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FF2017 Int. Race Pace Plane: Beck-Mahoney Sorceress

willsonman

Builder Extraordinare
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#21
Making great progress, Dan! And folks, this is our classic example of 3-views and plans not really being drawn by an engineer, but by an artist. Sometimes it takes an artist to interpret an artist. :rolleyes:
 

wilmracer

I build things that fly (sometimes)
Mentor
#22
Awesome! I'm always amazed what people can do with sketchup and other cad programs. I know "how" it works but it still looks like magic to me :)

I still owe you a few shots. I'm going to set myself a deadline to make it over there by COB Thursday...
 

Craftydan

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#23
Awesome! I'm always amazed what people can do with sketchup and other cad programs. I know "how" it works but it still looks like magic to me :)

I still owe you a few shots. I'm going to set myself a deadline to make it over there by COB Thursday...
In fairness, every time I pick it up after leaving it for a while I wonder how anyone gets anything done in a timely manner with a CAD . . . then it starts to come back and it all becomes a big geometric toybox :) Half of any good CAD class is how to press buttons and which tool does what . . . the other is how to bend geometry to your will. The math teacher may teach you the rules of geometry, but it's the draftsman who is the master of it ;)

Whenever you can get them, they'll be appreciated :)

While you're at it, see if you can eyeball the bottom wing and see if you can confirm the dihedral on the outer wing panel . . . It's probably there, but it's subtle and surprised me when I drew it out . . . and if you get to chat with a curator again, anything he has I'd be thrilled to see.
 

willsonman

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#24
I good front shot and analysis in Sketchup can reveal an approximation of dihedral. Of course, documentation from the national archives will be far superior to that ;)
 

wilmracer

I build things that fly (sometimes)
Mentor
#25
So I got out there on Thursday and grabbed a lot more shots. The lighting in there is really bad for photographers of my "skill" level. I attempted to get shots from every angle, some of which are somewhat obscured. The archivist wasn't there that day but the volunteer answering the phones gave me her direct line and recommended I call back on Monday to see what resources they have about this one. I'm eager to learn the process for getting my hands on their info so I'm definitely going to reach out to her.

Looking at her in person it does look to me like there is a touch of dihedral. How much is hard to say, but I doubt anyone would fault you for adding an extra degree or two. I tried to get several shots straight on and hopefully it is visible from those. I'll let you know what the archivist has to say on Monday :)

https://www.dropbox.com/sh/p3atmqi1kt0zgzf/AAAWg4lPG3UJf8wZgqUsRmcKa?dl=0
 

Craftydan

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#26
wilmracer,

Keep this up and I'm going to have to actually build this tiny beast!

Does look like a challenge for lighting, but you've popped out some details I hadn't seen elsewhere. Dunno how many photos I'd seen of her, that was the first time I noticed the fuel vent in front of the cockpit, the pitot tube under the right wingtip or the drain tube under the lower air scoop. Nice little scale details lost in the clutter of fancier photos.


Back to the plans . . . a bit more drawing and I have a tail!

BMS.skp - SketchUp Make 2016 1222017 95858 PM.jpg

I'll need to extend the rudder down a bit more when the fuse gets simplified (not to mention scale up a bit too), but for now, that will do.

So pulled up some dimensions and drew out the classic 3S 2200mAh to see roughly how it fits inside . . .

BMS.skp - SketchUp Make 2016 1222017 100010 PM.jpg

Wow. Swallowed it whole and plenty of space to boot. So a larger pack is not the challenge I thought it would be.

The scale is starting to sink in, so pulled out a power pod from the Sketchup library:

BMS.skp - SketchUp Make 2016 1222017 100404 PM.jpg

Not sure I'm going to mount a power pod in there, but there's plenty of room to do so. Measured the spinner and a 2-1/4" spinner will do the trick . . .

BMS.skp - SketchUp Make 2016 1222017 100544 PM.jpg

So that's where the model stands now. Now that I've got something fairly realistic, next we simplify the model to something foldable. In the mean time I've got to wrap up contemplating the wing cross-section . . . and servos . . .
 

Craftydan

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#28
. . . two more weeks pass, and what do I have to show for it?!?

The models aren't much different, but I've been peeling back the onion of airfoil selection.

First off, let's get the obvious out of the way: Any airfoil will work. Reading up, it was repeated by nearly every source and we've seen it time and again in model airplanes. Good or bad, so long as it's somewhat symmetric along roll (and sometimes not even that), with enough power and control throw it can be made to work.

That being said, I've seen a few unruly planes made worse by their aerodynamics, so "it will fly . . . probably" is a bit wider target than I hope for. A bit of care can make it better or worse . . . so if I'm going to go to the trouble instead of folding one of the TLAR wings, what can I hope to gain? What could I loose from a poor choice?

- Stall tendency (how gently or aggressively it enters stall)
- balance between lift capability (how much lift it can generate) and slipperiness (or draggieness)
- lower pitching moment (how sensitive the wing is to CG vs. how big do the tail feathers need to be)

And the rabbit hole goes even deeper than this, but we'll start here.

The exact scale airfoil is an unknown (at least to me . . . perhaps the archivists know, but that's not going to impact my choice much) but it is symmetrical. While the wing form is pretty well locked in, I don't have to go scale with this airfoil -- my Re is in the low hundreds of thousands not the mid-to-high millions, I can't expect the same performance -- so I can be a bit more selective on what I want to get out of it.

So what do I want?

I'd like a fast-but-friendly airfoil, high speed/low drag is nice, but I'm more interested in gentler stall and lower pitching moment. I've got to be able to fly this thing, so I'd rather it creep up on a stall rather than drop like it hit a wall. I'm also concerned the tail surfaces aren't enough, even after enlargement. I can be careful about balance, but it's hard to make up for a small tail.

I also want to build it out of foamboard (duh) and leave enough space to embed the aileron servos in the top wing . . . preferably enough to encase the servo and linkages (more on that later).

That's all worth a bit more effort, so into the deep I go . . .


After digging into the available info online (Buy a book? Maybe later -- There's textbooks and class notes online. links below) I've worked out a few useful details:

- The sharpness of the airfoil's leading edge implies the sharpness of the stall . . . a quick look at AoA v. C[SUB]L[/SUB] also implies that . . . and they tend to agree . . . as far as I've seen.

- careful inspection of airfoil plots can show the candidate cruising angle of attack (when the airframe is level, what's the best angle for the wing), compare relative stall speeds, and how negative (stable) the pitching moment is over AoA.

- the pitching moment is a function of how far off the aerodynamic center (a geometric center of the wing) and the Center of Pressure is (an aerodynamic center of the wing). What brings them apart? How far off the airfoil is from symmetric. Having a close-to-zero pitching moment means smaller tail, but lower pitch stability overall, and lower useful static margin.

- NACA 4 and 5 series foils are easy to draw but were spec'ed by an easy way to draw them, not how well they flew . . . so they're not that great. Popular, but not that great. NACA 6 series is spec'ed on actual performance (max lift and drag bucket size) but the shape is a bit . . . odder.

Muddy enough yet? it goes deeper . . .

. . . but we won't go there today.

Fundamentally I have to build this wing, and if I want the benefits, it has to have decent accuracy. For "how" I have some ideas, but "what" is more important at this stage.

So which foil have I chosen?!?

Dunno yet :p

All that being said, all I've done is rule foils out :eek:

While I'm concerned with the smaller tail, the 0 pitching moment of the symmetrical wing sounds like a disaster in the making. Sure, the tail is now more sensitive, but there's nothing helping to self-stabilize in pitch either. Symmetrical is out on the "friendly" goal.

The scale t/c ratio (how thick it is relative to the cord) is right on 8%. Pretty thin wing. For the scale wing cord, to fit the wing in 3 foam sheets high is about 9.5%, and 4 is 12.5%. Anything in that region will do, but if I want to embed a standard servo (looking at the TP-MG90D's), 4 layers will give me an extra layer of foam, where 3 layers is nearly the width of the wing . . . on the fence, but 9%-13% t/c is my search range.


Ugh. Lots of rambling, no resolution and no pics :( sometimes progress is more engaging than others, and today is the others. I'm getting close, so soon I'll have a foil picked, hopefully some plots I can wave my hands over (if you know of such things, feel free to suggest -- I reserve the right to say no . . . and maybe rib you if it's bad enough), a pic or two of the servo layout and spars, and maybe . . . just maybe . . . a few wings that aren't flat.

I've got the fuselage fold-up already in mind, so add on some wings and the fuse should follow quickly after that.


For those gluttons for making brains smoke, here's a few links I've found enlightening. Some are more . . . readable than others:

General -
http://airpigz.com/blog/2010/9/9/airfoil-shapes-in-the-fast-lane-world-of-the-reno-air-races.html
Picking the “right” airfoil for model aircraft

Theory -
Airfoil Selection - Matronics Forum[.doc]
Chapter 5: Wing Design

Airfoils -
The Incomplete Guide to Airfoil Usage
https://flyingfoam.com/knowledgebase/airfoil-basics/
http://airfoiltools.com/index
 

Craftydan

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#30
I believe your verbage is a bit too copius for my diminuative comprehension Dan. Fold it and let 'er rip!
Come on Abieex! I addressed that in the third and fourth paragraph! . . . and the 25th . . .

Yeah, I knew it was dense. Check out some of those links and you'll see it's just the cliff notes ;)
 

Craftydan

Hostage Taker of Quads
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#34
Thanks dpalmer!

hopefully when I get the foil sorted out I can get some traction back.



Just scanning through the database of airfoils I have access to, looking for simple-to-form shapes between 9% and 13%, I'm down to 12 candidates. There's some funky foil shapes in there -- particulalrly among the propeller foils -- but the 12 left over have nearly flat bottoms, and almost no curve up on the trailing edge -- at most I have to worry about shaping the top and leading edge. While I was sorting through the list I glanced at a quick comparison between that airfoil, the AG-35 and Clark Y (known good performers), and the Aquilla airfoil (nice oldschool glider, and an early contender). Obvious under-performers (high moment requirements, low L/D, low or sharp stall) were cut. So those that made the first cut:

Foil

T/C (%)

T (mm)

AQUILA

9.4

14.9

Fage & Collins 4

12.7

20.2

Gottingen 12K airfoil

11.1

17.6

Gottingen 285 airfoil

11.7

18.6

Gottingen 611 airfoil

12.9

20.5

N-10 airfoil

11.2

17.8

PT-40 trainer airfoil

11.6

18.9

RAF-6 airfoil

10

15.9

Rhode St. Genese 32

11.9

18.9

S4094 (E-flite UMX ASK-21 Root airfoil)

9.9

15.7

Selig S7055

10.6

16.7

Clark Y

11.7

18.6





Foamboard only comes in 5mm widths, so the thickness isn't as much a range between 9-13% so much as it's "close" to either 15mm OR 20mm thick for the 6.25in cord . . . so limiting it to 1mm off, we're down to 5, and limiting to 0.5mm we're down to 3:


Foil

T (mm)

ΔT (mm)

AQUILA

14.92 (thin)

0.078

Fage & Collins 4

20.16 (thick)

0.16

Gottingen 611 airfoil

20.48 (thick)

0.48

S4094 (E-flite UMX ASK-21 Root airfoil)

15.71 (thin)

0.71

RAF-6 airfoil

15.88 (thin)

0.88



This is all with only a cursory glance at the actual performance to eliminate the obvious.

We're down to two model glider airfoils, a prop airfoil (RAF-6) and two that I've seen referred to as "classic" airfoils, but aren't used on any airframe I can tell.

The prop is already at the bottom of the list, so we're down to four . . . Next I'll compare the polars and some of the more practical concerns of how to build the wing.
 

willsonman

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#35
If it were me? GOE611. I've used GOE foils before and they work quite well. Probably not what you want or care to hear but just my $0.02.
 

Craftydan

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#36
Ack! Wilsonman uses them?!? Tainted goods -- take them from my presence!!!!

I need to take the time to look at the polars (really, write-up what I've seen from the polars). I do appreciate the vote of confidence (it hasn't fallen on deaf ears), but the whole point of this was to get beyond the "I like 'em because so-in-so said it was good". If that were the methodology I'd probably already be using NACA 23012.5, while being oblivious about not having a clue as to why.

I could always have stopped at flat plate, after all. "Better", by it's nature is relative, and I wanted to take this excuse to quantify "better". "Where", similar conditions and relative performance are a bit harder to quantify . . . but these 4 have already passed the sanity check of fit the envelope and performance at the Re is not outrageously bad.

Finding the foil in use somewhere else (full scale or model) is a good warm-fuzzy, but the proof is in the pudding ;)
 

willsonman

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#37
I totally respect that. I think its a good process to go through here. Its not a topic that is often discussed openly.
 

Craftydan

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#38
So time for the final elimination round!!!!!

Who lives? Who dies? Who soars above the rest and who crumples under the forces . . .

So let's have a look at our contenders!

Aquilla:
aquilasm-il_m.jpg

Fage & Collins 4:
fg4-il_m.jpg

Gottingen 611:
goe611-il_m.jpg

S4094:
s4094-il_m.jpg

All fine airfoils, pretty flat bottomed, with roughly different points on the cord for the max peak, some thicker, some thinner.

One final piece of mystery for all of this . . . Reynolds number. This is a dimensionless measure of how thick the air appears to a wing of it's size relative how quickly it wants to push through. Since this is an exploration through graphs (No! I'm not doing the math!), we'll use a Re Calculator for this.

Plug in 0.15875 for the cord (6.25" in m), use the higher default temp for the Kenematic Viscosity . . .

v(mph)

Re

~Re

10

47,275

50k

20

94,025

100k

40

188,050

200k

100

469,915

500k



In this case, for warm(ish) air, at 10mph, Re is roughly 50k . . . so why did I hit those speeds? Each set of polar plots are made for a specific Re and Ncrit (turbulance in the air), and I have polar plots pre-generated at 50k, 100k, 200k, and 500k (and now we know roughly what airspeed that means for this wing on these plots). For Ncrit, we're going to assume the air is smooth -- pick the lowest -- because it makes the charts easier to read (it's good to go back and examine higher Ncrit to see if your selected wing may have trouble in turbulence)

So, 100mph (500k) is crazy fast (it might be able to do it) and 10mph (50k) is painfully near stall, but 20mph (100k) should be comfortably within the envelope. Since I'd be pleased with 20mph for a slow cruise, we can pick Re = 100k data sets and generate the polars . . .

For all of the polar plots below, Aquilla is teal, FG4 is blue, goe611 is red, and S4094 is tan.

First up:
CLvARe100k.jpg

Think of C[SUB]L[/SUB] as "how much lift", and Alpha as Angle of Attack. So all else kept equal, held at at ~20mph, if you change the pitch, this is how lift will increase.

For comparison, three things to look for:

- The farther right peak the peak is, the higher the stall angle (generally, the lower the stall speed)
- The flatter the top, the more gently it will approach stall (the more warning you get)
- the higher the peak, the more liifty the wing is.

In this case, Teal and Red have fairly flat stalls, but of the two, Teal is much further right. Tan is further right and higher, but it's peak pretty sharp. So Tan > Red > Teal in liftyness, Tan > Teal > red in stall, but Teal = Red > Tan in gentleness of stall. Blue is erratic and under-performing in this plot, so it's the big looser for this comparison.

There's more we can glean from this plot, but for comparison, we're ready to move on to . . .

CDvARe100k.jpg

Placed here for completeness -- C[SUB]d[/SUB] is "how much drag" vs. Alpha, which is still AoA. Most everything you see here can be found easier on other plots . . . but for this plot, a lower and wider "U" is preferable. Also notice the U can be shifted off center to the right from 0-Alpha this means you'll get more drag pointed down than up.

Not much more to see here, so we'll move on to . . .

CLCDvARe100k.jpg

Now we're getting somewhere! C[SUB]L[/SUB]/C[SUB]d[/SUB] is the lift:drag ratio.

two things to notice:
- the higher the peak is, the more efficiently the wing can work.
- the wider the curve, the less sensitive the wing's efficiency will be to AoA -- a wider "happy place".

looking at these plots, Red and Teal are once again neck-and-neck for the top, blue makes a good effort for third where Tan is abysmally low. Red is a touch wider, but just a touch.

So for L/D, Red = Teal > Blue > Tan.

We'll come back to this plot later with the winner foil, but for now, it's time to move on to:

CLvCDRe100k.jpg

This may be the oddest of plots but for flight envelope, it's one of the more telling.

Three things to look for:

- the farther left the vertical part of the "C" is, the less draggy the wing
- the wider the "C" is top-to-bottom, the wider the flight envelope the wing will have.
- the lower the bottom part of the C reaches, the better the inverted performance will be.

In this plot, Tan shines. It's just a bit more draggy than the rest, but it performs "a little draggy" over a much wider range, both normal and inverted, where as none of the other wings will do well inverted -- they'll need a LOT of back-pressure.

Blue once again under-performs (honest, I didn't try to setup this foil for failure ;) ), but orange and teal have similar behavior -- orange having a bit more lift, but teal picking up a *slight* advantage in inverted flight and lower drag.

So in low dragginess Teal > Red > Blue >Tan, but in envelope, Tan > Teal = Red > Blue

And finally . . .

CmvARe100k.jpg

This is the weird one. C[SUB]M[/SUB] -- the pitching moment -- is a measure of how much pitch stabilization this wing will need from the tail. What makes this weird, the closer C[SUB]M[/SUB] near 0 means smaller the tail, and more elevator authority, but less self-correction in pitch and narrower the tolerable CG range. The more negative C[SUB]M[/SUB] is the stronger it will self-correct any wobbles in pitch, and the wider your happy CG range can be, but the less authority your elevator will have.

things to look for:

- Never positive. When pitch is disturbed, you'd rather it resist the pitch than add to it. Where it's positive, you can have your CG right, and the wing will act unstably at that AoA like it was tail heavy.
- Sharp, strong transitions (Blue, I'm looking at you) show AoA's that may cause the wing to quickly switch from docile to hyper sensitive -- like a flying wing crossing over it's center-line.
- the lower the curve, the more stable, but the bigger tail it will need.

So for this plot, Tan is a big looser for the instability out on the negative AoA -- pitch down too much and the wing add to the pitch over -- not friendly. Blue is eratic . . . not thrilled with that. Red, as you'd expect from a liftier wing is a lot more stable, but will need a bigger tail to balance itself out. Teal stays negative but not too negative, has some erratic to it, but not as erratic . . .

So Teal > Red > Blue = Tan



So after all that . . . IMO, Teal (Aquilla) and Red (GOE611) are neck in neck. I'm liking the Aquilla foil a touch better because of it's smaller pitching moment and slightly better inverted performance, but I think it's worth pulling each into the model and seeing how they look on the layout.

That's next . . . we'll see how each look on the plane, get a feel for where the servo will hide, and start pondering the foam-board construction.

Sounds like a great thing to pick up . . . mañana . . .
 

Tench745

Well-known member
#39
Awesome overview, CraftyDan! I've never really understood what goes into airfoil selection (usually just use TLAR or suggestions from someone else.) This gives me a whole new venue for my neurosis to follow when designing. ;)
 

Craftydan

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#40
Tench,

Glad you liked it. Since it's just comparing graphs on a grand scale, it's still a TLAR method, but at least it gets the designer "looking" at slices of how the wing performs opposed to how appealing the geometry is.



One point (probably among many) I forgot to mention above . . . an underlying assumption of these charts is the wing represented by this cross-section is uniform, and infinitely long. For the Scorceresss, the first part is not bad (the inner wing panels are tapering in cord, but the other panels are uniform), the second is a polite fiction so we can ignore wingtip effects. The full scale Sorceress mounted a wing-fence on each wingtip to help manage the wingtip vortices -- which I plan to copy -- so I'm choosing to ignore those effects. Wings with significant taper or sweep can still use these techniques to compare airfoils, but the stronger these geometric features are, the less these charts say about the "whole story".

To go further down that rabbit hole, there are analysis tools designed to model the entire wing and predict how the air flows over them, and the pressures/forces that arise. These have been used to generate impressive results -- there are a few modern designs in the DLG community that are amazingly more effective than the previous "state of the art" because of this -- but being able to take a glance at the airfoil polars to make a few comparisons is an not-so-hard step in the right direction.

Long story short, this method is better than nothing -- sometimes MUCH better -- but take it with a grain of salt ;)