Tail plane in ground effect during rotation.
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Interesting points. Nice picture, too!
I do query whether the dear old Dak was getting any downwash effect at 95' above the sea (as I recall, the wingspan), so I am surprised at the quoted 1 x wingspan being the height at which downwash becomes useful.
Also, so far as the drag reduction is concerned, I think (from memory, but it's late) that the effect of the downwash reduction at low level is to rotate the effective lift vector forward, thereby reducing the component pulling rearward (i.e. drag) (I think it also fractionally increases lift).
This is different from the drag reduction gained from reducing wing tip vortex by flying low.
er, I think...
I do query whether the dear old Dak was getting any downwash effect at 95' above the sea (as I recall, the wingspan), so I am surprised at the quoted 1 x wingspan being the height at which downwash becomes useful.
Also, so far as the drag reduction is concerned, I think (from memory, but it's late) that the effect of the downwash reduction at low level is to rotate the effective lift vector forward, thereby reducing the component pulling rearward (i.e. drag) (I think it also fractionally increases lift).
This is different from the drag reduction gained from reducing wing tip vortex by flying low.
er, I think...
Warning Toxic!
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If you flew the 747, did you not find on an ideal approach that at a height of around 400' agl, a small backward tweak on the thrust levers kept you nicely in the slot? I reckon the ground effect on a 747 started being felt at 400'. Anyone else have similar perceptions?
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Rainboe,
Being rather new on the 74, I tend to agree, though I thought it was lower than 400. If turbulence and wind change is not a factor, I seem to go high on either or both of speed and glide slope unless I do a slight power reduction. When considering the span of the 747, it does make sense that it occurs at a few hundred feet.
Regarding what initial attitude to rotate to, going beyond 12 degrees in a swift rotation would give a tail strike (except for the SP), but as long as the rate of rotation is the recommended 2 to 3 degrees pr second, by the time the pitch goes through 12 degrees, lift of should have occured anyway.
Which leads to another relevant question of how the climbout performance may be affected by a slower rotation, as opposed to a faster one. Some will say to stop rotation at 10 degrees until positive rate, so would this technique alter the climb gradient? Waiting for positive rate at 10 degrees will often give a speed higher than V2+10. When airborne, however, that speed could be traded for climb, and thus one should be back on the same climb profile, more or less. Or what? I am not talking about gross underrotation, just rotate 10 degrees, then lift to whatever pitch brings the speed back to V2+10 to 20. If neither obstacles nor runway length is an issue, I'd rather be sure to avoid a tail strike.
How many degrees is this?
http://www.airliners.net/open.file/869836/M/
Tail strike:
http://www.airliners.net/open.file/336680/M/
Vmu testing
http://www.airliners.net/open.file/219906/M/
Other close examples
http://www.airliners.net/open.file/539269/M/
http://www.airliners.net/open.file/562519/M/
http://www.airliners.net/open.file/292539/M/
Tail strike on landing, when going below Vref is another similar issue
http://www.airliners.net/open.file/815565/M/
Being rather new on the 74, I tend to agree, though I thought it was lower than 400. If turbulence and wind change is not a factor, I seem to go high on either or both of speed and glide slope unless I do a slight power reduction. When considering the span of the 747, it does make sense that it occurs at a few hundred feet.
Regarding what initial attitude to rotate to, going beyond 12 degrees in a swift rotation would give a tail strike (except for the SP), but as long as the rate of rotation is the recommended 2 to 3 degrees pr second, by the time the pitch goes through 12 degrees, lift of should have occured anyway.
Which leads to another relevant question of how the climbout performance may be affected by a slower rotation, as opposed to a faster one. Some will say to stop rotation at 10 degrees until positive rate, so would this technique alter the climb gradient? Waiting for positive rate at 10 degrees will often give a speed higher than V2+10. When airborne, however, that speed could be traded for climb, and thus one should be back on the same climb profile, more or less. Or what? I am not talking about gross underrotation, just rotate 10 degrees, then lift to whatever pitch brings the speed back to V2+10 to 20. If neither obstacles nor runway length is an issue, I'd rather be sure to avoid a tail strike.
How many degrees is this?
http://www.airliners.net/open.file/869836/M/
Tail strike:
http://www.airliners.net/open.file/336680/M/
Vmu testing
http://www.airliners.net/open.file/219906/M/
Other close examples
http://www.airliners.net/open.file/539269/M/
http://www.airliners.net/open.file/562519/M/
http://www.airliners.net/open.file/292539/M/
Tail strike on landing, when going below Vref is another similar issue
http://www.airliners.net/open.file/815565/M/
Last edited by LGB; 7th Jul 2005 at 05:22.
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Not familiar with the 737, but in the 757 I've read that the wings blank out the tailplane during rotation, so going through 8-10 degrees the rotation tends to slow down so a little extra pull is needed to keep the rotation rate constant. Then, passing this 8-10 deg, you have to ease up a bit, again to maintain the proper rotation rate. Much more noticable in the sim, though.
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Linton,
I might misunderstand you, but I really don't understand your reference to Downwash in a Ground Effect context.
The reason you get a drag reduction is;
The LATERAL flow of air (along the wing axis, not longitudal like downwash) towards the wing tip
due to pressure difference below and above the wing will get slower. This is due to the space of movement gets smaller closer to ground.
THIS FLOW is the same flow that creates wingtip vortex -
or better known as DRAG.
So, e.g an aircraft with WINGLETS will get less drag reduction than one without...
Now, when getting into the Chord effect, as you know there is a lift increase due to RAM pressure. Whenever you have an
increase in lift, you will have an increase in INDUCED DRAG. Again, downwash is not an issue..
Cheers,
M
I might misunderstand you, but I really don't understand your reference to Downwash in a Ground Effect context.
The reason you get a drag reduction is;
The LATERAL flow of air (along the wing axis, not longitudal like downwash) towards the wing tip
due to pressure difference below and above the wing will get slower. This is due to the space of movement gets smaller closer to ground.
THIS FLOW is the same flow that creates wingtip vortex -
or better known as DRAG.
So, e.g an aircraft with WINGLETS will get less drag reduction than one without...
Now, when getting into the Chord effect, as you know there is a lift increase due to RAM pressure. Whenever you have an
increase in lift, you will have an increase in INDUCED DRAG. Again, downwash is not an issue..
Cheers,
M
Last edited by XPMorten; 7th Jul 2005 at 16:37.
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XP
I agree you are right with your two main points.
However, there is a theory which (I believe) goes as follows (I just know I'm going to regret this!):
Although the air approaches the wing horizontally, the air off the back of the wing departs the wing with a downward vector (downwash).
As a result, the relative total airflow past the wing is not horizontal, but descending.
As the lift is perpendicular to the relative air flow, the overall lift is not purely vertical, but has a rearward component - this rearward component is drag.
Ground effect removes the downward component of the relative airflow (the air leaves the wing horizontally) which means the lift is all vertical - i.e. less drag...
...so I'm told.
I agree you are right with your two main points.
However, there is a theory which (I believe) goes as follows (I just know I'm going to regret this!):
Although the air approaches the wing horizontally, the air off the back of the wing departs the wing with a downward vector (downwash).
As a result, the relative total airflow past the wing is not horizontal, but descending.
As the lift is perpendicular to the relative air flow, the overall lift is not purely vertical, but has a rearward component - this rearward component is drag.
Ground effect removes the downward component of the relative airflow (the air leaves the wing horizontally) which means the lift is all vertical - i.e. less drag...
...so I'm told.
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Just to comment on the 757......yep there is definitely a dead-zone at around 8-10 degrees....usually alleviated by continuing to rotate, its the first I've heard that its the mainplane though, I thought it was the unstick..and an increase in induced drag...with a corresponding movement in the CP.
Probably rubbish though
Probably rubbish though
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First of all, whatever theory is most correct, no-one says it only has to be one single effect causing this phenomena.
Whilst rotating and lifting off and going from zero lift to enough lift to accelerate the aircraft off the ground, several things happens almost at once.
In the end, though, all is down to Newton and his inertia as well as acceleration laws. Whatever molecules are accelerated downwords gives lift, whatever molecules are accelerated rearwards gives thrust, and that is the end of it, really. Drag and counterproductive lift works against thrust and lift, like the side effect of induced drag makes some of the molecules accelerate upwards. Closer to the ground, this effect is reduced by the ground blocking the rotating moment of the slipstream, ground effect of course. The ground effect is yet another hot discussion, ranging from "a pillow of air", deflected downwash, tilted lift vector etc, but again the end of it is the fact that the sum of acceleration of all molecules determines the acceleration of the aircraft.
If we look at the pitching moment only, these shold be the factors able to influence it:
1) Amount of lift (as center of lift is not colocated with the CG)
2) Center of pressure moving fore/aft
3) Downforce produced by the tail
4) Maybe the CG, as the aircraft tilts aft, gravity works on CG at a more rearwards angle, but this might not be significant. CG should be the same, but the direction of the force acting on it changes.
Factors not influencing it in this case
Gear/flap retraction (since we are only talking about the rotation itself)
So, wrong me if I am correct ...
Whilst rotating and lifting off and going from zero lift to enough lift to accelerate the aircraft off the ground, several things happens almost at once.
In the end, though, all is down to Newton and his inertia as well as acceleration laws. Whatever molecules are accelerated downwords gives lift, whatever molecules are accelerated rearwards gives thrust, and that is the end of it, really. Drag and counterproductive lift works against thrust and lift, like the side effect of induced drag makes some of the molecules accelerate upwards. Closer to the ground, this effect is reduced by the ground blocking the rotating moment of the slipstream, ground effect of course. The ground effect is yet another hot discussion, ranging from "a pillow of air", deflected downwash, tilted lift vector etc, but again the end of it is the fact that the sum of acceleration of all molecules determines the acceleration of the aircraft.
If we look at the pitching moment only, these shold be the factors able to influence it:
1) Amount of lift (as center of lift is not colocated with the CG)
2) Center of pressure moving fore/aft
3) Downforce produced by the tail
4) Maybe the CG, as the aircraft tilts aft, gravity works on CG at a more rearwards angle, but this might not be significant. CG should be the same, but the direction of the force acting on it changes.
Factors not influencing it in this case
Gear/flap retraction (since we are only talking about the rotation itself)
So, wrong me if I am correct ...
Tailplane in Ground Effect or Pitching Moments?
As LGB says, there are many factors involved and I for one would be the first to admit that I am uncertain about which may be dominant. Having said that I offer the following comments to create some discussion on the contribution of the effect of the change in pitching moments due to change in lift during rotation and liftoff.
We accept that in flight the mainplane of a conventional (stable) aircraft produces a strong downward pitching moment due to the lift distribution over the mainplane. To counteract this and control the aircraft around the pitch axis the tailplane produces a balancing downforce and adjustment of the elevator angle allows the pilot to refine the precise amount to give effective pitch control.
Obviously at zero airspeed prior to commencing the takeoff roll the mainplane is producing zero lift and zero pitching moment. And also obviously the tailplane is also producing zero downforce. As rotation speed is approached at zero pitch attitude the mainplane has started to produce a small amount of lift and therefore negligible pitching moment. However with the tailplane correctly trimmed for takeoff it will start to produce significant downforce on the end of a long moment arm and this can cause the nose wheel to become light and skippy. (Different aircraft types recommend varying techniques on the use of elevator during the takeoff roll.)
At the rotation speed, the stick force to initiate the rotation about the main wheels (aft of the cg) is normally quite low. As the pitch attitude approaches around 8-10 degrees the mainplane generates enough lift to overcome the weight of the aircraft and liftoff is imminent. Coincident with the generation of this lift by the mainplane a nosedown pitching moment is also generated by the fore/aft lift distribution. This centre of pressure is also aft of the cg. Therefore it is logical that this would require an increase in backpressure on the stick to create the corresponding increase in tailplane downforce to balance the new and additional pitching moments (either to maintain the desired pitch rate or maintain the pitch attitude).
Standing by to be corrected…!
The following discussion applies to Airbus brethren only:
We might be careful about jumping the gun in applying the observation of the reduction in rotation rate to Airbus operations, The Airbus A330/340 FCOMs specifically warn about increasing backstick at the point of, or immediately after liftoff due to the increased possibility of tailstrike directly resulting from this action. However, at liftoff after a V1 engine failure at heavy weight (in the A330 in particular), the increase in nosedown pitching moment can easily cause the main wheels to skip along the runway or even the aircraft to settled back to terra firma sometime after liftoff (especially embarrassing after passing the end of the runway or after gear retraction!).
Practically it does seem that some very small increase in sidestick backforce may be required to ensure that the pitch rate does not tend towards zero degrees/sec before the target pitch attitude is achieved especially at high weights. Shortly after airbourne the flight control laws change from ground law to normal law (auto pitch trimming) and the stick pitch force to maintain a selected pitch attitude is zeroed. This changeover from a sidestick backforce required to zero backforce can cause a tiny pilot induced pitch bobble that may be noticeable during engine-out work at high weights where performance margins are critical and demand that very accurate pitch attitudes need to be flown.
Perhaps the experienced and skillful Airbus driver would guard against the possibility of tailstrike by following the FCOM guidance and accurately flying the recommended pitch rate to the recommended engine-out pitch target of 12.5 degrees.
As LGB says, there are many factors involved and I for one would be the first to admit that I am uncertain about which may be dominant. Having said that I offer the following comments to create some discussion on the contribution of the effect of the change in pitching moments due to change in lift during rotation and liftoff.
We accept that in flight the mainplane of a conventional (stable) aircraft produces a strong downward pitching moment due to the lift distribution over the mainplane. To counteract this and control the aircraft around the pitch axis the tailplane produces a balancing downforce and adjustment of the elevator angle allows the pilot to refine the precise amount to give effective pitch control.
Obviously at zero airspeed prior to commencing the takeoff roll the mainplane is producing zero lift and zero pitching moment. And also obviously the tailplane is also producing zero downforce. As rotation speed is approached at zero pitch attitude the mainplane has started to produce a small amount of lift and therefore negligible pitching moment. However with the tailplane correctly trimmed for takeoff it will start to produce significant downforce on the end of a long moment arm and this can cause the nose wheel to become light and skippy. (Different aircraft types recommend varying techniques on the use of elevator during the takeoff roll.)
At the rotation speed, the stick force to initiate the rotation about the main wheels (aft of the cg) is normally quite low. As the pitch attitude approaches around 8-10 degrees the mainplane generates enough lift to overcome the weight of the aircraft and liftoff is imminent. Coincident with the generation of this lift by the mainplane a nosedown pitching moment is also generated by the fore/aft lift distribution. This centre of pressure is also aft of the cg. Therefore it is logical that this would require an increase in backpressure on the stick to create the corresponding increase in tailplane downforce to balance the new and additional pitching moments (either to maintain the desired pitch rate or maintain the pitch attitude).
Standing by to be corrected…!
The following discussion applies to Airbus brethren only:
We might be careful about jumping the gun in applying the observation of the reduction in rotation rate to Airbus operations, The Airbus A330/340 FCOMs specifically warn about increasing backstick at the point of, or immediately after liftoff due to the increased possibility of tailstrike directly resulting from this action. However, at liftoff after a V1 engine failure at heavy weight (in the A330 in particular), the increase in nosedown pitching moment can easily cause the main wheels to skip along the runway or even the aircraft to settled back to terra firma sometime after liftoff (especially embarrassing after passing the end of the runway or after gear retraction!).
Practically it does seem that some very small increase in sidestick backforce may be required to ensure that the pitch rate does not tend towards zero degrees/sec before the target pitch attitude is achieved especially at high weights. Shortly after airbourne the flight control laws change from ground law to normal law (auto pitch trimming) and the stick pitch force to maintain a selected pitch attitude is zeroed. This changeover from a sidestick backforce required to zero backforce can cause a tiny pilot induced pitch bobble that may be noticeable during engine-out work at high weights where performance margins are critical and demand that very accurate pitch attitudes need to be flown.
Perhaps the experienced and skillful Airbus driver would guard against the possibility of tailstrike by following the FCOM guidance and accurately flying the recommended pitch rate to the recommended engine-out pitch target of 12.5 degrees.
