Constant speed or constant attitude?

[Jim]

I would like to hear what other glider pilots have found to be helpful
in trying to center thermals.

It is common in gliding books to read that a constant diameter circle,
as a product of flying a constant airspeed and a constant bank, is
important in centering thermals. If the airspeed and / or bank is
allowed to vary significantly the thermal circle becomes an inconstant
oval, which can make locating and centering a thermal more difficult.

I understand this, and I think it helps in staying in contact with a
thermal once I have identified the stronger area in a thermal and am
more or less centered in it.

But while I am exploring a thermal, while I am trying to get an idea
of how the lift is varying around the circle I'm flying, while I'm
working my way towards the core of the thermal, I find it more
informative to try to fly a constant attitude and bank and allow the
airspeed to rise and fall as the lift comes and goes, and not lower
and raise the nose in response to the airspeed changes resulting from
the lift changes. I find that flying a constant attitude rather than
a constant airspeed in this task greatly simplifies the task of
locating the stronger lift and moving to it.

If I do try to keep the airspeed constant by raising and lowering the
nose as I fly through the changing lift around the thermal I usually
end up behind the changes in lift and confused about the thermal's
structure.

By keeping a constant attitude while investigating a thermal I seem to
be better able to use the lift-created airspeed changes as markers of
the thermal's structure.

Am I goofing-up my thermalling this way?

[Bill Daniels]

"Jim" > wrote in message
...
> I would like to hear what other glider pilots have found to be helpful
> in trying to center thermals.
>
> It is common in gliding books to read that a constant diameter circle,
> as a product of flying a constant airspeed and a constant bank, is
> important in centering thermals. If the airspeed and / or bank is
> allowed to vary significantly the thermal circle becomes an inconstant
> oval, which can make locating and centering a thermal more difficult.
>
> I understand this, and I think it helps in staying in contact with a
> thermal once I have identified the stronger area in a thermal and am
> more or less centered in it.
>
> But while I am exploring a thermal, while I am trying to get an idea
> of how the lift is varying around the circle I'm flying, while I'm
> working my way towards the core of the thermal, I find it more
> informative to try to fly a constant attitude and bank and allow the
> airspeed to rise and fall as the lift comes and goes, and not lower
> and raise the nose in response to the airspeed changes resulting from
> the lift changes. I find that flying a constant attitude rather than
> a constant airspeed in this task greatly simplifies the task of
> locating the stronger lift and moving to it.
>
> If I do try to keep the airspeed constant by raising and lowering the
> nose as I fly through the changing lift around the thermal I usually
> end up behind the changes in lift and confused about the thermal's
> structure.
>
> By keeping a constant attitude while investigating a thermal I seem to
> be better able to use the lift-created airspeed changes as markers of
> the thermal's structure.
>
> Am I goofing-up my thermalling this way?

That's pretty much what I do. Keeping a stable attitude does seem to help
visualize the location of the thermal core. However, small speed changes
don't affect the turn diameter nearly as much as small bank changes. See:
http://home.twcny.rr.com/ghernandez/turn_rad.htm

I try to carefully adjust the bank angle to move the center of my turn
towards the stronger lift. Using the typical bank angles and airspeeds, a
15 degree change in bank will either double or halve the turn radius. I
will reduce the bank 15 degrees at the weakest point in the turn and after
an interval of about 8 second have elapsed, steepen the bank 15 degrees.
Plotting this to scale shows that it moves the circle about one diameter
toward the lift. This is not my idea, I got it from someone else.

Bill Daniels

[Al]

fly the airspeed.

That way you take advantage of the gusts as you fly through stronger or
weaker lift.

Al

"Jim" > wrote in message
...
> I would like to hear what other glider pilots have found to be helpful
> in trying to center thermals.
>
> It is common in gliding books to read that a constant diameter circle,
> as a product of flying a constant airspeed and a constant bank, is
> important in centering thermals. If the airspeed and / or bank is
> allowed to vary significantly the thermal circle becomes an inconstant
> oval, which can make locating and centering a thermal more difficult.
>
> I understand this, and I think it helps in staying in contact with a
> thermal once I have identified the stronger area in a thermal and am
> more or less centered in it.
>
> But while I am exploring a thermal, while I am trying to get an idea
> of how the lift is varying around the circle I'm flying, while I'm
> working my way towards the core of the thermal, I find it more
> informative to try to fly a constant attitude and bank and allow the
> airspeed to rise and fall as the lift comes and goes, and not lower
> and raise the nose in response to the airspeed changes resulting from
> the lift changes. I find that flying a constant attitude rather than
> a constant airspeed in this task greatly simplifies the task of
> locating the stronger lift and moving to it.
>
> If I do try to keep the airspeed constant by raising and lowering the
> nose as I fly through the changing lift around the thermal I usually
> end up behind the changes in lift and confused about the thermal's
> structure.
>
> By keeping a constant attitude while investigating a thermal I seem to
> be better able to use the lift-created airspeed changes as markers of
> the thermal's structure.
>
> Am I goofing-up my thermalling this way?

[Udo Rumpf]

> As for this part, Dick Johnson gave an interesting talk about it at a
safety
> talk at the Region 9 Championships this year. He gave aerodynamic and
> safety reasons to always use a small slip while thermaling. I can't
recall
> all the details, but have been doing it ever since then and I like it.
>
> Larry Pardue 2I

I used a little slip ( inner wing forward) before I was flying with
winglets.
Now with winglets it is different. Most winglets only tolerate max + or - 5
deg.
change in angle of attack from optimum. I try to fly now with the string as
closely as possible in the centre.
Udo

[Mark James Boyd]

When I thermal, there generally seems to be wind,
and since the glider doesn't climb quite as
fast as the thermal, I seem to do better slipping
or changing bank angle to fly a little into the
headwind during each turn (kind of like
turns around a point in power flying).

I started using slips in thermals after I flew
with a competition pilot and noticed he did this
a little when thermalling in steeper banks. I
also sometimes slip a little "into" a thermal.
I suspect this happens mostly because I'm
getting fatigued and my coordination is
getting worse and I'd rather slip into
it than skid. But there may be some
aerodynamic reason for slipping a little.
It seems that at the extreme (in a 90 degree bank),
a slip is better than coordinated flight since it
exposes more fusealage area to the thermal when
C.G. is forward.

There is also some coriolis(SP?) effect, so I notice
on .igc traces of extended thermalling that the
thermal circles a little as it rises.

I also fly by the rule that the center third
altitudes of a thermal often provide the best lift.
Sometimes I fly in the upper third if I expect
to cross a sink area, but the middle third
has been pretty good for me.

I definitely trim for the thermal, and I've
never had a consistent thermal greater than 8 knots,
and I haven't found constant banks greater than 50
degrees useful.

Some of this comes from discussions with Serge Serfaty,
a fellow glider pilot. But he wasn't a big fan of
any uncoordinated flight :P

I've also had to use shallower banks or leave thermals
because I was getting dizzy or tired or hot or couldn't
track a fellow glider. I think these are other
factors that vary based on the pilot.

[Bill Daniels]

"Larry Pardue" > wrote in message
...
>

> As for this part, Dick Johnson gave an interesting talk about it at a
safety
> talk at the Region 9 Championships this year. He gave aerodynamic and
> safety reasons to always use a small slip while thermaling. I can't
recall
> all the details, but have been doing it ever since then and I like it.
>
> Larry Pardue 2I
>
Dick's point was that a glider with dihedral has an increased angle of
attack on the leading wing in a slip. (Try bending a #10 envelope to
simulate a wing with dihedral. Look at it from the front and then yaw it as
in a slip - you'll see the AOA difference between the wings.) If you use a
slip instead of top aileron, the wing's airfoil works better without the
deflection of the ailerons. You are also holding top rudder, which in the
event of a stall, would make the glider roll out of the turn - easier to
recover.

Really good talk, Dick.

Bill Daniels

[Mark James Boyd]

>> fast as the thermal, I seem to do better slipping
>> or changing bank angle to fly a little into the
>> headwind during each turn (kind of like
>> turns around a point in power flying).

>Hmm! Don't understand this and haven't ever done it.

The best way to explain this is to show an extreme example.
Assume a constant wind from the North at 10 knots, and
a stationary source thermal. Also assume that the lift in the
thermal is exactly the same at every altitude, and
the thermal has constant diameter.

The thermal is now a column that tilts south as it rises,
but the column never moves, rather like a tall leaning
tower of Piza (sp?). It remains fixed relative to the
ground.

Now assume that a perfectly centered glider in the thermal
has just enough lift to remain at a given altitude.

If the glider keeps exactly the same bank angle and pitch and
rudder on every 360, the glider will drift with the
wind and exit downwind of the thermal and begin to sink.

So the pilot should extend the upwind time and decrease the
downwind time, to fly a perfect ground reference circle and
remain in the thermal. The ASEL practical test asks pilots to
shallow the bank into the wind and steepen it on
downwind to accomplish this.

I'd guess a lot of competition pilots probably don't
consciously think of this, as their actions to center a thermal
are so subtle that changes in airspeed and direction with
altitude override the primitive assumptions presented here.

Mark Boyd

[Bert Willing]

Your example is not working.

If there is no wind, a perfect thermal would rise vertically and you fly
constant circles to stay in.
If you have a constant wind, the whole airmass - including the thermal -
drifts with the wind. If you stay with your constant circles, you drift at
the same speed as the thermal so you stay perfectly centered.
Just basic vector addition.

Corrections are made because there is no ideal thermal, but corrections are
made into the core, regardless of the direction of wind.
Corrections into the wind are made if the thermal is of orographic, i.e.
rotors.

--
Bert Willing

ASW20 "TW"


"Mark James Boyd" > a écrit dans le message de
...
> >> fast as the thermal, I seem to do better slipping
> >> or changing bank angle to fly a little into the
> >> headwind during each turn (kind of like
> >> turns around a point in power flying).
>
> >Hmm! Don't understand this and haven't ever done it.
>
> The best way to explain this is to show an extreme example.
> Assume a constant wind from the North at 10 knots, and
> a stationary source thermal. Also assume that the lift in the
> thermal is exactly the same at every altitude, and
> the thermal has constant diameter.
>
> The thermal is now a column that tilts south as it rises,
> but the column never moves, rather like a tall leaning
> tower of Piza (sp?). It remains fixed relative to the
> ground.
>
> Now assume that a perfectly centered glider in the thermal
> has just enough lift to remain at a given altitude.
>
> If the glider keeps exactly the same bank angle and pitch and
> rudder on every 360, the glider will drift with the
> wind and exit downwind of the thermal and begin to sink.
>
> So the pilot should extend the upwind time and decrease the
> downwind time, to fly a perfect ground reference circle and
> remain in the thermal. The ASEL practical test asks pilots to
> shallow the bank into the wind and steepen it on
> downwind to accomplish this.
>
> I'd guess a lot of competition pilots probably don't
> consciously think of this, as their actions to center a thermal
> are so subtle that changes in airspeed and direction with
> altitude override the primitive assumptions presented here.
>
> Mark Boyd

[CH]

> If you have a constant wind, the whole airmass - including the thermal -
> drifts with the wind.

Depends
Ground disconnected bubbles may drift.
Ground connected weaker thermals may have a slight tilt.
Ground related strong thermals can be rock steady, with
the wind blowing around it
Chris
Melbourne



"Bert Willing" > wrote in
message ...
> Your example is not working.
>
> If there is no wind, a perfect thermal would rise vertically and you fly
> constant circles to stay in.
> If you have a constant wind, the whole airmass - including the thermal -
> drifts with the wind. If you stay with your constant circles, you drift at
> the same speed as the thermal so you stay perfectly centered.
> Just basic vector addition.
>
> Corrections are made because there is no ideal thermal, but corrections
are
> made into the core, regardless of the direction of wind.
> Corrections into the wind are made if the thermal is of orographic, i.e.
> rotors.
>
> --
> Bert Willing
>
> ASW20 "TW"
>
>
> "Mark James Boyd" > a écrit dans le message de
> ...
> > >> fast as the thermal, I seem to do better slipping
> > >> or changing bank angle to fly a little into the
> > >> headwind during each turn (kind of like
> > >> turns around a point in power flying).
> >
> > >Hmm! Don't understand this and haven't ever done it.
> >
> > The best way to explain this is to show an extreme example.
> > Assume a constant wind from the North at 10 knots, and
> > a stationary source thermal. Also assume that the lift in the
> > thermal is exactly the same at every altitude, and
> > the thermal has constant diameter.
> >
> > The thermal is now a column that tilts south as it rises,
> > but the column never moves, rather like a tall leaning
> > tower of Piza (sp?). It remains fixed relative to the
> > ground.
> >
> > Now assume that a perfectly centered glider in the thermal
> > has just enough lift to remain at a given altitude.
> >
> > If the glider keeps exactly the same bank angle and pitch and
> > rudder on every 360, the glider will drift with the
> > wind and exit downwind of the thermal and begin to sink.
> >
> > So the pilot should extend the upwind time and decrease the
> > downwind time, to fly a perfect ground reference circle and
> > remain in the thermal. The ASEL practical test asks pilots to
> > shallow the bank into the wind and steepen it on
> > downwind to accomplish this.
> >
> > I'd guess a lot of competition pilots probably don't
> > consciously think of this, as their actions to center a thermal
> > are so subtle that changes in airspeed and direction with
> > altitude override the primitive assumptions presented here.
> >
> > Mark Boyd
>
>

[Robert Ehrlich]

Bert Willing wrote:
>
> Your example is not working.
>
> If there is no wind, a perfect thermal would rise vertically and you fly
> constant circles to stay in.
> If you have a constant wind, the whole airmass - including the thermal -
> drifts with the wind. If you stay with your constant circles, you drift at
> the same speed as the thermal so you stay perfectly centered.
> Just basic vector addition.
>
> Corrections are made because there is no ideal thermal, but corrections are
> made into the core, regardless of the direction of wind.
> Corrections into the wind are made if the thermal is of orographic, i.e.
> rotors.
>

Nevertheless Helmut Reichman in his famous book says the same thing
as Mark James Boyd. Except he mentions also a case where you have to
do the opposite. You are right that the thermal drifts with the wind
but the glider sinks in the thermal. You may either figure the thermal
as an oblique column of rising or a sequence of bubbles rising while
drifting downwind and so connected by an oblique line. In both cases
sinking in the thermal will bring you below it, and in order to get back
into the column or the next bubble, you have to move upwind. The case
above mentionned where you have to do the opposite is the case of a
continuous column with a strong maximum, below which the lift is weaker
and converging, as well a weaker and diverging above it. In this case,
despite your sink in the thermal, it will bring the glider in the upper
part of weaker lift, and at this height the stronger lift is downwind.

A thing that Mark James Boyd made me discover is that the needed upwind
move is higher with lower climb speed, up to completely cancelling the
drift when the climb speed is zero.

[Mark James Boyd]

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...

[Martin Gregorie]

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 :

[Jim]

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...)
>
>

[Mark James Boyd]

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...)

[Jim]

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.

[Joe Lacour]

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

[John Galloway]

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.
>
>
>
>

[Mike Borgelt]

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

[Robin Birch]

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

[Martin Gregorie]

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 :

[Martin Gregorie]

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 :

[Ian Cant]

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

[Mark James Boyd]

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.

[Martin Gregorie]

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 :

[dhofstee]

> 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

[E. A. Grens]

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 >,
> 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)

[Mike Borgelt]

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

[Eric Greenwell]

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. 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)

[Mike Borgelt]

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

[Martin Gregorie]

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 :

[TIM WARD]

"Martin Gregorie" > wrote in message
...
> On Mon, 18 Aug 2003 10:53:46 -0700, Eric Greenwell
> > wrote:
<snippage>
> >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.

Martin, you are using the wrong frame of reference. The "sinking speed
velocity" you are using is relative to the ground. Relative to the glider,
the air it is sinking through is rising past it. As the glider flies into
"lift" (relative to the ground) the vertical velocity of the air rising past
it is _increased_ by the velocity of the lift. So the AOA is increased
momentarily.

Tim Ward

[Finbar]

Two quick comments on this discussion:

1. There's a timing difference between the main wing and the tail.
Entering lift there is a transition region where the lift is growing
stronger as the glider moves forward. The main wing will be about 15
feet ahead of the tail. As a result the main wing will be at a higher
angle of attack than the tail during the transition into the lift,
creating a pitch-up tendency until the aircraft has gone through the
transition region. This would tend to offset the pitch-down tendency
that's been discussed. I've personally flown aircraft that pitched
down, others that pitched up. Flex-wing hang gliders tend to pitch up
very strongly on entering lift. However, I used to fly a rigid-wing
hang glider (flying wing) that pitched down. I never could explain
the difference.

2. There's been a few mentions of the download on the tail, and Martin
referred to an upload on the tail of a model but said this was
"probably unsuitable for a manned plane." Not at all: at thermaling
speeds, manned sailplanes have a substantial upload on the tail
(contrary to everything my instructors told me!). This was a topic of
discussion some months ago on RAS. Since Martin is familiar with
doing this in models, it may surprise him rather less than it did me -
but I ran the numbers and, indeed, it is so!

[Martin Gregorie]

On Tue, 19 Aug 2003 07:17:43 -0700, "TIM WARD" > wrote:

>
>"Martin Gregorie" > wrote in message
...
>> On Mon, 18 Aug 2003 10:53:46 -0700, Eric Greenwell
>> > wrote:
><snippage>
>> >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.
>
>Martin, you are using the wrong frame of reference. The "sinking speed
>velocity" you are using is relative to the ground. Relative to the glider,
>the air it is sinking through is rising past it. As the glider flies into
>"lift" (relative to the ground) the vertical velocity of the air rising past
>it is _increased_ by the velocity of the lift. So the AOA is increased
>momentarily.
>

I'm using the air mass as the reference frame - hence my reference to
a change of reference frame as the glider moves from neutral air into
rising air. If I was using the ground or the glider as reference frame
I would not have mentioned a change of reference frame.

The air mass is the usual one for discussions of stable flight - as in
the periodic circling in a wind discussion.


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

[John Galloway]

At 15:54 19 August 2003, Finbar wrote:

.....'There's a timing difference between the main
wing and the tail.
>Entering lift there is a transition region where the
>lift is growing
>stronger as the glider moves forward. The main wing
>will be about 15
>feet ahead of the tail. As a result the main wing
>will be at a higher
>angle of attack than the tail during the transition
>into the lift,
>creating a pitch-up tendency until the aircraft has
>gone through the
>transition region. This would tend to offset the pitch-down
>tendency
>that's been discussed. I've personally flown aircraft
>that pitched
>down, others that pitched up. Flex-wing hang gliders
>tend to pitch up
>very strongly on entering lift. However, I used to
>fly a rigid-wing
>hang glider (flying wing) that pitched down. I never
>could explain
>the difference.
>
>
I can confirm that, at least in the Discus and Duo
Discus flying with mid to aft C of G , if you simply
cruise with a rigidly fixed elevator position - set
for a reasonable median cruise speed of your choice
- then the glider slowly pitches up and slows under
positive acceleration as you enter regions of lift
and pitches down and speeds up under reduced acceleration
as you enter regions of sink. I have often flown this
way in the last few years since reading about the technique
as an aside in Reichmann (seventh edition in English,
pages 64 and 133). He discusses it in connection with
methods of trying to optimize g loading in transitional
phases of flight between lift and sink and refers to
it as 'the near optimal solution of simply flying with
the controls locked'. I have often wondered why it
works when the Yates effect would at first sight tend
to have the opposite effect so thanks to Finbar for
the obsevation above.

Flying fixed elevator results in very nice gentle speed
variation without the divergence you get flying hands
off but it takes a surprising amount of concentration
to keep the elevator fixed. (Perhaps a little 'dead
man's handle' on the stick that temporarily fixed the
elevator control alone would help.) It works best
when the lift and sink are continually changing because
if there is a long period of steady lift or sink without
vertical acceleration from the airmass then the airspeed
tends to settle back at the cruise speed for the elevator
setting chosen. In those circumstances you need to
depart from the fixed elevator.

John Galloway

[root]

Martin Gregorie wrote:
> ...
> 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.
> ...

But this is near like adding apples an oranges. The vector you are
interested in is the relative wind vector, i.e. the velocity vector
of the airmass seen from the aircraft as reference frame, for what
concerns the AOA. Before entering lift, you can consider it as the
sum of 2 components, a horizontal one, opposite of the horizontal speed
of the glider, and a vertical one, opposite of the sinking speed.
When lift is entered, a 3rd componemt is added, the lift vector. This
3rd component has the same direction as the 2nd one, i.e. both are
upward. So the new relative wind clearly causes a higher AOA. Adding
the sinking speed vector and the rising air vector, while mathematically
possible, has no physical sense.

[Martin Gregorie]

On Wed, 27 Aug 2003 19:20:48 +0000, root >
wrote:

>Martin Gregorie wrote:
>> ...
>> 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.
>> ...
>
>But this is near like adding apples an oranges. The vector you are
>interested in is the relative wind vector, i.e. the velocity vector
>of the airmass seen from the aircraft as reference frame, for what
>concerns the AOA. Before entering lift, you can consider it as the
>sum of 2 components, a horizontal one, opposite of the horizontal speed
>of the glider, and a vertical one, opposite of the sinking speed.
>When lift is entered, a 3rd componemt is added, the lift vector. This
>3rd component has the same direction as the 2nd one, i.e. both are
>upward. So the new relative wind clearly causes a higher AOA. Adding
>the sinking speed vector and the rising air vector, while mathematically
>possible, has no physical sense.

That makes sense.

Presumably the pitch down is the aircraft correcting its trimmed AOA,
but where does the commonly seen airspeed increase come from?


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

[Mike Borgelt]

On Tue, 02 Sep 2003 17:34:17 +0100, Martin Gregorie
> wrote:

>On Wed, 27 Aug 2003 19:20:48 +0000, root >
>wrote:
>
>>Martin Gregorie wrote:
>>> ...
>>> 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.
>>> ...
>>
>>But this is near like adding apples an oranges. The vector you are
>>interested in is the relative wind vector, i.e. the velocity vector
>>of the airmass seen from the aircraft as reference frame, for what
>>concerns the AOA. Before entering lift, you can consider it as the
>>sum of 2 components, a horizontal one, opposite of the horizontal speed
>>of the glider, and a vertical one, opposite of the sinking speed.
>>When lift is entered, a 3rd componemt is added, the lift vector. This
>>3rd component has the same direction as the 2nd one, i.e. both are
>>upward. So the new relative wind clearly causes a higher AOA. Adding
>>the sinking speed vector and the rising air vector, while mathematically
>>possible, has no physical sense.
>
>That makes sense.
>
>Presumably the pitch down is the aircraft correcting its trimmed AOA,
>but where does the commonly seen airspeed increase come from?


The Yates effect as the lift vector tilts forward(and increases in
magnitude).

Mike Borgelt

[Martin Gregorie]

On Wed, 03 Sep 2003 19:19:13 +1000, Mike Borgelt
> wrote:

>On Tue, 02 Sep 2003 17:34:17 +0100, Martin Gregorie
> wrote:
>
>>On Wed, 27 Aug 2003 19:20:48 +0000, root >
>>wrote:
>>
>>>Martin Gregorie wrote:
>>>> ...
>>>> 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.
>>>> ...
>>>
>>>But this is near like adding apples an oranges. The vector you are
>>>interested in is the relative wind vector, i.e. the velocity vector
>>>of the airmass seen from the aircraft as reference frame, for what
>>>concerns the AOA. Before entering lift, you can consider it as the
>>>sum of 2 components, a horizontal one, opposite of the horizontal speed
>>>of the glider, and a vertical one, opposite of the sinking speed.
>>>When lift is entered, a 3rd componemt is added, the lift vector. This
>>>3rd component has the same direction as the 2nd one, i.e. both are
>>>upward. So the new relative wind clearly causes a higher AOA. Adding
>>>the sinking speed vector and the rising air vector, while mathematically
>>>possible, has no physical sense.
>>
>>That makes sense.
>>
>>Presumably the pitch down is the aircraft correcting its trimmed AOA,
>>but where does the commonly seen airspeed increase come from?
>
>
>The Yates effect as the lift vector tilts forward(and increases in
>magnitude).
>

Thanks

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