Horizontal stabiliser without a trim tab?

I believe the Cri-Cri and Luciole (Michel Colomban) use springs in the same way as the Tomahawk. I never had any issues with the tommie.

My Druine Turbulent has control forces light enough not to need trimming during flight, though there is an option of installing a trimmer that I may well take up – I will miss the extra lever otherwise. There’s a fixed trim tab nontheless.

Why do you need a trim tab when you can move the whole empennage (Mooney)?

There are quite a few different trim arrangements, this thread gives an overview.

the elevator + trim tab mechanism is effectively a negative feedback control system for airspeed

Is it? I would expect that at least considering only first order effects, the airspeed should cancel out of the equation

To be precise: I believe it’s the main wing + CG position + elevator + trim tab which are effectively a negative feedback control system for angle of attack.

But in normal flight, the wing and CG don’t move much, and airspeed and angle of attack track each other nicely when pitch is stable, so I can see why Peter wrote what he did.

The Phugoid oscillation is good example of a stable angle of attack with varying airspeed.

No trim tab on a Mooney….that would add too much drag!

that would add too much drag!

Even the Super Cub has that arrangement, and a Cub is not exactly low drag

A trim tab adds drag, a trimable/movable tail does not, but it’s also more complicated structurally, so it will add weight and cost. A spring system also work it it is dimensioned properly, and the trim lever is a turning wheel, not just a lever. There really is no difference between that and a movable tail, except that the simple spring system will also add drag.

…but it’s also more complicated structurally

And generally non-survivable in case of failure. See for example Alaska Airlines Flight 261

Gliders generally have spring trims, rather than trim tabs.

Although trim tabs/tailplanes are very useful for trimming the airplane, they are more vital in some types in allowing the pilot to trim out of a dive if they have to. The control forces in some aircraft (amphibian C 185 for example) can be such that in some conditions of flight, trimming out is easier than applying huge pull forces.

As the topic is trimming, I’ll repost a previously posted article I wrote on a very startling trimming/pitch control lesson I learned:

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!

The Piper TriPacer has a jackscrew that moves the horizontal tail surface rather than an elevator trimtab. As noted many gliders use a spring, but one glider I’ve flown (Schreider HP-11) had a bit of bungee cord and hooks on the stick in which to put the bungee! Probably the cheapest form of trimmer I’ve seen.

The HP-11 was an interesting machine. The one I flew had flaps that went from -5 degrees to +90 degrees, and were about 3/4 span (on an 18m span glider). The glider was all metal, but it looked like fibreglass, all flush riveted and the rivet heads bondoed over, and long high aspect ratio wings, with not very much chord even at the root (most metal gliders have big fat wings and terrible performance, usually made for training – but the HP-11 was made for competition). In the air it was nice to fly but the ground handling was a complete pig, and because they are cheap many new glider pilots get them as their first glider and provide hours of entertainment to other club members.

But it is designed as a contest glider, and things like out landings are common when flying long cross countries. It has no airbrakes, but these huge -5 to +90 flaps do the job. Past 45 degrees, it’s all drag, and they are operated by a crank on the left side of the cockpit. My friend who checked me out in this glider (it’s a single seater so the checkout is just a briefing) told me to take a tow up to 4000 feet so I could have ample time to play with the flaps, and do some practise approaches before my first real one. He then told me that when you land, come over the threshold at 300 feet and get 90 degrees of flaps in – and you will touch down on the touchdown marker – which was only about 300 feet from the threshold! He basically said to time your roundout, wait until you are certain you are going to crash, count to three, then flare… and guess what, it worked. It wasn’t so bad when you got used to standing on the rudder pedals as you came down at an insanely steep angle with 90 degrees of flaps out. But what it meant is you could absolutely nail a spot landing without a lot of trouble – very useful if you have to land out in a small field with little room for error.