RV-4 HS incidence

I have a question for those familiar with changing the horizontal stabilizer angle of incidence on RV's - and RV4's particularly. I believe the angle of incidence isn't set correctly on my RV4. I'm the third owner of this airplane, and it's been flying for 16 years, so maybe I should just leave it alone, but if it can be changed to improve its aerodynamics a little, I might pick up a little speed. Here's what I have: In straight and level trimmed flight at cruise speed (~155K) my elevator is slightly deflected trailing edge down about 2 degrees (nose down pitch trim)to maintain level flight. My CG in this condition is pretty close to the middle of the CG range. The leading edge of my elevator counterweight horn is about 1/2" above the leading edge of the HS. If I streamline the elevator to make it even with the HS, I'm climbing at 1000 fpm. It seems to me that to correct this, I should raise the leading edge of the HS up a little with washers, or spacer under the HS front spar where the 4 bolts are connected to the aft fuselage plate and longerons. The plans call for two 1/8" spacers to be used here, however, the builder of this RV4 used 1/4" spacers, so it's already higher than plans call for. I'm thinking I need to move it up even more to fly with a streamlined elevator.

I don't want to create a problem with stability or structural integrity, and I want to make sure I move it in the proper direction. I'll have to guess at how much. It's not easy to do this. I'll either have to remove my elevator push-pull tube in the tail and squeeze my not so limber body into the tail cone to get to the nuts on those bolts - or, remove my rudder and VS to allow access to the bolts/nuts through the small lightening hole in the aft turtledeck bulkhead. Either way, I'll have to re-align my rudder because the top of the VS will be slightly further aft. And what does this do to the aft spar of the VS that bolts firmly to the aft fuselage bulkhead, but now slightly 'tilted' from where it used to be? .........sounds like I've almost talked myself out of doing anything....... If anyone has experience in doing something like this, or recommendations, please let me know.

Thanks

2 degrees is not much, and a very slight amount of down elevator was specifically designed in by the designer (Van) and considered desirable. The slight down elevator improves pitch stability at the cost of a very slight amount of drag.

If you do change the stab incidence, you have to redo the vertical stab fwd spar attachment. You cant just bend the aft spar back slightly. The three hinge points for the rudder have to stay in alignment.

I have an rv4 i bought a year ago that is very similar. I run out of nose down trim with a passenger and even solo i am almost all the way forward on the trim and in cruise the counterbalances of the elevators are about 3/4"-1" above the front of the Stab. My plane only has the 1/8" washers so i may remove those and install a piece of 1/4" aluminum. It seems that having a solid surface drilled for the bolts would be better than washers.

Scott, this has been discussed in detailed on VAF. You may want to use the search function for a more detailed answer. The short answer is to move the horizontal stab's leading edge by half the distance in the direction of change.

If it takes 1/2 inch of travel to line up the stab and the elevator counter weight horn, then you would need to raise the horizontal stab by half that distance or 1/4 inch. That will get you close quickly.

Hope that helps.

Btw, you may have to redo the empenage fairing.

Thanks for the responses, it's just what I was looking for. What Scott M said is something to consider and something I'm going to talk to my experienced buddies at HAO about. And I'll be contacting the builder of this new tail section to find out why he used 1/4" spacers. As Scott said, I'll have to redo the VS attachment to the HS spar so I don't mess up the VS aft spar attachment/rudder alignment. This will probably require new metal, or doublers, or both,so I'll have to weigh the possible advantages v.s. possible pitch stability changes. I'll look at this tomorrow. Axel - good gouge on the amount of adjustment I'll need of if I decide to go this way. -I was aware of the need to modify my tail fairing if I make a change, but I was thinking about doing that anyway....... it doesn't meet my requirement for a tight fit anyway. I don't see why we need as many as 12 screws on each side of the fairing when 4 or 5 will do if it's a tight fit - and it just looks better that way. I've done it many times.