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Reset Horizontal Stabilizer?

S

SHIPCHIEF

Well Known Member
Yesterday I was blasting along solo at full throttle, wishing I could go faster. I looked out the back at my horizontal stab, and the counterweights were about 1/2" higher at the front than the horizontal stab.
Later, when I was flying home with the cooler and other stuff in the back (more aft CG) the mismatch was greater, and my airspeed was down.
Has anyone changed the incidence of the horizontal stab after construction, and is there previous discussion here on the forum that I missed (poor search technique on my part?)
My goal is a light and slick 180 HP fixed pitch cross country ship that does occasional IAC competition.
 
Horizontal

I don't think moving the Horizontal Stab is feasable after initial
install ,egg shaped holes ???
What is your CG when flying solo ? Can you put the extra stuff in the front baggage compartment ? My CG is in the forward 25% and at 175 knt cruise the elevator is in perfect trail . I have not looked at it at top speed .
 
I have wondered the same thing for the same reason. The only time mine is in trail is at dash speed, which i rarely do. But I abandoned the idea because it would involve reworking the fiberglass fairing and paint.
 
Shimming the HS is quite simple at anytime. On your 8, there are 4 bolts holding the leading edge of the HS spar to the deck. It usually has a shim already. You have a little bit of leading edge HS down available, and all the up you want. Be advised that moving this changes your emp fairing placement/fit. Also if you shim more than ~3/8 of an inch, you are placing stress on the VS spar attach at the deck. You can add thin washers to the read spar attach between the HS spar and the vertical forks it bolts to. Best solution here is to loosen those rear spar attach bolts. Shim the front to where you want it. Then check to see if there is any gap between the rear spar and the fork attach. If there is, use thin washers to take up the gap space.
 
Changing the Horizontal Incidence

You can change it after construction. If I remember correctly, there is some info in the back of my RV9a build manual on doing just that. Solo, I had about 3/8ths of an inch of the front counter wt showing. I talked with Ken Krueger at Vans (this was in 2007 or 2008), we calculated the front attach point shim thickness required, and I made the change. I wanted "in trail" elevator with my wife and I aboard and about 45 lbs baggage---cross country configuration. I did have to modify my intersection fairing and have it repainted. Don't expect much in the way of speed change----my increase was less than 1 kt!!!

Hope this helps.

Cheers,

db
 
If I understand correctly you are referring to the elevator counter weights.
I think that Vans designs the elevator counterweight to be about a 1/4 in or a little more above the horizontal in level flight. At least that is what I was told in 1995 by a Vans Rep/Pilot while he was flying next to our RV6
I would check with them before shimming. Larry
 
Be careful!

There is more to the horizontal tail incidence than just a little bit of drag from the counter weights sticking up. Changing the incidence will change the stick force gradient, or "stick-free" stability.

Increasing incidence (leading edge up) reduces the stick-force gradient.

Flying an airplane with too little stick force gradient isn't fun, and if you get negative stick-force gradient, it is downright miserable. The faster you go, the more aft pressure on the stick is required. Bad.

As noted by other posters, the drag is insignificant, and all the trouble redoing your fairing after you change the incidence is not worth it, it won't change your speed.

If you really want to do it (experimental for education and enjoyment) then just make a small change (1/8" shim) and see how it feels before you do more.
 
This is nothing new!

SHIPCHIEF said:
Yesterday I was blasting along solo at full throttle, wishing I could go faster. I looked out the back at my horizontal stab, and the counterweights were about 1/2" higher at the front than the horizontal stab.
Later, when I was flying home with the cooler and other stuff in the back (more aft CG) the mismatch was greater, and my airspeed was down.
Has anyone changed the incidence of the horizontal stab after construction, and is there previous discussion here on the forum that I missed (poor search technique on my part?)
My goal is a light and slick 180 HP fixed pitch cross country ship that does occasional IAC competition.
Click to expand...

Yours is not the 1st ship to be affected with this malady. There is speed to be gained , but there are a few downsides. The rule of thumb is to raise the LE of the stab 50% of the elev horn offset. As Mike mentioned, the fin aft spar suffers if you do not re-straighten it after shimming the H stab.

And, the emp fairing wil no longer fit.:eek:

You might as well try it - it is practically free: shims needed. Go ahead and fly with a duct tape fairing (you won't be the 1st) to see if it suits your mission.

I have seen gains of 7kt by making this adjustment. But, 3-point landings are a bit different after this mod. Small steps are recommended.

Carry on!
Mark
 
Thanks for the insight and pointers.
I was running at about 1450 pounds, and about 1/2 back from fwd CG, doesn't seem like the elevator should be lifting the tail in this condition. I would expect the elevator to be neutral.
I'll workout more with the weight & balance program & experiment with various loads, CGs & airspeeds. Perhaps there is an airspeed less than full power level flight where the stab & elevator are in trail.
I'm very pleased with the aircraft over-all. I expect to tweak a few things here and there to get it optimized.
Right now my 'steam' airspeed indicator and Dynon D-10a are in close agreement, but the Garmin says they are optimistic by about 5-10 kts.
I need to fly the full speed formula instead of the simple upwind-downwind run.
Any additional tips and experiences are welcome.
 
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SHIPCHIEF said:
Thanks for the insight and pointers.
I was running at about 1450 pounds, and about 1/2 back from fwd CG, doesn't seem like the elevator should be lifting the tail in this condition. I would expect the elevator to be neutral............
Click to expand...

I would make the point that the elevator is not "lifting" the tail. It is only slightly reducing the "down force" that the elevator and HS are providing. The lift vector of the tail is always down in a conventional aircraft.

Semantics! :)
 
PCHunt said:
...The lift vector of the tail is always down in a conventional aircraft...
Click to expand...

Usually? Yes. Always? No, not true. Steve Smith (scsmith who posted to this thread earlier) can add more if he wants.

The most common counterexample is the high-performance sailplane. In low speed flight, the horizontal tail is actually lifting upward. However, positive stability and positive stick force gradient prevail.

Thanks, Bob K.
 
Conventional Aircraft

When Pete said "conventional aircraft" I assumed he meant front engine propeller driven machines like our RV's---not sail planes. As we learn in Private Pilot training, the cg range on a particular design is placed so that, should the wing stall, the nose will drop, the airframe will pick up speed, and the wing will again create lift. To counter this nose down force in level flight the designers must build in some down force in the HS. Should the down force be excessive due to wt distribution on a particular build (engine size/equipment/etc) the elevator may be trimmed down in level flight so as to "reduce the down force" as Pete states.

Maybe Scott or some of our Aero E's can explain this better than I--or correct my misunderstanding!

Cheers,

db
 
49clipper

I read all these threads on HS incidence, but have not seen anyone with my same issues. My counter weight up distance is also about 1/2", but at high cruise in my -6 (O-320)my trim tab is up 29 degrees (by measurement on the ground). My aero eng at boeing said change the hs incidence. i tried 1/8" more with no joy. Went back to stock. anyone know why?
Jim
RV-6
 
Having recently worked on this problem. I raised my LE 5/8" to get the flight characteristics I desired, not to put the elevator or trim in any particular location. Be sure your chasing the right problem. 1/8" may not have been enough to change the desired behavior. But if you were meticulous in measurements in both speed, elev position, trim, and stick force, I would expect there to be a measurable difference in 1/8" increments.
 
I have done this on a couple of rv8s. The problem I noticed most was the plane felt tail heavy on final unless you held power and also the counterbalance arms were up in cruise.
I raised them 1/8", if I remember correctly, and it solved the problems. The plane flew much better especially on short final.
Make sure you compare apples and apples before and after the adjustment. Set the plane up the same for cg, fuel, etc. for a true comparison. Depending on weight distribution in your plane the elev weights may be up in cruise, especially if you have a rear passenger. Depends on how your plane is built.
You may have to redo the empennage fairing if you change the HS incidence.
 
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db1yg said:
... the designers must build in some down force in the HS...
Click to expand...

Well, still no.

This Steve Smith post explains it better than I could. The important part is:

Static Stability: Probably the most widely held misconception in aviation is that the tail must have downward lift to make the airplane stable, and that zero lift on the tail would correspond to neutral stability. This is just simply not true. What IS true is that for a statically stable airplane, the tail can never lift upward at a higher lift coefficient (lift/unit area) than the wing, and including the effects of wing camber, a trimmed, stable airplane may only have an upward lift coefficient that is a fair bit less than the wing lift coefficient. BUT, it is often true that at low speeds, many airplanes have small amount of upward lift if the c.g. is toward the aft third of the c.g. envelope. This is especially true of sailplanes where performance is so important that we always balance the airplane in the aft third of the c.g. range, and because with a short wing chord, the effects of wing camber are lessened, allowing more tail upload. Anyone that doesn't believe this, I would invite you to get any good text on aircraft stability and control and work through the equations yourself.
Click to expand...

That post also offers some good advice that is very pertinent to the topic at hand. Steve is a professional aero engineer with over 30 years experience at NASA, he's built an RV-8, and he has contributed to the design of several high-performance aircraft beyond his NASA tenure, so I think he knows what he's talking about.

Edit add: Steve is also the guy who developed the wing profile for the RV-10. So Van seems to think he knows what he's talking about.

Thanks, Bob K.
 
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sf3543 said:
I have done this on a couple of rv8s. The problem I noticed most was the plane felt tail heavy on final unless you held power and also the counterbalance arms were up in cruise.
I raised them 1/8", if I remember correctly, and it solved the problems. The plane flew much better especially on short final.
Make sure you compare apples and apples before and after the adjustment. Set the plane up the same for cg, fuel, etc. for a true comparison. Depending on weight distribution in your plane the elev weights may be up in cruise, especially if you have a rear passenger. Depends on how your plane is built.
You may have to redo the empennage fairing if you change the HS incidence.
Click to expand...

Besides refitting the fiberglass empenage fairing, any change in stab incidence will also require redoing the attach point of the vertical stab. fwd spar to the horizontal stab. fwd spar.
If you don't, the rudder hinge line will no longer be aligned and it will be tweaking/binding your rudder anytime it is moved from neutral.
 
Well, still No

I don't think anyone is disagreeing with Steve's post on this subject--quite the contrary.

Let's look at the third sentence in the Steve's quoted posting: "What IS true is that for a statically stable airplane, the tail can never lift upward at a higher lift coefficient (lift/units) than the wing........"

Now lets think about our RV's--if the angle of incidence of the wing is set at say 3 degrees positive (speaking from memory for the 9a) and the HS is set to 0 degrees (again from memory), what forces are exerted on the airframe by the HS relative to the wing in straight and level high speed cruise flight?

Now go to the paragraph in Steve's posting immediately following the one which was quoted--it states:

"As a stable airplane is trimmed at higher speed, more downforce on the tail is needed. ....at cruise speed when the lift coefficient is very low the tail is most always lifting downward."

I'm dun!!

Cheers,

db
 
Bob Kuykendall said:
Well, still no.

This Steve Smith post explains it better than I could. The important part is:


"Static Stability: Probably the most widely held misconception in aviation is that the tail must have downward lift to make the airplane stable, and that zero lift on the tail would correspond to neutral stability. This is just simply not true. What IS true is that for a statically stable airplane, the tail can never lift upward at a higher lift coefficient (lift/unit area) than the wing, and including the effects of wing camber, a trimmed, stable airplane may only have an upward lift coefficient that is a fair bit less than the wing lift coefficient. BUT, it is often true that at low speeds, many airplanes have small amount of upward lift if the c.g. is toward the aft third of the c.g. envelope. This is especially true of sailplanes where performance is so important that we always balance the airplane in the aft third of the c.g. range, and because with a short wing chord, the effects of wing camber are lessened, allowing more tail upload. Anyone that doesn't believe this, I would invite you to get any good text on aircraft stability and control and work through the equations yourself."



That post also offers some good advice that is very pertinent to the topic at hand. Steve is a professional aero engineer with over 30 years experience at NASA, he's built an RV-8, and he has contributed to the design of several high-performance aircraft beyond his NASA tenure, so I think he knows what he's talking about.

Edit add: Steve is also the guy who developed the wing profile for the RV-10. So Van seems to think he knows what he's talking about.

Thanks, Bob K.
Click to expand...

That's very interesting. I'm glad I made the post, because I am learning something! :)

Now if we could narrow the discussion to RV-xx series aircraft, I would be interested in the answer to the same question.
 
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