ThePRGuy
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Help With Physics/forces On Cruising Jet Airliner

Sun Jan 28, 2007 3:41 am

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
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FredT
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Jan 28, 2007 4:54 pm

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
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speedracer1407
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Jan 28, 2007 5:33 pm

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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FredT
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Jan 28, 2007 6:05 pm

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
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ThePRGuy
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Jan 28, 2007 6:12 pm

Thanks guys, really great stuff. Is it possible for any ball-park quantities of the amount of drag/lift that an aeroplane experiences?
Thanks
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FredT
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RE: Help With Physics/forces On Cruising Jet Airli

Sun Jan 28, 2007 7:08 pm

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]
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kaddyuk
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Jan 28, 2007 9:05 pm

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

...and weight should equal the lift...  Wink
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FredT
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Jan 28, 2007 9:18 pm

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
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Jan 28, 2007 9:32 pm

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Areopagus
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RE: Help With Physics/forces On Cruising Jet Airliner

Tue Jan 30, 2007 7:39 am

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?
 
FredT
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RE: Help With Physics/forces On Cruising Jet Airliner

Wed Jan 31, 2007 2:55 am

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
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metroliner
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RE: Help With Physics/forces On Cruising Jet Airliner

Wed Jan 31, 2007 3:13 am

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
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vikkyvik
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RE: Help With Physics/forces On Cruising Jet Airliner

Wed Jan 31, 2007 4:08 am

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.
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SlamClick
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RE: Help With Physics/forces On Cruising Jet Airliner

Wed Jan 31, 2007 4:58 am

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.
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FredT
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RE: Help With Physics/forces On Cruising Jet Airliner

Wed Jan 31, 2007 5:26 am

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
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AirWillie6475
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RE: Help With Physics/forces On Cruising Jet Airliner

Thu Feb 01, 2007 4:00 am

There's also Newton's 3rd law, every action has an equal and opposite reaction. This has a major effect on flight.
 
RIHNOSAUR
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Feb 11, 2007 12:38 pm

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
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zeke
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Feb 11, 2007 8:13 pm

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
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BoeingOnFinal
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RE: Help With Physics/forces On Cruising Jet Airliner

Sun Feb 11, 2007 8:38 pm

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
 
BAE146QT
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RE: Help With Physics/forces On Cruising Jet Airliner

Mon Feb 12, 2007 7:46 pm

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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vikkyvik
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RE: Help With Physics/forces On Cruising Jet Airliner

Tue Feb 13, 2007 6:42 am

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
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BAE146QT
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RE: Help With Physics/forces On Cruising Jet Airliner

Tue Feb 13, 2007 6:52 pm

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).
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vikkyvik
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RE: Help With Physics/forces On Cruising Jet Airliner

Tue Feb 13, 2007 11:05 pm

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
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