BH 4A Horizontal Stab Incidence/VG Questions

rv8bldr

4A Scratch Builder
Hi all

I've been flying my scratch build 4A since February (except for a three month, 17,000km/10,500mi across Canada campervan trip this summer) and have a question for those of you who have a.) set their horizontal stab incidence to something other than what Bob recommends in the plans and/or b.) have put VGs on the underside of their hstab. I am finding the pitch ... heavy ... on mine. That might just be perception as most of my PIC time is on an RV-8 and a Maule M5-235C.

Here are (what I think are) the relevant numbers and notes on my build:

Engine/Prop: O-540/Hartzell 2 blade
Empty Weight: 1546lbs/701kg (IFR panel)
Empty CG: 11.5in
Typical weight and CG: 2170lb 15.4in
Hstab type: wood profiled ribs
Hstab incidence: -3deg
StolSpeed VGs: On wings, not on Hstab

Questions:
- If you have your Hstab at something other than 4deg (flat) or 3 deg (profiled) what testing/analysis lead you to that number?
- What were the benefits (consequences) of the lower incidence angle?
- If you have profiled ribs on your hstab, did you fit VGs as well ? If so, as above, what were the benefits (consequences) ?

The slowest I can trim the aircraft is a little above 60kts so I have to hold back pressure on the stick to get it any slower on short final. I'm afraid that, if I decrease the hstab incidence to improve the pitch handling at higher speeds that I may just make things worse at the low end...

Any thoughts would be appreciated.
 
Hi Mark, lots of things here. First, it's great to hear that you are getting some hours in your plane, and we'd always love to hear more! I will share some opinions but I would definitely recommend you have a chat with Bob as well. He is extremely knowledgeable about all of this, and also the practical ways to address it. I'll add a lot of words but the short version is you might be able to solve all of this with a little weight in the baggage area.

On running out of Elevator:
First, here's how Bob directed me to test the BH5 for its forward CG limit back when we were testing the prototype. The first flights were a little ballasted, then we would incrementally remove ballast, a little at a time. Each time, I would fly a few landings and see if I had enough elevator authority to get it slow in the flare. When we reached a point where that was no longer the case, that was the forward CG limit. If you were to apply that method, if you aren't able to slow down as you wish on final, it is fair to say that you are forward of the practical limit. Does it improve, or, to what degree does it improve, as you move the CG aft a little? The plane will fly totally differently at the aft limit in many ways, especially elevator authority in the flare, stick force, and pitch stability.

Second, when you say the slowest you can trim is a little above 60 knots, what is keeping you from being able to trim slower? Are you out of chain links or something? If your trim cable tension or friction are insufficient, the trim tab can change its commanded position, so that is something to ensure isn't happening as well, but you probably would have already noticed that. I was out of phase 1 before I noticed this problem, which is why I bring it up.

Third, let's talk about elevator throw/travel. There may have been some differences over the years. My plans set is from about 20 years ago (that sounds older than I expected it to) and it depicts 30 degrees up and 20 down. Do you see the same on your plans, and have you measured your actual travel in a way that leads you to believe that you are getting the full 30 up? Do you have good methodology here? You'll reach a point where more travel won't get you more result, something easily validated in setting up RC models for extreme maneuvers. But it may be worth a chat with Bob to see if he is still recommending 30 and not 35 degrees with the profiled tail. The airfoil shape might allow more deflection before the airflow separates, but again ask Bob about this.

On pitch being heavy:
I wish I had Maule time to compare notes, but I don't. I do have a little RV time and definitely the RV is going to be lighter in all axes. But the Bearhawk shouldn't feel like a truck. Unless you eliminate the servo action of the trim tab, as some have done. Assuming that isn't the case here, the same slim chance of uncommanded trim tab movement could be reducing the servo action. You would be able to see if this is happening by looking at your trim wheel in flight and seeing it move when you make stick inputs.

CG is King
The biggest single factor that is going to drive pitch feel is the CG. If you are flying at your forward limit, you should feel a totally different airplane than at the aft limit. When I am at the forward limit, I can roll to 45 degrees of bank and pull the stick as hard as I want, and I can't exceed 2g. If I'm at the aft limit, I can "bap" the stick in level flight and the plane will strongly pitch up at 2g until I chicken out and intervene.

At a forward CG, the amount of trim displacement required between 60 knots and cruise will be substantial, maybe a 3/4 of a turn of the wheel. At an aft cg, that will be reduced. Theoretically you can get the cg so far aft that you won't really need any trim changes at all as speed changes.

On setting the tail incidence:
We can only really optimize the airplane for a single CG, airspeed, and power setting. When you are flying in your most usual configuration, look back at the tail. Is the elevator in trail with the horizontal stab? Or is it displaced? If it is displaced, adjust the incidence to make it in trail. Once that is done, if you fly at a more aft CG, you can look back and see that your elevator is deflected stick-forward relative to the horizontal stab. If you fly at a more forward cg, you can look back and see a stick-aft displacement. Generally, my understanding is that the baseline incidence setting for a profiled tail was to be 2 degrees, vs 4 degrees for a flat tail. But again this depends on how you want to rig the airplane, because you can only make it perfect for a single set of conditions and everything else is a compromise.

Setting of the tail angle should not impact pitch stick force at all. Setting of the tail angle could impact your low speed pitch authority slightly, in the sense that if you had too nose-low of an angle set, it would require more up-elevator displacement to yield the same number of pounds of tail down force. Naturally this "total pounds of tail down force" is what really matters. Some of those pounds come from the horizontal stab (assuming the incidence isn't zero) and the rest comes from the elevator. Pre-loading some of that into the horizontal stab would ease the airflow transition at the hinge line, but would be terribly inefficient in cruise because you'd be plowing more of a wedge through the sky.

Vortex Generators:
If the loss of pitch authority is due to airflow separation at the elevator hinge line (which it likely is), VGs on the underside of the stabilizer can mitigate that. They can help the air turn the sharp corner and stay attached.

Moving the CG aft reduces the total demand on the HS/Elevator system. In other words, you need less total pounds of tail down force to achieve the same pitch change effect. Many owners of very nose-heavy planes find that they are more delightful for solo ops with a little weight in the tail. This applies to big-engine Bearhawks, C182s, etc. Airplanes are all about compromise, and configuring to optimize for a single mission often costs us optimization for another mission.
 
Without repeating anything which Jared has covered above, here's a few more datapoints for you:

In summary, I think your aircraft is performing as-designed, and your CG location is very useful.

I would not say that a "heavy elevator" = running out of trim. It sounds like you are running out of trim.

During the build, I biased my trim system towards downward tab deflection, it can travel -150% down and +50% up. I need maximum trim at full forward CG, to fly a stabilised approach at 1.1 Vso, and I still need to hold a little back pressure to really hold the plane at minimum speeds - around 37 KIAS (as slow as it'll fly), and even then - I need some pilot-trickery to really slow it up - explained below.

Your questions answered:
- Preamble, my setup is very similar to yours, all specific details are materially the same, apart from CG. My CG is a few inches further aft - but I would pay $$$$ to have it closer to your CG location!
- I have -2 degrees stab profile, chasing more airspeed / less drag, it was the recommended angle at the time (per Bob's design) for those with a profiled tailplane. I don't think the angle is your issue.
- I don't have VGs fitted under the tailplane. I looked at gap seal tape which I hear has the same effect, but practically I haven't bothered doing either.

As Jared says, the trim required varies with CG location. At very forward CG locations, you are going to need a little thrust over a stock elevator setup, to maintain control authority. You also need all the trim tab deflection you can get.

People have obsessed over engineering a solution to remote or mitigate this design feature, but there's always a trade-off and "no free lunch" in aviation. The easiest solution is ballast - moving your tie-downs, emergency gear, covers etc. to keep CG aft. But I hasten to add, it's not hard to fly a stabilised approach with a forward CG, using with good technique. It's a skill pilots benefit from practicing.

My trick is slowing it down behind the drag curve (indicated airspeeds below 45 kts with maximum flap), pulling the nose up to increase drag further, then applying a trickle of power to keep the elevator effective and flying - plus stabilising the approach, without accelerating. It's not great for the sight picture, but it's a great STOL approach. Clearly you need to have ample control of airspeed and pitch for this to work. It's also unsuitable for crosswind landings or situations where you need to see exactly where you're going to impact the ground. But it's a very relevant skill nonetheless.

In terms of elevator weight in general - in my experience - the elevator should feel lighter than aileron, but not as light as the rudder. It's not as heavy as a Cessna or Piper, but it's no Cub at the other end of the spectrum.
 
Hey Jared and Jonathon

Thanks a lot for your input. It is greatly appreciated getting advice from two of the most experienced Bearhawk guys around.

Ballast: I do have about 25 lbs of ballast (tools and The Claw tie down kit) tied down in the aft cabin, but maybe I should add some more

Elevator Trim: I have electric trim (Ray Allen servo) but it drives the Bob-designed trim system (torque tubes out to arms which then drive the tabs). I think I just have the plans-specified throws but I am going to figure out how to get it closer to what Jonathon uses. Fussing around with the pushrod length from the servo, and then the arm lengths to the tabs should allow me to do that.

One thing I noticed is that as you get closer and closer to full up elevator (on the stick) with full nose-up trim, your amount of nose-up trim (downward deflection of the tab) actually decreases due to the geometry of the pushrod. Not a lot, but some.

Flaps: The slowest I have had the airplane down to is about 45kts and I haven't yet put FULL flaps on. I haven't needed it (or so I thought) and it takes a LOT of pull to get it there. However, I am now going to try that out at altitude and see how it goes. As you (Jonathon) say, I won't be able to do it until I get the thing slowed down below 45kts which I haven't yet (required too much stick pull to be comfortable and keep the airplane stable)

I am also going to change the Hstab incidence to -2deg. I had found conflicting advice on the interwebs and had settled on -3deg to be safe. However, being a retired software engineer, I don't want to change more than one thing at a time so the plan will be:

1. Change the Hstab incidence to -2deg. Test fly and, if all good, do a slow flight/stall sequence.
2. Assuming all good, modify the trim system rigging to get more nose-up trim. Test fly. If all good and I can get < 45kts , perform slow flight with FULL flaps.
3. If I am still not happy with the pitch weight, add more weight 25 lbs at a time to the aft tie down (just in front of the rear cabin bulkhead) test flying each time.

Comments are more than welcome from anyone.

BTW, I took my first passenger in the Bearhawk the other day and he made a short video of it if anyone is interested. Thankfully he didn't include the landing because it was ugly :-)

 
The electric trim changes the discussion and I would definitely focus there for resolving the pitch issue. I'm not sure there have been published trim tab travel specs? With the elevator in trail with the horizontal stab, my takeoff trim position is with the tab also in trail with the elevator. My full nose up travel, measured when the elevator is in trail, is 1.5" measured from center to center at the trailing edge tubes. I could use a little more at times but it's mostly good enough. I'm curious what your travel is like while on the ground, and wonder how confident are you that the inflight displacement is also true to the zero airspeed displacement. Like is there any flexing going on in the linkage between the servo and torque tube horn that is allowing the tab to relax its position?

Your flap force experience matches mine. The last notch requires a very low speed. I find that the Bearhawk flaps mostly increase drag and just slightly increase lift. Stall speed goes down a little, but at the larger displacements they are most useful for diving over an obstacle without building speed.

There is a lot of good wisdom in Battson's post above especially concerning thrust. With our powerful engines and big props, we can easily increase the airspeed at the tail well above the airspeed at the pitot tube by adding a little power. Stalls with "mag check" throttle vs idle throttle are totally different.
 
Hi Mark,

Some excellent advice above. My Bearhawk is similar to yours (although 4B): empty weight 1,508 lbs and empty CG 10.5”. I noticed the same characteristics early on, particularly limited nose-up trim and elevator authority at very low speeds and low power with the CG forward of about 14”.

The heavier IO-540 installation tends to put the empty CG further forward than an IO-360. That gives more scope to carry heavier loads aft before reaching the aft CG limit, but may require ballast when lightly loaded. For me, that loading flexibility is a worthwhile trade-off.

I’ve tried several changes:
  • Biasing the trim for more nose-up trim helped the trim range, but didn’t resolve the elevator authority limitation.
  • VGs beneath the tailplane gave a small improvement.
  • Changing tailplane incidence from −3° to −4° gave a minor handling improvement, but wasn’t a cure. The main aim was better elevator alignment in cruise at my usual forward CG.
The biggest improvement came from carrying about 50 lbs of survival and tie-down gear aft. Flying without it recently, at 13” CG, I needed full aft stick and more power on landing, with a higher touchdown speed and longer rollout. With the kit aboard and CG at 14.7”, low-speed handling was noticeably better.

Mine feels best around 16” CG, and I now treat 14” as my practical forward limit for normal flying. I still value the flexibility the forward empty CG gives me when carrying heavier loads.

In your position, I’d explore the advice this forum gave me: check where around 50 lbs of securely restrained weight aft would put your weight and balance, using your mate and half fuel as the loading example. (Loved the vid, by the way!)

Assess the handling at a safe altitude first, then establish an approach speed that retains a comfortable margin of elevator authority at low power. A modest CG shift made much more difference on mine than any of the other changes.

Cheers !
 
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Elevator Trim: I have electric trim (Ray Allen servo) but it drives the Bob-designed trim system (torque tubes out to arms which then drive the tabs). I think I just have the plans-specified throws but I am going to figure out how to get it closer to what Jonathon uses. Fussing around with the pushrod length from the servo, and then the arm lengths to the tabs should allow me to do that.

Flaps: The slowest I have had the airplane down to is about 45kts and I haven't yet put FULL flaps on. I haven't needed it (or so I thought) and it takes a LOT of pull to get it there. However, I am now going to try that out at altitude and see how it goes. As you (Jonathon) say, I won't be able to do it until I get the thing slowed down below 45kts which I haven't yet (required too much stick pull to be comfortable and keep the airplane stable)
You are too kind!

I found these comments really interesting. I have almost exactly the same trim system, based on your description, down to the servo make.

I always use full flaps for every landing, apart from crosswind landings where I use 50% flap. I also rigged my flaps for max extension. The Bearhawk just lands better and stops flying more positively and completely, with maximum drag and slower touchdowns. I normally "flop" it onto the grass at about 37 KIAS, rather than dancing delicately down the surface, but I would not advocate that approach on a sealed or paved runway.

Perhaps off topic now, but I found the flaps were problematically heavy on the last notch. It's been a journey but I am happy with it now. I needed to get a professional to weld on a longer flap handle, to reduce the force required to extend the flaps.

Now I can deploy full flap at 55 KIAS as Bob specifies in his design, easily without a second thought. Compared to the original handle, which was pulling my aging shoulder muscles out of place. Especially on hot days where I get a little sweat on my palms.
 
Well, you guys rock.

Trim throws: it turns out I only had 1 1/8" (28mm) of "up elevator" trim throw. The way I have the Ray Allen servo is connected to the trim arm is through a custom pushrod, threaded at either end, and fork ends (the trim servo is mounted on a plate underneath the HStab forward attach). I only made the bottom arm of the trim arms in the centre fuselage (scratch build...), and drilled a hole at the plans defined spot towards the end of the arm. Just on spec, I had drilled another hole about halfway up the arm towards the centre in the off chance I might need it some day. Today was the day. I moved the trim pushrod to the inner hole and adjusted the fork ends to give me the same centre trim position. Full "up elevator" trim now resulted in 1 3/4" (44mm) of throw.

HStab incidence: I increased the incidence from -3deg to -3.5deg (or is it decreased because we're going negative ? :-)

With just those two changes my full "up elevator" trim speed with a little bit of power is about 45kts and it still feels pretty solid. I'm going to do more experimenting at that speed and 3 notches of flap before going full flap. The nose is REALLY high and I need to get comfortable with it before using it in anger near the ground. That speed and nose angle feels VERRRRY WROOONNG after 600 hours of RV-8 flying. Even my Maule was nose heavy and I couldn't really get it that slow (it had a big 3 blade McCauley on the front...) Just need to practice.

So far I haven't felt the need for more weight in the back. As I said, I have about 20 lbs at the back of the cabin right now and I'll play with that if I feel I need to.

As a performance data point, I normally tool around at 55-60% power because AVGAS is so frickin' expensive. When I got my engine back from overhaul I took off the carb and put on a used RSA/Bendix fuel injection system from an RV-10. With that, and the 6 channel EIS from Garmin, I can lean pretty aggressively and keep all of temps reasonable. So, what does that mean for speed/fuel consumption? I average about 110kts/126mph/203kph on 9.7gph/37lph. I am a bit schizophrenic on the units because, even though Canada is a metric nation, everything aviation here is Canada is mph/feet except for fuel purchases, which is in litres. I show lph on my G3X because that is how I buy my fuel and have calibrated my tanks.

As I get some more super slow flight experience, and finally take this thing on a $100 hamburger trip, I'll post more data. I've only got about 40 hours on it so it is early days yet....

Cheers
 
With just those two changes my full "up elevator" trim speed with a little bit of power is about 45kts and it still feels pretty solid. I'm going to do more experimenting at that speed and 3 notches of flap before going full flap. The nose is REALLY high and I need to get comfortable with it before using it in anger near the ground.
That sounds pretty much bang-on, to me.
 
110kts true. About 106kts indicated (I think I was around 3000' asl). I'm going to redo my cruise/performance testing with the adjusted Hstab incidence and figure out my optimum xcountry settings.
Those figures match mine, LOP.
If I'm going somewhere then 115kts if I increase to 10.5gph.
ROP 120-130kts, but I don't usually cruise ROP, it's hard on the wallet.
 
Another performance datapoint. I went on my first (short) cross country in the Bearhawk yesterday (about 100nm/185km) to a flyin.

24"MP/2000RPM
3500'
11C
120kts TAS
10.5gph/40 lph LOP

Good enough for me

Note that I have girly size 7.00x6 tires so that makes a difference. I'm not manly (or rich) enough to put those giant Bushwheels on it :-)
 
Another performance datapoint. I went on my first (short) cross country in the Bearhawk yesterday (about 100nm/185km) to a flyin.

24"MP/2000RPM
3500'
11C
120kts TAS
10.5gph/40 lph LOP

Good enough for me

Note that I have girly size 7.00x6 tires so that makes a difference. I'm not manly (or rich) enough to put those giant Bushwheels on it :-)
Keep working at it. I think you can squeeze 130 KTAS out of it, for around 40L/hr, in round numbers. That's one of the main advantages of the smaller tires, you get to go fast for cheap :)
 
Keep working at it. I think you can squeeze 130 KTAS out of it, for around 40L/hr, in round numbers. That's one of the main advantages of the smaller tires, you get to go fast for cheap :)
Absolutely, although I question your definition of "cheap" 😉😆
 
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