Are stretched aircraft less stable?

[Doors to Automatic]

I have read a lot of reports on this forum and elsewhare about a variety of landing problems with stretched aircraft.

The MD-11 has ended up on its back on more than one occasion whilst the A321 appears to be prone to developing high sink rates on short final often resulting in a tailstrike when last minute corrective action is taken.

Most recently I have heard a number of anecdotal reports of very hard landings in B737-800 aircraft.

This leads me to ask whether stretching an aircraft makes it less stable and more difficult to land?

As always any comments are appreciated.

[Mike Oscar]

It certainly makes them more trim-critical, for obvious reasons. Most 'short' versions (737-500, A319, ATR42) are much easier to trim, and more forgiving than their stretched family members (737-400/800, A321, ATR72)

Mike Oscar

[twistedenginestarter]

I'm talking off the top of my head here - but it's not against the rules. I can't see why length of plane affects stability. The greater the distance between cg/cp and the rear stabilising assembly, the greater the stability - other things being equal. But there is no reason why they should be equal. I would have thought Boeing or Airbus would adjust tail surfaces for each model in order to optimally meet stability targets.

Longer planes are probably more sensitive to attitude control as they seem to have the same size of landing gear and thus have more limited allowable pitch ranges.

['%MAC']

Are we talking about a problem in control or stability?

An increase in fuselage length increases the weight of the aircraft proportionally. With an unchanged wing area this will increase the wing loading. The relationship between wing loading and dynamic pressure is ½ dv^2, so a higher wing loading will give a higher stall speed which will necessitate higher landing speeds. Lower ceilings and cruise speeds are also present when increasing the fuselage length without changing the wing. The higher the weight per unit length of wingspan, the faster the downwash, and the less efficient the horizontal stabilizer if it is affected by wing downwash (i.e. not a T-tail).

The larger the aircraft in terms of mass and inertia the larger the tail control and stabilizing surfaces required. Increased inertia makes the aircraft more sluggish in response to control inputs. If upset from its original state, greater control authority is needed to damp and oppose the motion. Just as it takes more of a force to stop a spinning car tire (tyre if you prefer) then a bicycle tire. F=mv^2/r

If we add appropriate plugs in the fuselage, without changing the control surfaces we will require larger deflections of those surfaces to initiate a change. This is as above, to overcome the increase in mass and inertia. As the change takes effect, a larger damping force will be required to bring the aircraft back to equilibrium. From the pilots perspective it will seem sluggish in control and the short period pitch oscillation (SPPO) will not be as highly damped.

[Edited: This is the second edit, changed format and key concepts, deleted phugoid and eigenvalue comments, thanks all for their tolerance]

[QAVION]

"Control: If we add appropriate plugs in the fuselage, without changing the control surfaces we will require larger deflections of those surfaces to initiate a change."

If this is the case, I'm just wondering why the stumpy version of the 747 Classic, the "747SP" had a larger tail :confused: Surely a longer "arm" would mean less input required to initiate a change?

Rgds.
Q.

[basil fawlty]

Stability on long bodied a/c is the same as on short bodied a/c. When the particular type is designed, the areas of horizontal stab and the fin are sized accordingly. Long a/c results in smaller surface areas due to the longer distance, and therefore moment, to the c of g, and vice versa.

[gofer]

As a PAX I can certainly "feel" a difference between the A319 the A320 and the A321 which I have never really been able to sense in such a clear way in the DC9/MD80 or in the DC10 & 747 families.

The A 319 seems to twist left & right around the center point more than the A 320, whereas the A 321 seems to pitch up and down around the center point more. The A 321 also shudders as it takes off just after rotation, which I have so far never noticed on the smaller planes. :o

[In trim]

QAVION - Are you sure you've got it the right way round.

Add plugs to a fuselage and the length of arm gets longer. As with any 'see-saw' the amount of pressure required to produce the required effect will REDUCE as the length of arm gets longer. i.e. To produce the required effect on an A321 will take less elevator deflection than an A319.

The other example above clearly illustrates this....when Boeing shrunk the 747 to produce the 747SP they had to increase the size of the tail in order to produce greater forces to compensate for the length of arm.

In trim

[Pilot Pete]

Quite a few of our captains hsve commented how the 757-300 is nicer to hand fly than the 200........mind you that could be just because they're brand new!!! I'm reserving judgement.

PP

['%MAC']

Qavion, I see your point. As you are familiar with the B-747 I do believe that the SP has a tail that is 10 feet wider then the –200 and of different aerodynamic design. It appears to have a higher sweep back angle with a possible extension of the chord (this latter part I’m not certain of). The tail volume is the tool used to determine CG range and is the product of the tail area and the moment arm about the aircraft CG. It is expressed mathematically as:

V = LSt/ cS

Where L is the tail arm, St is the area of the tail, c is the wing chord, and S is the wing area

Let’s say Boeing wants the same tail volume which is proportional to stability. If they decrease L by shortening the fuselage the stability will be directly affected, the CG range will be decreased, so they increase St to compensate. So much for stability and CG range.

The increased tail size on the SP is from many different factors, one being the tail volume, the other being the desire to retain an effective force moment with a reduced arm. You are correct. If they stretch the 747 again then the tail will also need to be bigger because of the increase in moment and inertia as discussed above. Tail size is not necessarily a linear relationship to fuselage length; there are many confounding factors.

I think it’s correct this time, thank you for your previous advisement.
:)

[QAVION]

"QAVION - Are you sure you've got it the right way round."

Er...I'm never sure of that, "In Trim" :D

However, I thought I was saying the same thing as you(?). :)

Cheers.
Q.

[BigJETS]

.....so youve left me wondering, how is pitch control in a 747 classic compared to the SP? Are you saying the shorter SP was/is actually more sluggish than say a -200?

['%MAC']

I'm not saying that, but I don't fly them. I think John Travolta just purchased one, if I meet him, I'll ask. :D

Who knows what the Boeing Boys did with the flight controls on the inside. Mathematically the SP should be less stable and more lively in control when compared to its big brother. When they shortened the fuselage, the neutral point was moved closer to the CG range, so the CG margin became less, meaning the longitudinal stability is less. SP after all is Special Performance, not Sluggish Performance. As a juxtaposition, ever hear of a PS (Pitts Stretched), no I haven't either. :)

[Nano 763]

"Longer planes are probably more sensitive to attitude control as they seem to have the same size of landing gear and thus have more limited allowable pitch ranges."

On the:
767-200 tail strike pitch attitude is 13.1 degrees
767-300 tail strike pitch attitude is 9.6 degrees
767-400 tail strike pitch attitude is 9.4 degrees

I hope this helps.