Question About Take Off Trim Value.

[Tony747-400 44]

Hi guys.

 

I understand the concept of setting the Take Off trim and why we do it. I've watched Kyles video on his You Tube page about Trim settings too.

 

What I don't really understand what trim value really is. For example the FMC may give me a Trim Value of say 5.25 but what is 5.25? I've heard pilots on videos call it Units of Trim. Is it an angle setting of a the stabalizer in degrees like flap settings? If it's measured in units of Trim how do we arrive at the figure 5.25?

 

Hope my question kind off makes sense, thanks for any help.

[downscc 1,327]

 

 

I've heard pilots on videos call it Units of Trim.

 

It is just a dimensionless scalar, just a number.  How trim is applied varies with control surface and aircraft but in the most basic sense it is just a method that reduces the force necessary to deflect a control surface.  How this is done could be a trim tab (a smaller control surface on a control surface), movement of an entire element such as a horizontal stabilizer or even the whole tail assembly as in the early single engine Moony's.  The term is probably derived from sailing ships where sails are trimmed.  See also http://en.wikipedia.org/wiki/Trim_tab

[Tony747-400 44]

Thanks for the reply.

 

I fully understand the concept of Trim and how it works but it's the value of the example I used of 5.25 that I don't understand, what is it a measurement / value of? It's easy just to set the trim to 5.25 on the stabalizer as per the FMC but what is the actual figure, is it like a MAC% or an angle in degrees setting? Basically I'm asking what is the figures meaning? 5.25 of what?

[downscc 1,327]

 

 

5.25 of what?

 

Not a measurement of anything, like I said, it is a dimensionless scalar. Probably not totally arbitrary but many aircraft don't have numbers at all, just a dial and one direction is nose up and there might be a green band that indicates the normal take-off setting.  The number just makes it easier to "index" instead of setting trim to 5 seconds of nose up from neutral to a number such as 5.25.  I notice the green band on the NG has "CG-% MAC" in it; but the FCOM refers to the dimension as just UNITS.

[kevinh 409]

Not a measurement of anything, like I said, it is a dimensionless scalar. Probably not totally arbitrary but many aircraft don't have numbers at all, just a dial and one direction is nose up and there might be a green band that indicates the normal take-off setting. The number just makes it easier to "index" instead of setting trim to 5 seconds of nose up from neutral to a number such as 5.25. I notice the green band on the NG has "CG-% MAC" in it; but the FCOM refers to the dimension as just UNITS.

In the case of stab trim the units do relate directly to the angle the stabiliser is set to. There is a scale painted on the rear fuselage to check the calibration.

 

The 737 has two trim scales. One in pilot units, the other showing the equivalent %CG. So if you know your takeoff CG position you can set trim to that value directly and not have to determine the trim units required for the CG.

 

As I understand it "units" of trim were used originally as an arbitrary index when trim tabs were the norm. Trim units would relate directly to tab deflection. Tab deflection deflects the surface by a variable amount depending on speed, aoa, etc. so the relationship with surface angle was indirect. With fully powered controls, trim directly offsets the control surface angle and so a unit of trim directly corresponds to x degrees deflection. Sometimes one to one correspondence. So a unit of trim is just another different angular definition, like degree, radian, gradian, etc.

[vololiberista 205]

The trim setting is in degrees. You might be able to find a graph. It would be a straight line graph with on one side a positive scale in inches and on the other a negative scale in inches. These scales represent the CoG datum. Look at the fifth post down and you will see an example.

http://www.vc10.net/forum/viewtopic.php?f=4&t=1096

You will see that the neutral trim setting in this case is a tad below 3.8. The greater the load fwd of the CoG the higher the tail plane incidence setting and vice versa.

[downscc 1,327]

Full names in the PMDG forums please.

 

Your reference is specifically for a VC-10.  Not convinced.

[CaptKornDog 281]

As stated above, Mr. Boeing made the trim unitless. Sure, from an engineering perspective it correlates to some unit value, but for pilots, it is purely just a number. Perhaps because the designers wanted pilots to stop asking questions or do math on their own and just use the values the book/computer tells them

 

And yes "units of trim" is one form of correct nomenclature.

 

Mr. Pilatus, Mr. Cessna, Mr. Piper, Mr. Dornier, and many others all have at least some aircraft with the same convention.

 

(Source: numerous colleagues of mine affiliated with Boeing and my own aviation background experience)

[kevinh 409]

Full names in the PMDG forums please.

 

Your reference is specifically for a VC-10. Not convinced.

Boeing stab trim settings equate to equivalent amounts in degrees too. No doubt a different scale to the VC10 but the same principle applies.

 

On every Boeing I've worked on stab trim units are zero relative to the fuselage reference line and measure the degrees of incidence relative to that. These convert to pilot units, as marked in the cockpit, usually by the following relationship.

 

Pilot Units = 3.0 - Stab Units

As stated above, Mr. Boeing made the trim unitless. Sure, from an engineering perspective it correlates to some unit value, but for pilots, it is purely just a number. Perhaps because the designers wanted pilots to stop asking questions or do math on their own and just use the values the book/computer tells them

 

And yes "units of trim" is one form of correct nomenclature.

 

Mr. Pilatus, Mr. Cessna, Mr. Piper, Mr. Dornier, and many others all have at least some aircraft with the same convention.

 

(Source: numerous colleagues of mine affiliated with Boeing and my own aviation background experience)

Mr Boeing did no such thing. It's a convention, Boeing didn't invent it.

 

All angular measures are dimensionless btw. Degrees, radians, units of trim. They are still units you can measure things by though.

[CaptKornDog 281]

Mr Boeing did no such thing. It's a convention, Boeing didn't invent it.

 

Didn't say he did.  All I said was they made trim unitless on a variety of their aircraft.

[downscc 1,327]

I'm not an A&P so my knowledge is mostly anecdotal; however, I think you have to stretch to get from degrees to units, there's problems with signs (positive vs negative degrees with respect to longitudal axis)... sure, there's a relationship but there is also a reason that it is a dimensionless scalar.  Dimensionless means no sign in this context, scalar means ..... well, it's not a vector.

[kevinh 409]

Didn't say he did. All I said was they made trim unitless on a variety of their aircraft.

The way you wrote the post sounded like you did. Boeing didn't make trim unitless. A unit of trim is by definition a unit. A dimensionless one but still a unit. As I said in my post, if it was degrees of trim it would still be dimensionless.

 

Marking trim tab wheels in arbitrary units was convenient for aircraft manufacturers. It's a reference point. The pilot doesn't need to know the actual deflection, even though there is a physical relationship to that.

I'm not an A&P so my knowledge is mostly anecdotal; however, I think you have to stretch to get from degrees to units, there's problems with signs (positive vs negative degrees with respect to longitudal axis)... sure, there's a relationship but there is also a reason that it is a dimensionless scalar. Dimensionless means no sign in this context, scalar means ..... well, it's not a vector.

Dan,

 

Are you saying my post about Boeing stab trim calibration is wrong?

 

Dimensionless does not mean no sign in any context. Nor is it anything to do with vector or scale quantities. The sign of a trim input is certainly important for directional and lateral trim. Stab trim angle relative to fuselage reference certainly does have a sign.

[downscc 1,327]

Kevin, we are word-smithing and I think this difference is context. You are mechanically orientated and I'm cockpit oriented.  The cockpit trim is presented in units, no dimensions and no sign. Thank goodness the alignment of the stabilator is more complex than that.  By the way, dimensionless really does mean no sign not to be confused with scalars that can be dimensionless but have sign.

[CaptKornDog 281]

Yep, now we are down to semantics.

 

I think the original question was answered sufficiently.  I'm out.

[kevinh 409]

Kevin, we are word-smithing and I think this difference is context. You are mechanically orientated and I'm cockpit oriented.  The cockpit trim is presented in units, no dimensions and no sign. Thank goodness the alignment of the stabilator is more complex than that.  By the way, dimensionless really does mean no sign not to be confused with scalars that can be dimensionless but have sign.

Dan,

 

Actually I'm not mechanically orientated at all. I'm a systems engineer and very much cockpit orientated too. I relate to pilots all the time in the course of my work. I understand you see it from a pilot's perspective and that for you the trim setting is an arbitrary number. But that doesn't mean it has no relationship to an actual angle, which you hinted at very strongly, saying you have to stretch things to link units to degrees. You don't have to stretch anything. That isn't word-smithing, you more or less said I was wrong. For Boeing the setting in units has a clear meaning in terms of aerodynamics, stablity and control. Stabiliser units relate directly to stabiliser angles (radians or degrees). Imagine how they could possibly design an aeroplane without that relationship defined mathematically.

 

Dimensionless really does not mean without sign. Angles are dimensionless and have a sign, for example. Having dimension means the unit can be expressed in terms of mass, length and time. Simple as that. If you think about it, trim setting must have a sign too, since the trim can be nose up or nose down. The fact is Boeing make it easier for pilots by calibrating the pilot units so it's always a positive number, but the equation I referred to:

 

Pilot units = 3.0 - Stab units

 

shows that 3 pilot units equates to 0 units of stabiliser units incidence. Zero pilot units is +3 stabiliser units. 6 pilot units is -3 stab units. Pilot units are what you are familiar with and what is marked on the pedestal.

 

Airbus don't do this conversion, which is why their trim setting units are +/- with zero meaning zero incidence. In other words just like Boeing's stabiliser units.

 

Rudder trim certainly changes sign (only for the pilot the sign is right/left not plus/minus). It's dimensionless though.

 

Sorry for harping on about this but as an engineer I felt it was important to point out that trim units aren't just an arbitrary number pilots have to set. It does equate to a physical position on the tail. Pilot's tend to rule on this forum but they aren't always technically correct and that can misinform people.

 

Best regards

 

Kevin

Yep, now we are down to semantics.

 

I think the original question was answered sufficiently.  I'm out.

As a pilot you must know that a clear understanding of what words mean (semantics) can save lives. Semantics are not trivial. However this is more about definitions of technical terms than semantics.