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

mtnflyr

Member
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.
 
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From your post, it sounds like you are a new (non-builder) owner. If so, hope the following helps:
1. Get a set of plans/drawings from Van's that will describe/show how the trim system works and is adjusted. All RV owners, builder or not, should have these plans and drawings. You will need them for basic maintenance and troubleshooting (and just to understand your plane) in the future. Can be found here:
https://shop.vansaircraft.com/cgi-bin/shop.cgi?browse=videos&product=fd_plans

2. Assuming manual trim on a 6, you adjust the trim back where the trim cable comes out of the bottom of the elevator. There is a double set of nuts on the trim cable that can adjust travel both up and down where it connects to a bracket on the inspection cover plate. Generally speaking, a neutral trim tab should occur when the trim knob in the cockpit has roughly half of its turnability in each direction, but the actually amount of needed travel up and down in degrees is probably listed in the 6's manual and/or drawings. From your description it sounds like you need to make this adjustment.

Good luck.
 
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The first thing I would do is a weight and balance. You might want to check the angle of incidence of the horizontal stabilizer also.
 
The first thing I would do is a weight and balance. You might want to check the angle of incidence of the horizontal stabilizer also.

Plus one for these two items to check. RVs require a fair amount of pitch trim when flaps are used but if they are rigged correctly or there isn't a big issue with CG, all well within trim range. My RV7A needed trim but even with nearly full weight in the baggage, we never ran out of trim.
 
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.
 
The first thing I would do is a weight and balance. You might want to check the angle of incidence of the horizontal stabilizer also.

I second this. Even if the trim was misrigged, it should be much more sensitive. But I?d start with the basics, weight and balance. An aft CG will require excessive elevator, thus excessive trim. W&B is a place many home builders come up short.

If loaded CG is within limits, the next problem area could be horizontal angle of incidence. Need the plans set for that.
Good luck.
 
yep, all good answers above.

Talking incidence, also check the wings incidence, then the HS incidence. The relation between those 2 is important.
 
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.

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).
 
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hmmm...

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.

So you are saying that you do not trim for different speeds? Highly unlikely...
 
So you are saying that you do not trim for different speeds? Highly unlikely...

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.
 
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
 
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
 
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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.
 
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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.
 
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.
 
Trim settings

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
 
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
 
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.
 
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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.
 
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My RV-4 builders' manual explicitly states the rigging requirements for HS incidence and trim tab maximum up/down deflections. Worked out perfectly for my Harmon Rocket with solo CofG Vref. The deflections are highly asymmetrical.

When all else fails, read the manual!
 
There is for many RV's, but it depends on a number of factors.

SNIP

This was in response to my comment about no such thing as hands-off approach. To clarify, I meant unless in perfectly still air, and RV always is needing control input to stay on speed and gs, but control forces on final will be light, even if the pitch can't be entirely trimmed.

I also now see that the OP is/was a military pilot (thanks for your service!!), so a transition to an RV is not a real factor.

Best advice is to have someone go fly with you that has RV experience to determine if it is normal or not.
 
Mine is more or less similar

I usually end up landing with full nose-up trim; roughly neutral for takeoff and climb.

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

I don?t know if it us not usual but it us not desirable and it us certainly possible to fix it so that you don?t run out if your cg is within limits. It wouldn?t be allowed on a certified airplane.
 
I don’t know if it us not usual but it us not desirable and it us certainly possible to fix it so that you don’t run out if your cg is within limits. It wouldn’t be allowed on a certified airplane.

RV-6 is not certified and could never be certified under 14 VFR Part 23. The RV has other issues with pitch stability. However that is why they are so light on controls and fun to fly... :D

Amateur built / kit / experimental planes can actually be scary and not close to certified planes. In the 80's there were several kits with aberrant and dubious flying qualities. For example the 1st Glasair was unstable and many foreign countries disallowed them. Later models were better (longer tail moment arm, larger empennage).

As far as my RV-4 with metal Hartzell CS Prop I was at forward CG.... On approach full nose up was needed. There was nothing to spare. With a passenger trim on approach was no issue. With passenger and baggage it was at the edge of slow speed control reversal where stick force was too light and forward stick needed. Still plenty of elevator authority. I rarely flew it aft CG. The great thing about the RV is it talks to you through the controls and seat of pants if you pay attention. It has full control authority at slow speed regardless of pitch trim... but you are right it could not be certified.

Running out of nose up trim is not a safety issue.
 
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proper tab lever for electric vs manual

Another item to check - did the proper tab lever get installed on the trim tab? I hope the OP got a plans set with the aircraft. Plans drawing 5PP at the top of the page, if the manual lever was installed on an electric trimmed elevator, the trim tab will not achieve the proper range of motion. If manual with electric lever, the reverse would be the result with over control.

It is very interesting to read all the trimming techniques (or not) pilots use for approaches, some sound very unstable and rely on heavy handedness.

I have seen this mostly in 'new to RV' pilots, not used to trimming or flying with light finger stick control. I find the more trimmed the plane, the smoother, less jerky the ride is for the passenger.
 
RV-6 is not certified and could never be certified under 14 VFR Part 23. The RV has other issues with pitch stability. However that is why they are so light on controls and fun to fly... :D

Amateur built / kit / experimental planes can actually be scary and not close to certified planes. In the 80's there were several kits with aberrant and dubious flying qualities. For example the 1st Glasair was unstable and many foreign countries disallowed them. Later models were better (longer tail moment arm, larger empennage).

As far as my RV-4 with metal Hartzell CS Prop I was at forward CG.... On approach full nose up was needed. There was nothing to spare. With a passenger trim on approach was no issue. With passenger and baggage it was at the edge of slow speed control reversal where stick force was too light and forward stick needed. Still plenty of elevator authority. I rarely flew it aft CG. The great thing about the RV is it talks to you through the controls and seat of pants if you pay attention. It has full control authority at slow speed regardless of pitch trim... but you are right it could not be certified.

Running out of nose up trim is not a safety issue.

I am well aware that my RV is not certified, nor required to be. I certify aircraft for a living and I fly an amateur built aircraft. What I said is that it is not desirable. The certification requirements are a good guideline and I imagine that Van used a lot of them when he designed the RV aircraft i.e. things like the Vn diagram, load limits, stick forces etc. If there is a rule, it is likely there for a reason. Yes, you can fly out of trim and lots of people do, and 99 times out of a 100 it is not a safety issue. But it could be under the right circumstances. And, since it is an easy thing to fix, why wouldn't you? Don't you want everything in your favor? People who don't usually stall airplanes stall them when they fly into OSH because they are mentally overloaded. That's the kind of scenario where having to hold force could get somebody into trouble - when they are unable to do everything being asked of them due to lack of currency, fatigue, what have you. It happens every couple of years there.

Your statement that it is not a safety issue is a matter of opinion. The opinion of people who's job it is to ensure that production aircraft are safe to fly, and who base that on decades of experience and research into the cause of accidents feel otherwise. I know of accidents where insufficient trim was a factor. Not the cause, but one of the factors that lead to distracting the pilot. And like I said, it is an easy thing to fix. Just offset the tab slightly. It's your airplane, you are free to do as you wish. But telling other people that it is perfectly safe in my view (another opinion) is questionable.
 
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Another item to check - did the proper tab lever get installed on the trim tab? I hope the OP got a plans set with the aircraft. Plans drawing 5PP at the top of the page, if the manual lever was installed on an electric trimmed elevator, the trim tab will not achieve the proper range of motion. If manual with electric lever, the reverse would be the result with over control.

It is very interesting to read all the trimming techniques (or not) pilots use for approaches, some sound very unstable and rely on heavy handedness.

I have seen this mostly in 'new to RV' pilots, not used to trimming or flying with light finger stick control. I find the more trimmed the plane, the smoother, less jerky the ride is for the passenger.

in the 50s pilot manuals had great illustrations. My Dad was a cold warrior and in one of his manuals there is a great cartoon of this jet jockey. His right arm was huge and muscular. His left arm was a scrawny atrophied limb. The caption was him saying "Trim? Never use it... ". I wish I knew where to find it. I don't think a lot of pilots understand the idea behind trim and positive stick force gradients. They don't have to be large, but you really want them to be positive.
 
+1 Scott

Had the luck to fly a number of different aircraft during my life and career. Only one comes to mind in which full trim was ever required... the VariEze. Not only is the pitch overly sensitive on those, but flying in the rain affected the canard wing enough to require full nose-up trim in the cruise and still require some wrist pull to maintain level flight.
Vans would surely agree that the use of full trim in an RV is neither normal nor desirable...
 
Inadvertent Fix

After checking everything I could, based on all the inputs, I gave up and decided to live with the out of trim condition in the landing configuration and work on other projects. One of those was relocating my oil cooler from the position the original builder placed it - under the left intake ramp - to a more traditional location behind the aft baffle. We've had high oil temperatures when flying at low airspeeds and high OATs. The relocation helped some with the oil temp, but pretty much resolved the trim issue. Turns out moving an oil cooler full of oil from the most forward position you can imagine to the most aft position possible has a noticeable effect on CG and trim . . .
 
Hi Rick, thanks for the update. As part of my "phase 1" I was required to fly my RV-8 on all corners of the W&B and CG envelope, and it was really interesting and informational. I learned a lot of about how the aircraft handles. Although in a flying RV it's of course not required to do this, but I've read many experienced guys recommend it to get comfortable with the handling under the various conditions. The EAA test cards are being updated to help owners get comfortable with their aircraft with differing loads. https://www.eaa.org/shop
 
After checking everything I could, based on all the inputs, I gave up and decided to live with the out of trim condition in the landing configuration and work on other projects. One of those was relocating my oil cooler from the position the original builder placed it - under the left intake ramp - to a more traditional location behind the aft baffle. We've had high oil temperatures when flying at low airspeeds and high OATs. The relocation helped some with the oil temp, but pretty much resolved the trim issue. Turns out moving an oil cooler full of oil from the most forward position you can imagine to the most aft position possible has a noticeable effect on CG and trim . . .

Producing that information in your first post would have saved you 18 months of grief......

Also, you can look at the starter size. Old starters are very heavy. Some Alternators weigh a lot more than others.

They all have one thing in common, they ALL mount on the nose of the aircraft, causing an increased FWD CG.
 
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