AerospaceFan
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Elevator Feel And Tailstrike Reduction Issues

Sat Jan 27, 2007 7:04 pm

I came across the following some moments ago:

Quote:
Boeing has done design work to reduce tail strikes, including implementing an improved elevator feel system in some airplanes. For example, the 747-100/-200/-300 has varied feel (column forces) throughout the center of gravity (CG) and weight envelope. The newer 747-400’s elevator feel system design provides a constant feel elevator pressure, which has reduced the potential of varied feel pressure on the yoke contributing to a tail strike. The 747-400 has a lower rate of tail strikes than the 747-100/-200/-300.

Source:

http://www.boeing.com/commercial/aer...rticles/qtr_1_07/article_02_1.html

To me, this is a bit counterintuitive, since I would have guessed that a varied elevator feel would have made tailstrikes less, not more, likely. This would be so because varied feel might allow the pilot to sense when his actions are about to cause a tailstrike. But the article says differently. Could someone please explain why?

Thank you in advance.
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fr8mech
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RE: Elevator Feel And Tailstrike Reduction Issues

Sun Jan 28, 2007 4:35 am

My non-pilot guess would be that in a variable feel system, on take-off, the stiffer the feel, the more back pressure the crew may apply to the column. A constant feel throughout the column movement requires constant pressure.
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AerospaceFan
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RE: Elevator Feel And Tailstrike Reduction Issues

Mon Jan 29, 2007 8:20 am

^^ I see. Thanks for the kind response; it is very much appreciated.
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Starlionblue
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RE: Elevator Feel And Tailstrike Reduction Issues

Mon Jan 29, 2007 11:10 am

Or you could just make the darned thing FBW and give the pilot the feel needed for that particular regime of flight.

Oh wait, another manufacturer already did that.  Wink
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n8076u
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RE: Elevator Feel And Tailstrike Reduction Issues

Mon Jan 29, 2007 12:45 pm

Quoting AerospaceFan (Thread starter):
For example, the 747-100/-200/-300 has varied feel (column forces) throughout the center of gravity (CG) and weight envelope.

I looked this up in an old PanAm 747-121 flight manual that I have, but unfortunately, I didn't find any helpful information regarding the variable elevator feel. But the way I read Boeing's article, the control column feel forces would be varied depending only on the aircraft's gross weight and where the CG falls in its range. They make no mention of the forces being varied in relation to where in the range of motion the control column happens to be or the aircraft's relation (angle) to the ground.

I can only guess that because a light 747 will react quicker to the same elevator control input than a heavier one in the same situation, an attempt was made to convey that through the feel system, and so the lighter plane would "feel" lighter.


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boeingfixer
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RE: Elevator Feel And Tailstrike Reduction Issues

Tue Jan 30, 2007 2:04 am

Quoting Starlionblue (Reply 3):
Or you could just make the darned thing FBW and give the pilot the feel needed for that particular regime of flight.

Oh wait, another manufacturer already did that.

Sidestick FBW doesn't give the pilot tactile feedback on what the aircraft is really doing. In the case of the A320, springs attached to the sidestick controller give artificial feed-back to the pilot to mostly preclude abrupt maneuvers. The quality of the spring feedback system is all the pilot feels. The sidestick does not provide a tactile feedback of what the controls are really feeling and the pilots are more dependant on the flight display information than sidestick feed-back.

In the above case, FBW does not improve tail strike protection.

Cheers,

John
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Pihero
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RE: Elevator Feel And Tailstrike Reduction Issues

Tue Jan 30, 2007 2:51 am

Quoting BoeingFixer (Reply 5):
Sidestick FBW doesn't give the pilot tactile feedback on what the aircraft is really doing. In the case of the A320, springs attached to the sidestick controller give artificial feed-back to the pilot to mostly preclude abrupt maneuvers. The quality of the spring feedback system is all the pilot feels. The sidestick does not provide a tactile feedback of what the controls are really feeling and the pilots are more dependant on the flight display information than sidestick feed-back.

Why not say very clearly that you do not like the side-stick, prefer the Boeing set-up with feed-back, feel etc...?
In this forum it is acceptable, you know.
BS is not, though, because here people are more technically minded .

Quoting BoeingFixer (Reply 5):
In the above case, FBW does not improve tail strike protection.

Apparently, the other solution doesn't either as the 777 flight control system has an anti-tail-strike limiter ( by the way, that is a "hard limit" too.)
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Starlionblue
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RE: Elevator Feel And Tailstrike Reduction Issues

Tue Jan 30, 2007 4:09 am

Quoting BoeingFixer (Reply 5):
Quoting Starlionblue (Reply 3):
Or you could just make the darned thing FBW and give the pilot the feel needed for that particular regime of flight.

Oh wait, another manufacturer already did that.

Sidestick FBW doesn't give the pilot tactile feedback on what the aircraft is really doing. In the case of the A320, springs attached to the sidestick controller give artificial feed-back to the pilot to mostly preclude abrupt maneuvers. The quality of the spring feedback system is all the pilot feels. The sidestick does not provide a tactile feedback of what the controls are really feeling and the pilots are more dependant on the flight display information than sidestick feed-back.

In the above case, FBW does not improve tail strike protection.

First of all, I was being humorous.

Anyway, FBW is a very broad definition. There is nothing stopping a tailstrike limited in FBW systems, and in fact this kind of system is already in operation.
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FredT
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RE: Elevator Feel And Tailstrike Reduction Issues

Tue Jan 30, 2007 5:32 am

Quoting BoeingFixer (Reply 5):
Sidestick FBW doesn't give the pilot tactile feedback on what the aircraft is really doing. In the case of the A320, springs attached to the sidestick controller give artificial feed-back to the pilot to mostly preclude abrupt maneuvers. The quality of the spring feedback system is all the pilot feels. The sidestick does not provide a tactile feedback of what the controls are really feeling and the pilots are more dependant on the flight display information than sidestick feed-back.

Oh, but it does! For a given force on the controls, the FBW makes the control surfaces do just the expected thing. Sometimes this will take more deflection of the control surfaces, sometimes it will take less deflection. The guy applying the force does not need worry about it, all he knows is that x lbf of pressure corresponds to y deg/sec of pitch or a load factor of n gs.

This as opposed to the traditional non-FBW q feel system, which gives a set amount of tactile feedback for a given amount of control deflection, scaled by the dynamic pressure. A lot better than having no feedback at all (which will usually end up in disaster), but not all that transparent in all flight regimes. It is less predictable as it does not take all factors into account.
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boeingfixer
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RE: Elevator Feel And Tailstrike Reduction Issues

Tue Jan 30, 2007 8:22 am

Quoting Pihero (Reply 6):
Why not say very clearly that you do not like the side-stick, prefer the Boeing set-up with feed-back, feel etc...?
In this forum it is acceptable, you know.
BS is not, though, because here people are more technically minded .

You are assuming that I don't like the side stick based on my example. Nowhere did I say that! My reply stated a fact about the side-stick controller and you're the one taking offence to it. Please take your obvious A Versus B BS out of my reply and don't put words in my mouth that I didn't put there myself.

Regards,

John
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Pihero
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RE: Elevator Feel And Tailstrike Reduction Issues

Tue Jan 30, 2007 10:19 pm

Quoting BoeingFixer (Reply 9):
My reply stated a fact about the side-stick controller and you're the one taking offence to it.

OooooooKay !,
if that's the case, please accept my apologies.
Problem is you are quite wrong on that five-line statement of yours :
-1/ "Feel" is a very subjective term . The traditional "feel" was to re-establish a necessary force on a control as an aircraft speed function (basically).
-The 320 "feel" is a g force function. It becomes obvious very quickly and can help any pilot with the most accurate flying he/she could ever dream of.
So forget the PC-game idea you seem to be harbouring.
See FredT's post above about pilot's dependence on flight display info than stick feed back.

-2/

Quoting BoeingFixer (Reply 5):
In the case of the A320, springs attached to the sidestick controller give artificial feed-back to the pilot to mostly preclude abrupt maneuvers.

Another assumption . Have a look at an incident report (date is 12 Feb 99, the aircraft FGJVG on a descent to Montpellier ) and tell me if any other aircraft could have managed such a roll rate right off AP disconnect .
So you see why I could smell another uninformed, prejudiced anti-A rat ...

If it's not the case, once again my apologies .

Kind regards
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metroliner
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RE: Elevator Feel And Tailstrike Reduction Issues

Wed Jan 31, 2007 3:07 am

hey guys,

i'm still not entirely clear on this, but if possible, could you clarify for me whether the pilot has a 'feel' for the controls through a 'force-feedback-esque' system with an airbus sidestick?

as an example, when flying gliders, i needed more force on the stick to initiate a bank at 100kt as compared to 50kt - does an airbus pilot feel this difference, too? and, for that matter, does a pilot with hydraulically actuated controls?

toni
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Starlionblue
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RE: Elevator Feel And Tailstrike Reduction Issues

Wed Jan 31, 2007 4:04 am

Quoting Metroliner (Reply 11):
i'm still not entirely clear on this, but if possible, could you clarify for me whether the pilot has a 'feel' for the controls through a 'force-feedback-esque' system with an airbus sidestick?

as an example, when flying gliders, i needed more force on the stick to initiate a bank at 100kt as compared to 50kt - does an airbus pilot feel this difference, too? and, for that matter, does a pilot with hydraulically actuated controls?

On the Airbus the force required to move the stick x millimeters is always exactly the same.

But note that the deflection of the surfaces for x at low speed is much larger than at high speed. The deflection of the stick is translated by the computers as an "intent" to move the plane by y, and thus the surfaces are moved to achieve that. The actual deflection is dependent on several factors, for example speed.

Without such a system, the pilot has to apply different amounts of force based on flight regime. At high speed, the pilot of a "traditional" aircraft would have to apply more force to the stick since the surfaces are acted on by the air. However, hydraulics mitigate this.
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metroliner
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RE: Elevator Feel And Tailstrike Reduction Issues

Wed Jan 31, 2007 9:51 am

okay, i get you!  Smile

great explanation - thank you very much.

toni
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jetmech
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RE: Elevator Feel And Tailstrike Reduction Issues

Wed Jan 31, 2007 10:30 am

Quoting AerospaceFan (Thread starter):

G'day AerospaceFan ?,

The blurb from the MM is along the lines of such;

The elevator feel computer modulates hydraulic pressure to a feel actuator. The output of the feel actuator is fed, via a cam mechanism to the elevator aft torque tube located behind the stabiliser centre torque box. The "feel" resistance is fed back to the pilots control columns via the elevator control cables that run the entire length of the fuselage in the roof cavity.

Elevator feel is a function of the stabiliser position (CofG) and dynamic air pressure. The elevator feel actuator output is transmitted to a centering cam. This cam is also acted upon by a spring loaded centering roller. The cam profile is designed such that an increasing resistance is felt by the pilots the further away from neutral that the column is pushed or pulled.

If I understand correctly, the resistance the pilots feel at any given moment is the sum of the resistance of the centering springs acting on the centering roller, plus any additional force applied directly to the cam by the feel actuator.

Thus, if I understand correctly, the B744 elevator feel is constant in that the additional resistance force away from neutral is always the same for a given column displacement regardless of the current flight regime. The elevator feel force is varied by changing the force required to "breakout" the control column from neutral, which is a function of the stabiliser position (CofG) and dynamic air pressure. I suspect that the neutral "breakout" force is increased for increasing dynamic pressure and aft CofG positions.

Regards, JetMech

[Edited 2007-01-31 02:44:04]
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AerospaceFan
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RE: Elevator Feel And Tailstrike Reduction Issues

Wed Jan 31, 2007 10:41 pm

JetMech, thanks for your message! I really appreciate it.

Everyone's responses in this thread are so interesting, I'm quite impressed. Thanks to all!
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troubleshooter
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RE: Elevator Feel And Tailstrike Reduction Issues

Wed Jan 31, 2007 10:56 pm

Quoting JetMech (Reply 14):
Elevator feel is a function of the stabiliser position (CofG) and dynamic air pressure.

Basically the same principle as on the B737.
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boeingfixer
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RE: Elevator Feel And Tailstrike Reduction Issues

Thu Feb 01, 2007 4:13 am

Quoting Pihero (Reply 10):
Quoting BoeingFixer (Reply 9):
My reply stated a fact about the side-stick controller and you're the one taking offence to it.

OooooooKay !,
if that's the case, please accept my apologies.
Problem is you are quite wrong on that five-line statement of yours :
-1/ "Feel" is a very subjective term . The traditional "feel" was to re-establish a necessary force on a control as an aircraft speed function (basically).
-The 320 "feel" is a g force function. It becomes obvious very quickly and can help any pilot with the most accurate flying he/she could ever dream of.
So forget the PC-game idea you seem to be harbouring.
See FredT's post above about pilot's dependence on flight display info than stick feed back.

-2/

Quoting BoeingFixer (Reply 5):
In the case of the A320, springs attached to the sidestick controller give artificial feed-back to the pilot to mostly preclude abrupt maneuvers.

Another assumption . Have a look at an incident report (date is 12 Feb 99, the aircraft FGJVG on a descent to Montpellier ) and tell me if any other aircraft could have managed such a roll rate right off AP disconnect .
So you see why I could smell another uninformed, prejudiced anti-A rat ...

If it's not the case, once again my apologies .

Kind regards

All of that is your opinion and I take exception to you calling my post a prejudiced anti-A rant. FYI, I am endorsed on the A300 A310 and have no ill feelings towards Airbus as you so outwardly have against Boeing.

Since I don't have first hand knowledge of the A320 sidestick, I did some research and simplified my answer accordingly. I am sorry it didn't meet with your, oh so high, technical standards.

I smell a pilot who has a bad attitude towards the people that keep you in the air. If that's not the case, my apologies! I'll be sure to keep clear of you in the future!
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RE: Elevator Feel And Tailstrike Reduction Issues

Thu Feb 01, 2007 5:24 am

Never accused you of a raNt
I was just smelling a rat.

Me, I have nothing against Boeing, or Lockheed (Ah! the Tristar !) or Douglas (Ah! those DC 4 and 6 and 7s !) and my daily bread doesn't depend on how many aiframes X,Y or Z sell.
A few people on this forum know that I prefer the Airbus philosophy, but that's about it, I just love airplanes.
You said you did some research and simplified your answer accordingly.
Fair enough, your post still contains as many errors as sentences, which makes me wonder where you found your reference.
Read it again.
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FredT
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RE: Elevator Feel And Tailstrike Reduction Issues

Thu Feb 01, 2007 6:02 am

If there is an obvious line between two sides in a debate, you do not have to specifically mention one of the two sides to take a stand.

If you turn yourself into a knot trying to one-sidedly talk down one design philosophy, it takes a bit of gall to accuse the people calling you on it of being the ones starting up the old, tired debate on which of the two philosophies is "better".

Yes. Old. Tired. More or less pointless.

This forum has a high standard. Professionals and interested members of the public exchanging thoughts on a multitude of subjects related to the technological and operational side of aviation. Keep the facts straight and argue the pros and cons of the two debated design philosophies, but please do not twist the facts trying to "win" for the side you have selected.

Arguing on the internet, and all that...

Last but not least: If your nick contains the A word or the B word, you need to be extra careful not taking sides in your posts unless you want to be called on it.

Now, get back to factual debate.
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OldAeroGuy
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RE: Elevator Feel And Tailstrike Reduction Issues

Fri Feb 02, 2007 1:27 am

Quoting Starlionblue (Reply 3):
Or you could just make the darned thing FBW and give the pilot the feel needed for that particular regime of flight.

Oh wait, another manufacturer already did that.

The A340 has had some interesting tail strikes so the condition has not been completely eliminated.

Quoting Pihero (Reply 6):
Apparently, the other solution doesn't either as the 777 flight control system has an anti-tail-strike limiter ( by the way, that is a "hard limit" too.)

The 777 Tail Strike Protection (TSP) system as implemented on the 773ER and the 772LR is not a hard limit system as the pilot has to be able to put the aft body on the runway to satisfy the Vmu condition. At normal operating speeds though, an aft body contact is very unlikely.
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Starlionblue
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RE: Elevator Feel And Tailstrike Reduction Issues

Fri Feb 02, 2007 6:51 am

Quoting OldAeroGuy (Reply 20):
The A340 has had some interesting tail strikes so the condition has not been completely eliminated.

But I thought there were hard limits?  Wink

Anyway isn't this only on the 340-500/600?
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OldAeroGuy
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RE: Elevator Feel And Tailstrike Reduction Issues

Fri Feb 02, 2007 12:07 pm

Quoting Starlionblue (Reply 21):
Anyway isn't this only on the 340-500/600?

The tail strikes or the system?
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Starlionblue
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RE: Elevator Feel And Tailstrike Reduction Issues

Fri Feb 02, 2007 12:56 pm

Quoting OldAeroGuy (Reply 22):
Quoting Starlionblue (Reply 21):
Anyway isn't this only on the 340-500/600?

The tail strikes or the system?

Lol. The tailstrike limiter thingy.
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Pihero
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RE: Elevator Feel And Tailstrike Reduction Issues

Sat Feb 03, 2007 6:53 pm

Quoting OldAeroGuy (Reply 20):
not a hard limit system as the pilot has to be able to put the aft body on the runway to satisfy the Vmu condition.

Here, the discussion is lively and very open. I'd appreciate a good source as apparently, even B TRIs are not in agreement.

Quoting Starlionblue (Reply 21):
Anyway isn't this only on the 340-500/600?

As far as I know, and very strangely, A. doesn't go with a TSP as B. did for the 773 ER. There is only a pitch command limit on the FD.
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OldAeroGuy
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RE: Elevator Feel And Tailstrike Reduction Issues

Sun Feb 04, 2007 3:26 am

Quoting Pihero (Reply 24):
Here, the discussion is lively and very open. I'd appreciate a good source as apparently, even B TRIs are not in agreement.

My comments are based on discussions with the groups of Boeing engineers responsible for writing the FBW code specifications and doing the performance calculations.

The amount of FBW elevator damping applied counter to pilot input is a function of the aft body height relative to the ground and the closure rate. If the closure rate is low, the pilot can touch the aft body to the runway and satisfy the Vmu attitude and speed established during certification. However, if the same slow rotation rate is applied near the correct Vr, lift off will occur prior to aft body contact due to airplane acceleration.

On the other hand, if the airplane is near the correct Vr and the pilot tries to rotate at an excessive rate, the FBW logic will reduce the amount of elevator he can input. The net effect is to produce a "standard" pitch rate that will re-produce the certified performance levels for either all engine or engine inoperative takeoffs.
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Pihero
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RE: Elevator Feel And Tailstrike Reduction Issues

Tue Feb 06, 2007 4:00 am

Thanks OldAeroGuy !

Regards
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RE: Elevator Feel And Tailstrike Reduction Issues

Tue Feb 06, 2007 8:57 am

Quoting Starlionblue (Reply 21):

But I thought there were hard limits? Wink

Anyway isn't this only on the 340-500/600?

The 345/6 does not have any hard limits, just a visual indication on the PFD of the max pitch angle for T.O. & Ldg.
 
OldAeroGuy
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RE: Elevator Feel And Tailstrike Reduction Issues

Mon Feb 12, 2007 5:05 am

At the end of the article in the link, there is a description of the 777 TSP.

http://boeing.com/commercial/aeromag...rticles/qtr_1_07/article_02_1.html

It agrees with what I said in Reply 25.
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