# How does washout increase static pitch stability in subsonic swept wings?

How does washout increase static pitch stability in subsonic swept wings, more specifically in slow moving aircraft (eg. ultralights), 15mph to 60mph?

I understand washout is used to delay full stalls ( at least in unswept wings), but isn't pitch stability related to cg and centre of pressure?

See picture below of a european designed Archon SF1 rg ultralight with swept wings and retractable landing gear. Mid engine single prop is shown forward of the twin vertical tails.

• Does this answer your question? How does the location of a propeller affect an aircraft's stability? May 8 '20 at 13:45
• Here is one who is not interested in motor glider-like fuel efficiency! Re Zeus comment: if you have retractable slats on a swept wing you move your center of lift back, allowing CG to be further back. When slats are retracted, you can cruise without tail downforce saving fuel. The washout is opposite, so designing washout in a swept wing will move the center of lift forward. The secret is that center of lift on the WING generally moves forward with increasing AOA so a "horizontal stabilizer", something to counteract this, must be added. May 8 '20 at 14:24
• @Manu H: I'm not asking about the affect of prop location on stability. The pic just shows the only ultralight that I know that has swept wings. I'm interested in finding out how to make swept wings more stable in pitch. I was told that washout helps. Just wasn't sure how that works.
– Fred
May 8 '20 at 16:43
• @Fred-- many ultralights have swept wings-- see en.wikipedia.org/wiki/Pterodactyl_Ascender -- note that variants have been flown with or without the canard-- the original Fledge/Pledge hang glider version had no canard-- undoubtedly this wing did have some washout-- May 8 '20 at 16:51
• May 8 '20 at 16:51

## 3 Answers

Since its earliest days during the pioneer era, washout on a swept wing has been accurately described as "putting the tail at the end of the wing." It works in exactly the same way as a tail stabilizer.

As long as the change in moment of the rear surface is greater than the change in moment of the forward surface the plane will be statically stable. It matters not whether the tail is inboard, outboard, moved ahead of the wing (as a canard), tacked on the end of a swept wing, or even tacked on the back of a straight wing in the form of a reflex trailing edge.

On a straight, unswept wing washout can delay the stall a little, but stalling is a complex subject and best treated separately.

It works the same way as a tail, the wingtips are further aft, so the pitch torque around the center of gravity is greater. $$Torque$$ = force x distance.

Yes, pitch stability is related to CG and center of pressure

To create "static pitch stability", the goal is to move the center of pressure further back with increasing AOA, which helps lower the nose. The most common way do this is to move weight forward and have a down lifting tail, but this is not the only way.

One can make lift nuetral, or even down lifting wing tips on swept wings. As AOA increases, lift here becomes increasingly positive.

So you can wash out the wing tips, and they will start pushing the nose down if AOA gets too high, just like a horizontal stabilizer/elevator does.

As AOA increases, tail "downforce" goes to 0 downforce, then to "upforce" lift, moving the center of pressure further and further back. This safeguards the wing from reaching stall AOA.

One of my mentors told me "the wing is the symphony, the tail is the conductor" With out a "tail", wing center of pressure typically moves forward with increasing AOA, which is "bad".

• The 2nd paragraph is incorrect. The wing tips without washout (like the tail without decalage) would have even more nose-down moment (until they stall, but we are talking about normal flight). The point is that they actually provide (comparatively) nose-up moment, which allows to move CG forward, creating a stable configuration overall.
– Zeus
May 8 '20 at 1:40
• @Zeus ??? "They provide nose up moment, which allows to move CG forward"??? Isn't that what got Max into trouble? Moving weight to solve an aerodynamic issue is "bad". When u washout swept wings, lift is balanced with CG, lift moves back, instead of forwards because of longer lever arm at higher AOA. In this respect, a longer fuselage and smaller downlifting tail is more efficient, with lower drag. Washed out swept wings worked for Dunne, and are good for futuristic looking models, but do not allow for optimal wing AOA across the entire wing. May 8 '20 at 2:07
• More scientificly put, if you wash out a swept wing pitch down torque increases as the longer lever arm of the wing tip begins to contribute lift force at a higher AOA. In the unstalled AOA range, increase in Coefficient of Lift is linear. So, the center of lift moves back. This is exactly what a properly designed H stab does, May 8 '20 at 2:18
• @Zeus I think you mean to say the nose up moment from the washout creates a more trimmable configuration? I don't really see how washout affects stability (Cma) by a great deal.
– JZYL
May 8 '20 at 2:26
• Stability is not just an 'aerodynamic issue'; it's a balance issue as well. Pitch stability margin is the distance between CG and aerodynamic centre (or neutral point) for the whole aircraft. If you add washout or reduce tail AoA, lift moves forward, of course (and it has to be balanced with CG). But I guess you wanted to say that lift increase as AoA increases shifts back; this is equivalent to say that AC moves back. This is what I meant: now, with this arrangement, you can have CG in front of AC, which is the whole point.
– Zeus
May 8 '20 at 2:27

Washing out the outer span (i.e. decreasing the geometric incidence of the local section) of a swept wing does not directly increase the static stability of the aircraft (see addendum). The analysis mentions flying wing, but is applicable to any swept wing.

## 1. Stability

Static stability of a low-speed and structurally rigid flying wing is defined by having a negative variation of pitching moment with increasing angle of attack ($$C_{m_\alpha}<0$$), and is primarily achieved by the planform and the airfoil distribution:

• Planform: the swept outer span offsets the aerodynamic center aft, just like a traditional tail.
• Airfoil: the use of reflex airfoil allows for smaller unstable pitch variation at quarter chord, which further improves stability.

I'm going to skip why a negative pitching moment slope is necessary for static pitch stability, as this has been well discussed here (e.g.). Since the swept portion is (hopefully) aft of the CG, washout acts like a tailplane incidence and plays very little into stability.

## 2. Trimmability

However, stability is not the only concern. Trimmability is the other (some may argue even more important) issue: at the desired wing lift (or angle of attack), we must ensure that the total pitching moment is zero. This is shown graphically below, where a fully trimmed state is point A:

Image ref: Etkins, Dynamics of Flight

Traditionally this is achieved via pitch surfaces such as tailplane incidence and elevators (by shifting the above $$C_m$$ curve up and down such that point A can lie at the desired angle of attack). For a flying wing, elevons can be put to use. But it's less effective due to shorter moment arm. If the pitching moment is too negative, we may run out of available elevons to trim or left to maneuver.

Help can come from two sources:

• Airfoil: reflex airfoil generally has positive pitching moment at zero angle of attack. This decreases the overall pitching moment of the wing.
• Washout: since the swept portion is (hopefully) aft of the CG, it effectively acts like a negative incidence on the tailplane, alleviating elevons travel.

With washout, a larger range of forward CG becomes possible due to elevons alleviation. This indirectly helps with stability if the static margin of available CG range had been too small without the proper washout.

## 3. Addendum

Since washout modifies the lift distribution on the wing, it changes the downwash on the outer span ($$\frac{\partial{\epsilon}}{\partial{\alpha}}$$), which does affect stability. However, this effect is secondary to what had been mentioned.

• Washout has been used to achieve stability since 1904 and was a principal feature of the first certified stable aircraft, flown in 1910. It has been used for this purpose on many aircraft since. Your initial claim is wholly without foundation. Think of washout as adding a lateral element to the airfoil distribution - an aspect you have not addressed. May 8 '20 at 13:18
• @Guy Have you even read my whole post?
– JZYL
May 8 '20 at 13:19
• You can qualify your claim with as much stuff as you care to, it does not alter that fact that "Washing out the outer span (i.e. decreasing the geometric incidence of the local section) of a swept wing does not directly increase the static stability of the aircraft" is just plain wrong. Adding that "washout acts like a tailplane incidence and plays very little into stability" implies that tailplanes contribute little to stability, which is equally wrong. And yes, if I had not read your post I would not have known that you fail to mention lateral redistribution of the airfoil. May 8 '20 at 16:45
• @GuyInchbald Did I say tailplane does not affect stability? I'm pretty sure there's the word "incidence" somewhere in that sentence. "You can qualify your claim with as much stuff as you care to", so no matter how many equations or authoritative references I find, there's no convincing you?
– JZYL
May 8 '20 at 16:48
• Yep, you'll find the word "incidence" copied in my quote. But if you can find a reliable authority who claims that tailplane incidence does not affect pitch stability, I will find ten who claim that it does. May 8 '20 at 17:06