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The MD Balance Bit  
User currently offline747-600X From United States of America, joined Jan 2000, 2829 posts, RR: 13
Posted (16 years 4 months 5 days 11 hours ago) and read 1418 times:

A newer view on some older topics...
Almost all Douglas aircraft are off balance. The DC-9, DC-10, MD-80, MD-11, and MD-90 are all designed to be off balance. I have spent years trying to figure this out, and with thousands of pounds of passenger you'd think it would make some difference. In my opinion, this is one reason that Douglas aircraft spend so much time crashing. The other thing is the wings. It was said that they are so far back to counter the weight of the engines, but that makes no sense because the DC-9's engines would then before forward of that gravity center. The MD-90, the longest of the DC-9 uglies, has its wings so far back that I seriously cannot imagine how it lifts up... imagine lifting a pole lengthwise and trying to point it upwards by lifting it from the back end... it just doesn't seem possible. Frankly, the entire off-balance design seems very poor.
So here's the question: Why did they design 'em that way?

17 replies: All unread, jump to last
User currently onlineSpacepope From Vatican City, joined Dec 1999, 3390 posts, RR: 1
Reply 1, posted (16 years 4 months 5 days 11 hours ago) and read 1239 times:

I disagree. Firstly, designing an aircraft to be intentionally offbalance yet still controllable would be so annoying to the engineers that it is reserved for fighter jet contracts that use expensive FBW control ssytems. With a swept back wing such as those of commercial aircraft, you must take into account the stall charachteristics of that wing whendetermining the position. the wingtips tend to flex and achieve a higher angle of attack than the roots, so that they tend to stall first. this naturally shifts the center of lift forward, making this situation (most prevalent at low power settings) a non issue. Under normal circumstances, any difference between location of center of gravity and center of lift (in mainly yaw and pitch directions) is compensated for by basic aerodynamics of the craft. But normally, this difference is kept at a minimum. If unstable aircraft were a benifit, then we'd be seeing 777's with 2 engines on one wing and none on the other.

The last of the famous international playboys
User currently offlineWingnut From United Kingdom, joined Jun 2005, 1 posts, RR: 0
Reply 2, posted (16 years 4 months 5 days 10 hours ago) and read 1230 times:

What are you talking about?? Douglas aircraft are not out of balance. Nobody in their right mind would design or load a aircraft in a out of balance condition. Imagine, if what you say were true, control surface deflection or trim would be required to maintain straight and level flight. The more control input, the more drag. The more drag, the more fuel burn. All aircraft are designed to fly hands off, straight and level at cruise.

User currently offline747-600X From United States of America, joined Jan 2000, 2829 posts, RR: 13
Reply 3, posted (16 years 4 months 5 days 1 hour ago) and read 1214 times:

Yes, obviously, and spacepope who are you disagreeing with? The aircraft ARE off balance whether you like it or not, as Matt D once said, the DC-9 megafamily has always been 2-3, the DC-10 and MD-11 collective is frequently 3-4-2. The wings on the DC-9 stayed where they were when they made the MD-80, they added 20 feet in front of them thus increasing stress on the wings. It's not my opinion, its fact, I was hoping someone could explain WHY THEY WOULD DO THAT, not your views on how to design airplanes.

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 27
Reply 4, posted (16 years 4 months 5 days 1 hour ago) and read 1218 times:

You have NO idea what your talking about. The reason the wings are so far aft is to keep the center of gravity at the center of lift.

The 20 foot plug you speak of would still allow the aircraft to be in the balance envelope.

The second supporting evidence that they are balanced is that they are flying. Haven't you noticed.

Weight X Arm = Moment. You do the math.


You don't understand the first thing about weight,moment and CG. This is evident. If you would like to ask someone to explain this to you instead of posting this nonsense I'm sure someone would be happy to.

Until then....

User currently offlineTEDSKI From , joined Dec 1969, posts, RR:
Reply 5, posted (16 years 4 months 5 days ago) and read 1214 times:

I don't agree with this crap!!! Apart from the DC-10/MD-11, MD had been building good quality military and commercial aircraft for years with great success. The DC-9/MD-80 have never crashed because of some kind of design flaw. The Air Force has the F-15 Eagle and C-17 Globemaster III that are reliable aircraft.

User currently offline747-600X From United States of America, joined Jan 2000, 2829 posts, RR: 13
Reply 6, posted (16 years 4 months 5 days ago) and read 1213 times:

How can you disagree with facts?

And yes, I understand the physics of it a-plenty. That's not the issue. The issue is - why would you design an aircraft to have more weight on one side than the other. Also, as far as being 'in an envolope' is concerned, wouldn't you still want to aim for the center of that envelope?

User currently offlineLHMark From United States of America, joined Jan 2000, 7255 posts, RR: 42
Reply 7, posted (16 years 4 months 5 days ago) and read 1205 times:

Facts are facts, but there are an awful lot of them, and they ALL have to be looked at. I doubt the aeronautical engineers at MD designed, rolled out and flight tested and sold two whole families of aircraft to most of the airlines around the world, and then slapped their foreheads and said "duh... we made the whole thing off-balance!"

"Sympathy is something that shouldn't be bestowed on the Yankees. Apparently it angers them." - Bob Feller
User currently offlineFxra From United States of America, joined Jul 1999, 736 posts, RR: 2
Reply 8, posted (16 years 4 months 4 days 23 hours ago) and read 1201 times:

As far as the DC-10/MD-11 goes, yes the airplane, when empty (i mean a freighter version.. no seats, galleys, anything) had a CG that is out the aft of the envelope. However, if your loading the plane, its very very easy to have a CG that runs to far forward. B727s are the same way. A310s are the opposite being nose heavy. Its not just a Douglas phenomenon.

As ffor the seating arrangment, its totally about putting as many people on the plane as possible. The 3-2 arrangement on the DC-9 or MD-80/90 is all about putting as many ppl in the alotted width. A 2-2 arrangement would be less economical and a 3-3 would be impossible if you want to keep an aisle (i think the FAA requires that) or you want to have seats for ppl who are bigger than the standard 5 year old. The DC-10 is the same way. The seats could as easily be 3-4-2 as 2-4-3. Actually, the only pax DC-10 i've been on had a 2-5-2 arrangement. You have roughly +/- 30,000 lbs of lateral imbalance to play with on these planes. The planes are not built laterally imbalanced to any great significance.


Visualize Whirled Peas
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 27
Reply 9, posted (16 years 4 months 4 days 21 hours ago) and read 1196 times:

The weight ditribution is roughly 50/50 give or take 1-3 % for the entire CG envelope for every aircraft ever flown. The weight distribution is centered around the center of gravity which is if you haven't guessed located where the wings are. They put them in that location to balance the aircraft. Are we following...

From the center of lift of an airfoil forward and rearward the the moment of the aircraft is the same. It has nothing to do with weight. Weight is not a factor. The moment is what is important. When you understand moment then this discussion will be over.

Moment is how weight acts when multiplied by the distance that given weight is from an object center of gravity.

10 pounds one foot from a cg has a moment of 10.

A 1 pound object 10 feet from a CG has the same moment of 10.

Even though the one pound object is much lighter it acts in the same way the 10 pound object does.

The wings are mounted at the aircrafts center of gravity. That is why they are so far rearward in aircraft having rear mounted engines. To reduce the arm compensating for the rear mounted engines.

As far as your correlation between the weight and balance of an aircraft and the recent crashes of MD aircraft I don't see your point. Show us one NTSB report of a MD aircraft having been due to a design flaw in the planes design regarding balance and I'll give you the credibility you deserve.

The FAA has a publication concerning weight and balance. I suggest you start your education and make a purchase at your local flight school.

User currently offlineMD-90 From United States of America, joined Jan 2000, 8530 posts, RR: 11
Reply 10, posted (16 years 4 months 4 days 18 hours ago) and read 1198 times:

The real problem with rear-mounted engines is that the cg shifts during flight as fuel is burned off. That ensures the need for some complicated systems to keep the cg within acceptable limits. I can assure you that they are NOT out of balance. They wouldn't get off the ground if they were. As for the seating being 2-3, all you do is carry more fuel in the wing on the side with only two seats. And that's a very minor amount of fuel, and in fact it can be totally trimmed out. The reason the MD-80 and MD-90 are so long is because the engines are HEAVY. The fuselage is basically a long hollow tube, and passengers don't weigh all that much. The fuel load generally takes about half of the available payload and it's all in the wings or very close to them in the fuselage. The only thing that could possibly be said to be bad about T-tailed airplanes is the fact that they CAN reach an unrecoverable "deep stall." However, this will only occur if the cg is behind the acceptable limits. The day a Douglas DC-9/MD-80/MD-90 enters an unrecoverable deep stall I'll quit flying for the rest of my life.
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User currently offline747-600X From United States of America, joined Jan 2000, 2829 posts, RR: 13
Reply 11, posted (16 years 4 months 4 days 14 hours ago) and read 1187 times:

For explaining the complexities of moment - which were already not too far fetched for my puny intellect. As it happens, the explanation you gave supports my wonderment at the stretch of fuselage forward of the wing. It is the concept I tried to explain with the pole lifting... anyone can lift a pole from it's middle if it weighs 10 pounds, but to lift the same pole from it's back end would be very hard because of that very thing. And I'm not arguing that the airplanes are critically flawed, I'm simply wondering why you would design an airplane with more weight on one side or forward of the wings than aft even if was only .05%. And MD-90 makes the point that as fuel burns off the CG changes (though only slightly). Furthermore, if the CG is balanced by fuselage in the front and engine in the back, then what keeps this balance from being upset on the -80 with it's stretched fuselage?

User currently offlineD L X From United States of America, joined exactly 17 years ago today! , 12438 posts, RR: 51
Reply 12, posted (16 years 4 months 4 days 13 hours ago) and read 1181 times:

"I'm simply wondering why you would design an airplane with more weight on one side or forward of the wings than aft even if was only .05%."

As other have tried to put it, these planes are not designed to have more weight on a side, or forward of the wings. They are in fact quite balanced. The wings are placed so that the length in front of the wings weighs about the same as the length behind with the engines. Your analogy of lifting a rod from its end is quite flawed, because in reality, the MD jets have a very significant counterweight helping you out. Yes, engines are comparatively small in volume, but they are easily the heaviest area of the plane.

Oh, just thought of the greatest example. In fact, the MD80 might be the most balanced plane on the planet. And it is this balance that causes problems especially when the weather is bad. On the ground, the plane relies on most of the weight being on the main wheels, but a good portion must be on the nosewheel in order to steer. The MD80 being so balanced along its roll axis cannot place much weight on the nosewheel. This has forced many MD80s to fall off the end of the runway while trying to steer onto a taxiway. If the plane was less balanced, and placed more weight on the nosewheel, this wouldn't be a concern.

As for the 3-2 thing? The plane's MTOW is around 150,000 lbs. Since you're so good at physics, you'll notice that the moment of 29 passengers displaced by 8.5 inches from center is quite insignificant. (.05% is insignificant. You shift the weight of a plane that much just getting up and going to the restroom. Obviously, that doesn't cause crashes.) Modern aircraft automatically balance the plane in flight based on fuel burn load positioning, etc. Someone who is a real pilot can hopefully back me up on that one.

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User currently onlineSpacepope From Vatican City, joined Dec 1999, 3390 posts, RR: 1
Reply 13, posted (16 years 4 months 4 days 12 hours ago) and read 1176 times:

The point i was trying to make earlier is that yes, there are some inherintly unstable aircraft manufactured. These are designed this way si that they will be more manouverable. Examples are the Grumman X-29, the F-16, and the F-117. Every commercial/cargo aircraft today (and ever) are inherently stable. just because they don't look "balanced) in the pasenger compartment or from the outside doesn't ean that there are other ways of compensating for the discrepencies that you percieve.


The last of the famous international playboys
User currently offlineHawaiian717 From United States of America, joined May 1999, 3241 posts, RR: 6
Reply 14, posted (16 years 4 months 4 days 12 hours ago) and read 1176 times:

Allow me to finish the job of refuting the pole analogy.

True, it is easier to lift a 10 pound pole from the center than from one side. This assumes that the mass of the pole is evenly distributed, which is generally the case with poles.

However, the mass is most definately not evenly distributed in any rear engined aircraft (such as DC-9, MD-80, MD-90, 717, 727, BAC 111, F.28, F.70, F.100, Tu-134, Tu-154, Tu-334, Il-62, VC-10, CRJ, ERJ-145, Caravelle, etc). The engines are very big and heavy, while the fuselage itself is relatively light (remember it's mostly hollow). Thus the center of gravity for these airplanes is not in the middle of the airplane, but towards the rear. Thus the place that it is easiest to lift is towards the rear.

Imagine that your 10 pound pole did not have it's mass evenly distributed, but was hollow and weighed three pounds, with a seven-pound weight glued into one end. Suddenly the easiest place to lift the pole is not in the middle, but like our rear-engined aircraft, towards the rear.

You also ask why the stretched fuselage of the MD-80 does not have even more of a problem. The answer is simple: The engines are heavier. The Pratt & Whitney JT8D-217C and JT8D-219 engines weigh more than the JT8D-5, JT8D-7, JT8D-9, JT8D-11, JT8D-15, and JT8D-17 engines used in the various members of the DC-9 family. Thus while the fuselage was stretched forward of the wings, the added weight of the engines aft of the wings compensates for this.

User currently offlineAcvitale From United States of America, joined Aug 2001, 922 posts, RR: 10
Reply 15, posted (16 years 4 months 4 days 9 hours ago) and read 1171 times:

Actually another point to add....

On the DC9-10/15/20/30/40/50/80 series as each was expanded the plug was added both in front of and behind the wind on all except the 15 and 20 series which to my knowledge are the same ext. dimensions as the 10.

Finally, the difference in weight in the coach cabin by the single offset seat as pointed out above is negligable but easily offset by avionic, baggage etc if it were more than the few pound here and there other items such as APU inlets/exhaust are more likely to add/remove drag that has a more meaningful effect. These too are basically adjusted by the use of trim tabs on the rudder.

The DC-9/MD80/90 series is one of the best airliners out there.

As far as the DC-10/MD-11/L1011 argument with seating...

first most common seating arrangements

F 2-2-2
C 2-3-2
Y 2-5-2

Other are out there but not that prevelant. No more than variants to the 777/747/A300/A310A330/A340

I have seen an A310 with the following seating on it...

F 2-2-2
C 2-2-2
Y 2-3-3

Why no idea (cheap seat avail on refurb I guess...)

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 27
Reply 16, posted (16 years 4 months 3 days 22 hours ago) and read 1157 times:

The reason the stretch versions of the DC9 series planes are not out of balance in reference to the original short fuselage is because of the CG envelope. The envelope is a range that the CG may be in and the plane still be within balance limits. It may be a given distance of 3 feet.

The aircraft may be loaded out of balance as long as it can be offset with stabiliser trim .The trim imparts an aerodynamic load counteracting the imbalance of the aircraft loading.

The CG must be within a certain range to be within limits. It does not have to be at a predetermined location but rather within a given distance.

User currently offline747-600X From United States of America, joined Jan 2000, 2829 posts, RR: 13
Reply 17, posted (16 years 4 months 3 days 15 hours ago) and read 1152 times:

Thank you all, very much.  

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