Airfoil selection for a flying plank, how to achieve statical equilibrium?

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nestofdragons

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I love tandem wings and biplanes, but I like a flying plank because it is in theory less draggy and this might result in a lighter aircraft.

I read an article by Al Blackstrom on nest of dragons. I have ordered Jim Marske book because he seems to enter into technical details regarding the aerodymamics of his slightly tapered wings; that seem very similar to a flying plank.
Ordering the book of Jim marske must be a good idea. Mike Whittaker told me that he would have done his flying plank differently if he had this book earlier. Especially the data about the airfoils. :bow:

About saving weight with flying planks: i still dream of a flying plank which is so fat in the middle that the pilot fits mostly in it. Fat airfoils give the possibility to make a high spar whcih can be made out of lighter material. But the parts are longer too. I wonder if there would be a weight reduction by using fat airfoils. Anyway ... it will not be a speedy airplane if using fat airfoil. But i am not looking for speed. Just fun flying.

Control surfaces: if fearing stalls, go see the control surfaces of the Mitchellwing. NOT THE SPOILERS IT HAD IN THE FIRST MODEL (the hangglider in which the pilot was partly in the wing). The control surfaces that hang under the main wing. Might be draggy, but ... must be very stall free. Would it also give more stability??? Just to prevent tumbling. 🤔
 

ragflyer

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Hi ragflyer,

I understand that you mean that flying planks do not have their CG on 18%, 20% of the chord. Do conventional aircraft have their center of gravity on that position?

The static margins are of far less value than what Aaerowerx was suggesting on?

I thought that the formulas that serve for a conventional aircraft also serve for a flying wing?
The CG for planks have to be within a very narrow range- 16% to 20% utmost. The aft limit is highly constrained as you cannot go beyond the AC of the wing (which is at about 25%) and the forward CG limit is constrained by the power of the elevator as the elevators are very close to the CG.

On a conventional tailback airplane the CG range is typically between 15% to 35% more or less depending on tail volume.

Yes the formulas are the same between all types of configurations.
 

jedi

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Just one opinion on how to work with the narrow CG range of the plank flying wing.

The Spratt control wing hangs the variable payload below the wing on a fixed pivot point. It adds a tail defeating the simplicity of the plank but the pilot needs some place to put the body regardless and this allows the pilot compartment more freedom to fit the needs of the pilot. It also allows a lifting tail without destroying the stability of the aircraft.

Combining the pendular pitch control of a hang glider with the tail of the Sprat Control Wing may be a good hybrid solution. My preference is to use the weight shift for trim and the tail for aerodynamic pitch control and additional lifting surface for a low speed landing.
 

jedi

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More flying wings need inflight adjustable CG.
Time for Henryk to comment again. That is how Kasperic did his stunt flying inflight tumbles and recovery and zero roll landings.

Off topic - What is the issue elluded to with Henryk's health. CMWTK - translation not necessary or useful.

J.L. Frusha - Not aware of your issues either. Praying for you all.
 

Aesquire

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If I do not recall it bad, on the wing tips lift was pointing downwards.
Yep, the book says that. But when I turn my head, in any condition including stall, I can SEE that the wing is off the sprogs. ( Aussie name ) and lifting. ( I love tuft testing! ) The tips only push "down " in inverted flight or extreme low angles of attack. They're for dive recovery and to reduce tumbles.



atoms aren't actually tiny solar systems either.
 

Lendo

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nestofdragons, just a heads-up you might want to additionally apply the 'Beam Buckling' cantilever formula to the number of WEB wraps. When the Web fails so does everything else.
George
 

oriol

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Thanks Ragflyer for the information,

I guess that at the end it is all about drawing a diagram with moments and forces that equal to zero. If the formuals are the same then everything I can read about conventional airplanes might be of use.

Hi nestofdragons,

The bad thing of using a fat airfoil is the drag penalty. More drag means more power and thus more weight and thus you loos the advantage of a small flying plank.

Hi Aesquire thanks again for the clarification,

Hopefully this weekend I would be reading more about aerodynamics and control.

Cheers,

Oriol
 

rotax618

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I started a thread “Aussie Plank”, the airfoil used was symmetrical. It seemed to fly pretty well.
 

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oriol

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Hi Rotax618!

I saw your thread. The prototype on your pictures is absolutely gorgeous!

I did a lot of aerodynamics reading this last days. Unfortunately most of the equations parameters require experimental data from a wind tunnel. Despite the limitations of theory, there is a lot that can be done by running preliminary numbers on paper.

If using a non reflex airfoil, with a moment around the aerodynamic center that is not very big. I believe that a high wing flying plank can be made dynamically stable; because of the pendulum effect of the pilot weight. This is just a theory, I am waiting for Marske to learn more about this specific subject.

Given that there is still a lot of work, before the final dimensions of the airplane can be set. I am looking forward to build a small RC plank to be able to test all the theory. Now I need to learn some basic balsa skills.

Cheers,

Oriol
 

Aesquire

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. I believe that a high wing flying plank can be made dynamically stable; because of the pendulum effect of the pilot weight.
that might not be so. What happens in microgravity when maneuvering or inverted?

considerable effort is put into controlling pitch with modern weight shift hang gliders in the edge case weight shift stops working. That's because we fly in turbulent air on purpose. Microgravity isn't an "if" it's a "when".

One of the Common reasons is "going over the falls". If you imagine a typical thermal as a boiling mushroom cloud. with your plane on top, going outward from center, with a sudden tail wind that is also a rotating cylinder segment trying to pitch the nose down and tail over into a tumble. In zero G. That's "over the falls".

It's quite exciting.
When my suspension straps go slack, you need to know what to do until control returns.

The paragliders do INTERESTING things when the lines go slack. They are basically planks with little sweep and an airflow inflated wing that works at a limited AOA.

Pitch stability has to work when pendulums don't.
 

rotax618

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The fastest build for models is styrofoam or Depron. Get a sheet of lightweight styrofoam, make yourself a hot wire bow and some experimental airfoil profiles and you can make a wing in minutes.
 

AeroER

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The fastest build for models is styrofoam or Depron. Get a sheet of lightweight styrofoam, make yourself a hot wire bow and some experimental airfoil profiles and you can make a wing in minutes.
The first cut with a 8 inch span doesn't need to be that fancy. Sand a short angle on the top of a flat sheet of balsa to form the leading edge, and little longer angle on the bottom surface to form the trailing edge.

I'm curious about "microgravity".
 

Aesquire

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Microgravity, typically free fall, but also consider the "less than 1 G" part of the envelope. Stalls, flying through sink, Vomit Comet climbs and dives.
 

oriol

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Thanks Aesquire for your sharing your thoughts!

Jim Marske book covering flying wings is on the way. I am curious to see how Marske deals with theory to achieve static and dynamic stability.

The equation I have come up for static stability is:
Neutral point = Aerodynamic center wing body + Stabilizer efficiency * H tail Vol Ratio * (slope stab/slope Wing body) * (1-0,35)

If the wing is rectangular and there is no tail then the equation is:
Neutral point = Aerodynamic center wing.

If the pilot is seating on the aerodynamic center of the wing. The only way to have a static margin different than zero. Is by moving the engine or by changing somehow the center of gravity of the airplane.

Given that high wings can be stable with 0 dihedral. I thought that, roughly speaking, by having a high wing I would be able to achieve both longitudinal and lateral stability.

I understand that the limits between dangerous or stable are not trivial. The risk of tumbling in case of turbulence is a big concern in a flying wing.

With the equations above, I am not able to tell if a fying plank can be made stable or not?
The only way to confirm, what intuition allow me to guess, without risk. Would be by flying an RC airplane.

This is what comes next!

I am thinking about the pros and cons of balsa and foam and watching a ton of tutorials!

The basic idea is to confirm the theory by flying a rectangular wing on a conventional airplane with tail. And next fly the same wing in a fuselage without tail, only a vertical stabilizer, and see if I am able to make it as stable as the other. Once both airplanes are made stable. I will have to mesure the difference perfos of each aricraft, to see if one is better than the other for a specific mission?

I am really excited about testing and RC DIY airplane. I fly multirotors and I have practiced with Wings, a simulator with different aircafts to see what configuration would suit me best, a flying plank looks really promising.

Anyway, I will share the result of my RC experiments. Thanks everyone all for the stimulating input!

Oriol
 

oriol

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Thanks Hot wings!

Seeing how wonderfully this aircraft flies. It is hard to tell why there are no more tailess gliders and sport planes flying around?

I talked to a glider pilot who flew a fauvel, he told me that the airplane was a nice airplane to fly. I was curious given that is a single seater, and the only training is some basic verbal advice before jumping into the cockpit. According to the pilot any sailplane pilot could handle the aircraft.

I guess that if you build an experiental proto, without plans, you have to test the airplane and fly it carefully at first, until you become familliar with it.

Oriol
 

bhooper360

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Seeing how wonderfully this aircraft flies. It is hard to tell why there are no more tailess gliders and sport planes flying around?
I flew the Genesis 2 glider in Condor so that makes me an expert, right?

Due to its flying wing influences, the wing has wonderful performance at low speeds. That is, it has wonderful performance in a straight line at low speeds, because there is not enough rudder authority to coordinate the turns with any decent roll rate. In practice that means, when you catch a thermal, you will have to struggle struggle to center the lift, not only in the initial turn but also for readjustments. it's very frustrating.

this problem can be easily solved -- put the cg more to the front, no problem. This way you have nice directional stability and good rudder authority. Of course this glider is now competitively worthless, because the additional downforce has removed the only advantage which is low wing loading.

Luckily the glider has fantastic performance , and excellent glide ratio, because it is a very advanced design. If you take the tail and put it on the trailing edge, then it's naturally a lower moment arm and higher chord. For this reason, relative to the the other standard class gliders (which are even more fantastic and excellent), the Condor Genesis has relatively terrible performance at high speed, because the additional chord makes the wing a plank.

Well, at least the glider has excellent performance in weak conditions on ridge tasks. This is a great niche! A very niche niche for a very plane plane. Well, at least this one may be easier to trailer or something...?

Disclaimer: None of my observations are consistent what we have known all this time, that flying wings are 100% wing, whereas the other airplanes are only 50% wing or maybe 80% wing tops, and everybody knows that a wing which is 100% wing is just plane better!
 

Aesquire

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In roll stability the 2 typical ways is dihedral, and to simulate dihedral with a high wing.

But it's NOT that the wing is "high" that makes simulated dihedral, it's the vertical fin of the fuselage, making the "T" shape that gives the effect.

Consider the simple Dart paper airplane. The addition of elevons to the trailing edge can change it from a ballistic dart to a glider. ( 2 simple angled folds ) Darts with the central "fin" down are roll stable, "fin" up are not, without adding dihedral to the wings. ( flown "upside down" )

Simple plank flying wings with & without dihedral/vertical fins are easy to demonstrate with paper models.


IMHO the "bible" is by Barnaby, Captain Ralph Stanton, just as Theory of Wing Sections and Stick & Rudder belong on the shelf of Any Pilot or would be designer.



Before you get elitist... ;) I point out a ream of good paper, a roll of tape, and a few hours time will teach you more about aerodynamics than a typical college course today that focuses on the Colonialism of mathematics. ( Sarcasm, Joke, Deep Truth )

And can save a small fortune in model building by pointing the way to effective design. ( and a Large fortune in human scale design )

Captain Barnaby ( test pilot, US Navy, amphibious gliders! Among other accomplishments. ) lays out the basics better than I can. Better word smith, with nice drawings!

I've spent many hours ( when I should have been studying ) folding paper into Gliders and developing the basic tests for stability. Drop nose down and observe flight path. Stall recovery. Symmetry in various axis. Etc.

and countless hours waiting for soarable conditions on hills all over the North East. The free flight models sent off have soared beyond sight countless times ( with far less money and time invested than my equally lost balsa & tissue models ) and often out soared RC gliders, albeit the RC guys got their gliders back far more often.

From Mt. Washington to college campuses ( Redacted for legal reasons, but think "westerly point" & Horneynell ) biodegradable gliders I've made have soared past classroom windows and flown for multiple minutes until out of sight. ( Sgt. Majors have an incredible vocabulary to describe personality deficits! )

Planks work. Simple, easy, and effective. But the same center of lift & center of gravity "rules" apply as a Kitfox or 747.
 
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