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Old Feb 21, 2015, 09:07 AM
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Ray
United States, CT, Plainfield
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50" Eliptical Planform

I've built several motorized plank-style planes such as this one and really like the way they fly. I've read here that an eliptical planform has advantages over the straight line leading edge planform. Should I expect the eliptical planfom, shown in the image below, to fly better than it's straight line counterpart? I would use multiple PW51 templates and wire cut the 12 sections out of EPP. I have a lot of scrap pieces that are just looking for a project.

Thanks!

RayBird (3 min 55 sec)
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Old Feb 21, 2015, 05:12 PM
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Ray
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Edited Planform

I edited the planform to more closely represent an ellipse. I'm interested if there's supporting opinion that this elliptical planform wil perform better than the straight line planform.

Thanks!
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Old Feb 21, 2015, 09:47 PM
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I read something many years ago by an aerodynamicist, where he said:

"The least drag results from the longest straight trailing edge".

Since then, we've seen wing planforms continue to reflect this line of thought.

In ye olden days, trailing edges often curved forward towards the leading edge.

Modern planforms are the opposite, with leading edges tapering and sweeping aft to meet the trailing edge.

Sailplane designers have to maximize aerodynamic efficiency to get more performance, whereas the motorheads usually just add more power.

Take a look in the sailplane forums. And yes, the elliptical planform is still the favored one.

Go to the Slope forum and check out the "Gizmo 36". Your design is similar. Folks are paying good $ and waiting in line to buy the Gizmo, so you might be on the right track.
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Old Feb 21, 2015, 11:44 PM
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You'll need to define what you mean by "better". Clearly you're not soaring and finding thermals with the style of flying I see in your video. So I'm going to assume that you're looking for fairly aerobatic response with no bad manners.

One thing that occurs with an elliptical layout at "our" size of design is a sensitivity to the low Reynolds numbers that occur as small chord lengths are flown slowly. So you may find that the raked half ellipse shape you sketched becomes more prone to tip stalling than you'd expect. Mind you with the raked back pattern it's really no longer a plank since the 25% chord line along the span is going to have a strong crescent shape. So really it's more of a crescent or elliptical sweep wing with strong taper that just happens to have a straight trailing edge. As such your crescent shape is more correctly a swept wing and the tips should be washed out strongly to reflect this and to fly well. And that would likely save the tips from stalling when flying slow. This assumes of course that you agree with this idea and arrange the segments to have the washout cut into the foam. If you don't then I'd expect the wing to suffer from bad tip stalling when flying slowly. Especially since the ailerons or elevons will be such a large portion of the chord at the tips.

So there's only one thing to do now. Make one...
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Old Feb 22, 2015, 10:34 AM
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dayhead: Thanks for the comments! I'll take a look at the Gizmo

BMathews: Thanks also for the comments! I wasn't aware of the definition of a plank so that's good to keep in mind. Reading between the lines it sounds like planks don't need washout whereas a swept wing does. I've not incorporated washout in any of my flying wings before so will look into that. I'm assuming the washout is a 'twist' with the trailing edge twisting upwards?

Ray
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Old Feb 22, 2015, 01:30 PM
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Yep, that's right Ray. A true plank won't gain from washout other than reduction in tip stalling. Instead a true plank relies on the airfoil shape having a positive pitching moment thanks to a reflexed camber line.

The key is the line along the 0.25 chord line of the wing. If that line sweeps back by much at all then you're looking at a swept wing configuration. It's up for personal taste of course since there are very few things in life that are absolute. But if I start to see a quarter chord line that sweeps back by more than 8 to 10 degrees I look at the wing as being a swept wing instead of a plank wing. Up to around that 10 degrees and any washout used is not going to be as effective as reflex in the airfoil for obtaining stability. Over 10 degrees and it's far more likely that washout would dominate over the effect of reflex.
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Old Feb 22, 2015, 03:15 PM
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BMathews: Thanks for the response! I viewed all of the Gizmo thread in the slope forum...yikes! That's scary fast. I still like the performance of planks and I think at this time I'm going to do a little more research in the slope and thermal forums and decide what direction I want to take. This has been helpful!
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Old Feb 22, 2015, 11:57 PM
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I type only to serve....

If I helped you to better understand then it's worth the wear and tear on my fingers.
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Old Feb 23, 2015, 02:42 PM
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I've read many of your posts in the last few days and you and many others in RCGroups are exceptionally helpful ... and us newbies appreciate it!

Ray
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Old Feb 26, 2015, 10:11 PM
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It would be nice to meet "BMatthews" someday. He sure is generous with his time and expertise. I know I appreciate his posts a lot.
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Old Mar 01, 2015, 11:59 AM
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Another "quick and dirty" method to achieve "washout" is to make the the chord of the elevon larger at the elevon tip than at the elevon root. This way, when you trim in your reflex your tip has lower AOA. I did this on the constant chord, swept wing below. There were other mistakes I made with that wing, but it nevertheless flies triple the amount of time than my other planes with similar weight and wing loading.
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Old Mar 01, 2015, 06:55 PM
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Quote:
Originally Posted by nuteman View Post
Another "quick and dirty" method to achieve "washout" is to make the the chord of the elevon larger at the elevon tip than at the elevon root. This way, when you trim in your reflex your tip has lower AOA. I did this on the constant chord, swept wing below. There were other mistakes I made with that wing, but it nevertheless flies triple the amount of time than my other planes with similar weight and wing loading.
I've read other posts where the elevon at the tip was actually trimmed back a bit to eliminate flutter. That's how I've normally built them. I'll give that a try some time, maybe even add a bit of material to an existing one. Thanks for that.

Ray
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Old Mar 01, 2015, 10:04 PM
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Originally Posted by raybea528 View Post
I've read other posts where the elevon at the tip was actually trimmed back a bit to eliminate flutter.
The flutter issue will only be minimally addressed by doing that. The overwhelming majority of the cause for flutter, in almost all cases, is an insufficiently stiff wing.
This is analogous to statements like "hey, just add a bit more reflex to the elevons to solve your tip stall problems, as it will lower your angle of attack in flight" - while this is "technically" correct to some degree, it pales in comparison to the effect of adding more chord to the wing so that the Reynolds number is increased. More chord = higher stall AOA.
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Old Mar 02, 2015, 10:14 AM
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Originally Posted by dayhead View Post
It would be nice to meet "BMatthews" someday. He sure is generous with his time and expertise. I know I appreciate his posts a lot.
Keep this up and my head will swell too much to fit through the doors!

There's so many factors that contribute to flutter. Certainly too twisty a wing or a control surface itself is one. And slop in the control surface linkage is another.

The odd thing is that it often takes surprisingly little change to make it go away. Things like adding a small mass balance or maybe a turbulator or any number of other things will alter either the resonant frequency of the system to some value that cant' be reached or will alter the airflow over the surfaces in a way that doesn't try to set up a resonance.

The wider surface and washout being built in is the idea behind the rather smart Paoli flying wing. This is a design where a regular flat wing is built up then the controlling elevons cut away and flipped side to side and upside down to build in the washout at the tips needed to obtain a good start on stability. Other than the curved edge on the upper side producing a need to slightly flex the elevon into place it's quite ingenious. And in fact a SMALL amount of flex needed to make the elevons fit correctly might just be something that works to avoid flutter.

http://scherrer.pagesperso-orange.fr...sh/paolie.html
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