Sailplane performance as a function of CG
Karl Giesen
gravity, i.e. it climbs best with max. CG to the back and has it´s best
L/D during cruising with max CG to nose.
Has anybody experience (practical or theoretical) how significant the
effect in cruising is (in climb it´s very importand, therefore most of
the competition gliders fly with full back CG)?
If it´s more than a few thousandth parts it would be worth to think
about a 'moveable mass' in the glider, i.e. a water reservoir in the
nose and the far back of the fuselage which could be filled/drained via
a pump during flight.
Has anybody made some practical experience with such a device?
uru...@mail.reach.net
the centre of pressure moves forward, therefore the
CG should be in an optimum position generally
forward of CP. As the speed increases the reverse is true.
Udo Rumpf
Sent via Deja.com http://www.deja.com/
Before you buy.
Richard B.
Karl Giesen <Karl....@icn.siemens.de> wrote:
>
> Has anybody made some practical experience with such a device?
>
>
ballast". Don't know any more than that tho. Should be some info
available.
Richard B.
--
http://homestead.deja.com/user.rabue/index.html
N33913
Timothy Taylor
Karl Giesen <Karl....@icn.siemens.de> wrote in message
news:3873424A...@icn.siemens.de...
snip...
> If it愀 more than a few thousandth parts it would be worth to think
> about a 'moveable mass' in the glider, i.e. a water reservoir in the
> nose and the far back of the fuselage which could be filled/drained via
> a pump during flight.
>
>
in the late 60's or early 70's as movable balance. I don't think we would
want to try that today, but moving a small mass of water from the tail to
the wing area might work.
Tim
Donald Ingraham
> That doesn't sound right. CG towards the nose means that the elevator has
> to supply more downforce, which means added drag for no useful lift,
> *plus* the main wing had to supply extra lift and therefore extra drag.
> This applies whether at cruise or when turning, but the effect is likely
> to be worse when going slow.
It is correct. But before the exotic aerodynamic formulas come raining
down (and I get lost ;-)), think of it this way:
When you are flying at min-sink IAS, how are you pressuring the stick
(let's
assume no trim help)? The pressure is back, the elevator is up, the
downward-lift at the tail is increased and this creates drag right?
Now, if
you could suddenly make the tail 6 or 8 lbs heavier, what would happen?
You
wouldn't need to be producing as much downward-lift with the elevator,
right?
You ease forward, the elevator lines up with the stabilizer, drag
reduces and
also pitch responsivness improves, or at least gets closer to being
balanced.
The reverse is true for cruising flight; the stick is forward, elevator
downward, lifting the tail up and causing drag. If you could reduce the
weight
in the tail for cruising flight, you would need less elevator and
therefore
cause less drag.
The original poster asks for practical experiences with drag reduction
in *cruising* flight by a weight-shift method. And of course, I have no
help at all for him! :-)))
I heard rumors of accidents during the early days of competition flying
as this was being experimented with. I heard the weights got stuck or
broke free or something...maybe someone reading this knows....
Don
Jjglider
aft CG is best for both climb and cruse. Your idea will work on a hang-glider
but not on a sailplane. JJ Sinclair
Bruce Hoult
> It is well konwn that sailplane performance depends on the center of
> gravity, i.e. it climbs best with max. CG to the back and has it愀 best
> L/D during cruising with max CG to nose.
That doesn't sound right. CG towards the nose means that the elevator has
to supply more downforce, which means added drag for no useful lift,
*plus* the main wing had to supply extra lift and therefore extra drag.
This applies whether at cruise or when turning, but the effect is likely
to be worse when going slow.
-- Bruce
Richard B.
Donald Ingraham <d...@sgi.com> wrote:
> Bruce Hoult wrote:
>
> > That doesn't sound right. CG towards the nose means that the
elevator has
> > to supply more downforce, which means added drag for no useful lift,
> > *plus* the main wing had to supply extra lift and therefore extra
drag.
> > This applies whether at cruise or when turning, but the effect is
likely
> > to be worse when going slow.
>
> down (and I get lost ;-)), think of it this way:
>
with the CG position, although for another purpose than drag reduction.
I did not have "in flight adjustment" but merely shifted the battery
pack and radio back to change the CG. Dug through my notebooks, and
didn't find much except the final notes which are not encouraging.
Probably the reason this is not done is the possibility of the weight
becoming stuck in the wrong place for the intended flight. With the CG
at the normal 30% position, the planes handled normally, (there were
three of them, they had short lives). By the time the CG was back to
45%, recovery from a spin, and a successful landing were problems. At
50%, landing was possible only at half throttle, and at 55% the plane
was an accident looking for a place to become a disaster. Being well
aware that wings can be scaled down, but air can't, I would assume that
the effects would remain somewhat the same, but on a full scale ship
would happen at much smaller displacements of the CG. I did notice that
at the 45% position, if spin recovery was applied immediately after the
spin started, it would recover within two or three turns. If it was NOT
applied immediately, there was no recovery, the spin went flat and
applying full opposite rudder, full down, and full throttle had no
effect before impact. The moral would seem to be that a forward CG and
the penalty paid for it is easier to face than the consequences.
John Giddy
In a hang-glider ???
They are controlled *normally* by moving the C/G in relation to the
wing, are they not ?? Doesn't need a "New Idea"
John G.
--
John Giddy ( ) ) Mangalore Gliding Club
5/287 Barkers Rd ) ) ) http://www.gfa.org.au/vic/mgc/
Kew, Victoria, 3101 ( ) ) _
Australia ( ) '------8------'
MOTION SHILOH
Karl Giesen wrote:
>
> It is well konwn that sailplane performance depends on the center of
> gravity, i.e. it climbs best with max. CG to the back and has it´s best
> L/D during cruising with max CG to nose.
>
> Has anybody experience (practical or theoretical) how significant the
> effect in cruising is (in climb it´s very importand, therefore most of
> the competition gliders fly with full back CG)?
>
> If it´s more than a few thousandth parts it would be worth to think
> about a 'moveable mass' in the glider, i.e. a water reservoir in the
> nose and the far back of the fuselage which could be filled/drained via
> a pump during flight.
>
> Has anybody made some practical experience with such a device?
As you move the CG back you do decrease induced drag because of the
decreased angle of attack needed with the CG change. This can give a
plane an increased speed range and depending on the airfoils design,
better LD and sink rate. However the trade off is instability. CG,s that
are farther forward produce better pitch and yaw stability.
Hand launch glider (balsa model) builders take advantage of this by
using CG just barely in front of the CP along with an elevator with an
airfoil with very small ampount of incidence. The harder it is thrown
the more lift is generated on the the elevator which counters the
increased lift of the wings at higher speed. With this a hand launch
glider can be thrown very hard without it looping and when the speed
scrubs off the glider will fly very nicely at a slow speed.
If you move the CG behind the the CP you end up with a plane that flys
backwards, ala a Canard.
Sailplane performance depends on a lot of different factors. Airfoil
design, wing area/aspect ratio, wing loading (total weight versus wing
area), streamlining (parasitic drag reduction), moment arm and
rudder/elevator to wing area ratios. Generally increasing performance
has it's trade offs with stability. The best way to go without
decreasing stabilty is getting the right wingloading for the given task
and reducing the parasitic drag as much as possible while increasing the
lift from a wing without increasing induced drag. Sounds easy huh!
Finally, as far as the CG is concerned, moving it farther back (no
further than recommended by the manufacturer) will increase overall
perfomance including speed. The idea that moving the CG forward
increases speed is only partially correct as far as it increase the wing
loading to have more weight in the nose. However, adding ballast to a
sailplane that has it's GC at the most rearward recommended CG will
allow it to outperform the same sailplane with the same flying weight
with it's GC forward and less ballast.
Doug Hoffman
modeling experimentation which may not translate directly to the
typical full scale. It is easy and safe to play with these
variables to the extreme with a flying model!
My models use full flying stabilators so there is no fixed decalage.
By moving the CG aft *and* trimming the stab I achieved best L/D.
The reason is obvious. Moving the CG aft requires the stab to
be trimmed to less decalage (angle between the wing and stab).
Minimum drag, and so best L/D is achieved with little or no angle.
The larger the angle the larger the "airbrake effect".
I suspect that the fixed decalage glider behaves similarly,
but may not achieve what is possible with a full flying stab.
Flying nose heavy in *any* glider would require elevator induced
drag, just like Bruce says. Nose heavy will not yield best L/D.
-Doug
In article <3873AA17...@sgi.com> Donald Ingraham, d...@sgi.com
writes:
>Bruce Hoult wrote:
>
>> That doesn't sound right. CG towards the nose means that the elevator has
>> to supply more downforce, which means added drag for no useful lift,
>> *plus* the main wing had to supply extra lift and therefore extra drag.
>> This applies whether at cruise or when turning, but the effect is likely
>> to be worse when going slow.
>
>It is correct. But before the exotic aerodynamic formulas come raining
>down (and I get lost ;-)), think of it this way:
>
Shaber CJ
>>
>> Centrair built a pegasus with CG shifter and found it didn't work because
>an
>> aft CG is best for both climb and cruse. Your idea will work on a
>hang-glider
>> but not on a sailplane. JJ
I thought a plane climbed better with a aft CG and run beter with a forward CG.
One very expericenced open class winner has told me that he puts his CG40% aft
of forward as in the open class you are spending most of your time running.
Eric Greenwell
In article <85157...@enews4.newsguy.com>, dhof...@oakland-info.com
says...
= I suspect that the fixed decalage glider behaves similarly,
= but may not achieve what is possible with a full flying stab.
=
= Flying nose heavy in *any* glider would require elevator induced
= drag, just like Bruce says. Nose heavy will not yield best L/D.
This true, but not relevant to the optimising cross-country speed, as
a glider is flown at best L/D only in the weakest conditions. In
strong conditions, the glider is flown far faster than best L/D speed.
It is in the high speed cruise that the forward CG provides an
improved L/D compared to a rearward CG at the same speed.
--
>>Delete the "REMOVE" from my e-mail address to reply by e-mail<<
Eric Greenwell
Eric Greenwell
In article <20000105185941...@ng-fi1.aol.com>,
jjgl...@aol.com says...
= Centrair built a pegasus with CG shifter and found it didn't work because an
= aft CG is best for both climb and cruse. Your idea will work on a hang-glider
= but not on a sailplane. JJ Sinclair
I don't think this true for all gliders, however. My ASH 26 E manual
shows the optimum CG for cruise to be towards the forward limit.
Martin Heide (the H in ASH) told me the water ballast is positioned to
move the CG ahead the approximate amount required for the optimum CG
location in stronger conditions. In other words, stronger conditions
means you can cruise faster and an extra few fpm in the climb is less
important, so the CG should be further forward than in weak
conditions. Achieving this with the ballast, which you add in strong
conditions anyway, is a simple way to achieve the CG shift.
Another factor I haven't seen mentioned: a flapped glider will benefit
less from a CG shift than an unflapped glider, as the change in
airfoil shape takes care of a lot the elevator change that would
otherwise be required at different speeds.
I'm under the impression weight-shifting (other than one-time shifts
due to ballast release) is banned in the Standard Class, and perhaps
all FAI classes. Has anyone read the rules recently?
Robert Ehrlich
>
> .... You're assuming that
> the tail is always producing negative lift. It's true
> that's the usual case, but it need not be. .....
>
This is something I heard several times, except for "it need not be".
As a consequence of this I would expect that tail surface would have
a non symetrical airfoil, with some camber with downward convexity.
With this idea in mind I had recently a close look on the tail surfaces
of the gliders in my club, which are all relatively modern glass sailplanes,
the oldest are LS1. I was surprised by finding that the tail surfaces were
nearly symetrical with a little camber in a direction opposite to what I
expected. More preciseley most of the tail surfaces are symmetrical except
near the trailing edge, where the underside exhibits a little concavity which
doesn't exist on the upper side.
Donald Ingraham
> You can think of it this way, but IMHO it's not nearly as
> simple or neat as this makes it sound. You're assuming that
> the tail is always producing negative lift. It's true
> be stable with zero lift or positive lift at the tail. Even
> in those cases, you will have to pull back on the stick when
> flying slow. Further, you can't equate pressure on the
> stick directly to increased drag.
Wouldn't this, positive lift on the tail at min-sink, only be
possible if the CG was actually aft of the center of lift? And
wouldn't this kind of design make stalls/spins nearly
unrecoverable?
Don
_______________________________________________________
Don Ingraham
SGI Friends help you move.
d...@sgi.com Real friends help you move bodies.
Steve Uzochukwu
Greenwell) wildly wibbled thus:
>I'm under the impression weight-shifting (other than one-time shifts
>due to ballast release) is banned in the Standard Class, and perhaps
>all FAI classes. Has anyone read the rules recently?
Not all. There are FAI Classes for Footlaunched HG 1-3 and Ultralight
sailplanes[4] IIRC, Class O?
Steve U., XJ900, Major Diversion ahead.
Eric Greenwell
In article <3874DCA5...@inria.fr>, Robert....@inria.fr
says...
I was surprised by finding that the tail surfaces were
= expected. More preciseley most of the tail surfaces are symmetrical except
= near the trailing edge, where the underside exhibits a little concavity which
= doesn't exist on the upper side.
It's a JAR requirement (I believe) to have nose a nose up tendency as
speed increases greatly. The elevators are made this way to induce
that nose up tendency, though it is a slight detriment to performance.
NAMacLean
REMOVEeg...@prodigy.net (Eric Greenwell) writes:
>I'm under the impression weight-shifting (other than one-time shifts
>due to ballast release) is banned in the Standard Class, and perhaps
>all FAI classes. Has anyone read the rules recently?
According to the 1999 issue of the rules, which I think is still current, any
method of shifting the C of G location in flight is specifically prohibited for
World Class (Section 7.7.5), but there is no mention of such a restriction for
Standard Class (7.7.4) or other classes (7.7.1 to 7.7.3).
Neil MacLean
SZD-55.
Andreas Maurer
wrote:
>Wouldn't this, positive lift on the tail at min-sink, only be
>possible if the CG was actually aft of the center of lift? And
>wouldn't this kind of design make stalls/spins nearly
>unrecoverable?
Not necessarily - the question is whether the tail could overcome the
increasing nose-up force of the wing with further increasing AOA.
Usually the CG and the tail are designed that the tail does have to
produce the slightest possible lift to keep the AOA constant -
depending on g-forces, weight&balance and airspeed (CL) the tail does
indeed produce lift.
Bye
Andreas
Andreas Maurer
<Karl....@icn.siemens.de> wrote:
>It is well konwn that sailplane performance depends on the center of
>gravity, i.e. it climbs best with max. CG to the back and has it´s best
>L/D during cruising with max CG to nose.
>
>Has anybody experience (practical or theoretical) how significant the
>effect in cruising is (in climb it´s very importand, therefore most of
>the competition gliders fly with full back CG)?
>
>If it´s more than a few thousandth parts it would be worth to think
>about a 'moveable mass' in the glider, i.e. a water reservoir in the
>nose and the far back of the fuselage which could be filled/drained via
>a pump during flight.
Hallo Karl,
auf Deutsch - das Heraussuchen der Fachtermina würde für mich zu lange
dauern... ;)
Prinzipiell ist vorgeschrieben, daß ein Flieger stabil um die
Querachse (längsstabil) fliegt.
Das Maß hierfür ist das sogenannte Stabilitätsmaß Sigma - zur
Erhaltung der Stabilität ist ein Sigma von mindestens 0.05
vorgeschrieben (Sigma 0 wäre abolut indifferent).
Die hinterste Schwerpunktlage ist bei allen mir bekannten
Konstruktionen so definiert, daß sich ein Sigma in der Gegend von
0.055 bis 0.07 ergibt. Ein weiteres Zurückschieben des Schwerpunktes
würde Sigma auf ein unzulässiges Maß reduzieren, womit sich Deine
Frage schon rein zulassungstechnisch erledigt... ;)
[Warum 0.55 statt 0.05? Ganz einfach - um Reserven bei zu niedrigem
Pilotengewicht zu haben.]
Flugmechanisch sieht es so aus, daß die Höhenleitwerkslast (der
Auftrieb, den das Höhenleitwerk erzeugt) eine Funktion von
Flugzeug-Geometrie (auch Profildaten) Schwerpunktlage, Abflugmasse,
Fluggeschwindigkeit und g-Belastung ist.
Abflugmasse und Schwerpunkt beeinflussen Sigma insofern, daß Sigm,a
bei weiter hinten liegendem Schwerpunkt kleiner wird.
BTW [lang]:
Normalerweise erzeugt ein Höhenleitwerk brauchbarer Auslegung NUR im
Bereich des besten Gleitens bei EINER optimalen Schwerpunktlage
keinerlei Auftrieb - im Schnellflug erzeugt es leichten Auftrieb, im
Langsamflug leichten Abtrieb.
Ich habe nur die Daten der AK-5 und AK-8, aber die sind, wenn ich mich
recht erinnere, so ausgelegt, daß bei einer mittleren Schwerpunktlage
das Höhenleitwerk keinen Auftrieb erzeugt.
[Bei Interesse kann ich Dir mal meine Aerodynamik-Auslegung der
"Impulse" schicken, das ist ein großes Excel-File, welches fast die
gesamte Berechnung eines Flugzeuges vornimmt. Ist allerdings sehr,
sehr komplex. Damit lassen sich aber bekannte Konstruktionen sehr
schön analäsieren... <g>]
Der Grund, warum diese optimale Schwerpunktlage von Flieger zu Flieger
unterschiedlich ist, ist relativ einfach: Gerade bei Segelfliegern,
die in unterschiedlichen Varianten existieren (Klappmotor,
Ansteckohren, Winglets, Wasserballast) muß ein Kompromiß gefunden
werden, das Stabilitätsmaß für alle möglichen Konfigurationen und
Zuladungen in einen halbwegs optimalen Bereich zu bringen.
[Sehr schön zeigt sich das bei Schleicher, wo die ASW-27 und ASH-26
zwei komplett verschiedene Flugzeuge sind, obwohl alle anderen
Hersteller Kompromißkonstruktionen bauen - der Ventus 2 ist deutlich
besser als der entsprechende Kompromiß 2c (der auch mit 18 Metern
geflogen werden kann). Schleicher hat schon immer etwas feiner
optimiert...]
Das führt dazu, daß speziell die Ausgeung des Höhenleitwerks nur einen
Kompromiß darstellt. Bei weitem am einfachsten auszulegen ist ein
reiner Standardklasse-Segler ohne Wasserballast...
Die Aussage, daß vordere Schwerpunktlage den besten Gleitwinkel
ermöglicher, erklärt sich daraus, daß eben mehr oder wenig zufällig
die Höhenleitwerkslast bei entsprechender Wölbklappenstellung bei n =
1 minimal ist. Bei n>1 steigt sie dann überproportional an, ergo wird
der Widerstand größer - das Höhenleitwerk ist ganz klar zu klein
definiert (hier zeigt sich, daß der Flieger dann mehr auf bestes
Gleiten als auf Steigen optimiert wurde).
[End BTW]
Die einzige Möglichkeit, die Höhenleitwerkslast beim Kurbeln zu
erniedrigen, ist Dein Vorschlag, den Schwerpunkt darin nach hinten zu
verlagern (die Gewichtskraft einen Teil der aerodynamischen
Höhenleitwerkskraft übernehmen zu lassen).
Ergo das Stabilitätsmaß zu erniedrigen.
Das hat allerdings den Nebeneffekt, daß aerodynamische Kräfte
plötzlich durch Gewichtskräfte ersetzt werden. Aerodynamische Kräfte
sind unabhängig von Beschleunigungen (deshalb fliegt ein Flugzeug auch
bei Turbulenz immer stabil weiter und wird nicht plötzlich instabil),
bei der Schwerpunkt-Methode würden sich allerdings die Kräfte, mit
denen der Schwanz des Fliegers nach unten gezogen wird, bei jeder
Turbulenz ändern- ausgesteuert muß es durch das Höhenleitwerk werden.
Das ist der Grund, warum schwanzlastige Flugzeuge empfindlich um die
Querachse werden.
Ich habe es mal überschlägig berechnet - würdest Du mit solch einem
schwanzlastigen Flieger, bei dem zwei Drittel der Höhenlwerkslast
durch einen hintenliegenden Schwerpunkt ersetzt wurden, aus einem
45°-Kreisflug aufrichten (und die g-Belasting von von 1,41 auf 1
reduzieren), dann wäre der Flieger lt. Stabilitätsmaß deutlich im
instabilen Bereich (Sigma <0) und unsteuerbar
Im Kreisflug wäre er dies, nebenbei erwähnt, auch (sigma ist
unabhängig von g-Belastung!)- er wäre nur dann kontrollierbar, wenn es
absolut keine Turbulenz gäbe. Viel Spaß.... ;)
Bye
Andreas
Donald Ingraham
>
> Donald Ingraham <d...@sgi.com> wrote:
>
> >Wouldn't this, positive lift on the tail at min-sink, only be
> >possible if the CG was actually aft of the center of lift?
>
I see what you mean. That is technically true, but I wonder
if there are any sailplane designs that actually do this; produce
positive lift on the tail at min-sink airspeed. (I'm picking
min-sink just to avoid high speed, attitude and G-force tangents).
Andreas?
Don
Doug Hoffman
egree...@prodigy.net writes:
>= Flying nose heavy in *any* glider would require elevator induced
>= drag, just like Bruce says. Nose heavy will not yield best L/D.
>
>This true, but not relevant to the optimising cross-country speed, as
>a glider is flown at best L/D only in the weakest conditions. In
>strong conditions, the glider is flown far faster than best L/D speed.
>It is in the high speed cruise that the forward CG provides an
>improved L/D compared to a rearward CG at the same speed.
>
I am referring to best L/D at *any speed*. I am having a hard time
believing that best L/D at *any speed* is achieved with *maximum
forward* CG. An earlier poster made this implication. That was
what I was responding to.
Perhaps at high speeds and/or strong lift conditions it is
desirable to move the CG somewhat forward of max aft. But I
doubt that full forward CG is what you normally want (unless
you have an overweight pilot issue and tailweight restrictions).
-Doug
Eric Greenwell
In article <855q0...@enews2.newsguy.com>, dhof...@oakland-info.com
says...
=
= I am referring to best L/D at *any speed*. I am having a hard time
= believing that best L/D at *any speed* is achieved with *maximum
= forward* CG. An earlier poster made this implication. That was
= what I was responding to.
=
= Perhaps at high speeds and/or strong lift conditions it is
= desirable to move the CG somewhat forward of max aft. But I
= doubt that full forward CG is what you normally want (unless
= you have an overweight pilot issue and tailweight restrictions).
I missed the earlier part about "maximum forward CG", and I agree the
best CG at a particular speed is not the maximum forward CG, until you
get to a very high speed.
Doug Hoffman
egree...@prodigy.net writes:
>I missed the earlier part about "maximum forward CG", and I agree the
>best CG at a particular speed is not the maximum forward CG, until you
>get to a very high speed.
I'm still not sure that I follow. Are you saying that at very high
speeds, I presume near Vne, one normally *wants* max forward CG?
If this is what you are saying then I still have to disagree. But
perhaps I am not following your thought...
-Doug
uru...@mail.reach.net
basic understanding of flight, it is called
"Mechanics of Flight" by A.C.Kermode / Pitman or similar
Any University library will have a book on the basics.
Maybe the Private Pilot Hand book will do as well but
I do not remember if it was covered. Since it was a long time ago,
making my pilot licence. Also check out this webpage
http://www.hanleyinnovations.com/
Udo Rumpf
--
http://www.wingdolly.reach.net
> I missed the earlier part about "maximum forward CG", and I agree the
> best CG at a particular speed is not the maximum forward CG, until you
> get to a very high speed.
Tango4
The 'best compromise' position appears to be about 37% of mean chord
standard and 15m classes regular tail arrangement) the optimum for T tails
is slightly further forward.
If in the std class the CG was moveable then the energy loss could be made
zero in both conditions of flight (climb/cruise). The time saved would be
about 7 seconds per hour. or 0.02% with a cruising speed of 70 kts.
to achieve this you need to shift 18 lbs through 16 feet going from
'slightly unstable' to positive stability.
Moving the CG in flight appears to be a profitless occupation.
If you want all the technical jargon behind this then may I suggest you buy
Franks book!
Ian
Andreas Maurer
(Todd Pattist) wrote:
>mau...@funsystem.de (Andreas Maurer) wrote:
>
>>Hallo Karl,
>>
>>auf Deutsch - das Heraussuchen der Fachtermina wŘrde fŘr mich zu lange
>>dauern... ;)
>
>The longest post in this thread and my poor language skills
>preclude me from understanding it. Would some kind soulbe
>willing to summarize it?
If I've got some time this weekend I'll try to do an English version
(the other article I've posted should tell it all).
Basically I described the theory how the stability of an aircraft is
determined and why replacing the negative lift of the tail by the
force of a weight will result in an unstable aircraft.
Bye
Andreas
Andreas Maurer
(Todd Pattist) wrote:
>I'd be very interested to know if there are any sailplanes
>that produce positive lift on the tail at any airspeed when
>the CG is legal but at the rearmost position. I've never
>been able to get hard facts - mostly I've heard opinions and
>a repeat of the "negative tail lift is stable" story.
The amount of lift that is actually produced by the tail (whether it
is directed upwars or downwards doesn't matter) during cruise
conditions (1 g) is usually very small - the CL that is needed is
usually below 0.1, therefore the induced drag of the tail is low. It
looks different for very high and very low speeds as well as for
non-1g-conditions such as pullig up or flying non-straight.
The tail does produce no lift for only one given airspeed , CG and
g-force. Shifting the CG will have the result that it does produce
lift to compensate the force of the shifted CG - therefore shifting
the CG backward will make the tail indeed produce some lift. But the
amount of lift will usually be minimal (see numbers below).
The Longitudinal stability is defined by the Longitudinal stability
coefficient (let's call it Roh).
According to JAR 22 Roh must be at least 0.05 to provide suffient
longitudinal stability (Roh= 0 would make the aircraft indifferent,
Roh < 0 would make it instable (and uncontrollable)).
The basic idea of this thread was to replace the aerodynamic force
produced by the tail by weight-force produced by gravity. Basically
this would be possible, but the slightest change in g-loading would
alter the weight-force which would have to be corrected by the
elevator.
[This is why planes with (too) rear CG become ...hmmm... agile ...
concerning the elevator authority.]
Since a rear GC is already close to the allowed specs (Roh = 0.05) a
further move backwards wouldn't make sense - any deviation form the
g-load that the current CG is optimized for would make the aircraft
longitudinal instable.
Here's an example for some real numbers. They are (small) part of the
initial calculations for the aircraft "Impulse" (see latest issue of
the German magazine "aerokurier" for the aircraft - it's some kind of
lighter "Lancair"). Although it is no glider, the numbers are similar,
and especially the load-distribution over different g-loadings and
CG-positions follows the same principle.
For the given CG of this calculation Roh is relatively high (0.259)
since the cg is about in the center of the allowed range.
Note: Positive P_0 points upwards
Note the situation at n=1 at 98,44 km/h and 64 km/h (the design
minimum airspeed) - at very low speeds the tail indeed produces lift.
N = Newton
kg = Kilograms
IAS: n [gravity] Roh ramp weight P_0 [tail force]
98,44 km/h 0,00 0,259 329,14 kg -43,06 N
98,44 km/h 1,00 0,259 329,14 kg -17,74 N
130,75 km/h 1,76 0,259 329,14 kg -31,29 N
163,06 km/h 2,74 0,259 329,14 kg -48,67 N
195,38 km/h 3,94 0,259 329,14 kg -69,86 N
227,69 km/h 5,35 0,259 329,14 kg -94,88 N
260,00 km/h 4,40 0,259 329,14 kg -188,97 N
402,50 km/h 4,40 0,259 329,14 kg -608,51 N
402,50 km/h -1,50 0,259 329,14 kg -757,95 N
156,62 km/h -2,20 0,259 329,14 kg -164,73 N
150,87 km/h -1,39 0,259 329,14 kg -136,41 N
145,12 km/h -1,29 0,259 329,14 kg -126,21 N
139,37 km/h -1,19 0,259 329,14 kg -116,41 N
133,63 km/h -1,09 0,259 329,14 kg -107,01 N
127,88 km/h -1,00 0,259 329,14 kg -98,00 N
127,88 km/h 0,00 0,259 329,14 kg -72,67 N
64,00 km/h 0,00 0,259 329,14 kg -18,20 N
64,00 km/h 1,00 0,259 329,14 kg 7,13 N
69,30 km/h 1,17 0,259 329,14 kg 8,36 N
74,60 km/h 1,36 0,259 329,14 kg 9,68 N
79,91 km/h 1,56 0,259 329,14 kg 11,11 N
85,21 km/h 1,77 0,259 329,14 kg 12,63 N
90,51 km/h 2,00 0,259 329,14 kg 14,25 N
160,00 km/h 2,00 0,259 329,14 kg -63,11 N
Now the same numbers for rear CG (Roh= 0.074). The rear CG is a
function of maximum cockpit load and empty fuel tanks, resulting in
higher ramp weight.
Note the much higher tail loads for high g-loadings.
98,44 0,00 0,074 479,14 -43,06
98,44 1,00 0,074 479,14 279,75
130,75 1,76 0,074 479,14 493,54
163,06 2,74 0,074 479,14 767,62
195,38 3,94 0,074 479,14 1101,97
227,69 5,35 0,074 479,14 1496,61
260,00 4,40 0,074 479,14 1119,98
402,50 4,40 0,074 479,14 700,44
402,50 -1,50 0,074 479,14 -1204,18
156,62 -2,20 0,074 479,14 -819,20
150,87 -1,39 0,074 479,14 -550,49
145,12 -1,29 0,074 479,14 -509,34
139,37 -1,19 0,074 479,14 -469,79
133,63 -1,09 0,074 479,14 -431,84
127,88 -1,00 0,074 479,14 -395,49
127,88 0,00 0,074 479,14 -72,67
64,00 0,00 0,074 479,14 -18,20
64,00 1,00 0,074 479,14 304,61
69,30 1,17 0,074 479,14 357,17
74,60 1,36 0,074 479,14 413,92
79,91 1,56 0,074 479,14 474,84
85,21 1,77 0,074 479,14 539,94
90,51 2,00 0,074 479,14 609,23
160,00 2,00 0,074 479,14 531,87
Now the summarized tail loads for different airspeeds and different
CG's during cruise flight (n=1) - these are the interesting numbers...
;)
Note that negative P_0 means that the tail force points downward.
The only thing you have to do now is to collect the P_0 for a given CG
and Ramp weight over all the airspeeds - then you can decide whether
the tail produces always lift or not.
For gliders the numbers are different, but the priciple is the same.
BTW:
This was the de-luxe version of my German article... <vbg>.
airspeed [km/h] n CG Roh ramp weight P_0
270,00 270,00 1,00 1,842 m 0,26 329,1 kg -298,6 N
270,00 1,00 1,823 m 0,28 389,1 kg -315,8 N
270,00 1,00 1,923 m 0,18 446,6 kg -179,6 N
270,00 1,00 1,963 m 0,14 447,1 kg -124,0 N
270,00 1,00 1,924 m 0,18 439,1 kg -180,5 N
270,00 1,00 1,842 m 0,26 329,1 kg -298,6 N
270,00 1,00 1,810 m 0,29 449,1 kg -333,0 N
270,00 1,00 1,924 m 0,18 569,1 kg -138,4 N
270,00 1,00 2,038 m 0,07 479,1 kg -1,2 N
270,00 1,00 1,975 m 0,13 599,1 kg -35,5 N
280,00 280,00 1,00 1,842 m 0,26 329,1 kg -323,1 N
280,00 1,00 1,823 m 0,28 389,1 kg -340,2 N
280,00 1,00 1,923 m 0,18 446,6 kg -204,1 N
280,00 1,00 1,963 m 0,14 447,1 kg -148,5 N
280,00 1,00 1,924 m 0,18 439,1 kg -204,9 N
280,00 1,00 1,842 m 0,26 329,1 kg -323,1 N
280,00 1,00 1,810 m 0,29 449,1 kg -357,4 N
280,00 1,00 1,924 m 0,18 569,1 kg -162,8 N
280,00 1,00 2,038 m 0,07 479,1 kg -25,6 N
280,00 1,00 1,975 m 0,13 599,1 kg -59,9 N
290,00 290,00 1,00 1,842 m 0,26 329,1 kg -348,4 N
290,00 1,00 1,823 m 0,28 389,1 kg -365,6 N
290,00 1,00 1,923 m 0,18 446,6 kg -229,4 N
290,00 1,00 1,963 m 0,14 447,1 kg -173,8 N
290,00 1,00 1,924 m 0,18 439,1 kg -230,3 N
290,00 1,00 1,842 m 0,26 329,1 kg -348,4 N
290,00 1,00 1,810 m 0,29 449,1 kg -382,7 N
290,00 1,00 1,924 m 0,18 569,1 kg -188,1 N
290,00 1,00 2,038 m 0,07 479,1 kg -50,9 N
290,00 1,00 1,975 m 0,13 599,1 kg -85,3 N
300,00 300,00 1,00 1,842 m 0,26 329,1 kg -374,6 N
300,00 1,00 1,823 m 0,28 389,1 kg -391,8 N
300,00 1,00 1,923 m 0,18 446,6 kg -255,6 N
300,00 1,00 1,963 m 0,14 447,1 kg -200,0 N
300,00 1,00 1,924 m 0,18 439,1 kg -256,5 N
300,00 1,00 1,842 m 0,26 329,1 kg -374,6 N
300,00 1,00 1,810 m 0,29 449,1 kg -409,0 N
300,00 1,00 1,924 m 0,18 569,1 kg -214,4 N
300,00 1,00 2,038 m 0,07 479,1 kg -77,1 N
300,00 1,00 1,975 m 0,13 599,1 kg -111,5 N
110,00 110,00 1,00 1,842 m 0,26 329,1 kg -28,4 N
110,00 1,00 1,823 m 0,28 389,1 kg -45,6 N
110,00 1,00 1,923 m 0,18 446,6 kg 90,6 N
110,00 1,00 1,963 m 0,14 447,1 kg 146,2 N
110,00 1,00 1,924 m 0,18 439,1 kg 89,7 N
110,00 1,00 1,842 m 0,26 329,1 kg -28,4 N
110,00 1,00 1,810 m 0,29 449,1 kg -62,8 N
110,00 1,00 1,924 m 0,18 569,1 kg 131,8 N
110,00 1,00 2,038 m 0,07 479,1 kg 269,0 N
110,00 1,00 1,975 m 0,13 599,1 kg 234,7 N
120,00 120,00 1,00 1,842 m 0,26 329,1 kg -38,7 N
120,00 1,00 1,823 m 0,28 389,1 kg -55,8 N
120,00 1,00 1,923 m 0,18 446,6 kg 80,3 N
120,00 1,00 1,963 m 0,14 447,1 kg 135,9 N
120,00 1,00 1,924 m 0,18 439,1 kg 79,5 N
120,00 1,00 1,842 m 0,26 329,1 kg -38,7 N
120,00 1,00 1,810 m 0,29 449,1 kg -73,0 N
120,00 1,00 1,924 m 0,18 569,1 kg 121,6 N
120,00 1,00 2,038 m 0,07 479,1 kg 258,8 N
120,00 1,00 1,975 m 0,13 599,1 kg 224,5 N
130,00 130,00 1,00 1,842 m 0,26 329,1 kg -49,8 N
130,00 1,00 1,823 m 0,28 389,1 kg -66,9 N
130,00 1,00 1,923 m 0,18 446,6 kg 69,2 N
130,00 1,00 1,963 m 0,14 447,1 kg 124,8 N
130,00 1,00 1,924 m 0,18 439,1 kg 68,4 N
130,00 1,00 1,842 m 0,26 329,1 kg -49,8 N
130,00 1,00 1,810 m 0,29 449,1 kg -84,1 N
130,00 1,00 1,924 m 0,18 569,1 kg 110,5 N
130,00 1,00 2,038 m 0,07 479,1 kg 247,7 N
130,00 1,00 1,975 m 0,13 599,1 kg 213,4 N
140,00 140,00 1,00 1,842 m 0,26 329,1 kg -61,8 N
140,00 1,00 1,823 m 0,28 389,1 kg -78,9 N
140,00 1,00 1,923 m 0,18 446,6 kg 57,2 N
140,00 1,00 1,963 m 0,14 447,1 kg 112,8 N
140,00 1,00 1,924 m 0,18 439,1 kg 56,4 N
140,00 1,00 1,842 m 0,26 329,1 kg -61,8 N
140,00 1,00 1,810 m 0,29 449,1 kg -96,1 N
140,00 1,00 1,924 m 0,18 569,1 kg 98,5 N
140,00 1,00 2,038 m 0,07 479,1 kg 235,7 N
140,00 1,00 1,975 m 0,13 599,1 kg 201,4 N
Bye
Andreas
Eric Greenwell
In article <858a1...@enews3.newsguy.com>, dhof...@oakland-info.com
says...
= In article <MPG.12e102f26...@news.prodigy.net> Eric Greenwell,
= egree...@prodigy.net writes:
= >I missed the earlier part about "maximum forward CG", and I agree the
= >best CG at a particular speed is not the maximum forward CG, until you
= >get to a very high speed.
=
= I'm still not sure that I follow. Are you saying that at very high
= speeds, I presume near Vne, one normally *wants* max forward CG?
= If this is what you are saying then I still have to disagree. But
= perhaps I am not following your thought...
Here is what I was trying to say:
"I missed the earlier part about "maximum forward CG", and I agree the
best CG at most speeds is not the maximum forward CG, except perhaps
when you get to a very high speed (near VNE, say)."
I'm sure it varies from glider to glider, in particular between
flapped and flapped gliders.
--
Donald Ingraham
> >minimum airspeed) - at very low speeds the tail indeed produces
> Thanks Andreas, I appreciate the time you took to post this,
Andreas,
Same here! Thanks. (BTW, private e-mail to you bounces...?)
I will need to print off your posting and study it a bit.
Your numbers suggest that if I were to drill a series of
holes at the min-sink center-of-lift along my main wing and
attach a rope in each hole...then lift the glider (with
pilot) off the ground by these ropes, the tail would drop,
not the nose! This also means that when I then slowly pull
back on the stick, approaching stall, the rising elevator
is actually *reducing* the positive lift that the tail is
already producing, not increasing negative lift?
Most bizarre. Then again, maybe I am here at work so that I
act as less of an impediment to the river of money that
would otherwise come my way! ;-)
Don
_______________________________________________________
Don Ingraham
SGI
d...@sgi.com
Andreas Maurer
wrote:
>BTW, private e-mail to you bounces...?)
I changed my e-mail address:
Now it is mau...@funsystem.de
>I will need to print off your posting and study it a bit.
I can provide you with lots of other numbers, if you are interested.
I've taken the numbers from a huge Excel-spreadsheet I created to
design the aerodynamics of the "Impulse". If you are interested, I
could send you a copy. It's complex, and it is in German. But it is
veeery interesting - it's basically a complete designer for an
aircraft's aerodynamic.
>Your numbers suggest that if I were to drill a series of
>holes at the min-sink center-of-lift along my main wing and
>attach a rope in each hole...then lift the glider (with
>pilot) off the ground by these ropes, the tail would drop,
>not the nose!
Perhaps not at the speed of minimum sink, but at low speed, yes.
>This also means that when I then slowly pull
>back on the stick, approaching stall, the rising elevator
>is actually *reducing* the positive lift that the tail is
>already producing, not increasing negative lift?
I a certain way, yes - at a certain AoA the CM_0 of the wing tends to
lower the tail, therefore the tail must deliver (slight) lift to keep
the plane stable. Without a tail it would try to fly a *very* small
positive loop. On the other hand - at low AoA a wing without a tail
would try tu decrease the AoA further - the tail must deliver negative
lift to provide longitudinal stability.
Just like all the basic gliding-textbooks say... ;)
BTW: I don't think that pulling further back reduces the positive lift
that the tail is already producing - the AoA of the tail rises,
therefore it's lift rises. Or not. Maybe. Maybe not. God knows.
I let the computer do the calculations - I find it hard to think
around five edges... ;)
>Most bizarre. Then again, maybe I am here at work so that I
>act as less of an impediment to the river of money that
>would otherwise come my way! ;-)
<vbg>
Bye
Andreas
mil...@cgpp.com
mau...@funsystem.de (Andreas Maurer) writes:
> BTW: I don't think that pulling further back reduces the positive lift
> that the tail is already producing - the AoA of the tail rises,
> therefore it's lift rises. Or not. Maybe. Maybe not. God knows.
I think this is basically right - depending on various factors, the
stab-Cl can change with trim airspeed, moving in the "positive" (up)
direction as the trim speed diminishes. This happens even though the
stick is coming aft. As you point out, the AOA is changing too.
This is one point in favor of all-flying tails: in a conventional
stab/elevator setup, it's basically a flapped airfoil, with the flap
deflection going increasingly negative as the stab Cl increases. This
is just the opposite of what you'd want to keep the stab airfoil
working properly.
If you'll forgive a quick plug: there's a discussion of this, with a
diagram or two, in "Fundamentals of Sailplane Design", see
http://www.cgpp.com
Judah
--
Judah Milgram mil...@cgpp.com
P.O. Box 8376, Langley Park, MD 20787
(301) 422-4626 (-3047 fax)
Donald Ingraham
> If you'll forgive a quick plug: there's a discussion of this, with a
> diagram or two, in "Fundamentals of Sailplane Design", see
> http://www.cgpp.com
A couple friends of mine have burrowed into this book
and are enjoying it very much. I suppose I should do the
same. Thanks for the explanations, Andreas, and the
pointer, Judah.
Don