Aeronautical question
 

Being a bit of an airplane buff (although mildly), I sometimes wonder how airplane manufacturers decide which basic design to use, the tail-mounted T-elevator (is that term accurate?) a la DC-9 vs the under wing mounted 2 or 4 engine a la DC-8, size of plane notwithstanding.

Anyone with aerodynamic or aeronautical expertise know what drives these choices?

Ford

I think the most important difference is the ground clearance.
Smaller planes tend to use tail mounted engines to keep the landing gear short and small. I'm sure there is many other factors.

Could be. What got me thinking about this is that Bombardier and Embraer are the two biggest players on the regional jet scene. Both started out producing smaller jets with the basic DC-9 design (CRJ100 and 200s vs ERJ135, 145 etc.) As they designed bigger planes, Embraer went to the underwing model (E75s and up) while Bombardier opted to produce the CR700 and 900 series. I would think that up to a certain passenger size, it wouldn't matter as MD90s competed with Boeing and Airbus for a while.

Eventually if you want a lot of seats, you end up with the DC-8 structure. I was wondering if there are performance advantages of one vs the other.

Ford

There is a stall mode, which I believe affected in particular the BAC111 early on. At a certain attitude, flow from the wings (possibly tip vortices?) could go directly to the horizintal stabilizer, leading to it to stall, leaving the plane in an unstable configuration.

http://www.boeing.com/commercial/dc-9/index.html

This link doesn't directly answer your question, but it eludes to STOL and close to the ground being important.

I'm no aeronautical engineer, but I'd guess there aren't powerful enough engines that can be fuselage mounted to propel a larger aircraft, thus the need to be wing mounted. For smaller aircraft, I can't say why one manufacturer chooses wing mounted, while another chooses fuselage mounted. In Bombardier's case, I'm sure part of it goes from that's the way they've always done it with business jets, and there was probably some cost savings for them in extending this for the regional jets.

Embraer could have decided they needed to be different. These are new jets and they probably wanted them to look different from the ERJ's.

Actually, engines are engines, regardless of where they are mounted. No matter what, it's going to be more expensive to have fuselage-mounted engines, because their weight has to be transferred to the wing. While if hung on the wing, they can be placed at the best location from the stndpoint of the wing design as a beam.

One advantage (of very few ones) of four-engined planes is that the wing they require is lighter than for two engines. As the 340 shares the 330 wing, it ends up overdesigned. That is, not as cheap and light (hence efficient) as it could have been.

There is no perfect design. Its all about compromises. A T-tail, for example, has surfaces that are kept well out of the airflow behind the wing, giving smoother flow, more predictable design characteristics, and better pitch control. However, you virtually cannot recover from a deep stall or a fully-developped spin.

What I was referring to was, say, hanging a couple GE90's off the side of a 777 is not likely going to work.

Related reading material
 

http://en.wikipedia.org/wiki/T-tail

http://en.wikipedia.org/wiki/Ilyushin_Il-62

:D :D :D :D :D This would be a very interesting sight!

Why not?

(It's just that it would be expensive.)

I love that, must use it in a sentence sometime soon.

"Yes dear, apparently uncle Steve got drunk at the legion and flipped his car, leaving it and him in an unstable configuration."

hehe

Usually, but not always. Before the DC-9 was even conceived, came both the VC-10 and the (likely copycat aka: industrial espoinage), Ilyushin-62, with T empennage & 4 engines attached to the rear fuselage. They both carry (ied) 150-200+ passengers, and interestingly, for both aircraft their first overseas flights were into YUL (from LHR & SVO, respectively)

Even before these were built, in the early 1950's, there was also the "Sud Aviation Caravelle" from France, with a similar T tail & engines. It was a bit of an odd aircraft and made funny noises! I remember flying on an Air Inter one years ago, and it had triangular windows.

We flew on BA's (then BOAC) VC-10 from either PIK or LHR to JFK in the late 1960's and early 70's. Very quiet, comfortable aircraft. Similar to the B707 except for the four fans mounted at the back.

IB had a large fleet of Caravelles around that time, and we also had a couple of flights on those birds. Flying from MAD to Palma, I remember that we'd fly almost directly over the island, then make a very steep approach to the airport.

Because of the tail-mounted engines, doesn't the center of gravity change dramatically, which explains the wings being mounted further back on those aircraft to compensate? Particularly noticeable on the EMB's and CRJ's.

Also, the B727's and BA Tridents (three in the rear) were found to have a very high "sink rate", which was given as the probable cause of an accident in Queens, N.Y. in the mid 60's. Approaches and landings often had to have increased power settings to make up for this.

bj-21.