Hot answers tagged

55

The plane would be lighter. Not by much. It cannot be hard to dedicate different planes to different routes. This adds to the logistics difficulties in scheduling the airframes. It's MUCH easier if all the airframes can be used on all the routes. Returning the life jackets each time the plane has to fly over water. This involves extra down-time/...


54

Airlines do care about your carry-on weight and even your weight. Many have carry-on weight limits, which low-cost carriers usually enforce, weighing the carry-on and tagging it as cabin luggage. Full-service airlines rarely bother. One reason they don't measure your weight is that most people wouldn't feel comfortable getting weighed in front of strangers, ...


36

Pre-911 I towed banners, and towed them with a C172 and a C182. Parasitic drag of the banner is the limiting factor. We had different letters, and for the C-172, the letters were about 1.5 meter block letters, during the spring and fall, when the temps were below 20C about 18 to 22 letters was the max. Climb performance in the 172 was about 150 FPM. In ...


35

Yes, the removal of payload will result in a measurable reduction in emissions. First, I have to correct some math: the engines don't run at 150 kN in cruise - that would indicate a L/D ratio of only 4-5, since a typical A320 weighs around 60 tons mid-flight (give or take 10). Fuel consumption is roughly proportional to weight, so 30 kg out of roughly 60,...


31

Your misunderstanding lies in your thought that lift is smaller than thrust, while in fact, lift is much larger than thrust. The lift is provided by the wings. Their purpose is exactly to create a lift force (upwards force) while requiring relatively little thrust (forwards force). How well they do this is expressed by their lift-to-drag ratio (L/D ratio). ...


28

Boeing Boeing would have used DU because it had the right combination of physical characteristics and cost. Their tests showed that the radiation exposure for workers was low (2.6% of the statutory "safe" level). In most cases the exposure was so low as to be not detectable. Radiation Passengers on aircraft are exposed to cosmic radiation at much higher ...


26

In the context of aircraft design and maintenance, the terms counterweight and ballast generally have separate and different meanings. Both concepts involve mass used to balance, dampen, or adjust forces about a rotational axis. While ballast could also be described as a type of counterweight, it is best understood as working within the frame of reference ...


25

Well, any reduction in Basic Operating Weight, which eliminating the trolley achieves, is an increase in efficiency, because anything not humans and their bags paying money, or kerosene, and not essential to getting from A to B, is ballast. So there is value in forgoing 65lb of ballast and whatever cash income it brings in (I suspect the real reason is it ...


21

[W]hy did Boeing (and McDonnell Douglas) decide to use such a hazardous (and potentially dangerous from a military perspective) material? Because it's not particularly hazardous and it's not very dangerous from a military perspective. Uranium comes as a mixture of two major forms (called isotopes). Uranium-235 accounts for about 0.7% of the mix and is the ...


19

Two things: Loads on the landing gear. This can be alleviated by carefully touching down, but you never know what exactly will happen in the next landing, so better don't count on it to be smooth Deceleration distance: The higher mass requires a higher approach and touch down speed (higher by the square root of the mass ratio between your actual mass and ...


18

Why do airlines not remove the life jackets from planes that fly over land? They do, at least some of them. I fly on intra-European routes regularly (mainly Eurowings) and have never seen life jackets on flights that don't cross the Mediterranean. Obviously, the safety instructions don't include the life jacket demonstration either. I would guess that ...


17

It seems I misspoke in my comment in which I gave the moments for 747-400BCF gear retraction. I didn't realize the component weights were further on in the document I referenced. The weights for the BCF are: body gear 6447 lbs (2924 kg) each wing gear 6508 lbs (2952 kg) each nose gear 3157 lbs (1432 kg) However, @Gurkan's comment is relevant in that you ...


17

Aerospace Engineering e-Mega Reference shows the following table for two aircraft (the large commercial jet, the B747-100, and the military jet, the C-5A). You can see that it differs quite a lot between the B747 and the C-5 already, so it's hard to give a definitive answer for all aircraft in general. Also, it depends a lot on how you break down the ...


17

The letters and spaces can be made of a soft nylon type material that is extremely light. (there are different materials available, but all are made of durable, light-weight material). The number of letters and spaces vary based on the type of tow airplane being used. My experience many years ago was with a Cessna 172 (150 HP) and it could tow up to 32 ...


16

Because this can still happen. That’s US Airways Flight 1549 on the Hudson River. They made a movie about it starring Tom Hanks. There’s no way to predict disasters, and had the plane been “efficient” having no life jackets, I suspect it could have meant a lot of people drowning. [But see comments.] So it’s better to plan for contingencies, especially ...


14

If you discount aerodynamic effects, and assume the same landing speed, on a theoretical level mass makes no difference. The additional kinetic energy to be dissipated is exactly cancelled by the additional normal force. The kinetic energy is given by the formula: E = 1/2 mv², where E is the kinetic energy m is the vehicle's mass v is the speed at the ...


11

When American Airlines switched from paper to iPad for the pilots, they saved 40 lbs (18 kg). This translated to "\$1.2 million of fuel annually". (forbes.com; 2013) Of course this is across AA's fleet, which is huge. 963 planes as of writing this due to the c. 2013 merger with US Airways. Back in 2013 they had 605 planes, so it's an annual saving of \$2,...


10

The counterweights in the original question were from a Boeing 747 rudder, so I will explain what their purpose is. John Walter's answer covers all applications of balances and ballast well enough, so I just want to explain why control surfaces need balances. The short answer is: To bring the center of gravity on or even ahead of the elastic line of the ...


10

Why do airlines not remove the life jackets from planes that fly over land? The plane would be lighter and the safety demonstration shorter. Some do. As a specific example, if you fly Delta from Minneapolis to Lincoln, Nebraska, you'll be on a Canadair RJ-something (actually run by Sky West) that will only have a couple of infant lifejackets on board. ...


10

The fuselage weight for 3/4 of a 747-300 Combi minus engines, fuel tanks, seats, catering, cargo handling, navigation gear, control surfaces, pumps, hose is 70,000 lbs, according to Big Imagination, the people that bought a 747, gutted it and took it to Burning Man. The 70,000 lb figure was used by the Nevada DOT to determine the oversized vehicle permit fee....


9

This is not a full answer: much is mentioned by Michel Touw, but I'd like to correct a common misconception. The mass growing as the third power of linear dimensions applies only to solid objects. But the airplane is not solid; rather, it's a thin-skinned frame. (Unless it's a solid foam RC model). Therefore, its mass should grow with the area, as the ...


9

Controllability and redundancy. Airliners are certified according CFR 14 Part 25, which specifies that upon engine fail the aircraft must still be able to fly and climb: it must have more than one engine. After an engine has failed, the rudder must be deflected in order to compensate for the asymmetric thrust of the remaining engine. With a V-tail, this ...


9

(Own work via boeing.com) Overlayed scaled drawings. Above you see similar sized planes with the same 3-class seating (± 1 seat). The newer-more-composite 787 is 16 tonnes heavier, yet delivers up to 20% fuel saving (route dependent). 3.4 of the 16 tonnes are the newer and bigger engines. And the rest is primarily bigger wings (which can be built bigger ...


8

On Planet Earth, 1kg of mass exerts 9.81 Newtons of force (weight). Because this is true anywhere on planet Earth to within a tiny fraction of a percent, we use kg as a convenient measure of weight, even though pedantically it is a measure of mass. If you are thinking of landing your C-172 on the Moon, Mars, or some other locale where G is different from 9....


8

The maximum landing weight given here is actually the mass, not weight. In general, the various 'weights' given for aircraft are masses, not weights. i.e. they are in kilograms, not Newtons. This is simply an extension of our everyday usage. As @mins pointed out, if someone asks your weight, you tell your mass (70 kg or whatever), not the weight itself (~...


8

The major issue with landing a plane that is overweight is over-running the runway. If you are trying to land a plane that is over it's rated landing weight, you will need to be going faster to keep enough airflow over the wings to generate the lift required to keep the plane in the air. This, of course, has the consequence that it will take longer to stop ...


8

A lot of things happen, depending from what perspective you look at it. I'll first start with some scaling laws. Note that ~ means 'scales with' or 'is proportional to'. Basic scaling laws: Lets start with: (1) length ~ length (2) density ~ 1 (is constant) From (1) we can derive that: (3) area ~ length^2 (4) volume ~ length^3 From (...


8

A number of LCCs don't carry cargo at all or have a more limited cargo business, especially in Europe and the US. This is particularly the case if they're flying smaller narrowbody aircraft on shorter routes, where freight has to be bulk loaded. Ryanair and easyJet aren't in the freight business, though Southwest and AirAsia are. LCCs have strong ...


7

Depleted Uranium is 68% denser than lead and costs much much less as DU is a discarded material. Therefore, there were both space requirement considerations as well as cost considerations when using DU as a ballast or counterweight in aircraft. Although precautions are taken, there are no substantial health concerns when using DU in this way. The ...


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