In-flight straight & level trim of old aircraft
 

Here's a basic stick-and-rudder question.

In short: I'm trying to get a 40-year old Cherokee to flight straight. :) It's a PA28-140 with the barn door wings.

Here's my procedure:

At level flight I keep the wings level with ailerons, then adjust rudder trim until it flies straight hands-free. However, the ball is half a ball width to the left, and the airplane feels like it's going slightly sideways, and the wings go in a slight bank. If I then fix the yaw (I'm aware the ball reading might not be accurate), then I can only get it to fly straight with a slight but notable bank.

Am I doing this right? Any suggestions of how to better keep it straight?

Also, the target should be to get it straight & level hands free and wings level, regardless of ball reading, right?

sounds like something is bent.

You should be able to fly straight and level and in balance and trim to keep it there in a PA28.

If you trim so you are straight and level and in balance, it sounds like you are then having to use the ailerons to hold the wings level, or perhaps not level, to keep it straight.

If you trim so you are wings level, and straight, then you are not in balance.

Something is wrong if this is happening in a PA28, get it looked at. There may be a tab on the ailerons which can be adjusted on the ground (technically should be done by an engineer) to stop it from rolling when you have wings level and in balance.

The target is to get it s&l with the ball in the middle. Anything else costs a couple of knots cruising speed.

As RTN11 writes, your aircraft needs adjusting... the PA28 gets adjusted in roll by rigging the flap position, which for a qualified mechanic is not hard to do. This assumes that nothing else is "bent".

Once fixed, the proper technique is to keep the wings level with ailerons, then adjust the rudder trim until the ball is in the middle without you having to press a rudder pedal. If after that you have a more than minimal roll tendency, get it adjusted [or just fly the lower tank half empty, or hand a weight of one side of the yoke ;-)]

On a dead calm & clear day (or at least no turb & pretty good vis):

* find a good aiming point on the horizon while flying into or out of wind (ie no drift) and a spot or dirt on the windscreen directly in front of you.
* At cruise power & speed in level flight, trim in pitch 1st then, while holding the wings level, use your feet to keep your chosen windscreen spot aligned with the horizon aim point. It may take a few minutes to work out what pressure you need if it is very slight. Don't worry about the ball - it could need adjustment too!
* trim the rudder to relieve your foot pressure while still holding the wings level.
* Identify if you need any aileron input to hold the wings level.

Note: Try to have the aircraft's lateral weight distributed evenly - so no near full one side vs near empty other side fuel tanks

On the ground get your maint. blokes to adjust the rigging and try again. If either the rudder trim or roll rigging is displaced a lot to achieve S&L trimmed flight then you might have a bent airframe.

It needs rerigging by someone who knows what they are doing. I had a similar problem with my Archer, rerigging at RGV in Gloucester at the annual had it fly straight and level in balance and in trim no problem.

HI Cobalt, you say...

As RTN11 writes, your aircraft needs adjusting... the PA28 gets adjusted in roll by rigging the flap position,

Which I think means that each FLAP has its back-stop adjusted to give the desired S+L flight.

Would this be the same for a C172 as we seem to have the same 'problems'.... I have heard someone say that the tips of the AILERONS can be tweaked, but this does not look to be possible (as there is no aileron trim tab.)

Don't cessnas have a tab on the rudder you can adjust :confused:

I am told on Cessnas the incidence of each wing can be adjusted by a cam at the trailing edge of the wing for roll. It is accessable from inside the cabin.

I suspect fine adjustment may be done with the elevators.

For yaw trim it will have a rudder trimmer or an adjustable tab on the rudder.

D.O.

Hi, yes they do. The aircraft trim is Ok at full power in a cruise climb; However in a S+L cruise it requires 2 notches of left rudder trim to get the ball in the centre... This then requires some pressure on the yoke to avoid rolling to the left.
I don't think this is related to Pax weight, as it occurs with any loading. just the same as the OPs PA 28 'problem'.

I have never seen a cam inside a Cessna to adjust wing incidence. The C120/140 has dual struts and roll is adjusted by screwing out one aft rod end on one side. I don't think any of the PA28's I've flown have been trimmed well in roll. Makes it hard for instrument work, so I have used rudder trim and lived with the poor coordination.
Bryan

Adjustment
 

Most of the stuff above about Cessna's it true there is a cam in the wing root mountings to adjust the incidence.

The only roll adjustment that you can use on a piper is the flaps very little movement of the flaps can produce a lot of trim, IMO this Piper needs all the control setting back to the factory settings and then adjustments made from the factory settings to get it to fly S+L? It should be remembered that you need to set the Aleron neutral settings with a rigging board and tool to set the position of the belcrank.

I have learned something, thanks. I figured the spar carrythrough design was the same on all 100 series cessnas.
Bryan

Have you considered the changing dynamic pressure on the fin as a result of changing propeller wash?

There will also be changing propeller torque effect.

Is it correct that the PA-28-161 (and Maybe other Cherokees) won't return to S&L hands off, whereas the 172R (and maybe other 172's) do? In other words, trim out for best S&L and let go of the controls. Warrior slowly but surely dives off one side or the other with increasing bank. 172 returns to S&L as defined by the trim, even if preturbed from level flight.

At least, this is what happens in the two aircraft that I'm familiar with. Engineers shrug and say, "Yes, that's what it does" about the Warrior. I'm not convinced. Maybe it's a safety feature to keep the pilot awake? In the 172 you could doze until the fuel runs out, but not the Warrior.

Incidentally, I can't see how trimming the flaps is any different from compensating with the ailerons? It will only fly straight on the rudder trim with the ball out by about half a ball. You can get better airspeed by compensating with aileron (requires constant pressure), but that is surely an aileron trim issue, nothing to do with flaps?

Any information gratefully received, since it's an irritating and long term issue.

Victorian
 

The ailerons are differential so any adjustment will effect the effect in the turn and make the aircraft turn more quickly in one direction than the other, this is achived by the ineffective crank effect of the belcrank design so Aleron adjustment is not a good idea.

Adjusting the flap slightly has a very measurable effect on the aircraft in roll without effecting the ability to turn, of course it will have secondary effects in yaw and pitch but it is the best way to get the roll issue sorted.......as I said the first thing to do is go back to the book settings and then find out what needs adjusting to get the aircraft to fly S+L.

On the other hand, how severe is the roll issue really? It might be that the aircraft is perfectly rigged for S&L flight with 2 or 4 POB. But if you're solo it might have a tendency to roll slightly to the left (assuming you fly LHS). As others said, you might want to look at the weight distribution first, and maybe run the left tank at a lower level than the right tank, to see if that helps.

In any case, rudder trim is not going to fix a roll issue. Rudder trim is there to fix a yaw issue (*). If you want to fix the roll issue you've got to do something with the wing rigging, with a fixed trim tab on an aileron, or maybe by setting up the flaps differentially, as suggested above.

(*) Let me rephrase that: An effective rudder trim will help you fix a yaw issue. But I don't consider the PA28 rudder trim particularly effective. AFAIK nobody uses it.

I was building a nice sized right thigh muscle until I started liberally using rudder trim in the Warrior. It's at least two full turns to the right for a straight 79 knot climb (Fla, summer). Almost a full two turns back at cruise to hold a heading (and the wings never do stay or return to level despite draining the heavy side tank first, distribution etc). Then some left trim for descents.

before you start stuffing around with the geometry of the aeroplane answer a few questions.

how old are the engine mount rubbers?
if they sag the engine thrust line will not be as designed.

why the hell do you convince yourself that the balance ball is out of whack?
the instrument is as simple as it could be. a ball in a fluid filled glass tube bolted solidly to the instrument panel. how can it be out of whack???

try flying the thing with the ball centred. I'll bet it flies 5 knots faster in cruise. and starts behaving itself.

cessnas I can trim to fly at an exact speed. pipers with the all flying tail are an impossibility for me. why you expect to trim a piper is a mystery to me.

Point of clarification: The early Cessna 120-140-170 series were "ragwing" (fabric covered) and so the wing itself had very little torsional stiffness. The lift strut was a "V" strut, and (in common with many other ragwing designs) the outer wing incidence was trimmed by an adjustable fitting on (usually) the aft member of the V strut. (It used to be fairly common to rig both wings with 1 or 2 degrees of washout, to force the wing root to stall first.)

But the 140A and 170A introduced the metal-skinned wing, with enough torsional stiffness that only a single lift strut was needed. Incidence angle was thus established at the wing root. (The C-120 was discontinued about this time)

The balance ball will read an error if the instrument isn't aligned in the panel. That's why there's the ability to rotate the instrument by a few degrees left & right.