Air mass movement calculation
11-25-2014, 11:45 AM
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Air mass movement calculation
Hi,
This was my best guess as to the appropriate forum - it's an oddball question but please bear with me. (The answer has been bugging me.) I am installing a wood stove in my home. I plan on buying one of these:
http://www.plowhearth.com/large-caframo-ecofan.htm
The details state:
Small Wood Stove Fan circulates up to 100cfm,
4-1/4" x 5-1/2" x 8-1/2"H
Large Wood Stove Fan circulates up to 150cfm,
4-1/4" x 5-1/2" x 9-1/4"H
I was thinking about getting crafty and trying to replace the aluminum blade with a wooden 2 or 3 blade prop for RC. I know I have to deal with the inner diameter of prop, possible weight issues etc. But, wouldn't it be nice to have that sitting on my stove! So, my question is how can I calculate square footage of air mass moved to pitch? I also realize that the actual blade size is not listed but the height of the unit is so it would have to be less than 8.5 or 9.25. Recommendations? Thank you very much.
This was my best guess as to the appropriate forum - it's an oddball question but please bear with me. (The answer has been bugging me.) I am installing a wood stove in my home. I plan on buying one of these:
http://www.plowhearth.com/large-caframo-ecofan.htm
The details state:
Small Wood Stove Fan circulates up to 100cfm,
4-1/4" x 5-1/2" x 8-1/2"H
Large Wood Stove Fan circulates up to 150cfm,
4-1/4" x 5-1/2" x 9-1/4"H
I was thinking about getting crafty and trying to replace the aluminum blade with a wooden 2 or 3 blade prop for RC. I know I have to deal with the inner diameter of prop, possible weight issues etc. But, wouldn't it be nice to have that sitting on my stove! So, my question is how can I calculate square footage of air mass moved to pitch? I also realize that the actual blade size is not listed but the height of the unit is so it would have to be less than 8.5 or 9.25. Recommendations? Thank you very much.
11-26-2014, 10:48 AM
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Well, since the air movement will be based on the RPM and the RPM will be based on how hot it gets I think you might want to just live with whatever it manages to move and be happy.
The best prop would be a fairly wide blade unit since the RPM is clearly rather low. And wider blades will simply work better at such low speeds. In fact instead of an engine prop what you really want is more like a rubber model prop or an electric prop.
One "gas engine" prop that would likely work not too badly is one of the wider blade B, Y&O replicas.
Now another glitch. You would have to mount it backwards so you won't see the maker's logo. So much of the uniqueness will be lost.
And finally if you go ahead with trials anyway I can say that you'll want to use a lower pitch value. There's obviously very little power available from this power and any blade pitch much over 4 to 5 inch in value will operate in a stalled condition. Way back in the early days of electric flying a buddy and I set up a static test rig to try motors and props to find what would work for powering sailplane like gliders. From that testing we found that the blades operated in a stalled state and drew a LOT more current once we got to a 6 inch pitch value. Now by modifying the blades to a thinner shape and carving in some undercamber we got good results up to around an 8 inch pitch. And based on this and the idea that even a glider is moving ahead in the climb we found that these modified blades with a 10 to 12 inch pitch worked well in the air.
So what all this has in common with your idea is that the thin sheet aluminium blade might well be the most efficient in this case. But if you want to try a few options it should be pretty simple to adapt a few different model props to the cause to try them out. It's not like there's going to be any dangerous amount of RPM or torque. So a simple bushing of tape to hold the prop with some friction is all that's needed.
Calculations won't mean much due to the low RPM. And since we know that we want to avoid the stalled condition with pitches of 6 or more inches you're pretty much stuck with a 4 to 5 inch value for pitch.
The motor appears to be located well above the mid point so I'm thinking that a 10x4 three blade prop would have about the best chance of success for matching the simple sheet metal blade. But really if it doesn't melt from the heat then a wide blade GWS or APC parkflier prop would be the best option.
The best prop would be a fairly wide blade unit since the RPM is clearly rather low. And wider blades will simply work better at such low speeds. In fact instead of an engine prop what you really want is more like a rubber model prop or an electric prop.
One "gas engine" prop that would likely work not too badly is one of the wider blade B, Y&O replicas.
Now another glitch. You would have to mount it backwards so you won't see the maker's logo. So much of the uniqueness will be lost.
And finally if you go ahead with trials anyway I can say that you'll want to use a lower pitch value. There's obviously very little power available from this power and any blade pitch much over 4 to 5 inch in value will operate in a stalled condition. Way back in the early days of electric flying a buddy and I set up a static test rig to try motors and props to find what would work for powering sailplane like gliders. From that testing we found that the blades operated in a stalled state and drew a LOT more current once we got to a 6 inch pitch value. Now by modifying the blades to a thinner shape and carving in some undercamber we got good results up to around an 8 inch pitch. And based on this and the idea that even a glider is moving ahead in the climb we found that these modified blades with a 10 to 12 inch pitch worked well in the air.
So what all this has in common with your idea is that the thin sheet aluminium blade might well be the most efficient in this case. But if you want to try a few options it should be pretty simple to adapt a few different model props to the cause to try them out. It's not like there's going to be any dangerous amount of RPM or torque. So a simple bushing of tape to hold the prop with some friction is all that's needed.
Calculations won't mean much due to the low RPM. And since we know that we want to avoid the stalled condition with pitches of 6 or more inches you're pretty much stuck with a 4 to 5 inch value for pitch.
The motor appears to be located well above the mid point so I'm thinking that a 10x4 three blade prop would have about the best chance of success for matching the simple sheet metal blade. But really if it doesn't melt from the heat then a wide blade GWS or APC parkflier prop would be the best option.
11-26-2014, 10:50 AM
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By the way. Home machinists have built Stirling engines that do the same job. And I suspect that there are some commercially produced Stirling cycle engine stove fans as well. This one is clearly electric. And that's fine. But a slick looking Stirling cycle engine turning the prop would REALLY be a snazzy conversation piece. And truly? A lot more entertaining AND productive to watch than the bevy of reality shows on TV these days.
Here's a link to a You Tube video on building and running what appears to be a reasonably easy to make Stirling fan.
https://www.youtube.com/watch?v=2hi2SB3IciI
Here's a link to a You Tube video on building and running what appears to be a reasonably easy to make Stirling fan.
https://www.youtube.com/watch?v=2hi2SB3IciI
Last edited by BMatthews; 11-26-2014 at 10:57 AM.
11-26-2014, 10:59 AM
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Thank you very much for your input. I was thinking there may be a source of large, wide props out there kind of like the one's I have seen on rubber band powered aircraft. I'll tinker with this idea some more. I have found some very nice (not to mention expensive) Stirling engines that I would love to have Santa bring me but it's not possible this year. I agree with your comment about the reality shows of TV - I ditched the idea of cable a couple of years ago and don't miss it a bit. Thanks again!
11-26-2014, 02:23 PM
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Just to cap my replies off I didn't get into any calculations for air movement because they'll all be based on how fast the prop turns compared to the fan. Which all means that in the end and with so many variables the answer settles down to "it depends....". Your best test would be simply to try options.
Some rubber model props have far too little pitch to be truly suitable for rubber flying. But because of that if you can find the right example of one it might just work for this use provided it can be made to fit the motor shaft. Don't ask me for brands and availability though. I carve my own and generally don't pay much attention to plastic brands and types.
Carving your own from bass wood might be an option. It could be a fun project.
Some rubber model props have far too little pitch to be truly suitable for rubber flying. But because of that if you can find the right example of one it might just work for this use provided it can be made to fit the motor shaft. Don't ask me for brands and availability though. I carve my own and generally don't pay much attention to plastic brands and types.
Carving your own from bass wood might be an option. It could be a fun project.
