influence of C/G on wing section

[Dandiy]

Hi guys
Can anyone tell me the effect C/G position has on the performance of a given aerofoil.
We all know that a rearward C/G position is a great aid to thermal recognition,and with a long tail moment and large elevator it is possible to get the C/G back as far as 50% but what does this do to the performance of the aerofoil?
Thanks Dandiy

[Robglover]

now there's a fine can of worms....

[dephela]

There's nothing on a Lift-Drag polar that suggests CG.

[nmasters]

Actually CM dose, it's just not very easy to read. The old NACA airfoil characteristics charts that showed CP instead of CM are more informative in this respect. In this old system you could simply read where the lift acted at a given AoA. Notice that this section has its best L/D at α=0 and the center of pressure at that AoA is about 57%c. By balancing the plane at 57% of MAC you can have zero trim drag at the angle of attack for best glide but a CG that far aft requires a lot of tail authority.

--Norm

[Dandiy]

Hi
To be a bit more specific the sections I am looking at is SD7032 and AG25
Hope that helps
Ta folks
Dandiy

[histarter]

Actually CM dose, it's just not very easy to read. The old NACA airfoil characteristics charts that showed CP instead of CM are more informative in this respect. In this old system you could simply read where the lift acted at a given AoA. Notice that this section has its best L/D at α=0 and the center of pressure at that AoA is about 57%c. By balancing the plane at 57% of MAC you can have zero trim drag at the angle of attack for best glide but a CG that far aft requires a lot of tail authority.

--Norm
Congradulations Norm. Great wisdom! :D

Tailplane authority mentioned I determine to be a velocity servo system to regulate airspeed for control of center AoA that the grounded pilot modulates. Of course that would reduce the size of the stab needed for compensation, and return it back to just damping factor projections/proportions. :)
[That is Doyle Modesto's and my thinking on the subject, using series thermisters in a pitot tube for the onboard computer's velocity sensor].

[Neil Stainton]

Can anyone tell me the effect C/G position has on the performance of a given aerofoil.

The airfloil doesn't know the CG position and isn't affected by it.

We all know that a rearward C/G position is a great aid to thermal recognition,and with a long tail moment and large elevator it is possible to get the C/G back as far as 50% but what does this do to the performance of the aerofoil?

CG doesn't affect the airfloil, but as others have said it does affect the forces on the tail, and thus the induced drag portion of the tail drag can be reduced with an appropriate CG. However this change in drag is tiny, and anyway max glider performance (which would need to be defined better) doesn't usually coincide with minimum tailplane induced drag. So set your tailplane size for sufficient damping. Set your CG for sufficient stability, and don't worry about the magnitude or direction of the tail forces.

Neil.

[histarter]

The airfloil doesn't know the CG position and isn't affected by it.



CG doesn't affect the airfloil, but as others have said it does affect the forces on the tail, and thus the induced drag portion of the tail drag can be reduced with an appropriate CG. However this change in drag is tiny, and anyway max glider performance (which would need to be defined better) doesn't usually coincide with minimum tailplane induced drag. So set your tailplane size for sufficient damping. Set your CG for sufficient stability, and don't worry about the magnitude or direction of the tail forces.

Neil.
Excellant FF phylosophy! You are a thinking man that enjoys a direct simple effective solution. Your concept works best with short coupling because of less induced error due to circular airflow, i.e. when thermalling at high bank angles.

Unfortunately Airfoils do have changes in CP as alpha is varied. Using CG to trim for slow flight with a pre-programmed decalage, can yield a regenerative tuck with some profiles because of a mechanical 'Mu' (amplification factor).

Those involved with the Golden Age (1936) were no dummys, and their info is totally viable today, because air has not changed much! :D :D

[Neil Stainton]

Excellant FF phylosophy!

No, excellent airplane philosophy.

You are a thinking man that enjoys a direct simple effective solution.

Correct.

Your concept works best with short coupling because of less induced error due to circular airflow, i.e. when thermalling at high bank angles.

No, it works well with all reasonable tail moment arms.

Unfortunately Airfoils do have changes in CP as alpha is varied.

Indeed, that is why we need a tailplane.

Using CG to trim for slow flight with a pre-programmed decalage, can yield a regenerative tuck with some profiles because of a mechanical 'Mu' (amplification factor).

Really? I think using variable decalage can cause terminal nip due to interstitial Zeta. OTOH I think Dandiy and I are flying Bubble Dancer/AVA type models with all flying tailplanes, so maybe our models can fly in your philosophy as well as mine, Horatio?

Neil.

[histarter]

No, excellent airplane philosophy.



Correct.



No, it works well with all reasonable tail moment arms.



Indeed, that is why we need a tailplane.



Really? I think using variable decalage can cause terminal nip due to interstitial Zeta. OTOH I think Dandiy and I are flying Bubble Dancer/AVA type models with all flying tailplanes, so maybe our models can fly in your philosophy as well as mine, Horatio?
Neil.
Your summation is absolutely correct, however when it comes to my technology, you seem really too prejudicial to actually evaluate what I have said. I gave the reasons why a judgment can go either way - without prejudice. You on the other hand implied that only your way is the right way; is it just because it is the opposite point of view of my favoring simplicity? Thus your oxymoronic argument of “you are wrong;” but either way is correct. Duh! Demonstrates a non-professional hateful attitude that is totally contrary to the gentlemanly respect that I expose to you.

Tony uses a basic formula that calculates IDEAL sink of a sailplane attempting to do a 50 ft spiral. I give him a more advanced kick-sort program that extrapolates more attributes while feeding in statistical data of airspeed (nominal that pilots fly at) into the hopper for factoring, while demonstrating pilot loading because of an offset error due to circular airflow, and he goes ballistic without respect for all the great mathematicians that approved of my technique. Hey, because the doctors of math don’t all fly model airplanes, they are stupid? This is a very foolish syndrome. :o

Enough criticism (that is not my forte’) and let’s grow up and utilize RC Groups as intended. A think tank for soaring improvements. :) :cool:

[bjr_93tz]

Jeeze highstarter, give it rest.

The bloke asked a basic question about cog and airfoil performance and a got a few basic straight forward answers. End of story.

There are a many aspects of life and Engineering (as you are one or were one) where simplifying a model is more than good enough to achive a desired practical outcome. We both know Ohms law is "close enough is good enough"

You have a wheelbarrow to push when it comes to soaring and I can appreciate that peoples concept of soaring and efficiency differ and that moldies may not be as "efficient" as gasbags under certain conditions and launch constraints but please don't let it contaminate any constructive comments and advice you or others might put forward..

By all means if someone has made a glaringly wrong statement then correct them..

Cheers

[Jurgen]

...but what does this do to the performance of the aerofoil?The performance of an airfoil is revealed by its polars, these do not change because they're only airfoil shape dependant. For obtaining the polars you do not have to input CG, nor does there exist different sets of polars for different CG's. So the potential performance is fixed, like a hummer stays same powerful car with same HP even when it is parked.

Only the point where you use this performance (meaning where on the polars are you working) can change at will. For flying steady slope wind you want more aft CG compared to flying thermals in turbulent air, so the "best appropriate" usage (is this also called "performance"?) of same wing is dependent on circumstances and not at least personal taste. With same wing, often a V-tail ask for a more forward CG than a X-tail so best performance of a whole plane is more than strictly airfoil matter.

CG change is changing plane behavior, one pilot might perform better this way, the other one not. There's no definite answer to your question. Give it a go yourselves, play around and look what works for you.
Jurgen.

[Dandiy]

To add to the debate does not the position of the C/G also have an effect on the AoA? and therefore the min sink speed?
And does the change in AoA have an effect on the efficiency of the chosen wing section?
Yes I do fly a model similar in design to Neil in as much as the plan form is the same, but my opinions are all my own.
Thanks for the input so far
Cheers Dandiy

[MarkusN]

To add to the debate does not the position of the C/G also have an effect on the AoA? and therefore the min sink speed?
Yes and no. AOA is dependent on wing moment, stab moment and CG. For any (flyable) CG you can trim all desired AOAs by varying decalage / elevator trim.

Of course, as others have mentioned, (induced) stab drag will change the overall drag slightly. So if an unloaded stab is possible for your desired AOA that gives best performance in that point.

However, questions of handling are more important when considering CG. It doesn't help if you have set your C/G for optimum performance, but fly far off the optimum set point because your plane handles badly.

[histarter]

Yes and no. AOA is dependent on wing moment, stab moment and CG. For any (flyable) CG you can trim all desired AOAs by varying decalage / elevator trim.

Of course, as others have mentioned, (induced) stab drag will change the overall drag slightly. So if an unloaded stab is possible for your desired AOA that gives best performance in that point.

However, questions of handling are more important when considering CG. It doesn't help if you have set your C/G for optimum performance, but fly far off the optimum set point because your plane handles badly.
You guys made me quite happy as a teacher. You're now engaging in think tank discussions rather then derogatory comments. Finally progress, to make for soaring with picante. :cool:
Flying the AVA or Topaz with ballast gets airspeed up to improve re, LD, and square law tailplane damping that makes CG location less critical, even though it hurts spiraling efficiency. In the upper air where thermals are larger, it is a win-win situation! :D
The bad thing about modern costly sailplanes is that isolates soaring butchers (like myself) from enhancing a design for personal satisfaction. Taking a Topaz and whacking off 1/2 the tail boom makes it worthless in the eyes of the experts..even if it were to place at the Nats. If it wins, everyone would want one! :D :D

[Dandiy]

Hi Guys
Thanks for the input so far,another question then
Can a sailplane be "overstabalized" I.E.can the stab size and moment be to large and effectively kill any deviations to level flight that would otherwise indicate thermal activity?
Put another way can the C/G be to far back with the help of to big a stab or to long a moment and effectively damp out any small thermal indicators?
Your opinions please
Thanks Dandiy

[dephela]

By having a large stab or a long tail moment you increase the "tail volume coefficient". A larger TVc moves the Neutral Point to the rear. It the CG is left in the same place as with a smaller TVc, the Static Margin of Stability has been increased. To keep the plane "bouncy" the CG would have to be set back and of course the incidence reset also, reduced slightly.

Dennis

[histarter]

Hi Guys
Thanks for the input so far,another question then
Can a sailplane be "overstabalized" I.E.can the stab size and moment be to large and effectively kill any deviations to level flight that would otherwise indicate thermal activity?
Put another way can the C/G be to far back with the help of to big a stab or to long a moment and effectively damp out any small thermal indicators?
Your opinions please
Thanks Dandiy
My 2 mtr Shuttle has a large 18% stab with an 18 inch TMA and a 10% vert stabilizer. Since the stab enlargements evolved by chord increases over the years, and with the aircraft dedicated to the short launch HAM system or 1/6 hp winch, I found only advantages. Launch height is unequaled against more modern conventional sophisticated designs, and no apparent degrade in performance – handling wind to 30 mph in competition. And the highest data string of Max’s, (9) recorded.

In seeking performance at low speed for thermalling enhancement, there seems to be pride in handling a tricky machine i.e. with all the design criteria centered upon max LD. Long moments mean high tailplane error in a spiral that the pilot has to override because of circular airflow, in addition to fighting excessive spiral stability. As a slow witted engineer that has to read an instrument panel serially instead of broad-sided, I needed all the help I could get when I re-designed my Shuttle for the year 2000. :D

My attempts at long moments with large stabs (overstabilizing) worked well at slow flight; however, with CG back they were 'clumsy' when doing speed runs to known lifting areas. ;)

My next design step would be to shorten the Shuttle TMA and duplicate the EPP Hawk configuration, but time has run out for me. I feel sorry for you guys with the expensive machines because you just are unable to enjoy butchery. Filtering out engineers and innovators truly seems to be soaring goals for big 'kids' to enjoy the available toys from corporations, instead of individual contributions.

[schrederman]

To the original question, and avoid the tail moment argument, let's turn this around. The CG doesn't influence the wing section, however, the wing section selection has a real effect on allowable CG range. We're trying to achieve a forces-in-equilibrium situation, where the center of lift and the center of gravity's pull are very near each other. My Nordics had very deeply-cambered airfoils, and balanced in the neighborhood of 50% MAC. They also had long moments and lifting stabs. We're fortunate not to have to deal with trying to balance a light nose and a lifting stab. Different airfoils produce their center of lift at different positions along their chords. Some have greater pitching moments, etc.

Chew on that one for a while.

Jack

[MarkusN]

To the original question, and avoid the tail moment argument, let's turn this around.
Thing is, you can't really do that, because:

The CG doesn't influence the wing section, however, the wing section selection has a real effect on allowable CG range. We're trying to achieve a forces-in-equilibrium situation, where the center of lift and the center of gravity's pull are very near each other.
True. Now deeply cambered wings have widely shifting center of pressure. (large pitching moments) To be able to trim that over a wide range of CL needs a large tail volume. Which in turn shifts the total aerodynamic center back and only that allows to have the CG so far aft. (Which in turn makes the lifting stab sensible. It's not really the lifting wing section of the stab that makes the CG shift back, but having the CG far back puts load on the stab, which makes it sensible to give it a cambered profile.)

You bring again forth the argument of optimising CG for CP location. Which depending on Cm and design Cl may not be not possible, as it may place the CG in a position where stability is not possible.

Still, many successful designs have CG close do desing CP of the wing. But you cannot use that approach as a design guideline, because the stability implications of CG location take precedence over performance implications.

[Neil Stainton]

But you cannot use that approach as a design guideline, because the stability implications of CG location take precedence over performance implications.

Thanks Marcus, for a very good and clear post. I particularly like the summary above.

Neil.