How to fix RV-6A that needs large changes in elevator trim

[rocketman1988]

True statements...but...how often does THAT happen...

[newt]

Originally Posted by scsmith

Unless his c.g. is very far aft. At the true neutral point, there would be no change in elevator position with speed. And if the horizontal tail incidence was chosen just right, there would be no change in hinge moment either.

For the OP, assuming your trim system is mechanically adjusted so that you are actually getting full tab deflection, then your observations strongly suggest you are very nose heavy.

Or perhaps misrigged.

A reweigh and a rigging check will work it out one way or the other, and suggest whether the airplane needs tail ballast or a HS angle of incidence adjustment.

- mark

[lr172]

Originally Posted by Kyle Boatright

That isn't my experience. Solo, my airplane requires full nose up trim to hold 75 knots with the flaps all the way down and power at idle. Effectively, this is my landing configuration. With a farther aft CG, the need for full up trim goes away. At the same (solo) CG and weight, trim is roughly neutral during full power takeoff and climb (assuming no flaps).

My trim varies a good bit as well in different low speed regimes, but I don't get real close to full up. That said, I set my trim to provid much more nose up travel than nose down.

The OP should definately observe the trim tab movement through the full servo travel and compare that with both the manual and recommendations on this site. I would also follow the manuals reference for setting incidence and use this to see how close or far off the HS incidence is, relative to the wing incidence. The manual explains, in detail, how this is done. Some folks like the elevator in a certain position in cruise and will play with the HS incidence to achieve that. Your plane could be one of those.

Also check that the buidler didn't put a speed reducer in the trim circuit. The 6 is hard to trim at cruise with full trim power (it is very pitch sensitive at high speed). I have a separate circuit with voltage reducer that I use for trim in cruise to make this more manageable. This type of voltage reducer will make you think you're adding a lot of trim when in fact it is just a little bit of trim that is taking a long time to put in. If you can trim easily without going back and forth a few times a cruise speed, it's a good bet there is a voltage reducer in the trim circuit. The 6 needs very little trim authority at cruise speeds and a decent amount of authority at speeds below 90 MPH. The speed reducers work nice in cruise but create problems at low speed. Steve may be correct, in theory, but my 6 needs a decent amount of up trim as I slow below 100 MPH and start adding flaps.


Larry

[Southern Pete]

Originally Posted by merlin3

I only trim based on different loads. I can takeoff, trim and never touch the trim again for rest of the flight. Something has to be majorly wrong.

My experience is similar in a 6A O-320 metal fixed pitch, take-off, establish climb speed (say 110kt), trim. Level off, accelerate, trim (usually doesn't need much for say 140kt cruise). Next time trim is moved will be when half flap is dropped down wind / base, again not very much. Trim again after selecting full flap on final. Only small movements required each time. CG is typically around aeros limit or slightly forward. On a long trip (2.5 hrs cruise) may have to adjust trim slightly. Never seen the autopilot asking for a trim movement.
Pete

PS With this config Lycoming O-360 with a Hartzell constant speed prop. Battery on the aft side of the firewall, the CG will be quite far forward so will need more trimming. Would second suggestion to weigh aircraft to establish current weight & cg position. Potentially do what you can to move CG aft.

[AlexPeterson]

Originally Posted by mtnflyr

I bought a 6A some time ago and need some help understanding what changes I can make to it to reduce the need for large changes in elevator trim.

On final, I can't trim out the stick force with full nose-up trim. It's manageable, but I like to trim to a stable hands-off state and I can't do that today.

For takeoff, flaps up, the airplane rotates nicely with a fair amount of nose-up trim, and I have to trim aggressively down as the airplane accelerates. This isn't a huge deal for a normal takeoff but makes the airplane a handful on a go-around.

The trim movements I need to go from 80 mph to 120 mph are significant - much more than I'd like to have to do.

What should I check? CG comes to mind - I'm guessing that the airplane has a CG that is further forward than optimum. Would forward CG cause the need to trim more than a properly balanced aircraft? I don't want to start messing with stuff without understanding the aerodynamics causing the behavior I'm seeing.

The set up: Lycoming O-360 with a Hartzell constant speed prop. Battery on the aft side of the firewall.

Without more information, it is impossible to say if this is normal or not. I have the same basic configuration as you do. Yours might be behaving completely normally.

There is no such RV as a "hands off" on final, not sure what you mean there. If I have no baggage, full fuel and single pilot, I cannot fully trim for idle/full flaps/70 knots. But, the stick force residual is trivial.

As a reference point, I always take off with half flaps, with the trim tab exactly neutral. Lots of threads here about this topic.

Re trimming nose down aggressively - yes, that is quite normal when re-configuring from idle/full flaps to flaps up and accelerating. It is extremely easy to blow through the flap speed if you don't get the nose up once power is added, which exacerbates the trim problem. You do not need to apply (in most go-around situations) full power all at once.

Touch and goes can be a handful to a new-to-RV pilot. My typical: final approach full flaps, a tad of power (normal glideslope). Throttle to idle over the threshhold. Mains touch, immediately start forward trim, while keeping the nose wheel off. Flaps up to half, touch of power to keep the nosewheel off. Slowly add power (while continuously putting nose down trim in and keeping the nose off). Once trim back to a reasonable place, slowly apply full power. I'm often airborne before full power applied. Obviously, this technique requires an appropriate runway, and requires a lot of practice if you have transitioned from a lower performance craft.

For go-arounds, I will not immediately add full power, but will smoothly add power, perhaps taking 5 to 8 seconds to apply. This gives me time to get the trim rolling forward and the flaps coming up.

If you have the mechanical trim and/or flaps, what I described is a bit more challenging. I have the flap switch accessible when my hand is on the throttle, and the trim is on the stick.

[rvbuilder2002]

Originally Posted by AlexPeterson

There is no such RV as a "hands off" on final, not sure what you mean there.

There is for many RV's, but it depends on a number of factors.

One of the biggest is how the trim tab travel is rigged.

It is fairly common for people to rig the tab travel for an equal amount up and down through the entire range. A lot of people apparently never notice that very little of that down travel is ever used. That means some of that part of the travel range could be put to work as nose up trim.

My suggestion is that the travel be rigged so that the tab travel is about 65% down (nose up trim) and 35% up (nose down trim). The limiting factor of what level of asymmetry you can achieve in the degrees of deflection is usually the down travel limit of the tab.

This will give a lot more nose up trim available during approach and in most situations allow for trimming hands off at Vref with full flaps.

I do not recommend doing this though. Especially in situations where you are landing near the aft C.G. limit. The elevator forces are pretty low in this condition. Leaving the aircraft trimmed a little bit nose heavy artificially adds some elevator feed back force that most people will agree helps make pitch control more precise maneuvering into the round out and flair.

I trim for hands off on base or just after turning final, when I am usually at just a slightly higher speed than I will be on short final and as I enter the round out. This will leave the airplane feeling just slightly nose heavy as I enter the round out because of the speed reduction.

This process also works well if being forced to fly a very big pattern or a very long straight in. Just trim for and fly a speed 5 kts higher than you normally would, then manually reduce the speed with pitch input on short final.

BTW, if you do rig your trim this way, keep in mind that the take-off setting will be in quite a bit different position than you are used to and in high speed cruise flight it will be near the nose down end of the trim range.

[664781]

Landing: Full nose up trim almost every time with full flaps, more than 1/2 fuel.
T/O: 1 turn nose down will get you real close on take off. Then fine tune for different climb out speeds.

Mechanical trim........Catto prop, o-360

[sblack]

The absolute trim tab angle is a function of CG, flap deflection and stab vs wing incidence. The relative tab deflection as a function of change of speed (which is really change of AOA) is a function pretty much of CG only i.e. the level of static longitudinal stability. Incidence is irrelevant. So if you are running a big engine and a CS prop you are going to need more tab as speed changes. If you are running a wood prop and have lots of baggage you will need considerably less and the rest will be in the middle somewhere. This is normal - it is basic static stability.


There is no reason why you should not be able to trim hands off on approach and in fact you should have that capability for safety reasons. On a commercial aircraft this is a certification requirement. We don`t have to comply, but certification requirements are usually there for a very good reason. Stick force is a direct indication to a pilot of how far he is from trim speed i.e. ideal approach speed. Having to hold force is a distraction and other distractions can cause a pilot to relax that force and change the speed and deviate from the desired flight path. If you trim up at Vref on final, on the desired glide path and don`t change power, you never really have to worry about checking the airspeed every 2 seconds like many pilots do. It won`t change. You can`t stall if you don`t pull. Just maintain a constant sight picture. That is harder to do if you always have to pull and worse if you have to push.

I agree fully with rvbuilder2002 that the tab rigging should be based on the trim requirement, not on some arbitrary zero, because every airplane will have variations. If the tab goes far beyond 20 deg deflection relative to the elevator you could get some non-linearity so try to limit the rigging if you can. This is a basic rule of thumb for tabs. But other than that, offset it as much as you need to trim on final at the fwd CG limit.

Scott

[rvbuilder2002]

I probably should have been more clear.....
I don't advocate flying an entire approach with the airplane out of trim when it could be.

Some people use trim even as they are slowing the airplane while entering the round out phase of the flair.
It is this portion of a landing that I suggest leaving the pitch trim where it is, and benefit from the small amount of additional pitch force that will provide.

[gmcjetpilot]

All the advice you got (weigh AC and check W&B, check the Horz Stab incidence, Trim Tab operation range) is spot on. However as everyone said not enough info. Agree. It would really take seeing and flying the plane in person.

Maxing out or running out of nose up trim on approach is not unusual as long as the control force is light.

When going from 55 -200 mph you expect to trim will change significantly.

Without seeing the plane, flying it, doing the above rigging checks it is not possible to answer. It is possible you have mis-alignment issues with things like wheel pants, gear leg fairings, etc... As you change speed you get different pitching moments.