How can I measure lift in a wind tunnel?

[jozef]

Aerodynamics -- Wind tunnel

I'm not sure if this goes under here since it's not actually homework but it's more of a general question for a paper I have to do, so if it's in the wrong place I apologize; simply tell me and I'll delete it or something.

Okay well for my research paper I'm thinking of doing how the design of aircraft wings affect lift. I'm planning on constructing a simple wind tunnel but assuming I get everything else to work, I'm not exactly sure how to measure lift inside the wind tunnel. If someone could be of help, it would greatly be appreciated. Thanks.

 

[rcgldr]

Some type of scales to measure the forces are normally used. You'll need to figure out some method of rigging the model or wing in order to hold it's position while being able to measure the forces and torques.

Normally the design of an airfoil is to optimize for it's intended use while compromising for the sake of production costs. Most powered aircraft operate well above best lift to drag ratio speed for the wings they use, but here the focus is a reasonable (within cost constraints) miminal amount of drag while producing enough lift for level flight at cruise speed.

Glider wing design, both full scale and models focus on lift to drag ratios at an intended range of speeds, but there's a limit as to what airfoils can accomplish, and large wingspans are the way to achieve very high lift to drag ratios; high end gliders with over 80 foot wing spans have 60:1 glide ratios. Most glider airfoils are thinned and cambered tear drop shapes.

Another issue is the wind tunnel itself. Low speeds result in more laminar airflow than higher speeds, and there is a Reynolds number factor also at play. Short (vertical) wind tunnels restrict vertical component of air flow affecting the outcome. The best wind tunnels are large (compared to model size) and relatively high speed. An alternative for high speed air tunnel is to a gas with low viscosity, to keep the Reynold factor about the same as the intended model.

 

[jozef]

Some type of scales to measure the forces are normally used. You'll need to figure out some method of rigging the model or wing in order to hold it's position while being able to measure the forces and torques.

For this, I thought about having hooks on either end of the wind tunnel and then tying it to the plane and then having the fan start up so the plane would receive a tailwind and I could measure the lift in the plane. However, I'm not sure as to where the scales come into play if I were to use the hooks.

Edit: I was going through some of the other posts and I was wondering if a spring scale would work to measure lift.

Glider wing design, both full scale and models focus on lift to drag ratios at an intended range of speeds, but there's a limit as to what airfoils can accomplish, and large wingspans are the way to achieve very high lift to drag ratios; high end gliders with over 80 foot wing spans have 60:1 glide ratios. Most glider airfoils are thinned and cambered tear drop shapes.

For the purpose of the experiment, I would only be looking at lift so would I then need to refocus my question to include the drag itself or would it be acceptable to simply focus on just the lift portion of the experiment.

Another issue is the wind tunnel itself. Low speeds result in more laminar airflow than higher speeds, and there is a Reynolds number factor also at play. Short (vertical) wind tunnels restrict vertical component of air flow affecting the outcome. The best wind tunnels are large (compared to model size) and relatively high speed. An alternative for high speed air tunnel is to a gas with low viscosity, to keep the Reynold factor about the same as the intended model.

At what velocity would you consider it being high speed as for the gas it might be a possible option for me to do as well but I'll have to ask my supervisor.

 

[rcgldr]

At what velocity would you consider it being high speed as for the gas it might be a possible option for me to do as well but I'll have to ask my supervisor.

Depends on what you're trying to model, mostly the size and expected speed range for the targeted aircraft of the model. There are indoor free flight models that fly at less than 5mph. Small outdoor models range between 10mph to 20mph. Full scale gliders range from 45mph to 70mph (for best lift to drag ratio speed). Powered aircraft fly faster still.

I'd recommend a web search on existing wind tunnels at universities, to get an idea of how they "instrument" (hook up force measuring devices) on their models. I'm not sure how this is done myself. Regarding Reynolds factor at low speeds, if I remember correctly, helium is a low viscosity gas used at slower speed to emulate air at higher speeds in wind tunnels, but you need a closed system type wind tunnel to use it.