be able to buy 89 mm CW/CCW pairs of impellers, but they are 5 blades only, while mainstream seems to be more like 12 blades. Is it just cosmetics, or there are more basic, performance issues?
Tom B.

[Alpha]

Hi Tom,
A 90mm quadcopter will get heavy, very quickly. There is a [currently] unavoidable point past which the law of diminishing returns rules all. The battery weight necessary to feed EDFs powerful enough to sustain their own weight operates at a point below the "optimal efficiency/performance" line. That's largely why multi-rotors are propped, not EDF'd.

In terms of "practical differences" between the number of blades on an EDF impeller, the general rule is that, for a given rpm, fewer blades promotes higher dynamic thrust while more blades promotes higher static thrust. We can't adjust the pitch of the blades in real time, relying only on rpm changes to modulate thrust output, the cost of which is 1) efficiency and 2) watts consumed (which directly impacts battery health and flight duration for a given watt-per-ounce to maintain hover and a target watt-per-ounce to maneuver/fly). There exists a sweet spot for any motor/impeller/amp/battery/thrust/rpm combo --the key is to implement a system whose strengths are extant along the majority of your intended performance curve.

If I were developing an EDF'd multi-rotor aircraft, I'd start by using three blade fans at higher voltage and lower amps, then test and see where the data points me toward to further refine. This is basically what Schubeler and BVM did in the early days with their high voltage low blade count setups, where speed and efficiency were priorities over "whoosh" sounds (see early 12s-14s BVM Electras). High Static Thrust setups, pioneered by Stumax with 12+ blade counts, sought the opposite end of the spectrum for thrust, acceleration, and [eventual] top speed (see 10s-12s CARF Spark). Jetfan came into the middle ground with 9 blade fans as an alternative to HET's 6, etc, etc, etc. Foam electric manufacturers like Freewing followed a similar evolution. 5-6 blades, then jump to 12, then settling at 9. This path is common because ultimately manufacturers are trying to find the "sweet spot" that satisfies the largest group on the Venn diagram of cost, performance, complexity, speed, thrust, consumption, etc.

My point in all this is that you're pursuing a moving target, and there isn't a universal Right Answer. Rather, the right setup will largely depend on the specific machine you're developing, its specific operating environment, its specific flight characteristics and performance targets, its specific budgetary/size/complexity constraints, etc.

I'd suggest evaluating your envisioned airframe, estimate its weight, its carrying capacity, etc, then arrive at the answers to two questions:

1) Will your airframe even benefit from CW/CCW setups? If yes, your options are limited. If not, your options are vast.

2) Will your airframe perform as you intend, with its projected thrust to weight ratio? If yes, proceed with prototyping. If not, redesign it until your weight/performance/duration targets are met.

Notice that in none of those initial questions are you concerned about how many blades you should use. That question comes much later. Data will lead the way.

[Tom_B]

Thanks Alpha, a lot of info to research...
Tom B.