How to Stop Tailwheel Shimmy

Tailwheel shimmy is typically caused by caster angle misalignment. Changes in passenger, baggage, and fuel load can significantly affect the tailwheel geometry.

Persistent shimmy can cause excessive wear on the tire, tailwheel assembly, attachment hardware, and fuselage frame.

Steps to Prevent Tailwheel Shimmy:

1. Check Tailwheel Caster Angle

• The pivot spindle should be perpendicular to the ground at gross weight and in the aft-most loading configuration.

• If necessary, increase the caster angle using spacers:

  • 1/8-inch aluminum spacer (used with Fabreeka pad)

  • 1/4-inch aluminum pad (used without Fabreeka pad)

• Do not re-arch the tailspring (heat-treated). Instead, replace on condition.

2. Inspect & Replace the Fabreeka Pad

• The Fabreeka pad between the leaf spring and tailpost is often missing or deteriorated.

• Original thickness: .232 inches (compresses slightly after installation).

• Replace the pad if worn or missing.

3. Consider a Heavy-Duty Tailwheel Spring (8GCBC Only)

• The main leaf thickness increases from .262 to .323 inches.

• Available as a retrofit option for the 8GCBC.

4. Check for Other Common Issues:

✅ Loose Tailwheel Attach Bolt – Torque AN7 attach bolt to 40 ft-lb ✅ Missing Bushing at Attach Bolt – Use 7/16-inch ID bushing ✅ Incorrect Steering Springs – Use tension-type 2-1525 springs ✅ Low Tire Pressure – Inflate tailwheel to 40-45 psi

By addressing these issues, you can eliminate tailwheel shimmy, improving handling and reducing long-term wear.

Resources on Tailwheel Shimmy

Below are some useful resources that explain the causes of tailwheel shimmy and provide solutions to eliminate it.

• Taming the Tail Wheel Shimmy – This article covers the common causes of tailwheel shimmy and offers practical solutions to reduce or eliminate it. 👉 Read the article

• Tailwheels, Pavement, Shimmy Oh My! – A forum discussion where pilots share their experiences with tailwheel shimmy, including troubleshooting tips and recommendations. 👉 Join the discussion

• Interesting Article on Tailwheel Shimmy – This thread on Backcountry Pilot explores various factors that contribute to shimmy, including geometry, tire pressure, and component wear. 👉 Check out the thread

These resources provide valuable insights into diagnosing and preventing tailwheel shimmy, helping to maintain smooth and stable ground handling.

Recommended Tire Pressures

Tire pressure varies depending on tire size. Use the following recommended pressures for optimal performance:

Main Tires:

• 6.00 x 6 → 24±2 psi (8KCAB: 29+2/-0 psi)

• 7.00 x 6 → 24±2 psi

• 8.00 x 6 → 23±2 psi

• 8.50 x 6 → 18±2 psi

• Alaska Bush wheels → 8 to 20 psi

Tailwheel:

• 40±5 psi

Ensure proper inflation to maintain handling, tire longevity, and safety.

Uneven Fuel Burn from Wing Tanks – Troubleshooting Guide

An imbalance of up to ¼ tank is considered normal. However, if the difference exceeds ¼ tank, there may be an issue with the fuel system or aircraft rigging. Follow these steps to diagnose and correct the problem:

1. Check Fuel Cap Gaskets

• Inspect fuel cap gaskets for cracks or leaks.

• A damaged gasket can create a pressure difference between tanks.

• Troubleshooting Tip:
  

  • Swap the left and right fuel caps and observe if the imbalance follows the cap.

  • If so, replace the gaskets.

• Improving the Seal:
  

  • Apply a small amount of fuel lube to both faces of the gasket.

  • Recommended: EZ-Turn Lubricant (Aircraft Spruce, P/N 09-00306) or an equivalent fuel lube.

2. Verify Check Valve Function

• Test each check valve with ½ tank or less:
  

  1. Remove one fuel cap.

  2. Apply lung pressure to the under-wing vent – the check valve should release and vent into the tank (some chattering is normal).

  3. Apply suction to the under-wing vent – the valve should allow for expansion in reverse, significantly reducing outflow from the tank.

  4. Repeat the test on the opposite check valve.

  5. If there is a noticeable difference in airflow or pressure between the tanks, replace both check valves with a matched set.

3. Consider Flight & Parking Conditions

• Fuel imbalance can result from:
  

  • Parking on a slope

  • Sustained uncoordinated flight

• In normal cruise, ensure: ✅ Wings are equal distance above the horizon ✅ No yaw present ✅ No aileron or rudder pressure required for level flight

If the aircraft flies wing low or requires constant control input, rigging adjustments may be necessary. See the rigging guide for further correction.

By following these steps, you can identify and resolve uneven fuel burn issues, ensuring balanced fuel consumption and safe flight operations. 🚀

1. Verify Cable Tensions & Controls

• Check cable tensions and control travels per the aircraft service manual.

• Ailerons should have a slight droop on the ground (airloads will align them in flight).

• Flaps should align with the bottom of the wing.

• Apply slight upward pressure at the trailing edge to ensure flaps rest against the up stop during adjustment.

2. Set Rear Struts for Neutral Wash

• Ensure no wing twist from the first rib outboard of the tank to the first rib inboard of the tip rib.

• If using a digital level, adjust the clevis on the rear strut to remove preload.

3. Adjust Spade Plates with Neutral Ailerons

• With ailerons neutral, align the spade plates parallel to the wing bottom.

• If needed, remove spade plates temporarily for better rigging.

• Spade plates only affect control feel if grossly misaligned.

4. Set Rudder Tab & Lubricate Tailwheel

• Set the rudder trim tab straight (flat); final adjustments come later.

• If rudder trim issues persist, check for:

  • Unequal rudder return spring tension

  • Tailwheel friction (lubricate and adjust castle nut preload for smooth pivoting).

5. Evaluate Aileron Position in Level Flight

• In cruise flight, shake the ailerons slightly to let them settle.

• Compare the inboard edge of the aileron to the adjacent wing rib.

• Note displacement:

  • If the left trails up, the right will trail down.

• Adjust by bending the ailerons trailing edge in the same direction as displacement.

  • Use both hands to apply gentle, even pressure.

  • A small twist creates a large change.

  • If necessary, use a wide flange tool near aileron ribs for extra leverage.

6. Correct Roll Tendency

• In level flight at cruise power, check for roll tendencies.

• Ensure the aircraft is not slipping or skidding while testing.

• If the aircraft rolls left:

  • Shorten the left strut clevis or lengthen the right.

• Adjust in 1-2 turn increments, then refine with ½-1 turn adjustments.

• Repeat until there is no roll tendency.

7. Adjust Rudder Trim for Yaw Correction

• In cruise flight, shake the rudder pedals slightly to let the rudder settle.

• If the aircraft yaws left, bend the rudder trim tab to the left.

• A small tab adjustment results in a large correction.

Final Flight Test

Your aircraft should now fly straight and level in cruise: ✅ Wings level – Equal distance above the horizon ✅ No left or right roll ✅ No yaw tendency

Fine-Tuning

If small corrections are needed:

• Adjust the rudder trim tab

• Modify rear strut length

• Fine-tune aileron trailing edge

⚠️ Note: At different airspeeds or power settings, minor control inputs may still be required to maintain level flight.

Yes, snap rolls are an approved maneuver. However, to minimize unnecessary wear and tear, follow these guidelines:

• Respect the aerobatic gross weight:
  

  • 8KCAB & 7GCBC: 1,800 lbs

  • Other Citabria models: 1,750 lbs

• Follow entry speed limits:
  

  • 8KCAB: 90 mph

  • Citabria models: 85 mph

• Limit acrobatic fuel load to half tanks or less

Wing Structure & Fuel Tank Considerations

A common follow-up question is about the impact of snap rolls on the wing structure and fuel tanks. Any aircraft used for aerobatics will have increased maintenance requirements, which owners and operators should expect.

American Champion Aircraft (ACA) has implemented several design improvements over the years to enhance durability and address structural concerns:

Modifications to 8KCAB Wings & Tanks

• 1996:
  

  • Increased butt rib thickness (.020 to .032 inch)

  • Doubled upper gap skin rivet spacing

  • Added an airworthiness limitation to strut fittings

• 2002:
  

  • Added four slosh holes, two welds, and a doubler to the tank center baffle

• 2004:
  

  • Changed tank beading from rolled to stamped with a deeper draw

  • Upgraded center baffle material from 3003-O to 5052-O

• 2010:
  

  • Revised fuselage fairings to reduce strain on the upper wing gap skin

Should You Avoid Snap Rolls?

These changes have significantly improved durability, but the exact impact is difficult to measure.

• Some operators, especially flight schools, prohibit snap rolls to reduce maintenance costs. Given that each student may perform the maneuver multiple times, this is a practical decision for high-usage aircraft.

• For a typical owner, snap rolls will not pose a significant maintenance concern.

By following the recommended limits and understanding the aircraft’s design evolution, pilots can confidently perform snap rolls while managing long-term maintenance.

A sticking manifold pressure gauge is often caused by fuel dye accumulating in the adjustable damping screw, especially due to excessive priming. Here’s how to address the issue:

Step-by-Step Cleaning Process

1. Remove & Clean the Manifold Pressure Line
   

   • Disconnect the 1/8-inch copper manifold line from both the engine and gauge.

   • Blow out the line with compressed air.

   • For better long-term performance, flush the line with a solvent before blowing it out (e.g., avgas, Stoddard solvent, acetone).

1. Access & Clean the Damping Screw
   

   • Remove the gauge from the instrument panel (disconnect fuel pressure line as well).

   • Remove the manifold pressure fitting from the instrument case.

   • Take out the damping screw, clean it thoroughly, and reinstall it.

   • Set the damping screw ¾ turn from the closed position.

1. Additional Cleaning (If Needed)
   

   • If a significant amount of dye is present, clean the orifice with a 7221T29 McMaster or equivalent nylon brush.

1. Reinstall & Test
   

   • Reinstall the gauge, fuel pressure line, and manifold pressure line.

   • Perform an engine run-up and check: ✅ Proper engine operation ✅ Smooth manifold pressure response to power changes ✅ No excessive needle oscillation ✅ No leaks

If the Issue Persists

If the problem continues or requires frequent service, American Champion recommends replacing the gauge with an Aerospace Logic FM200 (STC SA02825NY) or another approved gauge. Installation data is available from the factory.

By following these steps, you can restore proper manifold pressure gauge function and ensure smooth, accurate readings. 🚀