Hot answers tagged

56 votes
Accepted

Why do all airliners have life jackets?

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 ...
RAC's user avatar
  • 4,616
55 votes

Why are airlines so concerned with checked baggage weight?

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-...
Therac's user avatar
  • 25.9k
38 votes

How large are the banners that are trailed behind small planes for advertising?

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 ...
mongo's user avatar
  • 17.7k
35 votes
Accepted

Can the removal of a duty-free sales trolley result in a measurable reduction in emissions?

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 ...
Therac's user avatar
  • 25.9k
32 votes

How can lift be less than thrust that is less than weight?

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 ...
Sanchises's user avatar
  • 12.9k
31 votes

Why are "counterweights" used in aircraft production?

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 ...
J W's user avatar
  • 16.6k
30 votes
Accepted

What is the reasoning behind using depleted uranium as counterweights in the 747?

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 "...
RedGrittyBrick's user avatar
25 votes

Can the removal of a duty-free sales trolley result in a measurable reduction in emissions?

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 ...
John K's user avatar
  • 130k
20 votes

What is the reasoning behind using depleted uranium as counterweights in the 747?

[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 ...
David Richerby's user avatar
19 votes

Why do all airliners have life jackets?

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 ...
dasdingonesin's user avatar
19 votes
Accepted

Why has structural efficiency (OEW / MTOW) not improved despite increasing use of carbon fiber?

Well, simply consulting Wikipedia you can calculate that an "old" A330 has an empty weight fraction of (in tons) 132/251=0.545. A recent A350 has 142/283=0.50 i.e. some 10% lighter. As a ...
sophit's user avatar
  • 10.9k
17 votes
Accepted

Why are "counterweights" used in aircraft production?

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 ...
Peter Kämpf's user avatar
17 votes

Why do all airliners have life jackets?

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 “...
NonCreature0714's user avatar
17 votes
Accepted

Is it possible for an airplane to enter another airplane?

Clearly the plane must enter and exit through the belly of the larger plane Why so? If you drop this condition, things become much easier, because you could attach the smaller planes externally and ...
Zeus's user avatar
  • 9,053
14 votes

How does aircraft weight affect landing distance?

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 ...
TomMcW's user avatar
  • 28.4k
11 votes

Why do all airliners have life jackets?

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 ...
David Richerby's user avatar
11 votes
Accepted

PoH says airplane climbs steeper at higher weight. Why? How?

There are three stages to a short field takeoff over an obstacle- the ground roll, where the airplane accelerates to the minimum airspeed necessary to leave the ground, a period of further ...
Chris's user avatar
  • 14.6k
10 votes

Why aren't there V-tail airliners?

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 ...
Koyovis's user avatar
  • 61.6k
10 votes

What is the weight of the fuselage (only) of the Boeing 747?

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, ...
gwally's user avatar
  • 2,354
9 votes

How large are the banners that are trailed behind small planes for advertising?

To add to mongo's and 757toga's answers, during my time towing in Central Florida in the late 00's, in addition to the 5' letter banners, we also had 8' letters that we often used when towing over ...
habu's user avatar
  • 2,892
9 votes

What are the physical laws for upscaling an RC model to 1:1?

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. ...
Zeus's user avatar
  • 9,053
9 votes
Accepted

Are there any airliners capable of vertical climb with engine thrust only by pointing the nose up?

No, there are few planes that can do this at all. In order to be able to climb out straight up you need a thrust-to-weight ratio greater than 1. In other words you need enough thrust to propel the ...
Dave's user avatar
  • 101k
9 votes

Why has structural efficiency (OEW / MTOW) not improved despite increasing use of carbon fiber?

To add to sophit's correct answer, calculating structural efficiency as MTOW/OEW is not always informative. It is informative if both aircraft designs aimed to maximize this ratio. For same weight, ...
Therac's user avatar
  • 25.9k
8 votes

What is the reasoning behind using depleted uranium as counterweights in the 747?

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 ...
G-Man's user avatar
  • 289
8 votes

What are the physical laws for upscaling an RC model to 1:1?

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 ...
Michel Touw's user avatar
8 votes
Accepted

Would a lighter aircraft require the same amount of power than a heavier one to fly at the same speed?

You are right. The heavier aircraft needs more power. With one exception. The power needed to keep an aircraft in the air is the product of flight speed and drag. When flying at the same speed, the ...
Peter Kämpf's user avatar

Only top scored, non community-wiki answers of a minimum length are eligible