Stabilizer Airfoil
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Stabilizer Airfoil
Hi all,
Not sure if this is the right place to ask, but I'll give it a try.
Does anyone know if Boeing and/or Airbus uses INVERTED airfoils on their horizontal stabilizers?
I have heard that Airbus does, but havn't been able to get this confirmed.
Also, I'd be very interested in general airfoil data on modern airliners - if anyone has any or knows
where to find it
Cheers,
M
Not sure if this is the right place to ask, but I'll give it a try.
Does anyone know if Boeing and/or Airbus uses INVERTED airfoils on their horizontal stabilizers?
I have heard that Airbus does, but havn't been able to get this confirmed.
Also, I'd be very interested in general airfoil data on modern airliners - if anyone has any or knows
where to find it
Cheers,
M
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To achieve natural longitudinal stability of the conventional aircraft there needs to be a download on the tail. This download can be as high as 30 tonnes for a heavy 747. The wings have to balance out the download with extra lift - a disappointing penalty.
Artificial stability allows the tail loads to come back to close to zero resulting in reductions in induced drag and for highly manoeuverable aircraft very rapid pitch rates.
The horizontal stabs on these aircraft are likely to be close to symmetrical or often just slabs which have a very busy time defeating the tendency to run away in pitch at the slightest disturbance.
A good stick and rudder pilot would find it hard going and probably impossible to fly one in manual so they do not have manual reversion. Triplicated flight control systems are considered to be enough insurance. If all systems fail - eject.
The early designs with artificial stability had the name of CCVs = Control Configured Vehicles. Perhas a better name is/has come out of the woodwork.
Artificial stability allows the tail loads to come back to close to zero resulting in reductions in induced drag and for highly manoeuverable aircraft very rapid pitch rates.
The horizontal stabs on these aircraft are likely to be close to symmetrical or often just slabs which have a very busy time defeating the tendency to run away in pitch at the slightest disturbance.
A good stick and rudder pilot would find it hard going and probably impossible to fly one in manual so they do not have manual reversion. Triplicated flight control systems are considered to be enough insurance. If all systems fail - eject.
The early designs with artificial stability had the name of CCVs = Control Configured Vehicles. Perhas a better name is/has come out of the woodwork.
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To achieve natural longitudinal stability of the conventional aircraft there needs to be a download on the tail.
Natural longitudinal stability depends on the variation of pitching moment with angle of attack - if an increase in angle of attack results in a tendency to pitch down again, you're stable. If instead you pitch up more, you're unstable.
Working through the equations to see how the overall aircraft pitching moment GRADIENT is affected by the tailplane/horizontal stabilizer, you'll find that what matters is the lift-curve-slope of the tailplane, and the relationship between aircraft AoA and tail AoA. The actual amount of download on the tail does not affect that calculation.
Where this fallacy comes from, I believe, is the fact that moving the cg forward increases both the download required for TRIM and also makes the aircraft more stable.
But consider this: if I magically move the tail further AFT, the download for trim reduces (due to increased tail arm) but the aircraft actually gets MORE stable.
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Thanks SMOC 
You don't happen to have any further information?
Milt,
Could you please clarify "..close to symmetrical or often just slabs.." Does that mean a "bit inverted"?
re stability, most airliners have an up cant on the engines of a couple of degrees (wing mounted) which will help.. .
Also, at transonic speeds, doesn't the center of lift move BACKWARDS on supercritical airfoils?
Cheers,
M
You don't happen to have any further information?
Milt,
Could you please clarify "..close to symmetrical or often just slabs.." Does that mean a "bit inverted"?
re stability, most airliners have an up cant on the engines of a couple of degrees (wing mounted) which will help.. .
Also, at transonic speeds, doesn't the center of lift move BACKWARDS on supercritical airfoils?
Cheers,
M
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Mad, I've heard the argument many times, but the facts remain:
1. The stab airfoil is indeed inverted on many planes. If the stab is slotted, the slot is inverted as well.
2. Aft CG is desirable, and Fwd CG undesirable, for fuel burn at a given weight/FL/SAT. Look it up.
3. If the stab fails (or even half of it), which way does the aircraft pitch? The answer is in a few accident reports. Look it up. (Hint: Lusaka, 707 freighter, 1977)
Until these are rebutted I cannot accept the "lifting tail" in principle.
1. The stab airfoil is indeed inverted on many planes. If the stab is slotted, the slot is inverted as well.
2. Aft CG is desirable, and Fwd CG undesirable, for fuel burn at a given weight/FL/SAT. Look it up.
3. If the stab fails (or even half of it), which way does the aircraft pitch? The answer is in a few accident reports. Look it up. (Hint: Lusaka, 707 freighter, 1977)
Until these are rebutted I cannot accept the "lifting tail" in principle.
Last edited by barit1; 10th Sep 2005 at 01:45.
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None of those have anything to do with STABILITY.
Aft cg is desirable for minimizing the trim load and hence trim drag. Nothing to do with increased, or decreased, stability.
The tail is often an inverted airfoil to more efficiently generate the TRIM download. Nothing to do with stability.
When the trim download is removed the aircraft will, of course, pitch nose down. Simple result of the remaining pitching moment on the aircraft.
I don't have to "look it up" by the way, but thanks for the thoughts.
Aft cg is desirable for minimizing the trim load and hence trim drag. Nothing to do with increased, or decreased, stability.
The tail is often an inverted airfoil to more efficiently generate the TRIM download. Nothing to do with stability.
When the trim download is removed the aircraft will, of course, pitch nose down. Simple result of the remaining pitching moment on the aircraft.
I don't have to "look it up" by the way, but thanks for the thoughts.
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Please be gentle with me, chaps, it's an awful long time since I did this theory work, and it's more than possible that I misunderstand the basics here. Added to that, I fly the type of aircraft with the propeller on top.
When we are talking about longitudinal stability, does that mean stability about the longitudinal axis? If it does, then don't the ailerons sort that out? Pitch is concerned with the lateral axis.
Or am I barking up the wrong tree?
Or simply barking?
Natural longitudinal stability depends on the variation of pitching moment with angle of attack
Or am I barking up the wrong tree?
Or simply barking?
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OK, Mad, but I believe Milt was referring to an aircraft's STATIC stability - its tendancy to re-establish its trim speed when disturbed.
Your point is well taken in regard to damping out the resulting Phugoid oscillation - i.e. DYNAMIC stability.
Your point is well taken in regard to damping out the resulting Phugoid oscillation - i.e. DYNAMIC stability.
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When we are talking about longitudinal stability, does that mean stability about the longitudinal axis? If it does, then don't the ailerons sort that out? Pitch is concerned with the lateral axis.
Or am I barking up the wrong tree?
Or am I barking up the wrong tree?
Longitudinal stability, and longitudinal motion, is considered to be motion in the fore-aft/up-down/pitch combinations. So that the phugoid and short period pitch oscillation modes are considered 'longitudinal'.
Lateral stability is to do with the roll axis primarily, although the couple nature of roll and yaw for most conventional designs means that we generally speak of Lateral-Directional motion, which includes the roll/yaw/sideways motions.
I believe Milt was referring to an aircraft\'s STATIC stability - its tendancy to re-establish its trim speed when disturbed.
For speed stability you\'ll find things like the derivatives of CL, CD and Cm with respect to both angle of attack and speed are what matter - and what matters from the tailplane in contributing to these derivatives are the tailplane lift-curve slope, the tail volume (coefficient) and any special interference effects. The actual amount of lift on the tail does not affect the stability.
Consider the following thought-experiment (which I\'m sure I\'ve mentioned before, but it\'s the simplest way I can visualise it for people):
You have an aircraft in-trim, at a given speed etc, with a download of 1t on the tail. You disturb it in speed and see what happens.
Now take the same aircraft, but now using a reaction-force-system - a compressed air vent or something, doesn\'t matter really - you relieve the load on the tail such that there\'s now 1/2t of trim lift, and 1/2t of \'puffer\' force, which together trim the aircraft. Introduce the same disturbance as before. You\'ll find (and if you think about the disturbance as a point by point event, much as a simulation would calculate it) that the forces generated by the disturbance are identical in both cases. Therefore the motion will bve the same, and the speed stability did not depend on absolute tail lift.
What matters is the DELTA forces and moments arising from the disturbance. And those don\'t depend on the absolute values, but on other design characteristics, such as those mentioned above.
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Tail download is not a prerequisite for speed stability either. This can be seen by considering that I can build a computer model which accurately predicts aircraft speed stability behaviour and response (or indeed an analytical model too) by reference solely to aircraft-level stability derivatives.
I guess the issue is - is the "stabilizer" a misnomer? Should it really be called "a component of an aircraft's stabilization design"??
And - of the hundred of thousands of aircraft built since 1903, how many DO NOT use negative stabilizer lift as a design characteristic?
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No, it's not a misnomer. Without the horizontal tail most conventional aircraft would be longitudinally unstable, at least at the rearwards portion of the cg envelope.
But the stabilizing effect comes from how the loads on the tail change with disturbances in angle-of-attack, not from the absolute load on the tail.
If you had movable ballast on a conventional jet, for instance, you could move the cg aftwards, eventually reaching a point where the tail load was zero for trim. You could then move it further aft, and the tail load would reverse, becoming an upload.
The cg for zero trim load is speed (Mach) dependent in most cases.
Now I can pretty much guarantee that most aircraft would still be STABLE at that cg position - damned hard to handfly, no doubt, as the stability is grossly reduced over what is normally considered acceptable, but nontheless stable. And that stability would still be being contributed to by the tailplane, regardless of the actual tail load.
But the stabilizing effect comes from how the loads on the tail change with disturbances in angle-of-attack, not from the absolute load on the tail.
If you had movable ballast on a conventional jet, for instance, you could move the cg aftwards, eventually reaching a point where the tail load was zero for trim. You could then move it further aft, and the tail load would reverse, becoming an upload.
The cg for zero trim load is speed (Mach) dependent in most cases.
Now I can pretty much guarantee that most aircraft would still be STABLE at that cg position - damned hard to handfly, no doubt, as the stability is grossly reduced over what is normally considered acceptable, but nontheless stable. And that stability would still be being contributed to by the tailplane, regardless of the actual tail load.
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Now I can pretty much guarantee that most aircraft would still be STABLE at that cg position - damned hard to handfly, no doubt, as the stability is grossly reduced over what is normally considered acceptable
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Correlation is not cause and effect.
Moving the cg aft also increases the minimum control speed. Would you argue that decreased tail trim load causes increased VMC? I hope not.
Consider this example:
I take a delta-winged type - let's say a Mirage - and put a tailplane on it (maybe top of the fin?)
Now, I trim the aircraft in level flight, using the conventional Mirage trim system of wing t/e devices. I control tailplane angle such that there is ZERO tail load.
Alongside I fly another Mirage, but with no added tailplane.
The tailplane is carrying no load, yet I absolutely guarantee that it is making the aircraft more stable in pitch, as you'd be able to see if both planes were hit by the same gust.
In fact, I could mistrim the t/e surfaces to induce either an upload or a download on the added tailplane, and the "tailed Mirage" would STILL be more stable than the conventional one.
Moving the cg aft also increases the minimum control speed. Would you argue that decreased tail trim load causes increased VMC? I hope not.
Consider this example:
I take a delta-winged type - let's say a Mirage - and put a tailplane on it (maybe top of the fin?)
Now, I trim the aircraft in level flight, using the conventional Mirage trim system of wing t/e devices. I control tailplane angle such that there is ZERO tail load.
Alongside I fly another Mirage, but with no added tailplane.
The tailplane is carrying no load, yet I absolutely guarantee that it is making the aircraft more stable in pitch, as you'd be able to see if both planes were hit by the same gust.
In fact, I could mistrim the t/e surfaces to induce either an upload or a download on the added tailplane, and the "tailed Mirage" would STILL be more stable than the conventional one.
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If you take an aerobatic aircraft with a non-symmetrical wing section (a Bucker Jungmeister would be nice, or CAP 10, or Zlin 526), roll it upside down and trim for inverted flight, the aircraft will be just as stable as right-side up. Viewed from the outside, however, the wing will now be producing a nose-up pitching moment and the tailplane will probably need to provide an up-load to trim.
A canard like the Rutan LongEz is an example where the 'tailplane' produces an upload in the 1g trim condition and the aircraft is stable.
A canard like the Rutan LongEz is an example where the 'tailplane' produces an upload in the 1g trim condition and the aircraft is stable.
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the wing will now be producing a nose-up pitching moment
the tailplane will probably need to provide an up-load to trim
Much as I'd love to, I've not flown any of these (Jungmeister, CAP 10, or Zlin 526). So I can't answer my own question.
A canard like the Rutan LongEz is an example where the \'tailplane\' produces an upload in the 1g trim condition and the aircraft is stable.
So the forward stabilizer is at a higher incidence than the mainplane (i.e. decalage) to insure it stalls first.
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I believe that what KZ8 means for his inverted aircraft is that when inverted:
The wing is now "lifting" UP wrt Earth and DOWN wrt the pilot
On an upright aircraft, a wing lifting UP wrt Earth (and pilot!) usually causes a nose down pitching moment (again wrt Earth and pilot).
Therefore on the inverted aircraft, the pitching moment is (still) nose UP wrt Earth, but nose DOWN as viewed by the pilot.
Similarly, an upright aircraft would have a tail download for trim (wrt earth and pilot)
So the inverted aircraft has (still) and earth-reference tail download, but a pilot reference tail UPLOAD.
The aircraft will obviously be at a significant negative AoA to generate the pilot-reference "down-lift" and the stick will be forward to generate the tail load for trim.
It's no different to an "upright" aircraft bunting to -1'g' in concept - negative AoA, stick forward.
The wing is now "lifting" UP wrt Earth and DOWN wrt the pilot
On an upright aircraft, a wing lifting UP wrt Earth (and pilot!) usually causes a nose down pitching moment (again wrt Earth and pilot).
Therefore on the inverted aircraft, the pitching moment is (still) nose UP wrt Earth, but nose DOWN as viewed by the pilot.
Similarly, an upright aircraft would have a tail download for trim (wrt earth and pilot)
So the inverted aircraft has (still) and earth-reference tail download, but a pilot reference tail UPLOAD.
The aircraft will obviously be at a significant negative AoA to generate the pilot-reference "down-lift" and the stick will be forward to generate the tail load for trim.
It's no different to an "upright" aircraft bunting to -1'g' in concept - negative AoA, stick forward.
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Yes, that's what I meant by outside the aircraft. Looking relative to the earth surface.
But the inverted aircraft may have a tail up-load relative to the earth surface (same direction as wing lift), if the wing aerofoil section has sufficient negative pitching moment characteristics.
The aeroplane will have the same static stability as when right-side-up.
And yes, the wing 'CP' will be forward of the aerodynamic centre.
I was trying to indicate that the tail does not have to develop a load opposite to the wing lift direction for the aircraft to be stable.
KZ8
But the inverted aircraft may have a tail up-load relative to the earth surface (same direction as wing lift), if the wing aerofoil section has sufficient negative pitching moment characteristics.
The aeroplane will have the same static stability as when right-side-up.
And yes, the wing 'CP' will be forward of the aerodynamic centre.
I was trying to indicate that the tail does not have to develop a load opposite to the wing lift direction for the aircraft to be stable.
KZ8
