Turbulence and wing design

[Mike-Bracknell]

A question I was pondering on a flight yesterday:

"Is there a specific design of wing that minimises the in-cabin effects of turbulence, and if so, is wing design dictated in part by these considerations?"

...and as a follow up question, "would it follow that seating at the wing root gives best comfort as a result?"

[BOAC]

Any wing section with a flat lift curve improves the ride in turbulence, but those sections are uncommon in airliners. Speed is the greatest influence we have.

Regarding 'motion' - over the wing you will just be 'jiggled' up and down mostly whereas at the ends you will probably get an 'end of a see-saw type' of motion.

[GlueBall]

The L1011-500 series TriStar featured ACS [Active Control System] which automatically deploys wing spoilers to dampen turbulence induced upsets.

[BOAC]

Clever, but presumably not able to cope with downdrafts?

[Bullethead]

Didn't the Tristar have active ailerons for gust response suppression while the spoilers were used on approach for direct lift control.

Regards,
BH.

[rudderrudderrat]

Hi BH,

Affirm. There was also RSB (recovery speed brake) on UK registered -500s which would automatically deploy at a high Mach No and required manual stowage. PFM.

[BOAC]

Apologies to MB for hijacking the thread but Bullethead - are you saying, then, Glueball is wrong? How did the ailerons work in lift control?

[rudderrudderrat]

How did the ailerons work in lift control?

When ACS detected a vertical acceleration beyond it's threshold, the ailerons would move upwards to reduce the lift at the extended wing outboard section. (-500 wing was extended to improve aspect ratio)

[Mr Optimistic]

Effectiveness

Were these 'active damping' schemes purely for passenger comfort ? If so, did they work in terms of the passenger experience ?

[WillDAQ]

"would it follow that seating at the wing root gives best comfort as a result?"

For dull rigid body mechanics reasons (as the aircraft translates up it pitches down and vice versa) the smallest turbulence upset will normally be felt at the front of the aircraft getting worse as you move back.

[rudderrudderrat]

Hi Mr. Optimistic,

Were these 'active damping' schemes purely for passenger comfort ?

I see you live up to your name - They were for wing bending relief to improve the fatigue life. A by product may have been the ride had less +ve g "up bump" - but as BOAC pointed out - the same -ve g "down bump".

[Mr Optimistic]

one lives in hope

Thanks.

[Mike-Bracknell]

For dull rigid body mechanics reasons (as the aircraft translates up it pitches down and vice versa) the smallest turbulence upset will normally be felt at the front of the aircraft getting worse as you move back.

Aha - this is something I hesitated to mention. I had a pretty good flight at the pointy end of a 757 on Thursday compared to a relatively poor flight at the rear of an A320 on Tuesday, and was wondering whether it was one of the reasons business class and first was nominally put at the front of the aircraft.

So for those of us paupers who don't normally travel in style, the best results can be seen at the front of the relevant section of cattle class, aligned with a liberal amount of praying to the gods of Bernoulli?

[Pugilistic Animus]

When the gust response analyis is being done the certification requirements mandate that the manufacturer observe the dynamic effects of an statistically alleviated gust at various stations on the airplane and around the airplane

§ 25.341 Gust and turbulence loads.

(a) Discrete Gust Design Criteria. The airplane is assumed to be subjected to symmetrical vertical and lateral gusts in level flight. Limit gust loads must be determined in accordance with the provisions:
(1) Loads on each part of the structure must be determined by dynamic analysis. The analysis must take into account unsteady aerodynamic characteristics and all significant structural degrees of freedom including rigid body motions.
(2) The shape of the gust must be:


for 0 ≤ s ≤ 2H
where—
s=distance penetrated into the gust (feet);
Uds=the design gust velocity in equivalent airspeed specified in paragraph (a)(4) of this section; and
H=the gust gradient which is the distance (feet) parallel to the airplane's flight path for the gust to reach its peak velocity.

(3) A sufficient number of gust gradient distances in the range 30 feet to 350 feet must be investigated to find the critical response for each load quantity.
(4) The design gust velocity must be:


where—
Uref=the reference gust velocity in equivalent airspeed defined in paragraph (a)(5) of this section.
Fg=the flight profile alleviation factor defined in paragraph (a)(6) of this section.

(5) The following reference gust velocities apply:
(i) At the airplane design speed VC: Positive and negative gusts with reference gust velocities of 56.0 ft/sec EAS must be considered at sea level. The reference gust velocity may be reduced linearly from 56.0 ft/sec EAS at sea level to 44.0 ft/sec EAS at 15000 feet. The reference gust velocity may be further reduced linearly from 44.0 ft/sec EAS at 15000 feet to 26.0 ft/sec EAS at 50000 feet.
(ii) At the airplane design speed VD: The reference gust velocity must be 0.5 times the value obtained under §25.341(a)(5)(i).
(6) The flight profile alleviation factor, Fg, must be increased linearly from the sea level value to a value of 1.0 at the maximum operating altitude defined in §25.1527. At sea level, the flight profile alleviation factor is determined by the following equation:


Zmo=Maximum operating altitude defined in §25.1527.

(7) When a stability augmentation system is included in the analysis, the effect of any significant system nonlinearities should be accounted for when deriving limit loads from limit gust conditions.
(b) Continuous Gust Design Criteria. The dynamic response of the airplane to vertical and lateral continuous turbulence must be taken into account. The continuous gust design criteria of appendix G of this part must be used to establish the dynamic response unless more rational criteria are shown.
[Doc. No. 27902, 61 FR 5221, Feb. 9, 1996; 61 FR 9533, Mar. 8, 1996]

[Checkboard]

Is there a specific design of wing that minimises the in-cabin effects of turbulence,

Yes - rotary wings have excellent ride in turbulence.

[Mike-Bracknell]

What I was really getting at was - "has ride performance in turbulence improved in later generation planes?" as I thought the ride on a 777 was great in turbulence, whereas the old 727 used to rattle my bones.

[grounded27]

What I was really getting at was - "has ride performance in turbulence improved in later generation planes?" as I thought the ride on a 777 was great in turbulence, whereas the old 727 used to rattle my bones.

Sure, later model aircraft provide a smoother ride as the wings are less rigid. The 777 wing allows for more flex as opposed to a rigid wing 727.. Been allways told that corperate jet's get beat up the worst for this reason, some commuters too. Stubby little wings that translate the up and down drafts at hard frequencies.

[Machinbird]

Based on personal experience, aircraft with higher wing loadings and with lower aspect ratios ride smoother in turbulence.

[CabinMaster]

Best gust response comes when you have very high wing loading and very flat lift curve slope. See: Tornado IDS or F-111.
As correctly pointed out, Gust Load Alleviation is done to drive down wing weight. As the certification requirements are looking at the upper end of gust intensity, I wouldn't expect such an aircraft to be more comfortable except maybe in really tough turbulence.

Been allways told that corperate jet's get beat up the worst for this reason, some commuters too. Stubby little wings that translate the up and down drafts at hard frequencies.

Those aircraft also have low wing loading.

[stilton]

I have never flown on any Aircraft that soaks up turbulence as well as the wonderful B727.


Rigid wing ! Have you ever watched it flap around ?



That wing was a masterpiece of design.



If you really want to find a modern Aircraft with a very bad ride in turbulence it is the B757.