RV-12 horizontal stabilizer tips

[Phantom30]

Originally Posted by peter52

Bad choice of word. The airplane undulates, up and down, not to lose control, but noticeable. All the other RV12 he has built and flown do not do that. The stick does not always stay in place during flight as with the other airplanes. The stab has a noticeable drop when you try to place in neutral position on the ground.

I find this interesting.....can not see how these tips could cause this...I would be looking for rigging issues (I haven't put them on mine); but they look cool.

[Tony_T]

The tips unbalance the stabilator. Without the tips the stab will stay put when positioned. With the tips, the stab wants to droop down at the rear on the ones that I have looked at.

Van's has carefully balanced the flight controls, when you change them you become a "test pilot" as has been said many times before.

Personally, I love the feel of the controls and like the looks of my stock tips. After all, it's a more like a Porsche 912 than a 911.

Just my 2? of course, not an aero. Many will disagree.

[Dvalcik]

I have them on my 12 from the first time she flew and have flown multiple stock 12s and don't notice a difference.

[bajapilot]

Originally Posted by peter52

Bad choice of word. The airplane undulates, up and down, not to lose control, but noticeable. All the other RV12 he has built and flown do not do that. The stick does not always stay in place during flight as with the other airplanes. The stab has a noticeable drop when you try to place in neutral position on the ground.

My control stick always falls forward at rest on the ground and I though this was normal as I have 160 hrs on now the the airplane flys great. I do notice a little (very little) tendency for the stab to oscillate when in turbulent air.

While helping a friend get his 12 ready for first flight I noticed his stick would park wherever you left it when moved. Mine always fell forward. How odd. I then check several other E & SLSA's and found that they too would park wherever placed. Now I was worried that something was wrong with mine.

I talked to Van's rep at OSH and they didn't have much to offer. Said it may be due to variations in balance weights for the stab but if it flys ok then don't worry about it.

I have had painted tips on my stab from day one and from this thread it appears that it adds weight to the rear and thus unbalances the stab. I'm going to have to rethink this and perhaps add some balance weight to offset unbalance. Going to contact Van's and see if they will weigh in on this.

Bill

[rvbuilder2002]

The balance of the stabilator is relative to where the hinge point is.
Note that 2/3's to 3/4's of the tip is aft of where the hinge point is.
That is going to add most of the tip weight to the aft portion of the stabilator and change the static balance.

I don't know how much these tip fairings weight so I don't know if the change in balance is an issue or not.

One consolation is that the stabilator is actually over balanced slightly with the standard balance weight supplied in the kit. If you disconnect the cables it will swing trailing edge up.
The reason it seems balanced with the cables connected is that the control column has mass and the hinge point of the control column is at the rear. So with the airplane at rest, the mass of the control column and sticks is attempting to swing forward, while the extra mass of the stabilator balance weight counters it trying to swing the sticks aft. The result is a closed loop system that as a whole is balanced.

The common opinion is that the balance of the movable control surface on its own is the primary concern. So, on RV-12's with stabilator tips installed, if the stab is balanced or still slightly nose heavy with the cables disconnected, it is probably still ok.

Disclaimer - The standard design of the RV-12 was flutter tested to prove a good margin of safety beyond VNE. Any modification (including adding stabilator tips) could have negatively altered the airplane so keep in mind that my statement above is just a personal opinion and until the flight test was repeated to prove it, the opinion is worth what you paid for it.
Anyone making modifications to the flight control system on an RV-12 must realize they are truly experimenting.

I personally would not casually add parts to a flight control system component without careful consideration of all the ways it might have an influence and at the very least confirming that it has not become negatively balanced.

Side comment - Even if changing the balance with the addition of stab tip fairings doesn't adversely effect the flutter margin, it could still change the pitch control feel slightly because you have altered the relationship of the control surface inertia vs the opposing force of the antiservo tab. Probably not enough to effect it in a negative way, but very possibly enough to notice a difference.

[bajapilot]

Thanks Scott for weighing in on this subject with your "balanced" opinion. You do bring up some very important considerations. Would you care to comment, with your personal opinion, on the subject of rebalancing the stab by the use of a few washers mounted to the nose of the outboard rib? I'm sure a very small amount of weight added to the counter weight arm would also accomplish this at considerable labor since you'd have to pull the stab for access.

I really like the finished look of the stab tips and would hate to have to pull them off having notice no adverse effects coming within 10 knots of Vne. But then, as you have pointed out I have noticed some stick oscillation (maybe 1/4 inch) fore and aft in rough air. This may just be a characteristic of a flying stab which I have very little experience with before the 12. The perils of being a test pilot.

Bill

[12vaitor]

Slow day at the hanger, so I pulled one of the stab tips to check the static balance point and to see what it would take to have the balance match the hinge rotation line.

Tip = 31" long
Static balance point is 13.5" aft of the leading edge
Stabilator hinge rotation line is 8" aft of the leading edge
2oz of weight just aft of the forward most attachment hole will move the tip static balance point to 8" aft of the leading edge. I used two 1oz self stick wheel weights. Could probably use a bit less if the weight is added directly behind the leading edge curve.

John Salak
RV-12 N896HS
120116

[DonFromTX]

This got my interest up, so I tried out the theory at the hangar. Mine stays in any position I put it! To be truthful I think the autopilot servo is providing enough resistance to keep it wherever I put it;

Originally Posted by rvbuilder2002

The balance of the stabilator is relative to where the hinge point is.
Note that 2/3's to 3/4's of the tip is aft of where the hinge point is.
That is going to add most of the tip weight to the aft portion of the stabilator and change the static balance.

I don't know how much these tip fairings weight so I don't know if the change in balance is an issue or not.

One consolation is that the stabilator is actually over balanced slightly with the standard balance weight supplied in the kit. If you disconnect the cables it will swing trailing edge up.
The reason it seems balanced with the cables connected is that the control column has mass and the hinge point of the control column is at the rear. So with the airplane at rest, the mass of the control column and sticks is attempting to swing forward, while the extra mass of the stabilator balance weight counters it trying to swing the sticks aft. The result is a closed loop system that as a whole is balanced.

The common opinion is that the balance of the movable control surface on its own is the primary concern. So, on RV-12's with stabilator tips installed, if the stab is balanced or still slightly nose heavy with the cables disconnected, it is probably still ok.

Disclaimer - The standard design of the RV-12 was flutter tested to prove a good margin of safety beyond VNE. Any modification (including adding stabilator tips) could have negatively altered the airplane so keep in mind that my statement above is just a personal opinion and until the flight test was repeated to prove it, the opinion is worth what you paid for it.
Anyone making modifications to the flight control system on an RV-12 must realize they are truly experimenting.

I personally would not casually add parts to a flight control system component without careful consideration of all the ways it might have an influence and at the very least confirming that it has not become negatively balanced.

Side comment - Even if changing the balance with the addition of stab tip fairings doesn't adversely effect the flutter margin, it could still change the pitch control feel slightly because you have altered the relationship of the control surface inertia vs the opposing force of the antiservo tab. Probably not enough to effect it in a negative way, but very possibly enough to notice a difference.

[rvbuilder2002]

Originally Posted by DonFromTX

This got my interest up, so I tried out the theory at the hangar. Mine stays in any position I put it! To be truthful I think the autopilot servo is providing enough resistance to keep it wherever I put it;

It could also be that (contrary to what the KAI cautions about) you have a lot of friction in the pivot bearings for the control column (or any number of other reasons).
That is why I recommended that any check be to evaluate the degree of balance with the cables disconnected.

[joedallas]

This will be different with paint
There is more paint on one side off the balance point

Joe Dallas