Frequently asked questions
How can I stop tailwheel shimmy?
Shimmy is most commonly caused by the caster angle of the tailwheel. A good description is available at www.pierceaero.net/tws.php. Increased loading due to passenger, baggage and fuel result in substantial geometry changes at the tailwheel. The pivot spindle should be perpendicular to the ground at gross weight and the aft most loading configuration. Persistent shimmy will result in wear and tear on the tire, tailwheel assembly, attachment hardware and fuselage frame.
Verify condition of the fabreeka pad between the leafspring and tailpost. The pad is commonly missing or in poor condition. The pad is originally .232 thick and will compress slightly during installation. Replace pad on condition.
To increase the caster angle of the tailwheel an 1/8 inch aluminum spacer can be used in addition to the fabreeka pad. A 1/4 inch aluminum pad is also available; the 1/4 inch pad is intended to be used without the fabreeka pad. It is not recommended to re-arch the tailspring due to its heat treatment, rather replace the spring on condition.
A heavy duty tailwheel spring is available for the 8GCBC. The main leaf thickness is increased from .262 to .323 inches. If not already equipped, this spring is available for retrofit on the 8GCBC.
Other common problems include the following:
Tailwheel attach bolt loose - torque AN7 attach bolt to 40 ft-lb
Missing bushing at attach bolt - use 7/16 in ID bushing in tailwheel
Wrong steering springs - use tension type 2-1525 springs
Low tire pressure - inflate tailwheel to 40 to 45 psi
What is the correct tire pressure?
Tire pressure varies based on tire size; the following pressures should be used.
6.00 x 6 - 24±2 psi (29+2/-0 psi for 8KCAB)
7.00 x 6 - 24±2 psi
8.00 x 6 - 23±2 psi
8.50 x 6 - 18±2 psi
Alaska Bushwheels - 8 to 20 psi
Tailwheel - 40±5 psi
Fuel burns unevenly from the wing tanks, what's wrong?
An imbalance of up to 1/4 tank is normal; a difference of more than 1/4 tank indicates a problem with the fuel system or rigging.
1. Verify the condition of fuel cap gaskets; a cracked or leaking gasket will cause a pressure difference between the fuel tanks. Troubleshoot by swapping the left and right fuel caps. The imbalance will follow the leaking cap. Replace the cap gaskets if necessary. Use a small amount of fuel lube on both faces of the gasket to improve the seal and ease of installation. EZ-Turn Lubricant (Aircraft Spruce, P/N 09-00306) or equivalent fuel lube is recommended.
2. Verify check valve function with fuel quantities of 1/2 tank or less. Remove one fuel cap and apply lung pressure to the under wing vent. The check valve should release and vent into the tank (some chattering is normal). Apply suction to the under-wing vent, the valve should allow for expansion in reverse. There should be a substantial decrease in the flow out of the tank. Repeat function test for the opposite check valve. If there is a noticeable difference in flow or pressure between the tanks, replace the check valves with a matched set.
3. Parking on a slope, sustained uncoordinated flight, etc., will cause fuel imbalance. In normal cruise the wings should be an equal distance above the horizon with no yaw present. If the airplane flies wing low or aileron/rudder pressure is necessary to maintain straight and level flight, rig in accordance with the following question.
My airplane does not fly straight, how should I adjust the rigging?
1. Verify the cable tensions and travels per the aircraft service manual. The ailerons should have a slight droop on the ground (airloads will align the aileron with wing in flight). Flaps should be adjusted such that they are aligned with the bottom of the wing. Apply a slight upward pressure at the trailing edge to place the flap against the up stop during adjustment.
2. Set the rear struts for neutral wash. There should be no twist in the wing as measured from the first rib outboard of the tank to the first rib inboard of the tip rib. If you do not have the means to adjust this with a digital level; set the clevis on the rear strut so there is no preload.
3. With the ailerons neutral (compare the inboard edge of the aileron to the adjacent wing rib) the spade plates should be parallel to the bottom of the wing. It is sometimes helpful to remove the spade plates for the purpose of rigging; generally they do not have a large effect unless they are grossly misaligned.
4. Set the rudder tab straight (flat), rudder trim will be adjusted later. Rudder trim issues may stem from rudder return springs that provide unequal tension on the cables or a tailwheel that is not free to pivot. Lubricate the tailwheel and decrease the castlenut preload as necessary for smooth operation.
5. Evaluate the stick free aileron position in level flight at cruise power. Due to friction in the control system shake the ailerons slightly until they settle into a consistent position. Compare the inboard edge of the aileron to the adjacent wing rib. Note the direction of displacement; if the left trails up, the right will trail down a similar amount. Bend the trailing edge of aileron in the direction of displacement; if the left aileron trails up, bend the trailing edge of the left aileron up and the right down. Using both hands twist the aileron trailing edge in desired direction along the entire length (a small twist results in a large change in displacement). If hand pressure is not sufficient a wide flange tool can be used where the aileron ribs meet the trailing edge (again, a small twist results in a large change in displacement). Repeat until the ailerons trail equally left to right.
6. Evaluate the roll tendency of the aircraft in level flight at cruise power. It is important that roll tendencies are evaluated without slipping or skidding the aircraft. Coordinate any roll with rudder to determine full effect of wing twist. If the aircraft has a left roll tendency shorten the left strut clevis or lengthen the right strut clevis. Adjust in 1 to 2 turn increments; refine the adjustment in ½ to 1 turn increments. Repeat until there is no roll tendency.
7. Adjust rudder trim in level flight at cruise power to correct any residual yaw. Due to friction in the control system shake the rudder pedals slightly until the rudder settles into a consistent position. Yaw to the left is corrected by bending the rudder tab to the left (a small change will result in a large correction).
Your aircraft should now fly straight with the wings level in cruise conditions. The left and right wings should visually be equal distance above the horizon. The aircraft should have no left or right roll or yaw tendencies. Small corrections can be made by to the rudder trim tab, rear strut length, or aileron trailing edge. Flight at any other airspeed or power setting will require slight control pressures to maintain wings level and zero sideslip.
Are snapped rolls an approved maneuver? (Acrobatic Models Only)
Yes, snap rolls are an approved maneuver. To reduce unnecessary wear and tear:
- Respect the aerobatic gross weight (1800 lb for the 8KCAB and 7GCBC, 1750 lb for the other Citabria models)
- Perform snaps at or below the listed entry speeds (90 mph for the 8KCAB, 85 mph for the Citabria models)
- Limit the acrobatic fuel load to half tanks or less
This question is often followed with questions about the wing structure and fuel tanks. Any aircraft used for aerobatics will have increased maintenance requirements; to an extent this should be expected by owners and operators. Where the design has not meet expectations, ACA has made changes to eliminate issues with strut fittings, fuel tanks, butt ribs and gaps skins. The following changes have been made to the 8KCAB wings and tanks:
1996 - The butt rib thickness was increased from .020 to .032 inch, the upper gap skin rivet spacing was doubled
and an airworthiness limitation was added to the strut fittings
2002 - Four slosh holes, two welds and a doubler were added to tank center baffle
2004 - The tank beading was changed from rolled to stamped with a deeper draw and the center baffle was
changed from 3003-O to 5052-O
2010 - Revised fuselage fairings to eliminate strain on upper wing gap skin
The changes have made significant improvement, exactly how much is hard to quantify. Some operators still chose to prohibit snaps in an effort to reduce maintenance costs. This is prudent for a flight school where each student is going to snap the aircraft multiple times. For a typical owner snap rolls will not create a maintenance concern.
Manifold pressure gauge sticks, how can this be corrected? (United Instruments 6335)
The manifold pressure gauge has an adjustable damping screw that tends to clog with fuel dye. Excessive priming contributes dye accumulation in the manifold pressure line and gauge. When sticking is observed, remove the 1/8 inch copper manifold line from the engine and gauge. Blow out the line using compressed air - use of a solvent prior to blowing out the line may improve the time between service (avgas, Stoddard, acetone, etc.). To access the damping screw, remove the gauge from instrument panel (the fuel pressure line must also be disconnected) and remove the manifold pressure fitting from the instrument case. Remove, clean, and reinstall the damping screw. Position the damping screw 3/4 turn from the closed position. If a large amount of dye is observed, it may also be necessary to clean the orifice with a 7221T29 (McMaster) or equivalent nylon brush. Reinstall gauge, fuel pressure line, and manifold pressure line. Conduct and engine run-up and observe for proper engine operation and fuel pressure and manifold pressure indications. Manifold pressure should respond smoothly to power changes without excessive needle oscillation. Check for leaks.
If the problem persists or the time between service is not acceptable American Champion recommends replacing the gauge with an Aerospace Logic FM200 (STC SA02825NY) or other approved gauge combination. Installation data is available from the factory.
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