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Thread: Truck top wind tunnel

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    Angry Truck top wind tunnel

    Has anyone outside of NASA built a truck top wind tunnel?
    I can see three options. All involve an automated test sequence while I drone down the local interstate in the small hours.
    1. A pole. This may not do a good job of getting out of the disturbed air from the trucks flow field. Going too far forward will give stability problems. Though a rear fin could help.
    2. A horizontal splitter plate. If larger than the truck, should give reasonably clean flow? Side cheeks will further help, but I need to keep their ac back to avoid instability.
    3. A venturi section. Should be the cleanest, but test section no good for full scale testing and the speed up not enough to get actual Re on scale models.

    I can go 8'6" wide and 12'6" high. My s10 sits 5'4" high empty.
    Comments?

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    Re: Truck top wind tunnel

    I understood that Burt Rutan "pioneered" the truck top wind tunnel in the Fresno, CA area before he moved from RC models to real sized aircraft. There are a series of videos of Burt's "oral history" in which he discussed the development of the TTWT.

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    Re: Truck top wind tunnel

    Rutan did a lot. I think the VV was high over the car. BD5 was on top of a truck but when he designed the BD 5 trainer that was on the front, he started using that. There is a picture of SpaceShip ones tail being tested that way. A model airplane guy in the 50's had one on the hood of his truck with a scale for airfoils. If you are trying to drive a traditional wind tunnel down the road, you are doing something way more complex.

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    Re: Truck top wind tunnel

    Quote Originally Posted by pictsidhe View Post
    Has anyone outside of NASA built a truck top wind tunnel?
    Scaled has done it fairly often. I used a truck version of a wind tunnel when I was there to do about 3 months worth of rocket nose-cone aerodynamic testing, and has been pointed out, we also used the system for both SS1 and SS2 component tests. At about 0:13 seconds into this:

    https://www.youtube.com/watch?v=_TTz4yAF7Yc

    you can see the type of thing we're talking about - this particular test was deep stall testing of a full scale Long-EZ.

    Quote Originally Posted by pictsidhe View Post
    I can see three options. All involve an automated test sequence while I drone down the local interstate in the small hours.
    Depending upon how large the item to be tested is, the most likely scenario will require building a frame that puts the test item well out in front of the truck to get it into relatively clean air. We built a steel frame that put the nose cone about 12 feet in front of the cab of the truck. We then put a LOT of ballast in the truck bed and tied it down firmly. We did NOT attempt to drive on public roads with this setup - we used the taxiway and runway at Mojave airport, with permission from the tower. Using about 6K ft. of runway, we could get data at 70 mph for almost a minute - maybe 30 seconds at top speed. I could do 10 - 20 runs in an hour and collect a lot of data.

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    Re: Truck top wind tunnel

    U.S. hang gliding manufacturers have been using truck tower open wind tunnels for years.

    http://www.hgma.net/

    The trucks are used for both pitch testing and load testing.

    I'll skip the pitch test part. ( even though I find it fascinating ) The load testing is far more dramatic to watch.

    the Negative 150 test actually drags the glider backwards and 30 deg. tail down through the air to simulate a tumble/tailslide that frankly would trash many "normal" airplanes. Sure, it's only at 32 mph. It's still a prime example of "don't try this at home".

    I remember the early days of truck tunnel testing, when Wills Wing couldn't get the truck up to speed with the drag of the glider. This was an era of badly designed pollution controls and sadly under powered American cars. One engine swap to a 454 cu inch hot rod engine....then the addition of nitrous oxide injection.... and they finally got up to the desired speed for the positive g load test, 65 mph.

    The glider then lifted the entire truck rig off the ground and it flew at low altitude down the runway until the speed bled off.

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    Re: Truck top wind tunnel


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    Re: Truck top wind tunnel

    I've done tests on 4 articles, all with normal (sedan) cars.

    We used a flat door for 2 of them, strapped to the roof beams and extending about 1.5 ft fwd of the window to provide clean, horizontal airflow.

    The other two were with another car and due to the speeds (170 mph over the ground), we limited ourselves to only putting the test section on the roof and securing it well.

    All intersection and interference tests, for the rest (airfoils, tails, fuselage), the books are good enough. No measurements, just flow patterns (with oil), showing whether detachment and the trigger points to turbulent flow were OK.

    All were carried out on public roads and fully legal...
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    Re: Truck top wind tunnel

    Thanks guys, some good pointers here. Mostly the stick it out on a pole type, but auto has done splitter plate tests. Unless I test a whole plane, I'll be doing road legal tests.
    A big reason for wanting to do tests is try and get laminar flow down the the length of a fan duct, the entrance is critical, I expect to make a few iterations. My design has currently evolved to something like the Horten IX, front fed fans as I will probably put floats on it. Duct drag is considerable if fully turbulent.

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    Re: Truck top wind tunnel

    It might be preferable, in some cases, to use a small model in water. Especially if you have a dock and a tidal current to work with. As I recall, at the same speeds, Reynolds numbers are much higher in water. Then again, so are the loads. Maybe if you know someone with a big boat. Lots of places to work that don't get in the way of other people. Better if not in the driftwood season, though.

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    Re: Truck top wind tunnel

    Can you use a leaf blower for any of it?

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    Re: Truck top wind tunnel

    Quote Originally Posted by pictsidhe View Post
    Thanks guys, some good pointers here. Mostly the stick it out on a pole type, but auto has done splitter plate tests. Unless I test a whole plane, I'll be doing road legal tests.
    This is how Rutan did it:
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Re: Truck top wind tunnel

    IIRC, I've seen Rutan use vehicle-mounted test rigs on top, in front, and even to the side (when there was a wide test area available).

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