SWEPT WING BALANCE POINT?

What is the proper way to determine the balance point when scratchbuilding a swept wing design? Please no hi-tech formulas here, just a simple method would be great.

These two sites show two different methods. I think the second one is correct.
http://www.lferc.com/Tips/mean_aerodynamic_chord.php
http://moleski.net/rc/WingMAC.htm
Pat MacKenzie

> These two sites show two different methods. I think the
>second one is correct.
>http://www.lferc.com/Tips/mean_aerodynamic_chord.php
>http://moleski.net/rc/WingMAC.htm
>Pat MacKenzie

I think you'll find that both methods define the same location of the MAC.

For design purposes you can consider the "MAC" thus located as "the" wing. Locate the CG at the MAC as you would for any "conventional" planform. Generally a CG in the neighborhood of 15 to 25% aft of the leading edge at the MAC will provide a flyable stunter.

Make your initial guesstimate of the CG location based on the size of the tail area relative to the wing area. A big tail (25% of the wing area or more) can easily stabilize and CG at 25% of the MAC. If the tail is only 15% of the wing area go for the 15% CG to start with. Fine tune from there.

Yep. Disregard the notes on those drawings that say "neutral point" and "cg". Other than that, they're OK. Then listen to Ted.

>Make your initial guesstimate of the CG location based on
>the size of the tail area relative to the wing area. A big
>tail (25% of the wing area or more) can easily stabilize and
>CG at 25% of the MAC. If the tail is only 15% of the wing
>area go for the 15% CG to start with. Fine tune from there.
>
>Ted Fancher

Everyone read this about 20 times.

We have found that the rudderlets as used on the Bears seems to add a couple of percentage points of tail area, because a Bear tail will function without stalling a at a CG position 2-3 percentage points more aft. I say stalling, and I know someone will rip me a new one, but I have no other explanantion for the phenomenon.

>
>>Make your initial guesstimate of the CG location based on
>>the size of the tail area relative to the wing area. A big
>>tail (25% of the wing area or more) can easily stabilize and
>>CG at 25% of the MAC. If the tail is only 15% of the wing
>>area go for the 15% CG to start with. Fine tune from there.
>>
>>Ted Fancher
>
>Everyone read this about 20 times.
>
>We have found that the rudderlets as used on the Bears seems
>to add a couple of percentage points of tail area, because a
>Bear tail will function without stalling a at a CG position
>2-3 percentage points more aft. I say stalling, and I know
>someone will rip me a new one, but I have no other
>explanantion for the phenomenon.

The only caveat I would add to 'zilla's comments is that once you get the CG back to 25% of the MAC moving it much further aft should be done with caution. Up to the point you get the CG and the Center of Lift of the wing in roughly the same longitudinal (fore and aft)location, i.e. right around 25% MAC, deviation from that point in either direction will generate a moment about the CG.

To wit:

If the Center of Lift moves aft of the CG (a forward CG location) the production of lift will cause a forward pitching moment which must be overcome by the tail before a pitch in the desired direction can be made.

If, by virtue of a very large or very effective tail ('zilla's rudderlet scenario) you can safely move the CG aft of the Center of Lift the opposite pitching moment would be induced. I.e., lift generated at the 25% MAC point would be acting in the desired pitch direction thus accelerating the pitch change. While, on the surface, this sounds desireable, I feel such acceleration results in the appearance of the vehicle "accelerating 'into'" the corner as opposed to "flying 'around' it". Not my personal preference.

IMHO the roughly co-located CG and CL are the more desireable circumstance because the development of the large amounts of lift demanded by rapid pitch change requirements will neither impede nor accelerate the anticipated rate of change the pilot desires. This
"balanced" lift vice load condition is a large part of what makes a well trimmed stunt ship retain its maneuverability when flying in high winds and turbulence.

Just another thought regarding all this talk.

It's fun to think about extremes of these lift/load/moments issues. Consider that we can continue to make the tail larger and larger and the Neutral point of the whole vehicle will continue aft as well. The more or less ultimate extreme of such is nothing other than the canard configuration where the rear surface is larger than the forward surface by anywhere from a tiny bit to a whole lot.

Note (just by thoughtful contemplation of the lever and arms of such a set-up...no big math required)that once you have a recognizable "canard" it become obvious even to the untrained eye that balancing at 25% of the forward surface is out of the question. Number one it would be almost impossible to balance any vehicle right at the virtual front of it and; #2 It is obvious that even if you could the tiny (comparatively) forward plane couldn't support it in level flight, let alone affect any aerobatic maneuvering.

This plain language understanding of what happens to the lift/CG relationship when a conventional planform translates to a canard is an easily understood example of a very effective way for the unschooled would be designer to determine the effectiveness of contemplated aerodynamic changes to an existing design or, like Wayne's interest in a swept wing, approaching the development of something unusual.

The likely effects of subtle changes about which a novice designer may find himself in a quandary about can be much more easily determined if you simply take the proposed change to an extreme. Aerodynamic is pretty much a game of balance. If you cange something dramatically it affects will be as plain to the untrained eye as the effects of handing one kid at the left end of a balance teeter totter a hundred pound bag of cement. You don't need a degree in physics to know that that end of the teeter totter is going to drop like...well, like a bag of cement,
ted

> These two sites show two different methods. I think the
>second one is correct.
>http://www.lferc.com/Tips/mean_aerodynamic_chord.php

I checked this site. It has a lot of other good stuff. Somebody wanted the formula for comparing 2- and 3-blade props. It's here. ALso 4- and 5- bladers.

Wayne:

What are you building with a swept wing anyway?

Nothing in the immediate future, just gathering info.

W.W.

Wayne-

You probably missed this, since it's been several months since the last time it was posted/cited on SSWF. This site, among others, will provide a more accurate, quick, and CONVENIENT determination of the information you seek than the graphical method cited, in which you must otherwise draw quite accurately so that obliquely intersecting lines meet near where they should:

http://www.palosrc.com/instructors/mac.htm

You may easily find the effects of varying sweep and taper here too. The formulae on which their results are based are valid.

You guys don't know Wayne yet. His wheels are always turning. The results are always classy and very well done. How about a profile T-28? Or a profile Hughes Racer made from a Sig Shoestring wing? Or a gorgeous Midwest P-51 named "Horsin' Around"? Or a Sig 1/2A Skyray in military trainer colors and Norvel power that will do the full pattern? May not sound like much, but they are very clever and the craftmanship is out of this world. Jeff, we need to get some pictures of Wayne's work up here.
Kelvin

I thought so. The Moleski site just gives you a chord half way between the root and tip. Tapering the wing moves the MAC closer to the center of the wing. The more taper the closer to the center. It isn't much on a typical stunter, but it can have an effect.

>Tapering the wing moves the MAC closer to the center of the wing. The more taper the closer
>to the center. It isn't much on a typical stunter, but it can have an effect.

Yes, and an important point.

The limit would be at 33 1/3% of the halfspan for a straight-tapered, pointed wing (i.e. triangular), a consequence of the old geometry theorem about where the medians of a triangle meet. Even with common tapers though, the MAC is often located between 44% and a bit over 47% of the half span from the root, making it larger than the average chord. So finding the actual value, rather than just taking the average chord, is worthwhile - especially when you only have to plug in dimensions on the Palos Verde sight for an immediate answer.