having a "blonde moment" when it comes to gyro procession and props

[xerj]

Pitching a clockwise spinning (seen from the cockpit) prop forward will
result in a yaw to the left, pitching it up will result in a right yaw.
Correct so far?

However, why does a yaw to the right cause a pitch up, and a yaw to the left
cause a pitch down? I thought it would be the other way, given that a yaw to
the right means applying a force to the left of the prop disc, which would
result in the precession force being applied 90 degrees later meaning that
it would pitch DOWN. What am I missing?

Thanks in advance.

[Leslie Watts]

I never noticed that much pitch up /down with yaw inputs with the
planes I fly, but I will try to give a reason for your observation.

I think it is due to p-factor rather than gyroscopic precession.

Let's take your right yaw. With the clockwise rotation that means the bottom
stroke of the prop is moving faster in the slipstream due to the forward
advancing
from the yaw. Higher speed = higher lift. The opposite would happen on the
top stroke.
This would give the pitch up.

I suspect the reason I never noticed it might be because the airframe and
motor mounting
might be designed to compensate, at least at one speed.

Les Watts

"xerj" > wrote in message
...
> Pitching a clockwise spinning (seen from the cockpit) prop forward will
> result in a yaw to the left, pitching it up will result in a right yaw.
> Correct so far?
>
> However, why does a yaw to the right cause a pitch up, and a yaw to the
left
> cause a pitch down? I thought it would be the other way, given that a yaw
to
> the right means applying a force to the left of the prop disc, which would
> result in the precession force being applied 90 degrees later meaning that
> it would pitch DOWN. What am I missing?
>
> Thanks in advance.
>
>

[Peter Duniho]

"xerj" > wrote in message
...
> [...]
> However, why does a yaw to the right cause a pitch up, and a yaw to the
left
> cause a pitch down? [...]

Who says it does? Right yaw should cause a pitching down moment, left yaw
should cause pitching up, in the scenario you describe, just as you have
concluded. What reference is telling you that you're wrong?

I have never noticed precession during attitude changes, but I hear that
it's noticeable in taildraggers during takeoff when the airspeed is low and
the tail is brought up during the initial takeoff run. I'm not sure you'd
ever notice it in a nosewheel airplane, where significant attitude changes
are always made at higher airspeeds.

In any case, your analysis is correct, and a reference that says otherwise
is incorrect. The force, inasmuch as it's present at all (and it's not
generally very noticeable, for typical GA airplanes), works the way you
describe.

Pete

[Friedrich Ostertag]

Hi xerj

> Pitching a clockwise spinning (seen from the cockpit) prop forward
> will result in a yaw to the left, pitching it up will result in a
> right yaw. Correct so far?

I believe it would be the other way round. Nose down will induce a yaw
to the right, nose up will induce yaw to the left. On the other hand,
yaw movement to the left will induce nose down and yaw to the right
will induce a nose up moment.

All this can be determined by the so called "rule of the right hand":
Extend the thumb, index finger and middle finger of your right hand in
a orthogonal manner. The thumb represents the vector of the initiated
rotary movement, e.g. pointing to the left for nose down. The index
finger represents the vector of the gyroscopic rotation (in this case
pointing in your flight direction - the vector of the propeller
rotation) while the middle finger gives you the vector of the rotary
force induced by the gyroscopic forces, pointing down meaning a
rotation or yaw to the right. (Rotation vectors assume a clockwise
movement when looking in the direction of the vector)

Whether all this is generally noticeable in a GA plane with a piston
engine and propeller I don't know. I suppose it was a big problem with
rotation engines of the very early days, where the whole cylinder block
rotated with the propeller, while the crank shaft was stationary and
mounted to the airplane.

> However, why does a yaw to the right cause a pitch up, and a yaw to
> the left cause a pitch down? I thought it would be the other way,
> given that a yaw to the right means applying a force to the left of
> the prop disc, which would result in the precession force being
> applied 90 degrees later meaning that it would pitch DOWN. What am I
> missing?

I don't know about this explanation. Why should a force be applied 90
degrees later?

regards,
Friedrich

--
for personal email please remove "entfernen" from my adress

[Teacherjh]

>>
I don't know about this explanation. Why should a force be applied 90
degrees later?
<<

To attampt to answer the question I think you're asking, consider a rotating
object (such as a propeller). Imagine it rotating in front of you, clockwise
from your point of view, and in the vertical plane. When the tip of one of the
blades is at the top of the disk, it is travelling to the right.

Note that (given the hub in front of you), having the tip travelling to the
right defines the disk - that is, there is only one orientation in which the
prop can be spinning which puts the hub in front of you, and the tip of the
prop at the top of the disk travelling (purely) to the right.

Now, apply an impulse, away from you, to the tip of the prop blade just as it
passes the top of the arc. (for symmetry, apply the same impulse, TOWARDS
you, to the other blade which is now at the bottom of the prop arc). This is
the same as applying a torque DOWNWARD (trying to point the nose down, if you
are envisioning yourself in the cockpit).

The tip of the prop blade at the top of the arc is now travelling not PURELY to
the right, but rather, to the right and away from you. The one at the bottom
is travelling to the left, and also towards you (due to the impulse you gave
it). Assuming the blades remain connected to the hub (!) the one at the top
will swing out as it goes to the right, and then swing back as it reaches the
bottom (where it will be travelling to the left and towards you)... while the
one at the bottom will swing towards you as it moves to the left, swinging
around and back away from you as it reaches the top.

The prop disk has rotated to the left, from your point of view, though you gave
it an impulse "down".

The key here is that when you push on a gyro, you are attempting to change the
VELOCITY of the rotating parts (making them move in a different direction).
This causes a change in POSITION which you can see or feel.

Mathematically, velocity isn't the same as position, it is the derivative (rate
of change) of position. For sine waves (and circular motion), derivatives are
90 degrees out of phase with each other (the deriviative of the sine is the
cosine, which is also the sine of the angle 90 degrees away from the original
angle)

Jose


--
(for Email, make the obvious changes in my address)

[G.R. Patterson III]

Friedrich Ostertag wrote:
>
> I believe it would be the other way round. Nose down will induce a yaw
> to the right, nose up will induce yaw to the left.

Nope. Bring the tail up during the takeoff roll on my plane, and she will try hard to
make a left turn. My prop is a tractor that rotates clockwise when seen from the
cockpit.

George Patterson
None of us is as dumb as all of us.

[G.R. Patterson III]

Peter Duniho wrote:
>
> I have never noticed precession during attitude changes, but I hear that
> it's noticeable in taildraggers during takeoff when the airspeed is low and
> the tail is brought up during the initial takeoff run.

That's correct. I typically start to bring the tail up at about 2/3 flying speed in
my plane. If the back seat is empty, I can raise the tail rapidly enough to require
full right rudder to counteract the effect. If I have a back-seat passenger, the tail
will rise so slowly that the precession effect is almost negligible. I have never
noticed precession effects in the air.

George Patterson
None of us is as dumb as all of us.

[xerj]

>>In any case, your analysis is correct, and a reference that says otherwise
is incorrect. The force, inasmuch as it's present at all (and it's not
generally very noticeable, for typical GA airplanes), works the way you
describe.<<

Thanks. I thought this was the case. Someone was trying to tell me
otherwise, and I was sure they were wrong.

[Friedrich Ostertag]

Hi George,

>> I believe it would be the other way round. Nose down will induce a
>> yaw to the right, nose up will induce yaw to the left.
>
> Nope. Bring the tail up during the takeoff roll on my plane, and she
> will try hard to make a left turn. My prop is a tractor that rotates
> clockwise when seen from the cockpit.

You are right!

I got it mixed up - for that mentioned "rule of the right hand" the
thumb has to be the axis of gyroscopic rotation (prop) while the index
finger represents the imposed rotation, in this case nose down resp
tail up, axis pointing to the left. Thus the middle finger is pointing
up, indicating an induced rotation or yaw to the left.

The same way it will also work for the other cases: nose up -> yaw to
the right, yaw right -> nose down, yaw left -> nose up.

This way it also figures with the explanation Jose has given.

regards,
Friedrich

--
for personal email please remove "entfernen" from my adress

[RT]

"Friedrich Ostertag" > wrote in message
...
> Hi George,
>
> >> I believe it would be the other way round. Nose down will induce a
> >> yaw to the right, nose up will induce yaw to the left.
> >
> > Nope. Bring the tail up during the takeoff roll on my plane, and she
> > will try hard to make a left turn. My prop is a tractor that rotates
> > clockwise when seen from the cockpit.
>
> You are right!
>
> I got it mixed up - for that mentioned "rule of the right hand" the
> thumb has to be the axis of gyroscopic rotation (prop) while the index
> finger represents the imposed rotation, in this case nose down resp
> tail up, axis pointing to the left. Thus the middle finger is pointing
> up, indicating an induced rotation or yaw to the left.

I remember it as: If you push on a gyroscope, where you push stays there
but the point 90 degrees around the periphery in the direction of the gyro
rotation moves in the direction of the push. (The spinning gyro drags the
push around the periphery 90 degrees before it gets a chance to work :-)