How do they do it?

[Siggypoo72]

Out of so many airfoil designs out there, how does a budding designer discern which airfoil best suits their homebuilt concept? I've asked the same question over at RC Groups, as I've been a scratch designer and builder since childhood. But I've always used the same methods for figuring out the airfoil that works for my RC models; trial and error, the TLAR Method, and SWAG Theory.
Having designed and flown many an RC plane of my own design, in many sizes and formats, I've been inspired by some projects on YouTube to try a man-carrying plane of a particular design of mine; an upright gull-wing reflexed airfoil with a single or dual RC turbine in a centre pod, or bulge. Wing span is 14' and wing chord is 3'.
The pilot lies prone on top and controls flight by shifting weight. Made of foam core with a fibreglass shell.
Basically a rigid powered hang glider with the pilot on top.
The airfoil I'm leaning towards, being low drag and high lift, has been the Roncz Marske 7, and it's worked nicely so far for the RC models of the same design.
The (perceived) benefit of using RC turbines is that a dummy human-weight could be placed (via servos) atop the airframe for flight testing, and the aircraft flown from the ground.
But, with hopes of having this homebuilt certified someday, I don't want to play the design by ear. I want real-world guides to help me narrow down which airfoil would best suit my needs.
I suppose each new aircraft design has risk because, ultimately, some poor schmuck has to take it up and in, right?
I want to know the best airfoil for my needs prior to wheels up, and I've no real-world aviation design education.
My ultimate question is, how do designers choose the right airfoil?
Any help, advice, or even flaming is very welcome.

Arthur

[Ron Blum]

Arthur:

The "right" airfoil is never just one airfoil because it is a compromise of lift, drag, pitching moment, Reynolds number, surface roughness, etc. High lift will come with the penalty of high drag, high pitching moment and lower cruising speeds. The airfoil section stall characteristics have little to do with how the airplane will stall ... that is determined more with wing planform and twist.

There are a lot of books and online information.

Good luck
Ron

[John McGinnis]

Arthur,

You don't need a special airfoil beyond the ones you may have seen, but I'd like to encourage you to wade right in and go for it anyway. Someone designing original aircraft needs to have that variable in play as they study the various trades available, and they should become very conversant with the terminology and variables of airfoil design.

For instance, your Roncz Marske 7 (http://www.profili2.com/eng/dett_profilo.asp?Id=2411) is a reflexed, flying wing airfoil, which can be trouble given a higher span loading (due to tip vortices and their hard-to-quantify, destabilizing influence) if you design as if the zero-pitching moment were a given. Perhaps a swept planform or a non-planar configuration can give you more benefits at that span, with greater stability. Neither would particularly need a reflexed airfoil.

Airfoils have been shrouded in too much mysticism for too long, and the subject matter today is infinitely easier to digest, thanks to the ability of low-priced tools to teach the things it took NACA decades to learn.

It's instructive and helpful to have Airfoil Optimizer (http://www.lancaironline.net/bookstore/lancair/airfoil_optimizer.htm), an older program by Gil Crouse... just don't trust the polars he uses for anything outside the program! (That mistake cost me eight months once, but forced me to finally learn how to design my own airfoils....so thank you, Dr. Crouse... my first design attempt beat its NASA benchmark airfoil by 40%!)

Get and read Theory of Wing Sections, by Abbot and von Doenhoff (http://www.amazon.com/Theory-Wing-Sections-Aeronautical-Engineering/dp/0486605868), then buy and read GA Airfoils by Harry Riblett (http://www.aircraftspruce.com/catalog/bvpages/ga_airfoils.php).

Neither will make half as much sense until you play around with Profili (http://www.profili2.com/eng/default.asp), which is an X-Foil re-packaging and a must-have software tool!

For your 42 sq ft wing, (assuming a decent stall!) the airfoil question will matter more if you get your induced drag under control first. At that span, a proper design will bury the Jetman (http://translogic.aolautos.com/2013/08/19/translogic-136-eaa-airventure/) in yesterday's news, and I invite you to consider the possibilities. Feel free to call, I'd be happy to chat about how you might proceed.

[Siggypoo72]

Wondrous insights, thank you all much. I'm not aware of issues with the Marske 7. John's comment on it only serves to cement my point in this topic. If the Marske 7 isn't right for my purposes, even with the three degree washout I build into it, how do I discern the right one, or ones? Because of your replies, I'll start precisely where you've mentioned.
I'll scan my sketches and upload them for your unending amusement.
Honestly, it's because of Jetman and the CriCri that I'm confident my concept at least has merit; I mean, the MC-10 has a wing span only 2.4' greater than my design, and a whole lot more mass to move around.

[Siggypoo72]

Here are my sketches, done when I was incapacitated for a while with no internet access. None of it is set in stone, and I would never suggest myself to be even remotely educated.
All of my test models (free flight glider, RC glider, RC EDF, all in different sizes) have flown wonderfully and seemed very stable.
Please try not to LOL too much, but your input is so welcome.
410741084109

[Frank Giger]

Hey, that's really using your noodle on design. Can't speak to airfoils, but I was wondering about counteracting adverse yaw through the use of drag.

You're applying drag to the wing that has less airflow (lift) to begin with in the turn (until it's coordinated) if I'm picturing things right and my mental physics aren't backwards. Seems like we're entering into the realm of the acrobat pilot (one of the reasons the Wright brothers were some of the only that could fly their Wright Flyers is that they were amateur acrobats on the ground and had really, really good balance).

Similarly, if this design ever gets into a spin it's unrecoverable. That's why the Jetman has stubby wings and a lot of horsepower - the drag of his body can counteract the asymmetrical lift of the wings with the power off if he ever got into one.

I'm not saying it won't work or is inherently flawed in design, I'm just ignorant on how it would work and hoping you'd enlighten me.

[Siggypoo72]

I had completely uneducated guesses while I was drawing this up, but the pods you see about half way down each wing have clamshells that cause drag for yaw control, but they'd open maybe a couple of inches? And that's only to augment the pilot swinging a leg out like a rudder. The main reason for the clamshells was strictly as air brakes to be used at the same time at touchdown that wouldn't effect the airfoil shape if used in flight. Again, no idea how realistic that idea is. If a left spin occurs, the right clamshell, combined with the pilot's swinging a leg right while banking right would (I hope) cause enough drag and imbalance that the spin couldn't be maintained, but would deteriorate into a descent that could be pulled out of, given sufficient altitude.
What I need to do is research what the varied unswept gliders out there do for spin control, and incorporate those lessons. The lower number of moving parts for this project the better.
Before the clamshells got voluntold as air brakes and yaw control, they were simply housings for the propane tanks I'd use for the turbine.
There are already a couple of Mehve designs out there, one popular design on YouTube (http://en.m.wikipedia.org/wiki/OpenSky_Aircraft_Project). It's a fan-based concept influenced by a fictional aircraft from an old animated movie. I kinda think my design holds closer to the artist's concept.

[rwanttaja]

The pilot lies prone on top and controls flight by shifting weight.
I'm not sure this will work.

Mind you, I'm not an aeronautical engineer, and my Physics instructors back in college had no great opinion of my capabilities. But I have trouble seeing how the pilot-on-top weight shift case can be anything but unstable.

Let's take a look at the traditional pilot-below case works. When the aircraft is in level flight, the pilot center of mass lies directly below the center of lift of the aircraft (when viewed from in front). When the pilot pushes to one side, this equilibrium is disturbed, and the aircraft rolls until the "balance vector" (my term, there's probably a "real" term for this) is pointed at the ground again. The aircraft is again balanced, and holds that degree of wing tilt for as long as the pilot displaces his mass. Of course, with the lift vector displaced to the side, the aircraft begins a turn.
4125
When the pilot's done with the turn, he just shifts his weight the other way and the aircraft recovers.

So, let's look at what happens when the pilot is on top of the wing. The pilot shifts his weight, and the aircraft dutifully drops a wing and starts hunting for equilibrium. But with the pilot on top, the tilt of the wing has the effect of pushing the pilot's weight MORE towards the lower wing. The system cannot obtain equilibrium; it keeps rolling until it can again line up the pilot's center of mass with the center of lift of the wing.

And THAT will happen when it rolls completely over and puts the pilot under the wing. To his great consternation, of course.
4126
By my (admittedly weak) comprehension, the pilot-on-top weight-shift case will only be stable if the wing has enough dihedral to put the center of lift above the pilot...so you'd be, essentially, in a pilot-below-the-wing case by then.

There have been planes flown with the pilot prone atop the fuselage... the Wee Bee comes to mind. But it had conventional controls.

I don't see it as being completely impossible, as long as the pilot doesn't let the bank angle get too far. But it's going to take fast reflexes, and LEARNING how to control it will be difficult. And probably painful.

The same problems hold for pitch control. Keep in mind, too, that with this setup the pilot cannot wear a seat belt! He's got to be loose to shift forward, back, left, and right. Conventional powered weight-shift aircraft strap the pilot in a small cockpit and have the pilot move the entire structure under the wing. Not an option, in this case, I think.

You're going to do RC testing, and that's a great idea. Make sure you make the control system as close to a human weight-shift as possible, with properly scaled weights. Also remember that the pilot is going to have to content with the mass of his own body when shifting his weight...no instantaneous shifts from 6" left to 6" right, followed by 3 inches forward/back....

Ron Wanttaja

[rwanttaja]

The other thing to consider is how uncomfortable the head-up prone position is, for any length of time. The "Superman" position is not natural (except if you're from Krypton, of course) and since it's a weight-shift aircraft, you can't even give the pilot a rest for his chin. The Wee Bee had a two-step pilot position that lowered the pilot's legs to relieve this to some extent; even with that, some of the picture show the pilot with his head down or turned to the side.

It's a neat design, though. I wonder if a semi-supine position might work... engines moved far enough to fit the pilot's body between them, with the pilot's feet at the lower front and his upper torso/head sticking out on top closer to aft. Appropriate headrest, of course. A switch to conventional controls would avoid the issues with weight-shift.

Ron Wanttaja

[Siggypoo72]

A semi-supine position might be much more comfortable. As for the pilot above weight shift prob, I was trying to wrap my head around what you said, Ron. Do you think greater dihedral would help counter that tendency?
Thanks for any feedback. I want to get this right, and am rather ignorant.

[rwanttaja]

A semi-supine position might be much more comfortable. As for the pilot above weight shift prob, I was trying to wrap my head around what you said, Ron. Do you think greater dihedral would help counter that tendency?
Thanks for any feedback. I want to get this right, and am rather ignorant.
Like I said, I'm not an expert...perhaps someone else here will chime in, who knows better.

A useful analogy would be a child's swing vs. a see-saw. The swing is like the conventional hang-below weight-shift design. You can sit in the swing, and via weight shift only, get the swing to oscillate. But if you stop pumping, the swing will return to the stable position, hanging directly below its attach point. If Peter Perturbation comes along and shoves you (the cad!), it'll displace you from the stable position but you'll quickly return to it without further action.

The pilot-on-top weight shift is like the see-saw. Imagine standing on it and straddling the pivot point. It doesn't take much to hold position. You can even lean over a bit (a bit!) and hold the plank in a slightly tilted position. But too far, and that end will come crashing to the ground without you being able to stop it. The same thing will happen if Gary Gustfront sneaks up and pushes one side up. If your reflexes are good, and you see him coming, and he doesn't push TOO hard, you might be able to recover. Odds are, no.

And recall, your design uses weight shift for both roll and pitch. The see-saw in this case doesn't have just a hinge across the middle, the whole plank is resting atop a cone. Any motion...fore and aft, left or right...is going to displace your position and will require a countering action to keep things from going goofy.

Yes, increased dihedral will help. If you run a computer program to determine how MUCH dihedral would be necessary for stability, it'll probably give you a value that puts the center of lift ABOVE the pilot. In other words, it becomes a classic weight-shift design.

There's a lot of argument about the need/desirability for stability. Some pilots believe that neutral stability produces more responsive aircraft. However, one must not forget that the Pitts Special is a fully Part 23-certified airplane...no one complains about ITS stability.

The problem is, it's my believe that this "inverted" weight shift arrangement produces negative stability. It'd require a very good pilot to keep it under control, and it'll probably require their full-time focused attention. Not fun.

I do like your design, though, Arthur, and look forward to hearing how your RC tests come out...with whatever configuration you end up with.

Ron Wanttaja

[Bill Berson]

I don't think weight shift works well in zero g maneuvers. Or upside down.
Weight shift does work at slow speed, that is good.
After Otto Lilienthal died, the Wright's set out to improve upon the limitations of weight shift.

I don't like weight shift.... Too much muscle work for me.

[Siggypoo72]

So sorry for the long wait between replies. I just returned from Cuba where I had my wedding.
My design has been has been edited to include significantly more dihedral and a four-degree washout at the wing tips. I've yet to finish construction, but the clamshells have been broadened to provide more drag when opened. The pilot no longer lays above the fuselage, but now directly on top. I expect with that there will be no exacerbation in pilot CG changes.
as I mentioned before, for the sake of construction simplicity and reliability there still are no control surfaces beyond the clamshells. I now have them opening from high-torque servos instead of a physical cable or push-rod connection.
with the advances in motor and fan design and efficiency I'm now also exploring electric ducted fan options instead of RC turbines. Less thrust but much less expensive and maybe more relatable. I'm still not decided on which airfoils to use, but the Marske 7 is still looking good. This isn't a high-speed design, and the high-lift/low-drag Marske 7 seems ideal. There's a much higher sink rate with it over other reflexed airfoil designs, so I'm thinking it bleeds airspeed faster than others too. With my models, as the velocity of the plane diminishes, I've had to add throttle to counter much earlier and to a higher degree than I did with other wings. But at speed it's so super stable, and forgiving.
My mindset is so old-school, I've taken the full size glider out and learned what changes to make that way.
here in Oshawa, there's one spot on Rossland Road where the road descends over 100 feet over a bridges over a creek, then back up hill towards Oshawa Airport (CYOO).
Around 2am I've taken the plane out there, laid down on it and had friends push me down the hill. Yah, I'm silly I know. But I'm a tactile kinda guy, lol.