Down Thrust
01-05-2004 | 06:16 PM
#2
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From: Lincoln,
NE
RE: Down Thrust
There are some interactions between thrust line, CG, wing/stab. Without going into full details, some planes tend to pitch up at higher speeds. Sometimes this is called ballooning. This is very noticeable on high lift flat bottom airfoils which are typically found in trainers.
Downthrust is used to help counteract some of the pitching moment. In other words, as you apply throttle and the plane goes faster, some thrust is used to pull the nose back down to help prevent the up pitch.
Not every plane design requires down thrust. Most aerobatic planes have zero down thrust, a lot of which has to do with their fully symmetrical airfoils that have no pitching moment. I've even seen planes designed with up-thrust. Just be sure to follow the directions as to thrust. Note: Most planes also have about 2.5 degrees of right thrust to counteract torque effects.
If you want the full technical low down on this subject, post this question in then aerodynamics forum.
Downthrust is used to help counteract some of the pitching moment. In other words, as you apply throttle and the plane goes faster, some thrust is used to pull the nose back down to help prevent the up pitch.
Not every plane design requires down thrust. Most aerobatic planes have zero down thrust, a lot of which has to do with their fully symmetrical airfoils that have no pitching moment. I've even seen planes designed with up-thrust. Just be sure to follow the directions as to thrust. Note: Most planes also have about 2.5 degrees of right thrust to counteract torque effects.
If you want the full technical low down on this subject, post this question in then aerodynamics forum.
01-05-2004 | 06:26 PM
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From: Laurel, MD,
RE: Down Thrust
John is right on.
Just to add one bit. You might be wondering why trainers do this, why not just adjust the wing and tail incidence to not require down thrust.
The reason is that for a trainer, it's desireable for the plane to try to pull itself out of a dive. In a dive, the engine thrust doesn't change (much), but the airspeed increases. The angle of the wing plus the higher airspeed in the dive causes an increase in lift, and the plane to lifts the nose and pulls out of the dive.
But this same effect causes the nose to really go up in a hurry when you add power, so, as John said, downthrust is used to reduce the amount the nose goes up when power is increased, but still keep the tendancy to pull out of a dive when power is not changed.
Just to add one bit. You might be wondering why trainers do this, why not just adjust the wing and tail incidence to not require down thrust.
The reason is that for a trainer, it's desireable for the plane to try to pull itself out of a dive. In a dive, the engine thrust doesn't change (much), but the airspeed increases. The angle of the wing plus the higher airspeed in the dive causes an increase in lift, and the plane to lifts the nose and pulls out of the dive.
But this same effect causes the nose to really go up in a hurry when you add power, so, as John said, downthrust is used to reduce the amount the nose goes up when power is increased, but still keep the tendancy to pull out of a dive when power is not changed.
