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Sep 07, 2019, 09:41 PM
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Montag DP's Avatar
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Effects of Re on neutral point


I am currently working on a new plane design. It will be a small parkflyer intended to be slow and light enough to do FPV in front of the house. The Reynolds number is around 100,000. A screenshot of what it looks like so far is attached. While trying to estimate the CG and neutral point early on in the design phase for overall layout and component placement, I came across what appears to be an interesting effect.

See below two sets of pitching moment curves, calculated by my panel code LORAAX. The first is inviscid, while the second is viscous with BL transition. The baseline CG location (denoted as xref in the plots) is 11.85 inches from the nose; this is the neutral point location that AVL reports. Running LORAAX inviscid with the CG at 11.85 inches, I get a Cm-vs-alpha curve that is pretty much flat, but has a slightly negative slope (small positive static margin), as shown in first set of pitching moment curves. Of course, moving the CG farther forward increases the static pitch stability (more negative slope), while moving it back decreases the static pitch stability. However, running it viscous with BL transition, I am finding some pretty significant differences. The baseline CG of 11.85 inches is now decidedly unstable (positive slope), and the neutral point looks to be forward of 11.55 inches (let's say 11.5) compared to something greater than 11.85 inches (let's say 11.9) when run inviscid.

So, my questions are:
1) Has this ever been documented before? Since it hasn't been verified by any real-world measurements or even another code, it is possible that it is not correct, but given the good correlations I've had with this code and higher-fidelity methods in the past, I believe that it could be true (and I'm certainly not willing to discount it for the plane I'm designing!).
2) Given that viscous effects -- especially BL transition -- appear to have the ability to significantly impact the neutral point, should we be always accounting for them in CG calculators? None of the calculators that you will find online do this, but I suppose it's kind of a moot point as long as they build conservatism into the estimate. For example, my own CG calculator spreadsheet gives the neutral point at 11.34 inches, which is pretty close to the LORAAX viscous result but a bit more conservative. However, these results suggest that just taking the neutral point from an inviscid panel code might not be a good idea at model scale.
Last edited by Montag DP; Sep 07, 2019 at 09:48 PM.
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Sep 08, 2019, 12:30 AM
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Some years ago I came to the very same conclusion. I am not a fluid mechanic specialist, but regarding the positioning of the CoG I have a tendency to be overly cautious when working on a new design. Not only for the (very) low Re numbers as you pointed out but up to the 500,000 (and above as well).

A lot of things are far from set when you design small(ish) aircraft with the pretty high Re variations they are supposed to work (fly) with... Above 800.000 to 1M Re the performances variations and stability/stabilities are much less influenced by the Reynolds number. It (re)starts to be interesting when approaching the subsonic/transonic regime...
Sep 08, 2019, 12:31 AM
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ShoeDLG's Avatar
Is the wake modeling the same in both cases? Could this be a d epsilon / d alpha effect?
Sep 08, 2019, 04:25 AM
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1. Many small slow flying FF and Old Timer models have enlarged empennages and I wonder if the decreased effectiveness as Re is lowered is accounted for in the above analysis.

2.Noticing the apparently constant chord planform, decades of experience designing and testing small models and toys has indicated that washout needs to be increased slightly exponentially (non-linearly) as Re is reduced when scaling down. As an off-the-cuff example, for the very approx. same efficiency (L/D) and stability (in tight increased-AoA turns, for ex.) if the washout angle measured at the tip of a 50 mm model was use-optimized at say 2, for a 25 mm scaled down model the washout might need to be increased to maybe 5 (actual angles determined by flight testing). Further, I have found that moving the region of max. twist inboard of the tip (as in the "Prandtl twist") actually seems to boost the washout effectiveness (L/D and stability).

However I have not separated the span from the twist when "playing" (developing & testing) a new design so I cannot swear that this is the theoretical Prandtl twist effect, rather just tried to keep the area and thus weight and strength of the wing similar (i.e., merely tried moving the region of max. twist inboard and flt. tested the prototype). But I can state that when I first did this to my fixed-span Hobie Hawks in the 70's, the improvement was obvious to other Hobie Hawk owners w.r.t. bettering L/D and stability in turns and at increased AoAs. In the Hobie case the original washout exponentially INCREASED as it approached the tapered tip and I reasoned that the extreme tip was thus far from effective, so I just made the washout increase more linear (SWAG).

Of course in general, when possible, even simple tip improvement never hurts, especially at low Re.
Sep 08, 2019, 07:02 AM
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Montag DP's Avatar
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Quote:
Originally Posted by ShoeDLG
Is the wake modeling the same in both cases? Could this be a d epsilon / d alpha effect?
The viscous case had a deforming wake, while the inviscid one had a fixed wake aligned with the freestream. I ran the inviscid case again with a deforming wake, and this was the result. It makes a small difference, but still pretty much the same.
Sep 08, 2019, 09:09 AM
Registered User
interesting point i once tried a theoretical concept i have been thinking about. made some video. it's not directly related but the test is at low Re. in the beginning i capture surfaces, lever arms, ratio of movement, angles, balance points. good flight @ 3:03 . i still am going to work out a RC test version. had a bit of turbulence past years but will have good occasion coming time for experiments.
concept proof of active aerodynamic stabilization concept (6 min 37 sec)


since i made this, positively stable around pitch axis test model, i am interested in any effects which may influence this. (and in general ofcourse).
Last edited by m4rc3l; Sep 08, 2019 at 12:02 PM.
Sep 08, 2019, 02:40 PM
Registered User
Cute idea. Of course it works in the same way as a stab but instead by also changing the decalage between wing & stab using inertia of the boom, etc. (in your model they move opposite to each other, with the added effect of increased drag with camber change). In an RC plane using AS3X with servos already installed, the stabilization effect is well-known. However in a small FF glider the added weight plus mechanical and aerodynamic entropy losses of that "complicated" system would seem to outweigh its benefit.

That is, in a small FF glider just increasing some combination of stab moment arm (boom length) and stab area/design, from my own experience of hand-gliding such FF gliders, for a non-articulated wing the same as in the videoed test model, they have a flatter and less sinusoidal glide-path due to less weight and fewer losses.

For the record, pendulum controlled (gravity more so than inertia per se) FF models have long been used as well as in model hang gliders and parasails using nothing but the pendulum effect without separate control surfaces or stab.

So the Re effects would involve the exact shape and area of the stab, and while those are design choices, contest-winning FF glider designs are not wildly different. And that goes for various size RC planes, depending on the design goal and purpose.

Am sure that Bruce has lots more expertise in this


sorry for OT
Last edited by xlcrlee; Sep 08, 2019 at 02:51 PM.
Sep 08, 2019, 04:32 PM
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exf3bguy's Avatar
To be honest here, your graphs are a bit over my head but I have to ask the question on if this is really nessesary at all. On thing that I did pick up on was your AOA and how you have it graphed to 8 degrees. It is actually quite rare that one of our models will fly straight and level with anything past 2-3 degrees AOA especially with a relatively flat bottom airfoil.

What I did notice in your illustration is a design that appears to be a bit short coupled and a little shy on vertical area. My thought is with all that fuselage side area you may loose some lateral tracking. Raising the stab maybe even a T tail would get the stab into the prop wash and help with slow speed handling. Just a couple thoughts.
Sep 08, 2019, 05:29 PM
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Quote:
Originally Posted by exf3bguy
.... What I did notice in your illustration is a design that appears to be a bit short coupled and a little shy on vertical area. My thought is with all that fuselage side area you may loose some lateral tracking. Raising the stab maybe even a T tail would get the stab into the prop wash and help with slow speed handling. Just a couple thoughts.
You mean like >

Sep 08, 2019, 05:37 PM
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Montag DP's Avatar
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Quote:
Originally Posted by exf3bguy
To be honest here, your graphs are a bit over my head but I have to ask the question on if this is really nessesary at all.
Quite possibly. As I mentioned, the effect seems to be there, but it is small enough that as long as you are conservative, you might never need to worry about it. In my case, I'm struggling a little bit with getting the estimated CG far enough forward for a decent static margin, so every little bit counts. That was actually why I ended up adding some sweep to the wings.

Quote:
Originally Posted by exf3bguy
On thing that I did pick up on was your AOA and how you have it graphed to 8 degrees. It is actually quite rare that one of our models will fly straight and level with anything past 2-3 degrees AOA especially with a relatively flat bottom airfoil.
Maybe not in level flight, but in a slow-speed landing, you definitely will hit that. Anyway, the important thing for stability is for the Cm curve to have a negative slope, so I wanted to carry it out far enough to be sure.

Quote:
Originally Posted by exf3bguy
What I did notice in your illustration is a design that appears to be a bit short coupled and a little shy on vertical area. My thought is with all that fuselage side area you may loose some lateral tracking. Raising the stab maybe even a T tail would get the stab into the prop wash and help with slow speed handling. Just a couple thoughts.
You might be right, though the perspective makes the vertical tail appear smaller. Here is a side view. Nonetheless, the planform isn't finalized yet, and the vertical tail was something on my list to check into some more. I was actually considering adding some holes to the fuselage in an effort to minimize its destabilizing effect. I do appreciate your input.
Sep 09, 2019, 02:54 AM
Registered User
@xlcrlee: yes, the camber changes opposite, to as it would in a Plank tailless wing. this widens CL envelope. more like a tailed glider with camber control. yet, the CG is well back (like on a tailed glider), and the tail is about half the length, and about half the horizontal surface, as it would be on a "passively" stabilized ff glider. i am going to try controlling it, by changing the length of the link between the boom and the wing flap(s). whereby roll is just differential between the both flaps. (one servo for each flap). also, the tailboom inline, and not on top like in the prototype. i may build a larger, ultralight version like this prototype, in that case i may use a tailboom on top just like this. but i also plan on low drag, fast gliding "normal" versions. which would have, as far as i can tell, a exceptionally wide speed range (wide drag bucket), lots of maximum lift for thermalling, yet low drag for moving inbetween thermals. work in progress.
Sep 09, 2019, 03:39 PM
AndyKunz's Avatar
Maybe this is why we start by balancing a plane slightly nose-heavy and then move it back to a comfortable position. I've never had any math that says "it should balance at X" match where it really feels best.

Nose heavy planes fly poorly; tail heavy planes fly once.

Andy
Sep 09, 2019, 06:49 PM
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exf3bguy's Avatar
Quote:
Originally Posted by AndyKunz
Maybe this is why we start by balancing a plane slightly nose-heavy and then move it back to a comfortable position. I've never had any math that says "it should balance at X" match where it really feels best.

Nose heavy planes fly poorly; tail heavy planes fly once.

Andy
Some truth there. When it comes to CG I always fine tune according to what is observed in flight after test flying with a CG that is known to be a bit forward. Same things when it comes to airfoils. I never use any plotted airfoils, just draw out something that has the cord, thickness and high point where I want it.
Sep 09, 2019, 07:57 PM
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Montag DP's Avatar
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Quote:
Originally Posted by AndyKunz
Nose heavy planes fly poorly; tail heavy planes fly once.
I had a plane once -- one of the first RC models I designed -- that was slightly unstable in pitch on the first flight. It wasn't unflyable, but it wasn't very fun either, and the landing was an "adventure." So there are degrees to pitch instability. I've heard that the Wright brothers purposely designed their planes to be slightly unstable in pitch, but I can't recall exactly why they thought that would be a good idea.
Sep 09, 2019, 08:01 PM
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exf3bguy's Avatar
There are various reasons for pitch instability that are not CG related.


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