Why are modern regional jet designs shifting towards under wing engines?

Question

Historically regional jets (eg, MD-95/B717, CRJ-700/900, ERJ-145, etc) have had their engines mounted near the rear of the fuselage. But, recently, regional jets are increasingly using an under wing configuration (E-170/175, Sukhoi SJ100, Mitsu MRJ700 to name a few.)

A couple related questions:

• Why might this configuration be advantageous for small jets as opposed to the rear mounting configuration commonly used before?
• If it's advantageous, I assume technical problems are what stopped it from being implemented before. So what has changed that allows manufacturers to use this configuration now?

Answer 1

The underwing engine location allows for a lighter airframe.

Placing the engines under/ahead of the wings brings these benefits:

• Bending relief: Engine mass is closer to where lift is created, so the structure has less stress to carry around.
• Flutter suppression: Placing the engines ahead of the elastic axis of the wing shifts the flutter speed up, so the wing can be built less stiff.
• Better evacuation: With wing-mounted engines a rear door is possible to create a second point for emergency egress.
• Lower noise level in the rear cabin.
• More useable cabin space at the rear.
• Easier access to the engines for maintenance.
• Shorter piping for fuel and bleed air. Especially in the narrow fuselages of regional jets the air ducts for ventilation take a lot of valuable space.
• And don't discount psychology: The underwing engine design looks more like the bigger jets, which can be a big factor for some passengers. The airplane looks safer and more mature.

With modern CFD it is much easier to reduce the interference between the engine nacelles and the wing, so the biggest disadvantage of an underwing engine location can be reduced.

Note that early designs for the Boeing 737 used rear-mounted engines. By relocating them under the wings, the structure could be made 700 kg lighter. Quote from www.b737.co.uk:

Overall, the wing-mounted layout had a weight saving of 700Kgs over the equivalent “T-tail” design and had performance advantages.

The rear-mounted engines were a fad from the late Fifties (started by the Sud Aviation Caravelle) to the mid-sixties and driven by the concern about the higher risk of low-mounted engines ingesting debris and the higher asymmetry in engine-out conditions. With more operational experience and more reliable engines, these concerns proved unfounded. Citing www.b737.co.uk again:

Initial worries about the low mounted engines ingesting debris proved unfounded, this was demonstrated by the Boeing 720B whose inboard engines are lower than the 737's and had been in service for four years without significant problems.

Small business jets still prefer rear-mounted engines so they can operate from more airfields (on which the runway might not be as clean as those of big airports) and because the weight penalty of rear-mounted engines is less pronounced. Instead of bending and thrust loads, handling loads and manufacturability drive the minimum skin gauges on smaller jets.

Answer 2

There are a few reasons for the placement of the engines under wings. As most of the pros and cons have been already discussed elsewhere, I don't want to go into them, but I'll just point out some reasons for this trend.

• One important shift in the last two decades in airline industry is the use of high bypass engines, which have higher diameters and are quite difficult to place in the rear. As the trend appears to be towards higher bypass ratios, it makes more sense to put the engines under wings where engine size increase could be better accomodated.

• Another point is that the aircraft today are not developed seperately for regional and national use, but developed as part of a family, which can be used in a variety of roles. Flighglobal points this out on its article on the Embraer E170:

The response reflects the shift in emphasis since 1998 away from a product geared solely to the regional airline market to a more robust family design with equal appeal to traditional mainline carriers.

Note that the 170 is not called ERJ 170, just the E-170; the dropping of the regional moniker is significant. Embraer considerd using a tail mounted engine for 170, but went with a wing mounted one as this design proved better.

• The mounting of engines on the wing helps in reducing weight. Mounting engines on the tail needs beefier fuselage for thrust transfer and usually necessiates a T-tail due to engine location. This is somewhat cancelled by a shorter landing gear, but as the aircraft size increases, the weight savings become significant.

• From the airline/customer point of view, the mounting of engine on the wing is better in two respects- reduced maintenance due to engine accesibility and reduced noise in cabin.

Answer 3

To answer your WHY, these are larger regional jets. Most regional jets are too small for engines under the wing.

As theses jets get larger (mini-737) there is such room. Thus these designs can take advantage of the benefits of wing mounted engines.

It's unlikely you will see many jet aircraft of the size of the Embraer ERJ series (30-40 passengers) with under wing engines.