Effect of CP on aircraft pitch attitude

[waela320]

Hi ,


Lowering the trailing edge flaps moves the CP aft of the CG and causes a pitch down moment but is this applicable to all aircraft types or only for T tail airplanes ?

What happens to the pitch attitude for an A320 for example when lowering the flaps ?

THANK YOU :)

[fizz57]

Pitching moment on Flap Extension [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-577603.html)

[FCeng84]

Care must be taken to distinguish between pitch attitude and pitching moment. Changing the CP (fore / aft) will induce a pitching moment change that must be balanced via the tail (elevator and/or stabilizer). The title of this thread, however, makes reference to pitch attitude, not pitching moment.

Trim pitch attitude for an airplane is more a function of the lift curve than the pitching moment curve. Extending flaps changes the Cl vs. Alpha characteristics by increasing the lift generated at a given speed / AOA. Whether flap extension moves the CP forward or aft, it will increase lift at a given AOA (assuming speed is held constant). As a result, trim pitch attitude will be lower following flap extension regardless of what happens to the CP. Note that on some airplanes this effect is accounted for within the control system such that flap position changes are directly compensated via commanded pitch attitude change to maintain lift and thus avoid disturbing the flight path angle.

[FE Hoppy]

You don't often increase flap without a concurrent decrease in speed.

With Slats thrown into the mix things get more tricky.
A good example is the Flap Fail/ Slat Fail procedure on the Ejets. Depending on the failed surface the optimum position of the non failed surface is as much to do with landing attitude as it is to do with stall speed.
Too nose high and the de-rotation eats up landing distance so the compromise is a slightly faster landing speed with a lower attitude on touch down.

Just saying ;-)

[FCeng84]

Leading edge slats have very little impact on the lift generated at low to moderate angles of attack. The motivation for leading edge slats is to extend the lift curve at higher angles of attack to push the stall point to higher AOA / higher lift. As such, extending leading edge slats has very little impact on trim pitch attitude.

Making an approach without slats extended will require a higher approach speed and thus a lower pitch attitude in order to preserve margin to stall.

[FCeng84]

The idea that landing at a higher speed improves landing performance by reducing the angle through which the airplane must derotate (and thus the time it takes to derotate) is a new one and seems counter intuitive to me. It seems to me that the required landing field length is much more a factor of touchdown speed than touchdown pitch attitude. Landing a little slower with the corresponding slightly higher pitch attitude would lead to stopping with a shorter rollout.

In my experience the limit to landing performance has been how slow one can approach as limited by approach attitude and the corresponding margin to tail strike.

[FE Hoppy]

I speak having discussed this at length with the manufacturer.

[FCeng84]

Do you have a reference for pilot procedure to fly faster in order to reduce touchdown pitch attitude? All I can find is increments above Vref as a function of flap position.

One of the keys to effective braking following landing touchdown is to get as much weight as possible on the wheels as quickly as possible. Touching down with a higher pitch attitude means that derotation by itself will put more weight on the wheels thus making braking more effective.

[Mad (Flt) Scientist]

@FCeng84
Touching down with a higher pitch attitude means that derotation by itself will put more weight on the wheels thus making braking more effective.

The brakes have no memory of where the aircraft was before - it's the instantaneous load on the gear that gives the brakes their effectiveness. If you landed at a high pitch attitude or a low pitch attitude, once you are derotated it's all the same - at speed V the load on the gears will be the same, however you got there.

What a higher attitude will affect is:
1. Most aircraft prohibit braking until nose is on the ground. higher nose at start = longer to get it on the ground = later start to braking. Obviously, the earlier you can start braking the better.
2. Higher attitude implies a lower speed all other things being equal. Lower speed is always better for stopping from.

For any given configuration/design there will be a sweet spot where the two combine to give the best stopping performance. of course, it may not be practical based on other considerations (min speeds, view over the nose, avoiding nose-down landings, ...)

[FCeng84]

MFS - I agree completely. My point about landing at a higher attitude is that brake effectiveness is directly related to dumping lift so that there is more weight on the wheels to make use of braking friction. As you state, landing at higher pitch attitude means landing slower. This provides runway length benefit in two ways. First, as you mention, slower to begin with is better. Second, slower to begin with also means more weight on the gear once derotated and thus greater brake effectiveness. As a side note, ground spoilers used as speed brakes result in a minor increase in drag while their main benefit is dumping lift to make the wheel brakes more effective.

I am still puzzled by the notion that landing as slow as permitted by touchdown pitch up to tail strike limits is not the optimal for using the minimum amount of pavement to stop. I have never seen any manuals suggesting that landing faster will require less runway!

[Mad (Flt) Scientist]

I am still puzzled by the notion that landing as slow as permitted by touchdown pitch up to tail strike limits is not the optimal for using the minimum amount of pavement to stop. I have never seen any manuals suggesting that landing faster will require less runway!

I'm thinking that there may be cases where the derotation constraints delay the start of braking too much.

[FCeng84]

MFS and TA - thanks for your comments regarding landing performance. Are you aware of how consideration of the touchdown attitude vs. touchdown speed sweet spot works its way into Vref or other parameters that serve to have flight crews landing at the optimal point?

I have always understood Vref to be based on a margin to stall and approach speeds to be determined starting with Vref. I know that probability of landing tail strike is also a consideration.

I have not run the numbers yet, but I am guessing that landing with the nose 2 deg lower would take a speed increase of at least 10 knots. If derotation rate is 2 deg/sec, the slower landing at higher attitude would lead to 1 sec longer derotation which works out to about 250 feet at typical transport max landing weight touchdown speeds. Slowing 10 knots in 1 second means about 15 ft/sec^2 which is half a g. That level of braking is not possible until speed is low enough to remove most of the lift and thus have most of the weight on the wheels.

[GlenQuagmire]

The brakes on my aircraft work before the nose is down. I never manually brake before the nose is down and tend to fly the nose on very gently but if the weather is crap i use the autobrake and that works one second after wheel spin up. If you use medium or high it reqires a pretty smart pull on the yoke to stop the nose banging down. Even low requires care.

Planes eh...

[Sidestick_n_Rudder]

There might be something to it. When I have transitioned from the A320 to A330 I have noticed that landing distances are roughly ~1000ft more for typical landing weights.

This is even though the approach speeds are very similar and autobrake deceleration is the same between these two types. Haven't seen any official Airbus data on this, but my gut feeling is that it's the de-rotation that accounts for the increased landing distance.

The only thing that is significantly different is the de-rotation, which takes significantly more time on the A330 compared to the 320.

[Mad (Flt) Scientist]

@S&R

Another factor is that "landing distances" are not just ground roll - there's an air distance component as well. That has to be established during certification flight test by various means, and could be another source of differences.

[PEI_3721]

Certification requires landing demonstration from Vref-5 at 50 ft without power adjustment; and without tail strike.
Landing performance considers the air distance from 50 ft. Is this to main wheel touchdown or all wheels?
AFAIR the time to activate deceleration devices is a fixed value, or a factored achieved value as with the air distance.

[Owain Glyndwr]

@PEI 3721

I don't think you are right there.
My version of JCAR 25 says:
JCAR25.125 (a) (2) A steady gliding approach with a calibrated airspeed of not less than 1.3Vs must be maintained down to the 50ft height
(a) (3) Changes in configuration , power or thrust, and speed must be made in accordance with the established procedures for service operations

Air distance is to main wheel touchdown

[PEI_3721]

OG, mistaken, possibly.
More likely confusing because of my intermixing of control and performance requirements.
I am not familiar with JCAR, but the ref appears to be similar to CS 25-125 performance requirements.

AC25-7C (www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2025-7C%20.pdf) (page 90 re 25-125) provides are more practical explanation of the flight test landing performance and control demonstrations.
Note that this refers to flight test procedures and not those used in daily operation, although some tests refer to the latter.

The lower speed (control) requirement is at Para (4) "...satisfactory flight characteristics should be demonstrated in the flare maneuver when a final approach speed of VREF-5 knots is maintained down to 50 feet."

The relevance to this thread is that the manufacturer can select the optimum configuration to achieve minimum landing distance based on Vref, but what ever this this (configuration / change of CP), then the aircraft must be both controllable to start and complete the flare from Vref-5 at the threshold, and not be limited by geometry (tailstrike) or control limit (full back stick).

Re attitude, the BAe146/RJ has a low approach and landing attitude, -3 deg pitch for a 3 deg GS (highlift wing, T tail, and no leading edge devices). Thus the attitude changes required to flare and then lower the nosewheel are small compared with other aircraft.
After touchdown, a reasonably fast nose lower could improve the flight test landing performance by minimising the air distance and the ground timing up to the point of using retarding devices.
However in daily operation, a more moderate rate of nose lowering might delay the WoW switching for spoiler deployment and braking because the wing still provides considerable lift - re MFS #9, - longer landing distance.

[Owain Glyndwr]

PEI 3721

I have no problem with that, my excuse is that the preceding discussions related to establishing distances.

[Chris Scott]

Hi waela320,

The title of your thread seems to have caused some confusion! I am guessing that you may not have expected the discussion to get so technical, and am not convinced that it has yet answered the questions you had in mind. (Forgive me if I'm wrong, as I don't know your background.)

Quote:
"Lowering the trailing edge flaps moves the CP aft of the CG and causes a pitch down moment but is this applicable to all aircraft types or only for T tail airplanes ?"

Staying with big jet transports that use tailplanes for horizontal stabilisation, in my experience on 6 types any difference caused by a T-tail is not noticeable to the pilot during flap extension. Taking the example of the B707/KC-135, which has a fuselage-mounted THS (trimable horizontal stabiliser), the HS has to be trimmed to a greater and greater negative angle of incidence (that is, nose-up trim) as the flaps are run out in stages on the approach, in order to allow the elevators to return to neutral. The same applied to the VC10, which is a similar-sized aeroplane with a T-tail, and all the others in my experience.

With conventional trailing-edge flaps, particularly those of the fowler type that all these jets have, the total negative lift required at final approach speed with full flap is presumably greater than at minimum-clean speed to stabilise the pitch attitude. But the lower IAS will itself demand a greater angle to produce the same negative lift, so those two additive factors explain why such a large change in the THS angle is needed.

A good, but sad illustration of the above was probably involved in a fatal accident to a B707-320C at Lusaka in 1975. When the crew selected full flap at about 4 miles from touchdown, the increase in load on the fatigued THS caused a failure on one side (I forget which) and the aircraft bunted so violently that it actually passed the vertical in pitch before hitting the ground.

(By the way, the CP is always behind the CG in a conventional aeroplane with a tailplane HS.)

Quote:
"What happens to the pitch attitude for an A320 for example when lowering the flaps ? "

The A320 is a bad example because the FBW control laws keep the pitch-attitude roughly constant as the flaps (and slats) run out. The B737 would have been better, but I haven't flown it. However, the first thing to remember is that, if you are trying to maintain a steady FPA (for example, maintaining the ILS glide-slope) the greater flap angle will initially demand a lower pitch-attitude until the aircraft slows down to the IAS appropriate for the new flap setting. If the pilot does not push the yoke or stick slightly forward to lower the nose (with down-elevator) while the flaps are running out, the a/c will seem at first to "balloon", and stop descending. (That applies equally on the A320.) Then, at a constant thrust, the IAS will decay rapidly and the nose will drop, partly because the CP has moved aft. So at that stage up-elevator will be needed until extra nose-up trim is selected on the THS.

There may come a point in the stages of flap extension, however, where the CP may not move as much for each degree extra. Perhaps the aerodynamicist(s) posting on this thread will comment?

[Chris Scott]

Quote from FE Hoppy:
"A good example is the Flap Fail/ Slat Fail procedure on the Ejets. Depending on the failed surface the optimum position of the non failed surface is as much to do with landing attitude as it is to do with stall speed.
Too nose high and the de-rotation eats up landing distance so the compromise is a slightly faster landing speed with a lower attitude on touch down."

With slats-only the possibility of tail strike certainly has to be taken into account when planning the standard FCOM increment pilots apply to the Vref (+25 kt, for example), due to the pitch-attitude on approach being much higher than normal. But with flaps-only the attitude is much lower than normal, because of the higher IAS (again, +25 would be typical). You and other posters imply that brakes cannot be used until nose-wheel touchdown. That doesn't necessarily apply to modern types, and I agree with GlenQuagmire: in my experience there is more than enough elevator authority on the A320, for example, to counter medium-autobrake and achieve a smooth nose-wheel touchdown, and that should apply at the higher touchdown IAS associated with partial absence of high-lift devices. (Must admit I never used the MAX setting on landing.)

Prior to nose-wheels touchdown, with reduced weight on the wheels despite the lift-dumpers/ground-spoilers, the anti-skid system should if necessary limit the intensity to whatever is achievable?

[FE Hoppy]

Chris Scott.
Take a look at the landing distance correction factors and the possible F/S combinations.
More is not always better.

Eg. E170 EASA Certification. Flap failed in position 1. The shortest landing distance correction factor comes with slats retracted. Factor is 1.46. If you choose to select slats (1,2,3) the correction factor increases to 2.0 as it does with slats (4,5,Full). In this case the Vref increment remains the same for all 3 options (VRef Full + 35) So same stall speed but higher pitch attitude resulting in longer landing distance. There are similar examples for all 4 versions both in EASA and FAA certifications.

In 2003 when we first started training the EJets on behalf of Embrear this was one of the first questions I brought to them. We asked if this might be a typo but the reply from their Perf Engineer was clear. Prolonged de-rotation due to High nose up at touchdown.


When discussing this with students I always draw a picture of the wing with both high lift devises in all positions and draw chord lines from Slat LE to Flap TE to show the different incidence on a fixed fuselage and then using Vref corrections discuss CL changes at different geometries.
This clearly shows those combinations where additional Slat does not increase Cl but simply changes the Incidence and thus the required attitude for a fixed Alpha.

[Chris Scott]

Hi FE Hoppy,

Funnily enough, as you will have noticed, we are having a discussion on the relative merits of slats and flaps for the WAT-limited take-off case in, of all places, AH&N:
http://www.pprune.org/aviation-history-nostalgia/590456-detailed-discussion-desired-flying-past-5.html#post9679119

Quote from FE Hoppy:
"When discussing this with students I always draw a picture of the wing with both high lift devises in all positions and draw chord lines from Slat LE to Flap TE to show the different incidence on a fixed fuselage and then using Vref corrections discuss CL changes at different geometries.
This clearly shows those combinations where additional Slat does not increase Cl but simply changes the Incidence and thus the required attitude for a fixed Alpha."

That seems a very useful way of presenting the subject. But two things on your E170 scenario - where the flaps are stuck in position 1 - remain unclear to me:
1) Is braking not permitted prior to nose-wheel touchdown?
2) Why is it necessary to maintain a speed increment of +35 kt to the Vref (full) regardless of slat position? In effect, slats lower the stalling speed, and (continuing to state the obvious) basic Vapp is normally a function of Vs.

Is (2) to reduce the risk of tail-scrape, due to unfortunate geometry?

[FE Hoppy]

Chris,
Braking is permitted at main wheel touchdown. At high nose attitudes there is still lots of lift even with spoiler so not much weight on the wheels. Hence higher pitch longer landing distance.

Speed increment remains the same because the slats are not increasing CL and the higher pitch for same Alpha doesn't help adding even more pitch for a bit more alpha (Cl) is counter productive as it extends the landing run.

Extra Slat is neither inhibited nor prohibited but the data is the data and the landings will be longer.
Clearly the aircraft isn't geometrically limited at slat (123) as the aircraft can land with slat (45full) and yet the Vref remains the same and the landing distance correction is considerably greater than with no Slat.

I can't really add anymore as I'm just recalling what Emb told me when I asked the question.

[Chris Scott]

Quote from FE Hoppy:
"Speed increment remains the same because the slats are not increasing CL and the higher pitch for same Alpha doesn't help adding even more pitch for a bit more alpha (Cl) is counter productive as it extends the landing run. [...] I can't really add anymore as I'm just recalling what Emb told me when I asked the question."

I appreciate your problem, but I simply don't understand the explanation you were given. Deploying the slats (assuming they are slats, rather than simple droop) should enable a higher alpha and, consequently, a lower approach speed. That increases the pitch attitude, as you say, which on some types would risk tail-strike. If that's not the issue on the E170, I suggest that the ability to lower the ground-speed on touchdown by, say, 10 - 15 knots would more than compensate for the reduced braking efficiency during the longer de-rotation.

Of course that assessment is at best only empirical, and I stand to be corrected. But it also reflects experience on four other jet types with slats.

[FCeng84]

If extending slats causes touchdown pitch attitude to be higher at the same speed the slats must decrease lift at a given angle-of-attack. Every set of lift curves I am familiar with shows almost no change in Cl as a function of slat extension at AOA less than that near stall. Are the E-jets somehow different in this regard?

[FE Hoppy]

I agree in the flaps extended case but I'm sure I've seen lower Cl on flap 0 + slat vs no slat.

Remember we are talking about off design geometry following a flap failure.

Here is an old but interesting report.
http://naca.central.cranfield.ac.uk/reports/1954/naca-tn-3129.pdf

[FCeng84]

FE Hoppy - thanks for the document you linked. Page 24 therein clearly shows a lift curve that shifts down/right as slats that extend forward and downward are deployed. For such a wing as that one extending the slats will lead to a higher pitch attitude at the same speed. As you make clear this is for a failure condition where slats are deploying without trailing edge flaps. We are much more familiar with flaps and slats extending in concert yielding a higher Cl and thus a lower pitch attitude.

For this failure case it might be possible to take advantage of the slats alone to permit a slower approach speed but that could yield a high enough pitch attitude to risk tail strike. As so often is the case the devil is in the details.

[mayam13]

@FCeng84
Higher speed landing induces more weight on landing gears,which ultimately reduces 'fatigue life' of main landing gears, if this is made a practice

[FCeng84]

Please explain. Airplane mass and touchdown sink rate affect gear load. How does speed get involved?

[Owain Glyndwr]

FE Hoppy, FCeng84

Just a comment chaps, but the slat geometry that gave that unusual result was set up with a divergent channel between slat underside and wing LE - something almost guaranteed to screw up the lift. No designer worth his salt would do it that way.

FCeng84

How does speed get involved?

Possibly through springback case; otherwise I am with you

[waela320]

Thank you all for your answers .

[vilas]

Derotation should not be an issue in Airbus because reversers are to be applied when main wheels touch down and brakes can also be applied taking care of the nose wheel snapping down.