Gear leg fairings instead of rudder trim tab?

In spite of the wedge rudder trim tab that came with the rudder, I'm having to apply a noticeable amount of right rudder.

The RV-4 builder's manual mentions that misaligned gear leg fairings can produce a significant deflection of the skid ball.

My question is: Can I use intentional misalignment of one or both gear leg fairings instead of an (additional) rudder trim tab?

Having a hard time visualizing which way they should be adjusted.

But also, what are the aerodynamic consequences (drag?) of doing this?

Finn

Is the trim tab on the correct side of the rudder? A tab on the left side adds right rudder.

Yes - gear leg fairings can significantly put the ball out if they are not properly rigged. And yes this does create unwanted drag.

Before you go chasing a fix I suggest you first identify the problem. Go fly with no wheel pants or gear leg fairings and see where the ball ends up.

Carl

Carl Froehlich said:

Is the trim tab on the correct side of the rudder? A tab on the left side adds right rudder.

Yes - gear leg fairings can significantly put the ball out if they are not properly rigged. And yes this does create unwanted drag.

Before you go chasing a fix I suggest you first identify the problem. Go fly with no wheel pants or gear leg fairings and see where the ball ends up.

Carl

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Wedge is on the left side.

Have only flown without fairings and pants so far.

So, would the unwanted drag be worse by using fairing alignment than using a bigger trim tab?

Finn

FinnFlyer said:

Wedge is on the left side.

Have only flown without fairings and pants so far.

So, would the unwanted drag be worse by using fairing alignment than using a bigger trim tab?

Finn

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It seems you have a root issue to resolve. I would first look at other rigging issues. For example:
- Is the VS really aligned fore and aft? Some tight strings and careful measuring will tell the tale.
- Do you have a roll issue as well?
- Any reason to think the engine is not at the right angle?

If all this checks out, I’d go with a larger trim tab instead of messing with gear leg fairings.

Carl

Thrust offset?

Carl Froehlich said:

It seems you have a root issue to resolve. I would first look at other rigging issues. For example:
- Is the VS really aligned fore and aft? Some tight strings and careful measuring will tell the tale.
- Do you have a roll issue as well?
- Any reason to think the engine is not at the right angle?

If all this checks out, I’d go with a larger trim tab instead of messing with gear leg fairings.

Carl

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Took quite a bit of care aligning the VS, but guess I could recheck it.
No roll issue noted. But then the Dynon (non-clutch, stepper motor) servos do have some friction and stick tends to stay where it's placed.

Perhaps the 1.25 degree thrust offset specified in drawing 33 is way to small?
I did ask here about the unequal left/right spinner clearance (3/16" left side, 3/8" right side) but was assured that that was "normal" with Van's cowlings.
Running a 76x88 prop. Don't know of that would make a possibly too small offset more of an issue?

At this point trying to increase the thrust offset (even with just adding washers at the firewall) might require front cowl rework. Right now the spinner is nicely centered left-right. Also not sure what that would do to wheel alignment.

But I would like to know what others with 180 HP engines have in terms of thrust offset angle.

Finn

FinnFlyer said:

In spite of the wedge rudder trim tab that came with the rudder, I'm having to apply a noticeable amount of right rudder.

The RV-4 builder's manual mentions that misaligned gear leg fairings can produce a significant deflection of the skid ball.

My question is: Can I use intentional misalignment of one or both gear leg fairings instead of an (additional) rudder trim tab?

Having a hard time visualizing which way they should be adjusted.

But also, what are the aerodynamic consequences (drag?) of doing this?

Finn

Click to expand...

The short answer is YES gear leg fairings can cause yaw and yes they cause aerodynamic drag. Both are true but we try to minimize it. Using them to adjust for something else that is out of trim or rig will cause drag but it may be very small. However everything hanging out in the breeze is drag regardless.

When you say "noticeable amount of right rudder", you mean in level cruise flight? Right? How is roll trim? Where is the elevator trim tab in level cruise flight (solo)?

When finishing my first RV-4 long ago. I endlessly adjusted, rigged to get wings level in cruise with no trim tab. I tried to get elevator pitch trim tab to have little to no tab deflection in cruise (solo). Last I wanted yaw zero in cruise flight without a tab. I was mostly successful. I ended up with a tiny rudder wedge. No shame in a small rudder trim tab.

GEAR LEGS are forward of CG. So looking down on the airplane, if you move gear leg trailing edge to the right it will will swing the nose LEFT (like pushing on the left rudder pedal and and making the skid ball go right.If you move the gear leg fairing trailing edge to the LEFT (looking down at the airplane) the opposite will occur. So if you have to push on the RIGHT rudder to make the plane not skid in wings level, level cruise flight, you would move one or both gear leg fairings trailing edges to the left. This pushes nose right.

Keep in mind (not to get too complicated) the spiral slip stream of from the prop will cause different effect on the left and right gear leg fairing and vertical Stab and Rudder even in level flight. So it is normal for the noise to be slightly nose left (slightly). The yaw will change with airspeed, but you are saying "noticeable amount of right rudder."

Check all your rigging. Make small adjustments. Take good flight test notes. Change one thing at a time. Have a flight test plan written out and don't get to focused on the yaw and not fly the plane. Good LUCK (and always fly safe).

PS LEFT TURNING tendencies (slip stream, gyro precision, torque and P-factor) is always there. In level flight it is mostly (spiral) slip stream and torque. Monster rudder tabs on RV's should not be necessary, assuming the vertical stab was installed per plans and the airframe is straight you should not have to apply noticeable rudder in level cruise.
Aileron and flap rigging can cause roll which can also induce yaw... so double check all the rigging.
Often over looked is engine mounting. Some times you need to SHIM engine to get thrust line correct, due to small tolerances in engine mount and firewall. The thrust line should be straight or slightly right to cancel left turning.

Whatever you do try to find neutral of gear leg fairings. By neutral I mean the chord line of fairing is in line with with airframe airflow in level flight. Air flow around plane is complex and variable, but for reference we use wing chord line or top edge of cockpit longeron. Finding the level chord line of gear fairings is not easy, due to gear leg angles and taper of fairing. Make sure when setting your references with no weight on wheels per plans. Set up some repeatable reference system so you know what you have now and what you are adjusting to. I chased my tail a few times rigging. Be methodical. Do easy stuff first. Don't adjust and pray. Do you flight test and record results. Wash, rinse, repeat. Again make sure the whole plane is in rig. If you did not build it check wing incidence angles and vertical stabilizer. Sometimes planes are built a little off (they are armature built after all.)

I bonded and glassed on wood stiffeners to my gear legs which set the fairings position in stone (by the way stiffeners made the plane feel solid and had no shimmy ever). I had no adjustments of fairing after that. However I flew the plane with no gear leg fairings or wheel pant fairings (which can cause yaw). So I knew the affect before and after. In the end a tiny wedge was bonded to the rudder. Consider taking the fairings off (gear leg and wheel pants) and flying like that. If you still have big rudder input with gear/wheel fairings off during level cruise flight you might have other issues.

FYI I had the original metal gear leg fairings which were narrow chord. Ones on later models were wider slightly. The aftermarket TEAM ROCKED gear leg fairings are even wider (chord) and have a greater effect in all axes. This is what I have for my RV-7. Richard Vangrunsven does NOT approve or like wide chord gear leg fairings on his airplanes designs, because it reduces the effectiveness of the rudder to get out of spins. The gear leg fairings act opposite of vertical stabilizer and reduces yaw stability. However the wide fairings do reduce drag and look cool. I did NOT fool with my gear leg fairings on later planes. I just tried to get the "straight" or parallel to wing chord line. Any other adjustments are done with vert stab, trim tab, rigging overall or engine thrust line as needed.

Thanks George.

Still have not finished and mounted the gear leg fairings (old Van's alum).
Need to do more testing at cruise speeds. Just noticed it at level flight at 120mph (speed I picked for testing cooling improvements) and occurred to me that using those fairings instead of rudder trim tab might be an option.

I don't understand how you can adjust the fairings left/right. I can only see adjusting the trailing edges up/down.

So far my attention has been on improving cooling. I picked smaller than "normal" radiators (lighter) and am paying for that by having to make incremental cooling improvements to make it cool during continuous high power climbs as OAT gets warmer (90F). Would have been easier to pick bigger (and thus heavier) radiators and then optimizing airflow to reduce drag. Leave it to me to do it the hard way

Finn

gmcjetpilot said:

With that said I had the old metal fairings which were narrow. The later ones are wider. The aftermarket TEAM ROCKED gear leg fairings are even wider (chord). They will have a greater effect. This is what I have for my RV-7. Richard Vangrunsven on his design because it reduces the effectiveness of the rudder to get out of spins.

Click to expand...

George,
1. What is this aftermarket team you're talking about? One of my (many) next projects is new maingear fairings and pants on my -6A.
2. I'm assuming you're saying RVG doesn't like the larger chord fairings?

LouFly said:

George,
1. What is this aftermarket team you're talking about? One of my (many) next projects is new maingear fairings and pants on my -6A.
2. I'm assuming you're saying RVG doesn't like the larger chord fairings?

Click to expand...

The whole long story, is there is a variation of the RV-4 called the Harmon Rocket II. It puts an (I)O-540 on the nose with new engine and gear mount. I recall it widens and stretches' the fuselage (which has a turtle deck), and the rest of it is RV-4. So you buy an RV-4 kit (when you could and ask to delete parts you did not need) and buy the Harmon II kit. You marry it together and make a Harmon Rocket II. Later a guy in Texas made a whole Harmon Rocket II kit and called it the F1, Team Rocket was born. You did not have to buy parts from Van's. I don't recall if Van was unhappy about all the shenanigan's, but I seem to recall they did stop selling RV-4 kits to Rocket builders with parts deleted (not needed on the Rocket). It is a much longer story but that is the background.

Team Rocket made not all the metal parts of the kit, but they made fiberglass parts like cowls, wingtip, ear leg fairings and wheel fairings. They were similar but different than Van's. The wing tip fairings would go on any 2 Place RV wing since the ROCKET or F1* used the same wing, and they were rumored to be a better shape. They looked cool. The gear leg fairings I talk about (wide chord) are not only wider to accommodate the longer Rocket gear but wider. They also can be trimmed to work on an RV 4. 6. or 7. Not sure about A-models. A wider chord gear leg fairing should have less drag. No one has proved this. They do look faster.

Team Rocket is kind of around and there web site is https://www.f1aircraft.com/
You can check to see if you can buy parts. I don't know.

My advice seriously is stick to Van's parts and plans. Use vans stuff and get'er flying. You start out sourcing and customizing you add time and money to the build. If you want to let your creative hair down do it with the paint, upholstery and panel. Put an aircraft engine and prop in per plans and don't mess with it. It ain't broke, don't fix it. Also the Model-A's, trikes, have the main gear behind and close to the CG, so the gear leg fairings affect yaw to a lesser degree....


* Fun fact Team Rocket when they were full steam ended up making quick build F1 kits. They then made a new model with their own wing design called the F1 Evo. It was a totally new wing with laminar flow airfoil and tapered chord and tapered thickness. It was raced at Reno and did OK but never realized a significant increase in speed as expected, but it was faster but not as good overall. Of course it was a lot harder to build and more expensive. In part the reason it did not perform as well as expected, was choice of laminar airfoil used, which was similar to the Glasair III. It is a long story but it was not as good of an airfoil as they thought on the F1 Rocket or Glasair III for that matter. Many laminar flow airfoils under perform in the real world due to many factors. Laminar flow is not the be all end all in all flight conditions. There are trade offs and Pros/Cons. The RV wing is a NACA 23000 series (23013.5) and has been used since the 40's. Many great well known and loved planes use the NACA 23000 series airfoil. Usually it is a NACA 23012 (12% thickness to chord, RV's are 13.5% for structural reasons). Some of the planes that use the same airfoil as used on the RV are the most iconic and best loved amazing planes ever, from DC-3, Bonanza to even the Citation I Biz jet (inner wing), on and on. It works and was used a lot. I know over the years new airfoils that are going to set the world on fire come and go, but this airfoil is still around. Richard Van is no dummy. He picked a good airfoil.

Chickened out and aligned the fairings and wheel pants as per plans and manual.

No noticeable difference in flight after adding them.

Still thinking that I didn't offset the engine enough.

Finn