Effect of CP on aircraft pitch attitude
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?
"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?
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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.
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.
Last edited by FE Hoppy; 20th Feb 2017 at 07:28.
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-histo...ml#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?
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-histo...ml#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?
Last edited by Chris Scott; 20th Feb 2017 at 15:01. Reason: Last para added.
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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.
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.
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.
"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.
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Slat must decrease Cl if touchdown pitch attitude increases
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?
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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/...ca-tn-3129.pdf
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/...ca-tn-3129.pdf
Last edited by FE Hoppy; 20th Feb 2017 at 22:01. Reason: added link
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Data shows a clear example
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.
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.
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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
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
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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.
