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Adjusting HS incidence?

airguy

Unrepentant fanboy
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On my 9A build I followed the instructions exactly to, in theory, arrive at 0 degrees incidence on the mounting of the horizontal stabilizer. Now at 330+ hours I'm thinking I want to adjust the stabilizer incidence to reduce the amount of elevator travel that is out-of-trail during cruise flight to get total drag down. Right now I'm seeing the leading edge of the elevator lifted by what looks to be about 3/4" in cruise flight (rough estimate - I'll make some distance markings for a real measurement and go fly), meaning the elevator as a whole is deflected downward, meaning I would need to raise the leading edge of the HS a small amount (or drop the trailing edge) to bring the elevator in trail or closer to it.

Obviously this means returning the aircraft to Phase I to properly test the modifications. My questions are directed to those of you (I'm sure there are some) who have done this, perhaps multiple times, and to get some "hints/tips/tricks" for how to accomplish it.

I'm expecting this to reduce cruise drag, but also believe it will change the low-speed handling characteristics considerably, especially close to the stall for landing. I would appreciate any feedback, even the ones that tell me I'm going to die screaming. So far I've managed to disappoint them but I still like to know how close I'm getting.

EDIT - as this will obviously relate closely to CG, my current full (standard) fuel solo CG with interior and oxygen tank as I normally fly it is between 80.2 and 80.5" aft of datum. This is where I'm seeing roughly 3/4" of the leading edge of the elevator counterweights raised above the HS in cruise flight.
 
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I had to raise the front of the stabilizer on my rv4 due to the same thing. I think i raised the front by 1/8-1/4" and it may a good difference. I probably could have raised it a little more but it was going to require redoing the fairing.
 
In the early days, this was a commonly "needed" adjustment on the RV-4. It is NOT common on later models.
Typically in cruise you should see about 1/4" to 3/8" of the counterbalance horns. 3/4" seems a bit excessive.
 
Did this on the 8A - added a 0.40? shim under the forward HS spar. It provided just enough reduction of HS downward force to bring the elevators in trail for cruise, but still had enough so that I did not run out of elevator up trim for landing when solo.

On the RV-10 I experimented with up to a 1/8? shim. It did not do much so I removed it.

Carl
 
......I'm expecting this to reduce cruise drag, but also believe it will change the low-speed handling characteristics considerably, especially close to the stall for landing.....

I could easily be wrong here, but I don't think this would change the induced drag. For a given CG location you're going to need the same down force regardless of how you get it. Seems like you're just going to move the required aerodynamic load (and the induced drag that goes with it) from the elevator to the horizontal stab.

Likewise with the low speed characteristics. Unless you get some kind of elevator blanking from the change, it seems like you wouldn't really see much difference unless you just run out of travel.
 
I could easily be wrong here, but I don't think this would change the induced drag. For a given CG location you're going to need the same down force regardless of how you get it. Seems like you're just going to move the required aerodynamic load (and the induced drag that goes with it) from the elevator to the horizontal stab.

Likewise with the low speed characteristics. Unless you get some kind of elevator blanking from the change, it seems like you wouldn't really see much difference unless you just run out of travel.

This is correct, you won't change the trimmed induced drag. You will see a very small change in the profile drag, and some reduction in pressure drag from the horns sticking up. These are going to be very small reductions. There are other reductions that can be achieved with less labor. If you shim the horizontal stab, you will probably need to fit a new empennage fairing.

One thing I expect you will notice is a reduction in "stick-force gradient", that is the control force to change speed away from a trimmed speed will be less. RV's already have fairly light control forces. We like that. But you can make the airplane feel overly sensitive if you change the "decalage" too much.
 
This is correct, you won't change the trimmed induced drag. You will see a very small change in the profile drag, and some reduction in pressure drag from the horns sticking up. These are going to be very small reductions. There are other reductions that can be achieved with less labor. If you shim the horizontal stab, you will probably need to fit a new empennage fairing.

One thing I expect you will notice is a reduction in "stick-force gradient", that is the control force to change speed away from a trimmed speed will be less. RV's already have fairly light control forces. We like that. But you can make the airplane feel overly sensitive if you change the "decalage" too much.

I realize I won't be affecting the induced drag without changing the CG - but I thought the pressure drag from the front-plate-area of the counterweight horns might be enough to do away with. I may very well be incorrect on that assumption.

You're certainly correct that there are other areas of "low-hanging fruit" for drag reduction, especially on my airplane, but I'm working through a list of them searching for a few more knots and better cruise efficiency, this was simply one item on the list.

Thanks to all for the comments.
 
Good discussion. As I mentioned I did add a shim on my 8A under the HS forward spar.

My thought on this was straight forward. If the HS was providing too much downward force in cruise, it would show up as the elevators trailing edge deflected down to provide enough upward force to achieve balance. The shim I added reduced the HS downward force just enough so the elevators where more or less in trail during cruise. So I assume this:
- Reduced the drag from the HS providing excessive downward force.
- Reduced the drag for the elevators providing upward force to counteract the HS.

I did note a change in trim. Solo before doing the shim I had more than enough up nose trim on landing. After the shim I found at full up trim I had just enough. So be careful on this - start off with small shims and work up.

On the RV-10 I tried this again. What I found out is the trail position of the elevators yields the horns up just a touch. Once I figured this out I found the HS angle of incidence about right if done by the plans.

All of this falls under “rigging the plane”. There are knots to be gained if you spend the time. But - HS angle of incidence is down the list of things that will do the most for you. I’ve seen several flying RVs with a low flap or two, wingtips out of alignment, poorly rigged gear and wheel pant fairings, etc. Rigging is not “one and done”.

I just cruised back from Florida in the RV-8A a couple of weeks ago. At 7500’ I was at 180kts TAS and 8.5gph (pushed it as I had a strong headwind). I can say with confidence the plane started out life a lot slower.

Carl
 
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All of this falls under “rigging the plane”. There are knots to be gained if you spend the time. But - HS angle of incidence is down the list of things that will do the most for you. I’ve seen several flying RVs with a low flap or two, wingtips out of alignment, poorly rigged gear and wheel pant fairings, etc. Rigging is not “one and done”.

I just cruised back from Florida in the RV-8A a couple of weeks ago. At 7500’ I was at 180kts TAS and 8.5gph (pushed it as I had a strong headwind). I can say with confidence the plane started out life a lot slower.

Carl

That's the program I have begun. I currently am cruising in the 14k-17k altitude range truing out 155 ktas on 6.6-6.8 gph (long-range econ cruise), and I am hoping to eventually get that up to 165 on the same fuel burn. That's a lot of drag reduction to get there, but you gotta have a target somewhere to shoot for, so that's the target for now.
 
Good discussion. As I mentioned I did add a shim on my 8A under the HS forward spar.

I just cruised back from Florida in the RV-8A a couple of weeks ago. At 7500? I was at 180kts TAS and 8.5gph (pushed it as I had a strong headwind). I can say with confidence the plane started out life a lot slower.

Carl

Nice numbers, Carl. Are you running LOP? These are the target numbers I had in mind when I started my build. Transition training starts tomorrow then I'll start on that journey to see what kind of numbers reality throws my way.

Carl
 
That's the program I have begun. I currently am cruising in the 14k-17k altitude range truing out 155 ktas on 6.6-6.8 gph (long-range econ cruise), and I am hoping to eventually get that up to 165 on the same fuel burn. That's a lot of drag reduction to get there, but you gotta have a target somewhere to shoot for, so that's the target for now.

Hey Greg - is what with full fuel tanks, or the original 36 gallons?
 
Hey Greg - is what with full fuel tanks, or the original 36 gallons?

Standard fuel 36 gallons, but I don't see much cruise speed decrease from the weight of the extra 31 gallons in the outboard tanks. The 9 wing airfoil is very forgiving of extra weight, I can honestly only see 1 or 2 knots decrease in cruise for carrying a full 67 gallons.

I'm always WOTLOP for cruise as well, there's horsepower left in the engine for higher speed but what I'm looking for is efficiency at this point. If I can add several knots of cruise speed at the same fuel burn, I'll need fewer gallons of fuel to make it across the oceans. I'm planning my ferry tanks for the longer trip legs so it's time to get real about drag reduction so I don't need air-to-air refueling...
 
I could easily be wrong here, but I don't think this would change the induced drag. For a given CG location you're going to need the same down force regardless of how you get it. Seems like you're just going to move the required aerodynamic load (and the induced drag that goes with it) from the elevator to the horizontal stab.

Likewise with the low speed characteristics. Unless you get some kind of elevator blanking from the change, it seems like you wouldn't really see much difference unless you just run out of travel.

This is correct, you won't change the trimmed induced drag. You will see a very small change in the profile drag, and some reduction in pressure drag from the horns sticking up. These are going to be very small reductions. There are other reductions that can be achieved with less labor. If you shim the horizontal stab, you will probably need to fit a new empennage fairing.

One thing I expect you will notice is a reduction in "stick-force gradient", that is the control force to change speed away from a trimmed speed will be less. RV's already have fairly light control forces. We like that. But you can make the airplane feel overly sensitive if you change the "decalage" too much.

These are the correct answers.....

The parasitic drag of the protruding horns is small. Some amount of protrusion is designed in for good pitch handling qualities (though 3/4 " is about double what you should see, assuming that is correct).
My guess is that any speed increase will be small enough that it can't be accurately measured, at the cost of handling quality and a lot of work.
 
Greg,
I noticed a rise in the elevator horn during Phase 1 and added a shim to the front of the HS attach points. (thicker than the one on the plans)
It was small as I did not want to modify the already painted fairing.
I saw no difference in speed or handling but I do still have some horn showing.
I felt it wasn't worth the effort to get it down even.
 
I've added 1/8" shims under the HS attach pads to raise my HS leading edge. I found that at 160+ knots it was taking lots of forward stick pressure to keep the nose down. My elevator trim would not relieve the stick pressure at those speeds ( I assume it was beyond the torque capacity of the servo). After inserting the shims I find that my forward stick pressure is significantly reduced and I still have 3/8" protrusion of the counter balance arms at normal cruise of 150kts. My intersection fairing fit just fine even after adding 1/8" under the HS attach pads.
 
Been there

Rule of thumb: move the LE of the stab 1/2 of what you see displaced to zero out the elev displacement. So you would raise the LE of the stab ~3/8". Might take a +3/16" shim?

Also, you will need to check the V Fin aft spar - likely you will need a different version of the fin aft spar upper attach bracket, as the fwd fin attach is raising along with the stab spar, slightly bending the spar.

Yes, the fairing will need some tweaking too. Probably not as much as you might think...
 
Stab

Just a thought as to your problem may be moving weight forward like a heavier prop or a battery that's bigger on the fire wall.
Bob
 
....My thought on this was straight forward. If the HS was providing too much downward force in cruise, it would show up as the elevators trailing edge deflected down to provide enough upward force to achieve balance. The shim I added reduced the HS downward force just enough so the elevators where more or less in trail during cruise. So I assume this:
- Reduced the drag from the HS providing excessive downward force.
- Reduced the drag for the elevators providing upward force to counteract the HS.

I did note a change in trim. Solo before doing the shim I had more than enough up nose trim on landing. After the shim I found at full up trim I had just enough. So be careful on this - start off with small shims and work up.
Carl

The net downforce of the stabilizer-elevator combination is dictated by c.g. location as the right downforce to trim in level flight.

If you have some elevator deflection to trim in level flight, it is as if you have a cambered airfoil lifting downward. That would not seem like a good idea, but for small angles of attack, modest amounts of camber from elevator deflection and small amounts of downforce, it is an almost unmeasurable difference in airfoil drag.

The trade-off in stick-force gradient is worth it.

It does sound like you have an unusually large amount of elevator deflection for trim -- 3/4" ? Really? You must be at a very aft c.g.? If it is really that large a deflection, it may be worth the effort in shimming. Note also Mark's comment about the change in the fin attachment position too.
 
One thing I expect you will notice is a reduction in "stick-force gradient", that is the control force to change speed away from a trimmed speed will be less. RV's already have fairly light control forces. We like that. But you can make the airplane feel overly sensitive if you change the "decalage" too much.

Changing wing or stab incidence won't affect the stability of the airplane, so there will be no change in stick force gradient. Moving CG will, but not incidence. It will just offset the elevator at 0 force. To change the gradient you need to either change the geometry of the control system (bellcranks etc), change the CG, or the stab size or wing size or move the stab position vertically to a different position in the downwash field.
 
Same here

Greg,
I?ve got the same thing as you do in cruise so I?ll be watching this thread to see if any gains can be found. Mine also appears to be ⅝ to ? high at cruise.

On my 9A build I followed the instructions exactly to, in theory, arrive at 0 degrees incidence on the mounting of the horizontal stabilizer. Now at 330+ hours I'm thinking I want to adjust the stabilizer incidence to reduce the amount of elevator travel that is out-of-trail during cruise flight to get total drag down. Right now I'm seeing the leading edge of the elevator lifted by what looks to be about 3/4" in cruise flight (rough estimate - I'll make some distance markings for a real measurement and go fly), meaning the elevator as a whole is deflected downward, meaning I would need to raise the leading edge of the HS a small amount (or drop the trailing edge) to bring the elevator in trail or closer to it.

Obviously this means returning the aircraft to Phase I to properly test the modifications. My questions are directed to those of you (I'm sure there are some) who have done this, perhaps multiple times, and to get some "hints/tips/tricks" for how to accomplish it.

I'm expecting this to reduce cruise drag, but also believe it will change the low-speed handling characteristics considerably, especially close to the stall for landing. I would appreciate any feedback, even the ones that tell me I'm going to die screaming. So far I've managed to disappoint them but I still like to know how close I'm getting.

EDIT - as this will obviously relate closely to CG, my current full (standard) fuel solo CG with interior and oxygen tank as I normally fly it is between 80.2 and 80.5" aft of datum. This is where I'm seeing roughly 3/4" of the leading edge of the elevator counterweights raised above the HS in cruise flight.
 
As a point of interest..
I had a similar issue on an RV8 I built.
I noticed two things, the counter weights were up about a half inch in level flight and the plane felt like it was behind the power curve on approach to landing. Basically felt too tail low and I had to keep the speed up a bit.
I shimmed up the leading edge of the HS by about 3/16" and it totally changed everything.
The elevators came in perfect trail at cruise and it handled much better on approach to landing.
CG loading will affect this however, so if you make adjustments, do it to match your usual flight configuration.
I also had to rework the empennage fairing to fit the change, but during testing I just taped it on at the leading edge. The rework was minor and limited to the front of the fairing only.
 
As a point of interest..
I had a similar issue on an RV8 I built.
I noticed two things, the counter weights were up about a half inch in level flight and the plane felt like it was behind the power curve on approach to landing. Basically felt too tail low and I had to keep the speed up a bit.
I shimmed up the leading edge of the HS by about 3/16" and it totally changed everything.
.

There is no reason why stab incidence would change anything except the trim. Being behind the power curve is a function of the basic L/D of the airframe and the engine thrust vs speed. Stab incidence won't change that at all. So you must have changed something else, or you evaluated the approach after the mod at a different speed or something. The drag difference you would get would be fractions of a percent.
 
Changing the stab incidence on my 4 gave a similar response, not only did it get the elevator closer to being in trail, but i also used to run out of nose down trim. Also after shimming the stabilizer it changed stall characteristics to where it used to break pretty rough and after it would just mush and usually wouldnt break at all.
 
There is no reason why stab incidence would change anything except the trim. Being behind the power curve is a function of the basic L/D of the airframe and the engine thrust vs speed. Stab incidence won't change that at all. So you must have changed something else, or you evaluated the approach after the mod at a different speed or something. The drag difference you would get would be fractions of a percent.

Scott: Please don't confuse basic static stability (stick fixed) with stick-free stability AKA stick-force gradient.

It is true that changing the decalage will not change the static stability.

It is false that it does not effect stick gradient. The stick-free stability is influenced by the change in elevator hinge moment with change in speed. The change in hinge moment with speed is influenced by the natural float position of the elevator when trimmed, which is strongly influenced by the trim-tab position.

As I think through it right now, I think leading-edge up on the horizontal stabilizer is the 'good' direction, that is, it is the change that should increase stick-force gradient, not reduce it. But I could easily be thinking about it incorrectly right now. In any case, the incidence DOES change the stick force gradients.
 
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Scott: Please don't confuse basic static stability (stick fixed) with stick-free stability AKA stick-force gradient.

It is true that changing the decalage will not change the static stability.

It is false that it does not effect stick gradient. The stick-free stability is influenced by the change in elevator hinge moment with change in speed. The change in hinge moment with speed is influenced by the natural float position of the elevator when trimmed, which is strongly influenced by the trim-tab position.

As I think through it right now, I think leading-edge up on the horizontal stabilizer is the 'good' direction, that is, it is the change that should increase stick-force gradient, not reduce it. But I could easily be thinking about it incorrectly right now. In any case, the incidence DOES change the stick force gradients.


In the hinge moment wind tunnel testing I have seen, the Hm is a reasonably linear function of surface deflection, be it elevator or tab, and surface aoa, with limits. To change that gradient we play with beads, tabs, wedges and other toys to tune the forces. I don't recall surface aoa, which in this case would be the same as tail incidence, having an impact on gradient. On the airplanes I've worked on, you trim by moving the stab. So if changing incidence changed the gradient you would have a different force gradient everytime you retrimmed the airplane. In this case we are talking about tiny chages to the incidence, say a degree or 2.

In any case, changing incidence would certainly not stop the airplane from being on the back side of the power curve at a given speed, as was being claimed.
 
In the hinge moment wind tunnel testing I have seen, the Hm is a reasonably linear function of surface deflection, be it elevator or tab, and surface aoa, with limits. To change that gradient we play with beads, tabs, wedges and other toys to tune the forces. I don't recall surface aoa, which in this case would be the same as tail incidence, having an impact on gradient. On the airplanes I've worked on, you trim by moving the stab. So if changing incidence changed the gradient you would have a different force gradient everytime you retrimmed the airplane. In this case we are talking about tiny chages to the incidence, say a degree or 2.

In any case, changing incidence would certainly not stop the airplane from being on the back side of the power curve at a given speed, as was being claimed.

To the second point, yes of course you are right, the incidence has nothing to do with being on the back side of the power curve. I think the comment was intended to convey a subjective stick feeling. A reversed stick force gradient feels like the slower you go, the more you have to push forward - as if you are fighting a tendency to slow down more. I think you can see the parallel notion to a case where the slower you go, the more power you have to add.


In the case of trimming by moving the stab, you are correct, the stick force gradient doesn't change. The elevator deflection angle for zero moment does change (which is why it works to "trim" the airplane). The deflection angle for zero moment is affected by the airplane angle of attack and the stabilizer angle.

In the case of trimming the airplane by moving a trim tab on the elevator, the elevator deflection angle for zero moment is affected by the angle of attack, the stabilizer angle, and the trim-tab angle. But also, because of the competing contributions to the hinge moment from the elevator and the trim tab, the hinge-moment gradient also changes. Please refer to Perkins and Hage Aircraft Performance, Stability and Control, section 6.7. Adjusting c.g. and/or adjusting stabilizer angle so that nose-up trim tab deflection is required for trim causes stable stick-force gradients (see fig. 6-18 and 6-19)

The upshot of all this is that increasing stabilizer incidence (leading edge up) will increase the amount of nose-up trim-tab deflection, and will improve stick force gradient at low speed. So to the OP, if your elevator horns are sticking up a lot at high speed, I think you are going in the right direction if you want to raise the leading edge of the stabilizer a little -- it will help, not hurt, the stick-force gradients.
 
Now the Million Dollar Question

Ok so it seems that this may be something to consider if building a 9 and potentially with other models?

What are other 9 drivers seeing? Seeing the same thing and effect at cruise along with trim effects at slow speed?

If that is the case, Why don't the plans and instructions call out an increased leading edge Horizontal stab incidence or shim? Or is it just a minor nuisance/preference?

I'm at a perfect place in the build to modify this, however, I think it unnecessary to change OEM design.

Thoughts?
 
If that is the case, Why don't the plans and instructions call out an increased leading edge Horizontal stab incidence or shim? Or is it just a minor nuisance/preference?

Because the elevator horns being a large amount out of alignment is not a common issue, but as mentioned previously, a small mount is specifically designed in and expected.

These are airplanes that are the majority of the time hand built built by amateur builders. This induces the chance of numerous minor variations that can have an influence on the flight specifics of each individual airplane.

Because of this, it would be foolish to make a change in H. stab incidence while building.
 
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