flight007
Topic Author
Posts: 33
Joined: Tue Jan 01, 2019 11:07 am

Wind tunnel drag test

-we use same airfoil and same AoA and airflow speed in both tests
-wind tunnel have adjustible walls so we can use it as 2d or 3d test,also meassure lift and drag...

Test "2D"
-AoA 15 degrees
-airfoil tips are "connect" with wind tunel walls,so there is no gap between tips and walls,(airfoil span is same as wind tunnel width),so called 2D test

test "3D"
-again AoA 15 degrees
-airfoil tips are apart from wind tunel walls,airflow can flow around tips,so called 3D test-real condition..

What do you think in which test will instruments show greater drag ?

flipdewaf
Posts: 2703
Joined: Thu Jul 20, 2006 6:28 am

Wind tunnel drag test

3D year will produce lower drag. The fact that that both edges of the aero foil are touching the wall make it more approximate to a wing of infinite span which assuming incompressible low speed flow means no lift induced drag.

Fred

Edit:Another assumption is that this airfoil is not a thin plate and not stalled in laminar flow.

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vikkyvik
Posts: 12423
Joined: Thu Jul 31, 2003 1:58 pm

Re: Wind tunnel drag test

flipdewaf wrote:
3D year will produce lower drag. The fact that that both edges of the aero foil are touching the wall make it more approximate to a wing of infinite span which assuming incompressible low speed flow means no lift induced drag.

Don't you mean 2D will produce lower drag?

I'd agree with that.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".

flipdewaf
Posts: 2703
Joined: Thu Jul 20, 2006 6:28 am

Re: Wind tunnel drag test

vikkyvik wrote:
flipdewaf wrote:
3D year will produce lower drag. The fact that that both edges of the aero foil are touching the wall make it more approximate to a wing of infinite span which assuming incompressible low speed flow means no lift induced drag.

Don't you mean 2D will produce lower drag?

I'd agree with that.

Lol! Yes. Too much wine!

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flight007
Topic Author
Posts: 33
Joined: Tue Jan 01, 2019 11:07 am

Re: Wind tunnel drag test

flipdewaf wrote:
vikkyvik wrote:
flipdewaf wrote:
3D year will produce lower drag. The fact that that both edges of the aero foil are touching the wall make it more approximate to a wing of infinite span which assuming incompressible low speed flow means no lift induced drag.

Don't you mean 2D will produce lower drag?

I'd agree with that.

Lol! Yes. Too much wine!

Sent from my iPhone using Tapatalk

3D test will show lower drag.
Airfoil tips in 3D test are open end,so there is pressure loss on this region,there is no wall(end plate) which will stop this pressure to "escape",so drag and lift are reduced.

(If we want have same amount of lift in both tests,than 3D will have greater drag.
Why?
3D airfoil have pressure loss on tips so lift is reduced compare to 2D airfoil..To compensate this reduction in lift we must increase AoA,now aerodynamic force is tilt back which has drag component,this is induced drag.)

Note, in 3Dtest induced drag do not exist,because we did not change AoA to compensate reduction in lift...

flipdewaf
Posts: 2703
Joined: Thu Jul 20, 2006 6:28 am

Re: Wind tunnel drag test

flight007 wrote:
flipdewaf wrote:
vikkyvik wrote:

Don't you mean 2D will produce lower drag?

I'd agree with that.

Lol! Yes. Too much wine!

Sent from my iPhone using Tapatalk

3D test will show lower drag.
Airfoil tips in 3D test are open end,so there is pressure loss on this region,there is no wall(end plate) which will stop this pressure to "escape",so drag and lift are reduced.

(If we want have same amount of lift in both tests,than 3D will have greater drag.
Why?
3D airfoil have pressure loss on tips so lift is reduced compare to 2D airfoil..To compensate this reduction in lift we must increase AoA,now aerodynamic force is tilt back which has drag component,this is induced drag.)

Note, in 3Dtest induced drag do not exist,because we did not change AoA to compensate reduction in lift...

Hang on... induced drag would exist in the 3D case as a function of lift coefficient ^2 however the lift coefficient would not increase at the same rate as the 2 d example due to edge effects. I.e. the lift-curve slope would be less than the 2d case at 2pi/rad.

The 2d case has no lift induced drag.

Fred

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flight007
Topic Author
Posts: 33
Joined: Tue Jan 01, 2019 11:07 am

Re: Wind tunnel drag test

.
Last edited by flight007 on Thu Jun 06, 2019 9:32 pm, edited 1 time in total.

flight007
Topic Author
Posts: 33
Joined: Tue Jan 01, 2019 11:07 am

Re: Wind tunnel drag test

flipdewaf wrote:
flight007 wrote:
flipdewaf wrote:
Lol! Yes. Too much wine!

Sent from my iPhone using Tapatalk

3D test will show lower drag.
Airfoil tips in 3D test are open end,so there is pressure loss on this region,there is no wall(end plate) which will stop this pressure to "escape",so drag and lift are reduced.

(If we want have same amount of lift in both tests,than 3D will have greater drag.
Why?
3D airfoil have pressure loss on tips so lift is reduced compare to 2D airfoil..To compensate this reduction in lift we must increase AoA,now aerodynamic force is tilt back which has drag component,this is induced drag.)

Note, in 3Dtest induced drag do not exist,because we did not change AoA to compensate reduction in lift...

Hang on... induced drag would exist in the 3D case as a function of lift coefficient ^2 however the lift coefficient would not increase at the same rate as the 2 d example due to edge effects. I.e. the lift-curve slope would be less than the 2d case at 2pi/rad.

The 2d case has no lift induced drag.

Fred

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My results from wind tunnel test are:

2D test
Lift:301
Drag:122

3D test
Lift:214
Drag:94

(Note once again,AoA is same for both tests,I didnt increase AoA on 3D airfoil to compensate lift reduction because of tip pressure loss..)

flipdewaf
Posts: 2703
Joined: Thu Jul 20, 2006 6:28 am

Re: Wind tunnel drag test

flight007 wrote:
flipdewaf wrote:
flight007 wrote:

3D test will show lower drag.
Airfoil tips in 3D test are open end,so there is pressure loss on this region,there is no wall(end plate) which will stop this pressure to "escape",so drag and lift are reduced.

(If we want have same amount of lift in both tests,than 3D will have greater drag.
Why?
3D airfoil have pressure loss on tips so lift is reduced compare to 2D airfoil..To compensate this reduction in lift we must increase AoA,now aerodynamic force is tilt back which has drag component,this is induced drag.)

Note, in 3Dtest induced drag do not exist,because we did not change AoA to compensate reduction in lift...

Hang on... induced drag would exist in the 3D case as a function of lift coefficient ^2 however the lift coefficient would not increase at the same rate as the 2 d example due to edge effects. I.e. the lift-curve slope would be less than the 2d case at 2pi/rad.

The 2d case has no lift induced drag.

Fred

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My results from wind tunnel test are:

2D test
Lift:301
Drag:122

3D test
Lift:214
Drag:94

(Note once again,AoA is same for both tests,I didnt increase AoA on 3D airfoil to compensate lift reduction because of tip pressure loss..)
do you have data for 0 degrees?

Fred

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flight007
Topic Author
Posts: 33
Joined: Tue Jan 01, 2019 11:07 am

Re: Wind tunnel drag test

flipdewaf wrote:
flight007 wrote:
flipdewaf wrote:
Hang on... induced drag would exist in the 3D case as a function of lift coefficient ^2 however the lift coefficient would not increase at the same rate as the 2 d example due to edge effects. I.e. the lift-curve slope would be less than the 2d case at 2pi/rad.

The 2d case has no lift induced drag.

Fred

Sent from my iPhone using Tapatalk

My results from wind tunnel test are:

2D test
Lift:301
Drag:122

3D test
Lift:214
Drag:94

(Note once again,AoA is same for both tests,I didnt increase AoA on 3D airfoil to compensate lift reduction because of tip pressure loss..)
do you have data for 0 degrees?

Fred

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I didnt meassure zero AoA,but I can do it..Why do you ask?

Can you explain why you think that 3D test must have greater drag ,what will increase drag if I didnt increase AoA on 3D airfoil?

flipdewaf
Posts: 2703
Joined: Thu Jul 20, 2006 6:28 am

Re: Wind tunnel drag test

flight007 wrote:
flipdewaf wrote:
flight007 wrote:

My results from wind tunnel test are:

2D test
Lift:301
Drag:122

3D test
Lift:214
Drag:94

(Note once again,AoA is same for both tests,I didnt increase AoA on 3D airfoil to compensate lift reduction because of tip pressure loss..)
do you have data for 0 degrees?

Fred

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I didnt meassure zero AoA,but I can do it..Why do you ask?

Can you explain why you think that 3D test must have greater drag ,what will increase drag if I didnt increase AoA on 3D airfoil?

Because of induced drag.

Are the aerofoils identical in profile and area? Are the measurements you have shown raw data ( they don’t look non dimensionalised).

Fred

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flight007
Topic Author
Posts: 33
Joined: Tue Jan 01, 2019 11:07 am

Re: Wind tunnel drag test

flipdewaf wrote:
flight007 wrote:
flipdewaf wrote:
do you have data for 0 degrees?

Fred

Sent from my iPhone using Tapatalk

I didnt meassure zero AoA,but I can do it..Why do you ask?

Can you explain why you think that 3D test must have greater drag ,what will increase drag if I didnt increase AoA on 3D airfoil?

Because of induced drag.

Are the aerofoils identical in profile and area? Are the measurements you have shown raw data ( they don’t look non dimensionalised).

Fred

Sent from my iPhone using Tapatalk

I use one same airfoil for both test..Numbers are in grams..

But you must know that this situation in my test do not exist in airplane real life,because lift must allways be same as weight,so lift is constant.When lift is reduced because of 3D effect -tip pressure loss ,you must increase AoA ,otherwise aircraft will descend..
All lift drag graphs are make for level flight,that mean lift must be constant,,

How you will descirbe induced drag with out incresing wing AoA?

flipdewaf
Posts: 2703
Joined: Thu Jul 20, 2006 6:28 am

Wind tunnel drag test

flight007 wrote:
flipdewaf wrote:
flight007 wrote:

I didnt meassure zero AoA,but I can do it..Why do you ask?

Can you explain why you think that 3D test must have greater drag ,what will increase drag if I didnt increase AoA on 3D airfoil?

Because of induced drag.

Are the aerofoils identical in profile and area? Are the measurements you have shown raw data ( they don’t look non dimensionalised).

Fred

Sent from my iPhone using Tapatalk

I use one same airfoil for both test..Numbers are in grams..

But you must know that this situation in my test do not exist in airplane real life,because lift must allways be same as weight,so lift is constant.When lift is reduced because of 3D effect -tip pressure loss ,you must increase AoA ,otherwise aircraft will descend..
All lift drag graphs are make for level flight,that mean lift must be constant,,

How you will descirbe induced drag with out incresing wing AoA?

It can’t be the same aerofoil surely? How does it reach across the entire test section for the 2d case and then be hanging for the 3D case?

You should try to non-dimensionalise the readings you have to lift and drag coefficients.

Is this wind tunnel at a university?

As for the climbing or descending or turning or whatever dynamic manoeuvre your trying to do in your plane it doesn’t matter, the wing produces the lift for the conditions it’s under.

Do you have any pictures of your set up?

Fred

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flight007
Topic Author
Posts: 33
Joined: Tue Jan 01, 2019 11:07 am

Re: Wind tunnel drag test

flipdewaf wrote:
flight007 wrote:
flipdewaf wrote:
Because of induced drag.

Are the aerofoils identical in profile and area? Are the measurements you have shown raw data ( they don’t look non dimensionalised).

Fred

Sent from my iPhone using Tapatalk

I use one same airfoil for both test..Numbers are in grams..

But you must know that this situation in my test do not exist in airplane real life,because lift must allways be same as weight,so lift is constant.When lift is reduced because of 3D effect -tip pressure loss ,you must increase AoA ,otherwise aircraft will descend..
All lift drag graphs are make for level flight,that mean lift must be constant,,

How you will descirbe induced drag with out incresing wing AoA?

It can’t be the same aerofoil surely? How does it reach across the entire test section for the 2d case and then be hanging for the 3D case?

You should try to non-dimensionalise the readings you have to lift and drag coefficients.

Is this wind tunnel at a university?

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It is same airfoil,just side walls of wind tunnel is adjustible,so I put walls in 2D test to touch airfoil tips.And adjust airflow speed in test section on 35m/s in both tests..

here is quote from book:
"To create a certain lift coefficient with the
airfoil section, a certain angle must exist between the airfoil chord line and the avcragc
relative wind. This angle of attack is a,,, the
section angle of attack. However, as this lift
is developed on the wing, downwash is incurred and the average relative wind is inclined. Thus, the wing must be given some
angle attack greater than the required section
angle of attack to account for the inclination of
the average relative wind. Since the wing
must be given this additional angle of attack
because of the induced flow, the angle between
the average reiative wind arid tlie remote fiCC
stream is termed the induced angle of attack,
ai."

page 66
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

flipdewaf
Posts: 2703
Joined: Thu Jul 20, 2006 6:28 am

Re: Wind tunnel drag test

Is the fact that the wall is now touching the tips affecting your readings?

You need to do multiple tests at various AoA to see if the 2d case confirms to the norms of a 2d aerofoil. Are you using a standard NACA profile? Does your data match with previous data taken, if not then you need to look at why.

Fred

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