commercial airliners and negative stability

Factual Questions
1

Conventional aircraft design places the center of mass slightly forward of the wings’ center of lift. During level cruise, the horizontal stabilizer provides downforce, preventing the plane from pitching down. It’s generally stable: if airspeed drops, tail downforce is reduced, the nose drops, the airplane gains speed, and it levels out again. An inattentive pilot is likely to end up with a phugoid flight path, but the plane won’t flip and tumble out of control.

The F-16 was the first fighter aircraft with fly-by-wire technology. A big benefit of FBW is that it enabled the aircraft to be designed with negative stability: the center of mass is placed behind the wings’ center of lift, and during level cruise, the horizontal stabilizer provides lift. This is basically unstable: if the aircraft slows, the tail lift is reduced, and the aircraft pitches up, further reducing speed (and causing the nose to pitch higher and higher) until the plane stalls. FBW allows a computer to make constant adjustments to the flight control surfaces to maintain controlled flight, in spite of this basic instability.

A fighter plane benefits from negative stability because it makes it really maneuverable. However, an added bonus for the negative stability configuration is more efficient cruise. Whereas the wings of a conventional aircraft have to produce enough lift to counter all of the weight of the aircraft PLUS the downforce from the tailplane, the wings on a negative-stability aircraft only have to produce enough lift to counter most of the weight of the weight of the aircraft (the tailplane provides a bit of lift to carry the rest of the weight). With less total lift being produced, there is less drag being produced, resulting in better fuel economy than you would achieve with an identically-shaped aircraft that has its center of mass forward of the wings’ center of lift.

With all of that out of the way, here finally is my question:
Many commercial airliners are now fly-by-wire. have they been designed with negative stability on order to make them more efficient? Or are they still being designed with the center of mass forward of the wings’ center of lift?

2

Boeing says that the trend in modern planes is to move the center of gravity back more and use computers to compensate for the stability issues. The reason they state is exactly what you said, to improve cruise efficiency.

Entire article has MUCH more info in it here:
http://www.boeing.com/commercial/aeromagazine/aero_02/textonly/fo01txt.html

(btw, many years ago I worked on the radar for the F-16)

3

No one will ever make a commercial jetliner with negative stability. You think the de Havilland Comet had a bad time of it? Try imagining the shitstorm when a plane designed to be unstable crashes, killing 300 people.

F-16 have ejection seats. And they use them.

4

I’m pretty sure that FBW airliners have some kind of manual reversion, so it still needs to be flyable manually. I have always encouraged our load staff to load us with an aft CofG though, for the reason you’ve pointed out, it is more efficient.

5

Very interesting article, thanks for the link.

Coincidence, my brother also worked on the F-16 many years ago, assessing its radar/acoustic/IR signatures.

You may be right. I would guess that the efficiency gained by moving the CoM aft of the wings’ CoL would be relatively little. But as engineer_comp_geek’s article indicates, they’re pushing the CoM farther back than they used to, to the point that they are having to use the flight control computer to augment its stability.

I can’t say I disagree with that sentence, but I’m not aware of any F-16 crashes that were due to a malfunction of the FBW flight control system.

6

The tail plane has a large down force ? WHAT ?

You’ve extrapolated from stunt aircraft to passenger liners…

7

Can I piggyback on this thread and ask a question I’ve had for years? I’ve heard that the horizontal stabilizer had to produce downforce, but I’m still not completely clear why.

When the plane slows down, the downforce generated by the stabilizer will decrease, but the lift generated by the wings will also decrease. Both of those will have the effect of lowering the nose.

Suppose instead that you had the CG aft of the wings’ CL, and the horizontal stabilizer was producing positive lift. Could you choose different airfoil shapes for the wing and stabilizer such that the wing was more sensitive to changes in airspeed and angle-of-attack? That is, as airspeed drops, the lift provided by the stabilizer decreases slightly (tending to make the nose rise), the lift from the wings decreases more (tending to make the nose sink). The wings (being more sensitive to changes) win out, the nose drops, the plane picks up speed and is stable.

But it sounds like they don’t do it that way. There’s something I’m not seeing; what is it?

8

No, he’s quite right. The tail plane has a down force.

Robot Arm, it’s better to think about it in terms of angle of attack rather than speed as lift is closely related to AoA. If you have your tail plane set so that in normal cruise flight its AoA is -2° and the wing’s AoA is 4° and a minor disturbance increases the AoA by 2°, the wing’s AoA goes to 6°, a 50% increase, while the tail plane goes to 0°, a 100% decrease. The effect at the tail is much more than the effect at the wing so there is a pitch down which reduces the AoA, thereby correcting the original disturbance.

IANAA but I think the effects produced by using AoA in this way is far greater than you could get by using the aerofoil shape.

Speed stability is more a product of drag and flying at speeds faster than the minimum drag speed (Vmd.) Total drag is made up of parasite drag due to friction which is proportional to the square of the speed, and induced drag which is related to angle of attack and is inversely proportional to the square of the speed. When plotted with drag on the Y axis and speed on the X axis the two drag sources combine to make the total drag curve which is shaped like the bottom of a “U”. At the very bottom of the U drag is at a minimum, go faster or slower and drag increases.

We normally fly faster than Vmd, in this speed range total drag increases the faster you go and decreases the slower you go. Consider an aeroplane in straight and level flight. Speed is constant so drag is constant and thrust required is constant. If engine thrust is unchanged and the aircraft is slowed slight by some external force (a bit of turbulence for example), the drag decreases, thrust is now in excess of that required to maintain the new speed so the aircraft speeds up until thrust equals drag again. If you got a bit fast the opposite happens, drag increases, thrust is less than required to maintain the new speed so the aircraft slows. So at these speeds the aircraft is speed stable, it will automatically tend to correct any deviations.

If you fly slower than the min drag speed you get into the speed unstable range. At these speeds drag increases as you get slower and decreases as you get faster. If you slow down, drag increases which slows you down more which causes drag to increase more. The opposite happens if you speed up. In this speed range there is a tendency for errors to compound and large changes in thrust may be needed to keep a desired speed. The only time you’d normally be in this speed range or close to it would be on approach to landing.

9

The OP’s clear explanation helps me better understand why merely pitching the nose down would have saved the day for that doomed Air France jet with inoperative sensors (the pilots didn’t know which sensors to trust) that went down off the coast of Brazil.

But, as a non-pilot, I can also understand why, when you’re scared, pointing the nose more toward the ground is not an easy thing to choose to do.

10

Current airline & former F-16 pilot …

Ref airliner manual reversion …

All Boeing’s aircraft have (so far) used FBW as an overgrown auto-trim system. You’re always flying in manual reversion with the FBW system “helping”. If it quits or goes insane, the amount of negative help it provides is small enough you can readily maintain control except perhaps in the extreme corners of the speed/altitude envelope.

All current Airbus aircraft have no manual reversion. You are always controlling the computer which in turn controls the airplane. If it goes 100% stupid the pilots become passengers too. Now it does have various less-smart modes intended to allow it to degrade gracefully in the face of sensor failures or computer failures. And it is very, very redundant & engineered for high reliability. To the best of my knowledge there’s never been a total FBW failure, and while there have been innumerable degraded-mode events, none prior to AF447 resulted in an accident.

In my book AF447 was a result of below-average skill meeting a situation where the cockpit engineers had designed a total cockpit system that required above-average cool to handle the malfunction some other engineers had built into the airplane’s subsystems. Were I King of Aviation there are several major things besides pitot heaters that I’d order changed as a result of the lessons learned.
As to F-16s & crash / ejections due to FBW …

In the very early days, say 1978-1983, there were many accidents attributed to the FBW system going stupid & the airplane going ballistic. The quad-redundant FBW computers had IIRC three normal & backup power sources but would occasionally all decide all those sources were no good and switch to their emergency internal batteries which only lasted a handful of minutes. Frequently the malfunction would not give any cockpit indication either since the normal & backup power supplies were in fact working fine.

When those batteries depleted the jet became an unguided dart; the control surfaces all moved to neutral. By an unhappy twist of engineering luck/non-skill, the neutral position of the tailplane created an instant -4G nose-over which was very difficult to eject from. There were very few survivors of these accidents.

In the early 1980s they did a major redesign of the FBW power supply to give it IIRC five independent power sources and much improved logic on source switching and improved cockpit warning. They also re-rigged the tailplane so if the FBW did go stupid and the jet went ballistic it performed a 2-3G zoom instead of the nose over.

By about 1983 all new jets had this improvement and by about 1986 all the existing fleet had been retrofitted. We all felt real good about this modification.

As I understand it, the F/a-18, Typhoon, Rafale, MIG-29, Su-27 and all subsequent first-line fighters by any manufacturer are also all pure FBW and doubtless have their own failure modes and horror stories.
For folks worried about RSS in airliners … Wait there’s more …

We also now have relaxed static strength. Traditionally aircraft were designed to be physically strong enough to fly through X level of turbulence / G forces once without something breaking. And to fly through Y lower levels of turbulence / G forces for umpteen years without fatigue cracking from the repeated bending.

But all that beefy structure weighs a lot which costs fuel = money to drag around for all those years just in case.

So now we have Taa Daa: computerized load alleviation. They build the wings and tailplane more flimsily and then have a computer monitor the turbulence loads on the wing and wiggle the various control surfaces to offset the bumps so the lighter wing isn’t overstressed.

So far this feature is used only out at the very edges of the envelope; it’s like they cut out a couple percent of redundant structure, not 50%. But as the pressure for every more efficient and ever more green aircraft continues, this will also be an emphasis area for ongoing improvement.

I’m not at all worried about this in the current generation of aircraft. I do wonder about the fresh designs they’ll be building in the 2040s. I guess we all get old enough to become fuddy duddy’s and I think I can see my time coming.

11

I’m at a loss to specify a relative magnitude, but the fact is that yes, for nearly 100 years, conventional aircraft were designed to be fundamentally pitch-stable by placing the aircraft’s center of mass forward of the wing’s center of lift. If the tailplane didn’t exist, then the aircraft would be a giant lawn dart: gravity would pull down on the CoM and cause the aircraft to pitch down until it was in a vertical dive. To prevent this, the tailplane exerts a downforce during straight/level flight.

Even stunt aircraft are designed this way. To my knowledge, there are no aerobatic aircraft presently available that employs negative stability (CoM aft of wing CoL, which requires fly-by-wire to make it flyable).

Anyone who has ever assembled a radio-controlled model aircraft knows that the instructions typically include a procedure for positioning the CoM forward of the wings. This includes aerobatic models, gliders, and everything in between. That said, this may have changed recently with the advent of cheap pitch-rate sensors and lightweight computational capability; I recall a YouTube video in which a hobbyist had built a prototype plane with computer-augmented pitch stability so he could engage in sustained extreme high-alpha flight.

12

That doesn’t completely answer my question. (And I may have had the details backwards before.) Suppose an airplane is designed so that the horizontal stabilizer generates lift. In level flight, both the wing and stabilizer are at a positive AoA. If something should nudge the nose upwards, the AoAs (for both wing and tail) would increase, and lift would increase. But it seems to me that if the lift of the stabilizer increases by a greater percentage than the wings, the tail would tend to lift more than the wings, that would bring the nose down and you’d return to level flight.

But I know next-to-nothing about how different airfoil shapes respond to changes in AoA (or airspeed). Maybe there aren’t shapes that have the kind of performance I’m trying to describe, or there’s something else I’m missing.

(I can see that the tail would need to be able to generate downforce on takeoff, otherwise you’d never be able to rotate. You’d need to move the main gear aft (or go back to taildraggers). If CG was behind the wings’ CL, and the main gear aft of that, it would certainly look different than current planes.)

13

The horizontal stabilizer produces negative lift in all phases of flight, except in a rapid nose-down pitch maneuver. Something has to balance the moment around the CG of the wing CP.

14

Operating from decades-old memory here, so grains of salt etc.:

The McD MD-11 moved CG rearward in cruise by pumping fuel into into the tail. I’m pretty sure it never approached an aft CG condition, but they did sacrifice stability at cruise for efficiency. As fuel was used, the CG gradually moved forward, preparing for the additional stability needed during approach and landing. I don’t know if other modern airliners use this form of CG control.

ISTR that step 1 of the system’s responses to inflight problems (ie. engine failure) was to quickly pump the fuel forward to regain stability. I worked on the FMS system back in the 90’s (Honeywell) and saw a lot of how the system worked in the simulators, but I admit my memory may be hazy.

15

Tail fuel tank(s) are a pretty common feature in long-haul jets. IIRC the later 747s, the 777, and 787 have such tanks at least as options. I vaguely remember the 767-400 having that as an option, but may be mistaken. Concorde also had tail fuel tank(s). I don’t know enough about Airbus products to hazard a guess.

16

The Concorde did pump fuel between tanks to control CG.

17

So if the trend continues (more reliance on FBW to achieve better fuel efficiency), what would be the ultimate end result? Airliners that look like the B-2 bomber?

18

The Concorde was unusual because the center of lift moved significantly aft during supersonic flight; the center of mass had to be adjusted by relocating fuel in order to avoid resorting to large trim inputs, which would have increased drag to an unacceptable degree.