# How much air flow is required for lifting a wing?

Gold Member
Given a wing that lifts a plane moving through it at velocity V, how do I figure the minimum volume of air flow necessary to lift the same wing if the plane is still and the air is blown over it? Would moving that volume of air over the wing artificially require more power than what is required to move the same plane through the air at the same speed? Thanks.

## Answers and Replies

berkeman
Mentor
Given a wing that lifts a plane moving through it at velocity V, how do I figure the minimum volume of air flow necessary to lift the same wing if the plane is still and the air is blown over it? Would moving that volume of air over the wing artificially require more power than what is required to move the same plane through the air at the same speed? Thanks.
WITW?

Are you asking what the differences are between flight tests and wind tunnel tests? Can you post the most recent links you have been reading about this question please?

russ_watters
jim hardy
Science Advisor
Gold Member
Dearly Missed
how do I figure the minimum volume of air flow necessary to lift the same wing if the plane is still and the air is blown over it?

Draw a picture.

At first reading it seems you have equated volume to velocity
and that's nonsense.

We are understandably puzzled by your words. I daresay they don't paint an accurate picture of what's in your mind.

A question well stated is half answered.
Were I to take your question literally, i'd have to answer
Use this formula to figure it
Volume of flow = area of flowstream X velocity of same

Perhaps you'll rephrase it ?

old jim

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russ_watters
Mentor
Given a wing that lifts a plane moving through it at velocity V, how do I figure the minimum volume of air flow necessary to lift the same wing if the plane is still and the air is blown over it? Would moving that volume of air over the wing artificially require more power than what is required to move the same plane through the air at the same speed? Thanks.
Yeah, like the others said, this implies to me plane moving in still air vs plane still and air moving (in a wind tunnel). Short answer is there is no difference.

jim hardy
FactChecker
Science Advisor
Gold Member
Are you trying to mimic the aerodynamic lift exactly or to just get the same total lift? One could use "ground effects" to get more lift from a stationary wing, but that would not be mimicking the aerodynamic lift of a moving wing high up in free stationary air.

Assuming that you want to mimic the aerodynamics of a moving wing at altitude:
The lift on a moving wing gets contributions due to air a significant distance away being redirected down. To completely mimic that by blowing a large volume of air at the wing would require a very large amount of air being blown at the wing. That would require a lot more energy. That is one reason why wind tunnels are so expensive to use and there are so few of them.

Gold Member
Interesting responses as I thought the question was clearly stated. If a plane is moving through air, it takes power. If you blow air over the plane instead, that takes power. To get the same lift with air moving at the same speed as the first case, you can assume for convenience a wind tunnel, how much volume of air must be involved? Does it take more power to move that air than it takes to move the plane assuming a perfectly matched wind tunnel cross section and motor power. Thanks.

Gold Member
Yeah, like the others said, this implies to me plane moving in still air vs plane still and air moving (in a wind tunnel). Short answer is there is no difference.
Are you saying the minimum power you need in a perfectly designed tunnel matched to this wing, with no waste, would have the same power as the plane needed to provide thrust to move the plane forward in the first case?

FactChecker
Science Advisor
Gold Member
Again, I will assume that you want to mimic the same aerodynamics. A wind tunnel for a full-sized wing would take a huge amount of power. It would have to create an air stream with a diameter significantly greater than the wing span. That is essentially unheard of for any airplane large enough to carry people. It is possible for small drones, but it would still require much more power than the drone requires to fly.

The reason is that the true aerodynamics of a wing in flight are affected by the flow of air quite far away.

Gold Member
Again, I will assume that you want to mimic the same aerodynamics. A wind tunnel for a full-sized wing would take a huge amount of power. It would have to create an air stream with a diameter significantly greater than the wing span. That is essentially unheard of for any airplane large enough to carry people. It is possible for small drones, but it would still require much more power than the drone requires to fly.

The reason is that the true aerodynamics of a wing in flight are affected by the flow of air quite far away.
You're saying the answer to my original question is that it takes far more power to move enough air over the wings to generate enough lift than moving the plane thought the air to generate the same lift. Thanks.

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FactChecker
Science Advisor
Gold Member
You're saying the answer to my original question is that it takes far more power to move enough air over the wings to generate enough lift than moving the plane thought the air to generate the same lift. Thanks.
To be more exact, I would say that it takes more to generate the same lift using the same aerodynamics. I can't say that there is no way to concentrate an airstream of smaller diameter to make the same total lift with different aerodynamics going on. I wouldn't know about that.

russ_watters
Mentor
Are you saying the minimum power you need in a perfectly designed tunnel matched to this wing, with no waste, would have the same power as the plane needed to provide thrust to move the plane forward in the first case?
You actually asked two questions; one was about airflow and the other about power. Let me separate them: the airflow (more specifically the airflow velocity) is the same. the power is higher in the wind tunnel than on the plane because in addition to moving the air past the plane, the wind tunnel fan has to move the air through the wind tunnel. And to be useful, the wind tunnel has to be big enough that the test model doesn't have a big impact on the flow though the tunnel.

The two scenarios are not well related in power.

CWatters and FactChecker
CWatters
Science Advisor
Homework Helper
Gold Member
The volumetric flow rate will be the cross sectional area of the wind tunnel multiplied by the velocity v.

FactChecker
Science Advisor
Gold Member
And to be useful, the wind tunnel has to be big enough that the test model doesn't have a big impact on the flow though the tunnel.
That is a good way of looking at it. It means that to mimic the flight aerodynamics the wind tunnel must be so big that it contains all the model disturbance. Ideally, the walls of the wind tunnel should not come into play at all and should parallel the airflow streamlines.
The two scenarios are not well related in power.
I agree. It doesn't take nearly as much power to fly a piloted airplane as it does to generate a huge-area airflow in a theoretical large wind tunnel. The two situations are not comparible. A wind tunnel model can be scaled down to as little as 1/100th size.

CWatters and russ_watters
jim hardy
Science Advisor
Gold Member
Dearly Missed
A wind tunnel model can be scaled down to as little as 1/100th size.
https://www.nationalmuseum.af.mil/V...icle/198082/wright-brothers-1901-wind-tunnel/

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bob012345, russ_watters and FactChecker
256bits
Gold Member
A wind tunnel model can be scaled down to as little as 1/100th size.
Somewhat non-intuitively problems arrive matching dimensionless quantities of fluid mechanics between scaled model and object.
Such as for instance Reynolds number, or Mach number.
Dimensional analysis comes into play. As does similitude.
Even for an actual slow airplane speed of say 200 mph, testing with a 1/100 scaled model, to match the Reynold's number requires a wind tunnel air speed in the neighborhood of 20000 mph ( velocity of object / scale of the model ),which is unrealistic in practice.
See https://en.wikipedia.org/wiki/Similitude_(model)

Testing a model in a wind tunnel, and applying that information to the actual object is not as straightforward as one would expect.

bob012345, anorlunda and FactChecker
Gold Member
To be more exact, I would say that it takes more to generate the same lift using the same aerodynamics. I can't say that there is no way to concentrate an airstream of smaller diameter to make the same total lift with different aerodynamics going on. I wouldn't know about that.
I agree. It may be possible to more efficiently flow air at higher speeds over smaller local areas to generate lift. They do this with 'blown wings' to some extent already.