Build log: DIY rotary sub for contemporary HT in 100 year old house? - Page 33

For the driver or the motor? I don't think it will take much either way, but I will have a bit of overkill on hand if necessary. The tough part is the higher freqs since mass is working against you. I think this driver will handle it though. The motor will likely see less than 200 watts, but I'll likely get a 3/4 HP motor or such.

Yeah, the Thigpen rotary is only running at 750 RPM or so....and the blades look to be about 4"x4". This thing is more like a box fan than a heli in power needs.

I should be able to go 45 degrees each direction.

I think the angle of attack should change with air flow. I have design/built/flown pitcherons (gliders with pivoting wings) and they do not stall with the blades at 45 degrees to each other. Also, the blades are like more like a ducted fan than a heli. Thigpen's rotary blades have about this much rotation.

I am able to change the pitching rotation through use of EQ and if I am very wrong I can make new parts. I don't intend to get it perfect on this first one, but it would be nice.

After I typed that, I realized that the wings are close to 90 degrees to each other....I don't know the exact angle as the plane isn't set up right now. Usually, a wing will stall at about 12 or so degrees of angle of attack, but the angle of attack is relative to the air flow. A total of 90 degrees of movement for the rotary's blades is not intuitive, but when air is flowing through the aperture, you need to increase the fan blades pitch.

No need to mention all those things and get everyone confused. I like to keep it simple and empirically test my ideas.

The fan speed does not change....at least you don't want it to change. Airflow and volume is related to pitch of the blade....if you want 10 hz louder than 20 hz, you need it to pitch more during the cycle. Realize that you need to move twice as much air anyway when you halve the pitch.

Now, if you want to adjust the "main" volume you can change the fan speed. Also, when the blades are pitching, the fan will probably slow down a little bit, but hopefully not a significant amount.

It starts getting pretty complicated and you just can't predict what will happen. I find if you build it, the answers will come faster than if you sit on your behind and think about it TOO MUCH. Some thought is good....more is better when the effort or cost is very high.

Basically, it all depends!

Ok, I see now... If you started bringing up Reynolds numbers, I was going to start salivating. Heh heh

I must say I love designing stuff though. I wish I could have a team of builders to boss around and make me stuff. Also, I get distracted with other cool projects.

Guilty! ...although I just spent time thinking about a marble machine and not building it. There are many others (and just as "weird") that you do not know of.

[A rotary sub sounds pretty weird to a lay person.]

I wanted to get metal today, but instead spent a LOT of time getting my little one warmed up enough to that scary red monster with the long gray beard to take a picture.

I picked up the steel today. I'll try to do some cutting and welding in the next couple of days.

I finally had some time to do a few minutes of work. I machined one of the rotary's supports. No pics because it really isn't that impressive. I should be able to do a bit this weekend.

I also received the top and two side seals for the door from Ted. They look pretty beefy. I have to trim them to size once I get the auto seal for the bottom of the door.

Also a few of us in the Albany area are meeting up Monday night. I know we are going to eat and then see MI. Ben or I will show off our work so far at one of our theaters before that if we can figure out scheduling and such.

I have a few other things to do so this is only tack welded together. Imagine the fan blades and driver between the two supports and the motor coupled to the shaft on the right.

Yeah, it is overbuilt, but that is way better than underbuilt. At least I won't have to rebuild it for strength/stiffness reasons. After seeing the video below I figured it could vibrate a decent amount. After re-watching it just now, my implementation makes the Thigpen look pretty wimpy! Hopefully mine will work.



As you may have notice, I am supporting the fan on both sides and not using the motor spindle. I think this is a better approach structurally, although not as clean aerodynamically....well except the Thigpen motor support is wider. I think the Thigpen is 17" diameter whereas mine can be 33"....I'm limited by how high a frequency I want to get to because of the mass of the blades/linkage.

RT: welding is actually pretty easy with this wire fed welder I have. Sure, a little practice is necessary to get the right speed, but not difficult at all. I can imagine they don't let beginners near any aerospace welding!

Yes, I have a Lincoln 110v wirefeed welder. I bought it maybe 7 years ago and used it for the CNC router I made. It worked very well!

Mikieson: http://lmgtfy.com/?q=Rotary+subwoofer

The rotary sub moves a lot more air than a regular sub and allows a lot more output at low frequencies down to 1 hz. They usually cost $25k installed.

Ha ha. You're welcome for the link, but you deserved it!

There is a lot of debate about <20hz. Here is my take on it from what I have read and it is FREE OPINIONATED ADVICE. It is worth every penny! Yes, you can hear <20hz. If you have sufficient volume, you'll hear 15hz and probably even 10hz. The thing is that very few systems will play loud enough to play 10hz. Not only do you need more decibels to hear low frequencies, it become exponentially more difficult to play at the same decibels as higher frequencies.

At some frequency, I agree that you switch from hearing to feeling. Mind you that you can feel 50hz quite well and it gives you that chest "thump". The ULF stuff still adds something to the experience. From what I have read about other's experience of the Thigpen rotary sub is that they certainly do know when it is turned on and off.

I like to tell people that ULF gives a sense of dread. It does not exist in nature unless something really bad is about to happen like an earthquake or volcanic eruption. Actually, that is not true...it occurs it a lot of stuff from slamming a door to any instantaneous sound like the plucking of a string.

Now, whether it is worth $25k....that depends on how much money you have and your priorities....like a LOT of what is on these forums. A $1000 projector??? That is CRAZY. I just bought a perfectly good 45" TV at Walmart for $300. These people will buy a $30K car to commute in. I don't think the rotary sub is worth $25K and that is why I'm building one. If I didn't want to build something so crazy, I'd put in $1000 of IB subs or $5k of sealed subs and call it a day and have an amazing system.

....but the rotary sub is the "ultimate" and so I would like to try it. I think it will only cost me around $500 with the driver that Penngray donated to the project.

Any time you have a large weight with 5 wings rotating and pitching multiple times a second being hung off a motor shaft -- expect some vibration. Supporting both ends of the shaft is much more rigid and (hopefully) requires much less precision in terms of aligning and balancing the entire unit. I don't know exactly what linkage is in Thigpen's rotary, but it may be more rigid than mine. Hopefully that won't be an issue as I have used less robust linkages in much larger flying surface moving much faster. They were not pivoting at 20hz, however.

Keep it down out there. You are getting TOO chatty. Ha ha

Looks like my auto bottom door seal will be here today. I've been closing my door to the theater and my wife "yells" down at me from the floor up to chat with me. I yell back, but she doesn't hear me. When I get the door sealed I don't think I will be able to hear her at all.

What do people do to communicate from their theaters? I guess my wife could always call me on the cell phone, but is there a better way? Just curious.

I will do some work on the rotary this afternoon. I've been busy making seafood paella and cookies since my parts arrived yesterday.