# Centre of lift in a swept wing?

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#### rtfm

##### Well-Known Member
Hi,
In a rectangular wing, finding the chord-wise centre of lift is simply a matter of finding the 25% position of the chord. However, in a swept wing, is it as simple as finding the 25% position of the MAC?

Duncan

#### Aesquire

##### Well-Known Member
Assuming no twist and moderate taper it's still about @ 25% mean chord. A little more math with triangles but same idea.

Iirc. Could be wrong.

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
...just don’t be misled by the static margin/CG figure it gives, which doesn’t take the other aerodynamic moments of the aircraft into account.

#### Aerowerx

##### Well-Known Member
Get a copy of Dan Raymer's "Simplified Aircraft Design for Homebuilders" He answers this very question.

Or you could just Google it. Lots of info out there in Cyberworld.

#### nickec

##### Well-Known Member
In all aircraft, only extensive flight testing can really nail down the practical limits of center of gravity - and the wisdom of using a particular planform.

As in so many things in life on earth, the edges are the trickiest things to define or navigate. Then again, the edges are sometimes the most interesting places - where new knowledge and power appears. The edge has risks AND rewards.

#### Dana

##### Super Moderator
Staff member
The center of lift is not necessarily at 25%. It moves with AOA. That said, when using airfoil data referencing the quarter chord point, yes, you use 25% of MAC just like any other planform.

#### TFF

##### Well-Known Member
It’s all about calculating, but is the wing swept or tapered?

#### Aerowerx

##### Well-Known Member
A simplified approach as given in the Raymer book.

1. Draw a scale drawing of the tapered swept wing.
2. At the tip, extend a line backwards equal to the root chord.
3. At the root, extend a line forwards equal to the tip chord.
4. Draw a line from the ends of these two lines.
5. Connect the quarter chord point of the root to the quarter chord point of the tip.
6. Where the lines from step 4 and step 5 cross is the center of lift. The chord at this point should also be the MAC.

#### Norman

##### Well-Known Member
HBA Supporter
A simplified approach as given in the Raymer book.

1. Draw a scale drawing of the tapered swept wing.
2. At the tip, extend a line backwards equal to the root chord.
3. At the root, extend a line forwards equal to the tip chord.
4. Draw a line from the ends of these two lines.
5. Connect the quarter chord point of the root to the quarter chord point of the tip.
6. Where the lines from step 4 and step 5 cross is the center of lift. The chord at this point should also be the MAC.
No. For this method you need two spanwise lines: one at 25% chord and another at 50%. Where the diagonal crosses the 50% chord line is the centroid of the wing and the spanwise location of the mean chord so draw a chord line through the centroid, that's MAC (to a first approximation) and the aerodynamic center of the wing is at 25% of that line.

AC and center of lift are totally unrelated but the OP meant AC

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#### Sockmonkey

##### Well-Known Member
Is someone thinking of doing a rear delta tandem?

#### Aerowerx

##### Well-Known Member
No. For this method you need two spanwise lines: one at 25% chord and another at 50%. Where the diagonal crosses the 50% chord line is the centroid of the wing and the spanwise location of the mean chord so draw a chord line through the centroid, that's MAC (to a first approximation) and the aerodynamic center of the wing is at 25% of that line.

AC and center of lift are totally unrelated but the OP meant AC
Thanks, Norm. I was working from memory, I should have got out my old beat up copy of Raymer when I typed that.

#### Bigshu

##### Well-Known Member
HBA Supporter
Is someone thinking of doing a rear delta tandem?
Yes, but a pusher version. For the twin, it will be two buried engines with counter rotating props. The singe engine version will be much like the Mini-Imp, or Teal designs, but using something other than a dynaflex to handle the torsional vibration on the driveshaft. I might actually start with a two seat side by side a'la Nomad or Cloudster II or Big Dipper. I'm quite taken with the idea of that LDA-1 as a tandem wing rather than a straight up canard. This discussion has been great for fleshing out the product line. I'll have a single seater, two seat LSA, four seat LPA (or whatever the FAA desides to call the performance based heavier LSAs) four seat twin and six seat tandem wing hauler. Some models will be available on floats as well, although once I figure it out, I've also got a SES design in the napkin drawing stage.

#### trimtab

##### Well-Known Member
All other things being unchanged (airfoil, twist, etc), because lift is directly proportional to the area,

...the center of lift will be at the lateral station of the geometric centroid. This doesn't include interference and tip effects. But if the center of pressure for a given Re and AoA is, say, 28%, then it will be located at 28% of chord at the lateral station that coincides with the geometric centroid. Because the wing can generally be thought of as a trapezoid in a lot of cases, it becomes straightforward to make this crude estimation:

Of course, the fore/aft (x) location of the actual geometric centroid is found on a line between the midpoints of (a) and (b). It does not matter how far swept the wing is for the model calculation as long as it has two parallel sides

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#### Bigshu

##### Well-Known Member
HBA Supporter
Maybe like so?
I prefer your low front wing, high delta looks. I need to post some art showing what I'm going for. It might break too many rules. This design is pretty attractive for a couple of my design criteria, but won't the downwash from the front wing negatively affect the delta's airflow?

#### Sockmonkey

##### Well-Known Member
I prefer your low front wing, high delta looks. I need to post some art showing what I'm going for. It might break too many rules. This design is pretty attractive for a couple of my design criteria, but won't the downwash from the front wing negatively affect the delta's airflow?
One of these then.

As long as there's enough vertical space between them, it shouldn't be a problem. Heck, Payen got away with having them on the same level.