Airfoils of one family

[funfly2]

Hi,

What makes the airfoils being of same family?
Have they the same thickness, the same chord or same camber?

Thanx

[Ollie]

"What makes the airfoils being of same family?"

The purpose of the designer.

"Have they the same thickness, the same chord or same camber?"

See for your self:
http://www.charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm
http://www.ae.uiuc.edu/m-selig/uiuc_lsat.html
Download Vol. 4

Other questions?

[funfly2]

Thanks Ollie for your response.
I regret but I still can't get it...

Maybe I should reformulate my question:
What should airfoils have in common to be considered being of the same family?

[Ollie]

"What should airfoils have in common to be considered being of the same family?"

The answer is "purpose"!

In one family of airfoils, it is an airfoil that is different from others (thickness, camber, reynolds number minimum use, etc.) in the same family. The purpose of the wing (like thermal soar, size, slope aerobatics, etc) is using airfoils in places in the wing span. The root of the wing uses one member of the family and another for the tip and and others used between.

Read it again till to you get it:
http://www.charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm

[funfly2]

Okay Ollie, I think I've got it now.
If I understood it refers to a flying surface’s different sections (airfoils), as long as they belong to the same flying surface they are of the same family… right?

Well, my initial question came up because I’ve read following in Model Aircraft Aerodynamics by Martin Simons:
…The more cambered airfoil of a family tends to reach a higher value of CL before stalling than the less cambered, but the geometric angle of attack at which the stall occurs is earlier. Only if the angle of attack is measured from the absolute zero does the more cambered profile stall later.
Does it mean that when flaps are deployed the new profile obtained still will be within the same family and make this part of the wing stall at a lower geometric angle of attack despite the increased lift coefficient CL?

[Ollie]

There is more.

Let's say that you design an aerobatic model. You might quick roll rate. So you make the tips light with wing taper of 0.5. The size and weight of the model and the airfoil family gives minimal air speed at stall. The minimum reynold numbers along the wing span from root to tip. Then pick the thickness tip airfoil and another root airfoil thickness of a zero camber family. You might pick the maximum thickness along the chord position for the tip airfoil and another chord position for the root airfoil.

[Arvid K. Gromhol]

"The more cambered airfoil of a family tends to reach a higher value of CL before stalling than the less cambered, but the geometric angle of attack at which the stall occurs is earlier. Only if the angle of attack is measured from the absolute zero does the more cambered profile stall later."

Ollie,
Could you please explain the difference between "geometric angle of attack" and "angle of attack measured from the absolute zero"?

[funfly2]

According to Martin Simons:
Geometric AoA: is the angle between the chord line and the flight direction.
Absolute AoA: is the angle between the chord line and the flight direction plus the negative angle at which the lift is zero.

For symmetrical profiles the geometric AoA is the same as the absolute.

[Ollie]

The name of an airfoil with flaps is a sub-family of airfoils. Each position of the flap is, depending on flap angle degree, is a member the sub-family with one name plus the angle number. One name plus one flap angle is one member with its only unique set of zero lift angle, polar curves, etc. A wing with flaps is airfoil family along the span and a sub-family of flap angle for each airfoil along the flap.

For a sub-family see SD7037 and how the flaps are measured for polars, see:
http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1078

[Ollie]

"angle of attack measured from the absolute zero"?"

Avid,
I think you mean, at zero lift the angle of attack is some number of degrees. For some airfoil with zero camber when the lift is zero and the angle of attack is zero. For a positive camber airfoil and at zero lift, the angle of attack is negative.

[funfly2]

For a sub-family see SD7037 and how the flaps are measured for polars, see:
http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1078They show a lot different Reynolds numbers but I found the Re 199100 having with and without flap alternatives.

Without flap:
CL 0.42 at 0 deg. AoA (alpha)
max CL 1.28 at 11 deg. AoA (geometric) and 14 deg. (absolute)


With 5 deg. flap:
CL 0.58 at 0 deg. AoA (alpha)
max CL 1.33 at 9.5 deg. AoA (geometric) and 15.5 deg. (absolute)

The flap increases CL and the absolute stall angle, but decreases the geometric stall angle.

[Ollie]

SD7037 flap is 0:
http://www.nasg.com/afdb/show-polar-e.phtml?id=627
http://www.nasg.com/afdb/show-polar-e.phtml?id=624
http://www.nasg.com/afdb/show-polar-e.phtml?id=625
http://www.nasg.com/afdb/show-polar-e.phtml?id=626

Flap at 5 degrees:
http://www.nasg.com/afdb/show-polar-e.phtml?id=615
http://www.nasg.com/afdb/show-polar-e.phtml?id=619
http://www.nasg.com/afdb/show-polar-e.phtml?id=606
http://www.nasg.com/afdb/show-polar-e.phtml?id=609

Flap at 10 degrees:
http://www.nasg.com/afdb/show-polar-e.phtml?id=613
http://www.nasg.com/afdb/show-polar-e.phtml?id=617
http://www.nasg.com/afdb/show-polar-e.phtml?id=605
http://www.nasg.com/afdb/show-polar-e.phtml?id=610

[funfly2]

Yes Ollie, but those have different Reynolds numbers, that's why I chose only two with the same Re 199100 for comparation purpose:

Flap zero:
http://www.nasg.com/afdb/show-polar-e.phtml?id=621

Flap at 5 deg.
http://www.nasg.com/afdb/show-polar-e.phtml?id=606

At same Re 199100

[Ollie]

"Yes Ollie, but those have different Reynolds numbers, that's why I chose only two with the same Re 199100 for comparation purpose"

A wing flies at at one speed if it has a range of reynolds numbers if it has taper. In any flight, a wing has a range of air speed have a range of reynolds numbers. A flight will change the flap range.

How can you use your comparation without consider many other comparations?

What is your purpose? I will be silent until you have better questions.

[funfly2]

Well, I thought that the best way to compare two different airfoils was by using the same Reynolds number.
I understand that airfoils in a tapered wing have slightly different Re, but my question was mainly to know what "same family" meant.
Now I wonder:
Assuming the same chord lenght does a cambered airfoil stall at a lower geometric AoA than a symmetrical one regardless their family?

[Ollie]

"Assuming the same chord lenght does a cambered airfoil stall at a lower geometric AoA than a symmetrical one regardless their family?"

I can't help you.