Front tail spar mount RV-4

GEM930 · #1 03-26-2013, 07:53 PM

In cruise flight my RV4 requires almost max forward trim to maintain level flight. it is not a weight and balance issue. I was thinking of changing the angle of incidence of the horizontal stab. when i looked at the mount for the front spar of the horizontal stab. it appears to have a piece of angle aluminium on the front and back of the spar bolting to the fuselage. The front piece of angle is riveted and bolted to the fuselage. I don't have the plans for this plane so i'm wondering if someone can help me out to determine weather it is supposed to be done like that or the builder of this plane decided to beef it up? if anyone has a set of plans handy so i could see how the stab is supposed to be mounted that would be awesome! it seems like the other RVs I've looked at only have one piece of angle mounting the horizontal stab to the fuselage but they arnt all 4's ive looked at.

GEM930 · #2 03-26-2013, 08:19 PM

I'll post the pic for my hangar mate that hacked my VAF account since his skills apparently fall short of hacking my photo bucket account.... By the way, if I were going to use the word "arn't" in a sentence (which I would never do), I would at least spell it correctly.

So everybody lets tell "Drew" to get his own VAF account!!!!

GEM930 · #3 03-27-2013, 12:02 AM

Crickets???Nothing???? Come on help a brother out????

WAM120RV · #4 03-27-2013, 04:15 AM

Hi,

Your photo is not very clear but the setup you have seems to vary from what is in the plans. I am not sure if there are any photos of this area in my blog but will take some today and post them there for you, and if I can a photo of the correct attachment from the plans.

From what I understand in your post you are having to trim the nose down in the cruise? I think most 4s have to do this. However, If I understand how the empenage has been attached on yours, I would expect this to have the effect of trimming the nose down further over standard........ but its hard to judge from the photos because you cannot see if the angle of attack of the horizontal stab has been changed.

Photos added to blog for you

Michael White · #5 03-27-2013, 05:03 AM

GEM,
Got a message for "Drew"...please pass it along.
According to page 30 of my RV-4 preview plans, the angle behind the vertical fin attachment plate is an extra (i.e. not shown on the plans), but I'm not sure that "extra" angle is your problem.

The plans show the horizontal stab should be mounted with the centerline of the stab 1-13/16" above the longerons, and that distance is consistant front to rear (i.e. 0 degrees incidence). It sounds like your stab may not be set at 0 degrees and may be a little trailing edge high (pushing the stab down in flight, requiring nose down trim). Tell "Drew" to check the distances to the longerons at his leading and trailing stab spars and see if they are the same. If not, he will either have to live with the situation or try to re-rig the horizontal stab. Sorry, but I don't have any great ideas for doing that...

Regards,

rvbuilder2002 · #6 03-27-2013, 12:54 PM

Part of your lack of response may be that there was an early version and a later version of hor. stab. attach.
I do not remember the specifics, but it looks like your airplane may be the early plans version.

AX-O · #7 03-27-2013, 02:19 PM

Sir,
I can try to help you out. I was going to post a pic of the plans last night but was not able to. The first thing I would say is that RV-3, 4 and 6 are unique. And depending on when the aircraft was built, it will have different configurations (many lessons-learned as time progressed). The original plans called out for a 90 deg angle to bolt directly to the fuse and to the front horizontal spar. As more people built and more combinations of engine/props were installed, ?people? started experimenting with the tail angle of incidence. People were putting washers under the 90 degree angle to raise the horizontal stab angle of incidence. That in turn changed the lift of the tail and the requirements of the elevator trim. The current plans from vans now have a spacer that goes in between the fuse and the 90 deg angle. I don?t have the plans in front of me so I can?t tell you the thickness (will do tonight).

Lets do some investigating:
-If you are almost running out of nose down trim, that means that you are forcing the tail to make more lift to counteract the nose up tendency.
-If you want to make more tail lift and your angle of incidence is fixed, then your only option is to move the trim in such a way that the elevator trailing edge goes down.
-You could also achieve more tail lift by changing the angle of incidence up from your current state. That will increase the lift and reduce the requirements for trim (at that particular airspeed; cruise).

That change will affect you throughout your speed envelope. It is something to be taken seriously and cautiously. Flight testing will be required and shims should be done in small increments. My current RV has a washer in between the fuse and the 90 angle. The 4 that I am building now, has a shim which raises my angle of incidence to 0 degrees with respect the canopy rail.

I hope that helps you out. Better pics of your particular set up would increase your chances of receiving additional responses/feedback. For example, a picture of the aft spar showing the fuse and 90 degree angle would be helpful.

F1Boss · #8 03-27-2013, 03:00 PM

A rule of thumb with the h stab setting: Look at your elev counterweights in cruise. The stab setting should be changed about 1/2 of the deflection value, in the same direction, measured at the LE of the stab. This will center the elevs, or put them in trail.
Do this testing with your normal load config. 2 up CG does not = solo!
My guess is you will end up at +0.5deg incidence on the stab.
If you choose to make stab adjustments, remember that the fwd spar of the fin is attached to the stab fwd spar. Moving that attach point will affect the fin aft spar (bending it). So, plan to shim at the deck/spar attach bracket while you are moving the H stab.
Now, it looks to me like the stab fwd spar is riveted to the #19 deck plate. This is non-standard on that design, but it's not a bad idea. If this is the case, adjustment is not gonna be an afternoon job.
Carry on!
Mark