Where is the center of gravity in a tailwheel airplane?
Introduction
In “Landing a Taildragger” (Flying, June 2007) Russell Munson wrote:
“Tricycle gears are more forgiving because the airplane’s center of gravity is in front of the main gear. On a “conventional gear” it is behind the main wheels.”
This statement is perfectly correct, but it may lead to a subtle misconception. One could get the wrong idea that the position of the center of gravity depends on where the wheels are.
The FAA fell into this trap when it published this diagram in the Airplane Flying Handbook (click on the picture to enlarge):
The FAA diagram shows the center of gravity located near the leading edge of the wing of the nosewheel airplane and near the trailing edge of the wing of the tailwheel airplane. Both are wrong.
Where the center of gravity is
The center of gravity (”CG”) in almost any airplane is located aft of the leading edge of the wing by about 10% to 30% of the distance from the leading edge to the trailing edge.
(In engineer talk the center of gravity lies between about 10% to 30% of the mean aerodynamic chord (”MAC”). The chord of the wing of a Cessna 172 is 5 feet 4 inches at the wing root and 3 feet 9 inches at the wing tip. The MAC is somewhere between these two distances.)
Airplanes are made stable in pitch by loading them such that the CG is forward of the center of lift of the wing. This leads to a nose down force that is balanced by a nose up (i.e. tail down) force generated by the horizontal stabilizer and elevator.
Notice that this has nothing to do with where the wheels are! For an airplane to fly properly, the CG must lie in a relatively narrow range of distances aft of the leading edge of the wing. It doesn’t make any difference if the landing gear is tricycle, tailwheel, skis or floats!
A better way to say the same thing
What if Flying had instead said:
“Tricycle gears are more forgiving because the main wheels are located behind the airplane’s center of gravity. On a “conventional gear” the wheels are located in front of it.”
This way one gets the correct impression: The CG is in the same place in both cases. The difference is that the wheels are attached in different places.
The location of the wheels with respect to the CG has a profound effect on ground handling of nosewheel versus tailwheel airplanes, but it’s the wheels that move around, not the CG!
Meaningless rant from the Department of Nitpicking or useful rewording to prevent a common misconception? You decide!
Until next time,
FLY MORE
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