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IO540
16th Feb 2008, 21:32
Flew on a 777 the other day.

These short ailerons move both up and down.

Interestingly, when their upward movement passes through a specific upwards angle, one of the speed brake surfaces pops up to help it...

I am wondering whether these inboard ailerons can move together in the same direction - to counter up/down draughts which the aircraft encounters. This would be a logical stability / passenger comfort enhancement. If an updraught causes the hull to move up, both ailerons could pop upwards and push the hull back down again. Their location is probably fairly close to the CofG for this to work.

Checkboard
16th Feb 2008, 21:38
Look up the Lockheed L-1011 TriStar's Direct Lift Control (DLC) system!

Millysmyth
16th Feb 2008, 23:33
IO540 i think you will find that the principle of this operation is far simpler to explain then your suggestion of a gust alleviation device or Checkboard’s suggestion of Lockheed’s DLC, which as the name suggests is LOCKHEED specific

For starters DLC was design to allow for smoother approaches a constant deck angle by deploying Spoilers symmetrically on both wings and causing a reduction in lift cause by flap usage but allowing the steeper approach angle that flaps provide, especially useful in the L-1011 that was known to have poor visibility on approach and low flap settings

Your observation in the 777 is simple control surface design and operation. When a turn is commanded (Via Yoke or A/P) the appropriate aileron will move to the required position. However if the Rate of input of this command, or the desired angle of bank is grater then the authority of this aileron. (i e: It cannot be achieved with just the aileron). Then the spoilers (flight only) will deploy, this will cause a reduction in lift over that wing, causing a wing drop and as a result establishing a turn (through many other aerodynamic processes)

Your idea of a gust alleviation system is impractical and illogical, because at the speeds aircraft fly, by the time the system had recognised, combated and adapted to gust the aircraft would be out of the gust area and would have to alter itself back to the “neutral” position.
The actual upward deployment of the ailerons would more likely act like spoilers/speed brakes and either cause a descent, or aircraft buffet, or because of their aft position relative to the wing centreline cause some other aerodynamic instability
On an interesting side note aircraft are usually designed to withstand updrafts of 50 to 100 feet per second as per FAR 23 and FAR 25 requirements

Hope this helps

Brian Abraham
16th Feb 2008, 23:51
A very insightful question IO540

Your idea of a gust alleviation system is impractical and illogical

Someone had better tell the folks who designed and built the B-1 and B-2 bombers and the Airbus airliners for they all have gust alleviation systems fitted.

Cue 411A for Lockheed info.

Millysmyth
17th Feb 2008, 00:03
Brian Abraham i whink you will find the B1, B2 and Airbus systems are Supressions and use harmonic dampening principles rather then alleviation as suggested...but dont ask me im just a Aerodynamic Systems Intergration Officer at EADS

Brian Abraham
17th Feb 2008, 04:07
Millysmyth - you may wish to take it up with these people including your boss in that case. If as you say

idea of a gust alleviation system is impractical and illogical

there seems to be a lot of extremely well educated people and manufacturers who use the term and state that such systems exist in the aircraft mentioned.

http://pdf.aiaa.org/preview/CDReadyMSDM2005_970/PV2005_1946.pdf

http://www.emeraldinsight.com/Insight/viewContentItem.do?contentType=Article&hdAction=lnkhtml&contentId=876460

Even Bae Filton seem who build the Airbus wings seem confused if what you say is true.

http://www.baesystems.com/Capabilities/Air/FlightControls/index.htm

Could you also expand on the difference between gust alleviation and harmonic dampening principles. I'm just a humble pilot so stand to be educated.

Millysmyth
17th Feb 2008, 05:22
Once again i think this may be a discussion that comes down to semantics.

First of all what are we taking Alleviat(ion) as:

1)To reduce severity off.....
2)To Remove.....
In the case of 1, I agree entirely in that there are systems available and in place in many aircraft that Alleviate the severity of events that cause instability of aircraft. This is demonstrated in the aircraft that you reference and alluded to in the references you have sighted.

However in the case of 2, I know of no such system that can entirely remove the effect of a disturbance. When i read IO540’s statement “counter up/down draughts which the aircraft encounters.” I took it to mean “Remove”

So perhaps this is the reason for a little dispute?
In response to your question on harmonic dampening principles. It is the same as ANR in headsets or any other waveform for that matter. By adding the opposite input, of what is currently being experienced, to the system a Null result can be achieved.
I refer to it as dampening because (and this is now talking specific to an airframe now) the input that introduced is often less the initial disturbance so as not to exacerbate the situation. The Disturbance will gradually get smaller and small till it is nullified or removed all together. The best way to think of this is like a dynamically stable aircraft.

I hope this clears things up...and do apologise if my language sounds abrupt, as English is not my first language

Brian Abraham
17th Feb 2008, 07:12
Millysmyth - Since you claim to be a "Aerodynamic Systems Intergration Officer at EADS" I would have thought you would be more than familiar than most about Airbus systems, rather than giving us a most emphatic,

a gust alleviation system is impractical and illogical

Taken from a A340 training manual.

Turbulence Damping Function
The purpose of the turbulence damping function is to damp the structural modes induced by atmospheric turbulence.
The function uses the Nz accelerometer and two dedicated Ny accelerometers. The PRIMs compute a turbulence damping command, which is added to the normal law command for the elevator and yaw damper.
This function is automatically monitored and becomes inoperative for the remainder of the flight, when a failure is detected. In addition, it may be manually inhibited by switching off the TURB DAMP pushbutton on the overhead panel, when it is considered that comfort is degraded instead of improved, and no failure is detected.
It is only available if the following conditions are met;
- Aircraft in flight
- Aircraft speed greater than 200 knots
- Autopilot engaged or normal law active
Aircraft within the normal flight envelope

IO540 - sorry could not find details on the 777, but certain documents do hint that it may be fitted with a similar system, but could find nothing definative.

gas path
17th Feb 2008, 07:39
The inboard ailerons as fitted to the 777 are actually Flaperons They are capable of moving together but only down (and obviously back to neutral!). This is a function of trailing edge flap position to increase the area of trailing edge flap. They will modulate around the redefined neutral as a command of roll.
The flaperons will also move down if the engine thrust is moved into the takeoff range (with flaps up!)

Millysmyth
17th Feb 2008, 08:32
Mr Abrahami conceeded in my last post that there are systems on avail. that can dampen the effects of turbulance etc on aircraft like the A340, as you alluded to. and that it was perhaps a misunderstanding on my behalf for which i apologised

Your most rescent example only helps to highlight my explanation of the dampening operating principle. I feel that your comment "would have thought you would be more than familiar than most about Airbus systems" is uncalled for and to take a page from your book "you as a pilot should know that aerodynamics concerns more then just the wings and control surfaces of an aircraft (or in you case rotor blades)".

I appologise (again) for the confusion and anguish i seamed to have caused you. Yes you were right that there are sytems that Alleviate it was that i mistook the context the word was used. I hope we can leave these professional attacks behind though.

Brian Abraham
17th Feb 2008, 12:38
No anguish Millysmyth. Its just that you went to great length to explain why a gust alleviation system is impractical and illogical.

because at the speeds aircraft fly, by the time the system had recognised, combated and adapted to gust the aircraft would be out of the gust area and would have to alter itself back to the “neutral” position.
The actual upward deployment of the ailerons would more likely act like spoilers/speed brakes and either cause a descent, or aircraft buffet, or because of their aft position relative to the wing centreline cause some other aerodynamic instability

Your answer indicates you knew exactly what IO540's question related to and no semantics were involved, neither were you offering an opinion. To suggest otherwise is disingenuous at best. Pprune gets plenty of imposters airing their so called knowledge, and while inquisitive posters such as IO540 are catered for, and welcome by all, for thats how we all increase our knowledge base, to pretend you have a particular talent or knowlege base is not welcome by anyone. I hope we can in future have professional facts.

IO540 - You will find an explanation of the A340 test gust alleviation here

http://www.awiator.net/task_3_1.html

I'm Martin Bakering.

IO540
17th Feb 2008, 14:37
Thank you all.

I often wonder about these things because airliners do seem to be more stable than the light aircraft I fly (a TB20).

Yet the only differences I can see are the much higher wing loading on jets, and their yaw damper.

I know wing loading makes a big difference - the TB20 is a lot more stable in turbulence than most other light aircraft as a result. But there are very few with a YD fitted and I have not found anybody with an experience on the same type with and without a YD. A YD should help because it blocks the yaw which is a side effect of the constant roll axis corrections which one gets when on autopilot.

Swedish Steve
18th Feb 2008, 11:08
Quoting Gaspath
The flaperons will also move down if the engine thrust is moved into the takeoff range (with flaps up!)

The flaperons do move around on take off as below.
Select take off flap, Flaperons move down.
Select take off thrust, the flaperons are disconnected electrically, and will float up, being pushed by the exhaust plume. They can be seen to flap up and down (Is this why they are called flaperons!) At 80kts the electrical signal is returned and they slowly move back down to the take off flap posn.
The reason behind this is to stop loads on the attach points from the engine exhaust when they are not required. The flaperon is in the direct line from the exhaust.

Blinkz
18th Feb 2008, 16:32
IO540, I think a lot of the stability your talking about that airliners have is down to the increased speed and momentum of the aircraft. It takes a larger gust to influence a 75+ ton aircraft then it does to a 5 ton GA aircraft. Also the increased speed of the airliner means that the gust will affect the aircraft for less time then in a GA aircraft.

TheGorrilla
18th Feb 2008, 23:13
According to my B777 manual all ailerons are "locked out" during the cruise and roll control is achieved by spoilers and flaperons (not inboard ailerons).