787 flaperon behaviour on takeoff roll
 

Hi Guys,

I've been watching some spotter videos on Youtube recently, featuring multiple takeoffs of the 787. I have noted an interesting behaviour oft he flaperons during the takeoff roll. At the begining of the roll, they seem to be positioned straight, around zero degrees, forming a visible gap between deflected flaps. Then, around 80-ish kt (?) they deflect downwards, flush with the flaps to seal the gap and provide additional lift.

An example can be seen here at 6:18, 11:42 and 20:50: - both flaperons deflect downwards symmetrically at some speed. It happens on all takeoffs and in my opinion does not have to do with the control wheel deflection due to crosswind.

Any idea behind this logic?

Also, a bonus question - does anyone have any more background info on how the cruise flaps work? Boeing devoted just one small paragraph in the FCOM to their operation, with virtually no explanation. Do they move the Center of Pressure, as in the A350, or do they modify Cl? Do they always deflect downwards, or can they move up as well?

Cheers,

SnR

Could it be, that the exhaust and bypass air-gas stream is expanded at low speeds but contracts at higher speeds? The wide gasstream could affect a deployed flaperon, so it's pulled up, but a narrower stream will allow the flaperon to be in line with the flaps, and increase the effectivity during take off.

Thanks, I was thinking along the same lines.

Another explanation I thought of was some sort of direct lift control via the flaperons - the takeoff technique on the 787 is to initiate the roll with a slight forward deflection of the control column and relax it at 80 knots - it more or less co-incides with the flaperons deflecting downwards. However, there’s no mention of anything like it in the FCOM.

Could well be related to this in the Spoilers section of the FCOM :"The flight control system automatically biases the spoilers up during engine start and taxi to prevent spoiler trailing edges from contacting the flaps."

I think the forward pressure on the stick is simply to help nosewheel adhesion at low speed and it can be relaxed once the rudder becomes effective.

Its got nothing to do with pilot input.

Essentially, hydraulic pressure is removed from them during takeoff which causes them to droop - then lift aerodynamically with increased airspeed. Then at x kts hydraulic pressure is restored and they return to the commanded position. I don't think its covered in the FCOM etc but I believe its to do with preventing unnecessary stress on the flaperon actuators.

Cruise flaps; black box technology as far as pilot manuals are concerned. It just works!

Thanks! I have watched a couple of wing-view videos and the seem to confirm it. The 787 wing lives a life of its own :) Does the AMM cover this logic?


Yup, Boeing FCOM provides very scant info on them and they are completely transparent to the pilot (other than a STATUS message when they break down). Still, would love to know more on how they work...

The cruise flaps are very similar to the wings of a fighter jet like for example the F-18 that use slats and flap as needed to fly at a commanded angle of attack , as a 787-9 pilot I have no idea how they work , as someone explained above they just work

Keeping the flaperons straight in the first part of the takeoff run would avoid adding drag before the additional lift is needed. Of course the drag below 80 knots wouldn't be huge, but there isn't an obvious downside.

Hmm. After the 737 Max, does it worry any 787 pilot to know that the aircraft has systems about which pilots say "Boeing FCOM provides very scant info on them and they are completely transparent to the pilot ... Still, would love to know more on how they work ..." (Sidestick_n_Rudder) or "as a 787-9 pilot I have no idea how they work, as someone explained above they just work." (Riskybis) ?

I’m also curious about a control surface that “just works”. Never flown a 787 though.

To be fair, there’s plenty unexplained logic in Airbus products as well - though Airbus manuals seem to be more in-depth, whereas Boeing manuals concentrate on how to use things, rather than how they are built. Each philosophy has its pros and cons...

Cruise Flaps, the function is called the Trailing Edge Variable Camber (TEVC) function. It actually has two functions, but the drag reduction function uses the inboard trailing edge to reduce drag during the cruise. It works above 25,000ft and an actuator between the inboard and outboard flaps disconnects and holds the outboard flaps in place. This then allows the inboard flaps to move using the electric motor on the flap power control unit. Can't remember the exact authority of the system but it is only a few degrees and moves in increments of 0.5 degrees if I recall right.

Finally a net positive and useful thread. Well done. I've learned few interesting things. Thank you.

Likewise.

A few seem concerned about having autonomous flight control surfaces, but have pilots ever been concerned about the yaw damper fitted to the aircraft they fly? The yaw damper is automatic and transparent to the pilots. It just works.

Any system should be properly developed, test-flown, and engineered. If a system has NOT been properly developed, test-flown and engineered - for example the Boeing MCAS - problems can arise.
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Isn't the same system also used on the 777?

Yes - but manuals usually explain HOW it works, and when it is inhibited.

Based on other responses and combined......

This is a design feature to protect the flaperon, and particularly its actuators, from vibration damage(fatigue) caused by engine thrust impingement loads. When the flaps are extended(such as during taxi), hydraulic pressure to the actuators cause the ailerons droop to the takeoff position in order to assist in providing lift.

When the engines are powered up at the start of the roll, the flaperon actuators are bypassed, removing the hydraulic pressure that has been holding them in position. Once the hydraulic pressure is bypassed, the flaperon can then free float in the airstream which reduces the vibration load on the actuators(extending their life).

With little forward airspeed, the flaperons droop downward from the takeoff position due to gravity(sort of like some airbus ailerons droop downward when depowered). As the airspeed increases, the increasing airflow causes the flaperons to rise(freefloat) to the horizontal position. Above a set speed(around 80 knots), the bypass is removed so the flaperon is hydraulically actuated again and is commanded downward to its drooped takeoff flap position.

So basically, what you are seeing is the result of the flaperons being hydraulically depowered during the first part of the takeoff roll and the subsequent affect on its free floating position at low and increasing speed until such point as they become powered again.









reduce the fatigue on those silly actuators when the thrust right in front of that surface is rather large. The flaperons will float as the lift on the wing increases.

With takeoff thrust, until reaching 80kts, the flaperons are in BYPASS mode so they droop all the way down due to gravity, then upon reaching 80kts, power to actuator is restored and they resume the preset drooped position.
With takeoff thrust, until reaching 80kts, the flaperons are in BYPASS mode so they droop all the way down due to gravity, then upon reaching 80kts, power to actuator is restored and they resume the preset drooped position.

Yes, the 777 has the same behaviour.

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Triple 7s have a function called “Modal Suppression,” totally automated and no info on the FCOM. You only find something on the AMM l