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

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#### Aircar

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

There is a big scale effect as noted (pilots don't scale either) -- extending the nose longer than needed for aerodynamics or packaging is a cheap insurance to flipping (with the nose wheel at the outer end of course) . The tail on the Cri Cri will do most of the aerodynamic resistance but as you can see from the RV6 photo that is often not enough either . One of my criteria for the Opal was to be able to land anywhere a typical glider could without any grief (that means no gunk or gravel into the prop as well as no flip and able to slide on the belly if gear up ) bicycle type gear helps with that too. The Cri Cri is unusual in terms of engine placement and avoids the usual long undercarriage by splitting the prop into two and then raising them above the nose height (and the pilot is not also then raised to see over the usual tractor engine location) all in all a good set of design decisions --maybe also good for a tailless design.

#### Norman

##### Well-Known Member
Supporting Member
Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

extending the nose longer than needed for aerodynamics or packaging is a cheap insurance to flipping
Unfortunately the nose is destabilizing in both pitch and yaw so any side area forward of the CG will have to be reflected in the tail fin.

The Cri Cri is unusual in terms of engine placement and avoids the usual long undercarriage by splitting the prop into two and then raising them above the nose height (and the pilot is not also then raised to see over the usual tractor engine location) all in all a good set of design decisions --maybe also good for a tailless design.
The CG of a flying wing must not be significantly above the aerodynamic center. Just as a CG below the wing will add pendulum stability a high CG will subtract pitch stability. Addaon ran into this with his swept 'wing and it took a while to figure out that the problem wasn't aerodynamic. A plank would have even less trim authority to deal with it.

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#### danmoser

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

Nickec..

Charles Fauvel's final airplane design was the AV-61.
Evolved from a dozen other designs over his lifetime.

It ended up looking very much like what you're proposing.

Charles Fauvel and his Flying Wings

#### nickec

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

Just a quick sketch to show a variation in planform. Downside is tricky rudder cable routing. No canopy shown.

Fuselage side and outer face of vertical surfaces might cooperate raising "apparent" area of surfaces.

An even steeper included angle between the verticals - than pictured - would lessen pitch up influence of rudders.

Or a trim system could be installed to rudders as a landing aid - when trimming the nose up is desirable. This would require moving the rudders in opposite directions - as opposed to normal operation when rudders would move in same direction.

It would be important to avoid stalling the rudders if a trim system as outlined above were implemented.

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#### nickec

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

Rear schematic view of fuselage. Only central wing panel shown. Probably too tight a cockpit. Wanted to see how tight I could go.

4'9" wide. 48" panels bolt to both sides. This is three piece wing option. Total span 153". Just like DA-11.

Red line is engine footprint. Blue circle is spinner. Blue ellipse is canopy. Trike pants likely too small.

Inner grey circle is small prop. Outer grey circle is for 38x38 prop which Leeon Davis used in DA-11.

If the methods used in the Colomban Cri Cri fuselage are imitated, the fuselage will be extremely light.

To reduce cost and complexity foam stringers can be replaced with aluminum at very small weight penalty.

Canopy greenhouse is smaller than Cri Cri.

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#### cluttonfred

##### Well-Known Member
Supporting Member
Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

First of all, I am a big fan of simple, light flying wings and wish you all the best in your project, but I do have a few questions.

Have you switched to an untapered (constant-chord) wing? If so, I strongly recommend that you track down a copy of Jim Marske's booklet on the development of his plank designs. In short, outboard elevons are the way to go with a constant chord wing, but for maximum effectiveness the should be inset from the wingtips enough to be out of the tip vortex (a trick Jim learned from Al Backstrom). Since the raised elevons wash reduce lift on part of the wing exactly when you need it most, you want short span, long chord elevons to minimize that effect, even if that makes them heavy.

Also, why the dual rudders? There is a risk of one rudder blanking the other in extreme cases, like a spin, essentially halving your rudder area when you need it most. A single, larger rudder would simplify the build and the control runs.

Cheers,

Matthew

#### danmoser

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

Another possible variation of the idea is shown in the tailless aircraft book by Nickel & Wohlfahrt.
A very mildly swept & tapered wing with two fixed vertical fins placed about 2/3 outboard.. and jump twist elevon section in the outboard 1/3 with extended chord..

The fins provide yaw stability, straighten the flow and prevent leakage from high to low pressure zones.
This model flies very well .. stable & high-performing with excellent lift distribution across the span at all speeds.
There's no reason you couldn't also make the fins into movable rudders to augment yaw control authority... that wasn't needed on my RC model .. which unfortunately was lost due to radio interference & strong winds years ago.

#### nickec

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

Have you switched to an untapered (constant-chord) wing? ...

I want to explore the constant chord planform. I admit the advantages of the Marske configuration.

... outboard elevons are the way to go with a constant chord wing, but for maximum effectiveness the should be inset from the wingtips enough to be out of the tip vortex ...

Some years ago Al Backstrom sent me a letter including an article he wrote about updating plank design. In it he depicted elevons of large chord which extended beyond the trailing edge of the rest of the wing. Surprisingly these were not set significantly inboard from the wingtips.

Thus I have the option of 25% chord elevons which can be enlarged past the trailing edge if need be.

Also, why the dual rudders?

The possibility that the flat sides of the fuselage will interact in a beneficial way with the dual fins.

For now I envision planning the airframe to allow easy repositioning of the vertical surfaces between these configurations:

1. one above, one below - like the NACA model
2. like the Winton Facet Opal
3. a single central surface
4. dual surfaces along the flat fuselage sides

Being able to reconfigure the verticals and the elevon size without rebuilding will encourage investigation and comparison. The elevons, fins, and rudders are all very small. Imagine one large fin and rudder, one smaller dual set, and two sizes of elevons. Total fabrication time and cost is very modest. Installing the various control linkages will consume some time which is a small price to pay.

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#### nickec

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

Another possible variation of the idea is shown in the tailless aircraft book by Nickel & Wohlfahrt.
A very mildly swept & tapered wing with two fixed vertical fins placed about 2/3 outboard.. and jump twist elevon section in the outboard 1/3 with extended chord..
View attachment 29903

...

Nickel & Wohlfahrt's book is a favorite of mine.

After the plank experiments your suggestion tempts.

#### Norman

##### Well-Known Member
Supporting Member
Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

In short, outboard elevons are the way to go with a constant chord wing, but for maximum effectiveness the should be inset from the wingtips enough to be out of the tip vortex (a trick Jim learned from Al Backstrom).

Oddly enough "Experiments In Flying Wing Sailplanes" is on my coffee-table right now. Backstrom's story was about the horizontal drag rudders not elevons. Extrapolating that lesson about drag devices to camber devices may not have been justified by evidence.

Since the raised elevons [] reduce lift on part of the wing exactly when you need it most, you want short span, long chord elevons to minimize that effect, even if that makes them heavy.

Planks control pitch by manipulating the airfoil pitching moment. Large chord surfaces produce larger changes in lift with less change in Cm than short chord surfaces so I'm dubious about this comment as well. Although there are several areas on an airfoil that you can manipulate to change the pitching moment the largest factor, by far, is the angle of the mean line at the trailing edge.

#### Aircar

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

Interesting thoughts Norman, this is the same as the conventional amphibian "engine on a stick' set up where the Cg moves aft and can destabilize the whole show (but does help raise the nose in the low speed high thrust case ) -- Low wing tailless aircraft definitely look wrong intuitively -- high wing tailless can face the other end of the pendulum effect in having to raise the Cg with increasing angle of attack and thus imposing a large amount of negative flap and still maybe not having enough power to flare ( the Twin Plank built in Australia apparently had something like this happening since it didn't soar worth a **** apparently --it was a High wing .

was that Fauvel the "Pinocchio" ? --wonder how well it flew ?

Nickec - check out photos of the F 18 and note that the rudders DO work together to aid nose up trim (before a catapult launch you can see this ) -the effect was enhanced by the pressure field being felt on the section of fuselage between the fins not just the fins themselves. Your Cg is still quite high and gear quite long in my opinion --just going on your sketch.

#### nickec

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

... Your CG is still quite high and gear quite long in my opinion --just going on your sketch.

Understood. I have Cri Cri plans. By superimposing an accurate drawing of the Colomban design atop the sketch I reason that the CG of the sketch must be lower than the CG of the Cri Cri. This is because the dual engines are higher, the flying horizontal is higher, and the pilot is more erect.

Fuselage bottom of Cri Cri is 2.5 inches lower than sketch.

Perhaps it is an optical illusion caused by the low relative perspective of the sketch. You would have to sit on the ground and bend low to achieve the viewpoint of the sketch.

I would like the gear as short as practical. Wing tip clearance constrains lower limit of gear length after prop strike and pitch rotation is factored in.

Maybe the lack of end wing panels makes the gear look longer. And the Cri Cri has very short gear.

Midwings might make gear look longer.

Here is a low wing view:

#### cluttonfred

##### Well-Known Member
Supporting Member
Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Some years ago Al Backstrom sent me a letter including an article he wrote about updating plank design. In it he depicted elevons of large chord which extended beyond the trailing edge of the rest of the wing. Surprisingly these were not set significantly inboard from the wingtips.

Like I said, plank flying wings are a favorite of mine and I have often thought that a simple, easy to build plank design would make a great Volksplane for the 21st century. If you can keep overall length to about 7' (likely by pulling a pin and folding the rudder 90 degrees) then it's not hard to imagine wheeling the plane on a spanwise trolley to hangar economically in a 20' shipping container (with carrier-style folding wings or perhaps just wingtips) or a 40' shipping container (without folding anything at all). A secure steel hangar for $2,000-$4,000 US would do a lot to reduce the cost of club flying.

nickec, I would be very grateful if you could share that Al Backstrom article and any relevant comments from his letter.

Cheers,

Matthew

#### nickec

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

... I would be very grateful if you could share that Al Backstrom article ...

Quality is best I can do right now.

These pictures were found posted elsewhere on the internet.

Click picture once to open, click again to open new tab with picture slightly larger, click on picture third time to see largest view size.

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#### nickec

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

A Flying Plank Sailplane For Today?

By Al Backstrom

What? An issue with main articles by Backstrom, Carmichael, Hall and Sunderland? Four of our favorites! Thanks to all of you! Here's Al's contribution:

In the early 50s I initiated design studies that became the EPB-1 Flying Plank. The intended performance criterion was to be equivalent to the WWII surplus two-place sailplanes that were common at the time. A simple easily built machine was desired. Others were pursuing similar thoughts in small sailplane designs. The Schweizer 1-26 and Stan Hall's Cherokee II am the best known and largely built of this thinking. The configuration of the prototype and plans versions of my Planks is shown in Figure I.

The EPB-1 met the basic design requirements but never achieved the popularity of the 1-26 or Cherokee II, although several welt built around the world. There were probably more built in Australia than any other country. Unfonu. newly, Australia had a change of policy on certification of homebuilt designs before any were finished and approved for flying. The untimely death of Fred Hoinville, who spearheaded the Australian activity, effectively ended work on the ships down under. A two-place Plank built by Reg Todhunter was certificated as a standard sailplane in Australia. It is still in existence in a museum there.

I am often questioned about the use of the plank configuration for a contemporary design. I feel that a small simple sailplane of acceptable performance can be designed using the plank layout, but not by copying the original closely. To start thinking of what to do on a new Plank. we should look at what mistakes were made on the earlier version. I think the primary errors were made in (I) using a wing span that was too low (25 feet on the drawings) and (2) building the machine in one piece. The short span made the span loading higher than desirable and produced a minimum sink rate that is not acceptable today. The one piece construction limited the areas that could be used for construction and made for very awkward trailering. The trailering was aggravated by the wing tip fins producing high drag that hindered trailering speed and reduced the mileage of the tow vehicle.

Secondary problems included inadequate approach control and a small cockpit. Approach control was by opening both drag rudders. This was not too effective and nervous pilots would fly around with both rudders extended. The general handling characteristics and control were very good. The prototype was flown in acrobatic demonstrations at air shows by Ted lanczarek with the most complex maneuver being eight point rolls.

Performance flight tests conducted at Mississippi State College showed a lower minimum drag coefficient and lower span efficiency factor than the design estimates. The former allowed good high speed L/D (for 1954) and the latter adversely affected the minimum sink rate (See Soaring. January - February 1957).

In designing a Plank for general soaring today. I would start by using a span of about 33 feet (10 meters). An empty weight of 150-200 pounds (68-91) kilograms) should be targeted - the 150 pounds for possible use as an uncertificated ultralight glider in the United States, and the higher weight for a more robust general purpose machine. In order to obtain low minimum speeds. a wing loading of about 4 pounds/square foot (psf) (20 kg/square meters) is reasonable. The structure should be designed for a weight higher than anticipated normal flying weight. Using an assumed maximum weight of 500 pounds, a wing area of 125 square feet will be required.

To simplify construction. the ship would be designed in three sections. i.e. the pod and two wing panels. With the wing panels of about 15 feet (4.5 meters) length, the ground handling will be crew friendly. The pod section would incorporate a fixed wheel for takeoff and landing plus ease ground handling.

September - October 2000 Sailplane Builder
Sailplanehomebuilders.com
Page 11

#### nickec

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

...

The departures from the previous designs are numerous but the primary are described below;

Airfoils are not covered here as there are many sections to choose from for Plank type tailless designs, or a special section could be designed for the Reynolds number range of the design.

The wing structure would use a constant chord geometry but the aerodynamic chord would be extended in the elevon area. The exact amount of increase would be determined by the airfoil selected and necessary spar depth for elevon structure. The increase of chord in the elevon area will produce better lift distribution at higher Cl in addition to reducing the required deflection angles for a given Cl. These two factors will tend to provide improved lift distribution at higher Cls and lower minimum sink values than that of the pure constant chord wing.

Instead of a pure cantilever wing, I would use a "semi-strut braced" arrangement. This would consist of a short strut that has about a 45-degree angle with the horizontal. A small drag penalty would result but this provides a very sig-nificant reduction in the maximum wing bending moments and allows a light simple carry-through structure in the pod.

Yaw control would be via drag rudders at the wing tip. These would be plate sections on the upper surface only. Deflection of a rudder would produce a yaw force and in addition a small proverse rolling moment and some nose up pitching moment. The rolling and nose up moments are both in the direction desired to coordinate the turn. Both rudders could be deflected together to assist approach control but would need a spring interconnect system so they would not both be deployed inadvertently as noted on the original machines.

To obtain directional stabiliry, a central fin would he used. The upper section of the fin can be designed to split to and provide drag for approach control. This drag force will produce a nose moment that can serve to reduce minimum flying speed but will require pilot awareness to prevent unwanted arrivals. A lower mounted drag flap arrangement would provide approach control and probably not reduce the minimum airspeed.

Selection of the vertical wing locations is a problem particularly if the lightest weight is desired. If you try to get the pilot's head above the wing for visibility, the spar always seems to be in just the wrong position. The best solution for a minimum weight design would be to place the pilot below the wing even though it restricts visibility. A design for higher empty weights would allow a pilot position in front of the main spar but will result in larger movement of CG due to different pilot weights.

Figure 2 illustrates most of the generalities discussed above. I have nor included any discussion of detail structure as this would require starting on an actual design project. There are many structural options today and persons skilled in specific materials and processes would want to work with their own favorites

Page 12
Sailplanehomebuilders.com
Sailplane Builder September - October 2000

#### nickec

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

Post 55 and 56 above will hopefully serve to make search engines aware of Al Backstrom's article.

I also hope the posts will make the article more readable than the poor quality scans of post 54.

You still need to refer to post 54 for the two pages of illustrations - particularly the top of the last page which covers the elevons and drag rudders.

Note the split vertical fin rudder for approach control and not for yaw control.

#### cluttonfred

##### Well-Known Member
Supporting Member
Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Here is the best I could in a cleaned up version of this article. Maybe someone can provide a better scan?

On the applications for this thread, yes, you can definitely see the idea behind the extended-chord, relatively short span elevons. I really like the "high wing" version and Al's point on the advantages of a strut-braced wing despite the small drag penalty, even for a sailplane. I'll also underline that Al was talking about a sailplane here, not a powered aircraft. Drag rudders are very effective with a long span and high aspect ratio, less so with spans and aspect ratios more typical of a powered plane. Ditto on the need for a the air brake built into the rudder. In a powered light plane a larger, moving rudder could handle yaw control and there would be no need to for the air brake.

I'll need to play around with some sketches and numbers to figure out what kind of wing area could work and still fit within a 20' shipping container (door dimensions 7′ 8 ⅛″ or 2.343 m wide x 7′ 5 ¾″ or 2.280 m high and interior length 18′ 8 13⁄16″ or 5.710 m). So basically it would need to fit inside a 7' x 7' x 18' envelope, say, with quick-removable wingtips and rudder. A 4' x 18' main wing with 2' wingtips tapering to 2' chord would give a total of 84 sq ft or 7.8 m2, which seems quite workable for a VW powered sportplane. Even without removing anything the whole plane would take up just 22' x 10' or so, easy to tuck in with another plane in a T-hangar, and 22' x 7.5' with the rudder folded, which would make for easy trailering and storage in single car garage.

Hmmm, you've got my creative juices flowing!

#### Attachments

• Backstrom article - SHA Newsletter - Sep-Oct 2000.pdf
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#### nickec

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

... In a powered light plane a larger, moving rudder could handle yaw control and there would be no need to for the air brake. ...

Backstrom suggests that the split fin brake aids landing. It would aid landing in a powered plane in the same way it aids a glider.

#### cluttonfred

##### Well-Known Member
Supporting Member
Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

It might, but without the floating on landing from a long, glider wing, you could probably do without it to simplify the design.