Why do some aircraft have a minimum flying weight? I can easily understand how too much weight stresses the airframe; I’ve never run into a situation with model aircraft where too light a plane meant it was weaker in flight. A friend who flies business jets said it had something to do with the loading, but he couldn’t be more specific - all he had to know was the minumum weight.
If you’re interested in a WAG*, here’s mine - maybe it has something to do with balance and specifically how the centre of mass shifts when the plane is loaded.
*Alternatively, sit back and wait for someone who knows what they are talking about.
Can you give us an example? I’d bet that any minimum weight just translates to dry weight plus minimum crew and fuel load.
I can’t say that I’ve ever had to worry about a minimum weight on big airplanes, except as a function of a minumum fuel quantity required by regulation or standard operating procedures.
I agree that it probably has to do with center of gravity, for instance some tandem two-place gliders have a minimum weight that must be present in the front seat for the center of gravity to be within acceptable range. Even if I fly this glider solo from the front seat, I have to either weigh more than say 140 lbs, or affix sufficient ballast in the cockpit to ensure that the CG is correct. This is just one example of an aircraft which has a minimum weight: empty weight of the glider plus at least 140 lbs (in the front seat). Similar I imagine to other tandem configured planes when flown solo - a J-3 Cub has to have someone in the back seat, and a Citabria someone in the front for instance.
An example of a non-tandem airplane with the same problem is the Australian-designed, US-certified Eagle 150B. Featuring a unique three-lifting surface configuration, it handles just like a conventionally-configured plane. If I remember correctly, for reasons associated with the location of the fuel tank and despite the side-by-side seating, there are some loading conditions which require burning no lower than a certain quantity of fuel to preserve an acceptable CG. Hence the minimum aircraft weight includes a minimum fuel quantity that is a function of cockpit loading.
For non-pilots/-aero guys, a CG too far forward invites reduced pitch control authority and increased stick forces, while an aft CG may yield a loss of pitch stability and possibly a resulting unrecoverable stall.
It isn’t just jets.
I once flew a “lightplane” of very minimal weight (535 lbs empty weight - and capable of carrying 400 lbs payload) that was balanced so that the pilot in the forward seat had to weigh a minimum of 150 lbs. If the forward seat had less than 150 lbs in it, the weight of the plane would be shifted too far to the rear making the airplane difficult to control at best, and possibly so uncontrollable that it would crash on take off.
At the time I only weighed 145 - meaning me alone in the front - even with weight in the rear seat - was unsafe.
Easily solved - we strapped a 10 lb sandbag in the nose between the rudder pedals. End of problem.
The “minimum weight” usually involves specific loading in specific places as well as an overall number in order to make sure the load is balance. This has to do with control, not strutural strength.
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- If an aircraft is not as “heavy” as it should be, then two things happen: the stall speed decreases but the top speed and rough-air penetration speed also decrease. The stall speed decreasing is no big deal, because the pilot will keep the airplane above that anyway–but the other two are bad because they happen at high speeds, and there is little time to react if the plane is upset.
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- If an aircraft is not as “heavy” as it should be, then two things happen: the stall speed decreases but the top speed and rough-air penetration speed also decrease. The stall speed decreasing is no big deal, because the pilot will keep the airplane above that anyway–but the other two are bad because they happen at high speeds, and there is little time to react if the plane is upset.
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Big jet driver weighing (har har) in …
For airliner-size aircraft the issue is, as stated above, one of longitudinal (fore/aft) balance, commonly called CG (center of gravity).
For a ferry flight without passengers or cargo we’ll normally have to load several thousand pounds of extra fuel beyond what we need for the flight itself. In the alternative we can load ballast in one or another cargo hold. Cargo or mail makes good ballast, as does cartons of airline magazines if there’s no revenue freight available.
But normally fuel is used since that saves the labor to unload it at the destination. All you’re doing is buying it a few hours sooner than you otherwise would. It’s also easy to meter the exact amount of weight required.
When fuel is carried as ballast, it often is carried with a different distribution amongst the tanks than is normal. That ensures the extra weight stays in the fore/aft location where it’s needed.
Each model of aircraft has its own specific requirements and procedures for when ballasting is required, how much, and where must be carried.
There is another minimum weight issue, and that’s FAA-required fuel load.
For every airplane there’s a minimum fuel that must always be on board at takeoff, even if all you intend to do is fly around the airport once & land again. This can be a matter of CG, but is also often a matter of ensuring there’s enough in the tanks so that flow to the engines is guarateed even while climbing / descending, etc. Most fuel is carried in the wings, where the tanks are very long & wide but only a few inches deep. So it doesn’t take much sloshing to uncover the fuel lines & start sucking air. That would be awkward.
Another minimum fuel requirement is to have enough to get where you’re going plus some extra calculated according to a pile of intricate rules.
For longer flights or older less efficient airplanes, the second fuel minimum is normally the greater value and hence the limit. But for a short hop in a late-model efficient airplane, the former limit can apply.
I’d imagine the business jet world sees a bunch of short hops, and so they encounter that limit fairly often.
Crud. After three years a question in my professional expertise FINALLY comes up (I’m an aerospace mass properties or “weights” engineer) and it’s answered perfectly before I get a chance to see it.
I’ll just be over here sulking and playing with my loadcells.
Then again, it’s pretty darn interesting that someone can devote an entire career to problems of this nature
Me, I gained a whole new level of appreciation for determining weight-and-balance for an aircraft when I was hanging out with homebuilders. Even working from a standard kit where you had some idea of where the CG would be, that final tweaking to get it exactly balanced and trimmed out was quite… um… tedious and daunting at times. Occassionally we’d resort to suspending an airframe from a single point to do some empirical problem-solving, but I can’t imagine that would be practical with, say, a Boeing 707.
Yeah, I’m a real hit at cocktail parties! 
Heh. It’s not as hard as you’d think. One way is to drive the aircraft up onto a set of overgrown bathroom scales. Another is to jack it up on its structural jacking points with electronic loadcells attached. Either method will give you three (or more) load points which can be used to triangulate the aircraft’s longitudinal and lateral centers of gravity.
You can’t measure the vertical cg this way unless you tilt the aircraft at a fairly severe angle, so that’s not usually done. However, because airplanes are more or less flattish (fuel tanks are basically big flat sheets, passengers and cargo are loaded in known locations, vertically), their vertical cg doesn’t move very much and the theoretical value is normally sufficient.
Mind you, in the commercial aircraft world, we don’t wait around until the plane’s built to figure out its weight. A tremendous amount of effort is put into predicting an aircraft’s weight and cg throughout its design and development phase, since weight is such a critical driver of aircraft performance. For example, I just finished counting all the fasteners in the wing of a major new commercial aircraft. Twice. You know how many there were? Over 90,000. Tedious doesn’t begin to describe it.