S

Steve Sampson

Well Known Member
With all the -4 flying in the USA is there any 'conventional wisdom' about offsetting the VS at the front spar to the left to reduce the need for right rudder in order to fly in trim?

I found a bit about it in the matronics archives back in '98 but nothing since. They were talking 1/4". The -9 is certainly designed this way and I expect the -7.

If yes, how much offset? Also does it mean you have to throw the VANS fairing out?

Thoughts?
 
A topic I have been researching and of interest as I am currently correcting the offset in my RV-6A. This is what I believe:

-Original RV6 were spec'd at 0 offset, believe the RV4s were also.
-RV7,9 have 1/4" left offset built in.
-Not sure about RV,8,10,12
-Have an RV6 friend (small VS & Rudder) who built in left offset (< 1/4") as much as the fairing would allow.
-My plans are to correct offset on my RV-6A to left offset (<= 1/4"), whatever a new fairing from Van's will allow.

Any other info. welcomed.
 
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Yes

We used the -7 Vertical fin attach plate with the 1/4" offset on our 6A and the Fairings-etc fairing and a great fit. Flies with no rudder trim.

Regards,
 
1/4" offset worked for my RV6. Required a little massaging of the fairing with the heat gun though.

Doug
 
Rudder offset and how to go faster?

I am wondering how to get a few more MPH for free?

It has lead me to look at the large wedge I have on the rudder. If I offset the stab to negate the need for the wedge, will it noticeably change the speed?

And that leads me to wonder if anyone has any more RV4 experience on how much to offset it since I started this thread? VANS said 'build it to plan', so I did, but I regret that now! I thought I saw a follow up discussion but I cant find it.

Thanks!
 
I offset mine and got rid of the rudder trim tab but I can not say that it increased speed. Either the VS is offset or the rudder is offset so the drag is probably close with either situation.
I tried to move the VS another 1/8", just to see what would happen and the plane did not handle well, so I went back to 1/4".
 
Steve,

Our -4 is built to plans and I've got a ground adjustable tab in the same location as your wedge. Works well for 55-75% cruise, feet on the floor. As you state, right rudder definitely required at high power, low airspeed however I just as frequently find myself using left rudder to coordinate (at both high speed and during maneuvering flight/acro).

I've been fortunate enough to fly some pretty well mechanized flight control systems and have also flown some less-than-stellar equipment, and I really enjoy the way the flight controls are harmonized on the four. Any piston-powered equipment requires you to think with your feet (as do old jets like the F-4) and if I find I'm using left rudder a commensurate amount of time as right, the overall rig must be fairly good.

Offseting the vertical fin is a time honored practice for propeller driven airplanes.

Just a couple of data points to plug into your decision matrix.

Cheers,

Vac
 
When I switched to a new RV-8 VS/rudder on my RV-6 I offset it 3/16"...before I had an 1/4"x8" trim tab and now I have no trim tab. The taller VS slowed the airplane down a hair.
 
RV-6A

Another RV-6A data point, maybe close enough to RV-4.

Bought a flying RV-6A, 1991 vintage, O320-D1A, 160 HP, Sensenich prop. After getting used to it, I felt it was not rigged correctly. In checking everything, discovered it actually had RIGHT VS offset built in & naturally needed a BIG left rudder trim tab. Controls did not feel right, plane felt very stiff & 'heavy'. Probably was flying in a crab.

Reworked the empennage, building in 1/4" LEFT offset for VS and removed the rudder trim tab. A totally different plane, controls light & responsive, faster, and flies straight.

Keep in mind this was an extreme case. I'm sure there was some drag before, that went away. But as others have mentioned, yaw trim varies across the full power range. If what you want is least drag on cruise, you may need to trim for that and then use your feet in other conditions.
 
Thanks for all the thoughtful and informative inputs.

At the moment, at cruise power setting, with the wedge, the ball is centred, but I cant help thinking there is a small fight going on between the VS and the rudder trying to do slightly different things. From the comments below it sounds as though I should not expect much of a speed change through from this 'small fight'!

Yes at other power settings one needs to use your feet, and that is fine, just in a straight line I would rather surrender to the autopilot with the ball naturally centred.

I will probably move it over 3/16th to 1/4", but with lots of aircraft things mulling for a while is useful also. I will report back in a while if I do something. It sounds as though the speed increase is minimal.

Thanks again, Steve.
 
I offset the Vstab on my 200HP RV4 a full 1/2 inch and it was spot-on perfect. This offset was determined by flight test for this specific airplane, so YMMV due to differences with your installed HP, prop, gear fairings, wt-bal, etc etc.

As for the Vans fairing, it was junk right out of the box. It was one of the old gel-coated fairings and had no chance of fitting well even if the stab had been left on center. Just bite the bullet and build your own - you'll be glad you did.

If you want to know how to do the flight test and get the exact offset for your airplane, drop me a line and I'll fill you in...
 
Aw man...great, just great, another thing I will have to consider…. :confused: Oh well, I like to know these things before I get there.

What else can you guys give me to worry about when it says to go with plans, but most others seem to go in an entirely different direction? :eek:
 
wrongway john said:
Aw man...great, just great, another thing I will have to consider?. :confused: Oh well, I like to know these things before I get there.

What else can you guys give me to worry about when it says to go with plans, but most others seem to go in an entirely different direction? :eek:
Click to expand...

I built mine straight (1996 plans) and added the rudder tab after duct taping a wedge on, for a few flights. If there is any speed advantage between an offset vertical stabilizer or a rudder tab............then it must be the tab! :D

L.Adamson --- RV6A
 
wrongway john said:
Aw man...great, just great, another thing I will have to consider?. :confused: Oh well, I like to know these things before I get there.

What else can you guys give me to worry about when it says to go with plans, but most others seem to go in an entirely different direction? :eek:
Click to expand...

Jon,

Just note that if the plans give you rigging advice, it is rarely wrong if you are building to the design configuration. I would have expected Bill to need to do something different with a 200 HP RV-4....that is a fine machine, but not what Van designed, so originality is required. I have seen folks say that they were going to offset the fin on their RV-8 because "that's the way it was done on the -6"....but on the -8, the motor mount is built with an offset, so you are SUPPOSED to mount the fin straight.

As much as I bemoan (and, admittedly, enjoy) the inaccuracies and omissions in the RV-3 plans and instructions, I will say that Van's is a learning organization, and their later manuals and plans are much more complete and accurate. My experience has been that I read the instructions THOROUGHLY first. If I can't find what I need, and it could mean a significant change to the airframe, I talk with Vans. I ask on forums to see what others have done, but I remember to check the inputs that I get for context.

I have no idea what the -4 calls for in offset - it might be primitive enough that it is completely left to the imagination. I do doubt, however, that there is significant difference in trim drag between an offset and a wedge - you are generating the roughly the same amount of lift (and its associated drag) either way.

Just thoughts,

Paul
 
Ironflight said:
I have seen folks say that they were going to offset the fin on their RV-8 because "that's the way it was done on the -6"....but on the -8, the motor mount is built with an offset, so you are SUPPOSED to mount the fin straight.
Click to expand...

The "6" motor is offset also.
edit: P.S. --- I agree with the "drag" using either method, because that's the way it seems to add up in, in flight comparisons. I just like to feel like my rudder tab method is faster.. :)

L.Adamson --- RV6A
 
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Thanks, Paul, I?m quite certain I?ll be going with the 0-320-160 HP Lycoming, that might get tweaked out to 170 HP with a P&P, so before I decide anything there, I?ll do just that: check this forum one more time, and also maybe a call to Van?s. That's interesting to note that the 8's offset their motor mounts.

I received all of my airframe kits, still haven?t inventoried it but will soon, and I?ll be taking a note on the plans with the VS to see what it says.

I hope they have done plenty of updates with the plans. Some say they haven?t done as good a job on the 4 plans as they have with some of the later RV designs.
 
Yes, the 4 has an offset mount. But it wasn't enough with 200HP in the nose in my case.

John, you might be glad to know the new fairings are much better quality. I had the idea that you were planning on offsetting the VS for your airplane due to more installed HP.

Just fly the plane with the VS on center and use the Vans fairing. See how it does, and if its not out of trim real bad you can just add a little wedgie tab on the rudder. My 4 flew way out of trim so that's why I did what I did.
 
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