Adjust angle of horizontal stabilizer

In cruise I need a significant nose down trim for level flight, such that my elevator is not streamline with my horizontal stabilizer. It is offset several degrees and to push the tail up.

Any thoughts on how to resolve? What if I built a shim and I offset the horizontal stabilizer up 1/2 to 1° versus the elevator pushing it up? Or, a weighted crush plate on the nose?

O320 with a wood prop.

Thanks

I had a similar issue with one of the rv8s I built. I saw the same offset elevators as you. Additionally it felt behind the power curve on final requiring extra power.
I added a 1/8” spacer under the front spar of the horizontal stabilizer and it fixed the problem.
YMMV
Good luck

Like Steve said. Start with a 1/8” shim and fly it. Keep adjusting the shim thicknesses until you get the desired result. That’s a very common issue with the -4’s. Both of mine required shims under the horizontal stab front spar.

There have been many threads on the subject. This is common across most of the RV models and designed in per "rvbuilder2002" (Scott M) in Post #2 of the first thread linked below:




My previous RV-8 has a bit of down elevator as shown by the mass balance horns sticking up a bit in the photo below. My current RV-8 also does.

Thanks for the comments and link to the prior discussions

In cruise my -4 has the exact amount of elevator horn displacement as seen in the OP’s pix.
I bet in one of Carl’s referenced threads there is some info regarding suggested increase in spacer thickness underneath the stab based on elevator displacement.
I considered making an adjustment to mine but didn’t think that the effort would be worth the gains.

Keep in mind the elevator horn may not be a perfect indication of the elevator in or not in trail. Do some careful measurements to find out. My RV-10 was like this - horns very slightly up with elevators in trail.

In the 8A I added a shim and I think it helped. In the current RV-8 the plane is flying so well I did not want to screw with it.

I suggest the “do I need a shim or not” data might best be associated with trim authority. If when nose heavy do you have enough trim tab when in slow flight? How about when tail heavy? What is the trim tab position when in cruise with normal loading?

The 8A shim translated to more nose up trim tab on landing solo - but still in the mid range. I know of an RV-4 that added a shim and just runs out of trim tab on landing - so had to pull as well.

Of course if adding/removing/changing HS shims was not such a PITA experimentation would be a lot easier!

If you try shims, don’t forget to adjust the rudder bearings to keep them lined up as the VS will be moved slightly.

Carl

Carl Froehlich said:

If you try shims, don’t forget to adjust the rudder bearings to keep them lined up as the VS will be moved slightly.

Carl

Click to expand...

If the stab incidence is changed without changing the attachment of the vert stab fwd spar to the horizontal stab., the rudder hinge brackets will get misaligned and simple readjusting the hinge rod ends will not solve the issue.

One of the issues is trim - I have sufficient trim in the range of flight, but the amount and pressure of trim to maintain level flight at speed puts force on the trim tab, which then tends to relax (cable moves forward, lessens trim tab displacement and starts a climb - and stick force gets high. Reset trim (nose down). It seems like there’s a lot of force on that trim tab. If I hold slight pressure on the trim lever, it doesn’t relax. The main goal would be to reduce the aerodynamic force on the trim tab and elevator in flight

It appears you don’t have enough friction on your trim lever pivot point. Or you have a loose locking nut at one of the cable anchor points. My bet is the former.

It's nice that this is a solvable problem with a RV, my 182 was like this and it drove me nuts. Fortunately a tornado in Little Rock solved that problem and provided me with the money to buy my house.

Yes, I realize that is a friction issue, but what is also apparent is the amount of force being applied to the system to keep it in level flight. With neutral trim the stick force is extremely high so the force on the trim system must equal, and I think having a flight a more relaxed position would be good. I realize the force-counter force add some stability, but a cruise setting would be nice to have it a bit more towards neutral, without as much trim tab deflection. I need a mounted camera because I haven’t been able to get a picture to see the amount of trim tab deflection that results in that elevator deflection.

I suggest this test:

-Trim it up in cruise.
-Come back and land without adjusting the trim from the cruise position. A no flap landing will make the stick forces more than manageable.
-Or you can make an accurate note of the lever position in cruise so you can place it there when back on deck.

I think you will be surprised at how little trim tab displacement is required to hold the elevator in cruise position. At cruise airspeeds, the stick forces to overcome even a small displacement of the trim tab will be high. The greatest deflection of the trim tab on any flight is the nose up trim required at landing speeds.

Thanks. I will do that and also see if I can adjust the friction force in the trim system. I’m not so worried about gaining speed, well speed is always good. My goal right now is a stable cross country platform.

Increased the clamping force on the trim lever (friction) and with nose down trim have about a 1/2 inch trialing edge displacement on the trim tab for level flight. Will fly it like that for now, but still interested in adjusting the stab to more of a neutral position, not as much trim tab input for level flight. Thanks Andoman for the suggestions and everyone else's comments.

Rudder brackets

I had assumed that adjusting the HS incidence by shimming would be fairly straightforward but per RVbuilder2002 comments the VS attachment to the HS will also need modification to compensate, otherwise the rudder support brackets will definitely be pushed out of alignment.

The HS on my RV-6A (still under construction) has an additional shim of 1/16" (total 3/16" shim) in order to meet the required incidence numbers per the construction manual. I tried temporarily removing the 1/16 shim to see what would happen and the rudder brackets were noticeably out of alignment. The emp fairing fit is also affected.

Re. the OP's discussion; there has been not much mention of W&B and it would be interesting to know the weight distribution and empty CG position in comparison to others.

Like everything these days, it seems that there is no single best answer and a bit of perspective can be helpful.

In my case, this change is not as simple as is often implied here because I have a very early -4, SN 107. The HS attach is a bit different than the current plans. Basically, the aluminum angle is forward of the F-10 bulkhead (which is the F-410 and F-410A bulkheads in the current plans) and that bulkhead is sandwiched between the HS spar and angle. (there is no F-410A bulkhead like in the current plans)



So, the first issue is getting at the bolts (that pass thru the aluminum angle/longeron) which can only be accessed from inside the tail. That will require removing the elevator push rod. I'm fairly confident (optimistic) that I can get in there. But... I seriously doubt I'll ever get out. My rotting corpse back there will seriously throw off the CG and decrease the value so, I probably need to rent a small/handy child.

Secondly, the aluminum angle and HS spar sandwhich the F-10 bulkhead. So, raising the HS is going to cause the bolt holes in the bulkhead to be elongated. Or, one must drill new holes (in different places) thru the entire sandwhich (angle, HS spar, and bulkhead).

I've seen an installation that moved the aluminum angle aft of F-10 which slightly improves access but one still has the sandwhich bolts to deal with. And, one is adding a couple of holes thru the longeron. Are all the additional holes ok? I don't know - thus I'm still flying with an out of trim elevator. :-(



I know... an email to the mothership is in order...