Aircraft model in a wind tunnel and the full size version

[mech-eng]

I try to understan this article: https://en.wikipedia.org/wiki/Reynolds_number

What does "... such as between an aircraft model in a wind tunnel and the full size version." mean? Why cannot we use a full size version in a wind tunnel or why cannot model be full size?

Thank you.

 

[BvU]

You'd need a really big wind tunnel...

 

[anorlunda]

Most wind tunnels aren't big enough. This is the biggest.

[/PLAIN]
The largest wind tunnel in the world is big enough to test a 737 airplane, and is part of NASA Ames Research Center’s state-of-the-art aerodynamics complex.

The wind tunnel, which is 80 feet by 120 feet, is actually one of two giant wind tunnels at NASA Ames National Full-Scale Aerodynamics Complex at Moffett Field. The “smaller” wind tunnel is a still-massive 40 feet by 80 feet, making it the second-largest wind tunnel in the world. The two wind tunnels were dedicated in 1987.

As it’s able to accommodate planes with wing spans of up to 100 feet, nearly all commercial aircraft made in the United States since 1987 have been tested in the subsonic tunnel. The tunnel can replicate the wind conditions of flight via six 22,500-horsepower motors with blades as tall as four-story buildings. It’s also used to test flight technology for space, such as parachutes for Mars missions.

 

[etudiant]

Note also that those big tunnels are only powerful enough to simulate low speed flight up to about take off speed.
Higher speed flight simulations all rely on small scale models, because the construction and operation of a full scale tunnel running at high speed, even well below supersonic, is financially and technically out of reach.

 

[PaulK2]

To add, you could certainly use a full-size "model" and simply observe the real behavior of the aircraft, as long as the tunnel was large enough and could handle the air velocities needed. But depending on the project, you may want to build variations of a model and that could be a lot of construction work! And anyway, like the others have said, it's unlikely that a wind tunnel is large enough for the size and weight of most full-scale aircraft. That's where Reynolds numbers are helpful, in compensating for fluid flow differences between full size and scale.

 

[Ion Aguirre]

Thats all a mater of MONEY. Relatively little tunnels, and models are a lot cheaper than the full scale option. As a teacher of mine said ... "Technical indecisivenes may exist, but economical one cannot".

 

[boneh3ad]

Why full scale test is not practical:
1. Cost
2. Size, construction, maintenance and operation of such a large facility
3. Unnecessary (required aerodynamic characteristics can be obtained with a well made scale model)
4. Cost of producing scale 1:1 prototypes
5. Handling and maneuvering scale 1:1 prototypes
6. Requirement of large enough fan and power to mimic actual speeds (health and safety issues too)
7. Electricity used to power that large fan (cost again)
8. and several other factors

Note: None of these limitations are technically infeasible and basically all boil down to cost effectiveness.

Why did NASA build a full scale wind tunnel if it is unpractical:
1) NASA can afford to build, operate and maintain such equipment because this is not the first large scale complex they have
2) This facility is not built for the purpose of testing commercial aircraft (scale 1:1), rather it is built for R&D purposes, majority of it is related to space activities. However, aircraft manufacturers may use this facility to test their aircraft (to appear in the news I would guess).

I disagree with this part of the post. The National Full-Scale Aerodynamics Complex (NFAC) does not exist just "because it can." It exists because there is still a need. In order to achieve full dynamic similarity between a scale model and its corresponding full-scale vehicle, you have to match all of the dimensionless parameters governing the flow field. This is not always easy to do. In fact, in many situations, we have to pick and choose which we match and have to let one or more sets be different. Usually you have to choose to match the ones that govern the phenomena that cannot be easily or accurately modeled on a computer, or you have to choose to match the parameters that let you isolate the physics you care about with a given test. Since designing a plane designed to haul people and other precious cargo requires a great deal of confidence and vetting of the design, sometimes you need a very special wind tunnel capable of matching more (or all) of those parameters in order to validate the design. That is why NFAC exists and is really the only facility of its kind.

It is so expensive to operate, in fact, that NASA actually decomissioned it in 2003. Computational tools had become so powerful and operating it became so expensive that they simply couldn't afford to keep it running. They certainly do not have the money to operate it. However, computers still aren't that powerful, and the US Air Force saw the value in continuing to operate the tunnel complex, so it was recomissioned and is now being operated and maintained by the USAF Arnold Engineering Development Complex (AEDC).

This facility is, in fact, built exactly for testing 1:1 models (or at least as near to 1:1 as is achievable). This is where the "Full-Scale" in its name comes from. That is its primary purpose.

 

[berkeman]

National Full-Scale Aerodynamics Complex (NFAC)

Was this at NASA Ames facility in Mountain View, California? I've seen the large wind tunnels there -- very impressive.

it was recomissioned and is now being operated and maintained by the USAF Arnold Engineering Development Complex (AEDC).

But this is not at NASA Ames -- it's back East, right? Did they move the big tunnels, or am I just confused...?

 

[boneh3ad]

Was this at NASA Ames facility in Mountain View, California? I've seen the large wind tunnels there -- very impressive.

But this is not at NASA Ames -- it's back East, right? Did they move the big tunnels, or am I just confused...?

It is located at NASA Ames. It is a pair of tunnels that are the two largest in the world. It got too expensive to run and they were shut down. AEDC, which is headquartered at Arnold AFB in Tullahoma, TN, then essentially bought the tunnels from NASA and operates them now. They are still located at Ames, but they are not funded by NASA anymore. AEDC also operates a large hypervelocity wind tunnel in Maryland that is not located on their main campus.

The actual main campus for AEDC itself is, by the way, extremely, extremely cool.