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Old Jun 18, 2011, 08:03 AM
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Propeller wind tunnel tests


Hi,

I’m the happy owner of a DIY wind tunnel for propellers tests. This allowed me to make thrust and power consumption measurements of props up to 9 or 10 inch diameter, at speeds up to 16 – 17 m/s. I’m just doing this because it’s an interesting challenge, but I guess this might be useful for designers…

In order to share my results I was thinking of a useful and easy way to present them.

So the question is “what do you think would be interesting to know about prop’s behavior in flight?”
and how can such data help in choosing the right prop for a given drive and airplane…

I can make an excel sheet that can simulate a drive quite well, giving amps, thrust and rpm figures at increased speed (see the pics).

Or many others calculated values…

This needs so much factors that it’s impossible to assume it really shows what would happen in flight, but it’s still a good illustration…

More, the way I get the prop’s data should be discussed and compared with other testers…

So, what do you think about all this?
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Old Jun 18, 2011, 03:40 PM
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Nice work!

Many have been waiting for years for this type of data.

The curves below demonstrate what NACA does.

One interesting data point is the Zero Thrust Speed.

From this it is possible to calculate the true pitch of the prop.
Old Jun 18, 2011, 06:21 PM
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Hi Martyn,

Which report is that? one from the 20s ? I love those stuff from the NACA… Reports #73 and #98 helped me a lot while I was building the wind tunnel, they’re from back in 1919…!
Here’s some pictures of what I have for an APC 10x6E, it’s very close to the one quoted “P/D = 0.6” on your graph.

At higher P/D it differs a bit, due to Reynolds I think, and prop shape. Also my airspeed measurement might be slightly higher than the actual airspeed at the prop in the test room.
Last edited by yomgui; Jun 18, 2011 at 06:31 PM.
Old Jun 18, 2011, 06:30 PM
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Originally Posted by yomgui View Post
Which report is that? one from the 20s ?
TR-237

http://hdl.handle.net/2060/19930091305
Old Jun 18, 2011, 06:31 PM
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I'd love to see some pictures of your set-up.
Old Jun 18, 2011, 06:57 PM
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Early wind tunnel tests.

Originally posted by a contributor in 2003.
Old Jun 18, 2011, 07:28 PM
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Originally Posted by Martyn McKinney View Post
Early wind tunnel tests.

Originally posted by a contributor in 2003.
Nice! I love the wirings... reminds me something !
Old Jun 18, 2011, 07:43 PM
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"..Originally posted by a contributor in 2003..."

Right! You're not trying to distance yourself from that effort by any chance, are you?

Jack
Old Jun 18, 2011, 08:05 PM
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Originally Posted by jackerbes View Post
"..Originally posted by a contributor in 2003..."

Right! You're not trying to distance yourself from that effort by any chance, are you?

Jack
Nah, I did all my testng by poking the fuselage out the car wndow.
Old Jun 18, 2011, 08:25 PM
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OK, I can visualize that.

I wonder if you can get an air start on a fuelie that way, and if so how much speed it takes?

I got to see a Reeve Aleutian Airways crew do an air start on one motor of a DC-4 out on Shemya Island in the Aleutians in 1965. Then I got on the plane and flew away to Adak on it. I always sort of wondered why they made the passengers get off for the air start if it was safe enough to board an airplane with one engine running.

Yomgui (or Martyn) if you guys can tell us what the definitions of the variables like "Cf" and "Ct" are in the spreadsheet and graphs it would be interesting.

This is pretty much over my head but I am fascinated by the the data and the potential value in it.

Of post a link to a sort of propellers 101 lecture...

Thanks,

Jack
Old Jun 18, 2011, 08:52 PM
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The Thrust(Ct) and Power(Cp) coefficients are dimensionless coefficients which allow you to determine propeller thrust and absorbed power if you know the RPM, Pitch and Diameter and the air density (rho).

The coefficients in the TR-237 Technical Report are determined using Imperial units (Pounds Thrust - Horsepower).

http://hdl.handle.net/2060/19930091305

From the Advance graphs, the Thrust and Power may be determined for any airspeed.

On the Advance Curves, the X-axis may be considered airspeed.

The Y-axis gives the value of the coefficients for that airspeed.

On the X-axis the Pitch Speed (RPM X Pitch) is equal to an Advance equal to the Pitch/Diameter ratio.

A prop with a P/D ratio of 0.8 has a Pitch Speed (RPM X Pitch) with an Advance of 0.8.

The Zero Thrust Speed is equal to the Pitch Speed Advance + 0.2.

For a prop with a P/D ratio of 0.8, the Zero Thrust Speed occurs at an Advance of 1.0.
Last edited by Martyn McKinney; Jun 19, 2011 at 09:43 AM.
Old Jun 18, 2011, 08:55 PM
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This explain Cp, Ct and V/nD... :

http://www.mh-aerotools.de/airfoils/index.htm

Things are not so easy unfortunately, Cp and Ct increase with rpm.

This numbers are useful to predict prop's behavior.
Old Jun 18, 2011, 08:58 PM
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Very nicely done !

Looks like you've got a cross section for the size of prop displayed -- no side wall influence.

I did an open jet version w/ a gambit of motors that could be place at either end:
Old Jun 18, 2011, 09:50 PM
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Martyn, yomgui, et. al.,

Thanks for the help and links. There is some interest there but the math is just too intensive for me. I'm still getting a lot of value out of the way Robert Goble has the prop data at FlyBrushless.com. That is about the level of interest and practicality I enjoy.

As a last question, knowing that each different testing setup can change the results a little, if you turn a given prop at a specific RPM with a number of different motors, wouldn't the thrust produced be about the same for every motor?

jrb,

I like the "hang in there baby!" testing setup, that is one I have used myself.

Jack


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