Characteristics of a straight-wing, tailless model in the Langley free-flight tunnel

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cluttonfred

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu


nickec, if the outboard and slightly extended control surface is your elevon, what is the purpose of the inboard control surface? Was that your "modular" component, in other words intended as a fixed surface but there to easily change the span of your mobile control surface if necessary without affecting the rest of the wing structure? Cheers, Matthew
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

The trailing edge components can act as control surfaces or as fixed trailing edges.

Inboard and outboard can be swapped.

Elevon is always outboard - full span elevons are NOT likely - so it is chord that changes, not span.

I will make one extra set of surfaces - the extended chord set - which can be installed inboard as elevator surfaces, or outboard as elevons.

You could configure the elements to result in full span elevons.

When the extended chord surfaces are elevons the configuration follows Al Backstrom's suggestion. When the extended chord surfaces are inboard and serve as elevators the configuration mimics Jim Marske's Pioneer series of flying wings - where root wing chord surpasses tip wing chord and the leading edge has no sweep. When no extended chord surface is installed and the trailing edge is straight, and only elevons control pitch and yaw, the configuration is like the EPB-1 which Easley, Powell, and Backstrom designed together.
 
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nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Preliminary Rib Weights.

NC11 rib weight.jpg

17 ribs would total 3.6 pounds.

21 would total 4.5 pounds.

More lightening holes would reduce figures very little.
 

sange

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

First time I am posting on this forum after several months of reading. I like the overall quality of the discussions. I have a Flying Plank project in mind similar to the one discussed here and therefore am very interested in this thread. For the moment I have a few questions on the above discussions :
I don’t understand why a larger elevon chord will help to get a better lift distribution at higher Cl. Can someone help ?
Link to this, does a section have a predetermine aileron / pitch chord (in %) ? Most of the flying plank have a 25% elevon chrod except the backstrom aircrafts which are closer to 33%. Only the Backstrom design use elevon. The other designs have separate control surfaces.
I am surprised by the position of the central rudder just fore of the pusher propller. Is that efficient ? Also the TE of the rudder should not be too close to the propeller to avoid noise, I not sure what should be the clearance, 4 inches ?
Thanks.

 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

... the TE of the rudder should not be too close to the propeller to avoid noise, I'm not sure what the clearance should be, 4 inches?

How close a fin or a rudder surface can be, for the proposed design, will require a little trial and error. This will be done on an engine test stand to test feedback to the rudder pedals. Easily done.

There are variables: prop area, prop speed, prop shape. Rather than guess I will test.

 

sange

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Here are the dimensions and weights of my flying plank project:


M, kg, km/hrft, Inch, pound, miles/hr
Wing span7,223' 7"
chord1,24'
aspect ratio66
Wing area8,6493
Empty weight81178
gross weight181399
Stall speed6540

Any comments?
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Using an online tool, which I have found to be ballpark accurate, I estimate your wing might lift about 450 pounds at 40 mph.

This assumes

an airfoil with about 2.5% camber
an angle of attack about 8 degrees
an altitude of about 1000 feet

Your wing would lift about 450 pounds at 76 mph at an altitude of 4000 feet with an angle of attack of about 0.5 degrees.

To comment further would require a drawing, more specifics about control surfaces areas and positions, weights, engine, propeller, and method of construction.

The numbers above are merely estimations and should not be taken as gospel - nor relied upon without flight testing to verify their accuracy.

Coefficient of lift at 40 mph is about 1.2 (in the scenario outlined above) which is definitely a reasonable expectation from a reflexed airfoil similar to the one at this link.

Click here to see polars at 8 degrees.
 

sange

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Nickec, Thanks for your interesting message.

Below is a drawing of my Flying wing. It is very similar to the Backstrom’s Powered Plank, but there are major diferences. It is a wood construction, except the wing main spar that will be made of carbone to gain weight. I am expecting 30kg / 66 pound for the wing including 2 wing tip rudders, 25 kg / 55 pound for the fuselage with a tail drag landing gear, and 25 kg / 55 pound for the engine.

The engine is located at the trailing edge of the wing with no shaft extension. So the engine must be light to get the CoG position right.

Regarding the Czmax, as you did i took 1.4 for an infinite wing span, and apply an Oswald coefficient of 0.75 for an wing aspect ratio of 6. It gives a 40 mph stall speed at 180 kg.

I am still not sure about the yaw control surface. I would prefer a central fin, but I am not convinced it will work on a pusher with an engine/propeller not located too far aft like teh NACA model. With my design a central would have a short lever arm and the rudder is not blown by the propeller.

Regarding the elevons, I was originally thinking of a 25% chord as the Fauvel’s design, but then move to 33% similar the Backstrom’s design. And then I understand extended further the elevon behing the wing trailing edge would further help. Not sure if i should stick to the 33% or extend further.

B8wAKS+MyyehwAAAABJRU5ErkJggg==

 

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Norman

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

I think there is some misunderstanding of the chord extension of the elevator section happening here. I haven't read Backstrom's comments about this but I do have "Tailless Aircraft In Theory And Practice" around here somewhere. What the authors describe is definitely not an extension of the elevator but rather a longer chord of the section of the wing that the elevator is attached too if you use separate ailerons and elevator. By having the wing jump to a higher chord in the elevator span that section of the wing has a smaller lift increment per change in pitching moment [delta CL/delta Cm] and thus distorts the lift distribution less which means less induced drag. You want as much lift increment per degree of deflection from the ailerons and tail of a conventional airplane as possible but not from the pitch trimmer of a plank. In the case of a plank flying wing all you want from the pitch trim surface is a pitching moment because the lift increment of the elevator is always in the wrong direction. As you can see in the attached graph elevator chords greater than 20% have practically no affect on Cm but there is a small cost in L/D for the longer surfaces ie for any given AoA the longer control surfaces produce less lift and more drag. You should do a comparison polar like this for your airfoil because many airfoils have different sweet spots depending on what effect you're trying to produce. Generally if a big lift increment per degree of deflection is the goal a forward hinge is better but that's not what will pitch a plank. You need pitching moment and shorter chord control surfaces are more efficient for that.

If I were building a constant chord plank 'wing I'd go with 15% full span elevons. That would have the least impact on the lift distribution.
 

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nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

... It is very similar to the Backstrom’s Powered Plank, but there are major diferences.

What you have outlined seems reasonable and achievable.

Backstrom wrote about his powered plank and the lessons he felt he learned. I will try to post links to the articles once I find them.

I remember two issues he discussed.

1. The thrust line must be adjusted to minimize pitch response to changes in thrust. Your design will require tipping your motor mounts up at the rear and down at the front.

2. Take off and landing procedures are different for planks. Backstrom and his building partner tried three landing gear configurations. They settled on trike gear - which required experimentation to find the best weight distribution for their airframe.

The short coupling of the taildragger landing gear you have drawn might be an issue. Only testing it can say for sure.
 

danmoser

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

.. Generally if a big lift increment per degree of deflection is the goal a forward hinge is better but that's not what will pitch a plank. You need pitching moment and shorter chord control surfaces are more efficient for that.

If I were building a constant chord plank 'wing I'd go with 15% full span elevons. That would have the least impact on the lift distribution.

Good points.
One of the unique and somewhat problematic aspects of the plank is the dynamics of pitch control.
Plank pilots coming in for a landing often experience a premature touchdown, because when they yank back on the stick quickly, the trailing edge goes up and zero-lift angle of the airfoil shifts such that there is a momentary loss of CL .. before the plank has had a chance to rotate to a higher angle of attack to restore some CL, the wheels are already on the ground.. and pilot is bewildered... "I had plenty of speed, so why did pulling back on the stick make me go down?"

Also common are stories of Pilot-Induced Oscillations on takeoff for much the same reason.. pulling back makes you go down briefly.. then it rotates nose up past a neutral point due to low pitch damping .. the pilot jams the stick forward to counteract.. it initially has more lift and goes up, then rotates nose down past the neutral point.. and the up & down rodeo begins!

My experience with plank RC models was frustrating at first until I learned to make small, slow control movements and anticipate aircraft response time... still, the low inertia model was a bad match for my reaction time, and I was never really able to master it.

Based on that alone, it seems logical to design a control system that minimizes delta CL/delta Cm

Marske & Fauvel came to the same solution: tapered wing, slight forward swept c/4, inboard extended elevator with longer lever arm to CG .. not a bad way to go.

It would be interesting to see how the rectangular planform, no sweep, short chord full-span elevon arrangement compares to the Marske/Fauvel types.
 
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nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

... I will try to post links to the articles once I find them. ...


The WPB-1 Flying Plank, by Al Backstrom

One of the articles is linked above. In it Al gives details about the trials, tribulations, success, and lessons learned from his particular design effort.

There is another instructive article which I have yet to relocate. I will post it in this thread as time allows.

In a phone conversation I had with Al he intimated that a simple plank planform will perform well when properly configured. This is really the case with various reasonable aircraft configurations. Some planforms have a wealth of repeated construction. For such craft much practical knowledge is available. For pure planks much less experience exists - compared to "standard tractor lightplanes".

Al Backstrom was a very experienced aircraft maven. He was a DAR. He was a avid airplane model builder and sailplane pilot.

He died before I got a chance to ask him about his opinion of the Scott Winton Facet Opal. He will continue to be missed.
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

To make my comment to Sange less ambiguous I post the image below from Al Backstrom.

Backstorm suggests mods to WPB.jpg

Note the canted thrust line.

Thread readers may already be aware of the importance of the vertical position of the Center of Gravity in all aircraft with respect to the thrust line. I post the picture above to emphasize the point as it relates to airframes close to Al Backstrom's WPB-1 in configuration.

As built, the WPB-1 suffered from a lack of such a canted thrust line. Thus engine throttle position strongly influenced pitch trim.

Since Sange avoids a prop extension shaft he will be able to adjust his thrust line much more easily than Backstrom and his building partner could make adjustment to the WPB-1's thrust line. They had to cope with a complex engine installation.

Read Sport Aviation article I linked to learn more.

You can also read the NACA report to understand more about thrust line issues.
 

sange

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Norman,
After a carefull reading, your comment on the pitch control vs chord depth makes a lot of sense. How did you produce these graphes ? I would love to play with the Fauvel and Marske sections I have selected. Now with a narrow depth elevon, will the roll control be effective enough? Because it optimises the Cm variation but not the Cl variation that is needed for roll control. Is my reasoning right ?

Nickec,
Thanks for the WPB-1 article and your comments. I will make sure the design of my aircraft will allow an easy adjustement of the thrust line and main landing gear position.
It is true that there are not so many experience on straight flying plank, but the Backstrom’s aircrafts. As anyone seen a test flight of the EPB-1 glider ? It might help understanding what to do to improve the original design. You might know this video : Flying plank.mpg - YouTube

 

danmoser

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Another possibility for plank pitch control is longitudinal weight shift.
This allows a pitching moment torque to be applied to the aircraft without changing the airfoil properties at all... in other words: (delta CL / delta CM) = 0

I believe some are already using weight shift mechanisms for pitch trim, but there's no reason it couldn't also be used for pitch control.
Flexwing trikes & hang gliders have used it for decades, and it works well.
It's responsive, positive pitch control without a tendency to over-control.

On a rigid plank, it could be implement in at least two different ways:
1. A moving weight coupled to the joystick .. this could be a small chunk of lead sliding along a bottom rail, geared to the stick such that full stick movement could move that weight the full length of the fuselage pod.
2. Moving the wing fore & aft relative to the fuselage .. this could be done by mounting the wing root attachment on a set of longitudinal rails, and linking the fore-aft stick movement to something like a rack & pinion mechanism, moving the wing fore & aft ... aileron linkage could get a bit tricky, but it could be done.
 

Norman

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

How did you produce these graphes ? I would love to play with the Fauvel and Marske sections I have selected.

I used Profili for that graph. It provides a user friendly interface to Xfoil and adds a handy rib design feature.
http://www.homebuiltairplanes.com/f...tions/10623-free-airfoil-design-software.html

Now with a narrow depth elevon, will the roll control be effective enough? Because it optimises the Cm variation but not the Cl variation that is needed for roll control. Is my reasoning right ?
That's part of the reason that I'd go with full span elevons. It's simple and fairly efficient. By making it full span you avoid abrupt changes in the lift distribution that cause induced drag and you minimize the control circuits that you have to design and build.
 

Norman

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Not much point posting private videos in a public forum. As members of the general public we can see the Youtube window but don't have permission to watch the video
 
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