longitudinal stability
06-22-2007, 03:57 PM
#1
Gets Weekends Off
Thread Starter
Joined APC: Mar 2007
Posts: 114
longitudinal stability
When the center of gravity is farther aft, the reason the plane is less stable is because of less downwash over the horizontal stabilizer? Since that there is less efficiency of the elevator it makes it harder for recovery if there is a disturbance.
Is this somewhat right? Could anybody add any knowledge on this that im leaving out or if I am totally wrong.
Is this somewhat right? Could anybody add any knowledge on this that im leaving out or if I am totally wrong.
06-22-2007, 05:24 PM
#2
Flying Farmer
Joined APC: Jul 2006
Position: Turbo-props' and John Deere's
Posts: 3,160
You are on the right path, Sorta.....
Stability is the aircrafts tendency after it is disturbed though. So aft CG mainly makes a stall/spin recovery a very difficult and and in some cases impossible.
Now talking about the down-wash, you are slightly confused or I am in what you are trying to say. The horizontal stabilizer produces down-force. It is merely an inverted wing. When you move the weight around in the plane, it doesn't effect the actual flow of air anywhere. Now what it will effect is the length from the CG to the "arm" of the stabilizer gets closer to each other, thus reducing the power the stabilizer(in conjuction with the elevators) provide.
Think of it as a teeter-totter that is movable. The wings at one end(Point A, and it's obviously fixed), the CG(B), and the elevator(C, also a fixed position) The first setting, the balance point is dead middle. Everything is easily moved. Now move the CG rear-ward by shifting some weight. Now the elevator/stabilizer, in level flight, actually have less work to do, because once again, it's job is down-force. Put more weight near it, less work. Now here comes the problem, when you need to provide "less" downforce, due to either a stall/spin or cross-wind landing. Now that the tail has all that weight shifted closer, it requires more effort in those other directions to get a reaction. Sometimes if it is extremely aft, it can actually be impossible to provide enough force to overcome some adverse situations.
So the lesson is, with an Aft-CG, cruise speeds go up because the reduced drag on the tail from requiring less downforce.(inverted lift still causes drag). Downsides are difficulty with the cross-winds and spin/stall recovery's as mentioned above. It's a lot in a little time, hopefully you have someone you can speak to in person that can give a clearer picture
Stability is the aircrafts tendency after it is disturbed though. So aft CG mainly makes a stall/spin recovery a very difficult and and in some cases impossible.
Now talking about the down-wash, you are slightly confused or I am in what you are trying to say. The horizontal stabilizer produces down-force. It is merely an inverted wing. When you move the weight around in the plane, it doesn't effect the actual flow of air anywhere. Now what it will effect is the length from the CG to the "arm" of the stabilizer gets closer to each other, thus reducing the power the stabilizer(in conjuction with the elevators) provide.
Think of it as a teeter-totter that is movable. The wings at one end(Point A, and it's obviously fixed), the CG(B), and the elevator(C, also a fixed position) The first setting, the balance point is dead middle. Everything is easily moved. Now move the CG rear-ward by shifting some weight. Now the elevator/stabilizer, in level flight, actually have less work to do, because once again, it's job is down-force. Put more weight near it, less work. Now here comes the problem, when you need to provide "less" downforce, due to either a stall/spin or cross-wind landing. Now that the tail has all that weight shifted closer, it requires more effort in those other directions to get a reaction. Sometimes if it is extremely aft, it can actually be impossible to provide enough force to overcome some adverse situations.
So the lesson is, with an Aft-CG, cruise speeds go up because the reduced drag on the tail from requiring less downforce.(inverted lift still causes drag). Downsides are difficulty with the cross-winds and spin/stall recovery's as mentioned above. It's a lot in a little time, hopefully you have someone you can speak to in person that can give a clearer picture
06-22-2007, 05:52 PM
#3
Gets Weekends Off
Thread Starter
Joined APC: Mar 2007
Posts: 114
Originally Posted by Ewfflyer
You are on the right path, Sorta.....
Stability is the aircrafts tendency after it is disturbed though. So aft CG mainly makes a stall/spin recovery a very difficult and and in some cases impossible.
Now talking about the down-wash, you are slightly confused or I am in what you are trying to say. The horizontal stabilizer produces down-force. It is merely an inverted wing. When you move the weight around in the plane, it doesn't effect the actual flow of air anywhere. Now what it will effect is the length from the CG to the "arm" of the stabilizer gets closer to each other, thus reducing the power the stabilizer(in conjuction with the elevators) provide.
Think of it as a teeter-totter that is movable. The wings at one end(Point A, and it's obviously fixed), the CG(B), and the elevator(C, also a fixed position) The first setting, the balance point is dead middle. Everything is easily moved. Now move the CG rear-ward by shifting some weight. Now the elevator/stabilizer, in level flight, actually have less work to do, because once again, it's job is down-force. Put more weight near it, less work. Now here comes the problem, when you need to provide "less" downforce, due to either a stall/spin or cross-wind landing. Now that the tail has all that weight shifted closer, it requires more effort in those other directions to get a reaction. Sometimes if it is extremely aft, it can actually be impossible to provide enough force to overcome some adverse situations.
So the lesson is, with an Aft-CG, cruise speeds go up because the reduced drag on the tail from requiring less downforce.(inverted lift still causes drag). Downsides are difficulty with the cross-winds and spin/stall recovery's as mentioned above. It's a lot in a little time, hopefully you have someone you can speak to in person that can give a clearer picture
Stability is the aircrafts tendency after it is disturbed though. So aft CG mainly makes a stall/spin recovery a very difficult and and in some cases impossible.
Now talking about the down-wash, you are slightly confused or I am in what you are trying to say. The horizontal stabilizer produces down-force. It is merely an inverted wing. When you move the weight around in the plane, it doesn't effect the actual flow of air anywhere. Now what it will effect is the length from the CG to the "arm" of the stabilizer gets closer to each other, thus reducing the power the stabilizer(in conjuction with the elevators) provide.
Think of it as a teeter-totter that is movable. The wings at one end(Point A, and it's obviously fixed), the CG(B), and the elevator(C, also a fixed position) The first setting, the balance point is dead middle. Everything is easily moved. Now move the CG rear-ward by shifting some weight. Now the elevator/stabilizer, in level flight, actually have less work to do, because once again, it's job is down-force. Put more weight near it, less work. Now here comes the problem, when you need to provide "less" downforce, due to either a stall/spin or cross-wind landing. Now that the tail has all that weight shifted closer, it requires more effort in those other directions to get a reaction. Sometimes if it is extremely aft, it can actually be impossible to provide enough force to overcome some adverse situations.
So the lesson is, with an Aft-CG, cruise speeds go up because the reduced drag on the tail from requiring less downforce.(inverted lift still causes drag). Downsides are difficulty with the cross-winds and spin/stall recovery's as mentioned above. It's a lot in a little time, hopefully you have someone you can speak to in person that can give a clearer picture
