How does reducing speed in turbulence improve the ride?
 

On the 777, I noticed that a lot of guys wind down the speed when there is light to moderate turbulence. They claim that by reducing the speed, the ride will be smoother. I'm not talking bout turbulence penetration speed here as that deals with severe turbulence.

Isnt ride relative to wing loading? In this case, wouldn't it be the same as flying through the rough weather with the current airspeed rather than winding it down? In fact since speed has no relation to wing loading, I would think that it would be much better to fly faster so as to leave the turbulent patch quicker.

I would be grateful if someone could provide me with an explanation and not use the "fast car over speed hump" analogy! Thanks! :)

- ah well! Don't expect too many replies then.

In our L1011, we normally cruise at M.84.
We do not slow down for light/moderate turbulence.
Heavy turbulence...we slow to the M.82-.84 recommended speed, target M.83

Turbulence is not heavy, it is severe or extreme.

If one views turbulence as being a short distance wind shear interaction with the aircraft, then the speed hump analogy is pretty useful. In the same manner that a car can reduce the vertical acceleration profile by running over the hump slower, rather than faster, the aircraft can adopt a similar strategy to achieve a similar result. This is driven by the desire for a comfortable ride for the occupants.

The turbulence penetration scenario looks at severe situations and endeavours to schedule a balance between certification structure reserves (at the faster end) and loss of control (at the slower end).

Wing loading is useful for comparing the likely ride qualities of one aircraft against another. Encounter speed is more useful for considering variation in accelerations on the one aircraft.

They are right. I don't want to get too technical, but the 'delta g' you get for a given vertical gust is proportional to:

Gust velocity * Aircraft velocity * wing lift curve slope / Wing loading

Everything else being equal reducing air speed reduces delta g on a one for one percentage basis. Increasing airspeed may reduce the exposure time but it will be a rougher ride for a shorter time. This is also why recommended speed in turbulence is lower - the loads on the wing are reduced.

If anyone wants a more detailed explanation just ask http://images.ibsrv.net/ibsrv/res/sr...lies/wink2.gif

Turbulence is caused by vertical gusts - that means changing vertical components of wind - updrafts and downdrafts. Because your aircraft has inertia, these changed vertical components of the wind will alter your wing's angle of attack, and consequently your lift. This is experienced in the aircraft as positive and negative g-forces.

For a given change in angle of attack, the change in lift is proportional to the true airspeed squared. That means, by reducing speed, you reduce the changes in lift produced by the given vertical gust, and consequently the shaking and rattling.

As bfisk and others have pointed out, it's all related to V in the lift formula:

CL x ½ Rho x V² x S

eg double the speed, 4 times the effect of the bumps.

Not quite true I'm afraid, because if you double the speed you halve the AoA change for a given vertical gust velocity (and therefore halve the CL change) so if you double the speed you double the effect of the bumps not 4 times.

OK...If you keep the same speed, the moderate turbulence has a tendency to remain exactly the same.
Reduce the speed..turbulence disapears...nobody knows exactly why.

PS
For light turbulence, do not reduce speed, just switch the "Seat Belts" on.
Cabin Servise stops about the same time the light turbulence stops...

question please
 

All of the above does use put a mass factor ..
Say if your B74 had less mass, say was 100 tonnes lighter would the effect be the same?

The math tells me that the heavier the aircraft the less the effect at the SAME velocity..yes?

Answer: from posted link in the next post
"So when operating at significantly lower weight (and thus lower wing loading) an encounter with a vertical gust at a particular flight speed will induce greater accelerations than when operating near MTOW, which obviously affects choice of speed in turbulent conditions. "

Was this ever indoubt?

Clive,
Thanks. I'll have a ponder over that at FL350 tomorrow! :ok:

Greenguard,
Explained here:
Wind shear and turbulence (xhtml w3c 12/09)
About half way down: "Effect of Speed".

Capt. Bloggs

OK - Table 6.1 of that reference you posted says it all nicely

Oh, and the bit on the effect of speed does as well so long as one remembers that I was talking about the increment in g above 1.0, not the total 'g'http://images.ibsrv.net/ibsrv/res/sr...ies/thumbs.gif

They talk about a turbulent airspeed(VA),but i think it is VB for jet pilots,VA being the speed at which use of full control forces will not damage the structure of the aircraft.

Turbulent air penetration speed in the 787 is .85 which is the highest I have seen in a transport category airplane.

SPOOKY

Wow! Gives us some idea just how strong the 787 is..thanks for the info.

Not really; Mach Number on its own tells you nothing about strength. Gust loads depend on airspeed not Mach Number so you need at least to know the altitude that goes with the Mach Number.

And besides, the 'rules' for VB say:

At altitudes where Vc is limited by Mach number –
(i) VB may be chosen to provide an optimum margin between low and high speed buffet boundaries; and,
(ii) VB need not be greater than VC.

So the chances are that the 0.85M value is based on buffet margins not structural capability.

Indeed so.
With the L1011, just as one example, the turbulence speed is M.82-.84, but limited to 290KIAS, regardless of mach number.
So, at lower altitudes, the IAS value overrides mach number target.
I would entirely agree, buffet onset margins are very important and offen override other considerations.

I don't see the logic in the above answers. In fact they are based on false premises.

How come you people think you can make up the rules of physics as you go along? Everytime I visit this type of thread I am always surprised at what you believe or have been taught.

If Boeing says the 'plane can do XYZ it will do XYZ : thats why they are late. Better late than have events like the A388 has suffered.

As far as the worry about buffeting goes maybe Boeing uses better math than Airbus does....

I am not saying that Boeing and Airbus know EVERYTHING about the products the make something like the 0,85 value does tell us about the airframe.

Every week there are quite serious events with people being thrown around in turbulence. Do you ever think about the stress on the airframe when these events happen?

All that means is that it has such a "critical" wing that it needs to be flown faster than most to prevent low speed buffet! :} IMO strength would have nothing to do with it given we're talking about high altitudes. Whilst .85 is only 260KIAS at 45k, it's 330KIAS ish at 30k. I don't know whether I'd want to be doing 330K in any aeroplane in severe turbulence, regardless of how plasticised it was.

Turbulence (I mean bad turbulence) means bounceys, injuries and structural damage. The lower the turbulence penetration speed, the better.

What is the IAS turb speed of the 787, or does it vary with altitude?