# Airfoil "Actual Location" of Aerodynamic center...(How to find it)

### Help Support HomeBuiltAirplanes.com:

#### clanon

##### Well-Known Member
Just found this presentation making (i guess) a relationship between real AC and location of Thickness on the MAC...
For the 23012 and the 64212

Is it the t/c ratio @30% on the 23012 moving the Actual Ac ahead and viceversa for the 64212 ?
Is there an easy way of calculating this for a novel airfoil ?

Thnkz

Last edited:

#### Norman

##### Well-Known Member
HBA Supporter
There's a spreadsheet to find the X offset here. You just need a couple of data points from the polars. The Y offset is a different matter and I don't know how to calculate it but it's a much smaller stability factor.

Last edited:

#### clanon

##### Well-Known Member
Thankz ; Norman.

CL for Alpha=4:
CM for Alpha=4:

Is that an arbitrary AoA ? 4 degreesonder:

PS:Is this the 0.25 - Cm/Cl formula...?
Thankz

Last edited:

#### Norman

##### Well-Known Member
HBA Supporter
The formula is attributed to John Roncz so I assume it's OK. I haven't checked it out. I had a conversation today that this would be applicable to so now I have a reason to run the spreadsheet but I'm not home this weekend so can't do it until Tuesday at the earliest. On a side note: I had a formula that turns Cm into C.P., since the center of pressure on a zero Cm airfoil is stationary within the linear range that could be used for crosschecking. That should be on a post-it note on one of me desks. If the cat hasn't run off with it I'll post it here next week.

#### Aerowerx

##### Well-Known Member
This PDF may be useful to you. All those math symbols just put me to sleep:tired:
www.aerostudents.com/files/flightDynamics/theAerodynamicCenter.pdf
I find things like this rather interesting. A student can graduate knowing how to crunch all the numbers to make an airplane fly, but can they actually make a plane fly?

I am a firm believer that, no matter what discipline you are in, you should educate yourself on the practical application of the fundamentals. In other words, an aero student should be required to take sailplane lessons, at least up to where they would solo. I say "sailplane" because they often fly on the edge of the flight envelope without an engine to get them out of trouble.

#### Birdman100

##### Well-Known Member
I find things like this rather interesting. A student can graduate knowing how to crunch all the numbers to make an airplane fly, but can they actually make a plane fly?

I am a firm believer that, no matter what discipline you are in, you should educate yourself on the practical application of the fundamentals. In other words, an aero student should be required to take sailplane lessons, at least up to where they would solo. I say "sailplane" because they often fly on the edge of the flight envelope without an engine to get them out of trouble.
Why do you think "crunching numbers" is not practical? Just because derivatives, functions, integrals and so on are not required in everyday life of the average person doesnt mean they are not required and practical in airplane design.

Clanon,

actual position of ac is rarely important. It is often from 24-27%, but in most cases we assume it in 25% and vast majority of Cm data and charts refer to that point. The main reason we adopted 0.25 as AC location is to avoid mess with uncertainty of the real ac position. 25% is not an assumption actually, but an agreement. One could choose any other point for which Cm would be determined, but 25% is from several reasons most logical and obvious choice.

#### WonderousMountain

##### Well-Known Member
I agree, there needs to be a balance between the virtual world and the practical,

That may be 80/20, but it's still better to have both.

#### Aerowerx

##### Well-Known Member
Why do you think "crunching numbers" is not practical? Just because derivatives, functions, integrals and so on are not required in everyday life of the average person doesnt mean they are not required and practical in airplane design.
I didn't say that crunching numbers wasn't important!

I said that is important to know the fundamentals of your profession on a practical application level.

Many people know all the theory and equations, but can't sit down and actually build something that works. For example, when I got my BSEE there were guys in my class that didn't know how to solder.

#### Topaz

##### Super Moderator
Staff member
Log Member
... Actual position of ac is rarely important. It is often from 24-27%, but in most cases we assume it in 25% and vast majority of Cm data and charts refer to that point. The main reason we adopted 0.25 as AC location is to avoid mess with uncertainty of the real ac position. 25% is not an assumption actually, but an agreement. One could choose any other point for which Cm would be determined, but 25% is from several reasons most logical and obvious choice.
This. The other problem is that AC tends to move about a bit with angle of attack, so if we were to use AC instead of the 0.25c convention, stability and control equations would have a different tail arm for each alpha. Could be done with spreadsheets or a program, but it's a lot easier to just assume 0.25c and have done. The error bars in the rest of the design process will largely engulf any regular variation in moment about the 0.25c point, except for big ones like flap deflections.

As for practical versus theoretical argument, IMHO, a lot of really great airplanes have been designed by engineers that didn't know how to make the parts or even fly the plane, so I don't think a "requirement" is needed. By the same token, knowing how the parts or made or what it's like to fly an airplane can only give an engineera better perspective. More education in your field is always a good thing, I believe.

#### clanon

##### Well-Known Member
Clanon,

actual position of ac is rarely important. It is often from 24-27%, but in most cases we assume it in 25% and vast majority of Cm data and charts refer to that point. The main reason we adopted 0.25 as AC location is to avoid mess with uncertainty of the real ac position. 25% is not an assumption actually, but an agreement. One could choose any other point for which Cm would be determined, but 25% is from several reasons most logical and obvious choice.
Wouldn't be nice to have a tubular spar right on it (Bull's eye).
For all the forces combined , i mean. (structural strength)...onder:

#### Norman

##### Well-Known Member
HBA Supporter
Wouldn't be nice to have a tubular spar right on it (Bull's eye).
For all the forces combined , i mean. (structural strength)...onder:
Nope. By far the largest force the spar sees is bending and a round tube is not efficient at all for that. For a given height and length an I-beam will bend much less than a tube under the same load and it will also hold more weight before it fails. A D-tube consisting of a leading edge skin and an I-beam is the simplest way to get both the bending stifnes of an I-beam and the torsional rigidity of a tube.

#### Norman

##### Well-Known Member
HBA Supporter
The choice of 25%c as the aerodynamic center isn't for convenience or because it's too hard to find the actual AC. It's because thin airfoil theory predicts that it will always be at 25%c and polars are confusing enough already. Unfortunately real airfoils aren't "thin" and air has viscosity. Once you have thickness and a boundary layer thin airfoil theory starts to deviate from reality but for airfoils less than 12% thick and airplanes with tails it's close enough but for a flying wing with an 18% thick airfoil that 1 or 2 %c can make a big difference.

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
I agree, there needs to be a balance between the virtual world and the practical,

That may be 80/20, but it's still better to have both.
I prefer 100/100. Both/and.

#### Birdman100

##### Well-Known Member
I said that is important to know the fundamentals of your profession on a practical application level.
Yes, and fundamentals of aircraft design is math. Of course it is desirable to have wide spectrum of knowledge (especially the ones related to field), but in a todays highly specialized world aerodynamicist doesnt need to know how to weld for example. Again, it would be probably good for him to know that as well, but that is not a professional requirement.

Homebuilding is a whole another story. We are much like aviation pioneers - we have to be pilots, engineers, welders, craftsmen, managers... - all in one; but because of that we are not building boeing but rather something that often resembles Bleriot XI - well, it depends on the overall skill of course.

As for practical versus theoretical argument, IMHO, a lot of really great airplanes have been designed by engineers that didn't know how to make the parts or even fly the plane, so I don't think a "requirement" is needed. By the same token, knowing how the parts or made or what it's like to fly an airplane can only give an engineera better perspective. More education in your field is always a good thing, I believe.
Exactly. I remember when I run across Job opportunities in Scaled Composites few years ago; among requirements were formal education titles (Msc, Bsc in engineering..), proficiency in 3D cad, experiences in CFD, and so on; and among DESIRED were pilots licence and airplane homebuilding experience.

Wouldn't be nice to have a tubular spar right on it (Bull's eye).
For all the forces combined , i mean. (structural strength)...
As Norman said tube is not good in bending as I-section is. A lot of wasted material in places we dont need it. Well, if we were limited to put single structural element in the wing, tubular section would be probably the one to choose; however in reality we can combine several structural elements to make an efficient composition. I-beam is good resisting bending, and rigid D-box or skin over entire wing is very good in torsion.

#### clanon

##### Well-Known Member
Would'n then , having the I section on the right CP center and the the D section ahead of it , make some structural gain. For the Aero forces anyway ? For a Monospar wing...onder:

#### clanon

##### Well-Known Member
Double post.(Edited)

So the airfoil shape could be Taylored ...using what tool ?
To keep the Aero Center on check.
Is this this pic from http://www.pprune.org/ right ?

Last edited:

#### Birdman100

##### Well-Known Member
Would'n then , having the I section on the right CP center and the the D section ahead of it , make some structural gain. For the Aero forces anyway ? For a Monospar wing...onder:
Thats the way many wings are built. Infact, you cant have spar positioned at CP, as CP is dependant on AoA or CL if you will (not a fixed point). As you increase AoA, CP moves forward (that for airfoils with positive camber and negative Cm). In some extreme cases when flying at low AoA (high speed) CP goes back close to TE or even beyond... these are cases with huge torsion acting on wing; you see, we cannot fight that issue (torsion) by putting spar at any particular position - thats why we assume spar caries no torsion load, then optimize it for bending, and use other structural elements to bear torsion.

one interesting specific example was Daedalus human powered aircraft. Main structural element of its wing was tubular carbon section - it carried both bending and torsion, and yes it was position in CP (which was somewhere at 30-35%, exactly where max airfoil thickness was). However that particular airplane was built for single purpose, to set distance record in human flight, and it was intended to fly in specific conditions - at optimum AoA (around 3-5 degs), and Cl (around 1.1-1.4).

"real" airplanes have to be more operational and to have some useful flight envelope.

#### clanon

##### Well-Known Member
Something like this

However that particular airplane was built for single purpose, to set distance record in human flight, and it was intended to fly in specific conditions - at optimum AoA (around 3-5 degs), and Cl (around 1.1-1.4).

"real" airplanes have to be more operational and to have some useful flight envelope.
PS: Wouldn't a very "humble" flying specs Ultralight fall very close to those specs...?

#### Attachments

• 74.4 KB Views: 126