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Help With Physics/forces On Cruising Jet Airliner  
User currently offlineThePRGuy From , joined Dec 1969, posts, RR:
Posted (7 years 7 months 4 weeks 1 day 11 hours ago) and read 3030 times:

When at cruising speed what sort of forces is the chasis of a plane under, as it goes through noticeable deformation? I do not need specifics for any type of plane, but if known this would be great.
This is a question that would greatly help with some physics I'm doing. Thank you

Regards

22 replies: All unread, jump to last
 
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (7 years 7 months 4 weeks 22 hours ago) and read 2976 times:

The forces acting on an aircraft is always the Big Four: Thrust, Drag, Weight and Lift.

Any structure under load is deformed. In airliners, the most noticeable is the bending of the wings due to the lift they generate, which is significant and viewable from the cabin. Is this what you are after?

In addition, you have turbulence which kan cause "flapping", clearly visible. This is, however, nothing more than variations in the lift generated due to updrafts and downdrafts.

I'm not sure this is what you are looking for, but I hope it can serve as a starting point for further specification of the problem you are looking at.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineSpeedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0
Reply 2, posted (7 years 7 months 4 weeks 21 hours ago) and read 2969 times:

Quoting ThePRGuy (Thread starter):
When at cruising speed what sort of forces is the chasis of a plane under, as it goes through noticeable deformation?

Everything on every trasportation vehicle's structure flexes to some degree. Since you use the word "chassis" in your question, it may be assumed that you mean fuselage. An automotive Chassis is supported by it's wheels and suspension, while an aeronautical "chassis" is supported by its wings. Just as a car's suspention moves to absorb road imperfections, so does a modern aircraft's wings flex. However, whereas an automotive chassis benefits from being as flex-free as possible, an airplane's does not. Furthermore, an airplane's fuselage flexes for different reasons. Turbulence will, of course, cause tortional flexion, but I suspect that the greatest "deformation" of an airliner has to do with pressurization. Take a look at pictures on this site of high-cycle aircraft, like old DC-10s and MD-80s. With the right light and a reasonably reflective paint job, you'll be able to see the shape of the fuselage's structure beneith the skin because the skin's many deformations due to pressurization with every cycle.



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User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 3, posted (7 years 7 months 4 weeks 21 hours ago) and read 2962 times:

And here's my entry for the nitpicker award 2007!  Wink

The buckling of skin panels on aircraft, which will eventually leave visible deformation in stressed areas over the life of many aircraft, comes mainly from the shear forces on the panels. That buckling is what leaves "ripples" diagonal to the frames/stringers. Pressurization does not cause shear forces.

That's the buckling usually seen. Pressurization does cause some bulging, which can also be seen at times.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineThePRGuy From , joined Dec 1969, posts, RR:
Reply 4, posted (7 years 7 months 4 weeks 21 hours ago) and read 2959 times:

Thanks guys, really great stuff. Is it possible for any ball-park quantities of the amount of drag/lift that an aeroplane experiences?
Thanks


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 5, posted (7 years 7 months 4 weeks 20 hours ago) and read 2952 times:

Lift is easy as it will equal the gross weight of the aircraft in straight and level flight (such as in cruise).

Once you know the lift, you are not far from knowing the drag. An aircraft has a lift/drag ratio, which is high for e g gliders and low for spam cans and jet fighters.

Assume a lift to drag ratio of about 15-20 to 1 for a realistic airliner ballpark figure for drag, i e one 20th to one 15th of the lift.

For level unaccelerated flight, thrust must equal the drag.

Rgds,
/Fred

[Edited 2007-01-28 11:10:21]


I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 25
Reply 6, posted (7 years 7 months 4 weeks 18 hours ago) and read 2927 times:

Quoting FredT (Reply 5):
For level unaccelerated flight, thrust must equal the drag.

...and weight should equal the lift...  Wink



Whoever said "laughter is the best medicine" never had Gonorrhea
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 7, posted (7 years 7 months 4 weeks 18 hours ago) and read 2922 times:

Quoting Kaddyuk (Reply 6):
...and weight should equal the lift...  Wink



Quoting FredT (Reply 5):
Lift is easy as it will equal the gross weight of the aircraft in straight and level flight (such as in cruise).

 Wink



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineQutaiba From Kuwait, joined Dec 2005, 47 posts, RR: 0
Reply 8, posted (7 years 7 months 4 weeks 17 hours ago) and read 2917 times:

How Airplanes Work

http://travel.howstuffworks.com/airplane.htm



When the tide comes in, all ships will rise
User currently offlineAreopagus From United States of America, joined Sep 2001, 1369 posts, RR: 1
Reply 9, posted (7 years 7 months 3 weeks 6 days 7 hours ago) and read 2793 times:

One thing I have been wondering about: Boeing shows nose-on views of the 787 showing that there is a very large amount of dihedral due to wing flex, on top of the dihedral built into the wings at the root. It seems to me that with lift being normal to the wings, they are fighting each other with inward lift components. The vertical component of lift falls off with the cosine of the dihedral, so a small amount wouldn't cause much loss. But when every percent of efficiency is a prize to be fought for, isn't the 787's pronounced dihedral a bit much?

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 10, posted (7 years 7 months 3 weeks 5 days 12 hours ago) and read 2717 times:

Quoting Areopagus (Reply 9):
isn't the 787's pronounced dihedral a bit much?

I suspect it is the result of a phenomenon known as Marketing, and that it will follow the shark fin stabilizer into Neverhappened land.  Wink



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineMetroliner From United Kingdom, joined Jan 2007, 1067 posts, RR: 1
Reply 11, posted (7 years 7 months 3 weeks 5 days 12 hours ago) and read 2706 times:

Quoting Areopagus (Reply 9):
The vertical component of lift falls off with the cosine of the dihedral, so a small amount wouldn't cause much loss. But when every percent of efficiency is a prize to be fought for, isn't the 787's pronounced dihedral a bit much?



Quoting FredT (Reply 10):
I suspect it is the result of a phenomenon known as Marketing, and that it will follow the shark fin stabilizer into Neverhappened land.

haha, interesting question and amusing conclusion - i did always wonder why boeing were touting the 787 like that - as if it were some sort of competitor to the larger, open-class gliders one sees from time to time.

do you think the large wing-flexing the boeing simulations show hints at a very high aspect ratio? aspect ratio, broadly speaking, is also an important factor in the efficiency of a wing. is the dreamliner going to feature a revolutionary wing design?

toni



Set the controls for the heart of the Sun
User currently offlineVikkyvik From United States of America, joined Jul 2003, 10035 posts, RR: 26
Reply 12, posted (7 years 7 months 3 weeks 5 days 11 hours ago) and read 2681 times:
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Quoting ThePRGuy (Reply 4):
Is it possible for any ball-park quantities of the amount of drag/lift that an aeroplane experiences?

Well, specifics will vary considerably, but for an airliner, you'll typically be looking somewhere in the low thousands-of-pounds range for cruise drag.

Consider that at cruise, a turbofan engine will be producing somewhere around 10-20% of sea-level thrust (I'll use 15%). And the engines will not be at max thrust (I'll use 75% thrust, but I don't actually know how accurate that is as N1 is not a reflection of actual thrust percentage).

So, for, say, a 737:

SLST =~ 22,000 lbs./engine
Thrust at cruise =~ 2,475 lbs./engine

So, total thrust = drag =~ 5,000 lbs.

That's just a very rough estimation, though. Just thought I'd give you an order-of-magnitude.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 13, posted (7 years 7 months 3 weeks 5 days 10 hours ago) and read 2671 times:

Also consider the fuselage in flight.

The lift comes from the wings, so the fuselage is a tube, being supported in the middle. The forward end - does it sag? This is called "hogging" in shipbuilding, and it must have a corollary in airplane design. Then the aft fuselage; depending on where the center of gravity is located, the horizontal stabilizer is either providing lift (thus, carrying part of the load) in aft CG conditions or providing downforce in forward CG loadings. The downforce so developed is part of the load the wings must then lift, in which case the wings must lift the sum of the gross weight plus the total downforce just to maintain level flight.

The wings must have a forward bending moment because of the thrust of the engines but also an aft bending moment because of drag.

During turning, gyroscopic precession must introduce lateral bending moments on the engine pylons.

How about rudder forces. Even in straight & level flight, the yaw damper is working, feeding rudder in, taking it out. The center of pressure of the rudder is well above the CG of the airplane so this must introduce bending moments to the rudder or even twisting moments to the entire aft fuselage.

I guess that is enough to think about for a while.

I know some of these things are so because I have installed drag, and anti-drag wires in Stearman wings as well as the fittings for the "flying" wires and "landing" wires between the wings.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 14, posted (7 years 7 months 3 weeks 5 days 9 hours ago) and read 2661 times:

It all flexes, all the time. Prop multi fuselages also oscillate in weird and wonderful shapes from the pressure waves off the blades. It is rather amusing to see it all amplified in a structure dynamic model!

Then there's flutter... no fun at all.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 15, posted (7 years 7 months 3 weeks 4 days 11 hours ago) and read 2563 times:

There's also Newton's 3rd law, every action has an equal and opposite reaction. This has a major effect on flight.

User currently offlineRIHNOSAUR From United States of America, joined Mar 2006, 362 posts, RR: 1
Reply 16, posted (7 years 7 months 2 weeks 1 day 2 hours ago) and read 2352 times:

I guess since were talking physics

I think it is also important to think about how the thrust of the engines is achieved. In a nut shell, it is just:

conservation of momentum.


thats why the gases that get bypassed through the fan cause the forward thrust in the plane.

ohh and on the side but most likely irrelevant: we cant forget that when an airliner gets hit by lightning there are electromagnetic forces in play!!  Smile  Smile  Wink



particles and waves are the same thing, but who knows what that thing is...
User currently offlineZeke From Hong Kong, joined Dec 2006, 9105 posts, RR: 75
Reply 17, posted (7 years 7 months 2 weeks 19 hours ago) and read 2326 times:

Quoting ThePRGuy (Thread starter):
I do not need specifics for any type of plane, but if known this would be great.

Have a look at "STATISTICAL LOADS DATA FOR THE BOEING 777-200ER AIRCRAFT IN COMMERCIAL OPERATIONS"

http://www.tc.faa.gov/its/worldpac/techrpt/ar06-11.pdf



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineBoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0
Reply 18, posted (7 years 7 months 2 weeks 18 hours ago) and read 2323 times:

Slightly of topic, but we had a great topic on physics, aviation related, a while back. At least I learned alot from it, I recommend you to check it out:

The Aviation Physics Thread



norwegianpilot.blogspot.com
User currently offlineBAe146QT From United Kingdom, joined Sep 2006, 996 posts, RR: 0
Reply 19, posted (7 years 7 months 1 week 6 days 19 hours ago) and read 2269 times:

Quoting SlamClick:
The lift comes from the wings, so the fuselage is a tube, being supported in the middle. The forward end - does it sag?

Probably overly complicating things here, but doesn't the fuse itself generate a certain amount of lift purely through its own angle of attack in the cruise?



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User currently offlineVikkyvik From United States of America, joined Jul 2003, 10035 posts, RR: 26
Reply 20, posted (7 years 7 months 1 week 6 days 8 hours ago) and read 2226 times:
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Quoting BAe146QT (Reply 19):
Probably overly complicating things here, but doesn't the fuse itself generate a certain amount of lift purely through its own angle of attack in the cruise?

I believe it does, but not nearly as much as the wings do.

~Vik



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineBAe146QT From United Kingdom, joined Sep 2006, 996 posts, RR: 0
Reply 21, posted (7 years 7 months 1 week 5 days 20 hours ago) and read 2166 times:

Quoting Vikkyvik:
I believe it does, but not nearly as much as the wings do.

Heh-heh quite. But I was thinking that it must relieve at least some of the bending moment caused by its own weight, (though I do understand Cpt. Click's point).



Todos mis dominós son totalmente pegajosos
User currently offlineVikkyvik From United States of America, joined Jul 2003, 10035 posts, RR: 26
Reply 22, posted (7 years 7 months 1 week 5 days 16 hours ago) and read 2151 times:
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Quoting BAe146QT (Reply 21):
Heh-heh quite. But I was thinking that it must relieve at least some of the bending moment caused by its own weight, (though I do understand Cpt. Click's point).

Well, not so much, actually, if my thinking is correct. Assuming that whatever lift the fuse generates is distributed relatively evenly on it, then it'll act on the whole length of the fuse, and won't change the bending moment. If it were localized at the ends of the fuse, then it would reduce the bending.

Now, the fuselage lift is probably NOT perfectly even, so I don't know the quality of that assumption. Corrections welcome.

~Vik



"Two and a Half Men" was filmed in front of a live ostrich.
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