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Author Topic: Wing sweep as a pitch stabilizing factor  (Read 827 times)

Online Mike Alimov

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Wing sweep as a pitch stabilizing factor
« on: September 21, 2021, 03:19:47 PM »
First, some definitions are in order.
 I will be referring to wing sweep as a sweep of its quarter chord (25% chord, including flaps) as viewed from the top; not the leading edge sweep as drawn on plans.
 Second, when I talk about stability, I will be talking about pitch stability (up/down as seen from the pilot's point of view).

Reference: in a 1980 article about his airplane, Anatoly Kolesnikov ('86 W.Ch) says that wings with low sweep contribute to better maneuverability, but does not explain why.

Data:  last weekend, I was trimming out two new-to-me airplanes.  In the end, both planes were trimmed out to give the same rate of turn (as or close as I could tell, flying them back to back on the same day).  Both planes have very similar wing area, airfoil, tail volume, weight, wing loading, powertrain and propeller, lines, lap times, etc.  Difference: one plane had wing that was swept forward (see definition above), and it ended up with CG at 23% of mean chord.  The one with the rear sweep ended up with CG at 25% of mean chord.

Based on this, it appears that rear wing sweep indeed makes them more stable in pitch compared to neutral or forward swept wings (thus requiring more rearward CG to turn just as well), but I can't explain why.  I can easily see how a swept back wing acts as a weathervane and helps with yaw stability, but why also pitch?

Online Mark wood

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Re: Wing sweep as a pitch stabilizing factor
« Reply #1 on: October 07, 2021, 06:42:10 AM »
That's an interesting observation but a question arises out of the gate. How similar are the airplanes? Also when you say Mean Chord, how did you derive mean chord which is important to know. Average chord and mean aerodynamic chord are not the same.

In terms of sweep and maneuverability this has been a long topic in many circles. The most common good reason to sweep a wing is for aerodynamic drag where the wing is swept for high mach number operations in order to reduce the T/C ratio. Sweeping has positive and negative effects on an airplane. Sweeping the wing can help with yaw damping and provide some yaw / roll coupling. Depending on the the application these are positive effects.

Longitudinally sweeping the wings can, in fact, change the pitching moment of the wing for a given area and aspect ratio. Similar to the way increasing aspect ratio of a wing increases the lift slope sweeping the wing aft steepens the slope of the pitching moment and forward sweep decreases the slope. How this translates to operation is that an aft sweep would tend to increase the stability of a given planform while forward sweep would tend to decrease the stability. For the nit pickers, the moment coefficient is a negative value generally and increasing sweep results in a more negative Cm slope providing greater pitch damping.

It's not clear to me which configuration did what in your post. However for small sweep angles I wouldn't expect much variation in CG WRT the MAC, if any, for a given planform. The results posted tend to indicate measurement of the CG from the LE at a fixed location such as the fuselage. Sweep would move the MAC aft and a CG of 23% MAC would move aft with it and appear to be a shift in CG at the root making it appear to be 25%. This then would draw the conclusion sweep has improved the longitudinal stability. The most common way of relating the CG is in terms of its location WRT the LE of the MAC or LEMAC. Using this convention eliminate confusion of where the CG resides aerodynamically. I'm sure the transport jet guys will chime in on this one.

I'm not sure the results actually indicate increased stability as that test isn't in the description. Steady state cases such as level flight turns continuous looping don't necessarily indicate stability. Stability is more better defined by the return to condition after perturbation. Places where increased stability will show up are in the exit of the corners where some flight path "ringing" can occur. That little bobble that cycles a few times and damps out, that's where the stability of the system will show up the best. System including all connections including the meat servo feedback driver in the center which often is out of phase generating greater gyrations and masking any inherent positive stability of the air vehicle. However, the air vehicle is a significant portion of this element and any lack of stability is best highlighted there. In full size testing we use a series of rapid control inputs to drive this and watch for the damping after cessation of the inputs.

My experience with wing sweep is somewhat limited but I have some with both full size and model size testing. My input on models is that sweep brings with it a couple negatives which cause me to not consider design which incorporate any sweep. Number one is that a wing with sweep, when it stalls, unless it is flying with zero yaw, very unlikely, one wing will stall first driving the airplane in to a "snap". People call it tip stall. Number two is yaw - roll coupling. Any yaw rate will generate a rolling moment. I know, many of the very best models have some level of sweep to them and it could be that the sweep compensate for the angle of the lines to some degree. However every airplane I've flown with sweep exhibited some level of this negative and airplanes without sweep demonstrably much less. Again, I know, this is a subjective observation and not a definitive A-B-A test.
 
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
The aerobatic airplane is a tool, a pencil, a paintbrush the artist uses to paint their aero art onto the tapestry air with. That art is the combined elements of figures drawn in space and time.  - M. Wood

Online Mike Alimov

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Re: Wing sweep as a pitch stabilizing factor
« Reply #2 on: October 11, 2021, 09:57:24 AM »
Mark, thank you for your input. Took me a while to digest all of that, but I think I got it and largely agree. 
Obviously, we are operating at deep subsonic (in fact, near-stall) speeds and vastly different Re numbers than any full-size aircraft.  Fruthermore, my comparison was not 100% scientific, because the two airplanes being compared are different designs, although they share many common numbers.  A pure test would have been to take a given airplane, and install wings at different sweep angles while maintaining CG location with regards to the mean chord and recording their response to abrupt control input.  I sent this request to my R&D department, but they said they are at least a year behind.  >:D

Offline Avaiojet

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Re: Wing sweep as a pitch stabilizing factor
« Reply #3 on: October 11, 2021, 12:13:53 PM »
Many of the EAA guys are steps ahead using sweep wings and finding no adverse flight characteristics with "some" of the designs which are currently flying. Many in fact.

Google "Synergy Aircraft," there should be plenty of reading plus photos plus information on this unusual aircraft, sweep wing design.

Years ago I developed an interest in this design, myself, being a washed up former commercial pilot, it wasn't difficult.  ;D

Sure, I contacted the man behind the design and with much in common, mostly the love of GA, (General Aviation,) John McGinnis sent me a great three view with the CG already placed. Sure, I had to promise a few things, never to make a commercial kit or sell plans of his design.

That's kind of funny because I know of no one interested in the concept or the design of this aircraft, except for John McGinnis' followers, employees and volunteers. Oh! and myself, LL~

Plenty of R/C guys have already copied the Synergy design and have great flying models.

John McGinnis built a large Electric powered model of his design which flies as smooth as silk. Google for the video.

I posted a few years back looking for help with the lead out location/bell crank location on this unusual sweep wing model. I finally decided to place the bell crank on the CG and place the lead outs in a favorable location.

This is not the kind of model many would be interested in, but I certainly enjoyed designing and framing this model. It's getting close to completion.

You can see the lead out location. I would have had a difficult time determining the actual CG if to wasn't for John McGinnis. What a guy!!  H^^

https://en.wikipedia.org/wiki/Synergy_Aircraft_Synergy

Charles
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Offline Howard Rush

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Re: Wing sweep as a pitch stabilizing factor
« Reply #4 on: October 13, 2021, 02:45:43 PM »
Sweeping the wing can help with yaw damping and provide some yaw / roll coupling. 

The old vacuum tube yaw dampers on early straight-wing jets were so unreliable that designers started sweeping wings so artificial yaw damping wouldn't be needed.  The reduced t/c was a bonus.
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Offline frank williams

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Re: Wing sweep as a pitch stabilizing factor
« Reply #5 on: November 13, 2021, 08:27:05 PM »
Did someone say Sweep?  Try this.

Online Bob Hunt

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Re: Wing sweep as a pitch stabilizing factor
« Reply #6 on: November 14, 2021, 05:32:47 AM »
...or this.


Offline Dennis Toth

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Re: Wing sweep as a pitch stabilizing factor
« Reply #7 on: November 14, 2021, 08:55:44 AM »
Hey Bob, what about your swept forward experiment, did that do anything crazy in yaw or roll? As I remember it was a pretty good flying concept.

Mark,
I appreciate you detail but for the sake of many guys that have not taken any aero courses please at least once in each post spell out the complete name of the acronym being used.

I followed Mike's description, it was pretty straight forward and made sense between the two models. Seems a little sweep is not a bad thing but both models were able to be trimmed just fine.

Best,   DennisT

 

Online Mark wood

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Re: Wing sweep as a pitch stabilizing factor
« Reply #8 on: November 14, 2021, 12:16:32 PM »
Hey Bob, what about your swept forward experiment, did that do anything crazy in yaw or roll? As I remember it was a pretty good flying concept.

Mark,
I appreciate you detail but for the sake of many guys that have not taken any aero courses please at least once in each post spell out the complete name of the acronym being used.

I followed Mike's description, it was pretty straight forward and made sense between the two models. Seems a little sweep is not a bad thing but both models were able to be trimmed just fine.

Best,   DennisT

My bad. I always the assumption that airplane people know what he various terms are. MAC Mean Aerodynamic chord (not the average chord most people use). LE leading edge. WRT With Respect To. LEMAC Leading Edge at Mean Aerodynamic Chord. T/C thickness ratio - airfoil thickness divided by chord.  Did I miss any? Life's rough for geeks who don't speak regular English.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
The aerobatic airplane is a tool, a pencil, a paintbrush the artist uses to paint their aero art onto the tapestry air with. That art is the combined elements of figures drawn in space and time.  - M. Wood

Offline Dennis Toth

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Re: Wing sweep as a pitch stabilizing factor
« Reply #9 on: November 14, 2021, 03:19:57 PM »
Mark,
Thanks, that helps us follow the thoughts in the post. I think you got them all.

Best,   DennisT

Online Mark wood

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Re: Wing sweep as a pitch stabilizing factor
« Reply #10 on: November 15, 2021, 08:53:50 AM »
Frank, Bob. Not that you didnít already know it, those planes are badasscooledness for sure.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
The aerobatic airplane is a tool, a pencil, a paintbrush the artist uses to paint their aero art onto the tapestry air with. That art is the combined elements of figures drawn in space and time.  - M. Wood

Offline Howard Rush

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Re: Wing sweep as a pitch stabilizing factor
« Reply #11 on: November 15, 2021, 02:02:06 PM »
I'd fear rolling moment due to sideslip with that much sweep, but Don Hutchinson's F-86 flies great. 
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Offline Serge_Krauss

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Re: Wing sweep as a pitch stabilizing factor
« Reply #12 on: November 21, 2021, 07:28:19 PM »
As I suggested in the personal e-mails, find the actual aero center of each wing by computation from valid equations based on applying the MAC definition to the shapes of your wings. The lateral position for elliptical wings is wrong on the internet sites I've found (it is 4/(3 pi) - or .4244 - times the half span for any elliptical wing). But you're interested in the longitudinal position. If you have a well-defined wing, compute the position from the equation I provided, but if not, just make a rigid, uniform-density card board or foam cut-out in the accurate, scaled shape of your wing, and find its cg (center of gravity) or enter it in CAD to find the cg. Compute the MAC and then measure or calculate 1/4 of that computed MAC of the wing ahead of that balance point. That is the aero center from the basic definition. 1/4 of the MAC for the ellipse is [2/(3 pi)]Cr or .2122 times the root chord, independent of sweep.

I say do this, because you may find that regardless of whatever you choose as sweep, these computed a.c.'s for your wings might actually explain your experimental results. If you have ribs aligned at some given percent chord, my 3rd equation will work. Otherwise, find the c.g. of the wing by CAD or the rigid model experiment. This is the same position computed by the usual MAC formulas. Then choose your c.g. positions an equal % of the MAC ahead of the quarter-MAC points of your wings and compare. I'd be interested in whether your chosen (experimentally determined) aircraft cg's differ uniformly from the computed ones, realizing of course that things like yaw damping, flap moments, tail volumes, etc., may ultimately have caused your different choices.

Note that the c.g. of the wing in the CAD or balancing is not the same as the chosen c.g. for the flying model.

I do realize that putting things like these into words is awkward.

Offline Air Ministry .

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Re: Wing sweep as a pitch stabilizing factor
« Reply #13 on: November 21, 2021, 09:36:20 PM »
The converse is a straight wing , where gusts etc rock the tips & you get yaw .

But sweeping it sounds a bit of a oversimplification when youd be better off elliptical .  VD~

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