Does a wing produce an appreciable amount of lift during the takeoff roll prior to rotation?

Question

I was hoping for some insight on this, perhaps also from pilots who have used AOA indicators. This because I assume that an AOA indicator is the only means to get at least an idea of how much lift is being produced.

The real question: In a crosswind, during the roll, it is friction from the tyres that keeps the aircraft tracking down the runway, since you can't crab before becoming airborne. If the wing starts to produce lift during the takeoff roll, that friction will reduce.

Is that lift significant, or is it minimal (on a tricycle-gear aircraft)? In other words, does the surface friction decrease by lots, or is it relatively unaffected throughout the ground roll?

As a follow-on, how does ground effect influence the lift that is generated during ground roll at essentially a fixed AOA until rotation?

I'm interested primarily in the business jet side of the house, takeoff weights between 20 and 35 tons, but would also like to know what the answers might be for airliners (Airbus, Boeing). I was going to exclude the aircraft that - with all due respect - "sit funny" on the ground such as the CRJ900 or the G280, but then thought that fuselage attitude might not be directly related to AOA.

Answer 1

Back to the basics: with credit to @jamesqf for the comment about soft/rough field takeoff ... "get weight off the wheels as soon as possible".

With a cross-wind take off, we leave weight on the wheels as much as possible, then rotate.

"Since you can't crab before becoming airborne"

Rudder and ailerons become effective as speed increases, even before lift off. The rudder in particular helps hold track while the wheel friction prevents side slipping. Once airborne, a bank (from the ailerons) replaces the tires! But aileron inputs should also be there during the take off roll to help hold the upwind wing down.

Generic recreational tricycle gear procedure:

Soft field: flaps down/yoke further back (increases AOA).
Cross-wind: flaps up (lower AOA) until takeoff, ailerons into wind, rudder (generally) away as needed, "brisk" rotation.

Tricycle gear aircraft are usually designed for minimal lift before rotation, because this configuration minimizes drag, shortening the take-off run.

But even with the same plane, flap/slat configuration and trim/yoke position can have a great influence on the amount of lift created before rotation, based on a given set of conditions.

Ground effects will also be much less at lower AOA.

As "25-30 ton business jets" tend to be rather expensive, one may seek training on that specific model for more definitive detail.

Airfoil specific Lift vs AOA information is also available at airfoiltools.com.

Answer 2

An 'appreciable amount of lift' is subjective, but with thanks to @RobertDiGiovanni's answer above, I was able to work out some example values, shown below. (I used the link to airfoiltools.com to get CL data to approximately solve the lift equation for a B737 - exact type not specified - at various speeds during the ground roll).

At an assumed constant (until rotation) AOA of 0 degrees (and thus a CL of 0.2), lift at 80 knots will be around 2000 Kg, and aprx 6500 Kg at 147 knots (147 is the Vr for a B737-500 at MTOW - 60 Tons - per b737.org.uk)

In this case, until just before rotation and the associated large increase in AOA, lift reaches a little over 10% of the weight of the aircraft.

At an AOA of 5 degrees (CL 0.6), I find lift to be 6000 Kg at 80 and 23000 Kg at 147 knots. That is a lot more significant in relation to MTOW.

Ultimately, I am looking into how the risk of lateral runway excursion on a 'slippery' crosswind takeoff evolves over time, from brake release to rotation. I should perhaps have included this info in the original question. Good unambiguous questions are quite hard to ask.

Answers are likely to remain approximate, as I've not found airfoil specifics for "my" aircraft.

The trim setting (usually set for aprx V2 for engine-out handling reasons) can cause unwanted rotation without pilot input, and may indeed need counteracting, especially in the above circumstances.

Edit: the airfoiltools.com data does not specify the flap configuration, these results may be without flaps - which is not the way most aircraft are configured for takeoff.

Answer 3

While on the ground run of an airplane during takeoff while still on the ground, especially a tricycle gear airplane, the angle of attack indicator will not move.

This does not mean the airplanes wings are not producing lift. Remember the lift formula. $L = C_{L} * \rho\ * SA * V^2$

The angle of incidence connecting the wings to the fuselage is most likely positive giving a positive $C_{L}$. However, the angle of attack indicator, which is typically mounted to the side of the fuselage is showing no lift.

We know the wings are producing lift because of the correct way to takeoff during a crosswind. The pilot must position the ailerons such that the wing is pushed down on the upwind side to prevent the wind from picking up the wing. We are also taught to keep flying the airplane once it lands and to positions the ailerons to keep the wings down.

A lot of jets have spoilers that will extend in the event of an aborted takeoff or during the landing rollout. The idea is to reduce the lift produced by the wings and to push the wings down allowing the tires to grip the asphalt better.

In the jets that I fly, we are taught to push forward on the yoke before VR so that the nose doesn't artificially rise before it is time. This is only possible if lift is being created on the wings before rotation. It also tells us the elevator is creating a nose down force during the takeoff roll.