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Professor Fog
26th Jun 2002, 23:14
Because if you do you maybe able to explain the following -


What are the advantages/disadvantages of rear mounted engines ?

Why do aircraft have winglets and how do they actually work ?


any answers would be most appreciated.....

MikeSamuel
26th Jun 2002, 23:26
One for the Tech Log methinks...:rolleyes:

Hand Solo
27th Jun 2002, 02:13
....but I'd offer:

1) Advantages:
Thrust acts closer to the aircraft centre line, thus reduced yaw in the case of assymetric thrust/engine failure, therefore smaller rudder/fin assembly required, therefore less weight.

Structurally simpler as the engines are effectively bolted on to the relatively solid fuselage instead of dangling from a flexible wing.

Shorter landing gear as no engine ground clearance required, saves space in the wing box and overall weight.

Disadvantages:

Aircraft is effectively restricted to a T-tail design which can be subject to deep stalls.

Noisy in the cabin between the engines (although for a given engine it'll generally be quieter in the cabin than with wing mounted engines)

Limited service access to rear of the aircraft - fiddly to get catering trucks to a reasonble size door.

2) Air under the wing is at higher than ambient pressure, air above the wing is at lower than ambient pressure. HP air escapes round the tip of the wing towards the upper LP area, reducing lift towards the wing tip and creating vortices (=drag). Winglets smooth the interaction of the airflows around the tip of the wing, reducing vortices (less drag) and eliminating disruption of the lift towards the end of the wing (effectively creates a longer, more efficient wing without increasing the actual wing span much). They also look quite futuristic and can be used to tart up an old aircraft design (MD11) even if their actual aerodynamic benefit is marginal in that particular instance.

Chuffer
27th Jun 2002, 08:43
Also useful to stop engineers walking of the end of the wing.

Professor Fog
27th Jun 2002, 10:58
Thanks for that Captain Solo



Any more offers ??

Gin Slinger
27th Jun 2002, 12:00
Having the engines at the back pushes the CG rearwards, therefore the wings are further back on the fuselage to compensate compared to for example a B737

The greater length of fuselage in front of the wing increases nose up moment in the stall due to amongst other things the lift provided by that long fuselage.

MorningGlory
27th Jun 2002, 13:16
Also.. With rear fuselage mounted engines, the wings need to be stronger due to further bending of the wings in flight (esp at MTOW on the climb out).

The wing mounted engines would help reduce the bending moment of the wing, due to their weight.

Field In Sight
27th Jun 2002, 14:35
Pod mounted engines on the wings also act as mass balances to counteract flutter.

This reduces even further the required strength of the wing.

Also rear mounted engines are a pain in the arse when you have to sit in Economy at the back. Like I used to have to do everyweek on MD80's when I worked in Sweden a while back.:mad:

Although they are excellent when you get to sit up at the front.
It is the quietest jet I've ever been on. :) or maybe that was the champagne I was drinking.

lomapaseo
27th Jun 2002, 17:57
Uncontained engine fragment hazard is much higher for the passengers in the back (lower for the flight crew of course).

Bird ingestion hazard decrease by a factor of 10. Tyreingestion hazard increases by a factor of 2. Of course if you dispatch with ice on the wings we all know what that means.

Nimbone
27th Jun 2002, 18:21
Winglets?

Everything you ever wanted know about winglets?


http://beadec1.ea.bs.dlr.de/Airfoils/winglt1.htm

...and beside, they look cool.

:cool:

Keith.Williams.
27th Jun 2002, 19:14
PF,

If you are preparing for your JAR POF exams these are the answers you need to know.

Rear mounted engines reduce the ptitching and yawing moments caused by power changes and the handling problems following engine failure. The down side is that they also require stronger wings, because wing mounted engines provide bending relief and flutter damping. Rear mounted engines are also more likely to surge when the aircraft goes into deep stall.

Winglets react with the wing tip vortices to produce a total reaction that is angled inwards and forwards. The inward component is unimportant, but the forward component adds a little thrust to reduce the effects of induced drag. Because the winglets extract energy from the tip vortices they also reduce the vortex intensity. But (existing ones) cannot be moved in flight, so they are most efficient at only one speed. They are therefore tuned to provided greatest effect in the cruise, where the aircraft spends most of its time.

Checkboard
28th Jun 2002, 06:28
Aircraft is effectively restricted to a T-tail design which can be subject to deep stalls.
Not necessarily - look at a lot of small business jets.

lunkenheimer
28th Jun 2002, 15:31
Pod-mounted engines can interfere with the aerodynamic design of the wing and thereby add some complexity to movable surfaces such as flaps, spoilers, and ailerons. The result can be a less clean wing, and less effective flaps, though not always.

GearUp CheerUp
1st Jul 2002, 16:01
There is also the problem of routing the fuel pipes from the wing tanks to the engines if there is a rear engine layout.

HOVIS
2nd Jul 2002, 22:36
The deep stall is a combination of T-tail and rear mounted engines. The wing effectively blankets the engines and tailplane/elevator when high AOA are achieved as in a stall. Thus the engines also stall/surge and therefore the a/c cannot power out of the stall plus the elevator/tailplane become ineffective due to the disturbed airflow created by the wing.

Having said that, the combination of a very clean wing and all the noise at the back gives a very comfortable ride for the First class punters up front. The VC10 is a good example.

Eff Oh
5th Jul 2002, 12:15
I remember seeing this on the flying club wall years ago.........

At 50 hrs a pilot WISHES he knew it all. At 500 hrs a pilot THINKS he knows it all. At 5000 hrs a pilot realises he will NEVER know it all!!!!

Eff Oh :D :D

Alpha Leader
5th Jul 2002, 14:36
One more:

Reduced escape chute ingestion hazard.

(On an LH A-320 I experienced the front right escape chute deploy at around 7000 ft just after take off and subsequently being chewed up by the starboard engine).:(

rottenlungs
5th Jul 2002, 14:44
Another drawback with rear mounted engines in the context of an uncontained failure is that there is a confluence of hydraulic/control systems in the tail area. Consequently, several redundant systems can be compromised by one initial failure, for example the Sioux City UAL Dc-10 accident.

lomapaseo
5th Jul 2002, 20:14
>Rear mounted engines better for the flight crew ? - there is at least on instance where an uncontained failure seriously injured a cabin crew member on a DC9, I believe. She was sitting on the 'jump seat' at the rear galley.
<

yes, Valujet @ ATL, but she was cabin crew not fight crew . The regulation reflects a difference in protection (25.903d1) The flight crew is typically outside the presumed 15 degree uncontained hazard zone.

moon11
5th Jul 2002, 22:42
Keith,
Greatest effect winglets provide in t/o and landing,not in cruise,because induced drag is the greates iat law speeds.