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Old Dec 01, 2011, 02:46 PM
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Multiplex Funjet in wind tunnel!

Hi everyone,

I spent the afternoon testing a Multiplex Funjet in a wind tunnel at Kingston University (London, UK) since I need this data for my dissertation project: I will be optimizing it for longest range/glide/altitude/climb rate with the help of Arduplane hardware (ardumega, shield, gps and airspeed sensor).

Within a couple of weeks I will be plotting similar curves for different elevator settings, as well as Turnigy 2836 performance (thrust vs current draw at different airspeeds, up to 34 m/s - 122.4 km/h) with APC 5x4.7 prop.

If anyone is into optimisation of this aircraft for long range UAV I'm sure you will find this data interesting, that's why I'm posting.

In a nutshell, this aircraft stalls when AoA reaches about 13 degrees. Best L/D obtained is 4.95 at 10.5 degrees AoA.

Interesting to notice the model was tested up to 128 km/h, thus generating about 8 kg of lift and 1.6 kg drag. The wake generated after reaching stall angle shakes both tail fins violently, I need to trim the video footage which will be on youtube soon.

You can have a look at the chart attached, taken at 20.5 m/s; this is a typical cruise speed for this model - 73.8 km/h or 46.1 mph.

Any suggestions for next tests are welcome!

Emiliano

London
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Old Dec 01, 2011, 10:54 PM
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Sounds cool. Do you have a picture of the aircraft?
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Old Dec 02, 2011, 06:20 AM
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Sounds cool. Do you have a picture of the aircraft?
Just google image "Multiplex Funjet" ;-)
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Old Dec 02, 2011, 10:25 AM
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Nice!

Do you have ideas of zero lift drag coeff ?

guillame
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Old Dec 02, 2011, 11:07 AM
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Iboxx83,

Very interesting. Your chart shows lift peaking at about 12 degrees. Somewhere (Martin Simon? Selig?) it was shown that lift of a typical airfoil at model sizes (12" chord) developed peak lift at 6 degrees. Your chart does not show appreciable increase in drag with increasing AOA up to 12 degrees. Hmm.

Jim R...
...whose drag is increasing with age...
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Old Dec 02, 2011, 11:16 AM
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Originally Posted by yomgui View Post
Nice!

Do you have ideas of zero lift drag coeff ?

guillame

Hi Guillame,

at air density obtained at 23C (1.292 kg/m3), Cd at zero lift approximates to 0.1064 (which is mostly skin friction drag)
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Old Dec 02, 2011, 11:53 AM
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Jim, I think you might refer to the optimum Lift/Drag AOA rather than maximum lift angle. 12 degrees seems about right for maximum lift, but drag at this sort of angle should be pretty high already.
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Old Dec 02, 2011, 12:11 PM
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Originally Posted by JRuggiero View Post
Iboxx83,

Very interesting. Your chart shows lift peaking at about 12 degrees. Somewhere (Martin Simon? Selig?) it was shown that lift of a typical airfoil at model sizes (12" chord) developed peak lift at 6 degrees. Your chart does not show appreciable increase in drag with increasing AOA up to 12 degrees. Hmm.

Jim R...
...whose drag is increasing with age...
I see where you're coming from, but bear in mind we are not testing an airfoil here, but a whole aircraft instead. That makes the whole picture a little more complicated, it's very hard to predict the exact pattern air molecules are taking on such an "arbitrary" shape. The fuselage itself must be generating some lift as well.
As you mentioned, the increase in drag doesn't seem high, but it makes sense considering that L/D increases going from 0 to 10.5. If L/D increases, it means the increase in drag must be smaller than the increase in lift.

Maximum Cl is normally obtained right before stall, hence if the model stalls about 13 degrees (lift starts to drop and both tail fins shake at this angle), it's reasonable to find maximum lift at approx 10 degrees right?
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Old Dec 02, 2011, 12:17 PM
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Jim, I think you might refer to the optimum Lift/Drag AOA rather than maximum lift angle. 12 degrees seems about right for maximum lift, but drag at this sort of angle should be pretty high already.
In order to have a noticeable increase in drag and massive drop in lift I think we should go well over stall angle.

Next session I'll plot the curves up to 25 degrees, we'll see
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Old Dec 02, 2011, 01:25 PM
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Originally Posted by Iboxx83 View Post
Hi Guillame,

at air density obtained at 23C (1.292 kg/m3), Cd at zero lift approximates to 0.1064 (which is mostly skin friction drag)
Thanks,

all this is very interesting!

But 0,1064 !! that is very high isn't it? half this value would already be high!

Remember this aircraft should be able to reach more than 130 km/h. It's a fast airplane.

My 1/12 Yak-3 has a zero lift Cd estimated around 0,03 to 0,04 (according to wind tunnel thrust data and in flight data).

http://aerotrash.over-blog.com/artic...-90225320.html


Don't be hurt, I'm just trying to help or understand where i may be wrong !
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Old Dec 02, 2011, 02:07 PM
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With the quite low aspect ratio of the funjet you don't expect to see the wing stalling at the same alfa as the airfoil does in the 2 dimensional case.

If the airfoil does feature a cl_max at alfa=8, the wing Cl_max may very well be at 10 or even 12.

biber
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Old Dec 02, 2011, 02:15 PM
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Originally Posted by yomgui View Post
Thanks,

all this is very interesting!

But 0,1064 !! that is very high isn't it? half this value would already be high!

Remember this aircraft should be able to reach more than 130 km/h. It's a fast airplane.

My 1/12 Yak-3 has a zero lift Cd estimated around 0,03 to 0,04 (according to wind tunnel thrust data and in flight data).

http://aerotrash.over-blog.com/artic...-90225320.html


Don't be hurt, I'm just trying to help or understand where i may be wrong !
Hey, questions never hurt :-) yours is an interesting point.

I need to find out whether the software that collects the data for this wind tunnel has any kind of calibration that offsets the drag given by the support arms.

I will ask for feedback to my supervisor before the next tests, there might be some kind of flaw which I need to investigate further, thanks for the heads up in the meanwhile
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Old Dec 02, 2011, 02:40 PM
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Well, I'm not sure ! just find it surprising
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Old Dec 02, 2011, 02:47 PM
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Originally Posted by yomgui View Post
Thanks,

!

Remember this aircraft should be able to reach more than 130 km/h. It's a fast airplane.
According to Multiplex, this airframe "Blows up" at about 130 MPH, 208kph, so they now make the Ultra FunJet, that should be good to 200MPH, 320 kph.

Keep up the wind tunnel tests, I love them.

Crunch
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Old Dec 03, 2011, 06:38 AM
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I wonder how you can reliably eliminate the drag from the support arms in the measurement. the drag they produce is not a fixed amount in that setup, they are affected by the model and its AOA. and since they are attached beneath the wing, they are in a higher pressure area where they will produce more drag.
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