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Constant speed or constant attitude?



 
 
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  #11  
Old August 14th 03, 09:03 PM
Mark James Boyd
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I also liked the comment to fly a certain airspeed.
Our local CFI at Avenal pointed out that if one hits
a gust up, the glider will speed up, because the
C.G. is forward of the center of lift, so the
nose drops. So he suggests quick back stick pressure
to prevent the speedup and translate it into lift
instead. He calls this "porpising" since that's
what the G forces feel like.

This same effect is subtle during landing. In ground effect,
lift increases, and the nose drops and the glider speeds
up. This is why transitioning to ground effect I need
more back pressure. I learned how to take off and land
a Cessna 172 using ONLY rudders and throttle, but was
having trouble because it landed flat all the time
when I couldn't use flaps. I couldn't set trim for
further pitch because it would stall when out of ground effect.
Instead I loaded for aft legal C.G., and the nose
down pitch during transition to landing was much less.

Oscillation was also obvious and needed throttle adjustments
to dampen. I suspect competition glider pilots anticipate
oscillations when encountering vertical gusts and
counter them instantly with the stick. I've noticed
myself getting wild pitch oscillations during the
first turn when entering a thermal when I don't
anticipate the oscillation...


  #12  
Old August 15th 03, 01:30 PM
Martin Gregorie
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On 14 Aug 2003 12:03:22 -0800, (Mark James Boyd)
wrote:

I also liked the comment to fly a certain airspeed.
Our local CFI at Avenal pointed out that if one hits
a gust up, the glider will speed up, because the
C.G. is forward of the center of lift, so the
nose drops.

This has nothing to do with CG position: F1A free flight competition
model gliders do this too - and they have a 55% CG and lifting stab
setup.

The explanation is easily seen if you draw a velocity vector triangle
for a glider flying in still air. Flying into rising air reduces the
vertical vector (sink speed minus rising air speed), which reduces the
AOA of the wing. A stable aircraft will react to this by pitching down
and increasing its airspeed. This is what you notice as the boot in
the bum as the glider surges forward on entering a thermal. The nose
drop is often quite noticeable as you point out.

The opposite effect happens as you fly out of the thermal or into sink
- the glider slows down and the nose rises. Again, a velocity vector
diagram shows what is happening - the increased AOA causes a stable
aircraft to correct by pitching up and shedding airspeed. The soggy
feeling as this happens is easy enough to notice, though for some
reason the pitch up is harder to see. Possibly its masked by the usual
reaction of stuffing the nose down to get more speed in the sink.


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

  #13  
Old August 15th 03, 09:25 PM
Jim
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On 15 Aug 2003 12:36:05 -0800, (Mark James Boyd)
wrote:

With regards to vertical gusts...

A stable aircraft will react to this by pitching down
and increasing its airspeed


I thought a stable aircraft has the C.G. forward of the center
of lift. If this is so, and this effect only happens if
the aircraft is stable, then C.G. is important, right?

If the C.G. and center of lift coincide, does this effect
still occur? If the C.G. is behind the center of lift (my
understanding of "unstable") does this occur?


My guess, and it sure is only a guess, is that the changes in
the indicated airspeed as a result of the glider flying into lift or
sink WOULD occur regardless of the stability or instability of
the aircraft. I'm guessing this is so because I'm also guessing
that THESE changes in the indicated airspeed are not the
result of instaneous pitch changes in the aircraft's attitude, but
rather are changes in dynamic and/or static pressure directly
created by the changes in lift and sink themselves.

I suppose another way to say this is that the changes in indicated
airspeed may be due to angle of attack changes that are not due
to changes in the aircraft's attitude, but rather due to changes to
the direction of the airflow (which are felt as changes in lift and
sink.

I dunno. This is absolutely wonderful stuff, but it leaves me
really wanting a wind tunnel so I could test these things.


Martin has interesting points, but I'm not understanding
them just yet (it may be I don't understand the
terminology quite yet...)



  #14  
Old August 15th 03, 09:36 PM
Mark James Boyd
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With regards to vertical gusts...

A stable aircraft will react to this by pitching down
and increasing its airspeed


I thought a stable aircraft has the C.G. forward of the center
of lift. If this is so, and this effect only happens if
the aircraft is stable, then C.G. is important, right?

If the C.G. and center of lift coincide, does this effect
still occur? If the C.G. is behind the center of lift (my
understanding of "unstable") does this occur?

Martin has interesting points, but I'm not understanding
them just yet (it may be I don't understand the
terminology quite yet...)



  #15  
Old August 15th 03, 09:59 PM
Jim
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On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote:

On 15 Aug 2003 12:36:05 -0800, (Mark James Boyd)
wrote:

With regards to vertical gusts...

A stable aircraft will react to this by pitching down
and increasing its airspeed


I thought a stable aircraft has the C.G. forward of the center
of lift. If this is so, and this effect only happens if
the aircraft is stable, then C.G. is important, right?

If the C.G. and center of lift coincide, does this effect
still occur? If the C.G. is behind the center of lift (my
understanding of "unstable") does this occur?



My guess, and it sure is only a guess, is that the changes in
the indicated airspeed as a result of the glider flying into lift or
sink WOULD occur regardless of the stability or instability of
the aircraft. I'm guessing this is so because I'm also guessing
that THESE changes in the indicated airspeed are not the
result of instaneous pitch changes in the aircraft's attitude, but
rather are changes in dynamic and/or static pressure directly
created by the changes in lift and sink themselves.

I suppose another way to say this is that the changes in indicated
airspeed may be due to angle of attack changes that are not due
to changes in the aircraft's attitude, but rather due to changes to
the direction of the airflow (which are felt as changes in lift and
sink.

I dunno. This is absolutely wonderful stuff, but it leaves me
really wanting a wind tunnel so I could test these things.



I think I only further muddled this by my saying "actual airspeed" may
not be changing. This is not at all the way to look at things.
Indicated airspeed DOES change as a glider flies into lift and sink.
Period. What I wanted to describe is a situation in which the changes
in indicated airspeed are reflective of changes in the airflow over
the glider created by the changed lift and sink, not of accelerations
of the glider itself.

Phooey. This probably only made it worse. I know what I want to say,
I just can't find the right way to say it.



  #16  
Old August 15th 03, 10:32 PM
Joe Lacour
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I am looking for a radio for my LS-4A and would like some
recommendations. Reliability, good features, power consumption, etc.
I might also be interested in a used radio if available. A 2.25" Ø
would be preferred. Thanks





  #17  
Old August 15th 03, 11:58 PM
John Galloway
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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.

John Galloway

At 21:42 15 August 2003, Jim wrote:
On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote:

On 15 Aug 2003 12:36:05 -0800,
(Mark James Boyd)
wrote:

With regards to vertical gusts...

A stable aircraft will react to this by pitching down
and increasing its airspeed

I thought a stable aircraft has the C.G. forward of
the center
of lift. If this is so, and this effect only happens
if
the aircraft is stable, then C.G. is important, right?

If the C.G. and center of lift coincide, does this
effect
still occur? If the C.G. is behind the center of lift
(my
understanding of 'unstable') does this occur?



My guess, and it sure is only a guess, is that the
changes in
the indicated airspeed as a result of the glider flying
into lift or
sink WOULD occur regardless of the stability or instability
of
the aircraft. I'm guessing this is so because I'm
also guessing
that THESE changes in the indicated airspeed are not
the
result of instaneous pitch changes in the aircraft's
attitude, but
rather are changes in dynamic and/or static pressure
directly
created by the changes in lift and sink themselves.

I suppose another way to say this is that the changes
in indicated
airspeed may be due to angle of attack changes that
are not due
to changes in the aircraft's attitude, but rather due
to changes to
the direction of the airflow (which are felt as changes
in lift and
sink.

I dunno. This is absolutely wonderful stuff, but it
leaves me
really wanting a wind tunnel so I could test these
things.



I think I only further muddled this by my saying 'actual
airspeed' may
not be changing. This is not at all the way to look
at things.
Indicated airspeed DOES change as a glider flies into
lift and sink.
Period. What I wanted to describe is a situation in
which the changes
in indicated airspeed are reflective of changes in
the airflow over
the glider created by the changed lift and sink, not
of accelerations
of the glider itself.

Phooey. This probably only made it worse. I know
what I want to say,
I just can't find the right way to say it.






  #18  
Old August 16th 03, 02:21 AM
Mike Borgelt
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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.

John Galloway


At least someone gets it.

Also mentioned by Doug Haluza in an article in "Soaring" a few years
ago.

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 Borgelt

Borgelt Instruments
www.borgeltinstruments.com




  #19  
Old August 17th 03, 11:04 AM
Robin Birch
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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.

Thanks

Robin
Mike Borgelt

Borgelt Instruments
www.borgeltinstruments.com





--
Robin Birch
  #20  
Old August 17th 03, 04:03 PM
Martin Gregorie
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On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote:

I suppose another way to say this is that the changes in indicated
airspeed may be due to angle of attack changes that are not due
to changes in the aircraft's attitude, but rather due to changes to
the direction of the airflow (which are felt as changes in lift and
sink.

That's pretty much what I was trying to say.

The change is AOA is instantaneous, but inertia effects will delay the
change in attitude and (probably) this delay is responsible for quite
a lot of the indicated airspeed increase on entering the thermal
because it makes the required correction bigger than an instantaneous
correction would require.

I'm sorry that I can't easily diagram the velocity vector using only
ASCII text! This was why I suggested you draw the still air vectors
for forward speed (l-r horizontal), sink in still air (downward) and
the resultant path (sloped down completing the triangle). There's a
simplifying assumption that the wing's AOA is given by the angle of
the resultant path. That's not strictly true, but doesn't affect the
argument. Now draw the thermal velocity vector (upward, starting from
the bottom of the sinking speed vector) and draw a new resultant
slope. This will have a lesser slope than the still air situation and
shows that the instantaneous AOA has been reduced, which reduces the
wing's lift. This is an unstable situation which must be corrected and
the normal reaction of a stable aircraft is to pitch down and
accelerate to restore the lost lift.

The attitude change in a free flight model is often quite obvious. Its
pitching inertia is minimal by design: large efforts are made to
concentrate its mass at the CG by shortening the nose as far as
possible and reducing the weight of the tail group and boom. I've
often seen them pitch down quite sharply on entering a thermal but not
noticed a parallel speed increase.

I dunno. This is absolutely wonderful stuff, but it leaves me
really wanting a wind tunnel so I could test these things.

This is actually quite difficult to show in a wind tunnel because it
is a dynamic effect. Wind tunnels, OTOH generally show static effects.
The best tools I know for showing dynamic effects are visualisation
tools, vector diagrams and carefully watching free flight model
planes.


Martin has interesting points, but I'm not understanding
them just yet (it may be I don't understand the
terminology quite yet...)



During a flight yesterday I realised that you can feel the pitch-up as
you enter sink when dolphinning: as well as the sudden soggy feeling
there is a distinct sensation that the rear of the glider is sinking
fastest. I still can't say I saw a pitch up, just that the tail feels
like its sinking faster. The resulting speed loss is almost certainly
masked by pushing forward accelerate and the resulting acceleration is
certainly slower than you can get by pushing over before leaving a
thermal.

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

 




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