[condor]

Originally Posted by niranjanprabhu

.. one of the last tests ...involves switching off the engines mid air and restarting them again. This test was to be done at 25000 feet but was wrongly conveyed to the pilots to be conducted at 10000 feet. This is what caused the tragedy

And for this human error, they shelved the whole project for 7 years ?? The ones who decided to scrap the project should have been called out for this.

[V.Narayan]

Originally Posted by niranjanprabhu

This test was to be done at 25000 feet but was wrongly conveyed to the pilots to be conducted at 10000 feet. This is what caused the tragedy

Originally Posted by condor

And for this human error, they shelved the whole project for 7 years ?? The ones who decided to scrap the project should have been called out for this.

This sounds like a lot more than a procedural error. Being a Govt. Dept. It is possible no one was hauled up or sacked. That unfortunately is our approach. Niranjanprabhu - thank you for sharing. We are very sorry for your loss.

Originally Posted by Sutripta

The same should hold for conventionally mounted (ie pods from main wing) engines, whether prop or jets.

Dear Sutripta, Not sure I understood the question. The airflow characteristics of a wing mounted engine are very deeply documented and known with a 85 year history to back it since wind tunnel tests started. A wing mounted prop cuts through still air (that's aerodynamically still not literally still) which is very very different from a pusher at the far rear thrashing through air disturbed by the airframe and throwing air (like a fan) on the vertical axis onto the tail fin.

Are the characteristics, and thus what is checked in the certification process, radically different.

Forgive me but my knowledge does not extend adequately to answer this. In every certification of a new aircraft or a significant modification a thousand things would be checked. Relative to this specific topic they would check in many different flying situations and on the ground the stability and grip of airflow over the flying and control surfaces, the fullness of air intake to the engines, the authority level of control the pilot has over the flying surfaces in different flying & ground positions, the manner in which the air breaks away over the wing especially the outer wing span sections, is the boundary layer breaking away too soon or too violently, what is the impact on stability and control when the landing gears are lowered,etc.

OT - Was the BAE 146 quiet by design (and if so, what elements of the design contributed to it. After all 4 engines!), or just turned out to be quiet by accident.

It was designed to be extra quiet for its day (1970s to 1990s). What does quiet mean is important here. Quiet is the noise footprint experienced by those under the flight path as it descends or climbs out. So two solutions both of which were used. First use the quietest engine then available which was the Avco Lycoming 502. The second was to give a wing and control surfaces such that the aircraft had a steep descent and climb thus reducing the footprint on the ground for a given decibel level. This second was achieved by (A) having a modestly swept wing that gave better control in descent and slower landing speeds (B) petal air brakes at the rear beneath the tail to slow the aircraft down in mid-air and thus increase the rate of controlled descent while engines were on a very low thrust (C) flaps that ran 3/4ths of the wing length and were blown by the 4 turbofans diverting some of the thrust downwards. This is common in military transports but not so in airliners.

Back to the 502 engines. These were, sadly, unreliable with many inflight shut downs and engine fumes leaking into the cabin pressurization system and thereby into the cabin. The 502 was the outcome of a American military research programme for a quiet engine and was the (to best of my knowledge) the first geared turbofan to enter real service. Which is exactly what the PW1100G is - causing Indigo and Go Air some headaches. The 502 had a good F.E., had low noise signature but was high maintenance. The engine proved to be the bête noire for the BAe146 which otherwise was a sensibly designed machine.

Hope this helps.


This is a small protrusion on the centre engine of the Lockheed L-1011 Tristar. This tiny change in design in later aircraft changed the way air flowed into the centre tail mounted engine so that it reduced noise very significantly. Common sense as we know it in daily life does not necessarily apply to airflows and thus aerodynamics.


Point #B above


Point #C above

[alpha1]

In addition to the points made by various people before me, I also have a gut feel that the prop pusher engine mounted at aft will not provide the "straight line stability" provided by a fore mounted engine. Essentially your rudder and elevators are so close to the point of propulsion that it will create an "unstable" motion regime.

[SmartCat]

Originally Posted by V.Narayan

This is a small protrusion on the centre engine of the Lockheed L-1011 Tristar. This tiny change in design in later aircraft changed the way air flowed into the centre tail mounted engine so that it reduced noise very significantly.

All Trijets are beautiful. What killed them? Noise?

Here is a Tupolev Tu-154 taking off. Turn up the speakers for a hair raising experience.

https://www.youtube.com/watch?v=wljRdyAqlrA

[amitk26]

Originally Posted by smartcat

All Trijets are beautiful. What killed them? Noise?

As I read Trijets were result of regulation in America which was largest aviation market in 60s - 70s. Reliability rules barred twin engine jets to cross over Atlantic and Pacific , Trijets such as MD10/DC 11 were designed such that they can overcome reliability regulations and also reduced cost.

Over a period of time reliability and thrust generated improved regulations were overhauled and and market of Trijets was taken over by twin engine jets.

4 Engine widerbody 747 could carry more passengers so market eventually divided between narrow body 2 engine and widebody 4 engine jets.

[anujmishra]

Originally Posted by smartcat

Most turboprop aircrafts have propeller infront of the engine like this -

But NAL Saras has propeller behind the engine.

Any idea why this is so? Does mounting propeller behind the engine have any particular cost advantages?

There are few reasons for putting propeller behind upwards of fuselage

1. Overall design needs to keep height of plane low for easy ingress & egress (small staircase). Putting propeller on wings means more height of fuselage.

2. The aircraft is low wing configuration, so there is no room for fitting propeller at safe distance from beneath (runway, parking). See the Honda Jet they have fitted turbofan engine above the wing of low wing aircraft.

What is difference between Boeing 737 & Airbus 320 family?

Have you marked turbofan engine housing of Boeing 737 made flat below? This is exactly same reason to keep maximum clearance from ground of low height aircraft. Designers have kept height of Boeing 737 as low as possible for loading, unloading cargo with ease. Whereas Airbus 320 family height much more than 737 so the round engine housing.

More such aircraft are (not necessarily turboprop aircraft): Embraer Phenom 300, Pilatus PC-24, Cessna Citation Jet with either engines are behind or over the wing of low wing configuration.

[sandeepmdas]

There were twin-jets, trijets and even quad-jets with aft-mounted configuration till late 60. Examples are the Caravelle, the 727 and the VC10. However, not a single large airliner in this configuration has been designed since '70s, except the MD-11. The DC-10 and L-1011 were entered in the service in the 70s, but designed in the '60s.

AFAIK, the reasons are:

• complexity of connections, both plumbing and routing
• difficulty in engine maintenance and routine observation (esp. in trijets)
• close proximity of engines (in quad-jets)
  
• flutter induced on a "clean" wing
  
• increase of weight due to extra-strong wings to counter flutter
• the small but inherent risk associated with a t-tail
• finding an ideal place for the APU

@Smartcat, the Tupolev is using a set of low bypass engines, and all low bypass engines WHEEEEEAMMMM!

[SmartCat]

Originally Posted by sandeepmdas

However, not a single large airliner in this configuration has been designed since '70s, except the MD-11. The DC-10 and L-1011 were entered in the service in the 70s, but designed in the '60s.

Not exactly an airliner of course, but did some search and found that Dassault Falcon 7X is the only aircraft still on sale with trijet configuration. From Dassault website:

Falcon tri-jets provide slower, safer approach and landing speeds. Their three-engine configuration gives peace of mind on long routes over water and uninhabited areas.

[blackasta]

Originally Posted by smartcat

All Trijets are beautiful. What killed them? Noise?

ETOPS & rising fuel costs.

[PGA]

NAL may be competent to build an aeroplane but is it capable of it, does it have wherewithal required for designing developing and testing the aeroplane and then selling it. Answer to why pusher type lies in there.

Secondly why IAF is flight testing an aeroplane built to civilian specs. Differentiation again about competence and capability is essential.

Military and civil aviation are different domains. While latter is staid and thorough the former is dynamic and result oriented.
Inquiry report is a must read for those wanting to understand if and why's of Saras crash and the huge systemic flaws. In the present ways we are most likely to come up with a great jugaad rather than a world beating and enduring product.

Frankly Saras in 2009 was nobody's baby, everybody tagged along wanting to claim fatherhood if it was a success and when we had one accident for which even though reasons came to be known within six months it was left as an orphan.

[V.Narayan]

Originally Posted by smartcat

Any idea how ISRO managed to prevent bureaucracy and politicians from ruining things?

ISRO and its predecessor organizations, for all practical purposes, have always reported directly to the Prime Minister.

[amitk26]

Originally Posted by PGA

Secondly why IAF is flight testing an aeroplane built to civilian specs. Differentiation again about competence and capability is essential.

IAF is first committed customer , Probably Navy and coastguard will be next. Civilian airliners are risk averse they wont buy a first product from a non-established player such as HAL ( NAL is design agency it wont do production). and I don't think any airline will buy Saras for foreseeable time until defense forces have used it for years.

Originally Posted by smartcat

Not exactly an airliner of course, but did some search and found that Dassault Falcon 7X is the only aircraft still on sale with trijet configuration. From Dassault website:

Many years ago I traveled from Copenhagen to Frankfurt in this I think now SAS retired their fleet of DC-10

[jraj]

Originally Posted by condor

And for this human error, they shelved the whole project for 7 years ?? The ones who decided to scrap the project should have been called out for this.

I believe it is much more complicated than just that.This the final investigation report on the ill fated crash in 2009.Page 42 has more info on how the SOP for the relight was derived from the SOI issued by the engine manufacturer.

http://dgca.nic.in/accident/reports/VT-XRM.pdf

However,it is highly debatable to see if it was just this crash which red flagged the programme. Since the time the first prototype was built,it weighed around 500kg more than the minimum weight specification.The one flown now also weighs pretty much the same and there is work to be done.

[Sutripta]

Originally Posted by V.Narayan

Dear Sutripta, Not sure I understood the question. The airflow characteristics of a wing mounted engine are very deeply documented and known with a 85 year history to back it since wind tunnel tests started.

Precisely the point I was seeking clarity on.
Knowing as much as we do about the different behaviour of props vs jets when wing mounted, do we still have to treat the rear mounted case as being entirely foreign, everything to learn anew, with the knowledge gained from wing positioned engines being of no use?

Regards
Sutripta

[V.Narayan]

When Brazil wanted to develop their own design & development capability for effectively the first time they started very practically with a simple unpressurized conventional design similar in size and performance to our Do228. This as you know was the EMB110 - first used by their Air Force and then introduced as a 19 seater regional commuter plane. They launched the project in 1965, first prototype flew in 1968, testing continued at full steam till 1973 and then production ran from 1973 to 1996 covering 501 examples sold to over a dozen countries. Our decision makers get carried away by grand projects that will make the world sit up and notice. In aeronautics we are ashamed to start small and go step by step and repeatedly lose 20 to 30 years spinning our wheels.

Originally Posted by Sutripta

Knowing as much as we do about the different behaviour of props vs jets when wing mounted, do we still have to treat the rear mounted case as being entirely foreign,

not entirely foreign; the principles of aerodynamics remain the same but each new design has to be thoroughly checked and tested

everything to learn anew, with the knowledge gained from wing positioned engines being of no use?

given that this is India's and NAL's first passenger aircraft design you cannot over test too much to gain customer confidence.

To illustrate my point - Question: would you or I go for heart surgery to a new doctor who is to perform his first heart surgery ever and who insists he is going to use a rare and not fully proven technique on you/me for his maiden heart surgery. That's exactly what's happening here. I am a fierce nationalist at heart and it pains me when I see these projects of national importance being pursued with such impracticality.

I'll stop my rant here. We should give the new team an opportunity to get organized. The political will seems to be there. Lets see.