Downwind Turning Effect on Takeoff - 777
 

Actually, this may apply to all big twins(and even smaller ones as well according to Boeing). Has anybody noticed during a strong crosswind takeoff, the need to use rudder input in one direction too maintain centerline during the initial takeoff roll transitioning to the requirement for opposite rudder input at higher speeds(classic weathervaning rudder input) to maintain the centerline?

I recently read an article about the 777x in Aviation Week which stated.....

"Another area of test focus will be whether the 777-9 has the potential for increased susceptibility to a phenomenon known as Downwind Turning Effect on Takeoff. First encountered during initial tests of the baseline 777, the effect occurs on the ground in strong crosswinds and manifests itself as a thrust-induced yawing moment in the windward direction. The effect, which increases with thrust, occurs only at the start if the takeoff run at speeds up to 70 knots and appears counterintuitive to pilots as it works against the aircraft’s natural aerodynamic weathercock yawing moment."

A few weeks later I went to a Boeing symposium and they discussed what they termed the Anti-Weathervaning Effect.

This is the effect according to Boeing.....

"High crosswinds at low airspeeds(less than 80 knots):
- Air is flowing across nacelle inlet
- A force is imparted on the inlet as air is drawn into the nacelle(from ‘across’ to ‘in’)
- As the nacelles are ahead of the main gear the force generates a yawing moment away from the wind.
- At these speeds, aerodynamic force on the vertical tail is small so the resulting yawing moment into the wind is small
- At low airspeeds, the yawing moment generated by the inlet flow outweighs the yawing moment of the vertical tail so the airplane turns away from the wind

At higher airspeeds:
- Airflow is more aligned with the inlet
- Force on the vertical tail increases
- Vertical tail yawing moment outweighs engine yawing moment and airplane weathervanes"


https://cimg1.ibsrv.net/gimg/pprune....c9c4343aa1.png

Also see
Safety First #27 pages 8-16
https://safetyfirst.airbus.com/app/t...y_first_27.pdf

Yes. I Noticed when I first started flying the 777 that required rudder inputs at low to medium speeds were sometimes counter-intuitive or a bit random. It still behaves “classically” on rotation.

While the Safety First article posted by safetypee doesn’t address this issue, the downwind turning effect does also occur on big Airbus twins. If your company has an Airbus relationship, speaking to their Flight Test department may be able to provide further information.

Nothing new, conceptually, albeit a new situation in which the effect is noticed. I must confess to not having read about it previously.

Any time you have a significant change in flow direction you will get a force generated.

Think about how a wing turns the airflow down and a force (lift) is directed up.

Think about those aircraft (eg piston to turbo conversions especially) with SAS add-ons to fix a resulting long stab problem. At low speed, high thrust (eg missed approach), the prop position (typically repositioned forward for CG concerns) provides a significant destabilising up force at the prop disk. Same thing can apply for underslung fans during the miss spin up.

Looking at the nacelle story, if the wind is from the L/R (ie crosswind) you get a lateral yawing moment and yawing turn due to the offset until the normal directional aspects get to take control.

It was called intake momentum drag in the Harrier. Sideslip needed to be kept to a minimum in the transition from Jet-borne to wing-borne and vice versa, with AOA at 8. There were pedal shakers but more importantly a "wind vane" on the nose in your line of sight to assist in controlling the sideslip.

Interesting. So to see if I have it clear... if in sideslip with relative wind coming from the right, intake accelerates air to the right and therefore produces force to the left, which is ahead of CG so it yaws to the left, which is away from the relative wind (i.e., anti-weathervaning) and therefore unstable in yaw. Is that it?

Vessbot, correct, it could then cause roll and the "puffer jets" on the wingtips would run out of ability to control te roll and instead of having 21000lb (potentially) thrust holding you up you had 21000lb throwing you at the ground!

Where's JF when you need him ? Bit late to ask him now, I guess.

intake accelerates air to the right and therefore produces force to the left

More a case of the airflow's coming from the right, gets sucked into the intake and, in so doing, turns a corner. The action of turning the corner produces a change in momentum and the lateral component of the force provides a yawing moment to the left and, hence, a problem for P/O Bloggs.

I.e., accelerates air to the right

Acceleration is a change in velocity, meaning a change in speed or direction. (When you take into account mass, it's also a change in momentum as you said.)

Don't think so.

The air, coming from the right, is turned aft ie no acceleration back toward the right (unless you are considering the decelerating component). It is the turning from moving left to aft that generates a force with forward and left components and a left yawing moment as a consequence ?

Interesting. The 787 FCTM indeed mentions this phenomenon (without going into detail), but personally, I have never noticed it. Maybe I'm too ham-fisted (ham-footed)? :}

It's either turned, or not turned. If it's turned, it's accelerated.

I'm seeing your misconception. ​​​​​You've gotta think of the change in direction, not just the final direction. You're correct that the final direction has no rightward component (it's just going straight down the intake, aka "aft") but that doesn't matter. It was originally going left, and now it's not, which is a change (i.e., acceleration) to the right.

Semantics ...

Well I don't know what else to tell ya. That's what acceleration means, and you took exception to it...