PA38 pitch down with yaw
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I recon it's because of 2 effects.
1. while using the rudder there is an amount of movement of air up the rudder which then hits the elevator which then pitches because the trimmer is a spring.
2. It has a relative large rudder and the drag causes a drop in airspeed which causes the nose to drop.
The t-tail does have a few differences compared to the other training air craft due to the fact its away from the prop wash as i am sure you have found out.
BTW personally I think the PA 38 is a fine aircraft for PPL training.
1. while using the rudder there is an amount of movement of air up the rudder which then hits the elevator which then pitches because the trimmer is a spring.
2. It has a relative large rudder and the drag causes a drop in airspeed which causes the nose to drop.
The t-tail does have a few differences compared to the other training air craft due to the fact its away from the prop wash as i am sure you have found out.
BTW personally I think the PA 38 is a fine aircraft for PPL training.
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tescoapp: Congratulations on being the first person to post a response which isn't a waste of space.
I'm trying to picture your explanation #1. If there is a movement of air up the rudder (presumably on the "windward" side), and it hits the elevator, surely that would push the elevator up which would then cause a pitch up? Or are you saying that the upward airflow on the rudder (however that may be caused) is changing the relative airflow over the tailplane from more or less horizontal, to something with a vertical component? That would cause the nose to pitch down.
But why would that upward flow on the rudder not also affect aircraft with a conventional tailplane?
NS
I'm trying to picture your explanation #1. If there is a movement of air up the rudder (presumably on the "windward" side), and it hits the elevator, surely that would push the elevator up which would then cause a pitch up? Or are you saying that the upward airflow on the rudder (however that may be caused) is changing the relative airflow over the tailplane from more or less horizontal, to something with a vertical component? That would cause the nose to pitch down.
But why would that upward flow on the rudder not also affect aircraft with a conventional tailplane?
NS
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I just did a turn round a walk back to edit post because I am talking rubbish.
I was trying to say that because the Elevator is sitting in relatively stable flow of air and that it is attached to not the worlds greatest trim system. It can be upset relatively easily compared to lower types with all the prop wash.
It could be completely the oppersite to what i originally said and its actually the low pressure on the rudder face which is sucking the elevator down. I suppose it could still be air migrating just depends on where the elevator pivot point is in relation to the air movement.
And to answer the question on what you term conventional tail plane the wash from the prop is going alot faster than the arflow over the wing, think about that picture in thom of the big arrows wrapping around the plane. This supplys a constant source of energised air. Means you can do short field takeoffs, grass field or act the idiot lifting the nose wheel up at relatively low air speeds. Where as in the tommy and other t planes you cant really lift the nose until you have enough airspeed which maybe 30 knots more than a pa28.
And the reason for the span migration of air along the rudder just think of the rudder as a swept wing some will have it more than others depending on there profile etc. You can see the vortex's spilling off the tip the fin of large jets in the right conditions
I was trying to say that because the Elevator is sitting in relatively stable flow of air and that it is attached to not the worlds greatest trim system. It can be upset relatively easily compared to lower types with all the prop wash.
It could be completely the oppersite to what i originally said and its actually the low pressure on the rudder face which is sucking the elevator down. I suppose it could still be air migrating just depends on where the elevator pivot point is in relation to the air movement.
And to answer the question on what you term conventional tail plane the wash from the prop is going alot faster than the arflow over the wing, think about that picture in thom of the big arrows wrapping around the plane. This supplys a constant source of energised air. Means you can do short field takeoffs, grass field or act the idiot lifting the nose wheel up at relatively low air speeds. Where as in the tommy and other t planes you cant really lift the nose until you have enough airspeed which maybe 30 knots more than a pa28.
And the reason for the span migration of air along the rudder just think of the rudder as a swept wing some will have it more than others depending on there profile etc. You can see the vortex's spilling off the tip the fin of large jets in the right conditions
Last edited by tescoapp; 5th Nov 2007 at 09:05.
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I asked this question a long time ago.
Personal theory, as the aircraft yaws say to the left, the fin blanks off the underside of the tailplane, thus its looses some of its downforce causing the nose to pitch down.
Probably why it has such interesting spin modes.
Brilliant for crosswind landing!
Lets face it the Tomahawk is the Mini Metro of the aviation world.
When the last one ends up as a coke can, it will be found jammed in the back of a Cessna 152 seat!
Personal theory, as the aircraft yaws say to the left, the fin blanks off the underside of the tailplane, thus its looses some of its downforce causing the nose to pitch down.
Probably why it has such interesting spin modes.
Brilliant for crosswind landing!
Lets face it the Tomahawk is the Mini Metro of the aviation world.
When the last one ends up as a coke can, it will be found jammed in the back of a Cessna 152 seat!
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that seems as good as my theorys. You would get a decrease in airspeed one side and an increase the other.
The amount of nose drop is related though to the state of the trimmer. I had one which you had to keep your hand on the wheel while doing anything more than balanced turns. Half a boot full and it spun the whole lot out and you got some serious nose drop.
Any chance northsouth you can speed tape some string onto the fin top middel bottom. just U it around the fin and tape so it can't escape. And tell us what it does while yawing.
The amount of nose drop is related though to the state of the trimmer. I had one which you had to keep your hand on the wheel while doing anything more than balanced turns. Half a boot full and it spun the whole lot out and you got some serious nose drop.
Any chance northsouth you can speed tape some string onto the fin top middel bottom. just U it around the fin and tape so it can't escape. And tell us what it does while yawing.
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Yaw and Pitch
Most single engine propellor aircraft will pitch following yaw, owing to the a 'gyroscopic effect' applied to the rotating propellor. Particularly easy to show in a Cessna but to varying degrees in other types.
The gyroscopic effect leads to (clockwise engines);
A) Yaw to the right - Roll right - Pitch Up
B) Yaw to the left - Roll left - Pitch down.
With anti-clockwise engines pitch up/down reverses. A simple experiment to demonstrate this is to use a bicycle wheel. Somehow hold onto the axle and spin the wheel clockwise - observe and feel the very powerful change in pitch. Repeat anti-clockwise.
I wouldn't want to argue as to whether there are any further aerodynamic effects acting on the airframe that causes pitch change in the case of a PA38..
The gyroscopic effect leads to (clockwise engines);
A) Yaw to the right - Roll right - Pitch Up
B) Yaw to the left - Roll left - Pitch down.
With anti-clockwise engines pitch up/down reverses. A simple experiment to demonstrate this is to use a bicycle wheel. Somehow hold onto the axle and spin the wheel clockwise - observe and feel the very powerful change in pitch. Repeat anti-clockwise.
I wouldn't want to argue as to whether there are any further aerodynamic effects acting on the airframe that causes pitch change in the case of a PA38..
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tescoapp:
Naaah I'm gonna get stude to take control, climb out the door, sit astride the rear fuselage and stick me hands in the airflow.
Seriously, what I've got so far is:
1) yaw left, "span"wise flow on right side of fin/rudder reduces (negative) AoA of tailplane/elevator, causing reduced downforce, causing nose drop.
Meanwhile....
2) left tailplane is blanked, causing loss of (negative) lift, causing reduced downforce therefore nose drop
The flaw seems to me to be that:
a) "span"wise flow should also occur in a conventionally tailplaned aeroplane - but perhaps it's ineffective due to being in the slipstream as you suggest tescoapp?
b) does blanking only have a significant effect when it's the low pressure side of the tailplane that's blanked - as it would be in the Tomahawk? In low-set tailplane aircraft it'll be the high pressure (upper) side that's blanked. But maybe the effects are balanced out by the slipstream. Must try yawing with closed throttle next time I fly PA38 to see what diff that makes.
NS
Any chance northsouth you can speed tape some string onto the fin top middel bottom. just U it around the fin and tape so it can't escape. And tell us what it does while yawing.
Seriously, what I've got so far is:
1) yaw left, "span"wise flow on right side of fin/rudder reduces (negative) AoA of tailplane/elevator, causing reduced downforce, causing nose drop.
Meanwhile....
2) left tailplane is blanked, causing loss of (negative) lift, causing reduced downforce therefore nose drop
The flaw seems to me to be that:
a) "span"wise flow should also occur in a conventionally tailplaned aeroplane - but perhaps it's ineffective due to being in the slipstream as you suggest tescoapp?
b) does blanking only have a significant effect when it's the low pressure side of the tailplane that's blanked - as it would be in the Tomahawk? In low-set tailplane aircraft it'll be the high pressure (upper) side that's blanked. But maybe the effects are balanced out by the slipstream. Must try yawing with closed throttle next time I fly PA38 to see what diff that makes.
NS
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It has been a long time since I flew a PA38, but I think that the rudder blanking the horizontal stab/elevator is a reasonable explanation.
In normal flight the stab/elevator is providing a downward force (nose-up pitching moment), with the most critical airflow being the underside. If the rudder interferes with that there will be a pitchover effect.
The aircraft I've flown most affected by this was a military PC9, which would give about a -0.5 g pushover if you put full rudder in at about 100K. That opened your eyes if you weren't ready for it!
Just my guess.
In normal flight the stab/elevator is providing a downward force (nose-up pitching moment), with the most critical airflow being the underside. If the rudder interferes with that there will be a pitchover effect.
The aircraft I've flown most affected by this was a military PC9, which would give about a -0.5 g pushover if you put full rudder in at about 100K. That opened your eyes if you weren't ready for it!
Just my guess.
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the throttle closed will have no effect on the T-tail. Hence you cant do wheelies for grass field takeoffs.
correct the prop wash negates nearly everything in normal flight conditions.
I don't think the blanking has that much to do with it if your demonstrating yaw maybe a good boot full but not a squeeze for a demo.
Personally I think we can discuss the dynamics of it all. But it will all come down to that bloody awful trim system.
Need to get someone to try it in a seminol to see if its just a vice of the tommy.
correct the prop wash negates nearly everything in normal flight conditions.
I don't think the blanking has that much to do with it if your demonstrating yaw maybe a good boot full but not a squeeze for a demo.
Personally I think we can discuss the dynamics of it all. But it will all come down to that bloody awful trim system.
Need to get someone to try it in a seminol to see if its just a vice of the tommy.
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Principle of rigidity in free space guys, as "Homeguard" mentioned....a force acting on a gyro, (prop in this case), acts at a point 90 degrees ahead of it's point of application in the direction of rotation; prop rotation that is....hence the resultant pitching up or down....been a while since I used the ole CFI privileges....correct me if I'm wrong please, anyone....
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Sounds like the gyro effect if you ask me, but you didn't.
I'll have to experiment with the DA40 next week, but would an aerodynamic blanking of one side of the stab not be compensated for by the increase in airflow over the other side as it moves into the stream?
Also, I'm not sure I found an answer to whether it pitches down with right and left rudder, does it?
I'll have to experiment with the DA40 next week, but would an aerodynamic blanking of one side of the stab not be compensated for by the increase in airflow over the other side as it moves into the stream?
Also, I'm not sure I found an answer to whether it pitches down with right and left rudder, does it?
