Tail plane in ground effect during rotation.

[Blip]

OK i've done a pprune and google search and couldn't find anything (which surprised me!).

I fly the B737 Classic and NG.

Whenever I'm in the simulator, during the take-off rotation I have the contol column displaced a certain amount to achieve a certain rate of rotation. Then, just as the main wheels are leaving the ground at around 10 degrees nose up, there is a prounounced reduction in rotation rate to the point where it virtually stops rotating. At this point only an extra yank backwards on the control column, re-establishes the original rate of rotation.

In the real aeroplane, the same characteristics are there but it is not nearly as severe as that. Obviously the simulator is over doing this characteristic a bit.

Anyway I'm wondering what it is that causes this characteristic. My guess is it has something to do with the tailplane and ground effect. Just as there is an increase in lift for an airfoil producing upward force when very close to the ground, there might be a reduction in "lift" from an airfoil producing downward force when very close to the ground.
i.e. The low pressure over the tailplane is actually underneath, between the tailplane and the ground, and this close proximity to the ground restricts the upwash and therefore reduces the downward "lift".

Does this sound right?

Is there anything on the internet you can refer to?

Thankyou.

[Tinstaafl]

If the change is conditional on the gear leaving the ground then I'd suspect it's due to a moment change. Instead of rotating about the gear when the a/c is wheel-borne the aircraft now rotates about the CG (or near the CG) once airborne.

[XPMorten]

Hi,

This is not Ground Effect or moment change.

It is caused by downwash from the wing/flap
hitting the Horizontal Stabilizer and forcing it up (working agains the rotate).
Closer to the ground the Hstb is, stronger the effect.

We have this in the X-Plane simulator :)
Could you try to discribe how/when it feels in real?
Have a feeling our XP model is a bit strong on the
effect as well... .

btw, if it was moment change, it would have the
OPOSITE effect and rotate faster since CG is in front of the gear... .

Cheers,

Morten

[Tinstaafl]

Good point re the longer moment for wing borne support.

[Rainboe]

With many years flying Boeing's finest, it is a strong effect that is very noticeable. You pull a rotation, and it stops at about the 10 degree mark. You quickly learn to give it another tweak approaching 10 degrees and leave it at that. That will keep the nose smoothly rotating up through 10 degrees.

I had assumed that it was the tailplane approaching the ground and running into compressibility effects of the air between the tailplane and ground, which at about 10 degrees are quite close. It may well be interaction off the flaps- I don't know.

[despegue]

Well, on all accounts, this is a safe thing.
I never pull above 10° pitch before I have a positive rate of 400-500 ft/min. Why? Because we have a tail my dears.
Maybe not that "fluent", but safe.

[Rainboe]

In a heavy, stopping at 10 degrees won't lift you off cleanly. It's important to keep the rotation going smoothly through to 12 degrees plus, so it is necessary on a heavyish 747 to give that extra pull.

[despegue]

Rainboe,

I have no experience on widebodies, but will keep that in mind thanx!

[BOAC]

I was taught that it is equally important to keep the rotation going in the 737 as well to ensure the flight path is as intended. Maintaining the rate you have been told to achieve (3 deg/sec in the 2-700) will NOT strike the tail if the speed is right. As Rainboe says, just adjust the pull (gently!). Another fault is people do not pitch into the pitch bar until well after rotate, presumably again 'worried' about a tail-strike?

[despegue]

BOAC,

Have to dissagree here.
Never follow the pitch-bar on initial climb-out as it only relates to the selected speed. Frequently, the F/D shows more than 20° up, which is an unusual attitude and can be catastrophic when you have an engine problem. Don't be a F/D slave, it is there to suggest you, not to command you, and in rotation, its suggestion is mostly incorrect.
The 3 degree per second is a good thing, but again, don't go passed 10° pitch on the 737 until 450'/min. The B737.800 will strike its tail at 11° pitch ( or just a bit more, forgot the exact number)
Just look at the Air Berlin, VEX, etc. SOP's. they write the same.

Good to share different opinions by the way. I'm only telling my view on the matter, in fact, there are many ways to fly the B737 safely.

[Rainboe]

I think this effect must be caused by the compression as the horizontal tailplane approaches the ground at about 10 degrees pitch. I can't see it being a flap interaction effect- it's exactly the same on the 737 as on the 747, and no doubt the 75/6/7 as well (is it?)

[BOAC]

NB I spoke only of 2-700 - I know nought about the 800! Re the pitch bar, I was referring to flying 'into it' in the climb away, not 'using it' for the rotation, and it generally drops down to meet the pitch demand. It is not uncommon to see folk fly 3 or so degrees below the pitch bar, with speed increasing quickly. A 'flat' climb-out is not always safe! Generally (in my experience) pilots limit pitch angle to 20 deg most of the time for passenger comfort. The +20 demand is normally reduced as you pitch towards it unless you are light.Just look at the Air Berlin, VEX, etc. SOP's. they write the same - yes, then follow SOPs. 2-700 does not specify that manoeuvre. Most manuals quote "a smooth continuous rotation at Vr towards 15 deg" and "Any improper rotation decreases initial climb flight path." l believe the tail strike angle for 400 is 11.5 deg and 15 deg is an 'ideal' climb pitch?

[Milt]

Back to basics with aerodynamics.

The horizontal tail flies as a little wing and with zero stick force during the early take off roll it will produce an increasing down/up force as V increases depending on the pitch trim setting. During this stage your little wing at the back end will be flying at close to zero angle of attack for minimum drag.

As you rotate you begin to change the angle of attack of the tail which opposes the rotation. It is stabilising. So if you only input a small stick force you will rotate only part way with the rotation rate decreasing towards zero as tail alpha inceases the wrong way to that desired.

So to produce a constant rate of rotation you have to input a rate of back stick input to continually and increasingly overcome the stability from the overall tail.

The whole thing is complicated somewhat by the increasing V and whether the tail is a slab or with elevator.

Not much different to pulling g for nose up pitch or into a turn.

The transition from rotation around the wheels initially to rotation around the cg at lift off should graduate smoothly as the wings increasingly take the load away from the wheels.

[PAXboy]

Non pilot here: In some of the NG 73s, ther is, I understand, a 'fully-flying' horizontal stab? Will this be moving as well, during rotation?

[XPMorten]

Rainboe,

Finally, with respect to the tail, the downwash from the wing is of considerable importance. Air is deflected downward when it leaves a wing, and this deflection of air results in the wing reaction force or lift. This deflected air flows rearward and hits the horizontal-tail plane. If the airplane is disturbed, it will change its angle of attack and the downwash angle also changes. The degree to which it changes directly affects the tail's effectiveness. Hence, it will reduce the stability of the airplane. For this reason, the horizontal tail is often located in a location such that it is exposed to as little downwash as possible, such as high on the tail assembly.

Adapted from Talay, Theodore A. Introduction to the Aerodynamics of Flight. SP-367, Scientific and Technical Information Office, National Aeronautics and Space Administration, Washington, D.C. 1975. Available at http://history.nasa.gov/SP-367/cover367.htm

In addition to the above, when on the ground, the
downwash has no place to escape and is forced back
up by the ground.

What happens is that the horizontal stab sort of "stalls" when it enters the wing downwash on rotate, and the effectiveness of the elevator decreases.

As for on what aircraft it feels more, I would guess
that the longer the distance between the wing and tail, less effect. Also, the vertical position of the Hstb relative to the wing would be a factor.

Cheers,

M

[Rainboe]

Interesting, chaps, and you may well be right. However, I am suspicious of the wing flap effects because even on different take-off flap settings, the halt in rotation at 10 degrees is identical. There is no effective downwash when your wheels are still firmly planted on the ground- there is nowhere for the air to go. So if the feeling is the same as you approach 10 degrees nose up for different flap settings and different types, it seems to me it is purely the interaction effect of the tailplane compressing the air as it gets near the ground.
Milt- the effect is quite sudden at 10 degrees. It doesn't appear to build up, just suddenly stop your rotation at 10 degrees. You pull a rotation and leave the stick fixed. The rotation is steady until you get to 8 or 9 degrees when the effect kicks in. With practice on the type you fly, you learn to subconsciously give an extra gentle pull as you approach 10 degrees to overcome it.

[XPMorten]

it seems to me it is purely the interaction effect of the tailplane compressing the air as it gets near the ground.


Sorry, but the "compression of air" you are talking
about is called RAM pressure or Chord Dominated Ground Effect. This does not occur until about 1 chord altitude above ground which I dont think it's possible to get a Hstb on a 737 - and definaately not at 10 deg pitch. In addition, the Hstab
is probably a symetric airfoil, so the effect would
be minimal!

http://www.pacificseaflight.com/graphics/chord_effect.jpg

The aircraft being on the ground is EXACTLY THE POINT! Had it been in the air, the downwash would go BELOW the Hstb..

M

MM

[Rainboe]

Have a look at this picture
http://www.airliners.net/open.file/869815/M/
from another thread here. See the tail skid? Well on a normal takeoff, that can easily go within 30-50cms of the runway. I'd say in a rotation, the tail can easily go pretty close! A heavy 747 can put the fuselage to about 18 inches (45 cms) of the runway, so the tailplane will easily go within a chord.
Take a look at this too:
http://www.airliners.net/open.file/869146/M/
That looks a little more than a chord, but skids can go a lot closer than that!

[Linton Chilcott]

As an aside, the instigator of this thread is, I believe, labouring under one common misapprehension: Downwash does not primarily increase lift, it primarily reduces drag - hence Wing In Ground Effect designs.

When I was part of a team using Dakotas for spraying oil slicks, we were almost certainly enjoying some of the benefits of downwash, as we were spraying at 25 feet over the water - well within the theoretical half the wingspan height at which it works (we were also enjoying various other aspects of the job too!)

As I remember, the aircraft would become a little more stable down at spray height and, rather than wanting to climb (more lift) it would simply accelerate very very slightly (acceleration normally being pretty slight at the best of times in the dear old gooney bird!). Although, I have to admit I haven't done it for some years, and memory may be playing optimistic tricks on me...

These links have something on downwash:

http://www.pilotsweb.com/principle/liftdrag
http://142.26.194.131/aerodynamics1/Drag

I've also flown the 747, and I must admit I had forgotten about the extra pull late in the rotate.

Cheers

[XPMorten]

It seems to be a common misconseption in these
forums about Ground Effect.

There is TWO TYPES of ground effect that
work on fixed wing aircraft!!!!
People seem to mix them up and think there is only one :ugh:

1. Span Dominated GE
This starts at about 1,0+ span altitude (not 0,5).
Due to less vortex buildup, we get DRAG REDUCTION Cd.
The reduction is logarithmic and is very significant close to ground (from 0,5 span and down). A high wing aircraft will not get
as much affected as a low wing.

2. Chord Dominated GE
This occurs at about 1,0 chord altitude.
Due to an increase in RAM pressure below the wing,
we get an INCREASE in LIFT Cl. The peak is around 0,05-0,2 chord - depending on airfoil etc.
Soo, THIS is the effect WIG aircraft mainly use.
They usually have a long chord and short wings.

http://aircraft.ru/Pictures/061-km.jpg

Note high tail to avoid downwash & wake from the wing, ground effect disturbances etc. The very reason for this topic...

Cheers,

M

[Linton Chilcott]

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...

[Rainboe]

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?

[LGB]

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/

[ww1]

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.

[XPMorten]

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

[Linton Chilcott]

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.

[haughtney1]

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:}

[LGB]

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 ...

[FlexibleResponse]

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.