Flying Wing Pros & Cons

[Ollie]

One of the dilemmas with flying wing configuration is that the more tail surface area and fuselage area that is eliminated, in order to reduce parasitic drag, the more difficult it is to see edge on. Another difficulty is that there are not a wide variety of suitable, low or positive pitching moment airfoils available that cover the necessary range of coefficients of lift while maintaining very low drag. When reflexed airfoils are operated at low angles of attack, the flow over the bottom of the reflexed portion of the airfoil tends to be very turbulent and even separated. Finding a suitable airfoil is an elusive goal.

Unswept and swept forward wings require vertical tails for yaw stability. They also require enough fuselage to put balancing weight ahead of the aerodynamic center and vertical tail area behind the aerodynamic center. The longer the fuselage the smaller the tail area can be and the smaller the balancing weight can be. A long fuselage increases moment of inertia in pitch.

The swept back, tailless configuration does not require any vertical surfaces for stability but is the stelthiest. It also is subject to very large torsional stresses.

Tailless aircraft tend to have low moment of inertia in pitch and are more easily upset in pitch.

It's not that flying wings are inherently unsuitable, it's that finding a balance between conflicting objectives that matches the DS mission is, perhaps, more difficult than for conventional configurations.

[h@bib]

hallo ollie

here is an example of a DS-swept flying-wing.

Dizzy (http://www.zanonia.de/dizzy.php)

the homepage is in german.

translate it with the http://babelfish.altavista.com/

i'm about to build an similar flying-Wing with a TL54-Airfoil. (TL54 coordinates) (http://www.aerodesign.de/profile/profile_s.htm#tl54)

regards from austria

h@bib

[surfimp]

Thanks Ollie! As always I really appreciate your responses!

Having hung out with a flying wing designer quite a bit, and being in the process of scratchbuidling a non-DS, forward swept tailless plane myself, I have really begun to appreciate the challenges inherent to the design. It will be interesting to see what flying wing proponents are able to come up with! :)

[Ollie]

In an effort to design a tailless DS machine that would break the current unofficial speed record of more than 200 MPH, I would accept the parasitic drag of a vertical tail for visibility and pilot orientation. I would use an unswept wing in order to avoid the torsional stresses associated with a sweptback wing. I have yet to find a laminar flow airfoil to operate in the reynolds number range of 1.5 to 3 million, with a positive pitching moment coefficient and low drag at lift coefficients between 0.1 and 0.3. The closest I have been able to come is the NACA 8-H-12. The 8-H-12 has an abrupt drag increase below a lift coefficient of 0.22. This limits how large a circle and lap time a plane with this airfoil can use while still operating at a good lift to drag ratio.

[osborn]

Ollie, have you looked at the EH series of airfoils? For example the EH 1,5-9,0 with 1.5 degrees of camber and 9 percent thickness. It has a bit less Clmax and higher drag at higher Cls than the NACA 8-8-12, but it is very well behaved in the 0 <= Cl <=.5 range, and near zero moment coefficent for +/- 5 degrees AOL, modestly positive in the cruising .5 <= AOL <= 2.5 degree range. In practice, with a degree or less of reflex trim, the moment coefficient would stay positive to provide the required stability. All this in the RE range of 1-3x10^6. :)

-- James

[Ollie]

James,

I'm hampered by the fact that I can't run X-Foil on my Mac.

[Craig Toutolmin]

I have a Mac and run Xfoil through Virtual PC on Profili.

[osborn]

Hi Craig,

So what do YOU think of the EH airfoils? I haven't compared them to the JW airfoils yet. But I would love to hear any other "pros & cons" commentary you might have on high speed plank design. Tim is going to show me the wreckage of your 100" plank so I can get some ideas!:D

I've heard the JW airfoils were designed with a very thick TE, supposedly because Pat Bowman did not want to have customers bother with thinning the elevon stock. So, did you make thick TE's on your bagged plank, or did you modify the airfoil to use a thin TE and confirm the performance spec?

-- James

[Craig Toutolmin]

Scott Hewett used many varieties of the EH foil on plank designs. He used the JW foil on at least one plane. He would be the best one to ask if you are comparing JW foils to many EH versions.

Here are some basic observations. Ity is tough to compare different foils and planforms.

I built two composite planks with the JW foils. They have outstanding inverted performance. I'd guestimate 80-90% of upright. This makes for a very fun plane. For ramp turn speed I'd go with the EH 2/10. A Zipper with an AUW of 40-50 oz is very fast. For DS, I'd build a traditional plane. I thought the 100" (JW foiled) plank that Tim Killed was the best performing DS wing to date. You had to fly it very smoothly to get the speed out of the plane.