Wing design

[Groundbased]

Haven’t found any answers by a search on this one.

My understanding is that swept wings are more suitable for higher speeds due to airflow considerations. Therefore I assume that most turboprop airliners don’t have swept wings because they don’t go so fast. What I am curious about is why the optimum design for wings on turboprops seems to utilise a small chord and a large span. Presumably it must be less effective to have a shorter more rectangular wing, but why?

If you consider the ERJ 145 and the SAAB 2000, which have similar Max TOW the SAAB has 4.72m more span than the ERJ.

I would appreciate it if someone could explain this to me.

Thanks.

[ft]

A wing with a high aspect ratio (square of the span divided by wing area) is more efficient and will create less (induced) drag for the same amount of lift, all other things being equal. Google for the Oswald efficiency factor.

That's why gliders have long wings.

Then there's lift distribution (an ideal wing is elliptical, take a gander at the Supermarine Spitfire) and quite a bit more to it. :)

Cheers,
Fred

[noisy]

As a body approaches the local speed of sound the drag begins to increase dramatically. The use of swept wings can delay this drag rise because the airfoil presented to the airflow effectively has a reduced thickness to chord ratio. (The effective chord line is longer).

High aspect ratio wings are used in order to reduce the strength of the vortex at the wing tip that is generated by high pressure air escaping from below the wingtip to the lower pressure region on the top. This sets up a big spiral of air which takes a good deal of energy (sorry, no numbers!) to sustain. This energy manifests itself as drag which directly translates into poorer fuel consumption.

High aspect ratio wings are used where drag must be minimised.
In the case of the U2/TR-1 in order to increase the range.
In the case of gliders to enable the maximum time in the air.
In the case of a turboprop, to reduce fuel use and improve the economics of operation.

The practicality of increasing the aspect ratio has to be balanced against structural considerations such as the rigidity of the wing etc.

[Volume]

As already said, high aspect ratios (high wingspan) reduces induced drag (mostly present at high CL), high wingsweep reduces transsonic drag.
So if you want to fly fast (for jet powered aircraft) you choose some ammount of wingsweep. This reduces your CL max, because at high CL (high suction peaks on the wings upper surface) some outbound spanwise airflow is induced on the upper surface, creating serious problems.
So if you have to accept lower CL due to the wingsweep, high aspect ratio is not so important.
On the other hand the wings torsional stiffness gets more important for high wingsweep because of aileron reversal effects. And higher aspect ratio means lower torsional stiffness.

´I assume that most turboprop airliners don’t have swept wings´
right, but there are some exceptions, for example the TU-114 (http://www.aerospaceweb.org/aircraft/jetliner/tu114/)