Elevator, rudder & aileron trim

[shumway76]

I have read that the trimming sequence for aircraft with trim on all primary flight controls is ERA - elevator, then rudder, then aileron.
What is the theory behind this?

[Pilot DAR]

My theory would be that sometimes people will write just because people might read, and some of what has been written does not really matter much (hooray PPRuNE!).

If I had a large out of trim, I'd be trimming that first, then going to the lesser out of trim, then tweaking each as required...

Aviation has too many rules as it is, we don't need one for trimming!

[Big Pistons Forever]

While this sounds like a silly question I woulds argue that for a low time pilot it is actually a teachable point.

Obviously most trimming involves the elevator, but many new CPL's will not see three axis trimming until they get their first job. I realized this many years ago when watching a new Navajo Copilot fuss with the rudder and aileron trim. The problem is, of course, the fact that rudder and aileron trim affect each other. After watching much fiddling with the trim wheels for a few minuites the light dawned and I realized what the problem was.

Things got much better when I told him to hold the wings level with the control wheel and centre the ball with the rudder trim. Then I got him to trim off the roll pressure. Pretty obvious to someone with some experience aircraft with 3 axis trim, but something not so obvious to a newbe and rather sad it had never been covered in his training.

The real problem was that as part of his preflight he had put all the trims to zero, apparently on the advice of some instructor in flight school fantasy land.

So here is a tip. If you are a new copilot and are preflighting an aircraft that just came in from a revenue flight like this guy was, and you find the rudder trim at 1.5 units right and the aileron trim at 1 unit left wing down, there was probably a good reason for it and maybe you don't want to mess with it.

Anyway after the copilot got the aircraft flying nice and straight guess where what the trim settings were :hmm:

[BEagle]

The real problem was that as part of his preflight he had put all the trims to zero, apparently on the advice of some instructor in flight school fantasy land.

What does the aircraft's POH require?

[Jumbo Driver]

So here is a tip. If you are a new copilot and are preflighting an aircraft that just came in from a revenue flight like this guy was, and you find the rudder trim at 1.5 units right and the aileron trim at 1 unit left wing down, there was probably a good reason for it and maybe you don't want to mess with it.

This is all very well, provided you don't subsequently discover that the aircraft flies wings level, but with the balance ball displaced to the left because of crossed controls as a result of trim set improperly by the previous incumbent.

Personally, therefore, I would favour resetting trims to zero on the pre-flight, after making a note of the setting(s) found, and then making any necessary adjustments as soon as stable in the cruise. The only exception would be if there was an existing Tech Log entry to the effect that trim had been found to be required to fly straight and level, in which case I would accept the recommended trim setting but confirm it myself by inspection in flight.

Incidentally, I agree with Big Pistons Forever about the order of 3-axis trimming. After trimming the elevator, hold the wings level with the ailerons and trim off any tendency to yaw (i.e. centre the ball) with the rudder trim; then (and only then) remove any roll tendency with aileron trim. If you trim in a different order, you are quite likely to end up with crossed-controls - such as in the situation posed in the quote above.


JD
:)

[maxred]

Whilst certain types may require 'zero', settings, as BEagle states, what about the POH???

A fair number of types I fly, require careful setting of trim position, pre take off.

This is all part of the pre take off checklist.

Obviously on the ground the order in which they are set probably does not matter, as long as they are correct when you get in the air. Then the sequence for me is elevator, followed by whatever requires minor adjustment (if any).

It is also more critical, GROUND SETTINGS, if your flight is straight into IMC.

[mad_jock]

Its one of the first things you bang into new FO's is trimming her properly. And for me its Elevator rudder then roll as well.

You have to watch though going for preflight settings same as cruise settings.

Some aircraft which are right at the edge of the envelope pre load for a engine failure of the critical engine. If you trim for the cruise you will come a cropper or the whole V1 cut will require considerable more work and leg forces.

On my type you have several trimming settings, every flight phase requires it to be changed and every aircraft is different. It also depends if you fly them on matched EGT's or matched torques or the really bizarre one matched levers.

But the setup for departure according to the book is the rudder slightly favouring a failure of the critical engine (half a unit), neutral roll and green band for the elevator. And for the other flavour of this type with smaller engines its everything neutral and green band for the elevator.

[Jumbo Driver]

Further to my post above, of course any trim settings required by the POH/FM for the specific take-off conditions should be used, modified if necessary by any relevant Tech Log entry.

In commercial flying, an initial pre-flight check will usually restore trim to a datum setting and the setting required for the conditions on the day will be set during the subsequent taxy or before take-off check.


JD
:)

[BackPacker]

While this sounds like a silly question I woulds argue that for a low time pilot it is actually a teachable point.

In what aircraft?

The majority of basic trainers, and thus also the majority of the for-hire fleet, only has elevator trim. The only spamcan I know of that has rudder trim is the PA28, and we were told not to touch them at all because they were set for cruise all the time. Aileron trim? Never seen it in any spamcan. (Although I've had a lot of solo flights in a DR400 where I would welcome aileron trim - the DR400 seems to be more susceptible to a lateral imbalance than for instance the PA28.)

[mad_jock]

I took this thread to be a discussion on flying twins.

Of course you can trim light aircraft in roll and yaw but it usually takes a series of flights by the same person to bend the trim tabs between flights so you get what you want. And you use the same method as if you were doing it internally ie yaw first then roll. The conditions that you trim for are set out in the POH. 2500ft pressure altitude and cruise rpm rings a bell for the ones I have done.

Its bloody fustrating though when some prat comes along afterwards and decides the tabs are too bent and bends them straight again. Then promptly complains that the plane is miss rigged as they have to fly everywhere with a boot full of rudder in. Low houred instructors are just as likely guilty doing this as PPL's more so because they think they know what they are doing.

Low hours in twins is anything below 100 sectors

They are just about usefull by 250 and know what they are doing by 500 depending how much single engine work they have done in that time.

[gasax]

My aircraft has a mauw of 560kg and an aileron trim.

The light weight and abreast seating mean that the aileron trim is actually very necessary to get it to fly correctly.

So for me it is elevator, make sure the ball is in the centre and then trim the aileron. If it is not done in that order then the aircraft tends to settle is a slightly crabbed orientation!

All of this is actually very noticeable as the control forces are very light - and so out of trim is very intrusive, masking what the aircraft is actually doing.

[IO540]

What a lot of people don't know is that before one does any bending of aileron tabs etc one must start with an accurately rigged plane.

The wings need to be screwed on exactly equally. The flaps must be within 1-2mm of each other when fully up. Then put a long spirit level across the sills of the two opposing doors (or whatever levelling reference points are defined by the factory) and make sure the AI is precisely level (obviously you need to run it to erect; this may need a vacuum pump). The AI should be adjustable to a small degree by loosening its four screws and moving it in the panel. If this adjustment is not sufficient, then get it overhauled by a company which can spell G Y R O :) Very very few of them in Europe. I recommend Castleberry Instruments in the USA, and avoid anything done by Mid Continent (I suspect they ship it out to Mexico ;) ).

No point in tweaking the TC because it is spring loaded to the central position ;)

Then, for a flight, the plane must be balanced. This means equal fuel tanks and two pilots of equal weights (or equivalent).

Only then you should think of bending aileron trim tabs.

You may well find that once the rest of the plane is sorted, the tabs are not needed. (Mine are straight).

When I fly alone, I run down the LH tank by about 1/4" on the fuel gauge (the gauges in the TB20GT are the very accurate capacitive type) and this gives a balanced situation. Obviously it is more quickly achieved if one does the takeoff+climb on the LH tank.

And load up the plane as fully to the rear as possible; this reduces the elevator AOA and thus reduces the elevator drag.

None of this is rocket science, and you get a few % more MPG that way. I've had some character on the Socata owner's group (from where I got kicked out in 2008 after upsetting the owner of the server and a bunch of bible carrying sycophants :E ) claim that my engine was illegally gas flowed because none of them seem to get that efficiency :)

[Big Pistons Forever]

I will stand by my point that 3 axis trimming is a teachable point. It is obviously not directly teachable in a Cessna trainer but that does not mean it cannot be discussed on the CPL course, so that you student doesn't have to ask for advice on pprune. Also what would blindly obvious to a very experienced pilot is not necessarily so to a low houred new CPL, so rubbishing the question as unworthy of an answer is IMO not really fair.

As for not zero-ing the rudder and aileron trim, well I have yet to meet an aircraft that flies perfectly wings level with the ball in centre with the trims set to zero. They all seem to need a little bit of tweeking to fly right. Therefore if a aircraft comes in from a flight flown by someone I know, which is the majority of the cases, I am going to leave the rudder and aileron trim where I found them. Obviously if after takeoff they appear to be mis set then I will zero them and retrim, but this in practice is hardly ever required.

It is not wrong to zero the trims particularly if they seem mis set, my only point is that mndlessly doing it before every flight is just the difference beween flight school theory and real world operations.

[Katamarino]

I don't think we need to add that the aircraft must remain inside the C.O.G. limits - that should be obvious to anyone. Where do we stop; do we need to start prefacing all advice, for example, that before trying this landing technique one must have a pilots licence, airworthy aircraft, permission to land (if required), etc? :ugh:

[goldeneaglepilot]

BPF I totally agree with you, it’s a sensible answer to the question.

From my own point of view the advice of IO540 is in part dangerous. Without doubt there will be a number of people reading the forum who are low houred pilots, looking for tips and advice to improve their own personal skills.

Had IO540 qualified his statement with a caution – “make sure the aircraft remains within the POH stated C of G limits by doing a weight and balance calculation” then that’s ok. However ANY aircraft with a C of G towards its rearwards limits of G of G envelope will be more sensitive to pitch (elevator response) and will be easier to spin. That’s the reason why some tandem aircraft (forgetting the issue of switches etc) state solo flight is only permitted with the pilot in the front seat.

Personally I would feel much safer in an aircraft with a slightly forward C of G than a rearward C of G, you can always fly with some up trim if the C of G is forward, but no amount of down trim will make up for the pitch instability of a rearward C of G.

Who cares if you gain a very small gain in economy - if the reason for gaining it is one that exposes a pilot to increased risk? Try applying the load into the rear logic to an AA5 and you’re likely to spin in on your turn to finals.

Just to qualify myself – I spent several years flying for different maintenance organisations doing C of A flight testing – both singles and twins and have experienced practical trimming and rigging of a number of different aircraft types, and I’m not a bible bashing sycophant!!

[IO540]

From my own point of view the advice of IO540 is in part dangerous

make sure the aircraft remains within the POH stated C of G limits by doing a weight and balance calculationActually I did consider suggesting that people fly 30% over MTOW as well, because that increases Va by approximately 15% which is useful because you can fly faster in turbulence :ugh:

Without doubt there will be a number of people reading the forum who are low houred pilots, looking for tips and advice to improve their own personal skills. Anybody who has got a PPL and doesn't know about loading ought to find his (or her - got to be careful here now ;) ) instructor and ask for a refund.

Who cares if you gain a very small gain in economy - if the reason for gaining it is one that exposes a pilot to increased risk? Try applying the load into the rear logic to an AA5 and you’re likely to spin in on your turn to finals.

Only if you have totally lost the plot when it comes to watching the airspeed. There is no way to unintentionally spin any normal certified plane otherwise.

[goldeneaglepilot]

IO540, I appear to have touched a nerve, sorry about that. My point is very simple, the handling of an aircraft does change dependent on where the C of G is. That is a fact. The envelope in the POH is a demonstrated RECOVERABLE envelope. The further aft the c of g is then the aircraft will be more sensitive in pitch.... We are not all super human pilots, and we sometimes make mistakes. Why deliberatly (for the sake of a very small increase in economy) take away a factor of safety? If we talk of extremes, look at the Pitts S2A, with C of G at the forward point of the envelope spins recover almost instantly (with correct control inputs of course) at the rear point it takes nearly a turn before it recovers. It also has a significant influence on flick roll entry. But of course you are very aware of all this - is the low houred PPL, who is reading this forum to expand their knowledge so aware?

I think we can all learn something from each other, however its hard to understand why someone gets upset when questioned? Dont forget that with things you take for granted because of your experience, others may not have yet reached that level of experience.

[IO540]

Not a problem, goldeneaglepilot, no offence taken.

I just think one is entitled to make some very basic assumptions about pilot training.

I have zero experience of aerobatic ops and accept that those types could be very sensitive to loading, etc, but at the same time anybody that's going to fly a Pitts ought to have enough brains to know they will need type specific training.

Loading a normal aircraft near the back of the envelope should not render it unstable or particularly difficult to fly.

My comment on fuel economy was meant to be in the context of getting the trim right. If one is trying to get the whole thing to fly straight, loading it for efficiency as well is not a bad idea.

Actually in most cases aft loading is not going to be optional; the most typical scenario would be flying 2-up in a 4-seater, and choosing to put luggage in the luggage compartment versus putting it on the back seat. The former option will improve MPG, ever so slightly.

I often wonder why the type I fly (a TB20) does almost the same speed (for a given fuel flow) across a wide range of total weights, and the only explanation I can come up with is that the elevator AoA (and drag) is indeed changing, and improves with larger total weights.

[24Carrot]

I often wonder why the type I fly (a TB20) does almost the same speed (for a given fuel flow) across a wide range of total weights, and the only explanation I can come up with is that the elevator AoA (and drag) is indeed changing, and improves with larger total weights.

I've been pondering this too, since you raised it in an earlier thread.

The only thing I came up with is IAS error. Most aircraft POHs have a table translating IAS to CAS, and for the C172 it just states assumptions of "Level Flight" or the mysterious "Max Power Descent". I assume a lot of this error is due to the AoA varying with speed, for a given weight. Of course the AoA will also need to change as the aircraft weight changes, and I suppose this must introduce similar errors.

I can't imagine it is big enough to make a difference, and it might not even be in the right direction, but I thought I would mention it.

[maxred]

I tend not to get involved technically on this forum, however, Theoretically as cg moves aft, the negative balancing lift from the tail can be reduced, and the aeroplane should become ''lighter'', and therefore more faster.

However, it must be understood that the price of increased airspeed is REDUCED LONGITUDANAL stability, and no speed gain should tempt anyone to violate an aeroplanes aft limit.

i.e no gain in speed, or MPG should involve a compromise with stability.

[IO540]

The only thing I came up with is IAS error

Wouldn't seem likely as the change in the aircraft pitch between say flying 1-up with 20USG in the tanks, and flying at MTOW (approx 1070kg to 1400kg) is not readily visible, at say 140kt IAS.

Also, one would tend to notice it in the GS, over time.

The change in IAS for a given fuel flow is at most 3kt i.e.2%.

no speed gain should tempt anyone to violate an aeroplanes aft limit.

Obviously, but you are entitled to load the plane anywhere inside the envelope.

If you feel otherwise, you should write to the FAA and to EASA and ask them to revoke the Type Certificate :)

[Big Pistons Forever]

I often wonder why the type I fly (a TB20) does almost the same speed (for a given fuel flow) across a wide range of total weights, and the only explanation I can come up with is that the elevator AoA (and drag) is indeed changing, and improves with larger total weights.

A Cessna 340 I occasionally fly, also seems to fly at about the same TAS regardless of the aircraft total weight. Your explanation seems reasonable. Also small changes in wing AoA may affect drag and because the airflow patterns over the whole aircraft may be driven in a slightly more favorable direction at higher aircraft weights therefore slightly reducing drag and offsetting the higher weight.

You have me intrigued now. Does anyone with a theoretical aerodynamics background care to comment ?

[24Carrot]

OK, one more uninformed speculation!

We think of these aircraft as rigid, but if you stick an extra 330kg in the middle of a 10m beam, (aka wing), you must get some deflection, and perhaps some twist.

I am sure the designers made sure that nothing bad happened as a result, but perhaps they managed to be cleverer than that?

PS the AoA change with those weights would be something like:
(1 - 1070/1400) x 3 degrees, maybe 0.7 degrees.
So the IAS error would indeed be small, and I think it is in the wrong direction anyway.

[maxred]

Yes, because at any given power setting the aircraft will be slower at high than at low weight. In steady level flight, thrust equals drag, and lift equals weight.

If therefore weight is added to an unaccelerated aeroplane, it would begin to descend. To compensate, you would have to increase lift, but with power held constant, the only recourse is to raise the nose, and therefore the AOA.

However, this also would increase induced drag, which slows you down.

In reality however, this speed reduction is quite small, and is generally because the addition of weight, generally moves the cg aft.

It may be in the two cases above 340, and TB20, that the cg does not move back far enough with increased weight, to alter the speed.

I am not an expert in theoretical aerodynamics however:uhoh:

[IO540]

Certainly, on the TB20, burning fuel has a negligible effect on the CofG. On the computer graph you can just about see a small shift.

The reality is that I can depart on a ~950nm flight with 86USG and land with say 22USG, and the IAS is the same for a given flow rate.