Friday, June 17, 2005

Wings and Things

The airplane I want to fly has one wing on each side, and at the back there's a tail, which pilots sometimes call the empennage, because otherwise people might understand. This blog entry is about all the hinged surfaces hanging off the wings and tail.

The primary flight controls--ailerons, elevator and rudder--are mechanically linked to the cockpit through cables. The ailerons are kind of big and heavy to move just by pulling on cables, especially if you have a cup of coffee in the other hand, so two geared servo tabs, one located on the inboard trailing edge of each aileron, deflect in the opposite direction to assist in the movement. The rudder also has a geared tab, at the lower edge, for the same purpose. The pilot is expected to have enough leverage to move the elevator herself, so the elevator has no geared tabs.

All three axes of control have cockpit adjustable trim, with one trim tab each, located at the outboard end of the left aileron, the top of the rudder, and at the left side of the elevator. The trim controls are conveniently located so that both pilots can reach them easily, especially if the captain can comfortably dislocate her own shoulder and the first officer has an extra long left arm. The purpose of trim is to use aerodymanic force to hold the controls in the desired position during flight. Elevator and rudder trim is mechanical, while aileron trim is electric, powered from the left DC bus. Easy to remember, because the trim tabs are all on the left, except for the rudder trim tab, but that would be the left if the airplane banked over towards the other trim tabs, to put them all together.

The difference between trim and geared tabs is that the geared tabs are used to help MOVE the controls, and trim is used to HOLD them in place. Almost all airplanes have trim, at least on the elevator. Larger ones have geared tabs or some other way to assist the pilots in moving the control surfaces.

On most airplanes the ailerons hang off the back of the wings, but these ailerons are a little different. One reference calls them "unique." The flaps run the entire span of the wing, except for the bit in the middle where the cabin is obviously in the way. The fore flaps are hinged directly off the wing rear spar. Don't be fooled by the term fore because they are still trailing the wing, it's just that there's another set of flaps aft of the fore flaps. The ailerons are actually attached to the outboard fore flaps, and their differential movement increases when flaps are extended.

If you can follow that, wait until you hear how the flaps work. Remember that the fore flaps hang off the back of the wing, so the trailing flaps hang off the back of the inboard fore flaps. The outboard fore flaps operate through a range of zero to twenty-six degrees, the inboard fore flaps extend up to forty degrees, and the inboard trailing flaps go from zero to sixty degrees. According to some peculiar rule of averaging, this equates to a nominal flap range of 0 to 37.5 degrees. You'd think the two inboard flap deflections would add up to a total deflection of 100 degress and curl back under itself, but when you look at the aft inboard flap, it seems to move from being about ten degrees below the horizontal to being about 85 degrees below horizontal. Now that's 75 degrees of travel, not 60 and not 100. So I can't quite figure out how the flap deflection is measured. There is only one flap lever for all these flaps, and it is graded from 0 to 40 degrees. I guess it's rounded off. The point is: this airplane has a heck of a lot of flaps.

When you extend flaps on an airplane, the nose has a tendency to pitch up. This airplane has a flap-elevator interconnect tab: a servo tab on the right side of the elevator that automatically deflects up as the flaps extend, to counteract the pitch up tendency. The tab has a range of 12 degrees up and down.

Oh and the flaps are hydraulically actuated, which means in the event of a hydraulic failure they will not extend, and may slowly retract due to leakage and aerodynamic pressure. They retract more slowly than they extend, and they don't extend all that fast, so you don't get massive pitch changes. If there is leakage in the poppet valve (whatever that is) of the flap control mechanism, the flaps may droop to the extended position while the airplane is parked overnight. It would be a good thing for an FO to notice a discrepancy between the flap selector postion and the flap position during the morning walkaround.

The other things trailing off the wings and tail are static wicks. I once encountered a Transport Canada inspector who literally pulled one of the static wicks off an airplane with his hand, and threatened to write up the airplane for missing it. Like the crooked cops in one of those old movies who pull you over and kick in your headlight, then cite you for driving with one headlight.


Anonymous said...


Ah methinks 'tis a Twotter!

A strange arrangement but it works beautifully.


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