Constant speed or constant attitude?

On Sun, 17 Aug 2003 11:04:23 +0100, Robin Birch
wrote:


I
On 15 Aug 2003 22:58:24 GMT, John Galloway
wrote:

What you guys are discussing is the 'Yates effect'
as described by Derek Piggot in 'Understanding Gliding'
Appendix A and also published in Gliding magazine in
1951 by Dr A.H. Yates.


One day I'll get round to reading that.

As you enter lift the glider accelerates forward due to the lift
vector tilting forward in the flight direction. Entering sink the
reverse effect occurs. This is a short lived effect for sharp edged
gusts with time constants of the order of 0 .15 to 0.5 seconds for
typical glider airspeeds and wing loadings.
It also has interesting effects on TE varios and is one of the reasons
that TE varios seem much quicker or more "nervous"in response than
uncompensated varios connected to static sources. The other is the
sensitivity of the TE vario to horizontal airmass changes"horizontal
gusts".
There is an article on our website about this.

Mike,
That is the clearest reason for it happening that I have ever seen.
When you sketch out the lift and drag vectors and then see what happens
when extra lift is added and removed it's obvious.

Agreed. I must try to consciously use constant attitude cruise. I'm
slowly approaching that way of flying in any case, but still have the
habit of clicking on 10 knots if the vario groans.... and am probably
going too fast as a result.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Martin,=20

I just can't resist a good argument.

Sorry, but your vector diagram as described gives AOA with sign =
reversed.

On encountering a rising airmass from stabilized flight, the =
instantaneous effect is an increase in AOA - the relative wind that was =
flowing from straight ahead is now flowing from ahead and below. This =
increase in AOA gives added lift at the wing and reduced pushdown at the =
tail. Since the CG is ahead of both CL and tail, both give a nosedown =
moment. The aircraft attitude is disturbed and it pitches nosedown.

Now the inherent stability starts to work through a series of =
transients. The pitch down reduces the AOA and the aircraft accelerates =
because the gravity vector is closer to where the nose is now pointing. =
The reduced AOA and the increased drag gradually restore the aircraft to =
its original stable attitude [maybe after a few oscillations, it is far =
from a deadbeat system] and sink rate through the airmass. The final =
result is an aircraft flying at exactly the same attitude, speed, L/D =
etc in the new airmass, but with a sink rate relative to the ground =
equal to the old value less the upward velocity of the new airmass.

Agree with your conclusion, just can't help nitpicking the argument..

Going back to the original question, whether to fly constant speed or =
constant attitude, both are difficult to achieve when hit by a gust. =
But if you are skilful enough to stop the nose drop then your attitude =
will remain constant and your speed will also be constant if the gust is =
vertical. If the gust is horizontal, there wil be relatively little =
change in AOA but an increase/decrease in total energy will be reflected =
in the vario and your airspeed will change. Holding attitude will let =
the aircraft stabilize itself, but trying to regain airspeed is a better =
bet. If the gust gives a speed increase, a little nose up will turn =
your inertia into more altitude sooner, and if the gust gives a speed =
decrease you might want to nose down a little to avoid a wind-shear =
stall situation.

In my case, gusts are always some unknown combination of vertical and =
horizontal, and my reactions are too slow to hold anything truly =
constant...

Ian

I didn't use a wind tunnel, just noticed this effect on
landing and takeoff in a 172. I suppose one could
trim for min sink, and then with no stick input watch
a plane or glider fly into ground effect.

From there you should see the nose initially drop. Due
to oscillation, it may come up again, but I always saw
an initial drop.

I saw the reverse when flying out of ground effect, although
it was also subtle because it was masked by the
osscillations a bit. Once out of ground effect, the nose
would pitch up, and the horn would be just chirping for
the stall. I'm guessing this is why the PTS soft field
power technique is to "remain in ground effect while accelerating
to Vx or Vy, as appropriate" after the wheels come off the ground.

As pointed out, this is a dynamic effect, and combined with
oscillation, pretty subtle.

On 17 Aug 2003 17:12:58 GMT, Ian Cant
wrote:

Martin,=20

I just can't resist a good argument.

Sorry, but your vector diagram as described gives AOA with sign =
reversed.

I must respectfully disagree. If you fly into a rising airmass from a
neutral or sinking one your instantaneous sinking speed is reduced by
the vertical velocity of the new air mass - hence the reduced AOA.

On encountering a rising airmass from stabilized flight, the =
instantaneous effect is an increase in AOA - the relative wind that was =
flowing from straight ahead is now flowing from ahead and below. This =
increase in AOA gives added lift at the wing and reduced pushdown at the =
tail. Since the CG is ahead of both CL and tail, both give a nosedown =
moment. The aircraft attitude is disturbed and it pitches nosedown.

Errrm. The CG may be ahead if the wing's CL, but it certainly is not
ahead of the CL of the entire aircraft: if that was the case you'd
soon be flying vertically downward!

An aircraft operating at its trimmed speed in still air has no
pitching moment about its CG. However, the CG is certainly ahead of
the NP (Neutral Point [1]) by the MSS (Margin of Static Stability) and
this must be so for stable flight. The size of the MSS controls the
rate of oscillation after disturbance: too small MSS and recovery
tends to deadbeat or no recovery (unstable) and to large MSS can cause
oscillation and divergent behaviour.

This is true for both aircraft with download on the tail (most manned
aircraft) and free flight duration models, which usually have a
lifting tail and rearward CG. A lifting tail may be unsuitable for a
manned plane but as a way of getting minimum sink from a fixed speed,
fixed trim unmanned aircraft its the best. However, the same NP
stability calculations apply to both of these - and to canards and
flying wings.

Now the inherent stability starts to work through a series of =
transients. The pitch down reduces the AOA and the aircraft accelerates =
because the gravity vector is closer to where the nose is now pointing. =
The reduced AOA and the increased drag gradually restore the aircraft to =
its original stable attitude [maybe after a few oscillations, it is far =
from a deadbeat system] and sink rate through the airmass. The final =
result is an aircraft flying at exactly the same attitude, speed, L/D =
etc in the new airmass, but with a sink rate relative to the ground =
equal to the old value less the upward velocity of the new airmass.

Agree with your conclusion, just can't help nitpicking the argument..

Going back to the original question, whether to fly constant speed or =
constant attitude, both are difficult to achieve when hit by a gust. =
But if you are skilful enough to stop the nose drop then your attitude =
will remain constant and your speed will also be constant if the gust is =
vertical. If the gust is horizontal, there wil be relatively little =
change in AOA but an increase/decrease in total energy will be reflected =
in the vario and your airspeed will change. Holding attitude will let =
the aircraft stabilize itself, but trying to regain airspeed is a better =
bet. If the gust gives a speed increase, a little nose up will turn =
your inertia into more altitude sooner, and if the gust gives a speed =
decrease you might want to nose down a little to avoid a wind-shear =
stall situation.

In my case, gusts are always some unknown combination of vertical and =
horizontal, and my reactions are too slow to hold anything truly =
constant...

Agreed, but we can work on it. :-)

One of the neatest demonstrations I've seen of vertical vector
addition is when, running a cloud street, you meet a region where the
vertical air velocity increases steadily over a relatively long
distance and you can continue to cruise for a surprisingly long time
while climbing with nose-high attitude and constant airspeed:
certainly long enough to look round and see your tailplane is on the
horizon - in a Pegasus that means at least 5 degrees of pitch up.
Ain't gliding fun!



[1] The NP is Lw * Mw - Lt * Mt = 0 at the current trim condition
where
Lw = lift from the wing
Mw = moment from wing CL to the NP
Lt = lift from tail
Mt = moment from tail CL to the NP

Free Flight model designers tend to think of the NP as a fixed point
because they always design for the minimum sinking speed (FF is a pure
duration contest, remember) but it isn't - in reality its trim
dependent and so will move with the trimmed airspeed.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Indicated airspeed is not necessarily the "actual" airspeed of the
glider.
Why not?

I suspect that the inertia of the glider will have the
"actual" airspeed (whatever that is) remaining unchanged
I think you want to say that "actual ground speed" remains unchanged.
Please correct me if I'm wrong. Bye,

David
NL

I think Eric has it right here. At speeds gliders fly they are subject to
classical (Newtonian) mechanics. Thus, if a glider in still air moves into
rising air (with no change in forward airspeed, to keep it simple), an
increased upward force is required to reduce its downward momentum and
restore normal descent in the new air mass. At constant airspeed this can
only come from increased AOA.

Ed Grens

Eric Greenwell wrote in message
...
In article ,
ess says...

I must respectfully disagree. If you fly into a rising airmass from a
neutral or sinking one your instantaneous sinking speed is reduced by
the vertical velocity of the new air mass - hence the reduced AOA.

The sinking speed is reduced with reference to the ground, but not
with respect to the airmass, which is what the glider flies in. The
glider has entered a new airmass, and for a few seconds is not in
equilibrium with this new airmass.

During the transition, the glider is accelerated upward, even though
it pitches downward (you can feel that acceleration upward!). The lift
on the wing must increase to provide this acceleration, and it
increase because the AOA increases momentarily due to the upward
velocity of the new airmass. The glider pitches downward, attempting
to reduce the AOA back to the trimmed AOA. If the transient AOA were
reduced, as you suggest, the natural stability of the glider would
pitch it upwards as it attempted to regain the AOA for which it was
trimmed.

Equilibrium is obtained when the AOA returns to what it was before,
the glider is no longer accelerated upward, and the sinking speed will
be the same as before (now referenced to the new air mass). Of course,
the sinking speed referenced to the ground will vary with the vertical
velocity of the airmass.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

On Sun, 17 Aug 2003 21:34:19 +0100, Martin Gregorie
wrote:

On 17 Aug 2003 17:12:58 GMT, Ian Cant
wrote:

Martin,=20

I just can't resist a good argument.

Sorry, but your vector diagram as described gives AOA with sign =
reversed.

I must respectfully disagree. If you fly into a rising airmass from a
neutral or sinking one your instantaneous sinking speed is reduced by
the vertical velocity of the new air mass - hence the reduced AOA.


Not so at the instant the airmass is entered.
After some time - about 0.75 to 2.5 seconds the glider will be close
enough to equilibrium in the new airmass.


On encountering a rising airmass from stabilized flight, the =
instantaneous effect is an increase in AOA - the relative wind that was =
flowing from straight ahead is now flowing from ahead and below. This =
increase in AOA gives added lift at the wing and reduced pushdown at the =
tail. Since the CG is ahead of both CL and tail, both give a nosedown =
moment. The aircraft attitude is disturbed and it pitches nosedown.


If like most sailplanes the longitudinal stability isn't excessive
this effect may not be all that large but yes the glider will tend to
align itself with the local airflow but this effect only lasts for a
short time. This can also be masked by mass balancing or lack thereof
in the elevator control system, not just the elevator and how hard you
are holding on to the stick. In any case you are probably going to try
to hold the attitude constant unless you are cruising at high speed
and decide to take the thermal.
I seem to remember the SB13 flying wing had a problem with this when
first flown at forward C of G. The low moment of inertia in pitch
resulted in unpleasant characteristics as the glider flew through lift
and sink.

Mike Borgelt

In article ,
ess says...

Not so at the instant the airmass is entered.
After some time - about 0.75 to 2.5 seconds the glider will be close
enough to equilibrium in the new airmass.

Indeed.

I misstated what I meant to say: that the instantaneous position is
that the glider has its original velocity vector but is now in rising
air, so has a reduced AOA until its in equilibrium with the rising
air.

Rising air comes from beneath the wing, and increases the AOA. The
increased AOA produces an increased lift, so the glider accelerates
upward. If the AOA decreased, the glider would begin to sink, and we
know that is not the case in rising air!
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

On Mon, 18 Aug 2003 10:37:27 +0100, Martin Gregorie
wrote:

On Mon, 18 Aug 2003 13:21:46 +1000, Mike Borgelt
wrote:

On Sun, 17 Aug 2003 21:34:19 +0100, Martin Gregorie
wrote:

On 17 Aug 2003 17:12:58 GMT, Ian Cant
wrote:

Martin,=20

I just can't resist a good argument.

Sorry, but your vector diagram as described gives AOA with sign =
reversed.

I must respectfully disagree. If you fly into a rising airmass from a
neutral or sinking one your instantaneous sinking speed is reduced by
the vertical velocity of the new air mass - hence the reduced AOA.
As Eric pojnted out you mean *increased* AOA until at equilbrium.

Mike Borgelt

On Mon, 18 Aug 2003 10:53:46 -0700, Eric Greenwell
wrote:

In article ,
says...

Not so at the instant the airmass is entered.
After some time - about 0.75 to 2.5 seconds the glider will be close
enough to equilibrium in the new airmass.

Indeed.

I misstated what I meant to say: that the instantaneous position is
that the glider has its original velocity vector but is now in rising
air, so has a reduced AOA until its in equilibrium with the rising
air.

Rising air comes from beneath the wing, and increases the AOA.

Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.

The
increased AOA produces an increased lift, so the glider accelerates
upward. If the AOA decreased, the glider would begin to sink, and we
know that is not the case in rising air!

The glider rises because the rising air is rising faster than the
glider's sinking speed. That's nothing to do with the AOA, which in
any case as the same as before once the glider is stabilised in the
rising air. There is a perceived (and actual) acceleration as the
glider's vertical velocity is decreased but this too has nothing to do
with what the AOA may or may not do during the transition from one
stable state to the next. If you prefer, the acceleration is the
transient result of changing the frame of reference from one air mass
to another.

I'm attempting to account for the AOA changes during that transition.
Fixed trim models definitely pitch down on entry to a thermal.
Transient AOA decrease explains this rather well and I find this
explanation also works for the sensations I experience in a glider.

You have a different account of what happens. That's fair enough, BUT
so far you haven't used it to explain why my model pitches down on
entering a thermal or why a glider surges forward on entering a
thermal and until you can do that I have to regard your account as
incomplete.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :