Flying Wing Washout vs Stability

[Slow Riser]

I have read a lot of posts of flying wings without a rudder; just elevons.

Is the stability of these wings, without the rudder or elevons, a result of the amount of washout in the wings?

If washout contributes to stability, what do you sacrifice in return? I would expect there is a loss of lift but does this adversely affect the overall performance of the wing?

Henry
--
New Canadian E-Tailer
www.zebrahobby.ca

[Terry S]

Are you sure you got that right ?
Do you mean without fin/rudder ?

Terry
UK

[Slow Riser]

Meant to say, "without a rudder, just elevons."

Henry
--
New Canadian E-Tailer
www.zebrahobby.ca

[Ollie]

Sweptback wings (with enough sweep) have some yaw stability and with enough sweepback can get by with out a rudder. However, they are stealthy and hard to see when edge on without fuselage and rudder. They are usually equipped with tip fins to enhance visibility for the R/C pilot even when the fins are not required for yaw stability.

Pitch stability requires that the CG be ahead of the neutral point. The neutral point for a swept back wing without tail is at the aerodynamic center of the wing is, depending on the airfoil(s), near 25% of the mean aerodynamic chord of the wing. The distance that the CG is ahead of the neutral point is called the static margin and is a measure of the static pitch stability. BTW, stability is defined as the tendency of the model to return to the trimmed flight condition when a disturbance (control input or gust) is over.

Washout in a tailless wing is functionally the same as decalage in a tailed configuration. The larger the static margin, the more washout is required to trim a tailless model. In other words, washout adjusts pitch trim.

Swept back and tapered wings have a tendency to tip stall. So, washout, sweep back, taper, airfoil selection, aspect ratio and CG location all have to be harmonized in a tailless model to achieve stable, trimmed flight. Harmonizing all six aspects of the design which are built into the model makes the successful design of a flying wing configuration more difficult than for a conventional, tailed configuration.

Jack Northrop had to add vertical fins to the jet version of his flying wing bomber to compensate for the loss of stability associated with removing the pusher props and their shaft housings. If the sweep angle had been greater, that might not have been required.

See:
http://aero.stanford.edu/WingCalc.html
http://www.b2streamlines.com/Culver.html
http://www.aerodesign.de/english/profile/profile_s.htm
http://www.b2streamlines.com/Panknin.html
http://www.b2streamlines.com/
http://home.planet.nl/~otten100/Whatsnew.html
http://perso.wanadoo.fr/scherrer/matthieu/english/mce.html
http://www.flying-wing.de/index2.htm

[DaveGherardini]

I have found from experiance that dihedral helps yaw tremedous in a delta wing config. I have built this plane without washout and without fins, Didnt need them. Very stable platform but hard to see in a turn. The sweep is also a big help as has been said. This is totaly different than a regular wing{non tapered}. Im sure washout would increase stability even more but i found it was not neccsary. I assume the dihedral is the reason.
I hope this is good input also


Dave G

http://www.geocities.com/xstratus/pyro.htm

[Slow Riser]

that washout creates sufficient drag to overcome yaw instability? Again this is with a swept wing.

As Ollie mentioned, having a motor/prop also adds to yaw stability.

I guess the easiest thing for me to try adding a lot of washout to see what the results are.

Thanks
--
New Canadian E-Tailer
www.zebrahobby.ca

[Ollie]

If the prop and motor are behind the aerodynamic center of the plane, they add yaw stability and if they are in front of the aerodynamic center, they add instability.

Dihedral results in yaw to roll coupling. So does sweepback but the amount of yaw to roll coupling varies with angle of attack or coefficient of lift (unlike dihedral).

It is not fruitful to consider only single items in flying wing design because everything affects everything else. All six features of a flying wing must be considered simultaneously or unintended effects will result.

In the case of a low aspect ratio delta, the entire wing is operating in the tip vortex. As a result, the angle of attack of any spanwise segment of the wing is operating at a different angle of attack than its neighbors. There are three things that affect angle of attack: the geometric angle, the zero lift angle of the airfoil and the induced angle of attack due to the up wash in front and the down wash behind. In low aspect ratio wings, instead of the axis of circulation being mostly spanwise it is mostly fore and aft. Furthermore, the strength of the circulation varies as the square of the coefficient of lift. In low aspect ratio wings the induced angle of attack effect takes over and the wing can achieve much higher geometric angles of attack without stalling. This is achieved at the expense of much greater induced drag.

The forces on a wing are focused on the 25% chord line of each wing panel. In the case of a swept back flying wing, where the 25% chord lines meet at an angle at the center line, the flow associated with one panel interferes with the flow from the other panel producing additional drag due to this "middle effect." The Horton brothers mitigated the middle effect by giving the planform a bat tail which caused the 25% chord line to bend around gracefully from one panel to the other rather than meeting at a sharp angle. You can also see this in commercial jets which have a sharp increase in chord near the wing center by increasing the chord at the trailing edge rather than interrupting the sweep angle of the leading edge.

I hope you are getting the message that tailless wing design is more complicated than meets the eye.

[Terry S]

Big subject with many ways to do it.
You keep talking about washout with no fin. A high aspect ratio wing with no fin benifits from lots of washout, maybe 7 or 8 deg as it counteracts adverse yaw. Be carefull not to add too much dihedral if you have no fin as it will probably dutch roll, you may see wings with the tip turned down to cure this. Low aspect ratio wings are much easier to make fly well but not as much fun !

Terry
UK

[daveswall]

Ollie does that same effect go for what I have designed. I have a 50" wing span. I found the mac and took 25% of that line. My root cord is 16in and tip is 71/2in. I have a large verticle fin. My mac line is 10 5/8in from the root cord and the cg is 3 1/8 from the leading edge. You seem to know your stuff and tell me what you think. Thank You

dave