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Horizontal stabilizer incidence

G

GoingHawgWyld

Member
I am at the point of mounting my horizontal stabilizer. Per the plans there is a 1/8 shim under the front which I have installed and set the horizontal stabilizer parallel to the top fuselage longeron. I was talking to another RV builder who said he read that one should change the horizontal stabilizer incidence to add a slightly more nose down pitch by now increasing the spacer to 1/4 inch? I have already drilled all the holes so pretty much changing the incidence will be by spacer at this point? I have searched in this forum and really have found nothing concrete... Is this something I should do now if in fact this is true or wait until I fly it and see if it is needed?

Ken
 
Per Plans

I have set mine per the plans, not flying yet.

I have never heard of anything like that, I would not change anything Untill Vans confirms it's ok:confused:
 
The horizontal stabilizer incidence for the -6 is correct per the plans.

Many of the RV-4s had to be altered during phase I testing, but not the RV-6.
 
Wing function

I am in the process of wing incidence on my 9a.
I read in the instructions that one can change the incidence
of the wing if need be in order to get the flaps (up) trailing smooth
and flush with the fuse underbody. It is stated if one does this
than the HS needs to follow suite to be parallel with the wing.
Would this not suggest they need to be parallel in straight and level flight?

However, I saw a thread once with a case to require straight and level flight
in which the elevator needed to be slightly down thus causing much drag.
The suggestion was to change the incidence of the HS in order to correct this problem.

I would also like it if someone could tune in on all of this?
 
Negative

Al, actually, a little down elevator makes the airplane faster. It's because whatever load the tail is carrying with down elevator, relieves the wing from carrying that portion, and flies at a lower angle of attack.

In my -10, it's very noticeable.

Best,
 
9A rigged per plans ...

... and in trimmed S&L flight, I have about 1/4" of the elevator counterweights showing above the top of the HS ... IOW, just a little bit of down elevator. All my tested speeds match Vans numbers. From what I read, that amount of top-exposed elevator counterweight is very common.
 
Rupester said:
... and in trimmed S&L flight, I have about 1/4" of the elevator counterweights showing above the top of the HS ... IOW, just a little bit of down elevator. All my tested speeds match Vans numbers. From what I read, that amount of top-exposed elevator counterweight is very common.
Click to expand...

It sure is. Even with the 6/6A
 
The original RV-6 instruction had a section on this issue and the possible need for resetting the stab incidence -



click to read
 
pierre smith said:
Al, actually, a little down elevator makes the airplane faster. It's because whatever load the tail is carrying with down elevator, relieves the wing from carrying that portion, and flies at a lower angle of attack.

In my -10, it's very noticeable.

Best,
Click to expand...

But isn't a little down elevator actually equivalent to a small reduction of the horizontal stab incidence?

Aerodynamically they aren't they same?
 
To me, it would seem that a little down elevator is equivalent to a little increase in the horizontal stab incidence angle. Either makes the airplane fly at a lower angle of attack.
 
Elevator position is CG dependent, so a person really can't make a determination until you're flying from forward to aft CG's with all rigging factors in play.
 
Interesting

Interesting. Man you guys are good. Who would have ever
Thought what appears to be a clear case of drag is actually
faster. Very cool. Thanks.
 
Unloading downforce on the tail, and as a result reducing work by the mainplane, for fuel economy is why I have never believed an attendant on a commuter when passengers were asked to move rearward for "weight and balance issues"
 
I think I will hold off till the flight testing phase. There are enough of these built that if there was an issue it would be discussed on here... Thanks for your feedback!
 
rvator51 said:
To me, it would seem that a little down elevator is equivalent to a little increase in the horizontal stab incidence angle. Either makes the airplane fly at a lower angle of attack.
Click to expand...

Correct, I think. I screwed up the direction of down elevator...:)

However, as stated by rv9av8tr it is specific to your plane and your usual loading conditions.

The Vans instructions for a check during the flight test phase make the most sense.
 
elevator trim

Mine has the trim tab up in flight 29 degrees at cruise. Elevator deflected downward so that the counterweight tips are up approximately .5" in flight. At 150 knots I have almost all the trim in for nose down. Always been this way. When I called Vans, they said thats normal, go fly and don't worry. I worried. I talked to a fellow test pilot/aero engineer at Boeing and he agreed I should add incidence. I tried 1/4" instead of 1/8" and saw no results except the plane flew slightly nose down at cruise. Trim tab still elevated up. I gave up. I know there has to lots of drag penaly with that trim tab sticking up like a spoiler.
JS
RV-6 O-320-D1A
 
For what it's worth, my Rocket does the same thing. Clearly requires nose down trim at cruise. If I had it to do over, I think I would increase the AOA of my horizontal stabilizer. If I don't re-trim from cruise flight to landing, when I shut down, the elevator trim tab trailing edge is up about an inch. In flight that results in the leading edge of the counterbalance horn being up at least a half inch.


Lee...
 
I added a second 1/8" shim on my -6A at about 50 hours. The trim was about 3/4 travel at cruise with one person and full gas.

The result was between 2-3 knots increase and the trim was about 1/4 travel. No change in stick forces or flying feel. The tail fairing still fit fairly well but I got a new one that is still on the shelf.
 
Just to bring an old discussion to date.
I just finished Phase one with my RV-9.
I found that I had a little elevator counter weight sticking up during cruise.
Started paying attention and found that I had to trim down a little during fast cruise. I decided to try the HS incidence adjustment.
I removed the forward spar 3/16" shims per plans and replaced with new 1/4" shims. This wasn't enough of a change to create a fairing problem.
I still have a little trim down at fast cruise but the trim tab is half what it used to be.
I think this was a worth while change.
Now off to the Rudder shimming.
 
I am curious here. I leveled my Hoirz stab by setting the it's incidence level with the longerons (my rear top skin was off, so this was easy) per the manual. For those of you that saw improvements by increasing the incidence, did you also level with the fuse/longerons? I am curious to know if I could benefit from increasing my incidence or if possibly your increase brought you to level with the fuse. My shims were a bit over 3/16 to get me level.

Larry
 
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