This thread is privately moderated by maguro, who may elect to delete unwanted replies.
Mar 13, 2012, 01:09 PM
RC beginner
New York
Joined Oct 2008
6,054 Posts
Quote:
 Originally Posted by teflyer Also I am planning to use simple basic equipment to measure lift and drag. However the problem is finding a way to do that. The wind tunnel will be 7x7in. I am open to suggestions
the cardboard wind tunnel i have folded up at my parents house is about 2'x18"x6'. i recall the fellow who gave it to me mentioned he wished it was bigger. i suspect 7" might not be enough. cost for this thing was less than \$20 including fan and scale so thats not a big issue. rolling the dozens of paper tubes might have taken some time but not more than an afternoon. i guess it depends on how fancy you want to get.

idk about drag but lift was pretty easy as you might gather from some experiments i posted in the science area. and you might not need a tunnel at all to do simple comparative analysis in steady wind:

 Mar 13, 2012, 01:47 PM Registered User Joined Feb 2012 70 Posts The reason it is so small is that I need the proper Reynolds number to test the airfoils. I should have said 7"x7"x36". The fan I have is a 20in. diameter and has a max windspeed of 6.2mph. I need the wind in the tunnel to be about 50mph to simulate the windspeed of RC aircraft when using equation of continuity and Reynolds Number equation. The problem with the wind is that is has updrafts and downdrafts and not very consistent. Can you give me a link on what you did for the lift calculations?
Mar 13, 2012, 02:47 PM
RC beginner
New York
Joined Oct 2008
6,054 Posts
Quote:
 Originally Posted by teflyer The reason it is so small is that I need the proper Reynolds number to test the airfoils. I should have said 7"x7"x36".
i dont see how tunnel size relates to re at all. seems wing size would be more relevant there. imo the bigger the tunnel the better. you gotta stay away from those walls. really big might be a storage problem though unless its collapsible like the one my buddy built.

if i were you id take avantage of the fan size and make it that big. also note that 2 fans are better than 1 in terms of homogeneous flow. probably 4 would be even better however...

Quote:
 Originally Posted by teflyer Can you give me a link on what you did for the lift calculations?
no cackle-ay-shuns at all for this test. i eyeballed scale readouts and attempting to make sense of the wildly jumping digits. tried to freeze them with photos but that didnt work too well due to lighting problems. intent was to compare two airfoils and that went pretty well in spite of the problems.
 Mar 13, 2012, 02:54 PM Registered User Joined Feb 2012 70 Posts Sorry should specify. With an RC plane moving about 15km/hr, if I want to make the wing 1/5th of the size of the original, I need a windspeed of 75km/h to maintain the same reynolds number. to get a windspeed of aout 75km/h, I need a smaller windtunnel compared to the size of the fan. Does that make sense? Maybe I am saying gibberish
 Mar 13, 2012, 03:01 PM RC beginner New York Joined Oct 2008 6,054 Posts ahhh yes.. fluid dynamics. the guys in the other thread finally convinced me that lines in those textbook drawings dont show compression but actually speed. im not sure how well high speed works out in our tunnels though. my buddy put a triac control to slow down the fans for smooth air. turbulence is our biggest enemy here. i found a pack of incense inside that was apparently intended to track the flow.
 Mar 13, 2012, 05:33 PM just Some Useless Geek Chicagoland Joined Oct 2008 2,551 Posts Don't forget that most of the guys building back-yard wind tunnels are using some sort of collimator (dave1993's paper tubes) to get the spiral motion removed from the air and make it move in a linear fashion. The collimator takes up a certain amount of room, so even if the wind tunnel test chamber is a meter long you probby need at least that much length of collimator tubing or grid or whatever to get the air de-turbulated. Smoothed out, whatever.
 Mar 13, 2012, 08:33 PM Registered User Joined Feb 2012 70 Posts yes, I do realize I need flow straighteners, thank you. It probably will only be a foot length though. I will be using straws.
 Mar 13, 2012, 10:12 PM Registered User Joined Feb 2012 70 Posts Sorry for double post, but notice that these guys also have rather small test sections for the wind tunnel. http://www.aerolab.com/Display_Pages/OCT.html looks pretty crazy! So to calculate lift of an airfoil, Can I just have a rod that is forced to move only up and down connected to a load cell? For drag, I suppose the same thing only horizontal?
 Mar 16, 2012, 09:06 PM Mitch China, Guangdong, Shenzhen Joined Nov 2010 64 Posts Perhaps for drag, you could measure the torque on the vertical rod used to measure the lift. The guide for the vertical rod could be mounted on a pivot, and the reading from the force cell (either above or below the pivot) could be multiplied by the inverse of the ratio of the distance from the pivot to the airfoil and the pivot to the force cell (I think this is right - just doing it in my head which isn't overly reliable). This would eliminate an extra rod. Just an idea. If the angular displacement at the pivot would be over 1 deg. then it might be necessary to rig a parallelogram linkage to adjust the airfoil angle to compensate.
 Mar 16, 2012, 09:27 PM Registered User Joined Feb 2012 70 Posts Can you please post a sketch for me? I am not sure if I completely get what you are saying. Thank you
 Mar 16, 2012, 10:48 PM Mitch China, Guangdong, Shenzhen Joined Nov 2010 64 Posts Almost bedtime - I'll try for tomorrow. The same idea would work for a horizontal rod - then the lift would be measured as a torque on the rod, and the drag would be measured as a force along the axis of the rod. Mitch
 Mar 16, 2012, 11:08 PM Mitch China, Guangdong, Shenzhen Joined Nov 2010 64 Posts Oops - probably created confusion by referring to a torque in the context of a rod. Sorry. Substitute bar for rod and it will probably be easier to visualize. The axis of rotation for the torque would have to be parallel to the long dimension of the wing.
 Apr 11, 2012, 07:03 AM Registered User Joined Feb 2012 70 Posts Is there any reason why there has not been a KFm3 version of the Kfm4 created? I know there is the KFm-6, but the step positions are not in the same placement.