Lift Question

[Typhoon]

Hi there,

Just a quick clarification question. I understand lift always acts perpendicular to the relative airflow (aka velocity of the aircraft). However, does it always act perpendicular to the lateral axis as well?

Thanks.

[loopa]

Someone can correct me cause I might be wrong.

But I believe the lateral axis to be the dimension of which you change position about (primarily pitching). And this dimension is a line connecting the wing tips. Say if the wings have a di-hedral effect, the lateral axis would still be horizontal. However, with a dihedral wing, the angle the lift vector makes with the wings would be different than the angle with the lateral axis.

That's my 2 cents... maybe somebody has a better explanation

[Typhoon]

Hmm I see what you are saying, if the wing isn't dihedral in nature the angle the lift forms with the lateral axis and wing should be the same (theoretically?). But with a dihedral wing, the lift will be perpendicular to the wing, but not necessarily the lateral axis?

[Big Pistons Forever]

Hmm I see what you are saying, if the wing isn't dihedral in nature the angle the lift forms with the lateral axis and wing should be the same (theoretically?). But with a dihedral wing, the lift will be perpendicular to the wing, but not necessarily the lateral axis?

Considering the small amount of dihedral in most aircraft, particularly high winged ones, I am struggling to understand how this factor could be of any practical (or even unpractical) use to a pilot.....

[into the blue]

The very fact that the lift of a wing with a dihedral is not perpendicular to the lateral axis of an airplane is what improves lateral (roll) stability of that airplane.

Example:
Let's say the airplane has a 3-deg. dihedral. It is momentarily upset by a moderate chop and rolls 10 deg. right (the angle formed between the airplane's lateral axis and the horizon, of course). Now the lift produced by the right wing is angled at 7 deg. relative to the absolute vertical, while the lift produced by the left wing is angled at 13 deg. For the sake of simplicity we may presume that in this situation both wings still produce exactly the same amount of lift; however, we cannot say the same about the vertical components of lift. Because the vertical component of lift for the left wing is less than that of the right wing, the wings will "want" to roll back to the left. Certainly, it's not the only factor that determines the degree of lateral stability of any given airplane, but it's important nevertheless.

[Typhoon]

The very fact that the lift of a wing with a dihedral is not perpendicular to the lateral axis of an airplane is what improves lateral (roll) stability of that airplane.

Example:
Let's say the airplane has a 3-deg. dihedral. It is momentarily upset by a moderate chop and rolls 10 deg. right (the angle formed between the airplane's lateral axis and the horizon, of course). Now the lift produced by the right wing is angled at 7 deg. relative to the absolute vertical, while the lift produced by the left wing is angled at 13 deg. For the sake of simplicity we may presume that in this situation both wings still produce exactly the same amount of lift; however, we cannot say the same about the vertical components of lift. Because the vertical component of lift for the left wing is less than that of the right wing, the wings will "want" to roll back to the left. Certainly, it's not the only factor that determines the degree of lateral stability of any given airplane, but it's important nevertheless.

Awesome, that makes perfect sense. Thanks a lot!

[Av_Av]

I understand lift always acts perpendicular to the relative airflow (aka velocity of the aircraft). However, does it always act perpendicular to the lateral axis as well?

It all depends on how you define your directions -- whcih is entirely a matter of convenience. The aerodynamic force acting on the aircraft can generally be considered to be a single force, but we divide it into directions (components) for our own convenience. The component of aerodynamic force acting parallel to the relative airflow is called drag. The component perpendicular to the RA and lateral axis is lift. The component parallel to the lateral axis, which we usually keep to zero, doesn't really have a name. You might call it "sideforce", or you might include it in lift -- defining your directions so that any force perpendicular to the RA is lift.

In practice, though, we usualy include this component as drag -- think of a forward slip in which we create a sideforce in order to increase drag and therefore increase our descent rate. On the other hand, if you fly a lot of aerobatics, you might think of the sideforce as lift -- in knife-edge flight, for example.

As for the effect of dihedral, in zero-slip flight (regardless of bank angle), the lateral components of lift from each wing cancel out, so the net force is still perpendicular to the lateral axis. Even in slipping flight, the amount of lateral "lift" due to dihedral would be very small. The main benefit of dihedral is that the total (perpendicular to the lateral axis) lift on each wing is different in a slip (due to the wings having different angles of attack), so that the aircraft tends to roll away from the slip.

[SAA340-600]

Lift is not perpendicular to lateral axis. Only to local relative airflow ( i.e sectional airflow for that cross section of the airfoil...due to circulation. With increasing circulation will come increasing upwash and downwash and hence the angle of the lift vector will tilt backwards or or forwards in relation to the lateral axis with varying AOA). The dihedral effect also brings up a great point.