Vertical CG limitations?

[CirrusF]

Do any aircraft have vertical CG limitations? Just wondering if, say, you massively load the top floor on an A380 and leave the lower floor empty, whether flying characteristics would be altered enough to matter?

[Genghis the Engineer]

It's critical for gyroplanes - the important thing being the relationship between the vertical CG and the propeller thrustline.

Can't say I've come across a limitation in any fixed wing aircraft, although it's occasionally calculated for handling qualities predictions.

G

[KiloB]

Modern (airliner) design practice of having the Thrust Line considerably lower than the Drag Line has bitten a couple of Crews recently when they discovered that there was a Vmc in pitch under TOGA Power settings. Presumably a heavy load 'upstairs' in a 380 would make this condition worse under GA acceleration.

I presume placing the Motors where they are gives easy maintenance access and provides a pitch-up moment during cruise (which reduces induced drag). But as A/C get larger and more powerful is this pitch-up moment going to become a problem? Perhaps the 146 with its thrust & drag in the same plane is the configuration of the future ;)

[keith smith]

KiloB,
1.The nose-up pitching moment due to a low thrust line has no direct effect on induced drag since this is proportional to lift-squared. However it will affect the distribution of lift between wing and tailplane, increasing tail lift, decreasing wing lift (in steady cruise the sum equals weight). I have no ideaas to which wins, or if the effect is significantin the first place. Best to ask an experienced performance engineer.
2.While the vertical position of the CG has zero effect on this phenomenum, it clearly can in a dynamic situation with sudden change in thrust alone. For this one best find a handling exppert or simulator engineer.

[Mark1234]

keith smith: unless I'm missing something the tail feathers are generally generating lift in a downward direction, resisting the inherent tendancy of the wing to pitch nose down, thus I would imagine the low thrustline 'unloading' the elevator might have some efficiency benefits.. whether it's significant is another matter.

However, in reality the thrustline / engine configuration has far more to do with the practicality of where to hang the things - high wings are more of a pain to engineer (a'la 146), and pod mounted engines seem to be the solution vs wing root/back end of fuselage (bending moments etc). the pitch couple is an (undesirable?) side effect that has to be lived with.

ISTR seeing something somewhere out there with the pod mount flipped upside down putting the engine over the wing... along with a bunch of marketing waffle that I can't remember about how much better that config was. Doesn't seem to have caught on however..

[ICT_SLB]

Mark wrote: "ISTR seeing something somewhere out there with the pod mount flipped upside down putting the engine over the wing... along with a bunch of marketing waffle that I can't remember about how much better that config was. Doesn't seem to have caught on however.. "

Beleive you're talking about the Fokker-VFW 614 19 seat regional jet. It had two MTU turbines mounted over the wing. IIRC the reason was "better shielding of overflight fan noise by the wing". I remember being just about knocked out of bed when staying at BAE's visitors accomodation at Filton when one did a run-up outside - and you thought a 1-11 was noisy! A similar layout is currently being used by Honda on the Hondajet.

[keith smith]

Mark1234, I am quite sure the normal tailplane loading is upwards because the CG isnormally behind the wing centre of pressure (quater chord point) but ahead of the static stability point. I remember it well from my ancient days in Aero Flight RAE.We did some trials on a Lancaster with a tailload restrictor that literally locked the elevators if the load exceeded a particular value. Result was that if you set the load limit below the value you might expectin a steady pullout or turn, you ended up with a situation from which you could not recover-if you pushed the stick forward you couldn't move it because transiently you would increase the upload transiently. Bet your life I once set the load limit too low, but fortuantely even in my reckles days I had included a simple switch that could disengage the whole wretched system.:)

[Mark1234]

I don't have your experience, but empirically there's an awful lot of aeroplanes that sport a cambered aerofoil at the back end - and it's stuck on upside down.. take a look at the back end of the next 737NG you see :)

I also struggle to see how the centre of gravity can be significantly behind the centre of pressure without the aircraft becoming pitch unstable - maybe a small distance, but I would imagine the COG range for the average airliner is rather significant with all those bodies wandering about.

[John Farley]

Dear Keith

I don't know when you were on Aero Flight but by 1964 all their aircraft used tailplanes that lifted downwards so I am with Mark on this one.

BTW I am glad to say that induced drag is not proportional to V squared (or we would all be in trouble). Induced drag increases as you go slower (as in not so fast) and is therefore proportional to one over V squared.

Regards

John

[keith smith]

Dear John and Mark,
1. You are right about induced drag being inversely proportional to Vsquared. Aslip of the pen
2.I still maintain that cg can be significantly behind wing cp because one purpose of tailplane is to make wing+tail cp behind cg,thus ensuring static stability. In such a case, for trim,tail lift would have to be upwards.
However, if you wanted the wing cp to be behind the cg, then the trim load on the tail would be downwards (I hope I have got that the right way round---I am sure somebody will tell me if I havn't)
Keith

[Mad (Flt) Scientist]

2.I still maintain that cg can be significantly behind wing cp because one purpose of tailplane is to make wing+tail cp behind cg,thus ensuring static stability. In such a case, for trim,tail lift would have to be upwards.
However, if you wanted the wing cp to be behind the cg, then the trim load on the tail would be downwards (I hope I have got that the right way round---I am sure somebody will tell me if I havn't)
Keith

Sorry, not true in a couple of regards.

The purpose of the tailplane is not to locate the CP in any particular place; it is to make the aircraft stable (and controllable, but lets stick with the "Stability" part of S&C for now).

An aircraft is stable in pitch when the gradient of pitching moment coefficient with respect to lift coefficient is negative - which means that if I disturb the aircraft so as to create more lift (and more CL - which USUALLY means more AOA) then I also get a more negative pitching moment, which will pitch me back nose down again, opposing the initial disturbance.

Therefore what matters in stability are the so-called neutral points - the notional cg position where d(Cm)/d(CL) is zero. At those points the aircraft is neutrally stable in pitch - any further aft and its unstable. There are two neutral points one may consider - one tail-off and one tail-on (for a conventional layout, naturally). The tail-on one includes the stabilising effect of the tail, as one would expect.

For a naturally stable aircraft the cg range MUST be ahead of the tail-on neutral point; it may, and often is, behind the tail-off neutral point. Most aircraft would be unstable if the tail fell off, therefore.

Now, all that only relates indirectly to both the load on the tailplane and the cp, because the relationship between cp and pitching moment is not direct - a cambered airfoil will generate a pitching moment even at zero lift - mechanically, a 'couple' - so some tail load is required for trim even where there is no wing lift (and where the wing cp is basically undefinable). And because the tailplane lift is a trim force, it depends on the actual magnitude of the pitching moment - whereas stability is concerned with the gradient of the pitching moment.

So while the cg can indeed be behind the wing cp, that says nothing special about the tail lift (and, indeed, since most airfoils will generate a nose-down pitching moment, it's likely that you'd need download on the tail, even if the cg were at or behind the wing cp. There ARE indeed circumstances where one can have an uploaded tail on a stable aircraft, but they are rather less common than your post suggests, and don't apply to all aircraft. There's been some discussion on this last point a number of times IIRC.

[barit1]

Tangential, but related:

The DC-10 & MD-11 have two donks low, one donk high -- and I am told that a #2 engine failure causes a worrisome nose-up pitch. It can readily be trimmed out, but it's a handful for a short time.

Also - 30 years ago a thoughtful F/E came up with a simple drag-reduction scheme for the DC-10: Run #2 engine at reduced thrust in cruise, so the stab won't have to work so hard fighting the nose-down moment of the high engine. It would have worked, were it not for the efficiency falloff of gas turbines at reduced throttle: thrust falls off faster than fuel burn. So, you lose more than you gain. :ugh:

[Mad (Flt) Scientist]

To answer more directly the original question.

Aircraft do have vertical cg limitations, but they are generally a "design consideration" only, with the intent being that the eventual operators don't have to worry about vertical (and, indeed, also lateral) cg loading considerations, because the OEM has considered a representative cg envelope in all three axes (plus associated inertia effects as appropriate) in developing both the airframe and component loads, and assessing the aircraft handling characteristics.

Certainly on a small aircraft carrying external loads representing a significant proportion of the overall mass, the resultant vertical cg variation was not insignificant and was considered as part of the conditions for each clearance.

For larger aircraft I'd expect the norm to be, as for lateral cg, to consider various nominal and worst case loading scenarios, and use them to create a design envelope. Generally speaking the effects are both smaller than the longitudinal case and also represent smaller variability, so there's little risk of an abnormal configuration taking you to an unsafe condition.

To address the question of low-slung engines and aircraft dynamics, cases that have to be considered include both the basic trim requirements of 25.161 (or equivalent) which cover a range of power settings (also implied by various other regs), plus dynamic cases including both the GA case mentioned and, quite important, stall recovery - you have to assume the pilots will be applying power to accelerate at the same time as either they or the SPS/FBW system is trying to lower the nose; the engines will therefore negate some of your recovery control power, and this must be considered in, for example, tail/elevator requirements.

[barit1]

Does anyone have numbers for the NASA 747 Shuttle carrier? Or the AN-225 for that matter?

[gr8shandini]

Sorry, no numbers, but I can say that it gets very complicated on something like the shuttle carrier (there's gotta be a good nickname for that thing) or an E-3. The strapped on bits for those aircraft are mounted at an AOA designed to give neutral lift (i.e. lift equal to their own weight) at cruise. CG doesn't change, obviously, but I wouldn't even know where to begin with the stability side of things.

Edit: Sorry, I just thought about that for a minute. I know the neutral lift statement is correct for the E-3, but obviously, for the shuttle to be hauling it's own weight at whatever the cruise for the carrier turns out to be, it's AOA would have to be in the 10s of degrees. See, I don't even know where to begin thinking about where to begin.

[Capt Pit Bull]

CG doesn't change, obviously,

Not quite sure what your saying there.

In relation to the original aircraft? Surely the CofG will be higher.

pb

[gr8shandini]

I was thinking of the complications and apparently not communicating well. I meant that the actual CG of the as assembled vehicle doesn't change throughout the flight, but weight on the mounting points that the original aircraft experiences changes with airspeed.

I'm not an S&C guy, so it seems complicated to me, but I guess if you consider it as one vehicle (albeit a very oddly shaped one) you can work out the stability derivatives just like anything else. But being a tester, I'd probably just head to the wind tunnel.

[barit1]

An example of a very low CG:

http://i17.photobucket.com/albums/b78/barit1/aw2511.jpg

With a half-ton of fuel under the floor, the Howard DGA was ultra-stable in roll. The ailerons seemed a bit lighter as you burned off some fuel.

[Pilot DAR]

I shall add to the discussion...

Consider the Lake LA-4 Amphibians, whose engine is pylon mounted entirely above the fuselage of an otherwise common layout aircraft. The H stab is logically located in the propwash, but other than that aid to pitch control, no special provisions seem to have been made to offset the affects of the engine/prop mass way up high. The plane flies very well, with power changes only slightly affecting pitch (mostly when operating on the water - but entirely controllable). Having done extensive flight testing including stalls, spins, and balked approaches, I can attest that there is no negative affect from the high C of G. Even the fuel is not in the belly! A well designed aircraft in this regard.

Pilot DAR

[CirrusF]

Aircraft do have vertical cg limitations, but they are generally a "design consideration" only, with the intent being that the eventual operators don't have to worry about vertical (and, indeed, also lateral) cg loading considerations, because the OEM has considered a representative cg envelope in all three axes (plus associated inertia effects as appropriate) in developing both the airframe and component loads, and assessing the aircraft handling characteristics


Thanks for addressing my question, rather than digressing into the question of vertical thrust/drag moments which is an issue obviously addressed at design stage. I'm still interested to know if (say) an A380 could be pushed outside of the flight control envelope by excessive loading on one deck. I suspect the answer is no, but am also interested to know if this has been modelled and tested?

[actus reus]

I
I believe that the now delayed A380F was to have the upper deck lowered approx 4" with reference to the pax version, to allow the use of large cargo pallets on the upper deck. The need here was to increase the available, combined total floor loading towards 10kg/square m. The flight control's authority to cope satisfactorily with the range of vertical CG was not in question IIRC.
I know you don't want to hear it but I can't help myself. I agree with MFS etc; the lift on the tailplane is negative in most civilian aircraft. If you wish to improve the SFC in the cruise, an aft CG will allow a reduction in trim drag on the tailplane. An aft CG will also help with the RTOW / Vspeed calcs.

If you wish to go even further but still shy away from exotic flight control systems such as the F16, then a reduction in the static margin will do even better (health warning on this though).
Under wing engines, amongst other things, allow the pylons by which they are attached to act as vertilons and the 'jet upset' recovery proceedure adopted by both BAC and AI (Mike C was the Boeing tp; I can't recall the AI tp) does call for the thrust to be reduced in the extreme nose high/low IAS situation
Actus

[werbil]

Re: LA-4 amphibian.

I am of the impression that the elevator trim on an LA-4 is set up to overcome the high thrust line. I recollect something to do with the size and rigging of the horizontal stabilizer trim tabs and location relative to the propeller wash causes the aircraft to pitch nose down on a power reduction (ie engine failure) despite the high thrust line. The cost I understand is cruise speed - I've heard of pilots using a bungee on the control column in cruise to reduce the trim tab offset resulting in a faster cruise.

Can anyone confirm or dispel this?

[barit1]

The trim drag must be considerable. See:
http://cdn-www.airliners.net/aviation-photos/small/6/6/1/1257166.jpg