Elevator trimming technique

[shumway76]

I guess all pilots are thought about the proper elevator trimming technique - hold the desired attitude with the control column, then trim off any pressures felt.

DO NOT use the trim controls to adjust the attitude.

My question is - why?
I find that very fine attitude adjustment sometimes can be done better with the hands off the control column & using only elevator trim.

Is it just a matter of technique?
Is there any detrimental effect to the trim system if it's done wrongly?

I remember reading in the POH, if elevator control is lost, your only other choice is to control pitch using elevator trim.

[AdamFrisch]

There's this whole idea that trim can overload and stress the structure if used to increase load factor. I don't worry about that myself and use it in sustained turns, climbs and descents all the time. Maybe that's wrong, but it makes for much more pleasant flying.

[The500man]

I think the idea is to trim until you reduce the control force. If you just use the trim control there is no control feedback for whether it was put to the desired position and it takes a bit of time for the aircraft to pitch and settle. That's not to say you couldn't learn how much to use in a given situation on a particular type of aircraft.

One of my PPL instructors taught me that in a C152 if your fan stops, 7 back trims will roughly give you 65 kts from a 90 kt/ 2300rpm cruise. Generally it works too.

With regards overstress, I imagine the cotrol mechanism will have some bearing on that. If you have a geared trim wheel it will be easier to make finer trim adjustments than with a lever and harder to make abrupt adjustments.

[Shaggy Sheep Driver]

When I was learning aeros many years ago the instructor would sometimes use the transit out to and back from the aeros area for 'no stick' flying. The idea was that aileron and elevator had failed, and the aeroplane had to be controlled by rudder, trim, throttle, and flaps.

After some practice (this was in the Chipmunk) I was able to fly the aeroplane home and position it (rather shakily) on to short final. I'd hate to have to actually land it like that but if the wind was reasonable and the air not too rough you'd walk away from the attempt OK I reckon.

I have been known to just give the trim a bit of a tweak up or down in the cruise if I thought it was called for. The procedures we learn in training are good for getting us up to PPL standard, but they are not the whole story on flying an aeroplane! Not by a long chalk!

[Genghis the Engineer]

Standard PPL/CPL test teaching is something along the lines of attitude, power, trim. In other words, set the right attitude, let the speed adjust, then adjust power to the cruise RPM, then finally trim out the pitch forces.

However, being realistic, small trim inputs in a long cruise can be an elegant way of making corrections to a more or less trimmed aeroplane - where you've plenty of time to get it right. I'd do this however as small corrections in the cruise, not initial trim conditioning. So the PPL/CPL method is best to initially set conditions.

Most aeroplanes, incidentally, have to be certified as controllable for a return, circuit, and landing - with any single primary flight control failed. So, the pitch trimmer has to be capable of being used to fly the aeroplane as far as a landing if for example an elevator pushrod decides to fail.

G

[AdamFrisch]

There's this delicate balance in my airplane because the trim is only on one side of the elevator (which is not the best of engineering ideas). I also have rudder trim. So when I trim the attitude ever so slightly, the airplane starts a minuscule roll depending on which way I do it. To counter that, I add the tiniest amount of rudder trim (as the secondary effect of yaw is roll). This is an ongoing, constant thing and I rarely get more than 5 minutes of perfect balance. It's actually quite fun to chase that elusive equilibrium - plus it keeps my hands busy.

[rapidshot]

İ think a pilot should adjust power, angle of attack and speed to put all the forces into equilibrium condition correctly and trim the aircraft slightly as required , because wrong setting of the above parameters and use of excessive trim should cause " trim drag" , this may be the first reason,

Second one is i guess it is the trim stall, applying sudden power with excessive trim should put the aircraft into nose up condition , at this point this attitude may take you to the back side of the power curve, as you know at this region there is less available power due to low advance ratio of the propeller ( low efficiency) and high induced drag due to sudden increase of angle of attack

[foxmoth]

Remembering of course, if the elevator is JAMMED the trim will work in the reverse sense.

[rkgpilot]

An instructor at my club teaches an 'Advanced PPL' course where one of the very many useful lessons covers flying without stick - only rudder, throttle and elevator trim are 'allowed'. After several circuits and low approaches, I actually landed it without using stick. Untidy yes, ugly even, but it can be done. I wouldn't ever want to have to do it for real, but at least I know it can be done. As he said, the objective in such a situation is to get down on the ground the right way up, and to be able to walk away.

Knowing what an elevator trim can do in the extreme has I think helped me be more 'delicate' with trim in normal flight.

[Pilot DAR]

Indeed, the trim to land without pitch control will usually get you to the ground with a reasonable chance of not being injured, there's no assurance that the plane will make it through too! Certainly good to practice this, but with extreme caution. A safety pilot can be handy!

Yes, the trim is there primarily to allow the pilot to trim out pesky control forces, and not to actually fly the plane. There are circumstances where the trim is used to fly (some auto pilots do), and it has been known to be used for dive recovery. That said, what were you doing in such a dive in the first place?

A trimmed aircraft might have drag resulting from trim applied by the pilot, but assuming the pilot would otherwise have made the plane fly as intended, that drag from control input would be there one way or another.

Yes, applying power to a trimmed aircraft should be expected to put the nose up, or otherwise create that force for the pilot to overcome. Bear in mind that (for certified aircraft) this force will be well within the limits for temporary application, and the pilot will be able to maintain control. The power application is not the only thing with will result in trim changes, so a smooth pilot will anticipate these pitch changes, and fly the aircraft first. If a pilot has allowed an aircraft to pitch and slow to the point of being on the back side of the power curve, from normal flight, that pilot should take some refresher training without delay.

As the subject is trimming, I will repost a cautionary story from a bad trim experience I had, as perhaps it will be new to some readers here....

So you’re going to do a maintenance test flight….

Part One - Which way is up?

A maintenance test flight was required, prior to the Transport Canada approved test flight for the evaluation of an external installation to the aircraft. I had flown this aircraft months earlier, for a very similar mod evaluation purpose, including spins and dives to 1.1 of Vne. Those flights were fine. It had not flown since, while it underwent inspection for a commercial C of A. This was the maintenance test flight following that inspection.

The aircraft is a Cessna U206F, with a Robertson STOL kit, and additional external equipment mounted. Following a review of the documents for the maintenance, a thorough preflight inspection, and normal start, I taxied out. Just before taking off, a final check of control freedom and direction, including the elevator trim tab – I have just read too many stories about flight control problems on test flights… Everything looked as it should from the pilot’s seat.

The aircraft was light weight at takeoff, and promptly leapt off the runway… Then immediately leapt more. A swift and large nose down control input seemed to help, but still it was heading for space! I confirmed that the pitch trim was set for takeoff (and it had been) then I rolled it all the way nose down. That helped a bit, but not really much.

By this point, landing back on the remaining runway was no longer an option. A circuit now lay ahead of me, which was going to be a muscle builder! I could lie on my back and hold my 30 pound daughter at arms length over me for a few minutes at a time. This flying was a lot more demanding than that! Flying with one hand so as retract flaps (very carefully) and adjust power, was manageable for only a few moments at a time. In downwind, I found that full flaps created the least uncomfortable configuration, probably simply because slower speed, less control force. I did not dare fly too slowly, being quite uncertain about how the aircraft would handle if stalled this way. During downwind, I was thinking about how I might jam my knee into the control wheel so as to relieve my tired arms – there really was no practical way..

Flaring for landing was an exercise in how to appropriately reduce the massive downforce I was applying to the controls. It worked. I taxied in, alternating thoughts of shock, and the old joke: I just flew in from – here! – and are my arms tired!” I also reminded myself how lucky I was there were no seats, occupants, or flight test ballast in the back for this first flight….

Knowing that I had had a serious pitch control problem, I asked for a check of the elevator and trim tab travels. The elevator was as it should be. The trim tab offered 3 degrees, where I would have expected to find five degrees according to the type certificate data sheet. I asked that the maximum travel in this direction be provided. I got 7 degrees. There was still a lack of certainty as to what the problem was, because I could not see how such a small travel limit error could produce such a dramatic effect. I invited the mechanic to join me in my next test flight. He reluctantly agreed, knowing that if he would not ride in it, why would I fly it. This time I was well prepared to abandon the takeoff, if things were not right. The takeoff was better, but the pitch control problem was still there. what had been a 40-50 pound push, was now 15-20. The mechanic now had no doubt that something had been very wrong on the first flight, as was still wrong now. I landed back.

After a rather puzzling review of the maintenance accomplished since my flight a few months earlier , the answer was found. The maintenance personnel had put a little too much thought into what they were doing…. The required maintenance had included the required overhaul of the trim actuator, thus it, and all the chain and cable, had been removed, and reinstalled. During the re-rigging, the technician had read the travel requirements for the tab in the maintenance manual. Instead of setting the tab for an up travel limit of 25 degrees, he set the travel to a “nose up” (tab down) limit of 25 degrees. This left the travel limit in the other direction of only 5 degrees, which I hereby attest is not anywhere close to the requirement!

By trying to “figure out” what the system required, the actual instruction was not followed as written. An unsafe condition was the result. The safety system further broke down, when the second signatory for the work accomplished did not detect the error. This was also a maintenance failing designed in by Cessna, as it was possible to mis-rig the system in the first place, and the manual did not give any warning to check for the mis-rigging.

Part Two – Is there enough?

With the trim error corrected, and many sets of eyes and minds making sure everything was just as it should be, I had the aircraft loaded so as to be at maximum gross weight, at the aft C of G limit. Off I went again. The elevator trim worked, well, though I was not completely sure of the indicator position. I was, though, satisfied that the aircraft was now conforming to its design.

I climbed the aircraft very high as I had done before, to do stalls and spins as required by the design approval test flight plan. The power off stall was very normal. While setting up for the maximum continuous power, full flap stall, things started to go wrong again…

With the power set, and the flaps selected to full, and passing 20 degrees deflection, I reached the forward control wheel travel stop. The nose was rising quite quickly now, with no ability to stop it, as the flaps continued to extend. The trim was set to full nose down, but that was really not a factor anymore. I had no more control! The only resolution I could think of was to retract the flaps as quickly as possible. Reducing power did not seem a good idea at such a nose high attitude. The stall warning was now screaming, and who knows what kind of stall recovery I would have if I could not lower the nose!

The flaps retracted back through 10 degrees just as the plane began to mush rather badly, settling downward quite nose high. I got it all sorted out, and resumed normal flight. Being up high, I decided to explore this situation, to try to fully understand it. Obviously another discussion with the maintenance crew would be in order, I’d better have something to tell them which was helpful…

I set up again for the power on stall, this time feeding in flap a little at a time. Sure enough, at 20 degrees flap the control wheel was at the forward stop again. I found that with lots of muscle, and the elevator trim set at full nose up (which caused a little more effectiveness of the elevator, because of the downward tab), I could get 30 degrees of flap down, and control aircraft pitch with slight flap setting changes. The strength required to fly this way prevented doing it for very long.

So I took the plane back to the mechanics, and reported that it still did not fly right. After a review of the loading for the flight, an error was found in the basic weight and balance. I had been 150 pounds too light! And 1 inch aft of the aft limit. Could this combination result in these poor flying characteristics? I thought not, but we reloaded, and I went again. Nope, it flew the same way. Back I went.

After a complete re-inspection of control travels, and the system, it was found that a previously undetected broken bearing in the elevator bellcrank was affecting the elevator travel. The cable tension made this not immediately apparent during a walk around control check. I would have thought, that such a defect would have been found during the recently completed commercial annual inspection. Oops!

It was also found that the horizontal stabilizer (which is not adjustable relative to the airframe) was more than one degree beyond its specified angle of incidence – but in the direction which would improve pushover control! There is no adjustment for the H stab on the tailboom. The bellcrank was removed, and the bearing replaced, with the expectation that the elevator travel would now be correct. It is noteworthy that this model year of the C206 specified a bushing, not a bearing, so it had been changed at some previous time, but no technical record entry could be found to describe this work.

I test flew again…

Part Three – Please sir, may I have more?

With the bellcrank reinstalled, and the elevator re-rigged, the problem was now worse! The elevator deflection with the maximum possible travel, and farthest stop setting, was even less than it had previously been. The broken bearing had been improving the elevator’s range of travel! I landed back.

All of the other elevator control system parts were checked and found to be correct applicability, and in good condition. There is no Cessna design provision to adjust out this problem. The only remaining possibility was a modification to the elevator control stops. It was possible to completely remove the Cessna installed elevator control stop block, leaving only the bolt as the stop. This did allow just enough elevator travel that the aircraft could be safely flown through all of the phases of flight. I tested very thoroughly, and found the aircraft to be acceptable.

It is sobering to realize that this aircraft had flown for years following the repair that had the horizontal stabilizer incorrectly installed, and the wrong parts put into the elevator system. In this configuration, had a pilot used full flap and high power at the same time, with an aft C of G, they would have run out of elevator effectivness instantly. Close to the ground, at could have been fatal. It is total luck that this never happened. In such a case, it is unlikely that the accident investigators could ever have determined that a bent airframe had not been correctly repaired years earlier. The cause might have been reported as “pilot failed to maintain control”, rather than the very different “pilot could not maintain control, due to airframe defect”. I also reminded myself that my month’s earlier test flight, which should have caught this defect, did not. I have to be more thorough, even though what I might find is not what I was up looking for.

I contacted a Transportation Safety Board friend of mine, to informally report that this had happened to me. After explaining the whole thing, he said “that happened to you too?”. Apparently this trim misrigging had also happed to Transport Canada’s own C 206, with the same scary, muscle building, yet accident free outcome.

It the subsequent times, I told everyone I met, who were associated with C 206’s about this, just as a word to the wise. Some time later, a fellow called me, and asked if I would come and fly his 206, it was doing the same thing ( though with less force), which he had remembered my describing. I went to the airport, and asked that the control and trim travel limits be measured in my presence. It was measured, and the trim was wrong (though not backward). I had the mechanics put it right, flew the plane, and all was fine!

Courtesy of another PPRuNer, I was very wisely told that entering a steep turn is a good way to temporarily alleviate the control forces while you deal with the problem (if possible)

[IO540]

The basic principle which should be taught in the PPL, but isn't, is that the plane is stable in pitch (phugoid excepted) and will glide at a given airspeed which is set by the elevator trim mechanism.

IOW, the trim wheel does not control altitude, rate of climb, or whatever else. It controls your airspeed.

Another way of stating it is that the plane has a negative feedback control system for the airspeed, and the trim wheel sets the setpoint

[Tinstaafl]

That's what I taught! Used to annoy me when other instructors would say that trim maintains an attitude. So easy to demonstrate, too. Trim then slowly change power by a small amount and watch the attitude change while the IAS remains steady. Need smooth conditions though.

[24Carrot]

Angle of attack, not airspeed.

Incidentally that's why, in a spiral dive, an aircraft's speed rises with its bank angle. The angle of attack is the same until the wings come off.

[Pull what]

IOW, the trim wheel does not control altitude, rate of climb, or whatever else. It controls your airspeed.

You must have an interesting stall recovery technique

[Tinstaafl]

For a given Load Factor such as in unaccelerated flight, AoA & airspeed are directly related. We lack AoA indicators so IAS is the only indication available that shows trim effect.

[Shaggy Sheep Driver]

Not much use in aeros. I can fly the Chippy unstalled at zero knots indicated, or stall it at 130 knots indicated.

Why don't light aircraft have AoA indicators?

[BackPacker]

Why don't light aircraft have AoA indicators?

Because the propwash around the fuselage would throw them off. You'd have to mount them far on the wing for them to be accurate.

I believe Dynon has a product which measures AoA not through a vane, but through the pressure differential between two pitot heads, positioned at a 45 degree angle or so.

But as others said, in *unaccellerated* (meaning +1G) flight, in light aircraft the IAS gives a pretty good indication of AoA. That's why we get away with talking about, and marking Vs on the ASI.

I can fly unstalled at zero knots too, and stalled at 130 knots. But not while maintaining +1G. And despite me and you trying to convert pilots to do aeros, most will still be perfectly happy flying around at +1G all day long.

[Thud105]

" Not much use in aeros. I can fly the Chippy unstalled at zero knots indicated, or stall it at 130 knots indicated."

Really? Stall a Chippy at 130ktsIAS? That's impressive. What's Va in a Chipmunk?

[Pull what]

IOW, the trim wheel does not control altitude, rate of climb, or whatever else. It controls your airspeed.

I think this is more impressive-we may have to rewrite our complete flying training programme

[Shaggy Sheep Driver]

Oops, sorry. Meant Yak52, not Chippy (the other one I had a share in!). Whatever, I'd gladly chuck out a basket full of standard cockpit instruments in exchange for an AoA meter for aeros.