What is the advantage of swept wings on a sub-sonic aircraft?

reduced lift slope = higher angle of attack capability
directional stability at high angles of attack ( swept back )

Depending on the taper ratio, possibly more optimized spanwise lift distribution

If too much sweep, loss of aileron control as outer wing stalls

Forward sweep has much more favorable stall characteristics and can therefore be combined with lots of taper

Otherwise, nothing

Most airliners are sub-sonic aircraft. The main purpose of their wing sweep is to reduce drag at high sub-sonic Mach numbers. Other than that there are very few benefits to wing sweep. You rarely see significant sweep on an airplane that doesn't fly above about 0.8 times the speed of sound.

For planes that are genuinly subsonic (i.e. as Shoe say's below about 0.8 mach) then there really arent anyreal advantages. Yes, sweep adds lateral and directional stability but not as efficiently as other methods (like dihedral for lateral stability for instance). And often too much lateral stability is a problem not an advantage. Yes it alters the spanwise lift distribution but it moves lift to the tips which is generally the last thing you want for safe handling or efficiency. It flattens the lift slope due to lower aspect ratio, but that's a bad thing because it increases induced drag.

In the model world the biggest reason for sweep is for scale reasons (on scale models) or in non-scale models because it looks 'cool' or the designer mistakenly thinks it will make his model fly faster.

There is another advantage. Sweep the wings back and suddenly the plane just got a lot narrower without losing any wing area. Makes it easier to transport.


Plus it looks cool

Really for the most part in a sub sonic airplane there is very little advantage gained with swept wings. Along with all of the other problems notes above it gives you an airplane that has to land hot (fast) to avoid tip stalls (look up "Saber Dance").
Yes it looks cool, but it has a lot of bad attributes that makes a plane harder to fly and land.
A highly swept wing should only be something you use when building a high speed design, for the most part for model aircraft swept wings are something you may want to avoid.

Gee whiz -a lot of 1930's bipes used swept panels and were stable yet very aerobatic and extremely easy to fly .
I am not any good on textbook theory (cain't reed) but some sweep INCREASES stability.

Your right!

I should have qualified what I said a bit better...
When I think of sweep (for speed type of planes) I thinking in the 30-60 degree area, a bit more than you typical design (when you have that type of sweep to the wings it can get a bit dicey).

If a wing is high enough AR and somewhat flexible, moving lift to towards the tips should induce a twisted in washout effect as loads increase because the tip CP is rearward of the root's CP, right?

RMH, wing sweep was used in some of the older airplanes to move the c.g. aft. This happened when power plant availability was changed during the mid construction phase.

Sweep was used on biplanes, usually on the top wing, to allow access for the pilot to get into the cockpit and to have good visibility while still having the CG in the right place. Similar in concept to why some two seat sailplanes use forward sweep.
Steve

Yep - very common fix on biplanes
Just like the British bomber which ended up with swept wings - to get CG correct- back in 1940
Much of the carefully engineered stuff got swept (pun) under the rug when necessity took precedence
for example the perfect ellipse Spit wings - got the tips axed on later Marks
The vaunted P51 airfoil?
again, swapped over to a rather humdrum profile after W11. just as good -
About the only perfect design concept has proven to be "make it as light as possible"

I'm sure i read somewhere that the first of the swept wing jets, the Me262, didnt actually have swept wings for aerodynamic reasons as such. It was apparently (according to the story i read) a fix to get the CG in the right placewithout having to alter the wing position of the fuselage.

I fly both swept and un-swept winged speed planes with .049 on up to .50 sized engines and there is no appreciable difference in speed between the 2 wing designs. I use Doppler, stop watch and onboard telemetry to compare speeds.
So, if there is a difference in speed potential between the 2 designs in the 100-200 mph range...I can't see what it is.

The 1/2A pylon racing course record at a local club field is to this day held by an old fashioned swept wing [ACE RC Company] GLH.
If you're a "fan" of this plane and of ACE RC in general, it's fun to see an ancient design still kicking butt out there.

As I understand it, sweep work as a Cl dependant dihedral.
This can be of use when designing a modell, at high angles of attack, i.e. high Cl the wing have dihedral.
At low AoA there is less dihedral. This way dihedral have become speed dependant too.
Another feature is that the dihedral effect always acording to the Cl, fly inverted and the dihedral is inverted, stabilizing the plane up side down.

wing sweep is like the "perfect airfoil"
It is simply a compromise in many cases
as one reduces weight all of these critical factors simply melt away.

But to the OP's question, for models, there is no advantage, and may be some small disadvantages. The fastest model airplane uses a high AR, straight wing. No engine, but even the FAI pylon models seem to be going that way.
Evan, WB #12.

Guys who race can see subtle differences based on results spread out over time. As the years go by they keep seeing little advantages that add up to pretty big gains when you look back at a decade or two's worth of progress.

To sweep or not to sweep - is that a question?
much of what makes up a wing shape is based on a far more simple need: strength and rigidity.
The classic double taper is very good at providing a strong wing.
The Delta is a a rigid layout but changes drag drag wildly as angles of attack change
The simple rectange is easy to build and actually very good except making certain it is strong and does not twist, is often an issue.
and so it goes

The purpose of wing sweep is to delay the speed at which sonic shock waves start to form. Shock waves are very high drag. Shock waves will form when the speed of sound is reached, which on many parts of a plane will be quite a lot sooner than the plane as a whole. Consider that the air flow over the top of a wing is speeded up, so it will reach the speed of sound and form shock waves while the rest of the plane is well below Mach 1. This region of speed in which parts of the flow are supersonic while the overall speed is subsonic, is the transonic region. What the design aims to do is to make the transonic region start at as high a speed as possible, so that all speeds below that transonic start will be subsonic over the entire aircraft. That means no sonic shocks forming and no shock wave drag. Airliners and subsonic fighters cruise at a speed that could potentially be transonic and suffer shock drag, so they use designs to push the transonic region just above their cruise speed and thus avoid shock wave drag.
One way to push the transonic region to a higher speed is to use a swept wing. There are two ways of explaining how it does it, both equally valid and both give the same results if you do the arithmentic.
1. The thickness ratio of the wing affects the speed at which a shock wave forms. The thickness ratio is the max thickness divided by the chord. A high ratio section will develop shock waves at a lower overall speed than a low ratio section. So to move the transonic region to a higher overall speed, use a low thickness ratio. But a wing has to be thick enough to be strong, carry fuel etc, so you can’t make it any thinner. You don’t want to increase the chord to lower the ratio as you don’t want more wing area. There is a geometric trick. Sweep the wing back and the chord that is parallel to the fuselage will increase as you are measuring at an angle across the wing instead of the shortest distance from leading to trailing edge. Thus without thinning the wing and without actually increasing the amount of wing, you have lowered the thickness ratio. The overall speed at which the flow over the top reaches Mach1 will be higher, so you can fly faster before you hit shock wave drag.
2. A wing section is the shortest line from leading to trailing edge, so the section sweeps as you sweep the wing. But ignoring the normal spanwise flows that a wing creates, the flow that goes over a swept wing, parallel to the fuselage, is partly moving from root to tip, and partly moving chordwise at 90 degrees from leading to trailing edge. Imagine a wing swept at 90 degrees, the entire flow would be from root to tip, not leading to trailing edge, so there would be no useful lift other than flat plate effect. So when air flows over a swept wing, the part that is spanwise gives no lift and the part that is chordwise gives lift. A quick diagram shows that the chordwise flow is cosine angle of sweep, so the effective speed over the chord is cosine angle of sweep times real speed. In other words, the speed over the chord is less than the real speed and thus you can get to a higher overall real speed before the speed over the chord reaches Mach 1. This also explains why swept wings have very high stall speeds, since lift is related to the square of the speed over the section, and the effective speed over a swept wing is reduced and the reduction is squared so a lot of lift is lost as a result.

Below transonic speed, swept wing has no drag advantage.

Going back to the advent of RC pylon in the mid '60's, most of the aerodynamic improvements were inside the engines. Airframes, maybe 10-15%.