[vann_d]

The fan operates at constant speed so the rotating mass of the motor structure is not an issue. It should actually help maintain constant rpm, thus requiring a less torque motor.

-v

 

[vann_d]

Also seems to me guys are a little hung up on a linear relationship between between stroke and rotation.

Cosine error for a 15° rotation is only 3.4%. For 30° it is 13.4%. Not sure anyone will notice 10% distortion at 7 Hz.

Finally, unless I'm missing something, a large xmax motor is not required here. If you have a 0.5 inch lever arm, you only need 7.2mm stroke for 30° rotation. Maybe it's just high B that people are after with Fi.

 

[Zeitgeist]

But a strong (driver) motor is key........right?

I just can't wrap my head around how you could get the force necessary to drive the linkage to change the blade angles - if you don't have a strong motor.

It seems like a lot of things can be fudged...... xmax... the rotational speed.. blade sizes....... But if you can't accurately control the linkage, nothing else matters.

 

[TristanK]

I think it is also important to balance the blades (making the surface area on each side of the rotational axle the same size) , in that way the airflow wont affect the nessary torque needed to alter the pitch of the blades

 

[vann_d]

But a strong (driver) motor is key........right?

I just can't wrap my head around how you could get the force necessary to drive the linkage to change the blade angles - if you don't have a strong motor.

It seems like a lot of things can be fudged...... xmax... the rotational speed.. blade sizes....... But if you can't accurately control the linkage, nothing else matters.

Yeah, I think you're right. I don't know what's the required force to overcome the blade resistance

 

[Binary]

its something you can calculate with physics and a little time.

The RPM of the fan, by the air displaced based on the pitch of the blades, and the average density of air. I bet it would equal less than the 450+ gram cones that most 18" drivers are using. Sometimes its better to assume and fail miserably i guess.
I'd say try a large powerful motor, aim for pro-sound as they are often more reliable long-term
And see if it works or not!

I know im armchair quarterbacking, but this isn't my project. just a push to say sometimes experimentation is required.

 

[memarcus]

To Vann d. The distortion of the wave form caused by this setup is not strictly due ot the cosine error you mentioned. That's probably a small portion of it. The reason I suggested 15 degree or less pitch is due to BL (boundary layer) separation on the fan blade. When the BL separates the flow goes turbulent and will cause noise due to high frequency vortice shed and collapse. The 15 degrees is for a neutral non lift airfoil shape also. A flat blade will have different (more problematic) results and prolly couldn't be pitched as much before thisp roblem occurred. The ideal is to make a pulsing column of air. I imagine you are correct on the distortion and audibility though. I imagine it would take a highly skilled ear to hear 10% or even 20% distortion that low. The prblem would be in the upper response band, say around 30Hz if it is fed signal that high.

Also the extra spinning weight is not just a problem at start up it also causes stress on the shaft and balance issues. I've not seen alot of motor magnets stacks that are aligned real well when glued. Also the ferrite magnet manufacturing process is powder based so you may have all kinds of density distributions in the magnet causing further balance issues. Although when spinning it will cause the same bending stresses as if it were stationary, when it spins with the shaft it will now add torsional transients that will cause the resultant stress profile to spike when the blades change pitch. A small increase in max stress in a fully reversed bending stress is a bad thing for fatigue. The same thing is at play for those gearheads who have installed a harmonic balancer on their crankshafts of their hot rods. 20lbs of magnet hanging on a 1/2" or so shaft could be an issue. It all really boils down to the bearing locations and where things are supported.

I suggest the stationary motor because you avoid these potential problems, a rotating electrical connection, and a potential high speed super heavy hockey puck to the head when the shaft breaks. I think the last time I looked the bearing I mentioned was like $10 or something so it really simplifies things for not much money. Just seems to me there's nbot to be gained by the rotation motor assembly other than headache and complication.

 

[memarcus]

Also the force required to pitch the blades could be roughly calculated knowing the blade geometry, fan diameter, fan speed, and the geometry of the linkage. Of course anything remotely close would require wind tunnel testing.

I beleive it would be similar to the forces exerted on your hand out of the window at 70mph but there would be various calculations to do to get it specific for the fan blade due to the different linkage. If I had to take a SWAG I'd guess it would require 10 in-lb torque to pitch the blade and if designing I would choose a motor that could do twice or three times that as a start.

 

[robbo266317]

Hi, I was just wondering where you were up to on this build as I haven't seen any activity for a while?
Any progress or problems that need solving?

 

[SteveCallas]

Maybe it's just high B that people are after with Fi.

Correct, they have some new, very high Bl drivers.

Scott at Fi first expressed a high level of interest initially, then when I narrowed down exactly what I needed and was ready to pay him, he stopped returning all emails - he wimped out. That setback soured me a bit, and to be honest, my interest has decreased quite a bit. This was more of an interesting project than a need for any performance improvement.

All that said, I still contend this is a fairly easy project. The hard part is getting a stripped down driver motor.

 

[caper26]

Not sure of the shaft RPM (SRPM) of this system, but when the blades deflect, drag is imposed. If keeping the SRPM constant is important, then the max drag/resistance needs to be accounted for and consistantly overcome, without burning out.

 

[sub_junkie]

Man, I sure hope this can be finished. If a person can (somewhat) mass produce these at a considerably cheaper price than the Thigpen, I think they could really make some dough. I'm not positive though...just a guestimation. Haha.

Apologies for asking because I'm positive it's been covered before (I couldn't find it), but what's the project cost for the finished project?

Sent from my iPhone using HTShack

 

[robbo266317]

Man, I sure hope this can be finished. If a person can (somewhat) mass produce these at a considerably cheaper price than the Thigpen, I think they could really make some dough. I'm not positive though...just a guestimation. Haha.

I think royalties may push the price up considerably if they were mass produced.

 

[sub_junkie]

I think royalties may push the price up considerably if they were mass produced.

That's what I was assuming as well. Too bad. I would love to try one of these things out. I have a large closet in my room and it would probably fit perfect in there. Haha :bigsmile:

Sent from my iPhone using HTShack

 

[Zeitgeist]

Not sure of the shaft RPM (SRPM) of this system, but when the blades deflect, drag is imposed. If keeping the SRPM constant is important, then the max drag/resistance needs to be accounted for and consistantly overcome, without burning out.

I thought that was the whole reason why the Thigpen uses a GE? motor/speed controller is to maintain RPMs?

 

[rotfan]

I thought that was the whole reason why the Thigpen uses a GE? motor/speed controller is to maintain RPMs?

As a owner of the Thigpen rotary subwoofer, the motor/speed controller is indeed to
maintain rpms, as well as to adjust the speed to match the size of the room (volume).

 

[caper26]

drool.....

 

[sub_junkie]

Ooooooooooooh I envy you rotfan. Haha.

Sent from my iPhone using HTShack

 

[memarcus]

I don't think royalties have anything to do with the price. The transducer isn't protected by any patent I've seen but I haven't done an exhaustive search. I think the price is high due to the tuning & tweaking required for each installation. Since you don't know where the customer will be you figure two round trip plane tickets across the US at $500 a pop. Two professionals a couple days work at $1000/day/person. Throw in a few hotel nights at $125 somewhere cheap, $1500-$2K for the device, $500 for lumber and other materials and a 300-400% margin due to the PIA factor for each install. I understand the $13K........

I looked into pricing some prototype molds for the plastic blades (been wanting to do an 8" flared port also), a plastic hub and such and think the components could be tooled in aluminum for much less than the price a single thigpen. I think these could be shipped as an "assemble it yourself kit" for a couple thousand using low volume tooling for the molded parts. The difficult part would be finding a plastic molder that would run the odd quantities. you'd be okay on the fan blades since it's multiplied by 5 or 6 per unit, but the not many molders would want to run an insert molded part by hand in the low volumes required. you start adding a pick and place robot and your fixed cost just went way up. Plus the do it yourself kit involve line voltage so someone would zap themselves or burn down their house and that equals lawsuit. Might be able to get around the legal mumbo with some finely crafted CYA disclaimers or require the use of a licensed electrician.

I used to be a product engineer for an appliance part maker that made thermostats for clothes dryers and everytime someone dryed a batch of shop rags from the garage or a gasoline soaked shirt and burned their house down we got sued since it was a UL listed safety device. They generally didn't win but we had a mark up percentage in our cost structure to cover legal expenses. We lost a few cases even when there were obvious levels of hydrocarbons inside the drum in the lab reports. OF course doing it right would require UL listing and you just tacked on a cool $100K or so to put the little white stamp on it.

Maybe after I finish my basement and honey-do list shrinks a bit I'll pick up a CAD program and throw something together and actually get some parts quoted. If you did it right you could use a lot of off the shelf componentry and minimize the high cost, custom machined/molded stuff. Some of it would be unavoidable though.

Whats peoples thoughts on what a reasonable/successful market price would be for such a device? Obviously $13K is not quite a mass appealing price point.

 

[robbo266317]

If you look at their website at http://www.rotarywoofer.com/howitworks.htm
It says One advantage of the Thigpen Rotary Woofer (patent pending) occurs in understanding the impedance match with the air in this approach to sound reproduction versus the impedance mismatch with the air in a cone woofer.
I think it would not be wise to try and commercially sell the whole unit on a mass scale without the fear of litigation.
However individual items that could be assembled to create a similar device may or may not be possible. :huh: