Dave Hodges
Well-Known Member
Can someone tell me how strong a tail boom for a 1000-pound airplane needs to be?
The aerodynamic center is defined as the point at which the pitching moments are constant. According to that definition, the tail would push down on the back of the airplane the same amount of force at a 1-G, straight and level, low angle of attack as it would at a 2-G higher angle of attack. The center of pressure moves forward as the angle of attack increases. The lift is greater. But the distance between the center of pressure and the aerodynamic center (which is where the center of gravity should be) decreases, which explains the constant pitching moment and consequently the same required downward force on the tail.
But some airfoil data show that, at about a 21-degree angle of attack, the center of pressure moves rearward. That blows the definition of aerodynamic center.
When I was testing the Steel Stork (an ultralight), tied to a trailer, without the tail, and with bathroom scales under the nose wheel, the scales bounced around between 30 and 40 pounds. So the 12-pound tail plus about 20 pounds of air force would push the back of the airplane down enough for straight and level flight.
But what if you're pulling some Gs at a high main wing angle of attack and you push the stick forward and subject the tail to a high angle of attack? Should the tail boom be able to withstand the upward force of the horizontal control surface at the maximum lift coefficient at lots of Gs?
The aerodynamic center is defined as the point at which the pitching moments are constant. According to that definition, the tail would push down on the back of the airplane the same amount of force at a 1-G, straight and level, low angle of attack as it would at a 2-G higher angle of attack. The center of pressure moves forward as the angle of attack increases. The lift is greater. But the distance between the center of pressure and the aerodynamic center (which is where the center of gravity should be) decreases, which explains the constant pitching moment and consequently the same required downward force on the tail.
But some airfoil data show that, at about a 21-degree angle of attack, the center of pressure moves rearward. That blows the definition of aerodynamic center.
When I was testing the Steel Stork (an ultralight), tied to a trailer, without the tail, and with bathroom scales under the nose wheel, the scales bounced around between 30 and 40 pounds. So the 12-pound tail plus about 20 pounds of air force would push the back of the airplane down enough for straight and level flight.
But what if you're pulling some Gs at a high main wing angle of attack and you push the stick forward and subject the tail to a high angle of attack? Should the tail boom be able to withstand the upward force of the horizontal control surface at the maximum lift coefficient at lots of Gs?
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