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Airfoil design study

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Old 01-30-2010, 12:43 PM
  #1
Jaspur_x
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Default Airfoil design study

Hello all,
I just made a purchase of a flying wing design r/c plane. ( more on that later)

I have never studied designs very much , I have only built a couple kits , and a couple "scratch built" that are really just modificatons of an existing design.
I have always loved flying wingdesigns.
The first time I even thought about building from plans was as a kid , I saw a "tailless flying wing" in flying models.
Itis still too complex for me to build , but every once in a while I read the article again and ponder attempting this build.

So here it comes , I am trying to learn more about wing airfoil designs.

I get the flat bottom arfoil and semi-symetrical designs easy enough. The tailless flying wing articleexplains the fact that this design has generous washout.
This is where you take a flat bottom airfoil and the bottom of the wing ribs , behind the cg ,dip upward toward the top of the rib , is this correct?



So , this flying wing I just got is a megatech B-3 ( as in the B-2 stealth bomber , yes it`s electric , but the question isn`t)

This design has a lot of this "washout"or at least I think that is what it is.

This little beauty has the bottom of the "wing" folllow the shape of the top pretty basically , from the leading edge the whole way back, judging by eye.

This would increase stability by my thoughts , but decrease lift a little by comparison to a flat bottom airfoil but not as much as a symetrical airfoil right?



Jared


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Old 01-30-2010, 01:01 PM
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Default RE: Airfoil design study

Flying wings are by far the least complex of all model designs to build. Build a wing and you're done.

Just build a wing with any smooth, flowing airfoil with no ripples, bumps, dips, twists, carbunkles, hickups......keep anything that sounds remotely eggheaded, hi-tech, high falootin', etc. completely out of the equation.
Just do a good job of building a wing that isn't warped or twisted, keep it light, set the CG between 15-18% MAC, get the vertical stabilizers on there with plenty of area AFT of the CG.
Then go have yourself a lot of fun!
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Old 01-30-2010, 02:14 PM
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Default RE: Airfoil design study

First Ditto what CP said.[sm=thumbup.gif]


Quote:
ORIGINAL: Jaspur_x

The tailless flying wing article explains the fact that this design has generous washout.
This is where you take a flat bottom airfoil and the bottom of the wing ribs , behind the cg ,dip upward toward the top of the rib , is this correct?
No. Washout is when you take and twist the wing along it's span so that the tip is flying at less angle of attach than the root.

What you described is called reflex, which is used to stabilize the flying wing if it's a plank type wing.

Quote:
This little beauty has the bottom of the ''wing'' follow the shape of the top pretty basically , from the leading edge the whole way back , judging by eye.
That sounds like washout. Swept wings don't need reflex in the airfoil, long story why, just believe.

Quote:
This would increase stability by my thoughts ,
Yes.

Quote:
but decrease lift a little
Yes.
Quote:

by comparison to a flat bottom airfoil but not as much as a symetrical airfoil right?
You probably couldn't tell the difference in the amount of lift. The aerodynamics on our little chord airfoils and at our speeds is very complex, but the short story is don't worry about the airfoil.
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Old 01-30-2010, 05:26 PM
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Default RE: Airfoil design study

CP ,
check out the " tailless gas model"   flying models, 1988 page 30............. looks way more complex than anything I have ever even atempted.
And the B2 stealth looks complex to me.

I`m used to building fewer pieces such as clancy aviation`s the little plane that ccould , or the ace littlest stick kit.

soarrich ,
see this why I have to ask stupid questions , I read stuff but do not understand some terms and stuff.

How about cg on a flying wing ( plank type) it is still at the thickest portion of the chord right?........ whereas with a  delta like the stealth it varies on the average chord and other variables right?


Any other pearls to share?

Thanks guys,


Jared
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Old 01-30-2010, 06:09 PM
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Default RE: Airfoil design study

A flying wing CG needs to be worked out just like any other aircraft. If the wing is higly swept, the CG might be near the rear of the wing at the root or even behind it.

Look here and fiddle with this a bit: [link=http://www.scaleaero.com/CG_Calculator.htm]CG calculator[/link]

The basic workup is this:

- figure out the MAC - mean average chord - and it's location along the wing panel.
- You now set the initial CG to a position ahead of this, determined by the static stability margin you choose. Since you likely don't have a number in mind, for starters I'll throw out 10%. It can be carefully moved back from there a bit to increase efficiency and manouverability, once you've checked out the initial setting.
- Unless you know more about the airfoil's properties than you usually do at this point, assume the neutral point on the airfoil is at 25% of the MAC*. That figure shows up a lot and generally works fine.
- Now measure your CG at 10% of the MAC ahead of this point. IOW, at the 15% point of the MAC. So if the MAC is 6", the CG would be located 0.6" ahead of the 25% chord point on the MAC, aka at 15% of the MAC. Follow?

NOTE: some folks express this point as a percent of the MAC ahead of the NP; this figure is called the static stability margin, it's the amount safely ahead of the NP you locate the CG.

Another way to express this is to do the arithmetic already and quote the actual position, as in "set it at 18% of the MAC", which is a 25% - 18% = 7% static stability margin. The routine linked above asks for the latter, the actual location on the MAC. So, for 10% static staiblity margin you would enter 15%, for 6% you would enter 19%, etc.***

That 10% static stability margin figure I mentioned would normally give you a fairly safe forward CG position to start, which would need just a little trim - in the form of reflex or elevon deflection, whatever.

You cannot simply apply a rule of thumb along the root chord, such as the high point or 1/3 or anything like that. Those rough approaches only work out in generalized applications with unswept wings. If you have a straight plank wing for example, unswept, a rough rule of thumb like at the 15-18% of the root would probably work fine, but do yourself a favor and review these basic procedures instead because they work for any planform.

If I may offer my personal recommendation - build yourself a little sheet balsa or Depron flying wing glider, and try this out for real. Nothing beats hands-on, and the only risk is a dinged leading edge or cracked dihedral joint on a $0.50 chuck glider - so what! Move the CG around, check out the effect. Make small elevons that you can tweak too. This is exactly what I did for a .15 powered delta I have under construction - I made a full scale chuck glider with Dollarama foam-core wings and vertical fin, with a fuselage cut and roughly carved from 2" thick foam.

MJD


* if you knew any more about the airfoil's pitching moment etc., then putting the horse before the cart you would already know how to work out MAC, NP, stability margin and the like, if you get my drift.

*** I just read CP's post - note that he did just that; 15-18% of MAC, or expressed differently, about 7-10% static margin. Pretty much the same numbers I tossed at you and so would others. Your hardest job - and it is easy - is to figure out what and where the MAC is, and that link will do it for you.
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Old 01-30-2010, 06:14 PM
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Default RE: Airfoil design study

Quote:
ORIGINAL: Jaspur_x

br/>soarrich ,
see this why I have to ask stupid questions , I read stuff but do not understand some terms and stuff.
No such thing as a stupid question.
Quote:

How about cg on a flying wing ( plank type) it is still at the thickest portion of the chord right?
NO. if you look at the airfoil in the picture it's max thickness is at 30%, but when I built a plank flying wing with it, it flew with the balance point at 21%.

Quote:

........ whereas with a delta like the stealth it varies on the average chord and other variables right?


Any other pearls to share?

Thanks guys,


Jared
First, there is only one balance point on a plane, no matter what type it is. Yes the platform shape, airfoil, sweep all have an effect on where you have to balance it. The plank is the crudest of flying wings, but easy to build. You can take any ugly stick ARF and leave the stab off of it, hook the ailerons up as elverons set with some reflex, and you have a plank flying wing.
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Old 01-30-2010, 07:57 PM
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Default RE: Airfoil design study

I got a shrike, I'm not using, you can play with if ya want?...Rog
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Old 01-30-2010, 08:10 PM
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Default RE: Airfoil design study

Jaspur,

Check this thread:

http://www.rcuniverse.com/forum/m_94...tm.htm#9412339

Just ask what you don't understand, there is no such a thing as a stupid question.

Regards!
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Old 01-30-2010, 08:28 PM
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Default RE: Airfoil design study

I read some threads some time ago on the Megadeth Stealth electric and it sounds like a hopeless piece of junk. Basically untrimmable. It can be flown in a defensive, counter-counteractive kind of way, but from all I've read about it it might be easier to balance a balloon on your nose while riding a unicycle....
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Old 01-31-2010, 12:58 AM
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Default RE: Airfoil design study

Flyingrod ,
I truly apreciate the offer my friend , but with my luck lately , I will have to pass. I mean no insult , but I would hate to see what my flying " skills" (and I use that term loosely on myself ), would do to it.I`m sticking to my frankensteined littlest stick( I won`t even get out the un-crashed version yet) I`m back to trainer mode for now til I get back into the swing of flying forward.
Is that the lanier shrike by the way?

I have`nt done any real ( forward flight) flying for quite a while. All I have for a runway is grass under a few inches of ice , and a frozen lake where the wind is higher.

CP ,
I am not suprised at all with theprice of the stealht , but I love the design , and its cheap I will not mind crashing it as much as something I took the time to build.


Thanks guys,
Jared
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Old 01-31-2010, 10:05 AM
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Default RE: Airfoil design study

Hi Jared,

I think your original question was about washout and tailless airfoil reflex.

Reflex refers to the airfoil shape that (as you said) has the bottom surface curve upward in the aft section to give pitch stability. The same effect can be had with a symmetrical wing and simply raising both elevons upward. This along with a forward CG (as the others said) will allow a previously horizontal stabilizer equipped aircraft to fly just fine without one.

Washout is the spanwise twist of a wing used to induce the root to stall before the tip to delay loss of roll control and sudden tip stalls. When viewed from the tip, the root is set at a higher AOA (Angle of Attack) than the tip; usually 1 to 3 degrees.

Nothing to do with the AOTD by the way. (Angle Of The Dangle.)
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Old 01-31-2010, 10:07 AM
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Default RE: Airfoil design study

If Great Planes still makes the "Slinger", that makes for a pretty neat/cheap flying wing that could probably be flown by anyone who can handle a trainer plane. And it's pretty crash resistant, mine finally got parked only after the top hatch came off in flight. My speed control had an "arming button" because it is old, most don't have those anymore. Anyway I mounted the button in the top hatch, when the hatch came off the wiring from the button yanked the speed control out, and that yanked the battery pack and the reciever out, and that yanked the wires out of the servos, with the motor chopping and slicing the plastic pieces on the way down. Still, the wing is flyable, if I feel like sticking the stuff back in it, just the plastic parts that cover the radio gear got destroyed, the wing itself has almost no damage. I bought it just for fun, I think it was $39, and I flew it with an 8cell nimh pack and the speed 400 brushed motor that came with it. Even with that modest setup it would loop, and roll and cruise around pretty good, but not as fast as it looked. On low rates it was very easy to fly, but not self correcting...so as long as you can fly an aileron trainer you should be able to fly it.

I have the same relationship with flying wings as I do with 3 channel aircraft. I get the urge now and then, I build one, after a while I get bored with the limitations...but I'll take it to the field every time to fly...eventually destroy it pushing the limits...(how low can you go inverted with rudder, lots of dihedral and no ailerons sort of thing) then after a year or so I'll say, "I miss my Gremlin..."

Combatpigg, are you saying I don't need the super duper top secret patented simitar airfoils on my flying wings? I've wasted years of research, and all this time I could have been using a simple NACA 0014 or something.... The first wing I built was just a "hots" airfoil, cut out 10, make it 40" long, 2 fins on the back, and a box for the nose to mount the tank in and motor mount on the firewall. With tricycle landing gear the mains were too close to the nose wheel and it was a handful on the ground, fortunately it didn't stay on the ground long. Now I'm too lazy for landing gear and just opt to hand launch....come to think of it, it's about time for another.

Austin

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Old 01-31-2010, 12:41 PM
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Default RE: Airfoil design study

Well , I like the looks of the "slinger" but maybe I will just do like you guys say and just build one.

i figure it would only take a handfull of wing ribs from the Carl goldberg gentle lady and a little box for controls.

Jared
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