travelavnut
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Handling Characteristics @ Higher/lower Thrust

Sun Nov 07, 2010 9:10 pm

One for the Pilots!

So I was watching this random take-off video at Youtube containing a lot of turns just after take-off, presumably at climb thrust. And this got me wondering; do the handling characteristics of 2 or 4 engined airliners change with higher/low thrust settings?

I can imagine that having 1 or 2 fixed points per wing where the pulling force of the engine can increase/decrease dramaticly have some effect on the way the airplane flies? Maybe pitch moments or some force on the controls?
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UAL747
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RE: Handling Characteristics @ Higher/lower Thrust

Sun Nov 07, 2010 9:48 pm

I'm not a pilot, but I think there is a pitch forward and back movement with thrust changes. But I think that's during the change only, as the energy changes. When you stabelize the thrust, it should have no bearing. (At least that's what I would assume).

Of course we all know that speed is the major factor in handling characteristics, and at higher speeds, the controls will be more "tight" so to speak.

I think manufacturers probably try to eliminate any type of handling issues that would arise from changes in thrust.
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Mir
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RE: Handling Characteristics @ Higher/lower Thrust

Sun Nov 07, 2010 10:43 pm

Quoting UAL747 (Reply 1):
I'm not a pilot, but I think there is a pitch forward and back movement with thrust changes

There is, depending on the location of the engines. And it's a pitch up or pitch down moment, not forward or back.

Quoting UAL747 (Reply 1):
But I think that's during the change only, as the energy changes. When you stabelize the thrust, it should have no bearing.

It's always there, actually. The only way to get rid of it is to trim off the additional control pressure, which is easy enough. But when you change power settings, you may need to re-trim if the change was significant enough.

-Mir
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Pihero
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RE: Handling Characteristics @ Higher/lower Thrust

Sun Nov 07, 2010 11:46 pm

Quoting TravelAVNut (Thread starter):
do the handling characteristics of 2 or 4 engined airliners change with higher/low thrust settings?

By design, the engine thrust lines are below the airplane CoG. Starting from a stabilised state,this means that an increased thrust will cause a pitch-up moment. and a decreased thrust a nose-down effect.
That is until you fly an FBW 'Bus in which the flight control systems keeps the aircraft in treim at all times.
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DiamondFlyer
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RE: Handling Characteristics @ Higher/lower Thrust

Sun Nov 07, 2010 11:57 pm

Quoting Pihero (Reply 3):
By design, the engine thrust lines are below the airplane CoG.

On some airplanes. Its not like there aren't airplanes out there that have the thrust line above the CG, which reverses the things you said about pitch changes caused by throttle adjustments.

-DiamondFlyer
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Pihero
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 12:19 am

Quoting DiamondFlyer (Reply 4):
Its not like there aren't airplanes out there that have the thrust line above the CG, which reverses the things you said about pitch changes caused by throttle adjustments.

Never heard of any airliner builkt that way. Could you provide some examples ?
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DiamondFlyer
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 12:56 am

Quoting Pihero (Reply 5):
Never heard of any airliner builkt that way. Could you provide some examples ?
Regards

Wow, so I figured this was obvious, but I'll go ahead and point them out. Any of the Dash 8 series, or the ATR 42/72. Surely you've heard of them.

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Mir
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 3:35 am

Quoting Pihero (Reply 5):
Never heard of any airliner builkt that way. Could you provide some examples ?

CRJs. Many business jets.

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
pilotpip
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 4:20 am

Handling is effected much more by airspeed than power changes. While it's true that thrust will effect depending on where the engines are placed on the aircraft some FBW aircraft have thrust compensation built into the software. The ERJ-170 has thrust compensation.
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UAL747
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 6:36 am

Quoting Pihero (Reply 5):
Never heard of any airliner builkt that way. Could you provide some examples ?

Wouldn't the L-1011, DC-10, and MD-11, MD-80, 717, or anything with tail mounted engines also have a pitch down moment when thrust is applied?

UAL
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Mir
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 6:47 am

Quoting UAL747 (Reply 9):
Wouldn't the L-1011, DC-10, and MD-11, MD-80, 717, or anything with tail mounted engines also have a pitch down moment when thrust is applied?

In the case of the L1011, DC-10 and MD-11, the two wing-mounted engines provide a counter to the tail engine, so you'd probably still get a nose-up pitch moment with increased thrust in normal operation. If you manipulated the thrust of the tail engine only, however, then you would see the nose-down moment with an increase in thrust.

-Mir
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Pihero
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 12:21 pm

Quoting DiamondFlyer (Reply 6):
Any of the Dash 8 series, or the ATR 42/72. Surely you've heard of them.

Don't know about the -8 but I do the ATRs : Funny enough, push the throttle and you have a pitch-up movement, mainly due, in this case to the augmented lift caused by the accelerated airflow. I would be very surprised to hear the -8 behaving differently.

Quoting UAL747 (Reply 9):
Wouldn't the L-1011, DC-10....,

I have some 4,000 hours on the 10-11 and she behaves as I said. The -10, I only know through the simulator. Nothing out of the ordinary here.

Quoting UAL747 (Reply 9):
...MD-80, 717, or anything with tail mounted engines also have a pitch down moment when thrust is applied?

Tested the MD-80 for a previous airline...Here again, nothing unexpected.
Let's face it, an airliner has to demonstrate *stability* in all axes. I therefore expect that airplane to show - on a short term reaction- a pitching moment to tend to keep the acftual airspeed steady.
Otherwise, the Sioux City DC-10 and the Baghdad A310 wouldn't have been so easy (!!!) to pilot with engines' thrust only.
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travelavnut
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 1:30 pm

Thanks everyone for all the replies!!
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NW747-400
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RE: Handling Characteristics @ Higher/lower Thrust

Mon Nov 08, 2010 2:37 pm

Quoting Pihero (Reply 5):
Never heard of any airliner builkt that way. Could you provide some examples ?

All CRJ models will pitch down when a thrust increase is commanded, and pitch up when the thrust is decreased. Seems like most airplanes with only tail mounted engines would behave in this manner? Anyone have any experience with the DC9/MD80 series? I'm curious to know about that.

To the OP, as long as all engines are operating normally and with equal thrust settings, there is no affect on the aircraft's turning forces. Should an engine fail, the aircraft will yaw about the vertical axis in the direction of the inoperative engine, which will in turn cause the aircraft to bank in said direction should no corrective rudder inputs be made.
 
tdscanuck
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RE: Handling Characteristics @ Higher/lower Thrust

Tue Nov 09, 2010 1:27 am

Quoting Pihero (Reply 3):
That is until you fly an FBW 'Bus in which the flight control systems keeps the aircraft in treim at all times.

Not limited to Airbus...anybody who uses a C*-esque pitch control law gets this benefit (F-16, A320 and onwards, 777, etc.). Good overview here: https://dspace.lib.cranfield.ac.uk/bitstream/1826/186/2/coareport9303.pdf

Quoting NW747-400 (Reply 13):
Should an engine fail, the aircraft will yaw about the vertical axis in the direction of the inoperative engine, which will in turn cause the aircraft to bank in said direction should no corrective rudder inputs be made.

Several FBW aircraft don't do this, or only do it because it's programmed in to provide pilot annunciation of an engine failure. Good yaw control laws will reject the disturbance of an engine failure.

Tom.
 
Pihero
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RE: Handling Characteristics @ Higher/lower Thrust

Wed Nov 10, 2010 11:40 am

Thanks for the link, Tom.
I'd wanted such a document for a very long time without spending 80 quid on a tech book.

Regards
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tdscanuck
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RE: Handling Characteristics @ Higher/lower Thrust

Thu Nov 11, 2010 4:18 am

Quoting Pihero (Reply 15):
I'd wanted such a document for a very long time without spending 80 quid on a tech book.

I was very surprised to find that one online without a fee...not sure why I haven't stumbled across it in the past when doing similar searches, but I'm thankful that it's there!

Tom.

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