Pitching moment on Flap Extension
 

Hey guys,

I have a question regarding the associated Pitching-moment with Flap Extension.

In my opinion to answer a question like "What pitching-moment will a Flap Extension cause?" I need to know where the CP in relation to the CG on that particular aircraft is.

If the CP is aft of the CG, then a flap extension should cause a pitch down even with the increased downwash which is reducing the AOA of the horizontal stabilizer?

Furthermore, I think that the CP of airliner usually is aft of the CG to have a positive longitudinal stability?

Maybe one of you can clarify..

Thanks!

Extension of trailing edge flaps will produce (at least) two competing effects:

Effect 1. The CP will move aft, causing a nose down pitching moment.

You may wish to argue that with CP ahead of CG the result will actually be a reduction in the nose up pitching moment (about the CG). But if the aircraft is originally in a trimmed condition, the result of the aft CP shift would be a nose down pitching moment.

Effect 2. If the tailplane is within the downwash from the mainplanes,the flap extension will cause the downwash over the tailplane to increase. This will cause a nose up pitching moment.

The overall result will depend upon the relative magnitudes of these two effects.

Alright, so the increased downwash caused by the trailing edge flap will increase the negative AOA of the tailplane and therefore the downward force of the tailplane which will result in a nose-up moment caused by the tailplane (if predominant over the nose-down moment caused by the flap extension).

However, lets assume we have a T-tail aircraft, where the horizontal tail is not in the downwash of the wing. Then I guess it will more likely be a pitch-down.

What I want to say. It´s not possible to answer a question like "What pitch moment will occur on flap extension" as I wouldn't know which moments are predominant?

On the 737: Typically first a tiny nose up followed by nose down. With underwing mounted enginges however, the increase in thrust followed by the increase in drag results in another nose up moment.

@keith williams

And those are the "pure" aerodynamic effects. On an aircraft with any kind of interconnect, the tailplane may be moving while the flap changes position. So the net result in terms of "does the aircraft pitch up or down" is further affected by the control system choices.

Depends whether the question is from the viewpoint of an aerodynamicist or a pilot.

MFS

You are of course correct. Some aircraft have systems which attempt to compensate for these effects, and some of these systems are more effective than others.

I must confess that having looked at the question and noted that this is the first post from this member, I concluded that he/she is preparing for the ppl or atpl exams. For many years the ATPL exam question bank has included this question, and the required answer was "pitch up moment".

In a previous life when teaching this subject I usually had some students shouting "my aircraft pitches nose up", while others shouted "but mine pitches nose down",and some even shouted "mine pitches up then down". This was very useful in that it demonstrated the range of possible effects.

So I suppose that my answer was based on the viewpoint of an ATPL student or groundschool instructor.

In general wing sections with more camber have a greater pitching moment so if the camber is increased by lowering flaps there will normally be a nose down pitch... unless something corrects for that.

keith williams:As a ground instructor myself, I absolutely agree. Regardless of the syllabus, good students will often learn as much or more FROM EACH OTHER than from the instructor, who may find himself somewhat bystander. Sorting out competing examples and theories, accepting that the "book" may be oversimplified!

:eek:
:D

You can not just treat the pitching moment independently...
Extending flaps also results in a higher lift coeficcient of the wing, so either speed will reduce, or AoA will decrease. AoA decrease in combination with reduced L/D (reduced glide path) will both bring pitch down. Especially on aircraft without leading edge devices, this nose down attitude may be quite impressive.

You also have to see the full picture of pitch being the sum of the flight path angle and the AoA, so a nose comming up or going down depends on both. The flight path angle is affected by engine thrust, just like the pitching moment.

With all these aspects, it becomes clear why every aircraft has individual ways of reacting to flaps.

Sounds like pitch up due to the downwash on the stabilizer being a little stronger than the pitching moment, followed by a reduced glide path angle due to higher drag...

B727 pitch couple with slat/flap extension
 

Anyone with previous experience on 727
can you narrate how miss piggy behaved with leading edge flap and then trailing edge extension?

Any peculiarities with Flap 40 extension?

Coupling effects of speed brake extension?