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Old Oct 11, 2005, 04:29 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
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Windspeed from rpm

At our pattern contest this weekend, one of the guys used a 10x6 prop on a free-running shaft and a tack to measure the wind speed, which at the time he was doing it was well past 20 mph.. and was packing enough dust to stop the event.
He said there had been an article in one of the magazines which went thru an extensive derivative of the factor needed to convert rpm to airspeed, including propellor efficiency.
The prop had to be installed in reverse on the shaft.
The rpms he was reading -appeared- to be close to the real wind speed...
BUT
He said the factor for a 6 inch prop was 176..
This is instantly recognizable as 2x88.
88 is the conversion factor from feet per second to miles per hour..
No efficiency factor at all.
I ginned up a chart based on this, for 4,6 and 8 pitch props.
Does anyone have a -reliable- wind speed gauge to test this idea with?
I have a Dwyer Wind gauge to use, but I'd like to see other types.
I mounted the props on MA gear boxes, for the shaft and the ball bearings for when I get some numbers... Should the wind -ever- come back to the Antelope Valley..
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Old Oct 11, 2005, 06:07 PM
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Again, proving simple is usually better.

I once thought that an anemometer could be made using a CD-ROM motor wound as a genrator and measure the voltage, but that would require calibration. I'm gong to make your version and use that chart, thanks.
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Old Oct 11, 2005, 06:10 PM
Ascended Master
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Palmdale, CA
Joined Oct 2000
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The wind did come up.... Funny how that happens every day of the year here.
With this setup, the speeds indicated by 10x4 and 10x6 props are within reading accuracy of the Dwyer Wind Gauge.
Tomorrow should the wind come back I'll try 9x6 and 11x6 props...
Any puzzlement with the article was why the prop had to be a 10x6.
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Old Oct 11, 2005, 06:14 PM
pushing both extremes
Zoom's Avatar
San Jose, California, United States
Joined Oct 2001
567 Posts
Maybe instead of an accurate wind speed gauge, try using a bike with an accurate speedometer on it on a windless morning. I say a bike becasue it doesn't have much much of an effect on the free stream like a car or other bigger vehicle would, and it would be easy to get a good reading.

Plus if you taped it to your handle bars, and made some motor noises... Well I wont go there.

- Dave -
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Old Oct 11, 2005, 06:18 PM
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Cambridge, MA USA
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Efficiency has little meaning for a freewheeling prop.

One way to calibrate the prop is to use a bike with a speedometer to move the prop/RPM counter rig at 5,10,15,20 mph when it's calm. Ride in both directions and average the results to cancel any slight wind which may be present.

There is also the complication that the aerodynamic pitch will vary somewhat with RPM, due to a Reynolds number influence on the effective camber of the blade airfoils. This will show up as a nonlinear RPM vs speed curve, but I don't know if it will be noticable. To minimize the Re effect, I would use a high P/D prop if possible. Higher P/D makes the prop's freewheeling RPM relatively less sensitive to blade airfoil changes.
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Old Oct 11, 2005, 06:50 PM
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Palmdale, CA
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Bike at 20 mph!!!
20 years ago I could do that.. actually did 20 miles in one hour..
But not any more!
I've got some 6,7,8,9, and 11 pitch props.. But those above 6" are the fancy MA electric props with undercambered back sides..
I'll run a test with the 10x6 MA I used today and the 10x6 MA electric to see if the results change to any noticeable degree.
Then try the higher pitches.
That 10x4 packs quite a sting when trying to stop it!
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Old Oct 11, 2005, 10:36 PM
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$50.00 anemometers only have +/- 5% accuracy. At wind speeds that might concern a modeler, say 5-10 mph, that is a range of 1/2 mph.

What effect do the ducted fan shrouds on the expensive handhelds have on reynolds effects?
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Old Oct 12, 2005, 11:17 AM
Ascended Master
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Palmdale, CA
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Sometimes you want to fly when the wind is a tad faster....
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Old Oct 12, 2005, 01:51 PM
Registered User
Cambridge, MA USA
Joined May 2001
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Quote:
Originally Posted by Sparky Paul
Bike at 20 mph!!!
20 years ago I could do that.. actually did 20 miles in one hour..
Umm... you don't need to go 20 mph for a whole hour for this test
Or you can cheat and find a slight downhill grade.


Quote:
I'll run a test with the 10x6 MA I used today and the 10x6 MA electric to see if the results change to any noticeable degree.
I'd avoid the thin undercambered props. These will have very nonlinear blade airfoil behavior at the negative AoA's they will see during freewheeling. The less blade airfoil camber the better. The little Zagi Gunther prop has almost zero blade camber, and hence should have a nice linear response.
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Old Oct 12, 2005, 02:11 PM
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Palmdale, CA
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Quote:
Originally Posted by markdrela
Umm... you don't need to go 20 mph for a whole hour for this test
Or you can cheat and find a slight downhill grade.

I'd avoid the thin undercambered props. These will have very nonlinear blade airfoil behavior at the negative AoA's they will see during freewheeling. The less blade airfoil camber the better. The little Zagi Gunther prop has almost zero blade camber, and hence should have a nice linear response.
.
By the time a good selection of props would be tested... an hour would be gone.. and my legs, and my lungs....
To say nothing of getting holding the test fixture and getting images at the same time...
.
I had a training video to use with my indoor wind trainer.. most of the rides began with the instruction.. "begin pedaling at 15 to 20 mph"...
The power required at 20 is way more than that required for 15!
The curve looks like any fast airplane's,... going up rapidly with speeds above 12 mph.
.
I'll look thru my collection of antique c/l props from way back when to get a pitch range of flat-backed props.
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Old Oct 13, 2005, 06:41 AM
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Quote:
Originally Posted by Sparky Paul
Sometimes you want to fly when the wind is a tad faster....
Maybe because I dont wear a speedo on the beach or a cyclists hat, you didnt read what I said. The range of error for store bought anemometer is 5%. You have yet to determine your error. The chart you provided is good enough for most purposes I imagine. I would only need to know is it 25 Mph or is it 5 Mph. I've flown my plane in 25 and it went inverted on takeoff. It required full power for any maneuver within my skills.

To really know wind speed requires a sophisticated model from data collected at the flying site. Kalman filtering probably. From what I can tell, that is a statistical method over time. Predict one value and measure another, determine the error, use that to predict the next value in time. Maybe the store bought ones I seen on the picture box use some kind of alegorithim.

No one has offered any of their own data points yet, besides Sparkypaul and I think that was the point. Compare results?
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Old Oct 13, 2005, 10:55 AM
Ascended Master
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Palmdale, CA
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The reliability within a couple miles per hour.. don't really need to know any more precise than that.
I found yesterday some props -won't- windmill in 10 mph winds!
Here's one.
APC 10x8 static, the Power Prop 9x6 on the other end going around nicely.
Several of the APC showed this.
And most of the 7 and 8 inch diameter props wouldn't rotate, even when "nudged".
The 40 year old Tornado Nylon 9x8 went around just fine.
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Old Dec 31, 2005, 02:49 PM
Got shenpa?
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It would be really nice to have a conversion factor that was as simple as shuffling the decimal point over a place or two on the tach, wouldn't it?

Using mph, rpm, and prop pitch P in inches, we have the well-worn formula for prop pitch speed:

Vp = RPM/1056 x P

So if P = 1056, then Vp = RPM. All we need is a prop with a pitch of 1056 inches.

How about this, though: if the prop pitch was 10.56 inches, then Vp = RPM/100, and all that is needed is to read the tach and move the decimal point over two places to get the wind speed.

APC makes a 11x10 Thin Electric prop (fairly low camber, as Dr. Drela suggested). That would introduce about a 5.6% error if everything was perfectly frictionless and the prop truly has a pitch of 10 inches - the prop would spin a bit too fast. However friction and aerodynamic losses will probably slow the prop down a few percent from that ideal value, and if we are lucky, it may be that the losses will amount to about 5% also, roughly cancelling out the +5.6% error from the prop pitch.

Anyone want to try a 11x10 prop and confirm that the rpm reading (in hundreds) equals the wind speed in mph?

BTW - that Hobbico tach is truly horrible, having a resolution only to 100 rpm. I bought one on sale from Hobby People a while ago, and when I discovered how bad the resolution was I took it right back (the very next day after I bought it). IIRC Tower Hobbies had a nice $20 tach with a resolution of +/- 1 RPM.

One other possibility crosses my mind: a 11x10 prop is inconveniently large to carry around as a tach. If, however, a prop with a smaller pitch was mounted in a gearbox of the right ratio, the rpm of the gearbox output could be scaled to match the wind speed. For instance a prop with 5.25" pitch geared down 2:1 would have the same result as a prop with 10.5" pitch and no gearing.

The nice part about this? The easiest way to "gear" by a factor of 2 would be to tell the tachometer that the prop has 4 blades, when it actually only has 2.

APC makes props with a 5" pitch down to a 5" diameter (5x5). One of these props freewheeling in the wind, with rpm measured with a tach set to measure a 4-blade prop, should yield a prop rpm (in hundreds) that is very close to the wind speed in mph.

-Flieslikeabeagle
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Old Dec 31, 2005, 04:02 PM
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Palmdale, CA
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I used gearboxes just to get the ball-bearing supported prop shaft.
The gears were just along for the ride.
Reading the rpm of the geared end would require something spinning there instead of in front.
I found that some high pitched props wouldn't turn at all in low winds.
Once the thing is constructed and tested, as long as nothing changes, the indicated rpms when converted to mph will be remembered.. as we're usually interested in ball-park numbers in lieu of precise values.
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Old Dec 31, 2005, 04:13 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Quote:
Originally Posted by Sparky Paul
.
By the time a good selection of props would be tested... an hour would be gone.. and my legs, and my lungs....
To say nothing of getting holding the test fixture and getting images at the same time...
.
I had a training video to use with my indoor wind trainer.. most of the rides began with the instruction.. "begin pedaling at 15 to 20 mph"...
The power required at 20 is way more than that required for 15!
The curve looks like any fast airplane's,... going up rapidly with speeds above 12 mph.
.
I'll look thru my collection of antique c/l props from way back when to get a pitch range of flat-backed props.
But Sparky. This is all for the SCIENCE man! Your life ending from a heart attack is a small price to pay if it'll help your fellow modellers! I can't believe you're being so selfish..... LOL

Even bicycling to work and back most days of the year I can only maintain 20 mph for short times as well. And I find that since I turned 50 my average speed is going down each year.

Now if we could strap all this onto my motorcycle......
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