When we add flaps, why........

Blip16

Blip16

Well-Known Member
In a Seminole (probably other multis as well) does the nose pitch down? It is probably simple, but i can't explain it other than it just does.

same with a warrior, why is the initial tendency to pitch up, then down?
 
I'll take a stab at this...

With the initial extension of flaps you are allowing the a/c to develop more lift. The warrior I know is a very stable airplane and being a very stable airplane it wishes to correct for any imbalances. Therefore it pitches down to correct. Eventually the oscillations will dampen out if the a/c exhibits positive static and dynamic stability, and all will be well.
 
Basically, the nose down comes from the trimmed state you where in before the flaps. The tail was trimmed for X knots, you added flaps and thus some drag, now the airplane (the tail) is trying to get back to that airspeed or more specifically that equalibrium point. The initial pitch up is, IMO, more from the ballooning of the momentary additional lift the flaps give than anything else.
 
It's more complex than is being described since not all airplanes exhibit this behavior. For example while, as Sidious points out, a PA-28 single will pitch down with the deployment of flaps, a CE-172 will pitch up until stabilized at the new configuration.

I think that the reason has to do with how the center of lift moves, the location of the drag vector, and how the airflow change affects the tail.

I once came up with an explanation that since the deployment of flaps in a low wing results in a drag vector below the aircraft CG, it's like puling someone standing up by the back of the belt - he'll "pitch down." On the other hand, if you pull him by the back of the shirt collar (like the drag vector in a high-wing) he'll "pitch up." It was fun for my CFI ride, but it's more complex than that.

I'm sure someone here, maybe tgrayson who seem to have a reak knack for aerodynic explanations, will have a good description of the phenoenon.
 
I was thinking of Both Dugie's and Midlife's answers and i couldn't decide. my MEI instructor asked me yesterday, and i just can't find any info in any books i have.
 
Here is my best shot.
In low wing aircraft like the Seminole when you extend the flap you in effect extend the wing. When you do so you shift the center of pressure (lift) farther aft from the CG. Because the center of lift is aft of the CG this causes a nose down pitching moment.
In high wing aircraft like the c-172 you have the same forces at work but the air flows is directed down off the flaps and strikes the horizontal stabilizer from the top. When that air flow pushes down on the tail there will be a nose up pitching moment.
 
BuickCFI said:
In a Seminole (probably other multis as well) does the nose pitch down? It is probably simple, but i can't explain it other than it just does.

same with a warrior, why is the initial tendency to pitch up, then down?
Click to expand...

All cambered airfoils have a nose down pitching moment. When you add flaps, the nose down pitching moment increases *a lot*, because the camber increases a lot. Absent any other effects, your airspeed will start to increase. This happens in a Mooney and other aircraft.

However, there are often other side effects. Flaps will change the downwash over the horizontal stabilizer. If the downwash increases (typical), then you may end up with a nose up moment which overwhelms the tendency of the aircraft to nose down with flaps. This latter is stronger in the Cessna's, less so in the Pipers. Also, as MidlifeFlyer indicated, there will be a drag vector that will either be above or below the CG. This will create either a nose up or nose down moment.

So the net result of nose up or nose down will the sum of the

1) Nose down pitching of a highly cambered (flapped) airfoil,
2) Change in downwash on the horizontal tail, and
3) Drag vector above or below the CG.

All these moments essentially retrim the airplane for a different airspeed. After all, all the trim tab does is adjust the angle of your elevator, which produces a pitching moment around the CG. Other forces can do the same thing.

Most important: don't confuse pitch with angle of attack. The pitching moments I've been talking about are angle of attack changes. Your net pitch, once equilibrium is established, will depend also on the drag changes on the aircraft. An initial pitch up (angle of attack increase) may result in a descending flight path if the overall drag of the airplane increases. That doesn't mean that the angle of attack has changed from the initial pitch up.
 
tgrayson said:
All cambered airfoils have a nose down pitching moment. When you add flaps, the nose down pitching moment increases *a lot*, because the camber increases a lot. Absent any other effects, your airspeed will start to increase. This happens in a Mooney and other aircraft.

However, there are often other side effects. Flaps will change the downwash over the horizontal stabilizer. If the downwash increases (typical), then you may end up with a nose up moment which overwhelms the tendency of the aircraft to nose down with flaps. This latter is stronger in the Cessna's, less so in the Pipers. Also, as MidlifeFlyer indicated, there will be a drag vector that will either be above or below the CG. This will create either a nose up or nose down moment.

So the net result of nose up or nose down will the sum of the

1) Nose down pitching of a highly cambered (flapped) airfoil,
2) Change in downwash on the horizontal tail, and
3) Drag vector above or below the CG.

All these moments essentially retrim the airplane for a different airspeed. After all, all the trim tab does is adjust the angle of your elevator, which produces a pitching moment around the CG. Other forces can do the same thing.

Most important: don't confuse pitch with angle of attack. The pitching moments I've been talking about are angle of attack changes. Your net pitch, once equilibrium is established, will depend also on the drag changes on the aircraft. An initial pitch up (angle of attack increase) may result in a descending flight path if the overall drag of the airplane increases. That doesn't mean that the angle of attack has changed from the initial pitch up.
Click to expand...
thanks that is a great explanation. i knew the center of pressure changes i just couldn't explain it. i think i will need to draw it out to fully understand it though
 
Thanks for those explanations Midlife and Tgrayson. I was aware of the center of pressure/lift and downwash on the tail considerations, but I had never considered the effect of the drag vector on CG as well.

:tmyk:
 
BuickCFI said:
i knew the center of pressure changes
Click to expand...

Yes, saying that the CP moves backwards and saying that the nose down pitching moment increases on an airfoil are saying the same thing.

However, I do avoid implying that the CP change is causal, because it's only a mathematical fiction, just like the idea of a Center of Gravity. Neither one can be detected by any instrument, only calculated. Probably more accurate to say that the CP moves because the nosedown pitching moment increases on an airfoil.
 
Logically derived from the other responses. . . when you add flaps you change the shape of the wing, which changes the frontal profile of the airplane. The air moving over the airplane forces the airplane to take the most efficient profile. This results in a pitch forward compared to the previous attitude.

I also like the explanation that the change in the center of lift moves further back. This seems logical too. The back of the wing (where the flaps are) are now creating more lift (they are also producing more drag, but that is not the issue here). The back of the wing is behind the CG.

A logical extension of this thinking is that in order to counteract the increased lift from the wing, the horizontal stabilizer (which is pushing down / creating negative lift) is now flying with a higher angle of attack, creating more negative lift, and keeping the airplane in equilibrium. The result is a more stable, lower performing airplane.
 
cre8flyer said:
Logically derived from the other responses.
Click to expand...

Logic is dangerous. :)

<<when you add flaps you change the shape of the wing, which changes the frontal profile of the airplane. >>

Not the best mental model...most of the drag is produced by the pressure differences around the aircraft, and those aren't necessarily related to the frontal area. Consider...when you go from cruise to best glide, you're increasing your AOA, increasing your frontal profile, but your overall drag decreases.

<<The air moving over the airplane forces the airplane to take the most efficient profile. This results in a pitch forward compared to the previous attitude.>>

The aircraft has no tendency to take the most efficient profile, otherwise it would assume best glide at all times. Rather, it assumes a profile whereby the pitching moments on the aircraft are zero.

<<This seems logical too. The back of the wing (where the flaps are) are now creating more lift (they are also producing more drag, but that is not the issue here). The back of the wing is behind the CG. >>

Several things here:
  1. Flaps affect the lift distribution *all over the wing*, not just where the flaps are, so the intuitive model isn't that accurate.
  2. Flaps don't increase lift, per se. Yes, there is an initial ballooning, but you control that with your elevators and retrim; once you're done, lift is equal to weight, just as it was before the flap deflection.
  3. Flaps produce a nose down moment no matter where the CG is located.
<<A logical extension of this thinking is that in order to counteract the increased lift from the wing, the horizontal stabilizer (which is pushing down / creating negative lift) is now flying with a higher angle of attack, creating more negative lift>>

Even though lift hasn't changed, the nose down pitching moment has increased, so yes there will be an increased tail down force. This doesn't always mean an increase in AOA, because an increase in tail down force can be achieved via the elevator. However, in this case it does, because at a given airspeed, the nose is lower (and the tail higher) when flaps are deployed. This is why you're more at risk in this condition for a tail stall in icing conditions.

<<The result is a more stable, lower performing airplane.>>

Not really more stable; flaps change the trim, but generally not the inherent static longitudinal stability of the aircraft, because that depends on the location of the CG with respect to the Aerodynamic Center (not the Center of Pressure). Fowler flaps may increase static longitudinal stability, because they increase the chordline, but I haven't seen any data.
 
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